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Front. Environ. Sci. Eng. 2020, 14(5): 76 https://doi.org/10.1007/s11783-020-1255-8 REVIEW ARTICLE Redox reactions of iron and manganese oxides in complex systems Jianzhi Huang, Huichun Zhang (✉) Department of Civil and Environmental Engineering, Case Western Reserve University, Cleveland, OH 44106, USA HIGH LIGHTS GRAPHIC A BSTRA C T � Mechanisms of redox reactions of Fe- and Mn- oxides were discussed. � Oxidative reactions of Mn- and Fe-oxides in complex systems were reviewed. � Reductive reaction of Fe(II)/iron oxides in complex systems was examined. � Future research on examining the redox reactivity in complex systems was suggested. AR TICL E I N F O Article history: ABSTRA CT Received 30 December 2019 Revised 28 February 2020 Conspectus: Redox reactions of Fe- and Mn-oxides play important roles in the fate and Accepted 21 March 2020 transformation of many contaminants in natural environments. Due to experimental and analytical Available online 16 May 2020 challenges associated with complex environments, there has been a limited understanding of the reaction kinetics and mechanisms in actual environmental systems, and most of the studies so far have only focused on simple model systems. To bridge the gap between simple model systems and complex environmental systems, it is necessary to increase the complexity of model systems and examine both Keywords: the involved interaction mechanisms and how the interactions affected contaminant transformation. In Iron oxides this Account, we primarily focused on (1) the oxidative reactivity of Mn- and Fe-oxides and (2) the Manganese oxides reductive reactivity of Fe(II)/iron oxides in complex model systems toward contaminant degradation. Reduction 2+ 2+ 2+ 3+ 2+ The effects of common metal ions such as Mn ,Ca ,Ni ,Cr and Cu , ligands such as small Oxidation anionic ligands and natural organic matter (NOM), and second metal oxides such as Al, Si and Ti Complex systems oxides on the redox reactivity of the systems are brieﬂy summarized. Reaction kinetics and mechanisms © The Author(s) 2020. This article is published with open access at link.springer.com and journal.hep. com.cn 2020 Zhang et al., 2008; Zhang and Weber, 2013) and the global 1 Introduction geochemical cycles of many elements, such as O, N, P and S (Borch et al., 2010; Sunda, 2010). These reactions have Iron and manganese are the ﬁrst and third most abundant been widely accepted as surface reactions, and the change transition metals in the earth’s crust (Martin, 2005). Redox in the oxide surface properties would signiﬁcantly reactions of Fe- and Mn-oxides play essential roles in the inﬂuence the overall reactivity (Anderson and Benjamin, fate and transformation of numerous organic and inorganic 1990b; Meng and Letterman, 1993; Taujale et al., 2016; contaminants in the environment (Klausen et al., 1995; Huang et al., 2019a; Huang et al., 2019b). They might also substantially alter contaminants’ solubility, toxicity, and ✉ Corresponding author bioavailability. Therefore, the redox reactions of Fe- and E-mail: firstname.lastname@example.org Mn-oxides should be considered in many areas such as site remediation and chemical risk assessments. Special Issue—Accounts of Aquatic Chemistry and Technology Fe- and Mn-oxides typically exist in mixtures with other Research (Responsible Editors: Jinyong Liu, Haoran Wei & Yin Wang) 2 Front. Environ. Sci. Eng. 2020, 14(5): 76 IV III þ metal oxides (e.g., Al- and Si-oxides), metal ions (e.g., ð> Mn ,ArXHÞÐðMn ,ArX$Þþ H (2) 2+ Zn ), and/or ligands (e.g., NOM) in aquatic and soil environments. Despite years of investigation on the redox III III ð> Mn ,ArX$ÞÐMn þ ArX$ (3) reactions of Fe- and Mn-oxides, to date, most previous studies have only focused on the roles of single metal III II > 2Mn ÐMnO þ > Mn (4) oxides (Klausen et al., 1995; Charlet et al., 1998; Zhang 2 and Huang, 2005; Huang et al., 2018). However, the surface and bulk properties of metal oxide mixtures behave ArX$Ðproducts (5) largely differently from those of single metal oxides in It is generally believed that either precursor complex natural aquatic environments (Anderson and Benjamin, formation (Eq. (1)) or electron transfer (Eq. (2)) is the rate- 1990b). For instance, metal oxides might undergo hetero- limiting step in the oxidative reactions of MnO . These aggregation and inﬂuence the adsorption capacity through reactions typically occur on the oxide surface; thus, the altering the number of surface sites and surface charges physiochemical properties of MnO , such as Mn(III) (Anderson and Benjamin, 1990a,b; Meng and Letterman, content, oxygen species, and surface area, can dramatically 1993; Huang et al., 2019a) and contaminant redox inﬂuence its oxidative reactivity (Nico and Zasoski, 2000; degradation (Taujale and Zhang, 2012; Huang et al., Simanova and Peña, 2015; Huang et al., 2018; Wang