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Tri-functional lanthanum-based biochar for efficient phosphorus recovery, bacterial inhibition, and soil fertility enhancement

Tri-functional lanthanum-based biochar for efficient phosphorus recovery, bacterial inhibition,... 1 Introduction However, current P resources can only meet human Phosphorus (P) is closely related to human produc- needs for a certain number of years (Cordell et  al. tion and is an essential element in agricultural fertiliz- 2011). If excess P in water bodies can be recovered and ers, pesticides, food additives, detergents, and various reused effectively, the P resource crisis can be alleviated chemicals (Zhao et  al. 2020). The widespread use of to some extent (He et al. 2022). P has been accompanied by an increasingly negative Common methods developed to remove P species impact on the environment, such as causing water from aquatic environments include chemical precipita- eutrophication and ecosystem imbalance, which seri- tion (Barca et al. 2012), microbial transformation (Geng ously threatens the health of humans and aquatic ani- et  al. 2018), enhanced biological phosphorus removal mals. Therefore, many regulatory authorities impose (EBPR), and adsorption (Ye et  al. 2017). Chemical pre- strict restrictions on the P content of water. For cipitation can usually reduce the concentration of P −1 example, the United States Environmental Protection in a solution to a range of 0.5−1 mg L (Mayer et  al. Agency (USEPA) requires that the total P concentra- 2013). It requires large quantities of metal ions, dis- −1 tion in water should not be higher than 50  μg L and posal of large quantities of sludge, and the recovery of Yao et  al. (2013) believes that to prevent water pollu- metals is cumbersome, making this process suitable for tion and eutrophication, the total P content should be treating effluents with higher P concentrations (Kumar −1 less than 20 μg L . Excess P in aquatic environments is et  al. 2019). The EBPR process can theoretically reduce −1 harmful, but paradoxically, there is a global P resource effluent  P concentrations to 0.1−0.2 mg L (Liu et  al. crisis. According to statistics, the human demand for P 2019a, b). However, due to factors such as organic fertilizer is tens of millions of tons per year (expected to loading and reactor operating parameters, most EBPR reach 50 million tons by 2023) (Almanassra et al. 2021). Jia  et al. Biochar (2023) 5:16 Page 3 of 17 −1 processes can only reduce P to 0.5−1 mg L in actual 2 Material and methods projects (López-Vázquez et  al. 2008). Microbial con- 2.1 Reagents and formulae version is susceptible to environmental conditions, The reagents used in this study are described in detail in such as the organic carrier on which it is loaded, reac- the Additional file  1, and the relevant formulae used in tion vessel, and other factors (Wu et al. 2020). Adsorp- this study are listed in Additional file 1: Table S1. tion is one of the most widely used P removal process, and it has a wide range of adsorbent materials, a fast 2.2 Adsorbent preparation P adsorption rate, high selectivity, a small amount of The method of preparing biochar with the mesocarp of equipment input, and adsorbent regeneration (Desmidt shaddock (MSB) is described in the Additional file  1. et al. 2015). MSB (3  g) was sonicated and dispersed in 500  mL of Lanthanum (La) is a rare earth element (REE) hav- water to form a suspension, and 1  g of LaCl ·7H O was 3 2 ing strong bonding ability with phosphate (Tang et  al. dissolved in 100  mL of water and added to this suspen- 2019). Among the REEs, La is second only to cerium sion. The suspension was stirred at 1000 rpm for 10 h and and neodymium in abundance in the  earth’s crust ammonia was added dropwise until the pH of the mix- (Tyler 2004) and is widely used to adsorb P from water. ture reached 11. After 12 h of stirring, the solid was col- However, because most P adsorption reactions involve lected by filtration, and the filter cake was washed with monolayer adsorption (Wu et al. 2020), the aggregation water to neutral pH and dried under vacuum at 80  °C of La can lead to a reduction in the adsorption capac- for 6  h. The product was named MSBL1. Other condi - ity. Recently, researchers have chosen to load La onto tions were kept constant, and only 1 g of L aCl ·7H O was 3 2 inorganic carriers to avoid its leaching and aggrega- replaced with 2, 3, and 4 g to obtain MSBL2, MSBL3, and tion, including activated carbon (Liu et  al. 2022), car- MSBL4, respectively. A schematic diagram of the prepa- bon nanotubes (Zhang et  al. 2022), and biochar (Chen ration of the shaddock mesocarp for La-loaded species et  al. 2022; Liu et  al. 2019a, b). Biochar is a carbon is shown in Fig. 1a. After going through the processes of material produced by the thermal processing of bio- peeling, freeze-drying, heat treatment, and La loading in mass under limited oxygen conditions; when biochar is turn, the P absorbing agent was successfully prepared. used as a soil amendment, it can serve to reduce soil density and enhance nutrient management. If the feed- 2.3 Adsorbent characterization stock is biological waste with a disposal fee (Stávková The preparation of the adsorbents at different ratios and and Maroušek 2021), the cost of recycling P does not the mechanism of adsorption of P by MSBL3 were ana- increase, which in turn increases the economic attrac- lyzed using Powder X-ray diffraction (PXRD), Brunauer– tiveness and competitiveness. Emmett–Teller theory (BET), X-ray  photoelectron Shaddocks are a variety of pomelos that are widely spectroscopy (XPS),  scanning electron microscopy grown in Southeast Asian countries. The mesocarp of (SEM),  transmission electron microscopy (TEM), spheri- shaddock refers to the part between the outer skin and cal aberration corrected transmission electron microscope the inner fruit, which is usually discarded as waste owing (ACTEM), and other methods. The instrument models are to the lack of food value. Therefore, it was selected as the described in the Additional file 1 . raw material for biochar in this study. Given the strong binding ability of La to phosphate, it was hypothesized 2.4 Batch adsorption experiments that the adsorption of P could be achieved by loading La P adsorption experiments were carried out in coni- on biochar. Therefore, one of the aims of this study was cal flasks, which were placed on a constant temperature to verify the adsorption effect of P in an aqueous envi - water shaker at 150  rpm. Unless otherwise stated, the −1 ronment by the shaddock mesocarp biochar after load- adsorbent dosage was 0.5 g L , the reaction temperature −1 ing of La and whether it could enhance the water quality was 25 ± 0.5  °C, initial P concentration was 50  mg L , of eutrophic water bodies. In addition, since biochar can and initial pH of the solution was 7. A polytetrafluoro - be used as a soil amendment, another aim was to verify ethylene rod of 0.5  cm diameter and 3  cm length was whether the adsorbent waste after P adsorption could be placed in a conical flask to ensure that the adsorbent did used as an effective slow-release P fertilizer to improve not collect at the bottom of the flask during centripetal soil fertility and plant growth. In conclusion, the material oscillation. During the experiments, samples were col- prepared in this study may be a promising P sorbent for lected at set time intervals and then filtered, after which application, and it is also hoped that the research meth- the concentration of P in the water samples  was deter- ods used in this paper can provide some ideas for schol- mined using ammonium molybdate spectrophotom- ars in related fields. etry and  inductively coupled plasma-optical emission Jia et al. Biochar (2023) 5:16 Page 4 of 17 Fig. 1 a Preparation diagram of adsorbents; b FTIR spectra of MSBL3 and MSBL3 + P; c High-resolution XPS spectra of La 3d; d N adsorption– desorption curve of adsorbents; e XRD pattern of adsorbents; f High-resolution XPS spectra of O1s spectroscopy (ICP-OES), respectively. All experiments River, all located in Kunming, China. The insoluble mate - were performed in triplicate, and the results were rial was filtered before preparing the P solution, and the reported as the mean with standard deviations. P concentration in natural water was determined. On the When exploring the effect of co-existing ions, only other hand, the P concentration was set at three differ - −1 −1 −1 the effect of ions with the same electrical properties as ent gradients (5 mg L , 10 mg L , and 20 mg L ) using 3− + PO was considered, where the cations were all K . The each of the four waters as solvent and the adsorbent per- 2− − 2− − initial concentrations of CO, NO, SO , and Cl formance in removing P was evaluated at neutral pH. 3 3 4 −1 −1 exhibited four different gradients (0  mg L , 20  mg L , Continuous fixed-bed experiments were performed in −1 −1 100 mg L , and 200 mg L ), respectively. For the cyclic an inorganic glass column with a length of 200  mm and experiments, after each adsorption, the collected adsor- outer diameter of 15  mm. Natural lake water samples bent was stirred in a NaOH solution (1 M) at 150 rpm for were prepared as feed solutions with a P content of 2 mg −1 10  h, washed to neutral pH, and dried. This procedure P L , and the flow rate was controlled using a peristal - −1 was repeated five times. tic pump at 20 bed volume (BV) h . The effluent solu - To assess the P sorption performance of the adsorbent tion was sampled at 0.5 h intervals. MSBL3 was desorbed in a natural water environment, water from different sce - in situ using a NaOH solution (1 M) as a resolving agent, narios was used as a solvent for P, including ultrapure, and the adsorption–desorption process was repeated municipal, lake, and river waters. Municipal water was three times. collected from the municipal water supply network, lake The eutrophication inhibition experiment using water from Dianchi Lake, and river water from Laoyu MSBL3 was set up with P-free, P-containing, and Jia  et al. Biochar (2023) 5:16 Page 5 of 17 3 Results and discussion P + MSBL3 groups, in which three different concentra - −1 −1 −1 3.1 Idea of material selection tions of P (20 mg P L , 50 mg P L , and 100 mg P L ) There was a thick fluffy mesocarp between the shad - were set in the P + MSBL3 group. The effectiveness of dock peel and pulp, which was considered useless and MSBL3 on P adsorption and inhibition of eutrophica- discarded together with the shaddock peel. In terms of tion in water bodies was verified by comparing the environmental management, it is expected that cheap growth status of water hyacinth. and readily available biomass can be locally sourced for The inhibitory properties of different MSBL3 concen - conversion to useful biochar. Considering that the mes- trations against E. coli and S. aureus were also inves- ocarp in the shaddock peel and pulp sandwich exhibits tigated. The antibacterial properties of MSBL3 were a fluffy and porous state from a macro perspective, we tested using the flat-plate coating method (Han et  al. used it as a biochar carrier for La species that can effec - 2021; Yu et al. 2019a, b). First, MSBL3 was fully ground, tively remove P. and then the ground samples were wrapped in tin foil and dried for 20  min (120  °C, dry heat sterilization of 3.2 Physicochemical state and morphology of adsorbents materials). E. coli and S. aureus were placed in shake The FTIR spectrum (Fig.  1b) shows that the MSBL con- flasks filled with Luria–Bertani (LB) agar medium for tains a rich variety of functional groups, such as –OH, 18  h (180  rpm, 35  °C). To prepare the inoculum, the –N–H, C–N, and C–O (Wang et  al. 2020). These spe - cultured E. coli and S. aureus solutions were diluted to –7 –8 cies are present in most biochar materials and can sig- 10 and 10 , respectively. 