Cr(OH)3(s) oxidation induced by surface catalyzed Mn(II) oxidation

Oxidative dissolution of Cr(OH)3 and mixed Fe-Cr(III) phases by aqueous Mn(III)-pyrophosphate complex

The key role of manganese (Mn) in the biogeochemical cycle of trace elements has been of great interest in recent years. Nevertheless, the redox properties of aqueous Mn(III) have been studied to a lesser extent. Mn(III) is not stable in solution by itself. However, when complexed with inorganic ligands, it has shown potential to oxidize and reduce trace elements. In the present study, we are exploring the redox characteristics of the complex Mn(III)-Pyrophosphate (Mn(III)-PP). This complex is stable over a wide range of pH values but requires the ratio of Mn:PP to be less than 1:6. Specifically, the redox reaction of chromium (Cr(III)) and Mn(III)-PP is investigated. A solid, Cr(OH)3, is used as a source of Cr(III).  For this reaction, environmentally relevant parameters, such as pH, ionic strength, ratio Mn(III)/Cr(III), and excess of ligand, were assessed. Results showed that Mn(III) can effectively oxidize Cr(III) to Cr(VI), taking about 15 days for the reaction to complete. This reaction occurs only under acidic conditions (pH 4), and with a low excess of Pyrophosphate. The initial Mn(III) concentration decreases as the Cr(VI) is produced, and Cr(VI) can be adsorbed back into the Cr(OH)3 surface, limiting the mobility of this toxic species. Despite this adsorption, significant amounts of Cr(VI) are release in the aqueous phase. This study shows the importance of a mobile species (Mn-PP complex) in the oxidation of Cr(III) and the release of Cr(VI) to the environment.

Is crystalline chromium phosphate environmentally stable? A study on the formation, dissolution and oxidation risk of CrPO4·6H2O

Hexavalent chromium (Cr(VI)) contamination in soil and groundwater is usually remediated via reduction techniques. The formation of crystalline chromium phosphate (CrPO4·6 H2O) occurs as a byproduct during Cr(VI) remediation processes in the presence of phosphate, yet its stability in the environment has received limited attention. In this study, the formation conditions, structure, properties, and risks associated with the dissolution and oxidation of CrPO4·6 H2O were comprehensively assessed. Results showed that crystalline CrPO4·6 H2O was formed under pH 5 - 7 at room temperature. CrPO4·6 H2O exhibits higher dissolution risk compared to Cr(OH)3·3 H2O due to a long Cr-P bond (4.2 Å). H+ and OH- increased the risk of dissolution at pH 5 and 11, respectively, owing to the formation of CrH2PO42+ and Cr(OH)4-. In addition, under faintly acidic conditions, the high solubility of CrPO4·6 H2O increases the risk of oxidation; under neutral and weakly alkaline conditions, the presence of positively charged Cr(H2O)63+ structures on the surface elevates its susceptibility to contact and oxidation by δ-MnO2 compared to Cr(OH)3·3 H2O. Specifically, at pH 11, the conversion of CrPO4·6 H2O to Cr(OH)3·3 H2O results in similar oxidation risks for both Cr(III) precipitates.

Interaction between chromite and Mn(II/IV) under anoxic, oxic and anoxic-oxic conditions: Dissolution, oxidation and pH dependence

Chromite oxidative dissolution has been recognized as an important process leading to elevated Cr(VI) in soil and groundwater. Under natural conditions, direct oxidation of Cr(III) by O2 is very unfavorable, and a critical determinant of Cr(VI) generation in soil and groundwater is the interaction between chromite and Mn(II) or Mn(III/IV) oxides. Here, the effects of Mn(II) or Mn(IV) on the oxidative dissolution of chromite were investigated at pH values of 5, 7 and 9 during anoxic, oxic and anoxic-oxic processes. The results showed that the direct oxidation of Cr(III) by O2 was slow in aqueous-phase system, while the Mn oxides in chromite could oxidize dissolved Cr(III). The added Mn(II) can be catalytically oxidized to MnOOH on the chromite surface only under alkaline oxidation conditions, and the catalytic efficiency is slow, which has less effect on chromite oxidative dissolution. Compared with the direct oxidation of O2 and catalytic oxidation of Mn(II), the synthesized biogenic Mn oxides drove the oxidative dissolution of chromite to release more Cr(VI) and were the main threat to the long-term stability of chromite in the environment. Overall, both acidic and alkaline environments are favorable to the catalytic oxidation of chromite by O2, Mn(II) and δ-MnO2, while neutral conditions are favorable to the long-term stability of chromite. These above processes may occur in soils and sediments with redox fluctuations (e.g., rice paddies, river floodplains, wetlands, and peatlands), and the presence of Mn(II) and Mn(III/IV) may play an important role in the oxidation and mobilization of Cr(III), leading to elevated Cr(VI) levels in soils and groundwater.

Geoenvironmental properties of a Cr(VI)-contaminated soil treated by alkali-activated GGBS under freeze-thaw cycles: Insights into Cr species transformation and microscopic mechanism

Stabilization/solidification is the most frequently used method for treating soils contaminated by heavy metals; however, degradation of the treatment will occur under freeze-thaw (F-T) cycles. In this paper, a low-carbon emission by-product, ground granulated blast furnace slag (GGBS), was adopted as a binder to treat Cr(VI)-contaminated soil after alkali excitation. Built on the usage scenarios of subgrade materials, the impact of F-T cycles and initial water content on the geoenvironmental properties of the treated soils, including leaching toxicity, unconfined compressive strength (UCS), pH, Eh, and permeability, were discussed. To investigate the mechanisms of the changing properties, this study analyzed the chemical morphology of Cr, the micromorphology of the reaction products, and the pore characteristics. The results demonstrated that negative impact of F-T cycles on treatment effectiveness was low at the optimal water content. After 28 F-T cycles, the Cr(VI) component increased by 6.4 %, and the leached Cr concentration showed a significant increase, especially for specimens with low water content. A new solid phase with mixed valence Mn(III/IV), mainly composed of birnessite and manganite, was observed via microscopic analysis. During the first 3 F-T cycles, the content of hydration gel increased by 0.18 %, and the cumulative pore volume decreased such that the UCS increased by an average of 1.2 MPa. This study demonstrated that a few F-T cycles would result in a secondary alkali-activated GGBS reaction, enhancing the treatment effect. However, additional F-T cycles would create an oxidizing environment under which the initially precipitated Cr(III) would react with manganese oxide, resulting in more Cr(VI) released. The degree of reoxidation was closely related to the initial water content of the solidified soil.