et al., 2019a). Therefore, it is imperative to investigate oxide 2019). Furthermore, the change on the oxide surface mixtures containing second metal oxides to enable more properties will likely affect either the precursor complex accurate prediction of the fate and transformation of formation or the electron transfer and, hence, the overall numerous contaminants in the environment. reactivity (Fig. 1). Natural organic matter (NOM) is a complex mixture of organic materials that vary from one source to another. It is 2.1 Effects of metal ions on the oxidative reactivity ubiquitous in natural environments and has a large impact on the fate and transformation of contaminants (Davis, 2+ 2+ 2+ 3+ 2+ Metal ions, such as Mn ,Ca ,Ni ,Cr and Cu , 1984; Redman et al., 2002). The adsorption of NOM onto could decrease the oxidative reactivity (Klausen et al., Fe- and Mn-oxides can inﬂuence their physicochemical 1997; Zhang and Huang, 2003; Barrett and McBride, properties, e.g., colloidal stability or electrophoretic 2005). (Zhang and Huang, 2003) showed that the mobility (Gu et al., 1994). Thus, studies should be carried inhibitory effect of metal ions deceased in the order: out to examine the interactions between NOM and metal 2+ 2+ 2+ Mn >Zn >Ca . This is because the adsorption of these oxides and to examine how NOM affects the redox three metal ions on MnO decreased in the same order reactivity of Fe- and Mn-oxides. This will enable the (Morgan and Stumm, 1964), inhibiting the reactivity. studied model systems to more accurately resemble actual 2+ Furthermore, the adsorbed Mn can block the reactive Mn environmental systems. (IV) surface sites, also decreasing the reactivity. Besides In this account, we focus on assessing recent develop- 2+ blocking reactive sites, the adsorbed Mn can lower the ments and current understanding in the redox reactions of reduction potential (E) to slow the electron transfer in the Fe- and Mn-oxides in complex systems, which will oxidation of contaminants (Eq. (6)) (Li et al., 2008; Zhang provide a bridge to connect the ﬁndings from simple et al., 2008): model systems with those from natural environmental systems. 2þ RT ½Mn E ¼ E – log (6) nF ½H 2 Oxidative reaction in complex systems where E is the initial redox potential of manganese oxides; R is the universal gas constant (8.314 J/mol/K), T is the Manganese oxides have high reduction potentials and are absolute temperature (Kelvin); n is the number of able to effectively oxidize a wide range of organic electrons in for the reaction; and F is the Faraday constant contaminants, including phenols (Stone, 1987), anilines (96,485 C/mol). (Laha and Luthy, 1990), and other aromatic compounds Besides, some metal ions might even be involved in the 3+ 3+ (Zhang and Huang, 2003; Zhang and Huang, 2005; Zhang redox reaction, such as Cr . The inhibitory effect of Cr 3+ et al., 2008; Huang et al., 2018). Many studies have results from the reaction between Cr and Mn(IV) to reported the surface involved reaction kinetics and consume Mn(IV), as the simplest case shown in Eq. (7) mechanisms for oxidative degradation of aromatic com- (Eary and Rai, 1987). pounds (ArXH) by single Mn oxides, as shown in Eqs. (1)– 3þ 2þ – (5) (Stone, 1987; Zhang et al., 2008; Huang and Zhang, 3MnO þ 2H O þ Cr ↕ ↓3Mn þ 2HCrO (7) 2 2 4 2019b). 2+ 2+ However, Wang et al. discovered that Ca and Mg IV IV promoted the oxidation of fulvic acid by birnessite at the > Mn þ ArXHÐðMn ,ArXHÞ (1) Jianzhi Huang & Huichun Zhang. Redox reactions of iron and manganese oxides in complex systems 3 Fig. 1 MnO surface-mediated oxidation of contaminants in complex systems. early stage, but the mechanism remained unclear (Wang 1996; Lu et al., 2011). et al., 2019). It should be noted that studies have The addition of PP has been demonstrated to inhibit the 3+ determined that different reactive sites, such as Mn(III, Cr oxidation by MnO , because PP can solidly complex IV) in MnO sheets, Mn(III,IV) at particle edges, and Mn with Mn(III) to lower the Mn(III) availability (Nico and (III) in interlayers, have different oxidizing abilities Zasoski, 2000). However, some researchers observed that (Manceau et al., 1997; Yu et al., 2012). Mn(III) has been PP enhanced the oxidation kinetics of bisphenol A and conﬁrmed to be more reactive than Mn(IV) in the oxidation triclosan (Gao et al., 2018; Huang et al., 2018). This is 3+ of bisphenol A (Huang et al., 2018), Cr (Nico and because the abundance of Mn(III) is key in determining the Zasoski, 2000), and ammonia (Anschutz et al., 2005), but oxidative reactivity, and PP can stabilize Mn(III) against less reactive in the oxidation of As(III) (Zhu et al., 2009). disproportionation (Eq. (8)) (Huang et al., 2018). Further Therefore, the interactions between metal ions and various studies are therefore needed to elucidate the above reactive sites on MnO and how they inﬂuence the electron different