8  mg of sterilized MSBL3 nificantly influence the physicochemical properties was added to 10  mL of LB liquid culture medium to of biochar, for example, by anchoring metallic species obtain a concentration of 0.0001  M MSBL3. Similarly, (metals or their compounds), changing the electrical the concentrations of MSBL3 with 0.001  M, 0.01  M properties of the biochar surface, or acting as hydrogen and 0.1 M were obtained with corresponding sterilized bond acceptors (or donors). In the powder X-ray diffrac - MSBL3. tion (PXRD) spectrum (Fig.  1e), MSBL1, MSBL2, and Maize was chosen as the target plant for the resource- MSBL3 exhibited signal peaks that were consistent with planning experiment after sorbent fatigue. According to the crystalline phase of La(OH) with a crystal group the Chinese soil classification standard (GB/T 17296- of P63/m (JCPDS:36-1481). Thus, a gradual increase in 2009), the type of soil in this study was Lateritic red soil, ammonia input allows for an increase in the pH of the which is widely distributed in the southern half of China 3+ solution, with La forming La(OH) insoluble matter and is a clayey soil with poor fertility. The physicochem - in a stronger alkaline environment and loading onto the ical characteristics of the soils are list in Table  1. Maize MSB biochar. In addition, the PXRD peak weakened with seed is the national trial high-yielding zhengdan 958, increasing amounts of LaCl . This phenomenon may be which is characterized by its salinity and drought toler- related to the size of the La(OH) crystals formed;  the ance. The seeds were soaked in warm water at approxi - higher the LaCl concentration, the smaller the La(OH) mately 55  °C for 8–12  h before planting. The seeds 3 3 particles obtained in the presence of ammonia. The were kept at a cultivation temperature of 23 ± 3  °C, XPS spectra were compared to the PXRD spectra. The soil humidity of 40–60%, air humidity of 40–50%, and chemical valence of La in MSBL (in the case of MSBL3) light at 10 ± 2  h per day. The growth conditions were was + 3. The signals at 834.5 eV and 838.0 eV were attrib - kept consistent and the growth of maize in soil samples uted to L a3d , and the signals at 851.4 eV and 854.9 eV doped and un-doped with MSBL3 was compared to ver- 5/2 were attributed to La3d (Fig.  1c). The satellite peaks ify whether MSBL3 could be recycled directly back into 3/2 of La3d and La3d of MSBL3 were at high binding the soil as an effective slow release P fertilizer after the 5/2 3/2 energies and had strong signals, which may be related to adsorption of P reached the fatigued state. the shake-up process resulting from the charge transfer Additional experimental procedures, statistical analy- between O2p-La4f. In the O binding energy region, the ses, and results are described in the Additional file 1 . Table 1 The soil physicochemical characteristics Colour Structure pH Organic carbon Total P Available P −1 −1 −1 (g kg )(g kg )(mg kg ) Blank control soil Brick red Granular 5.95 2.951 0.674 6.000 Soil + MSBL3 Greyish red Granular 5.89 30.057 7.221 83.250 Jia et al. Biochar (2023) 5:16 Page 6 of 17 XPS-acquired signal peaks can be obtained by coupling the amount of LaCl input increased. This may be due three fitted peaks at 530.1  eV, 530.8  eV, and 531.5  eV, to the introduction of La(OH) , resulting in the filling which correspond to the signals of the O–H, La–O, and and covering of the original pore and base surfaces of C–O–C bonds (Yang et  al. 2019), respectively (Fig.  1f ). the MSB, leading to a continuous decrease in the BET In addition, the effect of different LaCl dosages on the values (Fig. 1d). It is worth noting that the introduction specific surface area of the biochar was derived from the of La(OH) fills the pore channels of the MSB to some nitrogen adsorption and desorption curves of MSBL1, extent and therefore the average pore sizes reported in MSBL2, and MSBL3. MSBL1, MSBL2, and MSBL3 should be derived from The specific surface areas (based on BET analysis) of the statistical results of the interstitial spaces between MSB, MSBL1, MSBL2, and MSBL3 were obtained from the La(OH) species. In the above discussion, the the nitrogen adsorption and desorption curves as 59.5, PXRD signal indicates that the larger the L aCl input, 39.5, 27.8, and 11.8, respectively. Clearly, the BET sur- the smaller the La(OH) particles that are likely to be face area of MSB decreased when loaded with La(OH) , formed and, therefore, more uniformly distributed. and this decreasing trend became more pronounced as In this case, the gaps between the smaller particles Fig. 2 SEM images of MSB (a), MSBL3 (b), MSBL2 (c), and MSBL1 (d); TEM images of MSB (e), MSBL3 (f), MSBL2 (g), and MSBL1 (h); (i) Schematic diagram of adsorbents morphology; EDS element mapping of MSBL3 (j, m), MSBL2 (k, n), and MSBL1 (l, o) Jia  et al. Biochar (2023) 5:16 Page 7 of 17 of La(OH) were tighter; therefore, the pore size was Elementals C, N, O, and La were all distributed in MSBL3 smaller. (Fig.  2j, m), MSBL2 (Fig.  2k, n), and MSBL1 (Fig.  2l, o) Electron microscopy imaging directly presents the based on elemental mapping imaging. The La signal was microscopic morphological features of the MSBL. Fig- more uniformly distributed on MSBL3, whereas it was ure  2a, e shows the SEM and TEM images of the MSB, the least distributed on MSBL1. However, it was difficult respectively. MSB had a slightly distorted two-dimen- to observe any new species in the TEM images of MSBL3, sional lamellar structure with a smooth and delicate base. where the morphology could be resolved. It was observed When loaded with La(OH) , the SEM and TEM images that the particle size of the La species (characterized as of MSBL3 (Fig.  2b, f ) were similar to those of MSB, and La(OH) by PXRD) gradually  decreased as the amount it was difficult to distinguish the formation of new mate -of LaCl input  increased. Thus, the nanorods in MSBL1 rial on the base surface of the biochar with the naked eye. had a larger particle size than those in MSBL2, whereas The difference is that, although the morphologies of both the La(OH) particles in MSBL3 may be uniformly dis- MSBL2 (Fig.  2c, g) and MSBL1 (Fig.  2d, h) were two- tributed to form a homogeneous film on the MSB surface dimensional lamellar structures, the lamellar base surface (Fig. 2i). To test this hypothesis, we used a more accurate became rough, and the roughness of MSBL1 was more ACTEM to observe the microscopic morphology of the pronounced than that of MSBL2. In the TEM images, a MSBL3. large number of nanorods were attached to the MSB sur- When the scale bar reached 200  nm and 20  nm, the face in MSBL2 and MSBL1, the length of the nanorods surface of the flake carrier of MSBL3 remained smooth in MSBL2 was approximately 100 nm and that in MSBL1 (Fig. 3a, b). However, when the field of view was enlarged was approximately 400 nm. Because the MSB surface was to a scale bar of 10  nm, various new species with good smooth and without any rods, the nanorods on the MSB crystallinity in the size (planar size) range of 2−10  nm substrate in MSBL2 and MSBL1 were La(OH) species. were seen distributed on the MSB surface, which were Fig. 3 AC-HRTEM with HAADF mode (a, b, d, e) of MSBL3; (c) crystal lattice model and lattice spacing of La(OH) on MSBL3 3 Jia et al. Biochar (2023) 5:16 Page 8 of 17 closely connected to each other with almost no obvious In general, La(OH) exhibits good adsorption of P spe- fault lines (Fig. 3d). The lattice spacing of these new spe - cies (e.g. phosphate) in water. The loading of La(OH) cies was close to 0.326  nm, which is consistent with the in MSBL1, MSBL2, and MSBL3 gradually increased, so lattice spacing of the 110 crystal face of La(OH) . There - that their P adsorption capacity gradually increased, in fore, the new species evenly distributed on the MSB line with theoretical predictions. However, compared to was La(OH) .  Using the high-angle annular dark field MSBL3, MSBL4 did not significantly increase the adsorp - (HAADF) image mode, it can be inferred that the bright tion capacity of P. Therefore, considering the effect of  P spots of atomic size in the lattice stripe in Fig.  3e are La adsorption and economic benefits, MSBL3 was the only atoms because of the stronger scattering of electrons by experimental group used for subsequent batch experi- metal atoms with larger atomic numbers. It can be found ments. In view of the complexity of the water column, the that the distance between adjacent bright spots on the effect of pH on P adsorption by MSBL3 was investigated, lattice line is about 4.25  Å, which is consistent with the and the adsorption capacity of P varied with the pH of La-La distance in the La(OH) crystal cell (Fig.  3c). This the solution (Fig. 4b). The maximum P adsorption capac - −1 evidence effectively verifies the above reasonable hypoth - ity of MSBL3 was 78.5 mg  g when the pH was approx- esis that the particle size of La(OH) on MSB is small imately 3. When the pH was below 3, the P adsorption in the order of MSBL1, MSBL2, and MSBL3, and that capacity of MSBL3 decreased sharply, which may be its distribution on MSBL3 exhibits a high dispersion of related to the dissociation of La(OH) in a strongly small particle sizes. acidic environment. When the pH was higher than 3, the adsorption capacity of MSBL3 for P slowly decreased, 3.3 P erformance study of P removal and this trend became more pronounced when the pH In detail, the ability of La(OH) loaded onto MSBs to was higher than 7 (Yin et al. 2022). The zeta potential dis - adsorb P species from water was evaluated. The pH of tribution (Fig.  4e) of MSBL3 shows that its surface elec- natural water is close to neutral; therefore, the ability trostatic potential was positive when the pH was below of MSBL1, MSBL2, MSBL3, and MSBL4 to adsorb P in 10. When the pH increased from 10 to 11, the MSBL3 −1 P solutions at pH7 (P concentration of 50  mg L ) was surface electrostatic potential reversed from positive to first evaluated. When the adsorption reached equilib - negative (Jia et  al. 2022). It can be assumed that a posi- rium, the P adsorption capacities of MSBL1, MSBL2, tive electrostatic potential indicates more H enrichment −1 −1 + MSBL3, and MSBL4 were 27.4 mg P g , 41.8 mg P g , or the presence of more protonated groups (e.g. NH ) −1 −1 60.5  mg P g , and 62.1  mg P g , respectively (Fig.  4a). on the material surface, whereas a negative electrostatic Fig. 4 a Comparison of q ; b Eec ff t of initial solution pH on P removal of MSBL3; c Sorption kinetic models; d Eec ff t of different P concentrations on −1 q ; e Zeta potential of MSBL3; f Langmuir model. (dosage of adsorbent: 0.5 g L ; T: 298 K) e Jia  et al. Biochar (2023) 5:16 Page 9 of 17 Langmuir model is more consistent with the  P attach- potential indicates more OH enrichment on the mate- ment behavior of MSBL3 as a single molecular layer. rial surface (Wang et  al. 2021a, b). As the pH gradually increased from 3, the OH content of the solution gradu- 3.4 Universality of practical application of adsorbent ally increased, which weakened the H or protonated 3.4.1 P racticality verification of MSBL3 groups on the material surface. This weakened the Cou - 3− Before the practical application of the adsorbent, it is lombic interaction of the negatively charged P O (or 2− − necessary to consider its effects, such as the type of actual HPO and H PO ) with the material surface, thus 4 2 4 water and co-existing ions. Water samples were collected reducing the adsorption capacity of MSBL3 for P (Wang from nearby waters and tested for P concentrations, with et al. 2021a, b). −1 −1 P concentrations of 3.0 μg L and 24.0 μg L in samples Based on the adsorption capacity–time curve (Fig. 4d), collected from Dianchi Lake and Laoyu River, respec- the adsorption behavior of P on MSBL3 can be explained tively (Fig.  5a). After adsorption by MSBL3, P was not using pseudo-first-order and pseudo-second-order detected in either water sample; therefore, the removal kinetic models (Fig.  4c), that is, physical adsorption and efficiency was 100%. Ultra-pure water, municipal water, chemical adsorption, respectively. The regression  coef - Dianchi Lake water, and Laoyu River water samples were ficients (R ) fitted by the pseudo-first-order model were used as solvents to prepare solutions with concentrations lower than those of the pseudo-second-order model −1 −1 −1 of 5 mg P L , 10 mg P L , and 20 mg P L , respectively. (Additional file  1: Table  S2). Therefore, the adsorption −1 At a concentration of 20  mg P L , the removal effi - of P on MSBL3 was more consistent with the pseudo- ciency of P by MSBL3 reached more than 99.9% for the second-order kinetic model, and was mainly chemisorp- first two water samples, whereas for Dianchi and Luoyu tion. In addition, the attachment state of P to MSBL3 River water samples, it was 94.5% and 94.1%, respectively, was investigated using Langmuir and Freundlich mod- which might be due to the presence of other compet- els. When the Freundlich model was fitted linearly to ing ions in the lake water samples that would inhibit P the isothermal adsorption data, the value of R was only removal to some extent (Fig. 5b). 