Behavior and Fate of Chromium and Carbon during Fe(II)-Induced Transformation of Ferrihydrite Organominerals

The mobility of chromium (Cr) is controlled by minerals, especially iron (oxyhydr)oxides. The influence of organic carbon (OC) on the mobility and fate of Cr(VI) during Fe(II)-induced transformation of iron (oxyhydr)oxide, however, is still unclear. We investigate how low-weight carboxyl-rich OC influences the transformation of ferrihydrite (Fh) and controls the mobility of Cr(VI/III) in reducing environments and how Cr influences the formation of secondary Fe minerals and the stabilization of OC. With respect to the transformation of Fe minerals, the presence of low-weight carboxyl-rich OC retards the growth of goethite crystals and stabilizes lepidocrocite for a longer time. With respect to the mobility of Cr, low-weight carboxyl-rich OC suppresses the Cr(III)non-extractable associated with Fe minerals, and this suppression is enhanced with increasing carboxyl-richness of OC and decreasing pH. The presence of Cr(III) mitigates the decrease in total C associated with Fe minerals and increases the Cnon-extractable especially for Fh organominerals made with carboxyl-rich OC. Our study sheds new light on the mobility and fate of Cr in reducing environments and suggests that there is a potential synergy between Cr(VI) remediation and OC stabilization.

Sulfite Poses a Risk of Hexavalent Chromium Rebound in Vadose Zone: A Challenge of the Stability of CrxFe1-x(OH)3

Cr(VI) rebound is the primary risk associated with the reduction remediation of Cr(VI)-contaminated soil. The potential impact of sulfites, which can be produced by microbial activities or originate from sulfur-containing remediation agents, on the Cr(VI) rebound in the vadose zone has been overlooked. When sulfites are present, the stability of CrxFe1-x(OH)3 is compromised and significantly inferior to that of Cr(OH)3, as demonstrated in this paper. First, Fe acts as a catalyst for the conversion of adsorbed sulfite to SO4·-, which subsequently triggers the oxidation of Cr(III) and results in the rebound of Cr(VI). The heterogeneous catalysis by Fe on the surface of CrxFe1-x(OH)3 plays a predominant role, contributing to 78% of the actual oxidation of Cr(III) among all employed catalytic processes. The presence of ambient Cl- can exacerbate the rebound effect of Cr(VI) by promoting the generation of HOCl. Furthermore, a portion of released Cr(VI) was reduced to Cr(III) by dissolved sulfite in the presence of dissolved Fe as a catalyst, thereby increasing the dissolution and migration risk associated with CrxFe1-x(OH)3. Hence, the presence of sulfites results in a significant increase in the Cr(VI) rebound and Cr(III) release from CrxFe1-x(OH)3. This challenges the conventional understanding of the stability of CrxFe1-x(OH)3.

Highly efficient removal of Cr(VI) from wastewater using electronegative SA/EGCG@Ti/SA/PVDF sandwich membrane

The use of green, non-toxic raw materials is of great significance to the sustainable development of the environment, among which epigallocatechin gallate (EGCG) is a renewable carbon source from plants. At present, there is a lack of research on the metal-polyphenol nanomaterials their use in water decontamination. In this study, a novel SA/EGCG@Ti/SA/PVDF (SESP) sandwich membrane was prepared to effectively solve the problems of difficult recovery of nanomaterials and the leaching of metal ions. The membrane was made by scraping SA on the surface of the PVDF substrate as the bottom protective layer, depositing EGCG@Ti NPs as the functional layer, then coating SA as the surface isolation layer, and finally cross-linking with anhydrous calcium chloride. Results showed that EGCG@Ti NPs dispersed well on the surface of the SA/PVDF basement membrane. SESP sandwich membrane had good hydrothermal and acid-base stability, and it can be applied to wastewater with multiple co-existing heavy metals (e.g., Cu, Pb, Cd, and Ni). The contact angle and pure water flux of the SESP sandwich membrane with a negatively charged surface were 14.0-15.6° and 171.40 L/m2 h, respectively. The pure water flux of the regenerated membrane after BSA pollution recovered to 98.68 L/m2 h, and the interception efficiency and the interception flux of Cr(VI) were 100 % and 72.92 L/m2 h at 40 min of interception, respectively. Additionally, the removal efficiency of Cr(VI) by SESP sandwich membrane was maintained above 83 % for simulated wastewater and 100 % for actual wastewater after five adsorption-desorption cycles. Cr(VI) and Cr(III) can be removed simultaneously with the negatively charged SESP sandwich membrane. EDS and XPS analysis showed that the removal of Cr(VI) was controlled by the Donnan effect, anion exchange, chelation/complexation, and reduction mechanism. In contrast, Cr(III) was mainly influenced by electrostatic attraction and chelation/complexation mechanisms. In conclusion, the newly prepared sandwich membrane has good application potential in treating Cr(VI) wastewater.

Linking groundwater microbiome and functional ecological clusters to geogenic high hexavalent chromium from deep aquifers in a loess plateau

Geogenic high hexavalent chromium [Cr(Ⅵ)] in groundwater is a global environmental problem. However, the groundwater microbiome and its linkage to geogenic high Cr(Ⅵ) from deep aquifers still need to be elucidated. Here, we evaluated geogenic Cr(Ⅵ), groundwater microbiome with featured functional ecological clusters and their interactive responses in groundwater from a deep aquifer in a loess plateau of Northern Shaanxi, China. We found that the compositions and structures of microbial communities in groundwater from the deep aquifer were significantly different between low Cr(Ⅵ) (LCG, < 50 μg/L) and high Cr(Ⅵ) groundwater (HCG, > 50 μg/L), with higher microbial diversity and richness in HCG (p < 0.05). Functional "specialists" related to Cr biotransformation, including Cr(Ⅵ) reducing bacteria (CRB) Rhodococcus, Nocardioides, Novosphingobium, and Acidovorax and Mn-oxidizing bacteria (MnOB) Sphingobium, and Ralstonia were positively correlated to total Cr and Cr(VI) concentrations in groundwater. Moreover, these CRB and MnOB were dominant in high Cr(VI) groundwater and associated by strong interspecific relation in an ecological cluster (p < 0.05), suggesting their indicator roles for high Cr(Ⅵ) and the contribution of MnOB mediated Cr(III) oxidation to Cr(VI) enrichment. RDA and path analysis further revealed that the geogenic Cr(Ⅵ) directly promoted the key Cr-related functional cluster with the groundwater depth, dissolved oxygen, and total dissolved solids as the cofactors indirectly influencing Cr(Ⅵ) and the functional clusters (p < 0.05). Collectively, our results highlight the significant roles of microbial ecological clusters especially functional "specialists" MnOB and CRB in groundwater Cr(Ⅵ) from deep aquifers in the loess plateau and provide a basis for sustainable management of high Cr(Ⅵ) groundwater.

Enhanced Oxidation of Cr(III)-Fe(III) Hydroxides by Oxygen in Dark and Alkaline Environments: Roles of Fe/Cr Ratio and Siderophore

The current understanding of Cr(III)-Fe(III) hydroxide (Cr1-xFex(OH)3) oxidation in the dark is primarily focused on strong oxidants, yet the role of oxygen has generally been overlooked. Meanwhile, the effects of organic ligands on the Cr(III) oxidation are poorly known. Herein, we determined the kinetics of Cr1-xFex(OH)3 oxidation by oxygen in the dark as a function of pH and Fe/Cr ratio with/without the presence of a representative organic ligand-siderophore. Results showed that the Cr(III) oxidation rate increased linearly with increasing pH and Fe/Cr ratio. Thermodynamic calculations suggested that the enhanced Cr1-xFex(OH)3 oxidation with increasing pH was primarily due to the decreased ΔG value (i.e., the Gibbs free energy change) at higher pH. The decreased redox potentials (Eh) of Cr1-xFex(OH)3 suspensions with increasing Fe/Cr ratio accounted for the enhanced Cr(III) oxidation of iron-rich Cr1-xFex(OH)3. The siderophore greatly accelerated the Cr1-xFex(OH)3 oxidation at alkaline pH by promoting the formation of soluble organically complexed Cr(III), which can be oxidized readily by oxygen via mineral-surface catalyzed oxidation. Overall, this study highlights the specific role of oxygen and its synergistic role with the siderophore in the oxidation of solid Cr1-xFex(OH)3, which should be taken into consideration in assessing the long-term stability of Cr(III)-Fe(III) hydroxides.

