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Synergy effect between tetracycline and Cr(VI) on combined pollution systems driving biochar-templatedFe3O4@SiO2/TiO2/g-C3N4 composites for enhanced removal of pollutants

Synergy effect between tetracycline and Cr(VI) on combined pollution systems driving... of ZnO in the visible region and the photoelectron-hole 1 Introduction separation efficiency through the interface charge trans - Nowadays, wastewater remediation is one of the critical fer (Wang et al. 2019). However, these photocatalysts are issues due to the complexity of water environment, which still unable to meet the requirements for practical appli- has been detected to contain various types of chemicals, cations, such as the development of a non-metal, green, such as heavy metals, organic matter, etc., posing serious high photocatalytic activity, and the facility in dispersion, threats to the ecological environment and even human separation and recovery of the photocatalysts from the health (Li et al. 2021a; Sellaoui et al. 2021; Yu et al. 2022). solution (Kumar et  al. 2020; Zhao et  al. 2020). Among Among these pollutants, Tetracycline (TC), an emerging heterojunctions, as we all know, Z − scheme heterojunc- antibiotic, and chromium Cr(VI), a common poisonous tion exhibits higher photocatalytic activity compared heavy metal, are difficult to remove from water by effec - with conventional type − II heterojunctions, due to high tive methods (Wang et  al. 2020). It is urged to develop redox potential and separation of the charge carriers efficient methods for the removal of TC and Cr(VI). (Huo et  al. 2021; Jourshabani et  al. 2020). The graphitic Alternatively, photocatalysis has shown considerable carbon nitride (g-C N ), as a nonmetallic photocatalyst, potential for pollutant removal due to its critical advan- 3 4 exhibits wide visible light response but low redox ability tages, such as the strong ability to completely degrade (Wen et al. 2017), and titanium dioxide ( TiO ), as a most the recalcitrant pollutants, and low environmental risks commonly used photocatalyst, shows low light use effi - without any oxidizer addition (Buzzetti et al. 2019; Long ciency but excellent redox ability (Guo et al. 2019). Based et  al. 2020; Ravelli et  al. 2009). Nevertheless, its prac- on the characteristics of high efficiency, low toxicity, and tical large-scale application is limited due to particu- particular band and electronic structures, g-C N and lar challenges, such as limited light use efficiency and 3 4 TiO are considered to be ideal components for the con- easy recombination of photo-generated charge carri- struction of Z − scheme heterojunction. ers (Hu et  al. 2021; Qiu et  al. 2021a, b, c). Based on the Recently, a series of TiO /g-C N -based heterojunc- energy band theory, with a heterojunction, the band gap 2 3 4 tions were developed to enhance the photocatalytic energy can be significantly reduced, so light absorption performance of pollutant removal. For example, a syn- and charge transfer will be increased (Wen et  al. 2017). thesized magnetically g-C N /TiO /Fe O @SiO nano- Therefore, the construction of heterojunction catalyst 3 4 2 3 4 2 photocatalyst exhibited the removal of 97% of ibuprofen has drawn broad attention of researchers owing to its within 15  min of visible light irradiance due to the low easy operation and high photocatalysis activity. The het - recombination rate of photogenerated carriers (Kumar erojunction photocatalysts (e.g., Bi MoO /ZnO, AgI/ 2 6 et  al. 2018a). Similarly, the construction of F e O @ CeO , ZnO-TiO and M oO /Bi O /g-C N ) have been 2 2 3 2 3 3 4 3 4 SiO @g-C N /TiO core–shell microsphere nanocom- used to remove pollutants in water (Das et  al. 2020; Li 2 3 4 2 posite exhibited 27.7-fold higher for Methyl Orange et  al. 2021b; Zhang et  al. 2019, 2021). The introduction degradation in comparison to T iO due to the modified of CdS heterojunction greatly improves the absorption 2 Y ang et al. Biochar (2023) 5:1 Page 3 of 20 band structure (Narzary et  al. 2020). In order to fur-anhydrous (Na SO ), potassium nitrate (KNO ), mono- 2 4 3 ther improve the photo-response and redox ability, the potassium phosphate (KH PO ), ammonium hydroxide 2 4 TiO /g-C N -based heterojunctions were modified to (NH OH, 28%), absolute ethyl alcohol (C H OH), diso- 2 3 4 4 2 5 synthesize a magnetically terephthalic acid functional- dium ethylenediamine tetraacetate ( C H N Na O ), 10 14 2 2 8 ized g-C N /TiO photocatalyst, which showed enhanced benzoquinone (C H O ), dinitrodiphenyl carbazide 3 4 2 6 4 2 degradation of pharmaceutical and personal care prod- (C H N O), potassium dichromate (K Cr O , AR), 13 14 4 2 2 7 ucts (PPCPs) (Kumar et  al. 2020). However, attention tetraethyl orthosilicate (C H O Si), tetrabutyl titanate 8 20 4 should still be paid to some challenges in the practical (C H O Ti), and isopropanol (C H O) were obtained 16 36 4 3 8 application of TiO /g-C N -based heterojunctions: (a) from Sino pharm Chemical Reagent Co., Ltd. Tetracy- 2 3 4 low adsorption activity stemming from agglomeration, cline (C H ClN O , 96%) was obtained from Shanghai 22 25 2 8 and (b) development of nonmetal heterojunction photo- Aladdin Biochemical Technology Co., Ltd. Ultrapure catalysts with lower costs (Qiu et al. 2021a). Recent years water (18.2 M cm) was used to prepare all the solutions. have witnessed the wide application of biochar-based materials in the field of photocatalysis for enhancing 2.2 Synthesis of biochar‑coupled Fe O @SiO /TiO /g‑C N 3 4 2 2 3 4 photocatalytic performance due to its excellent physico- composites chemical properties, such as large specific surface area, 2.2.1 Preparation of  Fe O nanoparticles 3 4 abundant surface functional groups and mineral com- Fe O nanoparticles were synthesized by a simple copre- 3 4 ponents (Kumar et  al. 2018b; Lyu et  al. 2020; Tam et  al. cipitation method. Briefly, A 50 mL mixture of Fe2+/ 2020). Accordingly, we supposed that the introduction of Fe3+ in the ratio of 1:1 was prepared with FeCl3·6H2O biochars could also improve the photocatalytic perfor- and FeSO4 ·7H2O, and vigorously stirred at 80 ℃ under mance of TiO /g-C N -based heterojunctions associated −1 2 3 4 the atmosphere of N for 10  min. Then, 0.048  g·mL of with the advantage of extensive availability and condu- trisodium citrate dihydrate solution was added into cive physio-chemical surface characteristics (Qiu et  al. the above mixture after pH was adjusted up to 10–11 2022; Yang et  al. 2022). Furthermore, biochar could be by adding 5  M NaOH solution, which was continued expected to act as a promising alternative to semiconduc- whisking for 1  h to obtain the precipitated black F e O 3 4 tor components of photocatalysts for reducing environ- nanoparticles. mental risks. Nonetheless, the effects of the introduction of biochars on the optical electronic properties of the 2.2.2 Preparation of  Fe O @SiO composites heterojunction, as well as the performance of pollutant 3 4 2 In order to prevent the corrosive effect from the photo - removal have been poorly explored. Numerous studies electrons, Fe O nanoparticles were covered by SiO via tend to focus on the single pollutant system merely, while 3 4 2 the sol–gel method. 0.50  g of Fe O nanoparticles were the photocatalytic removal of pollutants in the combined 3 4 dispersed into 80 mL of ethanol, and then 10 mL of 28% pollution systems has been barely characterized. ammonium hydroxide and 18  mL of deionized water In this study, the biochar-coupled F e O @SiO /TiO /g- 3 4 2 2 were added to the above mixture. The mixture was added C N composites were successfully synthesized through 3 4 with 5 mL of tetraethyl orthosilicate and kept stirring for simple strategies for the photocatalytic oxidation of 4  h. Finally, the composites were washed with ethanol TC and removal of Cr(VI). To compared nonmetallic and water several times and dried at 60 ℃. g-C N -based photocatalysts, metallic CdS-based pho- 3 4 tocatalysts were also prepared. The effects of controlled parameters, including calcination temperature, pH, and 2.2.3 Synthesis of biochar‑coupled Fe O @SiO /TiO 3 4 2 2 ion strength, on the removal performance of TC were composites studied. The removal performance, possible mechanisms, The biochar-coupled Fe O @SiO /TiO (FSBT) photo- 3 4 2 2 degradation pathways of TC, as well as reusability of the catalyst was prepared by the similar method. 0.20  g of synthesized composites in the combined pollution sys- Fe O @SiO nanoparticles and 0.50  g of modified rice 3 4 2 tems were investigated. straw-derived biochar prepared in our previous work (Dai et al. 2020) were dispersed in 50 mL of ethanol and 2 Materials and methods sonicated for 20  min, respectively. After that, they were 2.1 Materials mixed, and 28% ammonium hydroxide was added drop- The biochar raw material rice straw used in this research wise into the suspension to adjust the pH to 8–9. 20 mL was obtained from Changzhou, Jiangsu Province of of titanium butoxide was added to the solution and vigor- China. Ferrous sulfate heptahydrate (F eSO ·7H O), fer- ously stirred at 80  °C for 2  h. Subsequently, the mixture 4 2 ric chloride hexahydrate (FeCl ·6H O), sodium hydrox- was heated hydrothermally by a Teflon-lined autoclave at 3 2 ide (NaOH), sodium chloride (NaCl), sodium sulphate 160 °C for 3 h. Yang et al. Biochar (2023) 5:1 Page 4 of 20 2.2.4 Synthesis of biochar‑coupled Fe O @SiO /TiO /g‑C N also investigated by adding contaminant-loaded pho- 3 4 2 2 3 4 composites tocatalysts to 0.5  M NaOH solution after each use, stir- Biochar-coupled Fe O @SiO /TiO /g-C N composites ring at 353 K for 1 h, and then replacing it with deionized 3 4 2 2 3 4 (FSBTG) were prepared via a modified in accordance water and stirring for another 1  h for complete desorp- with the previous study (Kumar et al. 2018a). The g-C N tion. The regenerative samples were washed, dried at 65 3 4 was prepared by thermal condensation of melamine mol- ℃ and applied for the next reusability test until removal ecules. Typically, melamine was calcined at 550 ℃ for efficiency was stabilized. Cr(VI) concentrations were −1 4  h in a muffle furnace at a heating rate of 20 ℃ min . detected via a diphenyl carbazide method using a UV– Finally, the powder from FSBT was milled with prepared vis spectrometer (UV-1901PC, Shanghai) at λ of 540 nm, g-C N and calcined (300, 450, and 600 ℃) for 1  h in a and TC concentrations also were detected at λ of 357 nm. 3 4 −1 tube furnace at a heating rate of 20 ℃ min to synthe- The photocatalytic intermediates in TC solution were size FSBGT. The biochar-coupled Fe O @SiO /TiO /CdS identified using high-performance liquid chromatog - 3 4 2 2 photocatalyst (FSBTS) was also prepared by a similar raphy coupled with mass (Thermo scientific Q Exactive method. The major steps are shown in Additional file  1: Ultimate 3000 UPLC-MS). Fig. S1. 3 Results and discussion 2.3 Characterization 3.1 Characterizations The characterization details of as-synthesized catalysts 3.1.1 Morpholo gy and textural characteristics are listed in the Additional file 1. The characters of the surface morphology of prepared composites were observed through SEM, EDS, TEM, and 2.4 Performance test HRTEM (Fig.  1; Additional file  1: Fig. S2). The g-C N 3 4 The adsorption and photocatalytic activities of as-syn - exhibited a bulk irregular layered structure, and agglom- thesized several photocatalysts were evaluated using high erated to each other, resulting from the polycondensation concentration TC solutions as the targeted contaminant. of melamine during the process of calcination (Li et  al. All adsorption experiments were conducted in a 250 mL 2019). The micrograph of FSBT (Fig.  1b) showed that conical flasks under the conditions of 100  mL of TC titanium dioxide covered F e O @SiO hybrid nanoparti- 3 4 2 solution, 0.10  g of catalyst dose, agitator speed of 150 r cles dispersed over the porous biochar matrix. It could be −1 min and reaction temperature of 298 K. In the adsorp- found that relatively small-sized g-C N nanosheets and 3 4 tion kinetic studies, the initial TC concentrations were cubic CdS particles existed, which were coated to bio- −1 set to 200 mg L , and the solutions were sampled every char-coupled FST nanoparticles to form biochar-coupled ten minutes for concentration measurement. Mean- FSTG and FSTS composites, respectively. In addition, the while, the initial TC concentrations were adjusted to the different elements were distributed on the surface of two −1 range from 10 to 200 mg L with an equilibrium time of catalysts (Fig.  1f-l; Additional file  1: Fig. S2f-l), obtained 60 min. All catalysts were stirred for 120 min in the dark via the EDS elemental mappings of the SEM images, indi- to ensure adsorption equilibrium before photocatalytic cating the successful construction of catalysts. The TEM degradation experiments. The photocatalytic activity was and high-resolution TEM (HRTEM) images clearly pre- assessed by the photodegradation of TC under simulated sented the different morphologies and phases of the com - solar light using a 200 W Xenon lamp as the light source. posite catalyst. It can be seen that the T iO nanoparticles −1 0.10  g photocatalyst was added to 200  mg L TC solu- wrap the CdS and g-C N to form two heterojunctions. 