effects, such as the oxidative reactivity of Mn transfer warrant further research. (III)-PP toward different chemicals. For instance, Mn(III)- PP has been known to be able to oxidize some chemicals, 2.2 Effects of ligands on the oxidative reactivity such as phenols (Jiang et al., 2009; Jiang et al., 2010; Gao et al., 2018), but not others, such as carbamazepine and The effects of anionic ligands on the oxidative reactivity of methyl p-tolyl sulfoxide (Gao et al., 2018). manganese oxides can be ligand speciﬁc. Both inorganic 2MnðIIIÞþ 2H O↕ ↓MnðIIÞþ MnO þ 4H (8) 2 2 (e.g., phosphate, sulfate, nitrate and chloride) and organic ligands (e.g., small ligands such as oxalic acid, citric acid, Humic acid (HA) displayed either promotive or malic acid, and pyrophosphate (PP) and NOM) have been inhibitive effects on the oxidative reactivity of MnO .On revealed to mostly decrease the oxidative reactivity of the one hand, HA exhibited a promotive effect in the manganese oxides (Klausen et al., 1997; Ge and Qu, 2003; degradation of contaminants including methylene blue, Zhang et al., 2008; Zhang et al., 2012). The inhibitory nonylhenol, 4-n-nonylphenol, 4-tert-octylphenol, and 17β- effects can be summarized as: (1) the ligands such as estradiol (Xu et al., 2008; Zhu et al., 2010; Lu and Gan, sulfate and nitrate compete with the chemicals for the 2013). This is because HA can strongly complex with Mn 2+ binding sites on MnO ; and/or (2) the ligands such as (II) to decrease the adsorption of Mn on MnO surfaces oxalic acid and humic acid cause the reductive dissolution and hence eliminate the large inhibitory effect of the 2+ 2+ of MnO to release Mn ions, which is a strong inhibitor, adsorbed Mn (Xu et al., 2008; Zhu et al., 2010). On the as mentioned in section 2.1 (Klausen et al., 1997; Zhang other hand, HA inhibited the oxidative reactivity of MnO 2– and Huang, 2003; Lu et al., 2011). HPO has been 4 to degrade contaminants, such as 4-chloroaniline, 17β- reported to have a higher inhibitory effect than other estradiol and lincosamide, because it blocked the surface – – 2– 2– inorganic ligands (Cl ,NO , and SO ) because HPO reactive sites to prevent the contaminants from being 3 4 4 can ﬁrmly adsorb onto the MnO surface (Yao and Millero, adsorbed and increased the extent of MnO dissolution 2 4 Front. Environ. Sci. Eng. 2020, 14(5): 76 3+ (Klausen et al., 1997; Chen et al., 2010; Sun et al., 2016). tion of (1) soluble Fe released from goethite dissolution HA is a complex compound containing various functional and (2) aggregation of goethite particles to the overall 3+ groups, which are known to exert different effects on redox reactivity. The released Fe had limited inhibitory effect reactions (Vindedahl et al., 2016). Therefore, more because of the poor solubility of iron oxides under a wide mechanistic investigation into the effect of the functional range of pH conditions, while the blocking of the surface groups of HA on the oxidative reactivity of MnO is reactive sites by the heteroaggregation between goethite required to understand these different results. and MnO (observed based on the sedimentation experi- ments and TEM images) was the major contributor to the decrease in the oxidative reactivity. The inhibitory effect 2.3 Effects of second metal oxides on the oxidative further increased as the particle size decreased or the reactivity goethite loading increased. Zhang’s group then increased the complexity of the Metal oxides have been shown to inﬂuence the oxidative model systems and investigated the effect of NOM on the reactivity of MnO , with different metal oxides exhibiting redox reactivity in ternary systems of two types of metal varying effects and mechanisms. These effects can be oxides plus NOM (Zhang et al., 2015; Taujale et al., 2016). summarized as: (1) the interactions between the second To differentiate the effects due to NOM from those due to metal oxides and MnO (e.g., heteroaggregation, surface the second metal oxides, the parameter P was introduced complexation/precipitation) mostly inhibited the MnO (Eq. (9)). A P value of 1 suggests that the NOM does not reactivity; and (2) the second metal oxides might compete have any additional effects on the reactivity, while a higher with MnO in adsorbing the chemical contaminants P value indicates higher oxidative reactivity of MnO and (Taujale and Zhang, 2012). For example, the inhibitory vice versa. effect on the oxidative reactivity of MnO by second metal oxides deceased in the order: Al O >SiO >TiO (Taujale k 2 3 2 2 with_NOM P ¼ 100% (9) and Zhang, 2012). This is because both Al O and SiO 2 3 2 without_NOM can release Al and Si ions into the aqueous phase due to where k is the reaction rate constant of the ternary their dissolution. When examining the relative contribution with_NOM system (metal oxide + MnO + NOM), while k of the released metal ions and metal oxide particles to the without_NOM