0.801 (Additional file  1: Fig. S1b), whereas the value of R 2− The presence of co-existing anions such as CO , obtained when the Langmuir model was used to fit the 2− − − SO, NO , and Cl in the P solution did not isothermal adsorption data was 0.999 (Fig. 4f ). u Th s, the 4 3 −1 Fig. 5 a Adsorption of P by MSBL3 in natural water; b Adsorption of P in four water bodies by MSBL3 (Solutions of 5, 10, and 20 mg P L were prepared using the four water samples as diluents); c Eec ff t of coexisting anions on q ; d Adsorption–desorption cycles. (dosage of adsorbent: −1 −1 0.5 g L ; T: 298 K); e P concentration in the effluent at different cycles. (C : 2 mg P L ; dosage of MSBL3: 0.25 g; T: 298 K); f Adsorption effect of −1 magnification experiment ( Volume of P solution: 25 L; dosage of MSBL3: 2.5 g; C : 25 μg L ; T: 298 K; The Y-axis represents the concentration of P in the solution) Jia et al. Biochar (2023) 5:16 Page 10 of 17 significantly affect the performance of MSBL3 for P on the ecological environment. Therefore, it is neces - adsorption (Fig.  5c). The high selectivity of MSBL3 for sary to focus on reducing La leakage from the adsor- 3− 2− − PO (or HPO and H PO ) can be explained by bent in subsequent studies. For example, improving the 4 4 2 4 the fact that the products LaCl , La(NO) and La (SO ) preparation method to firmly fix La in the adsorbent or 3 3 2 4 3 3+ − − 2− formed by L a with Cl, NO , and SO are all water- reducing the loading of La while ensuring the removal 3 4 soluble compounds and, therefore, have a small tendency efficiency of P. to react with MSBL3. When the co-existing ion was 2− CO , the adsorption of P by MSBL3 slightly decreased 3+ 2− because La easily combined with CO to form a 3.4.3 Magnification experiment La (CO ) solid, but the pKsp of L a (CO ) was smaller In practical applications, the separation cost after the 2 3 3 2 3 3 2− than that of La(OH) , making it difficult for CO to reaction is considerably higher if the powdered adsor- 3 3 3+ capture most of the La sites (Yu et al. 2019a, b). bent is sprayed directly in water. Additionally, incom- Reproducibility of the adsorbent is particularly impor- plete separation produces a large amount of sludge. tant for practical applications. Considering the complete Therefore, we conducted a scale-up experiment to conversion of L aCl to La(OH) on MSB, NaOH can be reduce the separation cost of the adsorbent and meet 3 3 used to provide a significant amount of OH to reconsti- the practical application requirements. As shown in tute the La(OH) species after P adsorption by MSBL3. Additional file  1: Fig. S5, filter packages containing After six repeated uses, MSBL3 was able to retain 76.7% MSBL3 were dosed into the P-containing water. Owing of its initial performance in terms of P adsorption capac- to the low mobility of the lake water, the water box was ity (Fig.  5d), which is better than that of most reported shaken slightly at fixed times to simulate water flow. P adsorbents, making MSBL3 economically relevant in Samples were taken at fixed times each day and the practical applications. P content of the solution was measured. As shown in Fig. 5f, after three days, P was not detected in the solu- tion. It can be seen that loading the adsorbent in the 3.4.2 Continuous fixed‑bed column experiment filter pack will prolong the time required for the reac - A fixed-bed adsorption experiment was designed to tion due to uneven dispersion, but will still achieve investigate the potential of the PL adsorbent for applica- the effect of adsorbing P. A filter package containing tion in flowing lake water. Both ends of the tower were MSBL3 can be easily removed after the adsorption is equipped with layers of quartz sand plates to prevent complete, which is more practical and allows for a wide PL loss (Additional file  1: Fig. S4). The water sample col - range of applications. lected from the Laoyu River was used as the feed solu- In terms of economic cost analysis, biomass is abundant tion. To visually verify the adsorption effect of PL in in nature and is environmentally and economically sus- natural water, the initial P concentration in the water −1 tainable (Maroušek and Maroušková 2021). In this study, sample was adjusted to 2 mg P L . −1 biochar was doped with La, which, although classified The breakthrough point for P was set at 20  μg L as a rare earth element, is abundant in the earth’s crust, because exceeding this value could result in lake second only to cerium and neodymium. Because of the eutrophication (Yao et  al. 2013). As shown in Fig.  5e, high requirements for elemental purity in the preliminary the first adsorption experiment reached a breakthrough tests, analytically pure L aCl ·7H O was chosen, which is 3 2 point at a water volume of 330 BV, and q was 29.3 mg −1 −1 slightly more expensive at 132.6 RMB 500  g . The prep - P g . Subsequently, a 1  M NaOH solution was used aration of 1  g of MSBL3 requires 0.7  g of LaCl ·7H O, 3 2 to desorb PL in a cyclic manner until the P concentra- 0.7  g of mesocarp of shaddock, and 25  mL of N H ·H O. 3 2 tion in the desorbed solution became constant, which The cost of the above reagents, in addition to the water took approximately 5.5  h. After desorption, MSBL3 and electricity used throughout the process, is approxi- was rinsed with deionized water until the effluent was mately 1 RMB, and it can reduce the P concentration in neutral, after which the next cycle was initiated. The 2.6 ton of Laoyu River water to below the eutrophication breakthrough point reached 280 BV and 240 BV for threshold. Although this cost is not very high, it is nec- the second and third times, respectively, with q of −1 −1 essary to continue to reduce the cost if large-scale prac- MSBL3 for P being 24.8  mg P g and 21.3  mg P g , tical applications are to be achieved (Durana et  al. 2021; respectively. The third time, q was 72.6% of the initial Maroušek and Trakal 2022). Market research shows that value. In addition, the leakage of La in the first effluent −1 −1 −1 commercially available L aCl sells for only 17 RMB kg , a was 207.7  μg L and 89.5  μg L after the third cycle. difference of approximately 15 times the price of the ana - Although there is no standard limit for La in the Chi- lytically pure grade, hence this is commercially promising. nese surface water environmental quality standard, it is unclear whether La leaching will have a negative impact Jia  et al. Biochar (2023) 5:16 Page 11 of 17 3.4.4 Inhibition of eutrophication in water bodies by MSBL3 there was no significant increase in the number of leaves, In eutrophic water bodies, algae and water hyacinth grow along with the presence of slightly more old leaves. This in large numbers, aggravating the deterioration of the suggests that too high a P concentration would have an water body. If P in a water body can be reduced below inhibitory effect on growth. The growth trends of water the eutrophication threshold, the occurrence or deterio- hyacinth in the MSBL3 and P-free groups were simi- ration of eutrophication in the water body can be effec - lar, and by day 25, the leaves had died in large numbers tively avoided. Therefore, we investigated the inhibitory and new leaf growth was insignificant. This verifies the effect of placing MSBL3 in P-containing water on the effectiveness of MSBL3 for P adsorption and shows that growth of water hyacinth. As shown in Fig.  6, during 25 MSBL3 has an inhibitory effect on the growth of water d of incubation, all water hyacinths in the group contain- hyacinth. It can be effectively used to prevent eutrophica - ing P grew well, with a significant increase in the number tion in water bodies and can also be applied to eutrophic of leaves; and most leaves were bright green. Higher the water to inhibit the excessive growth of water hyacinth or P content, greater the number of new leaves; however, algae, alleviate the level of eutrophication, and maintain −1 when the P concentration was increased to 100  mg L , the ecological balance of water. Fig. 6 The growth trend of water hyacinth at different P levels Jia et al. Biochar (2023) 5:16 Page 12 of 17 3.4.5 Antibacterial test such as positively charged NH , and the cell membrane A number of P-containing water bodies, such as live- surface of bacteria is negatively charged; therefore, it stock wastewater (Wang et  al. 2021a, b), urban sewage can attract more bacteria through electrostatic grav- plant wastewater (Bonetta et  al. 2016), and even water ity, causing a large loss of cytoplasmic components and networks (Holinger et al. 2014), contain large amounts of disrupting their normal physiological function, leading bacteria; therefore, if the adsorbent can act as a bacterial to bacterial death (Wei et  al. 1994). The above results inhibitor in the process of P removal, then this material show that MSBL3 can inhibit the growth of E. coli and S. can achieve both P recovery and improved water qual- aureus in the water column; therefore, the application of ity. Therefore, E. coli and S. aureus, which are commonly MSBL3 to recover P from the water column can simulta- found in water bodies, were chosen as the target species neously improve water quality at the same time. to assess the bacterial inhibition performance of MSBL3. The inhibitory effects of MSBL3 on E. coli and S. aureus As shown in Fig. 7b, c, the growth of E. coli and S. aureus, involve two main mechanisms. The first is direct inhibi - was barely inhibited when MSBL3 was excluded. Com- tion. Studies have shown that La ions have an inhibitory pared to the blank group, when the concentration of effect on microbial growth. When the concentration of −1 −1 MSBL3 was 0.0001 M, the inhibition against E. coli and S. La ions was 1000  μg L and 100  μg L , respectively, it aureus was 87.3% and 75.0%, respectively. When the con- was able to kill 83% and 39% of E. coli in just 1 min (He centration of MSBL3 reached 0.1  M, inhibition against et  al. 2015). In this study, the leaching amount of La in −1 both bacteria reached 98.7% and 85.0%, respectively. MSBL3 was approximately 200  μg L , which would This may be because MSBL3 is rich in functional groups, Fig. 7 a Flow chart of the antibacterial test; b, c Inhibition of E. coli and S. aureus at different concentrations of MSBL3 Jia  et al. Biochar (2023) 5:16 Page 13 of 17 directly inhibit bacterial growth to some extent. The sec - reduce the cost of treating pollutants by adsorption and ond is indirect inhibition: in the growth of bacteria, phos- has the potential to add value to the waste resources of phorus and organic carbon sources