The overlooked role of Cr(VI) in micropollutant degradation under solar light irradiation

Hexavalent chromium (Cr(VI)) is ubiquitous in natural environments, whereas its role in the transformation of coexisting contaminants may have been overlooked. In this work, it was reported for the first time that the irradiation of Cr(VI) by solar light (solar light/Cr(VI) system) could effectively degrade various micropollutants with different structures. The removal efficiency of selected micropollutants was increased by 13.3-64.8% by the solar light/Cr(VI) system compared to that by direct solar photolysis. Meanwhile, the oxidation rates were enhanced by 2.2-21.5 folds, while they were negligible by Cr(VI) oxidation alone. Experiments by specific scavengers, probe compounds, fluorescence absorbance, and electron spin resonance analysis demonstrated that hydroxyl radical (•OH) was the major reactive species in the solar light/Cr(VI) system. Further experiments showed that the generation of •OH was closely related to the intermediate Cr(V) generated from Cr(VI) reduction, and Cr(V) could be re-oxidized back to Cr(VI). Increasing solution pH negatively affected model micropollutant (carbamazepine (CBZ)) degradation by the solar light/Cr(VI) system, mainly due to the decreased quantum yield of •OH at higher pH. Coexisting sulfate ions showed negligible effect on CBZ degradation in the solar light/Cr(VI) system, while the presence of bicarbonate, chloride, and humic acid inhibited CBZ degradation to varying degrees, owing to their diverse scavenging effects on •OH. Furthermore, moderate CBZ degradation was also achieved by natural solar light photolysis of Cr(VI). This study demonstrated the pivotal role of Cr(VI) in the transformation of micropollutants under solar irradiation, which advances the understanding of the fate of micropollutants in natural environments.

Induced transformation of antimony trioxide by Mn(II) oxidation and their co-transformed mechanism

Antimony (Sb) is a toxic and carcinogenic element that often enters soil in the form of antimony trioxide (Sb₂O₃) and coexists with manganese (Mn) in weakly alkaline conditions. Mn oxides such as birnessite have been found to promote the oxidative dissolution of Sb₂O₃, but few researches concerned the co-transformations of Sb₂O₃ and Mn(II) in environment. This study investigated the mutual effect of abiotic oxidation of Mn(II) and the coupled oxidative dissolution of Sb₂O₃. The influencing factors, such as Mn(II) concentrations, pH and oxygen were also discussed. Furthermore, their co-transformed mechanism was also explored based on the analysis of Mn(II) oxidation products with or without Sb₂O₃ using XRD, SEM and XPS. The results showed that the oxidative dissolution of Sb₂O₃ was enhanced under higher pH and higher Mn(II) loadings. With a lower Mn(II) concentration such as 0.01 mmol/L Mn(II) at pH 9.0, the improved dissolution of Sb₂O₃ was attributed to the generation of dissolved intermediate Mn(III) species with strong oxidation capacity. However, under higher Mn(II) concentrations, both amorphous Mn(III) oxides and intermediate Mn(III) species were responsible for promoting the oxidative dissolution of Sb₂O₃. Most released Sb (∼72%) was immobilized by Mn oxides and Sb(V) was dominant in the adsorbed and dissolved total Sb. Meanwhile, the presence of Sb₂O₃ not only inhibited the removal of Mn(II) by reducing Mn(III) to Mn(II) but also affected the final products of Mn oxides. For example, amorphous Mn oxides were formed instead of crystalline Mn(III) oxides, such as MnOOH. Furthermore, rhodochrosite (MnCO₃) was formed with the high Mn(II)/Sb₂O₃ ratio_, but without being observed in the low Mn(II)/Sb₂O₃ ratio. The results of study could help provide more understanding about the fate of Sb in the environment and the redox transformation of Mn.

The occurrence of "yellowing" phenomenon and its main driving factors after the remediation of chromium (Cr)-contaminated soils: A literature review

Chromium (Cr) is a highly toxic element, which is widely present in environment due to industrial activities. One of most applicable technique to clean up Cr pollution is chemical reduction. However, the Cr(VI) concentration in soil increases again after remediation, and meanwhile the yellow soil would appear, which is commonly called as "yellowing" phenomenon. To date, the reason behind the phenomenon has been disputed for decades. This study aimed to introduce the possible "yellowing" mechanism and the influencing factors based on the extensive literature review. In this work, the concept of "yellowing" phenomenon was explained, and the most potential reasons include the reoxidation of manganese (Mn) oxides and mass transfer were summarized. Based on the reported finding and results, the large area of "yellowing" is likely to be caused by the re-migration of Cr(VI), since it could not sufficiently contact with the reductant under the effects of the mass transfer. In addition, other driving factors also control the occurrence of "yellowing" phenomenon. This review provides valuable reference for the academic peers participating in the Cr-contaminated sites remediation.

Increasing soil Mn abundance promotes the dissolution and oxidation of Cr(III) and increases the accumulation of Cr in rice grains

Hexavalent chromium (Cr(VI)) is more readily taken up by plants than trivalent chromium (Cr(III)) due to its similar chemical structure to phosphate and sulfate. In paddy soils, Cr(VI) of natural origin are mainly produced from Cr(III) oxidized by O2 and Mn(III/IV) oxides, which are affected by rice radial oxygen loss (ROL) and Mn(II)-oxidizing microorganisms (MOM). However, little is known about the effect of ROL and Mn abundance on rice Cr uptake. Here, we investigated the effects on Cr(VI) generation and the subsequent Cr uptake and accumulation with the involvement of two rice cultivars with distinct ROL capacities by increasing soil Mn abundance. Results showed that Mn(II) addition to the soil led to more Cr(III) being released into the pore water, and the dissolved Cr(III) was oxidized to Cr(VI) by ROL and biogenic Mn(III/IV) oxides. The concentration of Cr(VI) in soil and pore water increased linearly with the addition of Mn(II) doses. Mn(II) addition promoted the root-to-shoot translocation and grain accumulation of Cr derived mainly from newly generated Cr(VI) in the soil. These results emphasize that rice ROL and MOM promote the oxidative dissolution of Cr(III) at a high level of soil Mn, resulting in more Cr accumulation in rice grains and increasing dietary Cr exposure risks.