overall inhibitory effect, it was found that the released Al is the reaction rate constant of the binary system (metal and Si ions had the dominant inhibitory effect (Fig. 2), oxide + MnO ). mainly due to the surface complexation/precipitation of the Using the obtained P values, the authors revealed that ions on the surface of MnO (Taujale and Zhang, 2012). higher concentrations of model NOM (Aldrich humic acid, TiO only decreased the oxidative reactivity of MnO alginate, or pyromellitic acid) in the ternary systems 2 2 when a limited amount of triclosan was present, because a (MnO + goethite + NOM) had larger P values, indicating strong adsorption of triclosan on TiO inhibited the that the NOM promoted the oxidative reactivity of MnO 2 2 precursor complex formation between triclosan and the in the ternary mixtures. This was ascribed to the MnO . observation that NOM enhanced the extent of homoag- Iron oxides have also been reported to inhibit the gregation within the iron oxides, which inhibited the extent oxidative reactivity of MnO (Zhang et al., 2015). The of heteroaggregation between MnO and iron oxides 2 2 authors also attempted to investigate the relative contribu- (Zhang et al., 2015). Fig. 2 Effect of (a) soluble Al ions and Al O and (b) soluble silicate and SiO particle on the oxidation of triclosan by MnO (Taujale 2 3 2 2 and Zhang, 2012). Jianzhi Huang & Huichun Zhang. Redox reactions of iron and manganese oxides in complex systems 5 The authors also studied another ternary system of competes with AH DS for the surface reactive sites on the 2+ MnO + Al O + HA and found that the reactivity of hematite surface; and (2) Fe as a reaction product 2 2 3 MnO + Al O + HA was different from that of MnO + decreases the Gibbs free energy of the reaction (Eq. (12)) 2 2 3 2 Al ions + HA, indicating that Al ions surprisingly did not (Liu et al., 2007). play a dominant role in inﬂuencing the oxidative reactivity 2– þ 2– 2þ AH DS þ Fe O þ 4H ¼ AQDS þ 2Fe þ 3H O 2 2 3 2 in the system of MnO + Al O + HA. This is drastically 2 2 3 (12) different from the binary system of MnO + Al O ,as 2 2 3 mentioned above, because HA inhibited the dissolution of In a recent work, Zhang et al. (2019) attempted to Al O (Zhang et al., 2015). Therefore, the change in the 2 3 elucidate the interactions between goethite and Al O , and 2 3 heteroaggregation pattern between Al O and MnO 2 3 2 to understand how the interactions affected the oxidative (based on sedimentation experiments) upon the addition reactivity of goethite (Zhang et al., 2019). Not surprisingly, of HA was of great importance in affecting the oxidative Al O exhibited a strong inhibitory effect on the oxidative 2 3 reactivity in the ternary systems. Similar results were also degradation of hydroquinone by goethite at pH 3. obtained for the ternary systems of MnO + Al O + 2 2 3 However, unlike the binary system of MnO + Al O 2 2 3 alginate and MnO + Al O + pyromellitic acid (Taujale 2 2 3 (Taujale and Zhang, 2012), the release of Al ions due to et al., 2016), suggesting that this may be a general Al O dissolution did not affect the reactivity of goethite. 2 3 phenomenon in similar ternary mixtures. 3+ Instead, the amount of Fe released from the goethite dissolution at pH 3 exhibited a good linear correlation with 2.4 Oxidative reactivity of iron oxides in complex systems the obtained reaction rate, indicating that the inhibitory 3+ effect was due to the decreasing amount of Fe in the Besides MnO , iron oxides have been used to oxidize a presence of Al O . This difference pointed to the important 2 3 wide range of contaminants, including phenols, aniline, role of solution conditions such as pH and the types of hydroquinones and ﬂuoroquinolone (LaKind and Stone, oxides in affecting the interactions between oxides. It also 1989; Zhang and Huang, 2007). Compared with MnO , serves as a reminder to future researchers that when iron oxides generally have lower oxidizing ability, with the studying complex mixtures, we should always keep the reduction potentials 0.66–0.67 V vs. 1.23 V for MnO complexity in mind and carefully consider all possible (Eqs. (10) and 11) (LaKind and Stone, 1989). reactions and reaction conditions. þ – 2þ 0 α– FeOOH þ 3H þ e ↕ ↓Fe þ 2H O, E ¼ 0:67V (10) 3 Reductive reactions in complex systems þ – 2þ 0 α– Fe O þ 6H þ 2e ↕ ↓2Fe þ 3H O, E ¼ 0:66V 2 3 2 Regarding reductive transformation of contaminants, sur- (11) face associated Fe(II) has been well documented to be a Similar to MnO , the oxidation of iron oxides is also major reductant and can reduce a wide range of considered as surface-related (LaKind and Stone, 1989; contaminants, including U(VI) (Liger et al., 1999), Zhang and Huang, 2007; Zhang et al., 2019). Therefore, technetium (Peretyazhko et al., 2009), nitroaromatic compounds (Klausen et al., 1995), CCl (Amonette et al., the addition of metal ions, ligands and second metal oxides 