3 4 tion (100  mL) with constant stirring at 298  K. The sam - The different phases identified by HRTEM images and ples were taken out periodically and filtered by 0.22  μm selected area electron diffraction (SAED) (Fig.  1n, o) have membrane filters. The effects of pH (3–11), and 0.1  M lattice fringe spacings of 0.32, 0.31 and 0.35  nm, respec- − − 2− 3− inorganic anions (Cl, NO, SO, PO ) on the pho- tively, which are assigned to the (002) and (101) crys- 3 4 4 −1 tocatalytic reaction of 100 mg L TC solution were also tal plane of g-C N and TiO , respectively (Wang et  al. 3 4 2 studied. To distinguish the role of the reactive species in 2018b). In terms of FSBTS catalyst, the lattice fringes of the photocatalytic reaction, scavengers, such as tert-butyl interplanar spacing of about 0.319, 0.332 and 0.35  nm alcohol (tBuOH), potassium dichromate (PD), EDTA- could be ascribed to the (002), (110) and (111) crystallites 2Na and benzoquinone (BQ), were added to the 100 mg of hexagonal CdS, respectively (Additional file  1: Fig. S2n, −1 L TC solution with catalysts for scavenger experi- o). The presence of high resolution lattice fringes and ments. Besides, the photocatalytic activity of catalysts in diffraction cycles of SAED patterns showed the highly −1 the combined pollution systems with TC of 100  mg L crystalline nature of the conductor nanoparticles of as- −1 and Cr(VI) of 20  mg L was further studied. The cata - synthesized composite catalysts (Kumar et al. 2018a). lyst reusability in the combined pollution systems was Y ang et al. Biochar (2023) 5:1 Page 5 of 20 Fig. 1 SEM micrographs of g-C N (a), FSBT (b), and FSBTG (c); EDS pattern (d), SEM–EDS layered image of FSBTG (e, f); EDS mapping of FSBTG for 3 4 the elements of C (g), N (h), O (i), Si (j), Ti (k) and Fe (l); TEM (m) and HRTEM (n) micrograph of FSBTG with corresponding SAED patterns in inset (o) The textural properties of catalysts were studied and Liu 2019). Conversely, the hysteresis pattern of bio- through N adsorption–desorption isotherms and pore chars is more likely to be a narrow silt-like structure size distribution curves (Additional file  1: Fig. S3). Com- (Mian and Liu 2019). Specifically, when P/P < 0.4, the pared with FST, FSBT and biochars, a higher capacity isotherm showed an upward trend, indicating the exist- of N adsorption–desorption was observed due to the ence of micropores, and when P/P > 0.4, the hysteresis 2 0 introduction of the heterojunction. All catalysts show loop increased, indicating the existence of abundant increasing hysteresis pattern without plateau at high rela- mesoporous structures. The adsorption capacity of com - tive pressure, indicating the shape might transform from posites containing biochar or heterojunctions was higher disorder lamellar, slit and wedge-shaped pore structure than that of other catalysts, which could be ascribed to to regular shape after secondary calcining. However, the development of more surface area and pore volume, the H3 loop characterized the structure of synthesized resulting from further calcination during the process of catalysts based on the inconspicuous steady trend (Mian preparation of heterojunctions. The continually rising Yang et al. Biochar (2023) 5:1 Page 6 of 20 trend isotherm of all catalysts suggested the presence of these peaks are attributed to the existence of g-C N 3 4 both microporous and mesoporous structures (Cai et al. (Kumar et  al. 2018b; Li et  al. 2019). The relatively wide −1 2013), and it was also further confirmed by pore size dis - peaks near 3140  cm stem from the bending vibration of tribution curves. The specific surface area (S) of all cata - terminal NH or NH groups at the defect sites of the aro- lysts accorded with the following rule: S (280.30 matic ring (Yan and Yang 2011). Besides the above peaks, FSTBG 2 −1 2 −1 2 m g ) > S (266.41 m g ) > S (255.07 m several new peaks indicated stretching vibrations of FSTG FSTBS −1 2 −1 2 −1 −1 g ) > S (247.07 m g ) > S (135.87 m g ) > S Ti–O, Ti–O-Ti and Si–O were observed at 400–600  cm FSTS FSBT FST 2 −1 2 −1 (111.05 m g ) > S (28.13 m g ), and a similar rule in as-synthesized catalysts (Nematollahzadeh et al. 2015). BC −1 −1 was also found in the total pore volume, which could be The peaks at 1500–1700  cm and 3340–3550  cm attributed to the aggregation of flaky g-C N and granu- are attributed to the stretching vibration of the H–O-H 3 4 lar CdS. The detachment of unstable impurities from the group and -OH group (Cheng et al. 2016; Dai et al. 2020), surface of composites could also improve the surface which are derived from the biochar, TiO , and CdS, sug- characteristics of synthesized catalysts (Table 1). gesting the successful construction of composite catalysts (Mian and Liu 2019). The elemental states of the synthesized photocatalysts 3.1.2 Composition and structural characteristics were analyzed via XPS in the C 1s, N 1s, Ti 2p and Fe 2p The crystal structures of the synthesized catalysts were binding energy regions. As shown in Fig.  2c–f, the C 1s analyzed via XRD (Fig.  2a; Additional file  1: Fig. S4a). peaks at 284.4 and 286.2 can be attributed to the C atoms In terms of Fe O , the peaks of 2θ at 30.1°, 35.5°, 43.1°, 3 4 in the C=C/C–C and C–OH bonds, respectively, which 57.0° and 62.6° correspond to the (220), (311), (400), (511) originate  from the graphitic and amorphous C of NaOH- and (440) planes, respectively. Due to amorphous char- activated biochar, whereas the peak centered at 288.5 eV acters (Kumar et  al. 2019), relevant peaks of SiO were indicates the C atoms in the structure of N = C-(N) (Li not found in Fe O @SiO . The peaks at 25.5° and 27.5° 3 4 2 et  al. 2019; Wang et al. 2020). The obvious N 1 s peak at correspond to the (101) and (002) lattice plane of g-C N 3 4 399.0  eV can be considered as the sp -hybridized aro- (Qiu et  al. 2021b). The TiO exhibited diffraction peaks matic N atoms derived from C=N–C, the weak peak at at 25.5°, 37.9°, 48. 0°, 54.0°, 55.1°, 62.8°, 68.9°, 70.2°, and 400.3  eV is assigned to (C) -N linking structural motifs 75.2°, which identify with the (101), (004), (200), (105), of C N , (C) -NH/C-NH , or Ti-ON, and the peak at 6 7 2 2 (211), (204), (116), (200), and (215) planes, respectively 401.1  eV is indicated to N atoms of aromatic cycles (Zhang et  al. 2017). The characteristic peaks at 26.6°, of N-(C) (Li et  al. 2019; Wang et  al. 2020; Zhang et  al. 44.0° and 52.0° of the original nano-CdS represent cubic 2018). As for Ti 2p, two obvious peaks located at 459.1 (111), (220) and (311) crystal planes and hexagonal (002), 3/2 1/2 and 464.9  eV are attributed to the Ti 2p and Ti 2p (110) and (311) (222), respectively. Same peaks could 4+ state of Ti , respectively (Ma et al. 2021). A satellite peak be observed in the catalysts containing TiO , g-C N , 2 3 4 at 472.1  eV can be clearly seen for the Ti(IV) oxidation or CdS, but the peak intensity decreased gradually due state (Herath et al. 2022). For Fe 2p, the peaks at around to the very low quantity detected on the surface of the 3/2 1/2 711.5 and 724.5 eV are assignable to Fe 2p and Fe 2p catalysts. Interestingly, the peak intensity of TiO -based spin–orbit, respectively (Djellabi et  al. 2019). Moreover, heterojunctions was enhanced with the increase of calci- it can be curve-fitted with five peaks. Among them, the nation temperature from 300 to 600 ℃, which could be ones at the binding energies of 710.9 and 724.4 eV are in ascribed to the decrease in thickness of g-C N (Li et al. 3 4 2+ accordance with Fe (Liu et al. 2020; Raha and Ahmaru- 2017). When the calcination temperature reached up to 3+ zzaman 2020), the one at 712.5 is consistent with Fe 600 ℃, new peaks at 36° and 41.1° were observed, indicat- (Mian et al. 2019), and the ones at 719.3 and 732.8 eV are ing the formation of rutile (101) and (111) plane under the satellites peaks of Fe O (Liu et al. 2020). 3 4 high calcination temperatures (Zhang et al. 2017). As the synthesis temperature of CdS-based catalyst increased to 600  °C, the composite catalyst generated new hexagonal 3.1.3 O ptical and electronic characteristics (101) and (103) crystal planes (Wang et al. 2018b). The optical properties of a catalyst directly influence The functional groups of the as-prepared catalysts were its photocatalytic activity. The light response char- studied by FT-IR spectra (Fig.  2b; Additional file  1: Fig. acters of the as-synthesized catalysts were analyzed −1  S4b). The peaks at 460 and 540  cm correspond with by ultraviolet–visible/diffuse reflection spectroscopy the stretching vibrations of Fe–O (Nematollahzadeh (UV–vis/DRS). In Fig.  3a, g-C N exhibits an obvious 3 4 –1 et  al. 2015). The peaks at 808  cm indicate the bending light response at 200–450 nm, indicating a certain vis- vibration of s-triazine units (Li et al. 2019), several peaks ible response. For g-C N -based photocatalysts, the 3 4 –1 in the range of 880–1335  cm can be attributed to the light absorption over the range of 200–800  nm can be stretching vibration of N–H/N-H , C–N and C=N, and attributed to the decreased reflectivity by introducing 2 Y ang et al. Biochar (2023) 5:1 Page 7 of 20 Fig. 2 XRD patterns (a); FT-IR spectra (b) of TiO , g-C N and several g-C N -based photocatalysts; high-resolution XPS spectra of C 1 s (c), N 1 s (d), 2 3 4 3 4 Ti 2p (e), and Fe 2p (f) of FSBTG-450 Yang et al. Biochar (2023) 5:1 Page 8 of 20 Fig. 3 UV–vis/DRS spectra, Tauc plots, Transient photocurrent response and EIS spectra of g-C N -based photocatalysts (a, c, e) and CdS-based 3 4 photocatalysts (b, d, f) under visible light illumination (> 420 nm) black biochars, which improved the electronic transi- that the band gap of composite catalysts is lower than tion efficiency by capturing photons (Li et  al. 2015). that of g-C N , which could be attributed to the for- 3 4 The band gap plots are also plotted according to Tauc mation of heterojunctions between the employed pho- relation (Kumar et al. 2017) in Fig.  3a. It can be found tocatalysts (Jourshabani et  al. 2020). In addition, the Y ang et al. Biochar (2023) 5:1 Page 9 of 20 band gap energy of semiconductors can be decreased possible reason could be that the heterojunction is ben- by the introduction of biochar. It can be interpreted as eficial to the migration of carriers but ineffective to the the reason that biochar can sensitize the semiconduc- separation of photogenerated electron/hole. tor with another composite, create a mid-gap energy state via doping non-metals such as N, S, C, O etc., and 3.2 Performance tests form a local trapping state below the conduction band, 3.2.1 R emoval of TC from water by adsorption meanwhile, the N can also reduce the band gap of the Adsorption plays an important role in the process of composite by forming a mid-gap energy state (Mian photocatalytic removal of pollutants by catalysts. The and Liu 2018). A red shift was found for CdS based kinetics and isotherm models were used to describe photocatalysts from Fig.  3b, indicating a stronger abil- the adsorption capacity and mechanism of TC with the ity to capture photons compared with CdS. This could synthesized catalysts (Fig.  4). In order to research the result from the formation of heterojunction among adsorption kinetic characteristics, the experimental data biochars, Fe O @SiO, TiO and CdS. were fitted by pseudo-first-order and pseudo-second- 3 4 2 2 The photocurrent responses and electrochemical order models, respectively. The higher correlation coef - impedance spectroscopy (EIS) were studied to evaluate ficients of the two models suggest that both physical and the separation efficiency of photo-generated carriers. It chemical mechanisms were involved in this adsorption can be seen in Fig.  3c that a steady photocurrent was process. TC sorption by catalysts rose rapidly during the generated, and the switching on of current was done first 20  min, then gradually slowed down until sorption after every 50  s. FSBTG-450 and FSTG-450 increase equilibrium after around 60  min. Compared with FST significantly in photocurrent compared to g-C N , sug- and FSBT,   the equilibrium adsorption capacity of the 3 4 gesting the reduction of the photogenerated charge catalysts introducing g-C N increased by 116.47 mg g−1 3 4 carrier recombination (Guo et  al. 2021). It could be and 102.34 mg g−1, respectively, and that of the catalysts deemed that introducing biochar can further improve introducing CdS increased by 119.61 mg g−1and 120.95 the separation of photo-generated charge carriers of mg g−1, respectively. Moreover, the adsorption capacity photocatalysts, in accordance with the phenomenon of FSBTS gradually decreased as calcination temperature that the FSBTG-450 shows a higher photocurrent increased, and the catalysts calcined at 300  °C showed intensity than FSTG-450. The results were also found in the best performance. Maximal adsorption capacity was previous studies (Lyu et al. 2020; Tam et al. 2020). Sig- presented under 450 °C for FSBTG. The results could be nificantly, a prominent decrease occurred in photocur - explained via the reasons that the structure and phys- rent of CdS-based photocatalysts in comparison to CdS icochemical properties connected with the adsorption (Fig.  3d), although biochar, F e O @SiO, TiO could of rice straw-derived biochar can be easily changed as 3 4 2 2 promote the absorption of visible light. The reduced calcination temperature increases. It has been suggested charge carrier recombination of the composite catalysts in previous studies that the biochars at high tempera- can be reconfirmed by EIS analysis in Fig.  3e, in which tures shows structural defects, blocked micropores on the composite catalysts exhibit lower charge transfer the surface (Rosales et  al. 2017), and a decrease of car- resistance and small Nyquist semicircle compared with boxylic functional groups (Zhao et al. 2017), resulting in single g-C N . It can be explained via the following the decrease of adsorption active sites. Similarly, both 3 4 main reasons: (1) the aromatic ring structure of biochar Langmuir and Freundlich isotherm models can fit the can accelerate the electron shuttling between different isotherm experimental data well. q gradually increased reactants, which could be mediated via surface redox- along with the increase of C due to the more powerful active moieties; (2) biochar containing large amounts of driving force and contact area between high concentra- quinone can act as an electron reservoir, while the elec- tion TC and adsorbent (Dai et al. 2020). FSBTS-300 and tron storage capacity of biochar depends on the types FSTS-300 showed maximum excellent adsorption capac- −1 −1 of biomass and the pyrolysis conditions; (3) the forma- ities reaching up to 173.32  mg g and 171.82  mg g , tion of heterojunction between the semiconductor and while lower maximum adsorption capacities of FSBTG- −1 biochar is responsible for potentiation in charge sepa- 450 and FSTG-450 came up to 147.96  mg g and −1 ration, and the similar increase in charge separation 104.70 mg g , respectively. Metal components decreased can occur at metal Fe or iron oxides and semiconductor and nonmetallic g-C N , barely adsorbing TC, replaced 3 4 heterojunctions, which can be promoted by light irra- Fe O @SiO and TiO to occupy the active sites, which 3 4 2 2 diation (Mian and Liu 2018). Interestingly, inconsistent caused the above phenomenon. In general, the Langmuir results are presented between Fig. 3d and Fig. 3f. FSBT- model had a higher nonlinear relationship coefficient, 300 and FSBTS-300 revealed lower charge transfer indicating that TC adsorption may be single-layer molec- resistance than CdS on account of Nyquist semicircle, a ular adsorption, and chemical adsorption is involved in Yang et al. Biochar (2023) 5:1 Page 10 of 20 this process (Zeng et  al. 2019). The key parameter n of rate on TC. Based on the characterizations of catalysts, the Freundlich model is related to the degree of hetero- it can be seen that the catalysts introducing g-C N 3 4 geneity of adsorption sites, and all n values in this study and CdS had larger specific surface area, wider visible were < 1, indicating the high heterogeneity of the solu- light response range, and high photogenerated car- tion during the TC adsorption process (Srivastava et  al. rier separation capacity. Different from CdS, introduc - 2006). The presence of oxygen-containing groups, such ing g-C N to FSBT showed lower adsorption capacity, 3 4 as hydroxyl and carboxyl groups, can contribute to the which improved photocatalytic activity at the cost of formation of H-bonding between photocatalysts and TC decreasing the adsorption activity. During the process (Rosales et  al. 2017). It is reasonable to assume that the of TC removal by FSBTG, photocatalytic oxidation, coating of nanoparticles on carbon structure could also which played a leading role, was relatively slow and dif- increase the adsorption sites, which can be confirmed ficult compared with adsorption. Therefore, FSBTG and via the result that the introduction of metal nanoparti- FSBT showed similar adsorption capacities. It can be cles, such as TiO, Fe O , and SiO , further improved the discovered that the incorporation of biochar had almost 2 3 4 2 adsorption of  biochar or biochar-based catalysts. Con- no effect on photocatalytic performance, although bio - sequently, besides H-bonding, the  interaction of surface char was considered as an electron reservoir in previ- complexes could be considered as a dominant mecha- ous studies (Mian and Liu 2018). As for temperature, nism in TC adsorption (Tan et al. 2016).  