Solar irradiation accelerates the oxidation of Cr(III) by δ-manganese dioxide

Cr(VI) has been observed to be released from Cr(III)-bearing natural sources or residues when they are found alongside manganese and manganese oxides. However, relevant mechanism studies normally ignore the effect of simulated solar irradiation on this oxidation reaction. Therefore, we investigated the photochemical reaction between Cr(OH)₃ and δ-MnO₂, the common species of chromium and manganese oxide found in the environment. At pH 11, the oxidation of Cr(OH)₃ by δ-MnO₂ was accelerated under simulated solar irradiation, which had an oxidation rate 2.7-fold greater than that in the dark condition. Further investigation revealed that δ-MnO₂, an n-type semiconductor with a 2.7 eV band gap, can be excited by light with wavelengths < 459 nm and produce photogenic electrons and holes. These photo-induced carriers reacted with surrounding molecules to form free radicals and participate the redox reactions. Free-radical quenching experiments indicated that hydroxyl radicals (•OH) are the main oxidants of Cr(III) under simulated solar irradiation. This work provides new mechanistic insight into the oxidation of Cr(III) to Cr(VI), which may help clarifying the environmental fate of Cr and the potential solar-triggered release of Cr(VI).

Insights into the evolution of Cr(VI) species in long-term hexavalent chromium contaminated soil

Compare to the content of Cr(VI), the distribution of specific Cr(VI) species in soil is rarely paid attention to, which may lead to an inaccurate environmental risk assessment of Cr(VI) contaminated soil or inability to meet stringent requirement for soil remediation. Herein, to reveal the primary mechanisms and factors controlling the evolution of Cr(VI) species in soil, the distribution of Cr(VI) and Cr(III) species in soils with different particle sizes and textures was systematically investigated by using a modified sequential extraction procedure and spectroscopy characterizations (e.g., SEM-EDS mapping). The results show that a significant proportion of Cr(VI) can be captured by minerals containing exchangeable calcium ions and metal oxide hydrates in the soil, forming a relatively stable adsorbed Cr(VI). Also, a small fraction of Cr(VI) can precipitate as calcium chromate with free calcium ion which is the most stable Cr(VI) species in the soil. The majority of Cr(VI) discharged into soil tends to be reduced by ferrous ions or minerals containing ferrous ions with a product of Fe(III)-Cr(III) coprecipitate. Therefore, the speciation of Cr in the soil is closely correlated to Fe and Ca. After the equilibrium of adsorption, precipitation, and reduction reactions of Cr(VI), the rest of Cr(VI) retains as the form of its original water-soluble state in soil. The evolution of Cr(VI) species and the content of specific Cr species in soil are mainly determined by the contents of iron, exchangeable calcium ions and metal oxide hydrates, which effect the Cr(VI) reduction, precipitation and adsorption, respectively.

Visible Light Accelerates Cr(III) Release and Oxidation in Cr-Fe Chromite Residues: An Overlooked Risk of Cr(VI) Reoccurrence

The reduced chromite ore processing residue (rCOPR) deposited in environments is susceptible to surrounding factors and causes reoccurrence of Cr(VI). However, the impact of natural sunlight on the stability of rCOPR is still unexplored. Herein, we investigated the dissolution and transformation behaviors of Cr(III)-Fe(III) hydroxide, a typical Cr(III)-containing component in rCOPR, under visible light. At acidic conditions, the release rate of Cr(III) under illumination markedly increased, up to 7 times higher than that in the dark, yet no Cr(VI) was produced. While at basic conditions, only Cr(VI) was obtained by photo-oxidation, with an oxidation rate of ∼7 times higher than that by δ-MnO₂ under dark conditions at pH 10, but no reactive oxygen species was generated. X-ray absorption near-edge structure and density functional theory analyses reveal that coexisting Fe in the solid plays a critical role in the pH-dependent release and transformation of Cr(III), where photogenerated Fe(II) accelerates Cr(III) produced at acidic conditions. Meanwhile, at basic conditions, the production of intermediate Cr(III)-Fe(III) clusters by light leads to the oxidation of Cr(III) into Cr(VI) through the nonradical "metal-to-metal charge transfer" mechanism. Our study provides a new insight into Cr(VI) reoccurrence in rCOPR and helps in predicting its environmental risk in nature.

Dissolution of Mn-bearing dolomite drives elevated Cr(VI) occurrence in a Permian redbed aquifer

Municipalities in central Oklahoma, U.S.A. increasingly rely on water drawn from the Central Oklahoma Aquifer (COA) as surface water resources have not grown in proportion to population and current water demands. However, water drawn from certain regions of the COA frequently contains elevated levels of naturally occurring hexavalent chromium. Rock samples from the Norman Arsenic Test Hole Core (NATHC) were investigated to identify the mineralogic host(s) of Cr and mechanisms of Cr(VI) release via bulk mineralogy and chemistry measurements, selective chemical extractions, and microscale elemental analyses. Results demonstrate most COA Cr is contained in Fe oxides and clays as isomorphic substitutions for Fe(III). Analyses of regional groundwater data, including hierarchical clustering methods and GIS, demonstrate the most intense Cr(VI) occurrence is linked to cation exchange with Na-clays at depth. Cation exchange allows dissolution of Mn-bearing dolomite, which in turn produces Mn oxides in otherwise dolomite-saturated groundwaters. Mn oxides in turn are known to oxidize Cr(III) to Cr(VI). In general, co-occurrence of Mn-bearing carbonates and exchangeable clays in any aquifer, particularly those with Cr(III) present in iron oxide cements, serve as ingredients for groundwater occurrences of oxidizable trace metals.

Molecular mechanisms of Chromium(III) sorption by organo-ferrihydrite coprecipitates induced by crop straws

Agricultural organo-ferrihydrite (Fh) coprecipitates (OFCs), resulting from the coprecipitation of Fe(III) and dissolved organic carbon (DOC) from returned straws, significantly affect the bioavailability of heavy metals in farmland. However, the molecular sorption mechanisms of Cr(III) by the OFCs remain unclear. Here, we explored the sorption behaviors of Cr(III) by the OFCs formed with wheat or maize straws derived-DOC (wheat-DOC or maize-DOC) under various environmental conditions, and further underlying molecular sorption mechanisms using Cr K-edge X-ray absorption near edge structure (XANES) spectroscopy. Results showed that high C loadings reduced the specific surface areas (SSAs) and Cr(III) sorption capacities of the OFCs, implying the blockage of binding sites by C loading. Additionally, although the wheat-DOC induced OFC had a smaller SSA than the maize-DOC induced OFC, their Cr(III) sorption were comparable, which was likely to be compensated by the more carboxyl in the wheat-DOC. Moreover, at a higher ionic strength, the increased or slightly decreased Cr(III) sorption indicated that the inner-sphere sorption was dominant regardless of high or low C loadings, which was also supported by the extremely low Cr(III) extraction percentage. The Cr K-edge XANES spectroscopy suggested that Cr(III) could be immobilized by both the Fh and organic fractions, with the Fh fractions playing a significant role. These findings contribute to a molecular-level mechanistic understanding of Cr(III) sorption by the OFC, which will aid in the prevention and control of Cr-contaminated agricultural soils.