serve as nutrients for adsorbent sludge. Therefore, we investigated the viability their growth and reproduction, and if the P source is con- of MSBL3 as a fertilizer for agricultural fields, following trolled, the growth of bacteria is inhibited (Zhang et  al. the adsorption of P species in water. Considering the high 2016). In this study, the significant adsorption of P by dependence of maize on P during growth, the growth of MSBL3 resulted in "P deficiency" and some microorgan - maize seeds in soil samples, with and without MSBL3, isms were unable to perform normal metabolism in this was studied. The other growth conditions were kept P-deficient environment, which led to their death. consistent and maize growth was recorded at the same time each day. As shown in Fig. 8, maize seeds started to 3.5 R esource planning after adsorbent fatigue sprout on the eighth day, and the growth rate of maize Adsorption is a simple and effective strategy for pollut - in the soil samples without MSBL3 was considerably ant treatment, but it often faces problems such as dif- slower than that in the soil samples with MSBL3. After a ficulties with post-treatment of the adsorbent sludge. few days, the maize seedlings in the MSBL3-doped group However, when the adsorbent is used several times, its were tall and thick, whereas those in the non-MSBL3- ability to adsorb pollutants decreases. When it reaches doped group were short and thin. Therefore, it can be its maximum service life, adsorbent sludge causes sec- tentatively determined from the growth status of corn ondary pollution to the environment and its disposal is seedlings that MSBL3 can be used as an effective slow- costly (Sornhiran et  al. 2022). In this study, MSBL3 was release P fertilizer. the biomass material, and when it was enriched with P in However, it has been demonstrated that moderate solution, it formed a complex with compositional proper- amounts of La can also have beneficial effects on plant ties similar to those of grass ash, which has a fertilizing growth, such as promoting photosynthesis and acceler- effect. Therefore, when MSBL3 reaches its final fatigue ating growth (Agathokleous et  al. 2018). Therefore, the state, it can be placed directly on nutrient-poor agricul- availability of P for plant nutrition in this study needs to tural land, instead of separating MSBL3 from P. This will be investigated further. The availability of P and La for Fig. 8 The growth trend of maize seeds in pure soil sample (two columns on the left) and soil sample mixed with MSBL3 + P (two columns on the right) Jia et al. Biochar (2023) 5:16 Page 14 of 17 plant nutrition can be explored in depth based on this which were homogeneously distributed, and a new sig- study. On the other hand, since MSBL3 is an adsorbent nal peak appeared in the FTIR spectrum (Fig.  1b) of waste after phosphorus adsorption, backfilling it directly MSBL3, which can be attributed to the P-O bond. In into the soil means that the disposal costs of this waste XPS (Fig.  1d), the binding energy of the 3d orbital of can be reduced, and therefore, its pricing is slightly nega- La shifted towards a higher binding energy than that tive (Stávková and Maroušek 2021). before the adsorption of P, but the valence remained at + 3. These two phenomena suggest that a new spe - 3.6 Mechanism discussion cies has been doped into MSBL3, which acts directly on 3+ 3.6.1 Differences in the crystallinity of La(OH) on the surface the La site. Further direct evidence was provided by of MSB PXRD, where the PXRD signal was consistent with that In the present study, the particle size of the La(OH) of LaPO (JCPDS:35-0731) after P adsorption by MBSL3 3 4 crystals in MSBL3, MSBL2, and MSBL1 increased in (Additional file  1: Fig. S3 h), and the La(OH) phase on that order, which is related to the amount of L aCl input. the MSB carrier was almost completely replaced by the In general, crystal formation consists of two processes, LaPO phase. 3+ nucleation formation and nucleation growth, the rates of La can form insoluble precipitates La(OH) (Eq.  3) − 3− which are heavily dependent on the reactant concentra-and LaPO (Eq. 4) with OH and PO , respectively, but 4 4 tion. These two processes can be described by Eqs.  1 and LaPO had a greater reaction trend with a pKsp as high 3+ 2, where Q is the supersaturation of La , s is the equilib- as 22.43. Thus, La(OH) undergoes a complex decom- 3+ 3− rium concentration when La is converted to La(OH) , k position reaction with P O to form L aPO (Eq.  4), 3 4 4 is the pre-exponential factor, A is the surface area of the resulting in the removal of P from water. H PO exists 2 4 3+ nucleus, D is the diffusion coefficient of the solute La , in various hydrolysis forms at different pH, such as – 2– 3– and δ is the distance the nucleus grows in a given direc- H PO, H PO, HPO , and PO . When the pH is 3 4 2 4 4 4 3+ – tion. When the input of L aCl is large, Q of L a is large, below 4, it is mainly present as both H PO and H PO ; 3 2 4 3 4 leading to a large v . At this point, the process of La(OH) when the pH is between 4 and 10, it is mainly absent as 1 3 3+ – 2– nucleation dominates and L a is consumed at a concen- H PO and H PO ; and when the pH is greater than 2 4 4 2– 3– tration close to s in a short time, leading to a small v for 10, it is mainly present as HPO and P O . At pH 7, P 2 4 4 – 2– the subsequent nucleation growth process. As a result, species are present in the form of H PO and HPO , 2 4 4 the La(OH) crystal particles in the MSBL were highly but PXRD showed that La(OH)   was converted almost 3 3 dispersed and ultrafine at high LaCl input (Additional entirely to LaPO ; therefore, the reaction equation for the 3 4 file  1: Fig. S2a). As the amount of LaCl input decreases, conversion of La(OH) to L aPO in a complex decompo- 3 3 4 Q gradually decreases, leading to a decrease in v . The sition reaction with La species may follow  Eqs.  5−8. As decrease in the rate of the nucleation process delays the seen from Additional file  1: Fig. S1c, when the initial pH 3+ time required La to reach the s state; thus, v increases, of the solution is greater than 4, the pH of the solution resulting in the continuous growth of La(OH) crystal after P adsorption by MSBL3 undergoes a certain degree nuclei (Additional file  1: Fig. S2b, c). The La(OH) crystal of increase because some or all of the OH in La(OH) is 3 3 size is the largest when the LaCl input is the smallest, released in the solution after anion exchange, thus lead- otherwise, the opposite is true. ing to an increase in pH. The loading of La on MSBL3 was 30.2 wt%, so the theo - (Q − s) retical value of 1 g MSBL3 for P adsorption according to v = k (1) s −1 La(OH) to LaPO was 66.91  mg  g . The experimental 3 4 −1 results showed that MSBL3 had a capacity of 78.5 mg  g (Q − s) (at pH3) for P adsorption. Therefore, anion exchange is v = DA (2) the main P adsorption pathway. Additionally, there were other interactions. The zeta potential of MSBL3 (Addi - tional file  1: Fig. S1d) showed that the electrostatic poten- 3.6.2 Mechanism of P adsorption by MSBL3 tial on the MSBL3 surface was positive when the pH After the adsorption of P by MSBL3, the SEM images was less than 10.6; therefore, the MSBL3 surface could showed that the surface of the distorted lamellar struc- 3– 2– – adsorb P species, such as PO, HPO , and H PO 4 4 2 4 ture of MSBL3 was no longer smooth and was replaced via Coulomb interactions. Furthermore, FTIR showed by a fluffy spongy layer of the attached material (Addi - that MSBL3 is rich in N- or O-containing functional tional file  1: Fig. S3 a, b). The elemental mapping groups; therefore, the MSBL3 surface can adsorb P spe- (Additional file  1: Fig. S3c–f), and energy-dispersive 3– cies through hydrogen bonding interactions with PO , spectroscopy (Additional file  1: Fig. S3 g) showed that 2– – HPO, H PO , and H PO . Notably, there is no direct 4 2 4 3 4 MSBL3 after P adsorption included C, N, O, La, and P, Jia  et al. Biochar (2023) 5:16 Page 15 of 17 evidence that MSBL3 can adsorb P species by pore-size La(OH) + H PO ⇀ LaPO + 3H O 3,(s) 3 4,(aq) 4,(s) 2 (8) matching, as this would be difficult to identify in the presence of other driving forces; therefore, this mecha- nism was not considered. Based on the above discussion, the concept map (Fig.  9) reflects the mechanism of P 4 Conclusions adsorption by MSBL3 more intuitively. Currently, the problem of eutrophication in water bod- ies is serious, and finding suitable adsorbents is the key to 3+ − La + OH → La(OH ) , pKsp = 18.7 3,(s) (3) (aq) (aq) applying adsorption methods to avoid eutrophication. In this study, La-based biochar showed good characteristics 3+ 3− for the adsorption of P, such as high efficiency, stability, La + PO → LaPO , pKsp = 22.43 4,(s0 (4) (aq) 4(aq) economy, and safety. Among these adsorbents, MSBL3 exhibited the most significant results. The continuous 3− fixed-bed column studied the inhibition of eutrophica - La(OH) + PO ⇀ LaPO + 3OH 3,(s) 4,(s) (5) 4(aq) tion, and amplification experiments demonstrated the advantages of MSBL3 in practical engineering applica- 2− La(OH) + HPO ⇀ La(OH)(HPO ) 3,(s) 4 tions. It also has good inhibitory properties against E. coli 4(aq) (6) and S. aureus and can therefore be used to improve water − − + 2OH ⇀ LaPO + H O + 2OH 4,(s) 2 (aq) (aq) quality. In addition, when MSBL3 reached a fatigued state after P adsorption, it was mixed with a common La(OH) + H PO ⇀ La(OH) (H PO ) soil sample to study its effect on the germination and 3,(s) 2 2 2 4 4(aq) growth of maize seeds. The results showed that it could (7) − − + OH ⇀ LaPO + 2H O + OH 4,(s) 2 (aq) (aq) be used not only as an effective P absorber but also as an Fig. 9 Adsorption mechanism Jia et al. Biochar (2023) 5:16 Page 16 of 17 Author details effective slow-release P fertilizer. On the other hand, the Institute for Ecological Research and Pollution Control of Plateau Lakes, preparation of 1  g of MSBL3 costs about 1 RMB, and it School of Ecology and Environmental Science, Yunnan University, Kun- can reduce the P concentration in 2.6 ton of Laoyu River ming 650504, China. College of Chemistry and Engineering, Yunnan Normal University, Kunming 650092, China. National Engineering Research Center water to below the eutrophication threshold, making this for Marine Aquaculture, Marine Science and Technology College, Zhejiang study economically advantageous compared to some pro- Ocean University, Zhoushan 316004, China. Key Laboratory of Water and Sed- cess-complex P removal technologies. In conclusion, the iment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China. K ey Laborator y La-based biochar prepared in this study is a promising P for City Cluster Environmental Safety and Green Development of the Ministry sorbent for application and is expected to improve the of Education, School of Ecology, Environment and Resources, Guangdong pollution status and the water quality of eutrophic water University of Technology, Guangzhou 510006, China. bodies. Future research will focus on reducing the eco- Received: 6 October 2022 Revised: 2 March 2023 Accepted: 3 March 2023 nomic cost and improving the stability of the adsorbent to achieve large-scale application of this technology. Supplementary Information References The online version contains supplementary material available at https:// doi. Agathokleous E, Kitao M, Calabrese EJ (2018) The rare earth element (REE) org/ 10. 1007/ s42773- 023- 00216-y. lanthanum (La) induces hormesis in plants. 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Environ Sci Technol 54(1):50–66. https:// doi. org/ 10. 1021/ acs. est. 9b055 69 Yang B, Feng Y, Yu Y, He S, Liu H, Xue L, Yang L (2019) Lanthanum ferrite nano- particles modification onto biochar: derivation from four different meth- ods and high performance for phosphate adsorption. Environ Sci Pollut R 26(21):22010–22020. https:// doi. org/ 10. 1007/ s11356- 019- 04553-z http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Biochar Springer Journals