the same results could be found in adsorption and pho- tocatalytic degradation. It was reported that biochar 3.2.2 Removal of TC from water by photocatalytic synthesized at high heat temperatures showed a higher degradation electron storage capacity (Kluepfel et  al. 2014). In this The photolysis experiments under simulated sun - study, the catalysts with the high calcination tempera- light with different as-synthesized photocatalysts were ture exhibited decreased performance, suggesting that conducted to investigate their photolysis efficiency. the electron storage capacity of biochar may be not the As shown in Fig.  5a, b, in the absence of any kinds of primary factor affecting photocatalytic performance, catalysts, the TC concentration remained unchanged and there is no obvious evidence about electron stor- with increasing irradiation time, indicating negligible age stemming from biochar. In addition, a previous photolysis of TC without photocatalyst. The compos - study showed that the catalytic activity of carbon-based ite catalysts exhibited relatively high photocatalytic materials also probably depends on the internal elec- efficiency, and FSBTG-450 and FSBTS-300 showed tronic state of the hybrid covalent system (Duan et  al. the most excellent efficiency of 88.20% and 91.88%, 2018). Oxygen-containing functional groups are the much higher than that of pure g-C N (12.65%) and main active sites for the catalytic oxidation of carbo- 3 4 CdS (47.21%). Similarly, the catalysts synthesized by naceous catalysts (Wang et  al. 2018b), and the intro- high-temperature calcination showed relatively low duction of metal oxides and biochars may also cause O removal performance, and  450 ℃ and 300 ℃ were doping to g-C N , resulting in the generation of more 3 4 the optimal temperatures for FSBTG and FSBTS, active sites and high photocatalytic activity. Compared respectively, which could be ascribed to the forma- with photocatalysts synthesized in recent studies, as tion of anatase TiO with higher weight and activity illustrated in Table  2, various photocatalysts exhibited at the suitable conditions (Zhang et  al. 2017). Due to significantly different removal performances for TC the faster rate of adsorption than that of degradation, due to the difference in characters of catalysts, reac - the TC removal efficiency rose rapidly during the first tion conditions, as well as photocatalytic mechanisms 20  min, subsequently, the activity sites were occupied (Saadati et  al. 2016). Above all, the as-synthesized by adsorbed TC and intermediates of incomplete deg- FSBTG-450 catalyst could be considered as an efficient radation, resulting in gradual slowness in removal material with high removal performance for high con- rate. This phenomenon also suggests that adsorption centration TC solutions. plays a critical role in all processes of TC removal. The removal of TC followed degradation kinetics, as shown 3.3 Eec ff t of solution factors on the removal of TC in Fig.  5c, d; Additional file  1: Table  S3, the removal 3.3.1 pH rate of catalyst follows the following order: FSTG-450 It was deemed that the ionization state of pollutants and −1 −1 (0.0071  min ), FSBTG-450 (0.0068  min ), BTG-450 the surface charge of the photocatalyst are affected via −1 −1 (0.0051  min ), FSBTG-300 (0.0025  min ), FSBTG- pH (Kumar et  al. 2018a). Similar results were exhibited −1 −1 600 (0.00027  min ), and g-C N (0.00026  min ). k for  g-C N - and CdS-based photocatalysts on the effect 3 4 3 4 value of FSBTG was 1.33 times that of BTG, suggesting of pH. The solution pH was adjusted using 0.1  M HCl that Fe O @SiO composites enhanced the degradation and NaOH, and the effect of pH on the photocatalytic 3 4 2 Y ang et al. Biochar (2023) 5:1 Page 11 of 20 Fig. 4 Adsorption kinetics and adsorption isotherm for TC adsorption by g-C N -based photocatalysts (a, c) and CdS-based photocatalysts (b, d) 3 4 −1 (TC: C = 200 mg L , 100 mL; photocatalyst dosage: 0.10 g, T = 298 K ) − − 2− 3− removal of TC by FSBTG-450 is shown in Additional 3.3.2 Anions (Cl , NO , SO , and PO ) 3 4 4 file  1: Fig. S5a. After 1  h of light irradiation, 88.78%, To preliminarily evaluate the applicability of FSBTG- 87.68%, 83.60%, 77.18%, 76.40% and 77.33% of TC were 450 in the actual water environment, various 0.1  M ani- − − 2− 3− removed when the pH of the solution was not adjusted, ons, including Cl , NO , SO , and PO , were used to 3 4 4 3, 5, 7, 9 and 11, respectively. Generally, the state of TC explore the effect on the photocatalytic removal of TC. is affected by the pH of the solution. When pH < 3.4, As shown in Additional file  1: Fig. S5c, the photocatalyst 3.4 < pH < 7.6, 7.6 < pH < 9.7 and pH > 9.7, TC exhibits exhibited 88.36% of removal efficiency during 1  h with - + − 2− the state of H TC, HTC, H TC and HTC , respec- 4 3 2 out any additional anion. The removal efficiency was tively (Jang and Kan 2019). It was found that the sur- 82.46%, 91.86%, 88.62%, and 9.47% in the presence of ani- face of adsorbents was positively charged below pH , zpc 2− − − 3− ons of SO , Cl , NO , and PO , respectively. Appar- 4 3 4 otherwise it was negatively charged (Dai et  al. 2020). ently, the effect of Cl on TC removal was positive, that From Additional file  1: Fig. S8, the zero point of charge of NO was negligible, while a slightly inhibitory effect (pH ) of FSBTG is 4.43. Obviously, the catalyst exhib- zpc 2− of SO on the reaction was observed, which can be ited a relatively high removal efficiency when pH < pH , zpc which could be attributed to the electrostatic interaction ascribed to the reactive species scavenging reaction from 2− 2− − ·− between the catalyst and TC ions or molecules, but activ- SO (Zhang et  al. 2017): ·OH + SO → OH + SO , 4 4 4 ity was inhibited when pH > pH due to the electrostatic + 2− ·− zpc h + SO → SO . Moreover, TiO existing on the sur- 4 4 repulsion. However, the effect of pH on the removal of 2− face of the composite catalyst can adsorb SO through TC was not very significant, indicating that other mecha - van der Waals forces and hydrogen bonds, occupying the nisms might play critical roles  in this process. Yang et al. Biochar (2023) 5:1 Page 12 of 20 2016; Wang et al. 2020). In addition, ·OH was caught and active sites of the catalyst (Xekoukoulotakis et  al. 2011). the transfer of h from g-C N and CdS to TiO through The strong inhibitory effect was observed in the presence 3 4 2 3− 3− biochars and Fe O @SiO was hindered (Kudlek et  al. of PO due to the strong adsorption affinity of PO for 3 4 2 4 4 2016). FSBTS-300 showed similar results. TiO and biochar, resulting in a large number of surface sites that were competitively occupied (Kudlek et  al. Table 1 The main surface characteristics of biochar and as-synthesized catalysts 2 −1 3 −1 Sample Specific surface area (m g ) Total pore volume (cm g ) Pore width (nm) Average pore diameter (nm) BC 28.13 0.031 – 9.45 FST 111.05 0.204 3.627 5.36 FSBT 135.87 0.326 4.543 7.33 FSTG 280.30 0.514 3.627 5.37 FSBTG 266.41 0.466 3.627 5.24 FSTS 247.07 0.456 3.627 5.12 FSBTS 255.07 0.431 3.627 4.78 Table 2 Comparison of photocatalytic removal of various catalysts for tetracycline degradation  Catalyst Reaction system Reaction  condition Removal performance Reactive species Refs. −1 − Biochar-coupled Fe O @ Simulated solar light [Catalyst] = 1.0 g L ; Removal 92% in 180 min, ·O , ·OH This study 3 4 −1 −1 SiO /TiO /g-C N com- [ TC] = 200 mg L ; k = 0.0068 min , 2 2 3 4 obs −1 posites pH 3–11; UV–Vis of 200 W Q = 147.96 mg g Xe lamp −1 + − Hollow tubular g-C N Vis [Catalyst] = 1.0 g L ; Removal 85% within 30 min h , · O , ·OH Liang et al.( 2021) 3 4 −1 isotype heterojunction [ TC] = 5–30 mg L ; pH 2–11; Vis of 300 W Xe lamp with light intensity of −2 100 mW cm −1 + − − N-TiO /O-doped N vacancy Vis [Catalyst] = 0.4 g L ; Removal 80% in 20 min, h , e , · O , ·OH Wang et al. ( 2020) −1 −1 g-C N [ TC] = 30 mg L ; k = 0.0170 min 3 4 obs pH 2.94–9.32; Vis of 300 W Xe lamp −1 + − OV-mediated sandwich-like Vis [Catalyst] = 0.5–2 g L ; Removal 88% in 90 min, h , · O , ·OH Ni et al. (2021) −1 −1 TiO /ultrathin g-C N / [ TC] = 5–20 mg L ; k = 0.0317 min 2-x 3 4 obs TiO pH 3–11; Vis of 300 W Xe 2-x lamp −1 + − Boron nitride-fullerene com- Vis [Catalyst] = 0.1 g L ; Removal 97% in 90 min, h , · Guo et al. (2021) −1 −1 posite (C /BN-U ) [ TC] = 20 mg L ; Q = 131.05 mg g 60 6 e pH 3–11; Vis of Xe lamp with intensity of 164.4 mW −2 cm Au-TiO /PVDF composite Vis 20 mL of TC aqueous solu- Removal 75% in 120 min, ·O , ·OH Yan et al. (2021) 2 2 −1 tion; k = 0.0121 min obs Vis of 300 W Xe lamp −1 + − − MnFe O -Au composites H O /Vis [Catalyst] = 0.1 g L ; Removal 88% in 90 min, h , e , · O , ·OH Qin et al. (2021) 2 4 2 2 2 −1 −1 [ TC] = 10–40 mg L ; k = 0.0231 min obs pH 3–9; Vis of 300 W Xe lamp with light inten- −2 sity of 434 mW cm ; [H O ] = 10–80 mM 2 2 3− −1 PO -Bi WO /PI Simulated solar light [Catalyst] = 1.0 g L ; Removal 65% in 20 min, ·O , ·OH Gao et al. (2021) 4 2 6 2 −1 −1 [ TC] = 20 mg L ; k = 0.0066 min obs pH 3.15–11.15; light of 150 W Xe lamp Y ang et al. Biochar (2023) 5:1 Page 13 of 20 Fig. 5 Photodegradation curves and degradation kinetic curves of TC with g-C N -based photocatalysts (a, c) and CdS-based photocatalysts (b, d) 3 4 −1 (TC: C = 200 mg L , 100 mL; photocatalyst dosage: 0.10 g) 3.3.3 Active substances during degradation of TC reduced greatly after the addition of tBuOH, elu- − + E and h are the main reactive species produced on cidating that ·OH was the primary active species dur- the surface of the photocatalysts during the photocata- ing the TC degradation process. To further validate the lytic removal process. After that, they combine with generation of active species during the reaction process, dissolved O and H O to form other reactive species the ESR spin-trap with DMPO technique was carried 2 2 including ·OH and ·O (Kumar et al. 2018a). In order to out. As shown in Additional file  1: Fig. S6a, the TEMPO identify the dominant active substances controlling the signal peak strength fell with longer illumination times, photocatalytic removal of TC under light irradiation, which indicated that FSBTG could generate electrons different scavengers of 1  mM, tBuOH (·OH scavenger), and holes due to the anaerobic condition of acetonitrile − + PD (e scavenger), EDTA-2Na (h scavenger) and BQ solvent, so the heterojunction enhanced the spatial sep- ·O ( 2 scavenger) were added into the reaction systems, aration of photo-generate carriers (Wang et  al. 2020). respectively (Kumari et  al. 2022). The experiment was Moreover, there was no EPR signal for the DMPO-·OH also conducted without any scavenger during degra- or DMPO-·O adducts under dark conditions, indicat- dation for reference. From Additional file  1: Fig. S5d, it ing that no active free radicals were generated. However, was discovered that the removal efficiency decreased a four characteristic peaks for DMPO-·OH and six charac- − + little after adding PD and EDTA-2Na, so e and h had teristic peaks for DMPO-·O were found in the presence a slight effect on the removal efficiency of TC. Never - of light and enhanced with increasing light irradiation theless, the photocatalytic activity reduced clearly due time (Li et  al. 2022a), demonstrating that FSBTG could to the introduction of BQ, indicating the importance of produce ·OH and ·O during the photocatalytic reac- ·O in TC removal. Moreover, the degradation efficiency tion. The contribution of reactive oxygen species (ROS) 2 Yang et al. Biochar (2023) 5:1 Page 14 of 20 2– in photodegradation was obtained in the following order: Although the presence of competition between Cr O 2 7 + − − + ·OH > ·O > h > e according to Additional file  1: Fig. and the photogenerated e /h   due to the high electron 2– S6d. As a result, ·OH and · O were generated during the cloud density of O atoms in Cr O , Cr (III) may adhere 2 7 photocatalytic degradation of TC, which was consist- to the surface of catalysts through adsorption, accelerat- ent with the results of the quenching experiments. H O ing the migration of photon-generated carriers (Li et  al. could react with h to produce ·OH, whereas ·O , gener- 2019). It can be supposed that a complexation interaction ated by the reaction between e and O , could react fur- occurred between the adsorbed Cr(III) on the surface ther with H O to produce ·OH (Chong et  al. 2010; Fang of catalysts and oxygen-containing functional groups of et al. 2015). Therefore, it is a revelation that the photocat - TC. In addition, the adsorption of Cr(III) on the surface alytic efficiency in TC solutions with high concentration of catalysts can improve the removal of Cr(VI) based on can be promoted by adjusting the ratio of H O and O for the disproportionation reaction between the adsorbed 2 2 the practical application of photocatalysts. Cr(III) and Cr(VI) (Wang et  al. 2020). As for FSBTS, Cr(VI) could be removed in the single system due to photo-generated electrons based on efficient separation 3.4 Simultaneous r emoval of TC and Cr(VI) of carriers. However, it was found that further enhanced in the combined pollution systems removal of Cr(VI) and slightly inhibited removal of TC The pollutant removal performance of FSBTG and FSBTS in the combined pollution systems, synergistic photoca- was further discussed in TC/Cr(VI) combined pollution 2– talysis effect and the competition between Cr O and 2 7 systems. The UV–vis spectroscopic spectra at differ - − + e /h might be responsible for the above phenomenon. ent times are shown in Fig.  6a, b. Obviously, during the The photocatalytic experiments were carried out for whole reaction course, except for the typical absorption five cycles in the TC/Cr(VI) combined pollution sys - peaks, new