2000), polyhalogenated methanes (Pecher et al., 2002), can inﬂuence the surface properties of iron oxides and thus 2+ and N-O containing compounds (Li et al., 2019). their reaction kinetics. Sulfate, phosphate and Ca at low Until now, the reasons on how iron oxides signiﬁcantly concentrations ([sulfate] < 0.1 mmol/L, [phosphate] = 2+ 0.001–1 mmol/L, and [Ca ] = 0.01–10 mmol/L) exhibited enhanced the reductive reactivity of Fe(II) are still not well a negligible effect on the oxidative reactivity of goethite, understood. Previous studies proposed that it might result but decreased the oxidative reactivity at high concentra- from the sorbed Fe(II) coordinated with O-donor atoms on tions ([sulfate]>1 mmol/L) (LaKind and Stone, 1989). the surface of iron oxides, similar to hydroxylated Fe(II) Such an inhibitory effect is because these ligands and metal species in aqueous solution (Wehrli et al., 1989). Surface ions might block the surface reactive sites toward the complexation modeling has been employed to examine the III II contaminants. This result is different from a later study on formed sorbed Fe(II) species, and Fe OFe OH was the effect of phosphate on the oxidative degradation of believed to be the dominant reactive species (Charlet anthraquinone-2,6-disulfonate (AH DS) by hematite (Liu et al., 1998; Liger et al., 1999). However, it has been et al., 2007). Here, phosphate was demonstrated to shown that stable sorbed Fe(II) did not exist because of fast signiﬁcantly inhibit the oxidative reactivity of hematite electron transfer between the sorbed Fe(II) and iron oxides due to its strong complexation with hematite (Liu et al., (Williams and Scherer, 2004; Larese-Casanova and 2+ 2007). In addition, Fe has been reported to decrease the Scherer, 2007). Moreover, negligible reduction of nitro- oxidative degradation of anthraquinone-2,6-disulfonate benzene was observed when aqueous Fe(II) was removed 2+ (AH DS) by hematite through two mechanisms: (1) Fe from the solution, indicating that aqueous Fe(II) was also 2 6 Front. Environ. Sci. Eng. 2020, 14(5): 76 involved in the reductive reaction (Williams and Scherer, competitive adsorption between these metal ions and 2+ 2+ 2004). In addition, Yanina and Rosso (2008) demonstrated Fe (Maithreepala and Doong, 2004). However, Cu a critical role of electron transport through the bulk solid in demonstrated a signiﬁcant enhancement in the reductive 2+ 2+ interfacial redox reactivity (Yanina and Rosso, 2008). reactivity, because Cu(I), reduced from Cu by Fe , Therefore, Gorski and Scherer proposed a revised acted as an additional reductant to enhance the reaction rate conceptual model for Fe(II) adsorption onto Fe(III) oxides (Maithreepala and Doong, 2004). In addition, the reductive 2+ based on a semiconductor model (Becker et al., 2001; Park reactivity of Fe /goethite might vary with the amount of 2+ and Dempsey, 2005; Barnes et al., 2009; Gorski and Cu added. The reactivity increased in the presence of 2+ Scherer, 2011). In the model, electrons from the sorbed Fe Cu with its concentration lower than 0.375 mmol/L, but 2+ (II) are transferred to the bulk iron oxide particles and decreased when the Cu concentration was between 0.375 effectively doped the iron oxides (considered as semi- and 1 mmol/L (Tao et al., 2013). The authors believed that conductors) with additional electrons, part of which are the reactivity was related to the density of the sorbed Fe(II) 2+ eventually transferred to reduce the contaminants. on goethite, and the addition of Cu increased it when 2+ Different factors, such as pH, the amount of Fe(II) [Cu ] was lower than 0.375 mmol/L, while decreased 2+ sorbed, and surface sorbed Fe(II) species, have been shown when [Cu ] was higher than 0.375 mmol/L. However, the 2+ to inﬂuence the reactivity (Elsner et al., 2004; Zhang and mechanism on how the addition of Cu inﬂuenced the Weber, 2013; Huang et al., 2019a; Huang et al., 2019b). density of sorbed Fe(II) is not clear yet. Typically, higher amount of sorbed Fe(II) would result in higher reactivity. Despite the large volume of research that 3.2 Effect of ligands on the reductive reactivity focused on simple model systems containing an Fe(III) 2+ mineral and soluble Fe , which fails to represent actual Anionic ligands can also inﬂuence the reductive reactivity 2+ environmental conditions, there has been only a few of Fe /iron oxides. One of the widely studied ligands is 2+ studies investigating the reductive reactivity of Fe /iron NOM, which exhibited either promotive or inhibitive 2+ oxides in complex systems (Fig. 3). effects on the reductive reactivity of Fe /iron oxides (Colón et al., 2008; Zhang and Weber, 2009; Vindedahl 3.1 Effect of metal ions on the reductive reactivity et al., 2016). On the one hand, NOM can enhance the reactivity. (Zhang and Weber, 2009) veriﬁed that the 2+ Metal ions can inﬂuence the reductive reactivity of Fe reduced