Tri-functional lanthanum-based biochar for efficient phosphorus recovery, bacterial inhibition, and soil fertility enhancement

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10.1007/s42773-023-00216-y
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Abstract

1 Introduction However, current P resources can only meet human Phosphorus (P) is closely related to human produc- needs for a certain number of years (Cordell et  al. tion and is an essential element in agricultural fertiliz- 2011). If excess P in water bodies can be recovered and ers, pesticides, food additives, detergents, and various reused effectively, the P resource crisis can be alleviated chemicals (Zhao et  al. 2020). The widespread use of to some extent (He et al. 2022). P has been accompanied by an increasingly negative Common methods developed to remove P species impact on the environment, such as causing water from aquatic environments include chemical precipita- eutrophication and ecosystem imbalance, which seri- tion (Barca et al. 2012), microbial transformation (Geng ously threatens the health of humans and aquatic ani- et  al. 2018), enhanced biological phosphorus removal mals. Therefore, many regulatory authorities impose (EBPR), and adsorption (Ye et  al. 2017). Chemical pre- strict restrictions on the P content of water. For cipitation can usually reduce the concentration of P −1 example, the United States Environmental Protection in a solution to a range of 0.5−1 mg L (Mayer et  al. Agency (USEPA) requires that the total P concentra- 2013). It requires large quantities of metal ions, dis- −1 tion in water should not be higher than 50  μg L and posal of large quantities of sludge, and the recovery of Yao et  al. (2013) believes that to prevent water pollu- metals is cumbersome, making this process suitable for tion and eutrophication, the total P content should be treating effluents with higher P concentrations (Kumar −1 less than 20 μg L . Excess P in aquatic environments is et  al. 2019). The EBPR process can theoretically reduce −1 harmful, but paradoxically, there is a global P resource effluent  P concentrations to 0.1−0.2 mg L (Liu et  al. crisis. According to statistics, the human demand for P 2019a, b). However, due to factors such as organic fertilizer is tens of millions of tons per year (expected to loading and reactor operating parameters, most EBPR reach 50 million tons by 2023) (Almanassra et al. 2021). Jia  et al. Biochar (2023) 5:16 Page 3 of 17 −1 processes can only reduce P to 0.5−1 mg L in actual 2 Material and methods projects (López-Vázquez et  al. 2008). Microbial con- 2.1 Reagents and formulae version is susceptible to environmental conditions, The reagents used in this study are described in detail in such as the organic carrier on which it is loaded, reac- the Additional file  1, and the relevant formulae used in tion vessel, and other factors (Wu et al. 2020). Adsorp- this study are listed in Additional file 1: Table S1. tion is one of the most widely used P removal process, and it has a wide range of adsorbent materials, a fast 2.2 Adsorbent preparation P adsorption rate, high selectivity, a small amount of The method of preparing biochar with the mesocarp of equipment input, and adsorbent regeneration (Desmidt shaddock (MSB) is described in the Additional file  1. et al. 2015). MSB (3  g) was sonicated and dispersed in 500  mL of Lanthanum (La) is a rare earth element (REE) hav- water to form a suspension, and 1  g of LaCl ·7H O was 3 2 ing strong bonding ability with phosphate (Tang et  al. dissolved in 100  mL of water and added to this suspen- 2019). Among the REEs, La is second only to cerium sion. The suspension was stirred at 1000 rpm for 10 h and and neodymium in abundance in the  earth’s crust ammonia was added dropwise until the pH of the mix- (Tyler 2004) and is widely used to adsorb P from water. ture reached 11. After 12 h of stirring, the solid was col- However, because most P adsorption reactions involve lected by filtration, and the filter cake was washed with monolayer adsorption (Wu et al. 2020), the aggregation water to neutral pH and dried under vacuum at 80  °C of La can lead to a reduction in the adsorption capac- for 6  h. The product was named MSBL1. Other condi - ity. Recently, researchers have chosen to load La onto tions were kept constant, and only 1 g of L aCl ·7H O was 3 2 inorganic carriers to avoid its leaching and aggrega- replaced with 2, 3, and 4 g to obtain MSBL2, MSBL3, and tion, including activated carbon (Liu et  al. 2022), car- MSBL4, respectively. A schematic diagram of the prepa- bon nanotubes (Zhang et  al. 2022), and biochar (Chen ration of the shaddock mesocarp for La-loaded species et  al. 2022; Liu et  al. 2019a, b). Biochar is a carbon is shown in Fig. 1a. After going through the processes of material produced by the thermal processing of bio- peeling, freeze-drying, heat treatment, and La loading in mass under limited oxygen conditions; when biochar is turn, the P absorbing agent was successfully prepared. used as a soil amendment, it can serve to reduce soil density and enhance nutrient management. If the feed- 2.3 Adsorbent characterization stock is biological waste with a disposal fee (Stávková The preparation of the adsorbents at different ratios and and Maroušek 2021), the cost of recycling P does not the mechanism of adsorption of P by MSBL3 were ana- increase, which in turn increases the economic attrac- lyzed using Powder X-ray diffraction (PXRD), Brunauer– tiveness and competitiveness. Emmett–Teller theory (BET), X-ray  photoelectron Shaddocks are a variety of pomelos that are widely spectroscopy (XPS),  scanning electron microscopy grown in Southeast Asian countries. The mesocarp of (SEM),  transmission electron microscopy (TEM), spheri- shaddock refers to the part between the outer skin and cal aberration corrected transmission electron microscope the inner fruit, which is usually discarded as waste owing (ACTEM), and other methods. The instrument models are to the lack of food value. Therefore, it was selected as the described in the Additional file 1 . raw material for biochar in this study. Given the strong binding ability of La to phosphate, it was hypothesized 2.4 Batch adsorption experiments that the adsorption of P could be achieved by loading La P adsorption experiments were carried out in coni- on biochar. Therefore, one of the aims of this study was cal flasks, which were placed on a constant temperature to verify the adsorption effect of P in an aqueous envi - water shaker at 150  rpm. Unless otherwise stated, the −1 ronment by the shaddock mesocarp biochar after load- adsorbent dosage was 0.5 g L , the reaction temperature −1 ing of La and whether it could enhance the water quality was 25 ± 0.5  °C, initial P concentration was 50  mg L , of eutrophic water bodies. In addition, since biochar can and initial pH of the solution was 7. A polytetrafluoro - be used as a soil amendment, another aim was to verify ethylene rod of 0.5  cm diameter and 3  cm length was whether the adsorbent waste after P adsorption could be placed in a conical flask to ensure that the adsorbent did used as an effective slow-release P fertilizer to improve not collect at the bottom of the flask during centripetal soil fertility and plant growth. In conclusion, the material oscillation. During the experiments, samples were col- prepared in this study may be a promising P sorbent for lected at set time intervals and then filtered, after which application, and it is also hoped that the research meth- the concentration of P in the water samples  was deter- ods used in this paper can provide some ideas for schol- mined using ammonium molybdate spectrophotom- ars in related fields. etry and  inductively coupled plasma-optical emission Jia et al. Biochar (2023) 5:16 Page 4 of 17 Fig. 1 a Preparation diagram of adsorbents; b FTIR spectra of MSBL3 and MSBL3 + P; c High-resolution XPS spectra of La 3d; d N adsorption– desorption curve of adsorbents; e XRD pattern of adsorbents; f High-resolution XPS spectra of O1s spectroscopy (ICP-OES), respectively. All experiments River, all located in Kunming, China. The insoluble mate - were performed in triplicate, and the results were rial was filtered before preparing the P solution, and the reported as the mean with standard deviations. P concentration in natural water was determined. On the When exploring the effect of co-existing ions, only other hand, the P concentration was set at three differ - −1 −1 −1 the effect of ions with the same electrical properties as ent gradients (5 mg L , 10 mg L , and 20 mg L ) using 3− + PO was considered, where the cations were all K . The each of the four waters as solvent and the adsorbent per- 2− − 2− − initial concentrations of CO, NO, SO , and Cl formance in removing P was evaluated at neutral pH. 3 3 4 −1 −1 exhibited four different gradients (0  mg L , 20  mg L , Continuous fixed-bed experiments were performed in −1 −1 100 mg L , and 200 mg L ), respectively. For the cyclic an inorganic glass column with a length of 200  mm and experiments, after each adsorption, the collected adsor- outer diameter of 15  mm. Natural lake water samples bent was stirred in a NaOH solution (1 M) at 150 rpm for were prepared as feed solutions with a P content of 2 mg −1 10  h, washed to neutral pH, and dried. This procedure P L , and the flow rate was controlled using a peristal - −1 was repeated five times. tic pump at 20 bed volume (BV) h . The effluent solu - To assess the P sorption performance of the adsorbent tion was sampled at 0.5 h intervals. MSBL3 was desorbed in a natural water environment, water from different sce - in situ using a NaOH solution (1 M) as a resolving agent, narios was used as a solvent for P, including ultrapure, and the adsorption–desorption process was repeated municipal, lake, and river waters. Municipal water was three times. collected from the municipal water supply network, lake The eutrophication inhibition experiment using water from Dianchi Lake, and river water from Laoyu MSBL3 was set up with P-free, P-containing, and Jia  et al. Biochar (2023) 5:16 Page 5 of 17 3 Results and discussion P + MSBL3 groups, in which three different concentra - −1 −1 −1 3.1 Idea of material selection tions of P (20 mg P L , 50 mg P L , and 100 mg P L ) There was a thick fluffy mesocarp between the shad - were set in the P + MSBL3 group. The effectiveness of dock peel and pulp, which was considered useless and MSBL3 on P adsorption and inhibition of eutrophica- discarded together with the shaddock peel. In terms of tion in water bodies was verified by comparing the environmental management, it is expected that cheap growth status of water hyacinth. and readily available biomass can be locally sourced for The inhibitory properties of different MSBL3 concen - conversion to useful biochar. Considering that the mes- trations against E. coli and S. aureus were also inves- ocarp in the shaddock peel and pulp sandwich exhibits tigated. The antibacterial properties of MSBL3 were a fluffy and porous state from a macro perspective, we tested using the flat-plate coating method (Han et  al. used it as a biochar carrier for La species that can effec - 2021; Yu et al. 2019a, b). First, MSBL3 was fully ground, tively remove P. and then the ground samples were wrapped in tin foil and dried for 20  min (120  °C, dry heat sterilization of 3.2 Physicochemical state and morphology of adsorbents materials). E. coli and S. aureus were placed in shake The FTIR spectrum (Fig.  1b) shows that the MSBL con- flasks filled with Luria–Bertani (LB) agar medium for tains a rich variety of functional groups, such as –OH, 18  h (180  rpm, 35  °C). To prepare the inoculum, the –N–H, C–N, and C–O (Wang et  al. 2020). These spe - cultured E. coli and S. aureus solutions were diluted to –7 –8 cies are present in most biochar materials and can sig- 10 and 10 , respectively. 