peaks emerged and these two peaks synchro- tems to evaluate the stability of the recycled catalysts. nously decreased with offset, suggesting that the pollut - After the first catalytic run, the recovered photocata - ants were altered, not merely adsorbed. The pollutant lysts were regenerated and then used for the next pho- removal performance of photocatalysts in single and TC/ tocatalytic experiment under the same conditions. As Cr(VI) combined pollution systems are shown in Fig. 6c, shown in Fig.  6e, f, the removal efficiency of TC and d. Interestingly, in terms of FSBTG, the removal of Cr(VI) Cr(VI) by the recycled catalyst exhibited a trend of in the single system without adjustment of pH can be gradual decrease  with the increase in cycle time, and disregarded, with only 2.7% of Cr(VI) being removed especially, a significant adverse effect on the removal of within 120  min under light irradiation. The significantly Cr(VI) was observed. After five photocatalytic cycles, increased removal of 69.99% and 77.95% occurred in the TC removal rate of FSBTG was reduced from 91.03% the combined pollution systems, as shown in Additional to 57.32%, and the removal rate of Cr(VI) was reduced file  1: Table S3, and the degradation rates, k value, could −1 −1 from 78.56% to 16.08%. This may be due to the reaction reach up to 0.0093  min for FSTG and 0.0108  min for between the hot alkaline solution and chromium ions in FSBTG, which were over 46 and 54 times more than that the catalyst regeneration process to form a precipitate of FSBTG in the single system. Likewise, the removal that is tightly combined with the catalyst, occupying the of TC in the combined pollution system also slightly active sites of the catalyst, and causing the catalytic activ- increased compared with the single system. k values were −1 ity to decrease with the frequency of recycling. Further 1.99 and 2.24 times for FSTG (0.0141  min ) and FSBTG −1 searching for suitable biochar-based catalyst regenerators (0.0152  min ), respectively. The significantly enhanced is needed to achieve effective regeneration of the catalyst removal of Cr(VI) can be explained by the presence of the without significant adverse effects on its activity. Also, synergistic photocatalysis effect in the combined pollu - the removal of TC and Cr(VI) for FSBTS was greatly tion systems. In a Cr(VI) single pollution system without influenced by cycle time. Besides the above reason, local adjustment of pH, the Cr(VI) is barely removed because 2– aggregation and deactivation of FSBTS during the regen- of the high electron cloud density of O atoms in Cr O , 2 7 eration process and the loss of photocatalysts during resulting in the difference in the generation of the reduc - wash/dry process could weaken the removal of pollutants ing active intermediate (RAI) and oxidizing active inter- (Huang et al. 2018). mediate (OAI). The redox between TC and Cr(VI) occurring in the combined pollution systems without adjustment of pH can be interpreted by the fact that the 3.5 Removal mechanism of photocatalyst for TC and Cr(VI) + 2– H originating from TC can promote Cr O to form The removal of TC and Cr(VI) was achieved via adsorp - 2 7 RAI and OAI (Li et  al. 2019). It was observed that TC tion and photocatalytic redox reactions of photocata- removal in the combined pollution systems was slightly lyst, in which huge surface area gave it countless active higher than that in the TC single pollution system. sites, and the possible mechanism is shown in Fig.  7. Y ang et al. Biochar (2023) 5:1 Page 15 of 20 Fig. 6 UV–vis spectra, simultaneous TC photodegradation with Cr( VI) removal in the TC/Cr( VI) combined pollution systems, recycling experiments using FSBTG-450 photocatalysts (a, c, e) and FSBTS-300 photocatalysts (b, d, f) Adsorption played a critical role in the whole removal interaction, based on the appropriate pore size distribu- process, in which both physisorption and chemisorp- tion and surface charge of photocatalysts, which contrib- tion were involved, with the latter as the dominant uted to the adsorption effect (Dai et  al. 2020; Jang et  al. via isotherm and kinetic analysis. Physical adsorption 2018). The hydrogen bonding force and surface compl - was dominated by pore filling effect and electrostatic exation were deemed to be the critical mechanisms for Yang et al. Biochar (2023) 5:1 Page 16 of 20 can be removed by the e generated in the CB of g-C N . 3 4 Although the presence of competition for e between the removal of Cr(VI) and the generation of ·O , the Cr(VI) can be removed effectively due to the introduction of H derived from TC. To analyze the degradation pathway of TC, the inter- mediate products during the photocatalytic process were detected by means of UPLC-MS. The results showed 16 major intermediates during the degrada- tion process. Several photocatalytic degradation path- ways of TC over FSBTG-450 are proposed in Fig.  8. There are three types of functional groups, including double bond, phenolic group and amine group, which are vulnerable to attack by radicals due to the high electron density (Wang et  al. 2018a). In pathway I, TC was attacked by reactive oxygen species to generate Fig. 7 The plausible removal mechanism of TC and Cr( VI) by FSBTG the intermediate with m/z of 427 via dehydroxylation and N-demethylation due to the low energy of N–C bond (Guo et  al. 2021; Qin et  al. 2021). Subsequently, TC adsorption due to the presence of oxygen-containing the intermediates with m/z of 385 and 302 were gen- functional groups and metal oxides, which were discov- erated by dehydroxylation, demethylation and benzene ered to enhance TC adsorption in this study (Dai et  al. ring opening reaction. The two intermediates can be 2020; Li et  al. 2022b). The surface complexation interac - further decomposed by similar reaction mechanisms tion between metal chromium ions and oxygen-contain- to produce the intermediates with m/z of 306, 282, and ing functional groups of catalysts was also considered 148. In pathway II, the formation of the intermediate as the main force for Cr(VI) adsorption (Liu et  al. 2022; of m/z = 496 was mainly caused by the dehydrogena- Xiao et al. 2019; Zhu et al. 2021). The adsorbed pollutants tion of -OH in Carbon 3 position, ‒N(CH ) oxidation, can be further decomposed by reactive oxygen species, and carboxylation of -CH in Carbon 8 position after generation of which is significantly dominated by utiliza - the breakage of double bond in Carbon 7–8 position. tion of light and transport efficiency of photogenerated The intermediate was further oxidized to produce the charges (Buzzetti et al. 2019). The enhanced visible light intermediate of m/z = 433, which subsequently was response and separation of photogenerated charges decomposed to low-molecular-weight ketone or car- obtained from UV–vis DRS, photocurrent response, boxylic acid compounds with m/z of 256, 253, 196, 151, and EIS analysis indicated improved optical and elec- 119, and 118 by ring cleavage along with hydroxyla- tronic activities, which contributed to the photocatalytic tion. Similarly, in pathway III, the low-molecular inter- reaction of catalysts. The ·OH and ·O were regarded mediates with m/z of 283 and 214 were produced by as the dominant reactive species based on the quench- rings-opening, along with additional decarbonylation − + ing experiment. The photogenerated e and h transfer and demethylation reactions (Qin et  al. 2021). Finally, process of FSBTG-450 can be considered as Z-scheme these low molecular intermediates could be completely rather than type II heterojunction structure due to the decomposed to CO and H O. 2 2 product of ·O (Wang et  al. 2020). In the as-synthe- sized magnetic biochar-based composite catalyst, the photogenerated h in the valence band (VB) of g-C N 3 4 4 Conclusions and the e in the conduction band (CB) of T iO   recom- The biochar-coupled Fe O @SiO @TiO /g-C N com- 3 4 2 2 3 4 bined, and the components of   biochar and F e O @SiO 3 4 2 posites have been successfully synthesized, and exhibited accelerated the transmission of electrons, while lots of extraordinary removal performance for high-concen- electrons and holes survived in the CB of g-C N and 3 4 tration TC. The introduction of biochar and magnetic the VB of TiO , respectively. The ·O generated in the 2 2 nanoparticles can improve visible light response, charge CB of g-C N via reaction between e and O , and ·OH 3 4 2 transfer, and separation of photo-generate carriers. generated in the VB of TiO via reaction between h and 3− Except for the significant inhibition of PO , other H O were deemed to be responsible for the oxidation of anions and pH had no obvious effect on removal per - TC (Guo et al. 2021; Kumar et al. 2020). The Cr(VI) also formance. The heterojunction enhanced the spatial Y ang et al. Biochar (2023) 5:1 Page 17 of 20 Fig. 8 The TC degradation pathway in simultaneous photocatalytic redox conversion of TC and Cr( VI) systems over FSBTG-450 + − − and Z-scheme photocatalytic reaction and the pathway separation of h and e , and ·OH and ·O were iden- of TC were proposed. This work can provide a reference tified as the main reactive species responsible for TC for the removal of high-concentration TC/Cr(VI) in com- degradation and Cr(VI) removal. Due to the synergistic bined pollutant systems and the potential application of photocatalysis effect, the removal efficiency of Cr(VI) was biochar-based materials in waste treatment. notably enhanced in the combined pollution systems. In addition, removal mechanisms involved in adsorption Yang et al. Biochar (2023) 5:1 Page 18 of 20 Abbreviations University, Guangxi 530004, China. School of Environmental and Safety BC : Biochar; FST: Fe O @SiO /TiO ; FSBT: Biochar-coupled F e O @SiO /TiO ; Engineering, Changzhou University, Changzhou 213164, China. Environmen- 3 4 2 2 3 4 2 2 BTG: Biochar-coupled TiO /g-C N ; BTS: Biochar-coupled TiO /CdS; FSTG: tal Technology Engineering Co.Ltd Subsidiary of Chinese Research Academy 2 3 4 2 Fe O @SiO /TiO /g-C NFSTS, Fe O @SiO /TiO /CdS; FSBTG: Biochar-coupled- of Environmental Sciences, Beijing 100012, China. 3 4 2 2 3 4 3 4 2 2 Fe O @SiO /TiO /g-C N ; FSBTS: Biochar-coupledFe O @SiO /TiO /CdS. 3 4 2 2 3 4 3 4 2 2 Received: 11 August 2022 Revised: 3 December 2022 Accepted: 12 December 2022 Supplementary Information The online version contains supplementary material available at https:// doi. org/ 10. 1007/ s42773- 022- 00197-4. References Additional file 1: Fig. S1. Synthetic process of biochar-coupled SiO2@ Buzzetti L, Crisenza GEM, Melchiorre P (2019) Mechanistic studies in photoca- Fe O /TiO /g-C N composite. Fig. S2. SEM micrographs of CdS (a), FSBT 3 4 2 3 4 talysis. Angew Chem Int Ed 58(12):3730–3747. https:// doi. org/ 10. 1002/ (b), and FSBTS (c); EDS pattern (d), SEM-EDS layered image of FSBTS (e, anie. 20180 9984 f ); EDS mapping of FSBTS for the elements of C (g), Cd (h),O (i),Si ( j), Cai CJ, Xu MW, Bao SJ, Ji CC, Lu ZJ, Jia DZ (2013) A green and facile route for Ti (k) and Fe(l); TEM (m) and HRTEM (n) micrograph of FSBTS with cor- constructing flower-shaped TiO nanocrystals assembled on gra- responding SAED patterns in inset (o). Fig. S3. N adsorption-desorption 2 phene oxide sheets for enhanced photocatalytic activity. Nanotechnol isotherms (a) and pore size distribution curves of as-prepared catalysts 24(27):602. https:// doi. org/ 10. 1088/ 0957- 4484/ 24/ 27/ 275602 (b). Fig. S4. XRD patterns (a); FT-IR spectrums of biochar, TiO ,CdS and Cheng Y, Zhou F, Li S, Chen Z (2016) Removal of mixed contaminants, crystal several CdS-based photocatalysts (b). Fig. S5. Photocatalytic removal violet, and heavy metal ions by using immobilized stains as the func- efficiency of FSBTG-450 (a) and FSBTS-300 (b) photocatalyst at different − – 2− 3− tional biomaterial. RSC Adv 6(72):67858–67865. https:// doi. org/ 10. 1039/ pH; in the presence of 0.1 M Cl, NO ,SO, PO ions (c); with different 3 4 4 c6ra1 3337a scavengers (d). Fig. S6. The ESR spectra of TEMPO (a), DMPO- OH (b), • - Chong MN, Jin B, Chow CWK, Saint C (2010) Recent developments in photo- DMPO- O (c) of FSBTG and the relative contribution of each reactive catalytic water treatment technology: a review. Water Res 44(10):2997– oxygen species (d). Fig. S7. Full XPS spectra of FSBTG-450. Fig. S8. Zeta 3027. https:// doi. org/ 10. 1016/j. watres. 2010. 02. 039 potential of catalysts under various pH. Fig. S9. 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J Hazard Mater 403:123860. https:// doi. org/ 10. 1016/j. jhazm Project of the Ministry of Science and Technology of China (Grant Numbers at. 2020. 123860 [2020IM020300]); and the Beijing–Tianjin–Hebei Collaborative Innovation Guo Q, Zhou C, Ma Z, Yang X (2019) Fundamentals of TiO photocatalysis: Con- Promotion Project of China (Grant Numbers [Z201100006720001]). We want to cepts, mechanisms, and challenges. Adv Mater 31(50):1901997. https:// acknowledge anonymous reviewers for their valuable comments. doi. org/ 10. 1002/ adma. 20190 1997 Guo Y, Yan C, Guo Y, Ji X (2021) UV-light promoted formation of boron nitride- Availability of data and materials fullerene composite and its photodegradation performance for antibiot- All date generated during the current study are available from the corre- ics under visible light irradiation. J Hazard Mater 410:12468. https:// doi. sponding author on reasonable request. org/ 10. 1016/j. jhazm at. 2020. 124628 Herath A, Navarathna C, Warren S, Perez F, Pittman CU Jr, Mlsna TE (2022) Iron/ Declarations titanium oxide-biochar (F e TiO /BC): a versatile adsorbent/photocatalyst 2 5 2+ − for aqueous Cr( VI), P b, F and methylene blue. J Colloid Interface Sci Ethics approval and consent to participate 614:603–616. https:// doi. org/ 10. 1016/j. jcis. 2022. 01. 067 Not applicable. Hu C, Tu S, Tian N, Ma T, Zhang Y, Huang H (2021) Photocatalysis enhanced by external fields. 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Synergy effect between tetracycline and Cr(VI) on combined pollution systems driving biochar-templatedFe3O4@SiO2/TiO2/g-C3N4 composites for enhanced removal of pollutants

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Abstract