Suwannee River NOM and reduced juglone 2+ 2+ complexed with iron oxides. For example, Co ,Ni , and enhanced the reductive degradation of 4-cyano-4’-ami- 2+ 2+ Zn exhibited inhibitory effects on the reductive reactivity noazobenzene by Fe /goethite, because they functioned 2+ of Fe treated goethite, which was ascribed to the as electron shuttles and were able to reduce the goethite Fig. 3 Reduction of contaminants by Fe(II)/iron oxides in complex systems. Jianzhi Huang & Huichun Zhang. Redox reactions of iron and manganese oxides in complex systems 7 surface to increase the formation of surface sorbed Fe(II). two major binary Fe(II) species (≡FeOFe and ≡FeO- On the other hand, NOM can decrease the reductive FeOH), one additional outer-spheric ternary species + 2– reactivity due to multiple reasons (Colón et al., 2008; ((≡FeOFe ) L ) formed in the presence of phthalic 2… Vindedahl et al., 2016). First, the reduction rates and acid, which might partly result in the reactivity decrease. In capacity of the system having Suwannee river humic acid addition, the decrease in the amount of reactive Fe(II) (SRHA) added after Fe(II) was equilibrated with goethite monohydroxo surface species (≡FeOFeOH) was another (G/Fe(II)/SRHA) was lower than that having Fe(II) added reason for the lower reactivity. However, the above two after SRHA was equilibrated with goethite (G/SRHA/Fe reasons only accounted for part of the reactivity decrease, (II)). This might be ascribed to (1) the oxidation and/or thus, the authors proposed two additional mechanisms: (1) complexation of the surface-sorbed Fe(II) by SRHA; and/ phthalic acid might have blocked the more reactive sites on or (2) SRHA blocked electron transfer from the surface the goethite surface; and (2) phthalic acid might have sorbed Fe(II) to the chemical probes (Colón et al., 2008). blocked the electron transfer from Fe(II) to the conduction Second, compared with (Colón et al., 2008), NOM in the band of iron oxides and then to the contaminant. study of (Zhang and Weber, 2009) had been chemically In addition, Latta et al. observed that some anions 3– 2– 4– reduced, so the oxidation state of the NOM should be the (PO ,CO , SiO , and HA) did not inhibit electron 4 3 4 2+ main reason for the different effects observed in these two transfer between Fe and goethite (Latta et al., 2012). studies. Third, the characteristics of NOM can also Note that although electron transfer in that study was not inﬂuence the reactivity. Vindedahl et al. discovered that completely inhibited, it might still have slowed down, the increased molecular weight and nitrogen, carbon, and which might have inﬂuenced the reductive transformation aromatic contents in NOM enhanced the reductive of various contaminants. In fact, Jones et al. demonstrated degradation rates, whereas increased carboxyl concentra- that Si and NOM decreased the rate and extent of isotope 2+ tion and oxygen, heteroaliphatic, and aliphatic contents exchange between Fe and iron oxides (Jones et al., decreased the reactivity. In addition, the authors found that 2009). Thus, it is necessary to examine the effect of the amount of Fe(II) sorbed on goethite was not affected by common anionic ligands on the electron transfer between 2+ NOM and the change in the aggregation state of goethite Fe and iron oxides in future studies. was not the reason for the reactivity decrease. Thus, they attributed the inhibitory effect of NOM to (1) changes in 3.3 Effect of second metal oxides on the reductive reactivity surface Fe(II) reactive species and (2) blocking of electron transfer to goethite surface (Vindedahl et al., 2016). Given Besides NOM and small anionic ligands, the effect of the complexity of NOM effects, future research should second metal oxides on the reductive reactivity should also focus on NOM of different types, origins, oxidation states, be considered. Along this line, Huang et al. have etc. investigated the effects of SiO and TiO on the reductive 2 2 2+ To better understand the formation of surface sorbed Fe reactivity of Fe /goethite (Huang et al., 2019a). For SiO , 2+ (II) species on iron oxides in the present of ligands and to it signiﬁcantly lowered the reductive reactivity of Fe / elucidate how these species affect the reductive reactivity goethite. Then, the authors found that the inhibitory effect 2+ of Fe /iron oxides, surface complexation modeling by the Si ions (released from SiO dissolution) was quite (SCM) has been employed to investigate the formation comparable to that by the SiO particles, suggesting that of surface Fe(II) species in the presence of phthalic acid the aqueous Si ions were the main reason for the overall (Huang et al., 2019b). As shown in Fig. 4, besides the inhibitory effect. Soluble Si ions might have competed 2+ Fig. 4 Triple layer modeling results for Fe and phthalic acid (L) adsorption onto goethite in the ternary systems (Huang et al., 2019a). 