8  mg of sterilized MSBL3 nificantly influence the physicochemical properties was added to 10  mL of LB liquid culture medium to of biochar, for example, by anchoring metallic species obtain a concentration of 0.0001  M MSBL3. Similarly, (metals or their compounds), changing the electrical the concentrations of MSBL3 with 0.001  M, 0.01  M properties of the biochar surface, or acting as hydrogen and 0.1 M were obtained with corresponding sterilized bond acceptors (or donors). In the powder X-ray diffrac - MSBL3. tion (PXRD) spectrum (Fig.  1e), MSBL1, MSBL2, and Maize was chosen as the target plant for the resource- MSBL3 exhibited signal peaks that were consistent with planning experiment after sorbent fatigue. According to the crystalline phase of La(OH) with a crystal group the Chinese soil classification standard (GB/T 17296- of P63/m (JCPDS:36-1481). Thus, a gradual increase in 2009), the type of soil in this study was Lateritic red soil, ammonia input allows for an increase in the pH of the which is widely distributed in the southern half of China 3+ solution, with La forming La(OH) insoluble matter and is a clayey soil with poor fertility. The physicochem - in a stronger alkaline environment and loading onto the ical characteristics of the soils are list in Table  1. Maize MSB biochar. In addition, the PXRD peak weakened with seed is the national trial high-yielding zhengdan 958, increasing amounts of LaCl . This phenomenon may be which is characterized by its salinity and drought toler- related to the size of the La(OH) crystals formed;  the ance. The seeds were soaked in warm water at approxi - higher the LaCl concentration, the smaller the La(OH) mately 55  °C for 8–12  h before planting. The seeds 3 3 particles obtained in the presence of ammonia. The were kept at a cultivation temperature of 23 ± 3  °C, XPS spectra were compared to the PXRD spectra. The soil humidity of 40–60%, air humidity of 40–50%, and chemical valence of La in MSBL (in the case of MSBL3) light at 10 ± 2  h per day. The growth conditions were was + 3. The signals at 834.5 eV and 838.0 eV were attrib - kept consistent and the growth of maize in soil samples uted to L a3d , and the signals at 851.4 eV and 854.9 eV doped and un-doped with MSBL3 was compared to ver- 5/2 were attributed to La3d (Fig.  1c). The satellite peaks ify whether MSBL3 could be recycled directly back into 3/2 of La3d and La3d of MSBL3 were at high binding the soil as an effective slow release P fertilizer after the 5/2 3/2 energies and had strong signals, which may be related to adsorption of P reached the fatigued state. the shake-up process resulting from the charge transfer Additional experimental procedures, statistical analy- between O2p-La4f. In the O binding energy region, the ses, and results are described in the Additional file 1 . Table 1 The soil physicochemical characteristics Colour Structure pH Organic carbon Total P Available P −1 −1 −1 (g kg )(g kg )(mg kg ) Blank control soil Brick red Granular 5.95 2.951 0.674 6.000 Soil + MSBL3 Greyish red Granular 5.89 30.057 7.221 83.250 Jia et al. Biochar (2023) 5:16 Page 6 of 17 XPS-acquired signal peaks can be obtained by coupling the amount of LaCl input increased. This may be due three fitted peaks at 530.1  eV, 530.8  eV, and 531.5  eV, to the introduction of La(OH) , resulting in the filling which correspond to the signals of the O–H, La–O, and and covering of the original pore and base surfaces of C–O–C bonds (Yang et  al. 2019), respectively (Fig.  1f ). the MSB, leading to a continuous decrease in the BET In addition, the effect of different LaCl dosages on the values (Fig. 1d). It is worth noting that the introduction specific surface area of the biochar was derived from the of La(OH) fills the pore channels of the MSB to some nitrogen adsorption and desorption curves of MSBL1, extent and therefore the average pore sizes reported in MSBL2, and MSBL3. MSBL1, MSBL2, and MSBL3 should be derived from The specific surface areas (based on BET analysis) of the statistical results of the interstitial spaces between MSB, MSBL1, MSBL2, and MSBL3 were obtained from the La(OH) species. In the above discussion, the the nitrogen adsorption and desorption curves as 59.5, PXRD signal indicates that the larger the L aCl input, 39.5, 27.8, and 11.8, respectively. Clearly, the BET sur- the smaller the La(OH) particles that are likely to be face area of MSB decreased when loaded with La(OH) , formed and, therefore, more uniformly distributed. and this decreasing trend became more pronounced as In this case, the gaps between the smaller particles Fig. 2 SEM images of MSB (a), MSBL3 (b), MSBL2 (c), and MSBL1 (d); TEM images of MSB (e), MSBL3 (f), MSBL2 (g), and MSBL1 (h); (i) Schematic diagram of adsorbents morphology; EDS element mapping of MSBL3 (j, m), MSBL2 (k, n), and MSBL1 (l, o) Jia  et al. Biochar (2023) 5:16 Page 7 of 17 of La(OH) were tighter; therefore, the pore size was Elementals C, N, O, and La were all distributed in MSBL3 smaller. (Fig.  2j, m), MSBL2 (Fig.  2k, n), and MSBL1 (Fig.  2l, o) Electron microscopy imaging directly presents the based on elemental mapping imaging. The La signal was microscopic morphological features of the MSBL. Fig- more uniformly distributed on MSBL3, whereas it was ure  2a, e shows the SEM and TEM images of the MSB, the least distributed on MSBL1. However, it was difficult respectively. MSB had a slightly distorted two-dimen- to observe any new species in the TEM images of MSBL3, sional lamellar structure with a smooth and delicate base. where the morphology could be resolved. It was observed When loaded with La(OH) , the SEM and TEM images that the particle size of the La species (characterized as of MSBL3 (Fig.  2b, f ) were similar to those of MSB, and La(OH) by PXRD) gradually  decreased as the amount it was difficult to distinguish the formation of new mate -of LaCl input  increased. Thus, the nanorods in MSBL1 rial on the base surface of the biochar with the naked eye. had a larger particle size than those in MSBL2, whereas The difference is that, although the morphologies of both the La(OH) particles in MSBL3 may be uniformly dis- MSBL2 (Fig.  2c, g) and MSBL1 (Fig.  2d, h) were two- tributed to form a homogeneous film on the MSB surface dimensional lamellar structures, the lamellar base surface (Fig. 2i). To test this hypothesis, we used a more accurate became rough, and the roughness of MSBL1 was more ACTEM to observe the microscopic morphology of the pronounced than that of MSBL2. In the TEM images, a MSBL3. large number of nanorods were attached to the MSB sur- When the scale bar reached 200  nm and 20  nm, the face in MSBL2 and MSBL1, the length of the nanorods surface of the flake carrier of MSBL3 remained smooth in MSBL2 was approximately 100 nm and that in MSBL1 (Fig. 3a, b). However, when the field of view was enlarged was approximately 400 nm. Because the MSB surface was to a scale bar of 10  nm, various new species with good smooth and without any rods, the nanorods on the MSB crystallinity in the size (planar size) range of 2−10  nm substrate in MSBL2 and MSBL1 were La(OH) species. were seen distributed on the MSB surface, which were Fig. 3 AC-HRTEM with HAADF mode (a, b, d, e) of MSBL3; (c) crystal lattice model and lattice spacing of La(OH) on MSBL3 3 Jia et al. Biochar (2023) 5:16 Page 8 of 17 closely connected to each other with almost no obvious In general, La(OH) exhibits good adsorption of P spe- fault lines (Fig. 3d). The lattice spacing of these new spe - cies (e.g. phosphate) in water. The loading of La(OH) cies was close to 0.326  nm, which is consistent with the in MSBL1, MSBL2, and MSBL3 gradually increased, so lattice spacing of the 110 crystal face of La(OH) . There - that their P adsorption capacity gradually increased, in fore, the new species evenly distributed on the MSB line with theoretical predictions. However, compared to was La(OH) .  Using the high-angle annular dark field MSBL3, MSBL4 did not significantly increase the adsorp - (HAADF) image mode, it can be inferred that the bright tion capacity of P. Therefore, considering the effect of  P spots of atomic size in the lattice stripe in Fig.  3e are La adsorption and economic benefits, MSBL3 was the only atoms because of the stronger scattering of electrons by experimental group used for subsequent batch experi- metal atoms with larger atomic numbers. It can be found ments. In view of the complexity of the water column, the that the distance between adjacent bright spots on the effect of pH on P adsorption by MSBL3 was investigated, lattice line is about 4.25  Å, which is consistent with the and the adsorption capacity of P varied with the pH of La-La distance in the La(OH) crystal cell (Fig.  3c). This the solution (Fig. 4b). The maximum P adsorption capac - −1 evidence effectively verifies the above reasonable hypoth - ity of MSBL3 was 78.5 mg  g when the pH was approx- esis that the particle size of La(OH) on MSB is small imately 3. When the pH was below 3, the P adsorption in the order of MSBL1, MSBL2, and MSBL3, and that capacity of MSBL3 decreased sharply, which may be its distribution on MSBL3 exhibits a high dispersion of related to the dissociation of La(OH) in a strongly small particle sizes. acidic environment. When the pH was higher than 3, the adsorption capacity of MSBL3 for P slowly decreased, 3.3 P erformance study of P removal and this trend became more pronounced when the pH In detail, the ability of La(OH) loaded onto MSBs to was higher than 7 (Yin et al. 2022). The zeta potential dis - adsorb P species from water was evaluated. The pH of tribution (Fig.  4e) of MSBL3 shows that its surface elec- natural water is close to neutral; therefore, the ability trostatic potential was positive when the pH was below of MSBL1, MSBL2, MSBL3, and MSBL4 to adsorb P in 10. When the pH increased from 10 to 11, the MSBL3 −1 P solutions at pH7 (P concentration of 50  mg L ) was surface electrostatic potential reversed from positive to first evaluated. When the adsorption reached equilib - negative (Jia et  al. 2022). It can be assumed that a posi- rium, the P adsorption capacities of MSBL1, MSBL2, tive electrostatic potential indicates more H enrichment −1 −1 + MSBL3, and MSBL4 were 27.4 mg P g , 41.8 mg P g , or the presence of more protonated groups (e.g. NH ) −1 −1 60.5  mg P g , and 62.1  mg P g , respectively (Fig.  4a). on the material surface, whereas a negative electrostatic Fig. 4 a Comparison of q ; b Eec ff t of initial solution pH on P removal of MSBL3; c Sorption kinetic models; d Eec ff t of different P concentrations on −1 q ; e Zeta potential of MSBL3; f Langmuir model. (dosage of adsorbent: 0.5 g L ; T: 298 K) e Jia  et al. Biochar (2023) 5:16 Page 9 of 17 Langmuir model is more consistent with the  P attach- potential indicates more OH enrichment on the mate- ment behavior of MSBL3 as a single molecular layer. rial surface (Wang et  al. 2021a, b). As the pH gradually increased from 3, the OH content of the solution gradu- 3.4 Universality of practical application of adsorbent ally increased, which weakened the H or protonated 3.4.1 P racticality verification of MSBL3 groups on the material surface. This weakened the Cou - 3− Before the practical application of the adsorbent, it is lombic interaction of the negatively charged P O (or 2− − necessary to consider its effects, such as the type of actual HPO and H PO ) with the material surface, thus 4 2 4 water and co-existing ions. Water samples were collected reducing the adsorption capacity of MSBL3 for P (Wang from nearby waters and tested for P concentrations, with et al. 2021a, b). −1 −1 P concentrations of 3.0 μg L and 24.0 μg L in samples Based on the adsorption capacity–time curve (Fig. 4d), collected from Dianchi Lake and Laoyu River, respec- the adsorption behavior of P on MSBL3 can be explained tively (Fig.  5a). After adsorption by MSBL3, P was not using pseudo-first-order and pseudo-second-order detected in either water sample; therefore, the removal kinetic models (Fig.  4c), that is, physical adsorption and efficiency was 100%. Ultra-pure water, municipal water, chemical adsorption, respectively. The regression  coef - Dianchi Lake water, and Laoyu River water samples were ficients (R ) fitted by the pseudo-first-order model were used as solvents to prepare solutions with concentrations lower than those of the pseudo-second-order model −1 −1 −1 of 5 mg P L , 10 mg P L , and 20 mg P L , respectively. (Additional file  1: Table  S2). Therefore, the adsorption −1 At a concentration of 20  mg P L , the removal effi - of P on MSBL3 was more consistent with the pseudo- ciency of P by MSBL3 reached more than 99.9% for the second-order kinetic model, and was mainly chemisorp- first two water samples, whereas for Dianchi and Luoyu tion. In addition, the attachment state of P to MSBL3 River water samples, it was 94.5% and 94.1%, respectively, was investigated using Langmuir and Freundlich mod- which might be due to the presence of other compet- els. When the Freundlich model was fitted linearly to ing ions in the lake water samples that would inhibit P the isothermal adsorption data, the value of R was only removal to some extent (Fig. 5b). 