of ZnO in the visible region and the photoelectron-hole 1 Introduction separation efficiency through the interface charge trans - Nowadays, wastewater remediation is one of the critical fer (Wang et al. 2019). However, these photocatalysts are issues due to the complexity of water environment, which still unable to meet the requirements for practical appli- has been detected to contain various types of chemicals, cations, such as the development of a non-metal, green, such as heavy metals, organic matter, etc., posing serious high photocatalytic activity, and the facility in dispersion, threats to the ecological environment and even human separation and recovery of the photocatalysts from the health (Li et al. 2021a; Sellaoui et al. 2021; Yu et al. 2022). solution (Kumar et  al. 2020; Zhao et  al. 2020). Among Among these pollutants, Tetracycline (TC), an emerging heterojunctions, as we all know, Z − scheme heterojunc- antibiotic, and chromium Cr(VI), a common poisonous tion exhibits higher photocatalytic activity compared heavy metal, are difficult to remove from water by effec - with conventional type − II heterojunctions, due to high tive methods (Wang et  al. 2020). It is urged to develop redox potential and separation of the charge carriers efficient methods for the removal of TC and Cr(VI). (Huo et  al. 2021; Jourshabani et  al. 2020). The graphitic Alternatively, photocatalysis has shown considerable carbon nitride (g-C N ), as a nonmetallic photocatalyst, potential for pollutant removal due to its critical advan- 3 4 exhibits wide visible light response but low redox ability tages, such as the strong ability to completely degrade (Wen et al. 2017), and titanium dioxide ( TiO ), as a most the recalcitrant pollutants, and low environmental risks commonly used photocatalyst, shows low light use effi - without any oxidizer addition (Buzzetti et al. 2019; Long ciency but excellent redox ability (Guo et al. 2019). Based et  al. 2020; Ravelli et  al. 2009). Nevertheless, its prac- on the characteristics of high efficiency, low toxicity, and tical large-scale application is limited due to particu- particular band and electronic structures, g-C N and lar challenges, such as limited light use efficiency and 3 4 TiO are considered to be ideal components for the con- easy recombination of photo-generated charge carri- struction of Z − scheme heterojunction. ers (Hu et  al. 2021; Qiu et  al. 2021a, b, c). Based on the Recently, a series of TiO /g-C N -based heterojunc- energy band theory, with a heterojunction, the band gap 2 3 4 tions were developed to enhance the photocatalytic energy can be significantly reduced, so light absorption performance of pollutant removal. For example, a syn- and charge transfer will be increased (Wen et  al. 2017). thesized magnetically g-C N /TiO /Fe O @SiO nano- Therefore, the construction of heterojunction catalyst 3 4 2 3 4 2 photocatalyst exhibited the removal of 97% of ibuprofen has drawn broad attention of researchers owing to its within 15  min of visible light irradiance due to the low easy operation and high photocatalysis activity. The het - recombination rate of photogenerated carriers (Kumar erojunction photocatalysts (e.g., Bi MoO /ZnO, AgI/ 2 6 et  al. 2018a). Similarly, the construction of F e O @ CeO , ZnO-TiO and M oO /Bi O /g-C N ) have been 2 2 3 2 3 3 4 3 4 SiO @g-C N /TiO core–shell microsphere nanocom- used to remove pollutants in water (Das et  al. 2020; Li 2 3 4 2 posite exhibited 27.7-fold higher for Methyl Orange et  al. 2021b; Zhang et  al. 2019, 2021). The introduction degradation in comparison to T iO due to the modified of CdS heterojunction greatly improves the absorption 2 Y ang et al. Biochar (2023) 5:1 Page 3 of 20 band structure (Narzary et  al. 2020). In order to fur-anhydrous (Na SO ), potassium nitrate (KNO ), mono- 2 4 3 ther improve the photo-response and redox ability, the potassium phosphate (KH PO ), ammonium hydroxide 2 4 TiO /g-C N -based heterojunctions were modified to (NH OH, 28%), absolute ethyl alcohol (C H OH), diso- 2 3 4 4 2 5 synthesize a magnetically terephthalic acid functional- dium ethylenediamine tetraacetate ( C H N Na O ), 10 14 2 2 8 ized g-C N /TiO photocatalyst, which showed enhanced benzoquinone (C H O ), dinitrodiphenyl carbazide 3 4 2 6 4 2 degradation of pharmaceutical and personal care prod- (C H N O), potassium dichromate (K Cr O , AR), 13 14 4 2 2 7 ucts (PPCPs) (Kumar et  al. 2020). However, attention tetraethyl orthosilicate (C H O Si), tetrabutyl titanate 8 20 4 should still be paid to some challenges in the practical (C H O Ti), and isopropanol (C H O) were obtained 16 36 4 3 8 application of TiO /g-C N -based heterojunctions: (a) from Sino pharm Chemical Reagent Co., Ltd. Tetracy- 2 3 4 low adsorption activity stemming from agglomeration, cline (C H ClN O , 96%) was obtained from Shanghai 22 25 2 8 and (b) development of nonmetal heterojunction photo- Aladdin Biochemical Technology Co., Ltd. Ultrapure catalysts with lower costs (Qiu et al. 2021a). Recent years water (18.2 M cm) was used to prepare all the solutions. have witnessed the wide application of biochar-based materials in the field of photocatalysis for enhancing 2.2 Synthesis of biochar‑coupled Fe O @SiO /TiO /g‑C N 3 4 2 2 3 4 photocatalytic performance due to its excellent physico- composites chemical properties, such as large specific surface area, 2.2.1 Preparation of  Fe O nanoparticles 3 4 abundant surface functional groups and mineral com- Fe O nanoparticles were synthesized by a simple copre- 3 4 ponents (Kumar et  al. 2018b; Lyu et  al. 2020; Tam et  al. cipitation method. Briefly, A 50 mL mixture of Fe2+/ 2020). Accordingly, we supposed that the introduction of Fe3+ in the ratio of 1:1 was prepared with FeCl3·6H2O biochars could also improve the photocatalytic perfor- and FeSO4 ·7H2O, and vigorously stirred at 80 ℃ under mance of TiO /g-C N -based heterojunctions associated −1 2 3 4 the atmosphere of N for 10  min. Then, 0.048  g·mL of with the advantage of extensive availability and condu- trisodium citrate dihydrate solution was added into cive physio-chemical surface characteristics (Qiu et  al. the above mixture after pH was adjusted up to 10–11 2022; Yang et  al. 2022). Furthermore, biochar could be by adding 5  M NaOH solution, which was continued expected to act as a promising alternative to semiconduc- whisking for 1  h to obtain the precipitated black F e O 3 4 tor components of photocatalysts for reducing environ- nanoparticles. mental risks. Nonetheless, the effects of the introduction of biochars on the optical electronic properties of the 2.2.2 Preparation of  Fe O @SiO composites heterojunction, as well as the performance of pollutant 3 4 2 In order to prevent the corrosive effect from the photo - removal have been poorly explored. Numerous studies electrons, Fe O nanoparticles were covered by SiO via tend to focus on the single pollutant system merely, while 3 4 2 the sol–gel method. 0.50  g of Fe O nanoparticles were the photocatalytic removal of pollutants in the combined 3 4 dispersed into 80 mL of ethanol, and then 10 mL of 28% pollution systems has been barely characterized. ammonium hydroxide and 18  mL of deionized water In this study, the biochar-coupled F e O @SiO /TiO /g- 3 4 2 2 were added to the above mixture. The mixture was added C N composites were successfully synthesized through 3 4 with 5 mL of tetraethyl orthosilicate and kept stirring for simple strategies for the photocatalytic oxidation of 4  h. Finally, the composites were washed with ethanol TC and removal of Cr(VI). To compared nonmetallic and water several times and dried at 60 ℃. g-C N -based photocatalysts, metallic CdS-based pho- 3 4 tocatalysts were also prepared. The effects of controlled parameters, including calcination temperature, pH, and 2.2.3 Synthesis of biochar‑coupled Fe O @SiO /TiO 3 4 2 2 ion strength, on the removal performance of TC were composites studied. The removal performance, possible mechanisms, The biochar-coupled Fe O @SiO /TiO (FSBT) photo- 3 4 2 2 degradation pathways of TC, as well as reusability of the catalyst was prepared by the similar method. 0.20  g of synthesized composites in the combined pollution sys- Fe O @SiO nanoparticles and 0.50  g of modified rice 3 4 2 tems were investigated. straw-derived biochar prepared in our previous work (Dai et al. 2020) were dispersed in 50 mL of ethanol and 2 Materials and methods sonicated for 20  min, respectively. After that, they were 2.1 Materials mixed, and 28% ammonium hydroxide was added drop- The biochar raw material rice straw used in this research wise into the suspension to adjust the pH to 8–9. 20 mL was obtained from Changzhou, Jiangsu Province of of titanium butoxide was added to the solution and vigor- China. Ferrous sulfate heptahydrate (F eSO ·7H O), fer- ously stirred at 80  °C for 2  h. Subsequently, the mixture 4 2 ric chloride hexahydrate (FeCl ·6H O), sodium hydrox- was heated hydrothermally by a Teflon-lined autoclave at 3 2 ide (NaOH), sodium chloride (NaCl), sodium sulphate 160 °C for 3 h. Yang et al. Biochar (2023) 5:1 Page 4 of 20 2.2.4 Synthesis of biochar‑coupled Fe O @SiO /TiO /g‑C N also investigated by adding contaminant-loaded pho- 3 4 2 2 3 4 composites tocatalysts to 0.5  M NaOH solution after each use, stir- Biochar-coupled Fe O @SiO /TiO /g-C N composites ring at 353 K for 1 h, and then replacing it with deionized 3 4 2 2 3 4 (FSBTG) were prepared via a modified in accordance water and stirring for another 1  h for complete desorp- with the previous study (Kumar et al. 2018a). The g-C N tion. The regenerative samples were washed, dried at 65 3 4 was prepared by thermal condensation of melamine mol- ℃ and applied for the next reusability test until removal ecules. Typically, melamine was calcined at 550 ℃ for efficiency was stabilized. Cr(VI) concentrations were −1 4  h in a muffle furnace at a heating rate of 20 ℃ min . detected via a diphenyl carbazide method using a UV– Finally, the powder from FSBT was milled with prepared vis spectrometer (UV-1901PC, Shanghai) at λ of 540 nm, g-C N and calcined (300, 450, and 600 ℃) for 1  h in a and TC concentrations also were detected at λ of 357 nm. 3 4 −1 tube furnace at a heating rate of 20 ℃ min to synthe- The photocatalytic intermediates in TC solution were size FSBGT. The biochar-coupled Fe O @SiO /TiO /CdS identified using high-performance liquid chromatog - 3 4 2 2 photocatalyst (FSBTS) was also prepared by a similar raphy coupled with mass (Thermo scientific Q Exactive method. The major steps are shown in Additional file  1: Ultimate 3000 UPLC-MS). Fig. S1. 3 Results and discussion 2.3 Characterization 3.1 Characterizations The characterization details of as-synthesized catalysts 3.1.1 Morpholo gy and textural characteristics are listed in the Additional file 1. The characters of the surface morphology of prepared composites were observed through SEM, EDS, TEM, and 2.4 Performance test HRTEM (Fig.  1; Additional file  1: Fig. S2). The g-C N 3 4 The adsorption and photocatalytic activities of as-syn - exhibited a bulk irregular layered structure, and agglom- thesized several photocatalysts were evaluated using high erated to each other, resulting from the polycondensation concentration TC solutions as the targeted contaminant. of melamine during the process of calcination (Li et  al. All adsorption experiments were conducted in a 250 mL 2019). The micrograph of FSBT (Fig.  1b) showed that conical flasks under the conditions of 100  mL of TC titanium dioxide covered F e O @SiO hybrid nanoparti- 3 4 2 solution, 0.10  g of catalyst dose, agitator speed of 150 r cles dispersed over the porous biochar matrix. It could be −1 min and reaction temperature of 298 K. In the adsorp- found that relatively small-sized g-C N nanosheets and 3 4 tion kinetic studies, the initial TC concentrations were cubic CdS particles existed, which were coated to bio- −1 set to 200 mg L , and the solutions were sampled every char-coupled FST nanoparticles to form biochar-coupled ten minutes for concentration measurement. Mean- FSTG and FSTS composites, respectively. In addition, the while, the initial TC concentrations were adjusted to the different elements were distributed on the surface of two −1 range from 10 to 200 mg L with an equilibrium time of catalysts (Fig.  1f-l; Additional file  1: Fig. S2f-l), obtained 60 min. All catalysts were stirred for 120 min in the dark via the EDS elemental mappings of the SEM images, indi- to ensure adsorption equilibrium before photocatalytic cating the successful construction of catalysts. The TEM degradation experiments. The photocatalytic activity was and high-resolution TEM (HRTEM) images clearly pre- assessed by the photodegradation of TC under simulated sented the different morphologies and phases of the com - solar light using a 200 W Xenon lamp as the light source. posite catalyst. It can be seen that the T iO nanoparticles −1 0.10  g photocatalyst was added to 200  mg L TC solu- wrap the CdS and g-C N to form two heterojunctions. 3 4 tion (100  mL) with constant stirring at 298  K. The sam - The different phases identified by HRTEM images and ples were taken out periodically and filtered by 0.22  μm selected area electron diffraction (SAED) (Fig.  1n, o) have membrane filters. The effects of pH (3–11), and 0.1  M lattice fringe spacings of 0.32, 0.31 and 0.35  nm, respec- − − 2− 3− inorganic anions (Cl, NO, SO, PO ) on the pho- tively, which are assigned to the (002) and (101) crys- 3 4 4 −1 tocatalytic reaction of 100 mg L TC solution were also tal plane of g-C N and TiO , respectively (Wang et  al. 3 4 2 studied. To distinguish the role of the reactive species in 2018b). In terms of FSBTS catalyst, the lattice fringes of the photocatalytic reaction, scavengers, such as tert-butyl interplanar spacing of about 0.319, 0.332 and 0.35  nm alcohol (tBuOH), potassium dichromate (PD), EDTA- could be ascribed to the (002), (110) and (111) crystallites 2Na and benzoquinone (BQ), were added to the 100 mg of hexagonal CdS, respectively (Additional file  1: Fig. S2n, −1 L TC solution with catalysts for scavenger experi- o). The presence of high resolution lattice fringes and ments. Besides, the photocatalytic activity of catalysts in diffraction cycles of SAED patterns showed the highly −1 the combined pollution systems with TC of 100  mg L crystalline nature of the conductor nanoparticles of as- −1 and Cr(VI) of 20  mg L was further studied. The cata - synthesized composite catalysts (Kumar et al. 2018a). lyst reusability in the combined pollution systems was Y ang et al. Biochar (2023) 5:1 Page 5 of 20 Fig. 1 SEM micrographs of g-C N (a), FSBT (b), and FSBTG (c); EDS pattern (d), SEM–EDS layered image of FSBTG (e, f); EDS mapping of FSBTG for 3 4 the elements of C (g), N (h), O (i), Si (j), Ti (k) and Fe (l); TEM (m) and HRTEM (n) micrograph of FSBTG with corresponding SAED patterns in inset (o) The textural properties of catalysts were studied and Liu 2019). Conversely, the hysteresis pattern of bio- through N adsorption–desorption isotherms and pore chars is more likely to be a narrow silt-like structure size distribution curves (Additional file  1: Fig. S3). Com- (Mian and Liu 2019). Specifically, when P/P < 0.4, the pared with FST, FSBT and biochars, a higher capacity isotherm showed an upward trend, indicating the exist- of N adsorption–desorption was observed due to the ence of micropores, and when P/P > 0.4, the hysteresis 2 0 introduction of the heterojunction. All catalysts show loop increased, indicating the existence of abundant increasing hysteresis pattern without