8 Front. Environ. Sci. Eng. 2020, 14(5): 76 2+ with Fe to decrease the amount of Fe(II) sorbed, speciﬁc times. For example, cryo-TEM has been used to subsequently leading to the reactivity decrease. examine the aggregation state of goethite in the mixture of Unlike SiO ,TiO surprisingly dramatically enhanced goethite and kaolinite (Strehlau et al., 2017). The cryo- 2 2 the reactivity, which was ascribed to interparticle electron TEM image (Fig. 5) surprisingly showed that the goethite transfer, that is, the electrons from Fe(II) went through the was in homoaggregates, independent of the kaolinite conduction band of TiO to that of goethite before reaching loading, even though the surface charges are opposite for the contaminant (Huang et al., 2019a). The reactivity of goethite (positive) and kaolinite (negative). The phenom- 2+ Fe /goethite + TiO system depended on the types of enon indicated that the inhibition of the reductive reactivity TiO , with the reaction rate decreased in the order: of Fe(II)/goethite by kaolinite was not because of rutile>TiO -P25>anatase. This order agreed well with heteroaggregation in the aqueous phase (Strehlau et al., the conduction band energy of TiO , suggesting that the 2017). However, the reasons why there is no heteroag- conduction band energy of semiconductor minerals might gregation between goethite and kaolinite are not clear. have affected the electron transfer. Then, the authors utilized a dialysis bag to prevent the direct contact between goethite and TiO , and found that the promoting effect of TiO disappeared. Based on the above results, interparticle electron transfer was for the ﬁrst time proposed to occur under dark conditions that enhanced the reductive reactivity. However, additional work is needed to further elucidate the interactions and mechanisms of interparticle electron transfer within different oxide mixtures, especially between iron oxides and titanium dioxides, and how such a new mechanism can be applied to develop new site remediation technologies. Besides metal oxides, clay minerals in soils and sediments also coexist with iron oxides (Tombácz et al., Fig. 5 Cryo-TEM images of (a) 0.325 g/L goethite (G) + 0.05 g/L kaolinite (K) suspension and (b) 0.325 g/L G+ 2g/L 2001; Dimirkou et al., 2002). Among them, kaolinite K suspension (Strehlau et al., 2017). (Al Si O (OH) )) has been demonstrated to decrease the 2 2 5 4 reductive reactivity, due to the competitive adsorption of 2+ Fe and the Al/Si ions released from kaolinite dissolution (Strehlau et al., 2017). Al and Si ions can also be 3.4 Reductive reactivity in sediments incorporated in goethite or on its surface, resulting in the reactivity decrease. Metal oxides are an important composition of soil and Note that dynamic light scattering (DLS), sedimentation sediments. Several studies have investigated the reductive reactivity in sediments. Zhang and Weber examined the experiments, and electron microscopy (SEM and TEM) reductive reactivity of 21 natural sediments and attempted have been frequently used to study the size and hetero- to correlate the sediment physicochemical properties, aggregation of particles. DLS can only be used in dilute including BET surface area, cation-exchange capacity, suspensions (< 0.2 g/L) for sub-micron particles (Cwiertny texture, Fe speciation and composition, and organic carbon et al., 2008; Zhang et al., 2015). For more concentrated content, to their reductive reactivity (Zhang and Weber, suspensions and larger particles, sedimentation experi- 2013). Based on the cluster and regression analysis, the ments have been employed to understand the extents of 2+ authors revealed that surface-associated Fe as well as heteroaggregation of metal oxides (Zhang et al., 2015; reduced DOC played a deciding role in the reductive Taujale et al., 2016; Huang et al., 2019a). During such reactivity of the anaerobic sediments. Other researchers experiments, sedimentation of mixed metal oxides was monitored at a certain wavenumber as a function of time have also conducted ﬁeld experiments in a landﬁll leachate with UV-vis spectrophotometry. Here, instead of measur- plume of a sandy aquifer to elucidate the contribution of ing the adsorption of light, the scattering of light was various reductants to the overall reactivity (Rügge et al., actually obtained (Huang et al., 2019a); and the faster the 1998). They found that the surface sorbed Fe(II) on iron sedimentation rate, the larger the formed aggregates in the (hydr)oxides was the dominant reductant, even though suspension. DOM had a high concentration. One likely reason is that In addition, electron microscopy, such as SEM and the DOM in the plume was mostly oxidized, and hence TEM, has been used to observe the heteroaggregation and lacking the reducing ability. Despite the above efforts, the 2+ homoaggregation states of mixed metal oxides. Compared relative contribution of Fe associated reductants and with traditional SEM and TEM that are for dry samples DOM (and possibly other reductants) to the overall only, cryo-TEM/SEM have unique advantages because reactivity of actual sediment systems is still mostly unclear, they are able to provide images of in situ aggregates at which warrants further research. Jianzhi Huang & Huichun Zhang. Redox reactions of iron and manganese oxides in complex systems 9 site remediation technologies and risk assessments. 