0.801 (Additional file  1: Fig. S1b), whereas the value of R 2− The presence of co-existing anions such as CO , obtained when the Langmuir model was used to fit the 2− − − SO, NO , and Cl in the P solution did not isothermal adsorption data was 0.999 (Fig. 4f ). u Th s, the 4 3 −1 Fig. 5 a Adsorption of P by MSBL3 in natural water; b Adsorption of P in four water bodies by MSBL3 (Solutions of 5, 10, and 20 mg P L were prepared using the four water samples as diluents); c Eec ff t of coexisting anions on q ; d Adsorption–desorption cycles. (dosage of adsorbent: −1 −1 0.5 g L ; T: 298 K); e P concentration in the effluent at different cycles. (C : 2 mg P L ; dosage of MSBL3: 0.25 g; T: 298 K); f Adsorption effect of −1 magnification experiment ( Volume of P solution: 25 L; dosage of MSBL3: 2.5 g; C : 25 μg L ; T: 298 K; The Y-axis represents the concentration of P in the solution) Jia et al. Biochar (2023) 5:16 Page 10 of 17 significantly affect the performance of MSBL3 for P on the ecological environment. Therefore, it is neces - adsorption (Fig.  5c). The high selectivity of MSBL3 for sary to focus on reducing La leakage from the adsor- 3− 2− − PO (or HPO and H PO ) can be explained by bent in subsequent studies. For example, improving the 4 4 2 4 the fact that the products LaCl , La(NO) and La (SO ) preparation method to firmly fix La in the adsorbent or 3 3 2 4 3 3+ − − 2− formed by L a with Cl, NO , and SO are all water- reducing the loading of La while ensuring the removal 3 4 soluble compounds and, therefore, have a small tendency efficiency of P. to react with MSBL3. When the co-existing ion was 2− CO , the adsorption of P by MSBL3 slightly decreased 3+ 2− because La easily combined with CO to form a 3.4.3 Magnification experiment La (CO ) solid, but the pKsp of L a (CO ) was smaller In practical applications, the separation cost after the 2 3 3 2 3 3 2− than that of La(OH) , making it difficult for CO to reaction is considerably higher if the powdered adsor- 3 3 3+ capture most of the La sites (Yu et al. 2019a, b). bent is sprayed directly in water. Additionally, incom- Reproducibility of the adsorbent is particularly impor- plete separation produces a large amount of sludge. tant for practical applications. Considering the complete Therefore, we conducted a scale-up experiment to conversion of L aCl to La(OH) on MSB, NaOH can be reduce the separation cost of the adsorbent and meet 3 3 used to provide a significant amount of OH to reconsti- the practical application requirements. As shown in tute the La(OH) species after P adsorption by MSBL3. Additional file  1: Fig. S5, filter packages containing After six repeated uses, MSBL3 was able to retain 76.7% MSBL3 were dosed into the P-containing water. Owing of its initial performance in terms of P adsorption capac- to the low mobility of the lake water, the water box was ity (Fig.  5d), which is better than that of most reported shaken slightly at fixed times to simulate water flow. P adsorbents, making MSBL3 economically relevant in Samples were taken at fixed times each day and the practical applications. P content of the solution was measured. As shown in Fig. 5f, after three days, P was not detected in the solu- tion. It can be seen that loading the adsorbent in the 3.4.2 Continuous fixed‑bed column experiment filter pack will prolong the time required for the reac - A fixed-bed adsorption experiment was designed to tion due to uneven dispersion, but will still achieve investigate the potential of the PL adsorbent for applica- the effect of adsorbing P. A filter package containing tion in flowing lake water. Both ends of the tower were MSBL3 can be easily removed after the adsorption is equipped with layers of quartz sand plates to prevent complete, which is more practical and allows for a wide PL loss (Additional file  1: Fig. S4). The water sample col - range of applications. lected from the Laoyu River was used as the feed solu- In terms of economic cost analysis, biomass is abundant tion. To visually verify the adsorption effect of PL in in nature and is environmentally and economically sus- natural water, the initial P concentration in the water −1 tainable (Maroušek and Maroušková 2021). In this study, sample was adjusted to 2 mg P L . −1 biochar was doped with La, which, although classified The breakthrough point for P was set at 20  μg L as a rare earth element, is abundant in the earth’s crust, because exceeding this value could result in lake second only to cerium and neodymium. Because of the eutrophication (Yao et  al. 2013). As shown in Fig.  5e, high requirements for elemental purity in the preliminary the first adsorption experiment reached a breakthrough tests, analytically pure L aCl ·7H O was chosen, which is 3 2 point at a water volume of 330 BV, and q was 29.3 mg −1 −1 slightly more expensive at 132.6 RMB 500  g . The prep - P g . Subsequently, a 1  M NaOH solution was used aration of 1  g of MSBL3 requires 0.7  g of LaCl ·7H O, 3 2 to desorb PL in a cyclic manner until the P concentra- 0.7  g of mesocarp of shaddock, and 25  mL of N H ·H O. 3 2 tion in the desorbed solution became constant, which The cost of the above reagents, in addition to the water took approximately 5.5  h. After desorption, MSBL3 and electricity used throughout the process, is approxi- was rinsed with deionized water until the effluent was mately 1 RMB, and it can reduce the P concentration in neutral, after which the next cycle was initiated. The 2.6 ton of Laoyu River water to below the eutrophication breakthrough point reached 280 BV and 240 BV for threshold. Although this cost is not very high, it is nec- the second and third times, respectively, with q of −1 −1 essary to continue to reduce the cost if large-scale prac- MSBL3 for P being 24.8  mg P g and 21.3  mg P g , tical applications are to be achieved (Durana et  al. 2021; respectively. The third time, q was 72.6% of the initial Maroušek and Trakal 2022). Market research shows that value. In addition, the leakage of La in the first effluent −1 −1 −1 commercially available L aCl sells for only 17 RMB kg , a was 207.7  μg L and 89.5  μg L after the third cycle. difference of approximately 15 times the price of the ana - Although there is no standard limit for La in the Chi- lytically pure grade, hence this is commercially promising. nese surface water environmental quality standard, it is unclear whether La leaching will have a negative impact Jia  et al. Biochar (2023) 5:16 Page 11 of 17 3.4.4 Inhibition of eutrophication in water bodies by MSBL3 there was no significant increase in the number of leaves, In eutrophic water bodies, algae and water hyacinth grow along with the presence of slightly more old leaves. This in large numbers, aggravating the deterioration of the suggests that too high a P concentration would have an water body. If P in a water body can be reduced below inhibitory effect on growth. The growth trends of water the eutrophication threshold, the occurrence or deterio- hyacinth in the MSBL3 and P-free groups were simi- ration of eutrophication in the water body can be effec - lar, and by day 25, the leaves had died in large numbers tively avoided. Therefore, we investigated the inhibitory and new leaf growth was insignificant. This verifies the effect of placing MSBL3 in P-containing water on the effectiveness of MSBL3 for P adsorption and shows that growth of water hyacinth. As shown in Fig.  6, during 25 MSBL3 has an inhibitory effect on the growth of water d of incubation, all water hyacinths in the group contain- hyacinth. It can be effectively used to prevent eutrophica - ing P grew well, with a significant increase in the number tion in water bodies and can also be applied to eutrophic of leaves; and most leaves were bright green. Higher the water to inhibit the excessive growth of water hyacinth or P content, greater the number of new leaves; however, algae, alleviate the level of eutrophication, and maintain −1 when the P concentration was increased to 100  mg L , the ecological balance of water. Fig. 6 The growth trend of water hyacinth at different P levels Jia et al. Biochar (2023) 5:16 Page 12 of 17 3.4.5 Antibacterial test such as positively charged NH , and the cell membrane A number of P-containing water bodies, such as live- surface of bacteria is negatively charged; therefore, it stock wastewater (Wang et  al. 2021a, b), urban sewage can attract more bacteria through electrostatic grav- plant wastewater (Bonetta et  al. 2016), and even water ity, causing a large loss of cytoplasmic components and networks (Holinger et al. 2014), contain large amounts of disrupting their normal physiological function, leading bacteria; therefore, if the adsorbent can act as a bacterial to bacterial death (Wei et  al. 1994). The above results inhibitor in the process of P removal, then this material show that MSBL3 can inhibit the growth of E. coli and S. can achieve both P recovery and improved water qual- aureus in the water column; therefore, the application of ity. Therefore, E. coli and S. aureus, which are commonly MSBL3 to recover P from the water column can simulta- found in water bodies, were chosen as the target species neously improve water quality at the same time. to assess the bacterial inhibition performance of MSBL3. The inhibitory effects of MSBL3 on E. coli and S. aureus As shown in Fig. 7b, c, the growth of E. coli and S. aureus, involve two main mechanisms. The first is direct inhibi - was barely inhibited when MSBL3 was excluded. Com- tion. Studies have shown that La ions have an inhibitory pared to the blank group, when the concentration of effect on microbial growth. When the concentration of −1 −1 MSBL3 was 0.0001 M, the inhibition against E. coli and S. La ions was 1000  μg L and 100  μg L , respectively, it aureus was 87.3% and 75.0%, respectively. When the con- was able to kill 83% and 39% of E. coli in just 1 min (He centration of MSBL3 reached 0.1  M, inhibition against et  al. 2015). In this study, the leaching amount of La in −1 both bacteria reached 98.7% and 85.0%, respectively. MSBL3 was approximately 200  μg L , which would This may be because MSBL3 is rich in functional groups, Fig. 7 a Flow chart of the antibacterial test; b, c Inhibition of E. coli and S. aureus at different concentrations of MSBL3 Jia  et al. Biochar (2023) 5:16 Page 13 of 17 directly inhibit bacterial growth to some extent. The sec - reduce the cost of treating pollutants by adsorption and ond is indirect inhibition: in the growth