plateau at high rela- mesoporous structures. The adsorption capacity of com - tive pressure, indicating the shape might transform from posites containing biochar or heterojunctions was higher disorder lamellar, slit and wedge-shaped pore structure than that of other catalysts, which could be ascribed to to regular shape after secondary calcining. However, the development of more surface area and pore volume, the H3 loop characterized the structure of synthesized resulting from further calcination during the process of catalysts based on the inconspicuous steady trend (Mian preparation of heterojunctions. The continually rising Yang et al. Biochar (2023) 5:1 Page 6 of 20 trend isotherm of all catalysts suggested the presence of these peaks are attributed to the existence of g-C N 3 4 both microporous and mesoporous structures (Cai et al. (Kumar et  al. 2018b; Li et  al. 2019). The relatively wide −1 2013), and it was also further confirmed by pore size dis - peaks near 3140  cm stem from the bending vibration of tribution curves. The specific surface area (S) of all cata - terminal NH or NH groups at the defect sites of the aro- lysts accorded with the following rule: S (280.30 matic ring (Yan and Yang 2011). Besides the above peaks, FSTBG 2 −1 2 −1 2 m g ) > S (266.41 m g ) > S (255.07 m several new peaks indicated stretching vibrations of FSTG FSTBS −1 2 −1 2 −1 −1 g ) > S (247.07 m g ) > S (135.87 m g ) > S Ti–O, Ti–O-Ti and Si–O were observed at 400–600  cm FSTS FSBT FST 2 −1 2 −1 (111.05 m g ) > S (28.13 m g ), and a similar rule in as-synthesized catalysts (Nematollahzadeh et al. 2015). BC −1 −1 was also found in the total pore volume, which could be The peaks at 1500–1700  cm and 3340–3550  cm attributed to the aggregation of flaky g-C N and granu- are attributed to the stretching vibration of the H–O-H 3 4 lar CdS. The detachment of unstable impurities from the group and -OH group (Cheng et al. 2016; Dai et al. 2020), surface of composites could also improve the surface which are derived from the biochar, TiO , and CdS, sug- characteristics of synthesized catalysts (Table 1). gesting the successful construction of composite catalysts (Mian and Liu 2019). The elemental states of the synthesized photocatalysts 3.1.2 Composition and structural characteristics were analyzed via XPS in the C 1s, N 1s, Ti 2p and Fe 2p The crystal structures of the synthesized catalysts were binding energy regions. As shown in Fig.  2c–f, the C 1s analyzed via XRD (Fig.  2a; Additional file  1: Fig. S4a). peaks at 284.4 and 286.2 can be attributed to the C atoms In terms of Fe O , the peaks of 2θ at 30.1°, 35.5°, 43.1°, 3 4 in the C=C/C–C and C–OH bonds, respectively, which 57.0° and 62.6° correspond to the (220), (311), (400), (511) originate  from the graphitic and amorphous C of NaOH- and (440) planes, respectively. Due to amorphous char- activated biochar, whereas the peak centered at 288.5 eV acters (Kumar et  al. 2019), relevant peaks of SiO were indicates the C atoms in the structure of N = C-(N) (Li not found in Fe O @SiO . The peaks at 25.5° and 27.5° 3 4 2 et  al. 2019; Wang et al. 2020). The obvious N 1 s peak at correspond to the (101) and (002) lattice plane of g-C N 3 4 399.0  eV can be considered as the sp -hybridized aro- (Qiu et  al. 2021b). The TiO exhibited diffraction peaks matic N atoms derived from C=N–C, the weak peak at at 25.5°, 37.9°, 48. 0°, 54.0°, 55.1°, 62.8°, 68.9°, 70.2°, and 400.3  eV is assigned to (C) -N linking structural motifs 75.2°, which identify with the (101), (004), (200), (105), of C N , (C) -NH/C-NH , or Ti-ON, and the peak at 6 7 2 2 (211), (204), (116), (200), and (215) planes, respectively 401.1  eV is indicated to N atoms of aromatic cycles (Zhang et  al. 2017). The characteristic peaks at 26.6°, of N-(C) (Li et  al. 2019; Wang et  al. 2020; Zhang et  al. 44.0° and 52.0° of the original nano-CdS represent cubic 2018). As for Ti 2p, two obvious peaks located at 459.1 (111), (220) and (311) crystal planes and hexagonal (002), 3/2 1/2 and 464.9  eV are attributed to the Ti 2p and Ti 2p (110) and (311) (222), respectively. Same peaks could 4+ state of Ti , respectively (Ma et al. 2021). A satellite peak be observed in the catalysts containing TiO , g-C N , 2 3 4 at 472.1  eV can be clearly seen for the Ti(IV) oxidation or CdS, but the peak intensity decreased gradually due state (Herath et al. 2022). For Fe 2p, the peaks at around to the very low quantity detected on the surface of the 3/2 1/2 711.5 and 724.5 eV are assignable to Fe 2p and Fe 2p catalysts. Interestingly, the peak intensity of TiO -based spin–orbit, respectively (Djellabi et  al. 2019). Moreover, heterojunctions was enhanced with the increase of calci- it can be curve-fitted with five peaks. Among them, the nation temperature from 300 to 600 ℃, which could be ones at the binding energies of 710.9 and 724.4 eV are in ascribed to the decrease in thickness of g-C N (Li et al. 3 4 2+ accordance with Fe (Liu et al. 2020; Raha and Ahmaru- 2017). When the calcination temperature reached up to 3+ zzaman 2020), the one at 712.5 is consistent with Fe 600 ℃, new peaks at 36° and 41.1° were observed, indicat- (Mian et al. 2019), and the ones at 719.3 and 732.8 eV are ing the formation of rutile (101) and (111) plane under the satellites peaks of Fe O (Liu et al. 2020). 3 4 high calcination temperatures (Zhang et al. 2017). As the synthesis temperature of CdS-based catalyst increased to 600  °C, the composite catalyst generated new hexagonal 3.1.3 O ptical and electronic characteristics (101) and (103) crystal planes (Wang et al. 2018b). The optical properties of a catalyst directly influence The functional groups of the as-prepared catalysts were its photocatalytic activity. The light response char- studied by FT-IR spectra (Fig.  2b; Additional file  1: Fig. acters of the as-synthesized catalysts were analyzed −1  S4b). The peaks at 460 and 540  cm correspond with by ultraviolet–visible/diffuse reflection spectroscopy the stretching vibrations of Fe–O (Nematollahzadeh (UV–vis/DRS). In Fig.  3a, g-C N exhibits an obvious 3 4 –1 et  al. 2015). The peaks at 808  cm indicate the bending light response at 200–450 nm, indicating a certain vis- vibration of s-triazine units (Li et al. 2019), several peaks ible response. For g-C N -based photocatalysts, the 3 4 –1 in the range of 880–1335  cm can be attributed to the light absorption over the range of 200–800  nm can be stretching vibration of N–H/N-H , C–N and C=N, and attributed to the decreased reflectivity by introducing 2 Y ang et al. Biochar (2023) 5:1 Page 7 of 20 Fig. 2 XRD patterns (a); FT-IR spectra (b) of TiO , g-C N and several g-C N -based photocatalysts; high-resolution XPS spectra of C 1 s (c), N 1 s (d), 2 3 4 3 4 Ti 2p (e), and Fe 2p (f) of FSBTG-450 Yang et al. Biochar (2023) 5:1 Page 8 of 20 Fig. 3 UV–vis/DRS spectra, Tauc plots, Transient photocurrent response and EIS spectra of g-C N -based photocatalysts (a, c, e) and CdS-based 3 4 photocatalysts (b, d, f) under visible light illumination (> 420 nm) black biochars, which improved the electronic transi- that the band gap of composite catalysts is lower than tion efficiency by capturing photons (Li et  al. 2015). that of g-C N , which could be attributed to the for- 3 4 The band gap plots are also plotted according to Tauc mation of heterojunctions between the employed pho- relation (Kumar et al. 2017) in Fig.  3a. It can be found tocatalysts (Jourshabani et  al. 2020). In addition, the Y ang et al. Biochar (2023) 5:1 Page 9 of 20 band gap energy of semiconductors can be decreased possible reason could be that the heterojunction is ben- by the introduction of biochar. It can be interpreted as eficial to the migration of carriers but ineffective to the the reason that biochar can sensitize the semiconduc- separation of photogenerated electron/hole. tor with another composite, create a mid-gap energy state via doping non-metals such as N, S, C, O etc., and 3.2 Performance tests form a local trapping state below the conduction band, 3.2.1 R emoval of TC from water by adsorption meanwhile, the N can also reduce the band gap of the Adsorption plays an important role in the process of composite by forming a mid-gap energy state (Mian photocatalytic removal of pollutants by catalysts. The and Liu 2018). A red shift was found for CdS based kinetics and isotherm models were used to describe photocatalysts from Fig.  3b, indicating a stronger abil- the adsorption capacity and mechanism of TC with the ity to capture photons compared with CdS. This could synthesized catalysts (Fig.  4). In order to research the result from the formation of heterojunction among adsorption kinetic characteristics, the experimental data biochars, Fe O @SiO, TiO and CdS. were fitted by pseudo-first-order and pseudo-second- 3 4 2 2 The photocurrent responses and electrochemical order models, respectively. The higher correlation coef - impedance spectroscopy (EIS) were studied to evaluate ficients of the two models suggest that both physical and the separation efficiency of photo-generated carriers. It chemical mechanisms were involved in this adsorption can be seen in Fig.  3c that a steady photocurrent was process. TC sorption by catalysts rose rapidly during the generated, and the switching on of current was done first 20  min, then gradually slowed down until sorption after every 50  s. FSBTG-450 and FSTG-450 increase equilibrium after around 60  min. Compared with FST significantly in photocurrent compared to g-C N , sug- and FSBT,   the equilibrium adsorption capacity of the 3 4 gesting the reduction of the photogenerated charge catalysts introducing g-C N increased by 116.47 mg g−1 3 4 carrier recombination (Guo et  al. 2021). It could be and 102.34 mg g−1, respectively, and that of the catalysts deemed that introducing biochar can further improve introducing CdS increased by 119.61 mg g−1and 120.95 the separation of photo-generated charge carriers of mg g−1, respectively. Moreover, the adsorption capacity photocatalysts, in accordance with the phenomenon of FSBTS gradually decreased as calcination temperature that the FSBTG-450 shows a higher photocurrent increased, and the catalysts calcined at 300  °C showed intensity than FSTG-450. The results were also found in the best performance. Maximal adsorption capacity was previous studies (Lyu et al. 2020; Tam et al. 2020). Sig- presented under 450 °C for FSBTG. The results could be nificantly, a prominent decrease occurred in photocur - explained via the reasons that the structure and phys- rent of CdS-based photocatalysts in comparison to CdS icochemical properties connected with the adsorption (Fig.  3d), although biochar, F e O @SiO, TiO could of rice straw-derived biochar can be easily changed as 3 4 2 2 promote the absorption of visible light. The reduced calcination temperature increases. It has been suggested charge carrier recombination of the composite catalysts in previous studies that the biochars at high tempera- can be reconfirmed by EIS analysis in Fig.  3e, in which tures shows structural defects, blocked micropores on the composite catalysts exhibit lower charge transfer the surface (Rosales et  al. 2017), and a decrease of car- resistance and small Nyquist semicircle compared with boxylic functional groups (Zhao et al. 2017), resulting in single g-C N . It can be explained via the following the decrease of adsorption active sites. Similarly, both 3 4 main reasons: (1) the aromatic ring structure of biochar Langmuir and Freundlich isotherm models can fit the can accelerate the electron shuttling between different isotherm experimental data well. q gradually increased reactants, which could be mediated via surface redox- along with the increase of C due to the more powerful active moieties; (2) biochar containing large amounts of driving force and contact area between high concentra- quinone can act as an electron reservoir, while the elec- tion TC and adsorbent (Dai et al. 2020). FSBTS-300 and tron storage capacity of biochar depends on the types FSTS-300 showed maximum excellent adsorption capac- −1 −1 of biomass and the pyrolysis conditions; (3) the forma- ities reaching up to 173.32  mg g and 171.82  mg g , tion of heterojunction between the semiconductor and while lower maximum adsorption capacities of FSBTG- −1 biochar is responsible for potentiation in charge sepa- 450 and FSTG-450 came up to 147.96  mg g and −1 ration, and the similar increase in charge separation 104.70 mg g , respectively. Metal components decreased can occur at metal Fe or iron oxides and semiconductor and nonmetallic g-C N , barely adsorbing TC, replaced 3 4 heterojunctions, which can be promoted by light irra- Fe O @SiO and TiO to occupy the active sites, which 3 4 2 2 diation (Mian and Liu 2018). Interestingly, inconsistent caused the above phenomenon. In general, the Langmuir results are presented between Fig. 3d and Fig. 3f. FSBT- model had a higher nonlinear relationship coefficient, 300 and FSBTS-300 revealed lower charge transfer indicating that TC adsorption may be single-layer molec- resistance than CdS on account of Nyquist semicircle, a ular adsorption, and chemical adsorption is involved in Yang et al. Biochar (2023) 5:1 Page 10 of 20 this process (Zeng et  al. 2019). The key parameter n of rate on TC. Based on the characterizations of catalysts, the Freundlich model is related to the degree of hetero- it can be seen that the catalysts introducing g-C N 3 4 geneity of adsorption sites, and all n values in this study and CdS had larger specific surface area, wider visible were < 1, indicating the high heterogeneity of the solu- light response range, and high photogenerated car- tion during the TC adsorption process (Srivastava et  al. rier separation capacity. Different from CdS, introduc - 2006). The presence of oxygen-containing groups, such ing g-C N to FSBT showed lower adsorption capacity, 3 4 as hydroxyl and carboxyl groups, can contribute to the which improved photocatalytic activity at the cost of formation of H-bonding between photocatalysts and TC decreasing the adsorption activity. During the process (Rosales et  al. 2017). It is reasonable to assume that the of TC removal by FSBTG, photocatalytic oxidation, coating of nanoparticles on carbon structure could also which played a leading role, was relatively slow and dif- increase the adsorption sites, which can be confirmed ficult compared with adsorption. Therefore, FSBTG and via the result that the introduction of metal nanoparti- FSBT showed similar adsorption capacities. It can be cles, such as TiO, Fe O , and SiO , further improved the discovered that the incorporation of biochar had almost 2 3 4 2 adsorption of  biochar or biochar-based catalysts. Con- no effect on photocatalytic performance, although bio - sequently, besides H-bonding, the  interaction of surface char was considered as an electron reservoir in previ- complexes could be considered as a dominant mecha- ous studies (Mian and Liu 2018). As for temperature, nism in TC adsorption (Tan et al. 2016).  