Natural source of Cr(VI) in soil: The anoxic oxidation of Cr(III) by Mn oxides

Cr(VI) from oxidation of geogenic Cr(III) minerals is gradually becoming the primary source of Cr(VI) in soils and groundwater instead of direct emissions. Thermodynamically, natural oxidants of Cr(III) are limited to O₂ and Mn oxides. The oxidation of Cr(III) occurs commonly in oxic soils but the difference in the oxidative dissolution of Cr(III) by Mn oxides in different redox soils (especially under anoxic conditions) is not fully understood and field evidence is lacking. Here, the relationship between Cr(VI) and Mn oxides in basalt-origin soil profiles under three different redox conditions (anoxic, suboxic and oxic) was studied. The oxidative dissolution of chromite was validated by synthesising δ-MnO₂ that was close to biogenic Mn oxides under anoxic and oxic conditions. In anoxic soils, high levels of Cr(VI) were detected in the same horizons as those where Cr(III)-minerals co-existed with Mn(III/IV) oxides, suggesting an exclusive pathway for Cr(VI) generation through oxidation by Mn oxides where there was a deficiency of other oxidants, such as O₂. In oxic soils, the highly abundant Fe oxides combined with Cr(III) to form Cr(III)-Fe(III) oxyhydroxides and Cr(VI) was generated mainly via slow oxidation by O₂. The chromite oxidation experiment results also indicated that a high abundance of Mn oxides could promote chromite oxidative dissolution to generate Cr(VI), even under anoxic conditions. Additionally, the form of Cr and the reactivity and abundance of Mn oxides and reducing agents controlled the net content of Cr(VI) in the soil. This study showed that, even under reducing conditions, Cr(III) is readily oxidised by Mn oxides to generate Cr(VI) in reductant-deficient and Mn-rich soils, which may lead to the continuous introduction of Cr(VI) into groundwater and agricultural soils.

Alteration of birnessite reactivity in dynamic anoxic/oxic environments

Although the oxidative capacity of manganese oxides has been widely investigated, potential changes of the surface reactivity in dynamic anoxic/oxic environments have been often overlooked. In this study, we showed that the reactivity of layer structured manganese oxide (birnessite) was highly sensitive to variable redox conditions within environmentally relevant ranges of pH (4.0 - 8.0), ionic strength (0-100 mM NaCl) and Mn(II)/MnO₂ molar ratio (0-0.58) using ofloxacine (OFL), a typical antibiotic, as a target contaminant. In oxic conditions, OFL removal was enhanced relative to anoxic environments under alkaline conditions. Surface-catalyzed oxidation of Mn(II) enabled the formation of more reactive Mn(III) sites for OFL oxidation. However, an increase in Mn(II)/MnO₂ molar ratio suppressed MnO₂ reactivity, probably because of competitive binding between Mn(II) and OFL and/or modification in MnO₂ surface charge. Monovalent cations (e.g., Na⁺) may compensate the charge deficiency caused by the presence of Mn(III), and affect the aggregation of MnO₂ particles, particularly under oxic conditions. An enhancement in the removal efficiency of OFL was then confirmed in the dynamic two-step anoxic/oxic process, which emulates oscillating redox conditions in environmental settings. These findings call for a thorough examination of the reactivity changes at environmental mineral surfaces (e.g., MnO₂) in natural systems that may be subjected to alternation between anaerobic and oxygenated conditions.

Oxidative dissolution of Sb2O3 mediated by surface Mn redox cycling in oxic aquatic systems

Antimony trioxide (Sb₂O₃) is one of the primary forms of Sb in the environment, and its dissolution significantly impacts the migration and bioavailability of Sb. However, the dissolution of Sb₂O₃ coupled with abiotic redox of Mn processes is unclear. Here, we investigated the kinetics of Sb₂O₃ dissolution in the presence of the ubiquitous Mn(II) by kinetic experiments, spectroscopies, density functional theory calculations and the chemical kinetic modeling. The oxidative dissolution of Sb₂O₃ was catalyzed by Mn(II) through the in-situ generated amorphous Mn oxides (MnO_x) under oxic conditions, during which the generation of Mn(III) is a critical step in Sb(V) release. The released Sb(V) was partially retained on MnO_x through bidentate-binuclear (corner-sharing) complexes as revealed by extended X-ray absorption fine structure analysis. The coexistent morphological forms of Sb₂O₃, i.e., senarmontite and valentinite exhibited distinct dissolution patterns. Valentinite showed higher activity in catalyzing Mn(II) oxidation and faster oxidative dissolution than senarmontite, due to its higher surface energy and lower conduction band minimum of its exposed facets. These abiotic processes can extrapolate to other metal(loid)s (hydr)oxides, further supplying for the comprehensive understanding of the redox transformation of Mn.

Genesis of high hexavalent chromium groundwater in deep aquifers from loess plateau of Northern Shaanxi, China

Hexavalent chromium (Cr(VI)) groundwater usually exists in shallow aquifers related to ultramafic and serpentine formations, but knowledge of the genesis of dissolved Cr(VI) in deep sandstone aquifers is limited. Both groundwater and aquifer sediments were taken from the Jingbian County in the Loess Plateau of Northwestern Shaanxi to investigate distribution and genesis mechanism of high Cr(VI) groundwater in deep sandstone aquifers. Results showed that the Cr concentrations (median 142 μg/L) in groundwater from deep aquifers (>100 m) were relatively high, while the Cr concentrations in shallow groundwater were low (median 33.8 μg/L). Dissolved Cr mainly existed in the species of Cr(VI) (average, 90%). Deep groundwater with higher Cr(VI) concentrations generally had higher pH, Eh, and DO than shallow groundwater, indicating that the high Cr(VI) groundwater existed in relatively oxic environment. Cretaceous sandstones in deep aquifers had anomalously high contents of total Cr (average 115 mg/kg), where Cr was mainly present in silicates-bound form, and secondly in strongly adsorbed form. There were positive correlations between Mn and Cr in the unweathered silicate-bound form and adsorbed form, which were conducive to Cr(III) oxidation into Cr(VI) in an alkaline-oxic environment. The different ionic ratios (i.e. (Ca²⁺ + Mg²⁺)/(HCO₃^- + SO₄^2-)) also supported silicate weathering as the dominant rock-water interactions in the deep groundwater, which enhanced the release of the unweathered silicate-bound Cr. Relatively high pH and ionic strength mobilized the adsorbed Cr(VI) into groundwater. This investigation emphasizes the geological origin of high Cr(VI) groundwater in deep sandstone aquifers containing Mn oxides, which deserves more concerns for the purpose of drinking water supply.