of bacteria, phos- has the potential to add value to the waste resources of phorus and organic carbon sources serve as nutrients for adsorbent sludge. Therefore, we investigated the viability their growth and reproduction, and if the P source is con- of MSBL3 as a fertilizer for agricultural fields, following trolled, the growth of bacteria is inhibited (Zhang et  al. the adsorption of P species in water. Considering the high 2016). In this study, the significant adsorption of P by dependence of maize on P during growth, the growth of MSBL3 resulted in "P deficiency" and some microorgan - maize seeds in soil samples, with and without MSBL3, isms were unable to perform normal metabolism in this was studied. The other growth conditions were kept P-deficient environment, which led to their death. consistent and maize growth was recorded at the same time each day. As shown in Fig. 8, maize seeds started to 3.5 R esource planning after adsorbent fatigue sprout on the eighth day, and the growth rate of maize Adsorption is a simple and effective strategy for pollut - in the soil samples without MSBL3 was considerably ant treatment, but it often faces problems such as dif- slower than that in the soil samples with MSBL3. After a ficulties with post-treatment of the adsorbent sludge. few days, the maize seedlings in the MSBL3-doped group However, when the adsorbent is used several times, its were tall and thick, whereas those in the non-MSBL3- ability to adsorb pollutants decreases. When it reaches doped group were short and thin. Therefore, it can be its maximum service life, adsorbent sludge causes sec- tentatively determined from the growth status of corn ondary pollution to the environment and its disposal is seedlings that MSBL3 can be used as an effective slow- costly (Sornhiran et  al. 2022). In this study, MSBL3 was release P fertilizer. the biomass material, and when it was enriched with P in However, it has been demonstrated that moderate solution, it formed a complex with compositional proper- amounts of La can also have beneficial effects on plant ties similar to those of grass ash, which has a fertilizing growth, such as promoting photosynthesis and acceler- effect. Therefore, when MSBL3 reaches its final fatigue ating growth (Agathokleous et  al. 2018). Therefore, the state, it can be placed directly on nutrient-poor agricul- availability of P for plant nutrition in this study needs to tural land, instead of separating MSBL3 from P. This will be investigated further. The availability of P and La for Fig. 8 The growth trend of maize seeds in pure soil sample (two columns on the left) and soil sample mixed with MSBL3 + P (two columns on the right) Jia et al. Biochar (2023) 5:16 Page 14 of 17 plant nutrition can be explored in depth based on this which were homogeneously distributed, and a new sig- study. On the other hand, since MSBL3 is an adsorbent nal peak appeared in the FTIR spectrum (Fig.  1b) of waste after phosphorus adsorption, backfilling it directly MSBL3, which can be attributed to the P-O bond. In into the soil means that the disposal costs of this waste XPS (Fig.  1d), the binding energy of the 3d orbital of can be reduced, and therefore, its pricing is slightly nega- La shifted towards a higher binding energy than that tive (Stávková and Maroušek 2021). before the adsorption of P, but the valence remained at + 3. These two phenomena suggest that a new spe - 3.6 Mechanism discussion cies has been doped into MSBL3, which acts directly on 3+ 3.6.1 Differences in the crystallinity of La(OH) on the surface the La site. Further direct evidence was provided by of MSB PXRD, where the PXRD signal was consistent with that In the present study, the particle size of the La(OH) of LaPO (JCPDS:35-0731) after P adsorption by MBSL3 3 4 crystals in MSBL3, MSBL2, and MSBL1 increased in (Additional file  1: Fig. S3 h), and the La(OH) phase on that order, which is related to the amount of L aCl input. the MSB carrier was almost completely replaced by the In general, crystal formation consists of two processes, LaPO phase. 3+ nucleation formation and nucleation growth, the rates of La can form insoluble precipitates La(OH) (Eq.  3) − 3− which are heavily dependent on the reactant concentra-and LaPO (Eq. 4) with OH and PO , respectively, but 4 4 tion. These two processes can be described by Eqs.  1 and LaPO had a greater reaction trend with a pKsp as high 3+ 2, where Q is the supersaturation of La , s is the equilib- as 22.43. Thus, La(OH) undergoes a complex decom- 3+ 3− rium concentration when La is converted to La(OH) , k position reaction with P O to form L aPO (Eq.  4), 3 4 4 is the pre-exponential factor, A is the surface area of the resulting in the removal of P from water. H PO exists 2 4 3+ nucleus, D is the diffusion coefficient of the solute La , in various hydrolysis forms at different pH, such as – 2– 3– and δ is the distance the nucleus grows in a given direc- H PO, H PO, HPO , and PO . When the pH is 3 4 2 4 4 4 3+ – tion. When the input of L aCl is large, Q of L a is large, below 4, it is mainly present as both H PO and H PO ; 3 2 4 3 4 leading to a large v . At this point, the process of La(OH) when the pH is between 4 and 10, it is mainly absent as 1 3 3+ – 2– nucleation dominates and L a is consumed at a concen- H PO and H PO ; and when the pH is greater than 2 4 4 2– 3– tration close to s in a short time, leading to a small v for 10, it is mainly present as HPO and P O . At pH 7, P 2 4 4 – 2– the subsequent nucleation growth process. As a result, species are present in the form of H PO and HPO , 2 4 4 the La(OH) crystal particles in the MSBL were highly but PXRD showed that La(OH)   was converted almost 3 3 dispersed and ultrafine at high LaCl input (Additional entirely to LaPO ; therefore, the reaction equation for the 3 4 file  1: Fig. S2a). As the amount of LaCl input decreases, conversion of La(OH) to L aPO in a complex decompo- 3 3 4 Q gradually decreases, leading to a decrease in v . The sition reaction with La species may follow  Eqs.  5−8. As decrease in the rate of the nucleation process delays the seen from Additional file  1: Fig. S1c, when the initial pH 3+ time required La to reach the s state; thus, v increases, of the solution is greater than 4, the pH of the solution resulting in the continuous growth of La(OH) crystal after P adsorption by MSBL3 undergoes a certain degree nuclei (Additional file  1: Fig. S2b, c). The La(OH) crystal of increase because some or all of the OH in La(OH) is 3 3 size is the largest when the LaCl input is the smallest, released in the solution after anion exchange, thus lead- otherwise, the opposite is true. ing to an increase in pH. The loading of La on MSBL3 was 30.2 wt%, so the theo - (Q − s) retical value of 1 g MSBL3 for P adsorption according to v = k (1) s −1 La(OH) to LaPO was 66.91  mg  g . The experimental 3 4 −1 results showed that MSBL3 had a capacity of 78.5 mg  g (Q − s) (at pH3) for P adsorption. Therefore, anion exchange is v = DA (2) the main P adsorption pathway. Additionally, there were other interactions. The zeta potential of MSBL3 (Addi - tional file  1: Fig. S1d) showed that the electrostatic poten- 3.6.2 Mechanism of P adsorption by MSBL3 tial on the MSBL3 surface was positive when the pH After the adsorption of P by MSBL3, the SEM images was less than 10.6; therefore, the MSBL3 surface could showed that the surface of the distorted lamellar struc- 3– 2– – adsorb P species, such as PO, HPO , and H PO 4 4 2 4 ture of MSBL3 was no longer smooth and was replaced via Coulomb interactions. Furthermore, FTIR showed by a fluffy spongy layer of the attached material (Addi - that MSBL3 is rich in N- or O-containing functional tional file  1: Fig. S3 a, b). The elemental mapping groups; therefore, the MSBL3 surface can adsorb P spe- (Additional file  1: Fig. S3c–f), and energy-dispersive 3– cies through hydrogen bonding interactions with PO , spectroscopy (Additional file  1: Fig. S3 g) showed that 2– – HPO, H PO , and H PO . Notably, there is no direct 4 2 4 3 4 MSBL3 after P adsorption included C, N, O, La, and P, Jia  et al. Biochar (2023) 5:16 Page 15 of 17 evidence that MSBL3 can adsorb P species by pore-size La(OH) + H PO ⇀ LaPO + 3H O 3,(s) 3 4,(aq) 4,(s) 2 (8) matching, as this would be difficult to identify in the presence of other driving forces; therefore, this mecha- nism was not considered. Based on the above discussion, the concept map (Fig.  9) reflects the mechanism of P 4 Conclusions adsorption by MSBL3 more intuitively. Currently, the problem of eutrophication in water bod- ies is serious, and finding suitable adsorbents is the key to 3+ − La + OH → La(OH ) , pKsp = 18.7 3,(s) (3) (aq) (aq) applying adsorption methods to avoid eutrophication. In this study, La-based biochar showed good characteristics 3+ 3− for the adsorption of P, such as high efficiency, stability, La + PO → LaPO , pKsp = 22.43 4,(s0 (4) (aq) 4(aq) economy, and safety. Among these adsorbents, MSBL3 exhibited the most significant results. The continuous 3− fixed-bed column studied the inhibition of eutrophica - La(OH) + PO ⇀ LaPO + 3OH 3,(s) 4,(s) (5) 4(aq) tion, and amplification experiments demonstrated the advantages of MSBL3 in practical engineering applica- 2− La(OH) + HPO ⇀ La(OH)(HPO ) 3,(s) 4 tions. It also has good inhibitory properties against E. coli 4(aq) (6) and S. aureus and can therefore be used to improve water − − + 2OH ⇀ LaPO + H O + 2OH 4,(s) 2 (aq) (aq) quality. In addition, when MSBL3 reached a fatigued state after P adsorption, it was mixed with a common La(OH) + H PO ⇀ La(OH) (H PO ) soil sample to study its effect on the germination and 3,(s) 2 2 2 4 4(aq) growth of maize seeds. The results showed that it could (7) − − + OH ⇀ LaPO + 2H O + OH 4,(s) 2 (aq) (aq) be used not only as an effective P absorber but also as an Fig. 9 Adsorption mechanism Jia et al. Biochar (2023) 5:16 Page 16 of 17 Author details effective slow-release P fertilizer. On the other hand, the Institute for Ecological Research and Pollution Control of Plateau Lakes, preparation of 1  g of MSBL3 costs about 1 RMB, and it School of Ecology and Environmental Science, Yunnan University, Kun- can reduce the P concentration in 2.6 ton of Laoyu River ming 650504, China. College of Chemistry and Engineering, Yunnan Normal University, Kunming 650092, China. National Engineering Research Center water to below the eutrophication threshold, making this for Marine Aquaculture, Marine Science and Technology College, Zhejiang study economically advantageous compared to some pro- Ocean University, Zhoushan 316004, China. Key Laboratory of Water and Sed- cess-complex P removal technologies. In conclusion, the iment Sciences, Ministry of Education, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China. K ey Laborator y La-based biochar prepared in this study is a promising P for City Cluster Environmental Safety and Green Development of the Ministry sorbent for application and is expected to improve the of Education, School of Ecology, Environment and Resources, Guangdong pollution status and the water quality of eutrophic water University of Technology, Guangzhou 510006, China. bodies. Future research will focus on reducing the eco- Received: 6 October 2022 Revised: 2 March 2023 Accepted: 3 March 2023 nomic cost and improving the stability of the adsorbent to achieve large-scale application of this technology. Supplementary Information References The online version contains supplementary material available at https:// doi. Agathokleous E, Kitao M, Calabrese EJ (2018) The rare earth element (REE) org/ 10. 1007/ s42773- 023- 00216-y. lanthanum (La) induces hormesis in plants. 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Journal

BiocharSpringer Journals

Published: Mar 20, 2023

Keywords: Two-dimensional ordered biochar; Lanthanum hydroxide phosphorus adsorbent; Adsorption sludge re-planning; Antibacterial

References