the same results could be found in adsorption and pho- tocatalytic degradation. It was reported that biochar 3.2.2 Removal of TC from water by photocatalytic synthesized at high heat temperatures showed a higher degradation electron storage capacity (Kluepfel et  al. 2014). In this The photolysis experiments under simulated sun - study, the catalysts with the high calcination tempera- light with different as-synthesized photocatalysts were ture exhibited decreased performance, suggesting that conducted to investigate their photolysis efficiency. the electron storage capacity of biochar may be not the As shown in Fig.  5a, b, in the absence of any kinds of primary factor affecting photocatalytic performance, catalysts, the TC concentration remained unchanged and there is no obvious evidence about electron stor- with increasing irradiation time, indicating negligible age stemming from biochar. In addition, a previous photolysis of TC without photocatalyst. The compos - study showed that the catalytic activity of carbon-based ite catalysts exhibited relatively high photocatalytic materials also probably depends on the internal elec- efficiency, and FSBTG-450 and FSBTS-300 showed tronic state of the hybrid covalent system (Duan et  al. the most excellent efficiency of 88.20% and 91.88%, 2018). Oxygen-containing functional groups are the much higher than that of pure g-C N (12.65%) and main active sites for the catalytic oxidation of carbo- 3 4 CdS (47.21%). Similarly, the catalysts synthesized by naceous catalysts (Wang et  al. 2018b), and the intro- high-temperature calcination showed relatively low duction of metal oxides and biochars may also cause O removal performance, and  450 ℃ and 300 ℃ were doping to g-C N , resulting in the generation of more 3 4 the optimal temperatures for FSBTG and FSBTS, active sites and high photocatalytic activity. Compared respectively, which could be ascribed to the forma- with photocatalysts synthesized in recent studies, as tion of anatase TiO with higher weight and activity illustrated in Table  2, various photocatalysts exhibited at the suitable conditions (Zhang et  al. 2017). Due to significantly different removal performances for TC the faster rate of adsorption than that of degradation, due to the difference in characters of catalysts, reac - the TC removal efficiency rose rapidly during the first tion conditions, as well as photocatalytic mechanisms 20  min, subsequently, the activity sites were occupied (Saadati et  al. 2016). Above all, the as-synthesized by adsorbed TC and intermediates of incomplete deg- FSBTG-450 catalyst could be considered as an efficient radation, resulting in gradual slowness in removal material with high removal performance for high con- rate. This phenomenon also suggests that adsorption centration TC solutions. plays a critical role in all processes of TC removal. The removal of TC followed degradation kinetics, as shown 3.3 Eec ff t of solution factors on the removal of TC in Fig.  5c, d; Additional file  1: Table  S3, the removal 3.3.1 pH rate of catalyst follows the following order: FSTG-450 It was deemed that the ionization state of pollutants and −1 −1 (0.0071  min ), FSBTG-450 (0.0068  min ), BTG-450 the surface charge of the photocatalyst are affected via −1 −1 (0.0051  min ), FSBTG-300 (0.0025  min ), FSBTG- pH (Kumar et  al. 2018a). Similar results were exhibited −1 −1 600 (0.00027  min ), and g-C N (0.00026  min ). k for  g-C N - and CdS-based photocatalysts on the effect 3 4 3 4 value of FSBTG was 1.33 times that of BTG, suggesting of pH. The solution pH was adjusted using 0.1  M HCl that Fe O @SiO composites enhanced the degradation and NaOH, and the effect of pH on the photocatalytic 3 4 2 Y ang et al. Biochar (2023) 5:1 Page 11 of 20 Fig. 4 Adsorption kinetics and adsorption isotherm for TC adsorption by g-C N -based photocatalysts (a, c) and CdS-based photocatalysts (b, d) 3 4 −1 (TC: C = 200 mg L , 100 mL; photocatalyst dosage: 0.10 g, T = 298 K ) − − 2− 3− removal of TC by FSBTG-450 is shown in Additional 3.3.2 Anions (Cl , NO , SO , and PO ) 3 4 4 file  1: Fig. S5a. After 1  h of light irradiation, 88.78%, To preliminarily evaluate the applicability of FSBTG- 87.68%, 83.60%, 77.18%, 76.40% and 77.33% of TC were 450 in the actual water environment, various 0.1  M ani- − − 2− 3− removed when the pH of the solution was not adjusted, ons, including Cl , NO , SO , and PO , were used to 3 4 4 3, 5, 7, 9 and 11, respectively. Generally, the state of TC explore the effect on the photocatalytic removal of TC. is affected by the pH of the solution. When pH < 3.4, As shown in Additional file  1: Fig. S5c, the photocatalyst 3.4 < pH < 7.6, 7.6 < pH < 9.7 and pH > 9.7, TC exhibits exhibited 88.36% of removal efficiency during 1  h with - + − 2− the state of H TC, HTC, H TC and HTC , respec- 4 3 2 out any additional anion. The removal efficiency was tively (Jang and Kan 2019). It was found that the sur- 82.46%, 91.86%, 88.62%, and 9.47% in the presence of ani- face of adsorbents was positively charged below pH , zpc 2− − − 3− ons of SO , Cl , NO , and PO , respectively. Appar- 4 3 4 otherwise it was negatively charged (Dai et  al. 2020). ently, the effect of Cl on TC removal was positive, that From Additional file  1: Fig. S8, the zero point of charge of NO was negligible, while a slightly inhibitory effect (pH ) of FSBTG is 4.43. Obviously, the catalyst exhib- zpc 2− of SO on the reaction was observed, which can be ited a relatively high removal efficiency when pH < pH , zpc which could be attributed to the electrostatic interaction ascribed to the reactive species scavenging reaction from 2− 2− − ·− between the catalyst and TC ions or molecules, but activ- SO (Zhang et  al. 2017): ·OH + SO → OH + SO , 4 4 4 ity was inhibited when pH > pH due to the electrostatic + 2− ·− zpc h + SO → SO . Moreover, TiO existing on the sur- 4 4 repulsion. However, the effect of pH on the removal of 2− face of the composite catalyst can adsorb SO through TC was not very significant, indicating that other mecha - van der Waals forces and hydrogen bonds, occupying the nisms might play critical roles  in this process. Yang et al. Biochar (2023) 5:1 Page 12 of 20 2016; Wang et al. 2020). In addition, ·OH was caught and active sites of the catalyst (Xekoukoulotakis et  al. 2011). the transfer of h from g-C N and CdS to TiO through The strong inhibitory effect was observed in the presence 3 4 2 3− 3− biochars and Fe O @SiO was hindered (Kudlek et  al. of PO due to the strong adsorption affinity of PO for 3 4 2 4 4 2016). FSBTS-300 showed similar results. TiO and biochar, resulting in a large number of surface sites that were competitively occupied (Kudlek et  al. Table 1 The main surface characteristics of biochar and as-synthesized catalysts 2 −1 3 −1 Sample Specific surface area (m g ) Total pore volume (cm g ) Pore width (nm) Average pore diameter (nm) BC 28.13 0.031 – 9.45 FST 111.05 0.204 3.627 5.36 FSBT 135.87 0.326 4.543 7.33 FSTG 280.30 0.514 3.627 5.37 FSBTG 266.41 0.466 3.627 5.24 FSTS 247.07 0.456 3.627 5.12 FSBTS 255.07 0.431 3.627 4.78 Table 2 Comparison of photocatalytic removal of various catalysts for tetracycline degradation  Catalyst Reaction system Reaction  condition Removal performance Reactive species Refs. −1 − Biochar-coupled Fe O @ Simulated solar light [Catalyst] = 1.0 g L ; Removal 92% in 180 min, ·O , ·OH This study 3 4 −1 −1 SiO /TiO /g-C N com- [ TC] = 200 mg L ; k = 0.0068 min , 2 2 3 4 obs −1 posites pH 3–11; UV–Vis of 200 W Q = 147.96 mg g Xe lamp −1 + − Hollow tubular g-C N Vis [Catalyst] = 1.0 g L ; Removal 85% within 30 min h , · O , ·OH Liang et al.( 2021) 3 4 −1 isotype heterojunction [ TC] = 5–30 mg L ; pH 2–11; Vis of 300 W Xe lamp with light intensity of −2 100 mW cm −1 + − − N-TiO /O-doped N vacancy Vis [Catalyst] = 0.4 g L ; Removal 80% in 20 min, h , e , · O , ·OH Wang et al. ( 2020) −1 −1 g-C N [ TC] = 30 mg L ; k = 0.0170 min 3 4 obs pH 2.94–9.32; Vis of 300 W Xe lamp −1 + − OV-mediated sandwich-like Vis [Catalyst] = 0.5–2 g L ; Removal 88% in 90 min, h , · O , ·OH Ni et al. (2021) −1 −1 TiO /ultrathin g-C N / [ TC] = 5–20 mg L ; k = 0.0317 min 2-x 3 4 obs TiO pH 3–11; Vis of 300 W Xe 2-x lamp −1 + − Boron nitride-fullerene com- Vis [Catalyst] = 0.1 g L ; Removal 97% in 90 min, h , · Guo et al. (2021) −1 −1 posite (C /BN-U ) [ TC] = 20 mg L ; Q = 131.05 mg g 60 6 e pH 3–11; Vis of Xe lamp with intensity of 164.4 mW −2 cm Au-TiO /PVDF composite Vis 20 mL of TC aqueous solu- Removal 75% in 120 min, ·O , ·OH Yan et al. (2021) 2 2 −1 tion; k = 0.0121 min obs Vis of 300 W Xe lamp −1 + − − MnFe O -Au composites H O /Vis [Catalyst] = 0.1 g L ; Removal 88% in 90 min, h , e , · O , ·OH Qin et al. (2021) 2 4 2 2 2 −1 −1 [ TC] = 10–40 mg L ; k = 0.0231 min obs pH 3–9; Vis of 300 W Xe lamp with light inten- −2 sity of 434 mW cm ; [H O ] = 10–80 mM 2 2 3− −1 PO -Bi WO /PI Simulated solar light [Catalyst] = 1.0 g L ; Removal 65% in 20 min, ·O , ·OH Gao et al. (2021) 4 2 6 2 −1 −1 [ TC] = 20 mg L ; k = 0.0066 min obs pH 3.15–11.15; light of 150 W Xe lamp Y ang et al. Biochar (2023) 5:1 Page 13 of 20 Fig. 5 Photodegradation curves and degradation kinetic curves of TC with g-C N -based photocatalysts (a, c) and CdS-based photocatalysts (b, d) 3 4 −1 (TC: C = 200 mg L , 100 mL; photocatalyst dosage: 0.10 g) 3.3.3 Active substances during degradation of TC reduced greatly after the addition of tBuOH, elu- − + E and h are the main reactive species produced on cidating that ·OH was the primary active species dur- the surface of the photocatalysts during the photocata- ing the TC degradation process. To further validate the lytic removal process. After that, they combine with generation of active species during the reaction process, dissolved O and H O to form other reactive species the ESR spin-trap with DMPO technique was carried 2 2 including ·OH and ·O (Kumar et al. 2018a). In order to out. As shown in Additional file  1: Fig. S6a, the TEMPO identify the dominant active substances controlling the signal peak strength fell with longer illumination times, photocatalytic removal of TC under light irradiation, which indicated that FSBTG could generate electrons different scavengers of 1  mM, tBuOH (·OH scavenger), and holes due to the anaerobic condition of acetonitrile − + PD (e scavenger), EDTA-2Na (h scavenger) and BQ solvent, so the heterojunction enhanced the spatial sep- ·O ( 2 scavenger) were added into the reaction systems, aration of photo-generate carriers (Wang et  al. 2020). respectively (Kumari et  al. 2022). The experiment was Moreover, there was no EPR signal for the DMPO-·OH also conducted without any scavenger during degra- or DMPO-·O adducts under dark conditions, indicat- dation for reference. From Additional file  1: Fig. S5d, it ing that no active free radicals were generated. However, was discovered that the removal efficiency decreased a four characteristic peaks for DMPO-·OH and six charac- − + little after adding PD and EDTA-2Na, so e and h had teristic peaks for DMPO-·O were found in the presence a slight effect on the removal efficiency of TC. Never - of light and enhanced with increasing light irradiation theless, the photocatalytic activity reduced clearly due time (Li et  al. 2022a), demonstrating that FSBTG could to the introduction of BQ, indicating the importance of produce ·OH and ·O during the photocatalytic reac- ·O in TC removal. Moreover, the degradation efficiency tion. The contribution of reactive oxygen species (ROS) 2 Yang et al. Biochar (2023) 5:1 Page 14 of 20 2– in photodegradation was obtained in the following order: Although the presence of competition between Cr O 2 7 + − − + ·OH > ·O > h > e according to Additional file  1: Fig. and the photogenerated e /h   due to the high electron 2– S6d. As a result, ·OH and · O were generated during the cloud density of O atoms in Cr O , Cr (III) may adhere 2 7 photocatalytic degradation of TC, which was consist- to the surface of catalysts through adsorption, accelerat- ent with the results of the quenching experiments. H O ing the migration of photon-generated carriers (Li et  al. could react with h to produce ·OH, whereas ·O , gener- 2019). It can be supposed that a complexation interaction ated by the reaction between e and O , could react fur- occurred between the adsorbed Cr(III) on the surface ther with H O to produce ·OH (Chong et  al. 2010; Fang of catalysts and oxygen-containing functional groups of et al. 2015). Therefore, it is a revelation that the photocat - TC. In addition, the adsorption of Cr(III) on the surface alytic efficiency in TC solutions with high concentration of catalysts can improve the removal of Cr(VI) based on can be promoted by adjusting the ratio of H O and O for the disproportionation reaction between the adsorbed 2 2 the practical application of photocatalysts. Cr(III) and Cr(VI) (Wang et  al. 2020). As for FSBTS, Cr(VI) could be removed in the single system due to photo-generated electrons based on efficient separation 3.4 Simultaneous r emoval of TC and Cr(VI) of carriers. However, it was found that further enhanced in the combined pollution systems removal of Cr(VI) and slightly inhibited removal of TC The pollutant removal performance of FSBTG and FSBTS in the combined pollution systems, synergistic photoca- was further discussed in TC/Cr(VI) combined pollution 2– talysis effect and the competition between Cr O and 2 7 systems. The UV–vis spectroscopic spectra at differ - − + e /h might be responsible for the above phenomenon. ent times are shown in Fig.  6a, b. Obviously, during the The photocatalytic experiments were carried out for whole reaction course, except for the typical absorption five cycles in the TC/Cr(VI) combined pollution sys - peaks, new peaks emerged and these two peaks synchro- tems to evaluate the stability of the recycled catalysts. nously decreased with offset, suggesting that the pollut - After the first catalytic run, the recovered photocata - ants were altered, not merely adsorbed. The pollutant lysts were regenerated and then used for the next pho- removal performance of photocatalysts in single and TC/ tocatalytic experiment under the same conditions. As Cr(VI) combined pollution systems are shown in Fig. 6c, shown in Fig.  6e, f, the removal efficiency of TC and d. Interestingly, in terms of FSBTG, the removal of Cr(VI) Cr(VI) by the recycled catalyst exhibited a trend of in the single system without adjustment of pH can be gradual decrease  with the increase in cycle time, and disregarded, with only 2.7% of Cr(VI) being removed especially, a significant adverse effect on the removal of within 120  min under light irradiation. The significantly Cr(VI) was observed. After five photocatalytic cycles, increased removal of 69.99% and 