Modified geosynthetic clay liners bentonite for barriers of Cr (VI) in contaminated soil

Hydroxy-Fe-Al and cetyltrimethylammonium bromide (CTMAB) were chosen to modified Na-bentonite (Na-bent). The characteristics of Na-bent and modified bentonites were determined with scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR) and zeta potential. It was found that CTMAB mainly entered the interlayer and hydroxy-Fe-Al groups were mostly loaded on the external surface of the Na-bent, respectively. The efficiency to remove Cr (VI) of Na-bent, organic modified bentonite (O-bent), inorganic modified bentonite (I-bent) and composite modified bentonite (Co-bent) followed the order: Co-bent > I-bent > O-bent > Na-bent. Adsorption experiments were carried out by the batch contact method. The highest removal rate of Cr (VI) by Co-bent was found to be 96.2% at optimal pH = 4. The Cr (VI) uptake on Co-bent from 50 mg/L solution rapidly attained equilibrium within 10 min, and the pesudo-second-order kinetic model could provide satisfactory fitting of the kinetic data (R² = 0.999) compared to the intraparticle diffusion model (R² = 0.585). The adsorption data were applied to the Langmuir, Freundlich, Temkin isotherm model. The Langmuir was found to be the most suitable equation to fit the experimental data (R² = 0.956) with a high Cr (VI) adsorption capacity of 27.472 mg/g, and R_L values (0.012-0.035) also indicated the adsorption could be accepted. The present study confirmed that Co-bent would be one of candidates for Cr (VI) adsorbent.

FeS-mediated mobilization and immobilization of Cr(III) in oxic aquatic systems

Reduction of soluble Cr(VI) into insoluble Cr(III) by iron sulfide (FeS) minerals under anoxic conditions has been widely observed in natural and engineered systems. Yet, information has been lacking on the FeS-mediated oxidation and remobilization potential of Cr(III) under varying environmental conditions. The objective of this study was to investigate FeS-mediated redox transformation of Cr(III) to Cr(VI) and the associated mobilization and immobilization when Cr(III)-FeS systems are exposed to atmospheric conditions. The results showed that FeS nanoparticles facilitated rapid and strong Fenton-like reactions during the early-stage oxygenation of FeS, resulting in rapid production of hydroxyl radicals (•OH). Consequently, Cr(III) was rapidly oxidized into Cr(VI). Yet, as the reactions proceeded, the oxidative potential was counteracted by competitive scavenging of •OH by Fe(II) and S(-II) from FeS and the reduction reactions by these electron donors. At equilibrium, all Cr(VI) was reduced back to Cr(III) at an FeS-Cr(III) molar ratio of 10:1, while a small fraction of Cr(VI) persisted in solid products of Cr(OH)₃(s) at an FeS-Cr(III) molar ratio of 1:1. Acidic conditions favored the generation of Cr(VI) and the equilibrium concentration of Cr(VI) in oxic FeS NPs systems at pH 5.0 was 1.7 times higher than at pH 9.0. Overall, the FeS-induced Fenton-like reactions and the oxidation of Cr(III) were favored in the early stage, but quenched in the later stage and outcompeted by the reduction of Cr(VI) if sufficient FeS was available. The findings provide new insights into the hydrochemical processes that can affect the speciation, toxicity, and mobility of Cr in aquatic systems containing FeS and Cr.

Chromium biogeochemical behaviour in soil-plant systems and remediation strategies: A critical review

Chromium (Cr) is a toxic heavy metal that is heavily discharged into the soil environment due to its widespread use and mining. High Cr levels may pose toxic hazards to plants, animals and humans, and thus have attracted global attention. Recently, much progress has been made in elucidating the mechanisms of Cr uptake, transport and accumulation in soil-plant systems, aiming to reduce the toxicity and ecological risk of Cr in soil; however, these topics have not been critically reviewed and summarised to date. Accordingly, based on available data-especially from the last five years (2017-2021)-this review traces a plausible link among Cr sources, levels, chemical forms, and phytoavailability in soil; Cr accumulation and translocation in plants; and Cr phytotoxicity and detoxification in plants. Additionally, given the toxicity and hazard posed by Cr(VI) in soils and the application of reductant materials to reduce Cr(VI) to Cr(III) for the remediation of Cr(VI)-contaminated soils, the reduction and immobilisation mechanisms by organic and inorganic reductants are summarised. Finally, some priority research challenges concerning the biogeochemical behaviour of Cr in soil-plant systems are highlighted, as well as the environmental impacts resulting from the application of reductive materials and potential research prospects.

Simultaneous removal of Cr and organic matters via coupling Cr-Fenton-like reaction with Cr flocculation: The key role of Cr flocs on coupling effect

Cr contamination is frequently combined with organic pollution. Cr flocculation using flocculants results in difficulty in Cr recovery and increase of salinity. Moreover, the fates of coexisted organic pollutants are rarely attended. In this study, the Fenton-like reaction based on Cr redox reaction (Cr-Fenton-like reaction) coupled with Cr flocculation was established using H₂O₂ as additives, which gave the opportunity to realize simultaneous removal of Cr and organic matters sustainably. In the coupling system, Cr-Fenton-like reaction generated more OH^- for Cr flocculation via the H₂O₂ decomposition, meanwhile, Cr flocculation provided heterogeneous catalytic regions for Cr-Fenton-like reaction. The formation of Cr flocs was the key to the coupling effect. They created partially alkaline regions, therefore Cr-Fenton-like reaction (reaction condition: pH > 5) and Cr flocculation (reaction condition: pH > 8) occurred in these heterogeneous regions, although the pH of the solutions was below 5. Besides, the Cr flocs in the coupling system tended to adsorb Cr(III) which also contributed to the coupling effect. Although Cr-Fenton-like reaction generated Cr(VI) inevitably, the dissolved Cr(VI) didn't accumulate due to the persistent acidic condition and the heterogeneous catalysis of Cr flocs in the coupling system. When the coupling effect was applied to a stimulated tannery wastewater with Cr and organic co-pollution, the simultaneous removal rates for Cr and total organic carbon were 81.2% and 41.34% respectively after an 8-h running. This study gives deep insights into the coupling effect and provides a sustainable and eco-friendly strategy for the remediation of wastewater with Cr and organic co-pollution.

Rhodochrosite Oxidation by Dissolved Oxygen and the Formation of Mn Oxide Product: The Impact of Goethite as a Foreign Solid Substrate

Rhodochrosite conversion to Mn (oxyhydr)oxides significantly affects the fate and transport of various substances in the environment. We examined rhodochrosite oxidation by dissolved oxygen and the oxidation product formation with an emphasis on the effects of goethite substrate. Without goethite, rhodochrosite oxidation was slow as no detectable change was observed for 28 d with microscopic and spectroscopic analyses, except a minor change in X-ray diffraction. Interestingly, by contrast, it was greatly accelerated and completed in 7 d in the presence of goethite, resulting in the heteroepitaxial growth of groutite (α-MnOOH)-like Mn oxides on the goethite (α-FeOOH) tip surfaces. The formation of this secondary Mn oxide likely induced the acidification of the microenvironment in the vicinity of rhodochrosite particles and thereby promoted their dissolution. Subsequently, their oxidative conversion to the Mn oxide was expedited by the surface catalyzed Mn(II) oxidation on the goethite tip. Our results revealed that goethite as a foreign substrate imparts a decisive control on not only the rate but also the type of the reaction product of rhodochrosite oxidation. This study presents a new insight into the geochemical roles of foreign particles on the dynamics of redox-sensitive solid phases in the environment.