4 Future outlook Finally, as mentioned in Introduction, numerous con- taminants can undergo redox reactions, which can The knowledge of the redox reactions of Fe- and Mn- inﬂuence their toxicity, solubility and bioavailability in oxides in different systems is important to understand the environment. Many studies on complex systems only biogeochemical processes. Future research focusing on focused on the abatement of parent compounds without examining their reactivity in complex systems is necessary, considering the intermediates or ﬁnal products, which as brieﬂy summarized below: requires further research. First, the redox reactions of Fe- and Mn-oxides in complex systems are remarkably different from simple model system. To better simulate actual environmental Acknowledgements This material is based upon work supported by the systems, it is necessary to increase the complexity of National Science Foundation under Grants CBET-1762691 and CHE- model systems sequentially (from binary systems to 1808406 to H. Zhang. The authors are thankful to Dr. Zheng Li at University of Washington for the assistance for TOC drawing. ternary systems, then to more complex systems) to study the redox reactions of organic contaminants with various Open Access This article is licensed under a Creative Commons functional groups and of different inorganic contaminants, Attribution 4.0 International License, which permits use, sharing, adaptation, which will be important for conducting site-speciﬁc distribution and reproduction in any medium or format, as long as you give chemical exposure assessments. appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images Second, most of previous studies only focused on d- or other third party material in this article are included in the article’s Creative MnO ; this is especially the case in complex systems Commons licence, unless indicated otherwise in a credit line to the material. (Taujale and Zhang, 2012; Zhang et al., 2015; Taujale If material is not included in the article’s Creative Commons licence and your et al., 2016). However, there are different phase structures intended use is not permitted by statutory regulation or exceeds the permitted of MnO , such as α-, β-, g-, d-, and l-MnO , and they 2 2 use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. demonstrated various catalytic and direct oxidative reactivities due to different surface and structural proper- ties (Meng et al., 2014; Huang et al., 2018; Huang and Zhang, 2019a). Therefore, it will be interesting and useful References to investigate the effects of metal ions, ligands and second metal oxides on the oxidative reactivity of these MnO of Amonette J E, Workman D J, Kennedy D W, Fruchter J S, Gorby Y A different phase structures. (2000). 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Her Zhang H, Rasamani K D, Zhong S, Taujale S, Baratta L R, Yang Z research focuses on the fate and transfor- (2019). Dissolution, adsorption, and redox reaction in ternary mation of contaminants in natural and mixtures of goethite, aluminum oxides, and hydroquinone. Journal engineered environments and water/was- of Physical Chemistry C, 123(7): 4371–4379 tewater treatment. Zhang H, Taujale S, Huang J, Lee G J (2015). Effects of NOM on oxidative reactivity of manganese dioxide in binary oxide mixtures Dr. Jianzhi Huang is a postdoc at the with goethite or hematite. Langmuir, 31(9): 2790–2799 University of Washington studying the Zhang H, Weber E J (2009). Elucidating the role of electron shuttles in properties of interfacial water at hydro- reductive transformations in anaerobic sediments. Environmental philic surface. He received Ph.D. in 2019 Science & Technology, 43(4): 1042–1048 from Case Western Reserve University Zhang H, Weber E J (2013). Identifying indicators of reactivity for under the guidance of Prof. Huichun chemical reductants in sediments. Environmental Science & (Judy) Zhang. His research interests Technology, 47(13): 6959–6968 focus on environmental interfacial chem- Zhang Y, Yang Y, Zhang Y, Zhang T, Ye M (2012). Heterogeneous istry and its applications. oxidation of naproxen in the presence of α-MnO nanostructures with
Frontiers of Environmental Science & Engineering – Springer Journals
Published: Oct 1, 2020
Keywords: Iron oxides; Manganese oxides; Reduction; Oxidation; Complex systems; Reaction kinetics and mechanisms
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