77.95% occurred in the TC removal rate of FSBTG was reduced from 91.03% the combined pollution systems, as shown in Additional to 57.32%, and the removal rate of Cr(VI) was reduced file  1: Table S3, and the degradation rates, k value, could −1 −1 from 78.56% to 16.08%. This may be due to the reaction reach up to 0.0093  min for FSTG and 0.0108  min for between the hot alkaline solution and chromium ions in FSBTG, which were over 46 and 54 times more than that the catalyst regeneration process to form a precipitate of FSBTG in the single system. Likewise, the removal that is tightly combined with the catalyst, occupying the of TC in the combined pollution system also slightly active sites of the catalyst, and causing the catalytic activ- increased compared with the single system. k values were −1 ity to decrease with the frequency of recycling. Further 1.99 and 2.24 times for FSTG (0.0141  min ) and FSBTG −1 searching for suitable biochar-based catalyst regenerators (0.0152  min ), respectively. The significantly enhanced is needed to achieve effective regeneration of the catalyst removal of Cr(VI) can be explained by the presence of the without significant adverse effects on its activity. Also, synergistic photocatalysis effect in the combined pollu - the removal of TC and Cr(VI) for FSBTS was greatly tion systems. In a Cr(VI) single pollution system without influenced by cycle time. Besides the above reason, local adjustment of pH, the Cr(VI) is barely removed because 2– aggregation and deactivation of FSBTS during the regen- of the high electron cloud density of O atoms in Cr O , 2 7 eration process and the loss of photocatalysts during resulting in the difference in the generation of the reduc - wash/dry process could weaken the removal of pollutants ing active intermediate (RAI) and oxidizing active inter- (Huang et al. 2018). mediate (OAI). The redox between TC and Cr(VI) occurring in the combined pollution systems without adjustment of pH can be interpreted by the fact that the 3.5 Removal mechanism of photocatalyst for TC and Cr(VI) + 2– H originating from TC can promote Cr O to form The removal of TC and Cr(VI) was achieved via adsorp - 2 7 RAI and OAI (Li et  al. 2019). It was observed that TC tion and photocatalytic redox reactions of photocata- removal in the combined pollution systems was slightly lyst, in which huge surface area gave it countless active higher than that in the TC single pollution system. sites, and the possible mechanism is shown in Fig.  7. Y ang et al. Biochar (2023) 5:1 Page 15 of 20 Fig. 6 UV–vis spectra, simultaneous TC photodegradation with Cr( VI) removal in the TC/Cr( VI) combined pollution systems, recycling experiments using FSBTG-450 photocatalysts (a, c, e) and FSBTS-300 photocatalysts (b, d, f) Adsorption played a critical role in the whole removal interaction, based on the appropriate pore size distribu- process, in which both physisorption and chemisorp- tion and surface charge of photocatalysts, which contrib- tion were involved, with the latter as the dominant uted to the adsorption effect (Dai et  al. 2020; Jang et  al. via isotherm and kinetic analysis. Physical adsorption 2018). The hydrogen bonding force and surface compl - was dominated by pore filling effect and electrostatic exation were deemed to be the critical mechanisms for Yang et al. Biochar (2023) 5:1 Page 16 of 20 can be removed by the e generated in the CB of g-C N . 3 4 Although the presence of competition for e between the removal of Cr(VI) and the generation of ·O , the Cr(VI) can be removed effectively due to the introduction of H derived from TC. To analyze the degradation pathway of TC, the inter- mediate products during the photocatalytic process were detected by means of UPLC-MS. The results showed 16 major intermediates during the degrada- tion process. Several photocatalytic degradation path- ways of TC over FSBTG-450 are proposed in Fig.  8. There are three types of functional groups, including double bond, phenolic group and amine group, which are vulnerable to attack by radicals due to the high electron density (Wang et  al. 2018a). In pathway I, TC was attacked by reactive oxygen species to generate Fig. 7 The plausible removal mechanism of TC and Cr( VI) by FSBTG the intermediate with m/z of 427 via dehydroxylation and N-demethylation due to the low energy of N–C bond (Guo et  al. 2021; Qin et  al. 2021). Subsequently, TC adsorption due to the presence of oxygen-containing the intermediates with m/z of 385 and 302 were gen- functional groups and metal oxides, which were discov- erated by dehydroxylation, demethylation and benzene ered to enhance TC adsorption in this study (Dai et  al. ring opening reaction. The two intermediates can be 2020; Li et  al. 2022b). The surface complexation interac - further decomposed by similar reaction mechanisms tion between metal chromium ions and oxygen-contain- to produce the intermediates with m/z of 306, 282, and ing functional groups of catalysts was also considered 148. In pathway II, the formation of the intermediate as the main force for Cr(VI) adsorption (Liu et  al. 2022; of m/z = 496 was mainly caused by the dehydrogena- Xiao et al. 2019; Zhu et al. 2021). The adsorbed pollutants tion of -OH in Carbon 3 position, ‒N(CH ) oxidation, can be further decomposed by reactive oxygen species, and carboxylation of -CH in Carbon 8 position after generation of which is significantly dominated by utiliza - the breakage of double bond in Carbon 7–8 position. tion of light and transport efficiency of photogenerated The intermediate was further oxidized to produce the charges (Buzzetti et al. 2019). The enhanced visible light intermediate of m/z = 433, which subsequently was response and separation of photogenerated charges decomposed to low-molecular-weight ketone or car- obtained from UV–vis DRS, photocurrent response, boxylic acid compounds with m/z of 256, 253, 196, 151, and EIS analysis indicated improved optical and elec- 119, and 118 by ring cleavage along with hydroxyla- tronic activities, which contributed to the photocatalytic tion. Similarly, in pathway III, the low-molecular inter- reaction of catalysts. The ·OH and ·O were regarded mediates with m/z of 283 and 214 were produced by as the dominant reactive species based on the quench- rings-opening, along with additional decarbonylation − + ing experiment. The photogenerated e and h transfer and demethylation reactions (Qin et  al. 2021). Finally, process of FSBTG-450 can be considered as Z-scheme these low molecular intermediates could be completely rather than type II heterojunction structure due to the decomposed to CO and H O. 2 2 product of ·O (Wang et  al. 2020). In the as-synthe- sized magnetic biochar-based composite catalyst, the photogenerated h in the valence band (VB) of g-C N 3 4 4 Conclusions and the e in the conduction band (CB) of T iO   recom- The biochar-coupled Fe O @SiO @TiO /g-C N com- 3 4 2 2 3 4 bined, and the components of   biochar and F e O @SiO 3 4 2 posites have been successfully synthesized, and exhibited accelerated the transmission of electrons, while lots of extraordinary removal performance for high-concen- electrons and holes survived in the CB of g-C N and 3 4 tration TC. The introduction of biochar and magnetic the VB of TiO , respectively. The ·O generated in the 2 2 nanoparticles can improve visible light response, charge CB of g-C N via reaction between e and O , and ·OH 3 4 2 transfer, and separation of photo-generate carriers. generated in the VB of TiO via reaction between h and 3− Except for the significant inhibition of PO , other H O were deemed to be responsible for the oxidation of anions and pH had no obvious effect on removal per - TC (Guo et al. 2021; Kumar et al. 2020). The Cr(VI) also formance. The heterojunction enhanced the spatial Y ang et al. Biochar (2023) 5:1 Page 17 of 20 Fig. 8 The TC degradation pathway in simultaneous photocatalytic redox conversion of TC and Cr( VI) systems over FSBTG-450 + − − and Z-scheme photocatalytic reaction and the pathway separation of h and e , and ·OH and ·O were iden- of TC were proposed. This work can provide a reference tified as the main reactive species responsible for TC for the removal of high-concentration TC/Cr(VI) in com- degradation and Cr(VI) removal. Due to the synergistic bined pollutant systems and the potential application of photocatalysis effect, the removal efficiency of Cr(VI) was biochar-based materials in waste treatment. notably enhanced in the combined pollution systems. In addition, removal mechanisms involved in adsorption Yang et al. Biochar (2023) 5:1 Page 18 of 20 Abbreviations University, Guangxi 530004, China. School of Environmental and Safety BC : Biochar; FST: Fe O @SiO /TiO ; FSBT: Biochar-coupled F e O @SiO /TiO ; Engineering, Changzhou University, Changzhou 213164, China. Environmen- 3 4 2 2 3 4 2 2 BTG: Biochar-coupled TiO /g-C N ; BTS: Biochar-coupled TiO /CdS; FSTG: tal Technology Engineering Co.Ltd Subsidiary of Chinese Research Academy 2 3 4 2 Fe O @SiO /TiO /g-C NFSTS, Fe O @SiO /TiO /CdS; FSBTG: Biochar-coupled- of Environmental Sciences, Beijing 100012, China. 3 4 2 2 3 4 3 4 2 2 Fe O @SiO /TiO /g-C N ; FSBTS: Biochar-coupledFe O @SiO /TiO /CdS. 3 4 2 2 3 4 3 4 2 2 Received: 11 August 2022 Revised: 3 December 2022 Accepted: 12 December 2022 Supplementary Information The online version contains supplementary material available at https:// doi. org/ 10. 1007/ s42773- 022- 00197-4. References Additional file 1: Fig. S1. Synthetic process of biochar-coupled SiO2@ Buzzetti L, Crisenza GEM, Melchiorre P (2019) Mechanistic studies in photoca- Fe O /TiO /g-C N composite. Fig. S2. SEM micrographs of CdS (a), FSBT 3 4 2 3 4 talysis. Angew Chem Int Ed 58(12):3730–3747. https:// doi. org/ 10. 1002/ (b), and FSBTS (c); EDS pattern (d), SEM-EDS layered image of FSBTS (e, anie. 20180 9984 f ); EDS mapping of FSBTS for the elements of C (g), Cd (h),O (i),Si ( j), Cai CJ, Xu MW, Bao SJ, Ji CC, Lu ZJ, Jia DZ (2013) A green and facile route for Ti (k) and Fe(l); TEM (m) and HRTEM (n) micrograph of FSBTS with cor- constructing flower-shaped TiO nanocrystals assembled on gra- responding SAED patterns in inset (o). Fig. S3. N adsorption-desorption 2 phene oxide sheets for enhanced photocatalytic activity. Nanotechnol isotherms (a) and pore size distribution curves of as-prepared catalysts 24(27):602. https:// doi. org/ 10. 1088/ 0957- 4484/ 24/ 27/ 275602 (b). Fig. S4. XRD patterns (a); FT-IR spectrums of biochar, TiO ,CdS and Cheng Y, Zhou F, Li S, Chen Z (2016) Removal of mixed contaminants, crystal several CdS-based photocatalysts (b). 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Cr( VI) reduction and TC Dai J, Meng X, Zhang Y, Huang Y (2020) Eec ff ts of modification and magneti- photodegradation kinetics with g-C N -based photocatalysts (a) and 3 4 zation of rice straw derived biochar on adsorption of tetracycline from CdS-based photocatalysts (b) in combined pollution systems. Table S1 water. Bioresour Technol 311:123455. https:// doi. org/ 10. 1016/j. biort ech. Adsorption kinetic parameters for TC adsorption by photocatalysts at 2020. 123455 298K. Table S2 Adsorption isotherm parameters for TC adsorption by Das A, Kumar PM, Bhagavathiachari M, Nair RG (2020) Hierarchical ZnO-TiO photocatalysts at 298K. Table S3 Photocatalytic performance parameters 2 nanoheterojunction: a strategy driven approach to boost the photocata- of different materials for TC photodegradation and Cr( VI) reduction in lytic performance through the synergy of improved surface area and different pollution systems. interfacial charge transport. Appl Surf Sci 534:147321. https:// doi. org/ 10. 1016/j. apsusc. 2020. 147321 Djellabi R, Yang B, Sharif HMA, Zhang JJ, Ali J, Zhao X (2019) Sustainable and Author contributions easy recoverable magnetic TiO -Lignocellulosic biomass@Fe O for solar BY: conceptualization, methodology, writing-original draft. JD: methodology, 2 3 4 photocatalytic water remediation. J Clean Prod 233:841–847. https:// doi. visualization, formal analysis. YZ: conceptualization, resources, supervision. ZW: org/ 10. 1016/j. jclep ro. 2019. 06. 125 visualization, data curation, resources. JW: validation, data curation. CJ: formal Duan X, Sun H, Wang S (2018) Metal-free carbocatalysis in advanced oxidation analysis, validation. YZ: funding acquisition, supervision, project adminis- reactions. Acc Chem Res 51(3):678–687. https:// doi. org/ 10. 1021/ acs. tration, validation, writing-review and editing. XP: methodology, formal accou nts. 7b005 35 analysis, writing - review and editing. All authors read and approved the final Fang G, Zhu C, Dionysiou DD, Gao J, Zhou D (2015) Mechanism of hydroxyl manuscript. radical generation from biochar suspensions: implications to diethyl phthalate degradation. Bioresour Technol 176:210–217. https:// doi. org/ Funding 10. 1016/j. biort ech. 2014. 11. 032 This work was supported by the National Natural Science Foundation of China Gao X, Niu J, Wang Y, Ji Y, Zhang Y (2021) Solar photocatalytic abatement of (Grant Numbers [42220104004]); the National Natural Science Foundation tetracycline over phosphate oxoanion decorated Bi WO /polyimide of China (Grant Numbers [41671331]); the Innovative Approaches Special 2 6 composites. J Hazard Mater 403:123860. https:// doi. org/ 10. 1016/j. jhazm Project of the Ministry of Science and Technology of China (Grant Numbers at. 2020. 123860 [2020IM020300]); and the Beijing–Tianjin–Hebei Collaborative Innovation Guo Q, Zhou C, Ma Z, Yang X (2019) Fundamentals of TiO photocatalysis: Con- Promotion Project of China (Grant Numbers [Z201100006720001]). We want to cepts, mechanisms, and challenges. Adv Mater 31(50):1901997. https:// acknowledge anonymous reviewers for their valuable comments. doi. org/ 10. 1002/ adma. 20190 1997 Guo Y, Yan C, Guo Y, Ji X (2021) UV-light promoted formation of boron nitride- Availability of data and materials fullerene composite and its photodegradation performance for antibiot- All date generated during the current study are available from the corre- ics under visible light irradiation. J Hazard Mater 410:12468. https:// doi. sponding author on reasonable request. org/ 10. 1016/j. jhazm at. 2020. 124628 Herath A, Navarathna C, Warren S, Perez F, Pittman CU Jr, Mlsna TE (2022) Iron/ Declarations titanium oxide-biochar (F e TiO /BC): a versatile adsorbent/photocatalyst 2 5 2+ − for aqueous Cr( VI), P b, F and methylene blue. J Colloid Interface Sci Ethics approval and consent to participate 614:603–616. https:// doi. org/ 10. 1016/j. jcis. 2022. 01. 067 Not applicable. Hu C, Tu S, Tian N, Ma T, Zhang Y, Huang H (2021) Photocatalysis enhanced by external fields. 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Journal

BiocharSpringer Journals

Published: Jan 3, 2023

Keywords: Biochar; Tetracycline; Hexavalent chromium; Combined pollution; Synergistic photocatalytic effect

References