Cr(VI) Formation via Oxyhalide-Induced Oxidative Dissolution of Chromium Oxide/Hydroxide in Aqueous and Frozen Solution

The oxidative dissolution of Cr(III) species (Cr₂O₃ and Cr(OH)₃) by oxyhalide species, which produces hexavalent chromium (Cr(VI)), was studied in aqueous and frozen solution. The oxyhalide-induced oxidation of Cr(III) in frozen solution showed a different trend from that in aqueous solution. Cr(VI) production was higher in frozen than aqueous solution with hypochlorous acid (HOCl) and bromate (BrO₃^-) but suppressed in frozen solution with hypobromous acid (HOBr) and periodate (IO₄^-). In particular, bromate markedly enhanced Cr(VI) production in frozen solution, whereas it had a negligible activity in aqueous solution. On the contrary, periodate produced Cr(VI) significantly in aqueous solution but greatly suppressed it in frozen solution. Bromate was found to be much more concentrated in the ice grain boundary than periodate according to both chemical and Raman spectral analyses. The oxidative transformation of Cr(III) to Cr(VI) was accompanied by the concurrent and stoichiometric reduction of oxyhalide species. Dissolved O₂ had little effect on the oxidative dissolution, but dissolved organic matter retarded the oxidation of Cr₂O₃ in both aqueous and frozen conditions. This study proposes that the oxyhalide-induced oxidation of Cr(III) (particularly by bromate) in frozen conditions might have a significant effect on the generation of Cr(VI) in the frozen environment.

Different Pathways for Cr(III) Oxidation: Implications for Cr(VI) Reoccurrence in Reduced Chromite Ore Processing Residue

Hexavalent chromium contamination is a global environmental issue and usually reoccurs in alkaline reduced chromite ore processing residues (rCOPR). The oxidation of Cr(III) solids in rCOPR is one possible cause but as yet little studied. Herein, we investigated the oxidation of Cr(OH)₃, a typical species of Cr(III) in rCOPR, at alkaline pH (9-11) with δ-MnO₂ under oxic/anoxic conditions. Results revealed three pathways for Cr(III) oxidation under oxic conditions: (1) oxidation by oxygen, (2) oxidation by δ-MnO₂, and (3) catalytic oxidation by Mn(II). Oxidations in the latter two were efficient, and oxidation via Pathway 3 was continuous and increased dramatically with increasing pH. XANES data indicated feitknechtite (β-MnOOH) and hausmannite (Mn₃O₄) were the reduction products and catalytic substances. Additionally, a kinetic model was established to describe the relative contributions of each pathway at a specific time. The simulation outcomes showed that Cr(VI) was mainly formed via Pathway 2 (>51%) over a short time frame (10 days), whereas in a longer-term (365 days), Pathway 3 predominated the oxidation (>78%) with an increasing proportion over time. These results suggest Cr(III) solids can be oxidized under alkaline oxic conditions even with a small amount of manganese oxides, providing new perspectives on Cr(VI) reoccurrence in rCOPR and emphasizing the environmental risks of Cr(III) solids in alkaline environments.

High Hexavalent Chromium Concentration in Groundwater from a Deep Aquifer in the Baiyangdian Basin of the North China Plain

Hexavalent chromium (Cr(VI)) is known to occur naturally in shallow oxic groundwater, typically from aquifers associated with mafic and ultramafic formations, but information on the occurrence of Cr(VI) in deep groundwater from large sedimentary basins is limited. This study shows that groundwater from the Baiyangdian Lake Basin (BYB), home to the future second capital city of China, had high Cr concentration (>10 μg/L, up to 86 μg/L) in the deep aquifer (>150 m), while shallow groundwater had lower Cr concentration (<10 μg/L). Chromium occurred predominantly as Cr(VI) (>95%). Shallow groundwater was characterized by higher Mn and Fe concentrations relative to deep groundwater, likely indicating more reducing conditions. Sequential extraction experiments from aquifer sediments suggest that Cr(VI) may derive from silicate weathering and that Mn oxides in the aquifer play a major role in the formation of Cr(VI) in groundwater. Inverse correlations between Mn and Cr(VI) suggest that reductive dissolution of Mn oxides constrains Cr(VI) mobilization in the shallow groundwater, while oxic-suboxic conditions in the deep aquifer limit Mn solubility, which enhances oxidation of Cr(III) to Cr(VI) and promotes desorption of Cr(VI) under alkaline conditions. This study demonstrates the potential geogenic occurrence of high Cr(VI) concentration in deep groundwater from a nonmafic, large sedimentary basin containing Mn oxides in the aquifer sediments.

Catalytic oxidation and adsorption of Cr(III) on iron-manganese nodules under oxic conditions

The speciation, toxicity and mobility of chromium (Cr) are significantly affected by natural iron-manganese nodules due to the adsorption and redox reactions in soils. However, the redox processes in oxic environments have received little attention. In this work, the interaction mechanism between Cr(III) and natural iron-manganese nodules was studied under oxic conditions, and the effects of chemical composition, dissolved oxygen concentration, pH, ionic strength and coexisting ions were further investigated. The results showed that iron-manganese nodules could effectively oxidize dissolved Cr(III), and most of the newly formed Cr(VI) was adsorbed on the surface of nodules. In iron-manganese nodules, manganese oxides mainly contributed to Cr(III) oxidation, and iron oxides facilitated the adsorption and immobilization of Cr(VI). In addition, Cr(III) could be catalytically oxidized to Cr(VI) on the surface of manganese oxides through the generation of Mn(III) intermediate or Mn(IV) oxides from released Mn(II) under oxic conditions. The oxidation rate of Cr(III) by the nodules decreased with increasing pH from 2.0 to 8.0, and increased with increasing ionic strength. This work reveals the adsorption and catalytic oxidation mechanism of Cr(III) by iron-manganese nodules in a simulated open system, and improves the understanding of the geochemical behavior of chromium in soils.

Occurrence, behaviour, and human exposure pathways and health risks of toxic geogenic contaminants in serpentinitic ultramafic geological environments (SUGEs): A medical geology perspective

Serpentinitic ultramafic geological environments (SUGEs) contain toxic geogenic contaminants (TGCs). Yet comprehensive reviews on the medical geology of SUGEs are still lacking. The current paper posits that TGCs occur widely in SUGEs, and pose human health risks. The objectives of the review are to: (1) highlight the nature, occurrence and behaviour of TGCs associated with SUGEs; (2) discuss the human intake pathways and health risks of TGCs; (4) identify the key risk factors predisposing human health to TGCs particularly in Africa; and (5) highlight key knowledge gaps and future research directions. TGCs of human health concern in SUGEs include chrysotile asbestos, toxic metals (Fe, Cr, Ni, Mn, Zn, Co), and rare earth elements. Human intake of TGCs occur via inhalation, and ingestion of contaminated drinking water, wild foods, medicinal plants, animal foods, and geophagic earths. Occupational exposure may occur in the mining, milling, sculpturing, engraving, and carving industries. African populations are particularly at high risk due to: (1) widespread consumption of wild foods, medicinal plants, untreated drinking water, and geophagic earths; (2) weak and poorly enforced environmental, occupational, and public health regulations; and (3) lack of human health surveillance systems. Human health risks of chrysotile include asbestosis, cancers, and mesothelioma. Toxic metals are redox active, thus generate reactive oxygen species causing oxidative stress. Dietary intake of iron and geophagy may increase the iron overload among native Africans who are genetically predisposed to such health risks. Synergistic interactions among TGCs particularly chrysotile and toxic metals may have adverse human health effects. The occurrence of SUGEs, coupled with the several risk factors in Africa, provides a unique and ideal setting for investigating the relationships between TGCs and human health risks. A conceptual framework for human health risk assessment and mitigation, and future research direction are highlighted.

Impact of dissolved O2 on phenol oxidation by δ-MnO2

Although redox reactions of organic contaminants with manganese oxides have been extensively studied, the role of dissolved O2 in these processes has largely been overlooked. In this study, the oxidative degradation of phenol by δ-MnO2 was investigated under both oxic and anoxic conditions. Dissolved O2 inhibited phenol degradation due to its promoting role in the reoxidation and precipitation of reduced Mn(ii) to Mn(iii) on the δ-MnO2 surface, resulting in partial transformation of δ-MnO2 to "c-disordered" H+-birnessite at pH 5.5 and feitknechtite, manganite, and hausmannite at pH 7.0 and 8.5. The reformed Mn(iii) phases could reduce phenol oxidation by blocking reactive sites of δ-MnO2. In addition, dissolved O2 caused a higher degree of particle agglomeration and a more severe specific surface area decrease, and hence lower reactivity of δ-MnO2. These findings revealed that after reductive dissolution by phenol and reoxidation by dissolved O2 throughout continuous redox cycling, δ-MnO2 became less reactive rather than being regenerated. These results can provide new insights into the understanding of the oxidation of organic contaminants by manganese oxides in the natural environment.

Effective removal of Cr(VI) by attapulgite-supported nanoscale zero-valent iron from aqueous solution: Enhanced adsorption and crystallization

The attapulgite supported nanoscale zero-valent iron composite (AT-nZVI) was synthesized and used for Cr(VI) removal. X-ray diffraction (XRD) and transmission electron microscope (TEM) indicated that nZVI particles were well distributed and immobilized on the attapulgite surface. Batch experiments of Cr(VI) removal were conducted at varying mass ratios, initial Cr(VI) concentrations and kinetics. The results indicated that the removal efficiency of Cr(VI) by AT-nZVI approaches 90.6%, being greater than that by non-supported nZVI (62.9%). The removal kinetics could be more accurately explained using pseudo second order kinetics model. The composite exhibited a synergistic interaction instead of simple mixture of AT and nZVI. Reduction was the dominant mechanism at low concentrations as opposed to adsorption at high concentrations. FeCr₂O₄ was the main reduction product by AT-nZVI, which was attributed to the reduction of Cr(VI) by nZVI and co-precipitation of CrFe oxides on the surface of AT. In the meantime, Fe(II) ion contributed to 64% for the Cr(VI) removal, which resulted from the dissolution of nZVI during the removal process. From the analysis of XRD and XPS results, the crystallization of FeCr₂O₄ is believed to be formed easily after the reaction of the AT-nZVI composite with Cr(VI) which is more stable and greatly reduce the risk of secondary pollution compared with nZVI. The introduction of AT enhanced adsorption of Cr(VI) and crystallization of the products. The above results suggested that AT-nZVI could be a promising remediation material for Cr(VI)-contaminated groundwater.

Effective Extraction of Cr(VI) from Hazardous Gypsum Sludge via Controlling the Phase Transformation and Chromium Species

Through controlling the phase transformation and chromium species under hydrothermal condition, the Cr(VI) was extracted fully from hazardous Cr(VI)-containing gypsum sludge, with a very high efficiency of more than 99.5%. Scanning transmission electron microscopy, X-ray absorption fine structure, and density functional theory calculation results revealed that the dissolution-recrystallization of CaSO₄·2H₂O into CaSO₄ was the key factor to fully release the encapsulated Cr(VI). Moreover, the mineralizer (persulfate salt) provided H⁺ and SO₄^2- ions, the former made an acidic condition to transform the released CrO₄^2- into the specie (Cr₂O₇^2-) with less similarity to SO₄^2-, which further prevented the recombination of the released Cr(VI) with gypsum; and the latter was essential to accelerate crystal growth of calcium sulfate so as to enhance Cr(VI) extraction. This work would provide an instructive guidance to fully extract heavy metals from hazardous solid wastes via the control of crystal transformation and the pollutant species.

Determination of Cr(III) solids formed by reduction of Cr(VI) in a contaminated fractured bedrock aquifer: Evidence for natural attenuation of Cr(VI)

Hexavalent chromium Cr(VI) is toxic and can be highly mobile in many aquifer systems. Redox reactions with naturally occurring minerals and organic compounds can reduce Cr(VI) to Cr(III), forming labile Cr(III) oxyhydroxide precipitates, which is a natural attenuation process. In fractured bedrock aquifers, reduction of Cr(VI) in the rock matrix can enhance attenuation beyond that from matrix diffusion only, and potentially reduce back diffusion if concentrations in fractures decline following source reduction via natural processes or engineered remediation. In this study, we develop an extraction method for labile Cr(III) precipitates from Cr(VI) reduction using 5% hydrogen peroxide (H₂O₂). Combining Cr(III) extractions with an established sodium hydroxide (NaOH) method for determination of Cr(VI) concentrations in rock porewater, a measure of the labile Cr(III) and Cr(VI) fractions in geologic samples is achieved. The methods were applied to cores from a contaminated groundwater system in fractured porous bedrock in order to assess the effectiveness of natural attenuation and whether Cr(VI) mass that diffused into the bedrock matrix was undergoing reduction. Detailed vertical distributions display two depth intervals with corresponding elevated concentrations of Cr(VI) in the porewater and extractable total Cr. The correspondence of Cr(VI) and labile Cr(III) provides evidence for reduction of Cr(VI) contamination in the bedrock matrix. Mineralogical analysis suggests that Fe(II)-bearing minerals, chlorite and biotite are the most likely candidates for natural reductants. This study provides evidence for the natural attenuation of anthropogenic Cr(VI) contamination in the porewater of a fractured bedrock aquifer, and it outlines a quantitative method for evaluating the effectiveness of natural attenuation in groundwater systems.

Efficient removal of Cr(III)-organic complexes from water using UV/Fe(III) system: Negligible Cr(VI) accumulation and mechanism

Most available processes are incapable of removing Cr(III)-organic complexes from water due to their high solubility, extremely slow decomplexation rate, and possible formation of more toxic Cr(VI) during oxidation. Herein, we proposed a new combined process, i.e., UV/Fe(III) followed by alkaline precipitation (namely UV/Fe(III)+OH), to achieve highly efficient and environmentally benign removal of Cr(III)-organic complexes from water. The combined process could remove Cr(III)-citrate from 10.4 mg Cr/L to 0.36 mg Cr/L and ∼60% total organic carbon as well. More attractively, negligible Cr(VI) (<0.06 mg/L) was formed during the process. In the viewpoint of mechanism, the added Fe(III) generates ·OH radicals to transform Cr(III) into Cr(VI) and simultaneously released the citrate ligand to form Fe(III)-citrate simultaneously. Then, the photolysis of Fe(III)-citrate under UV irradiation involved the citrate degradation and the production of massive Fe(II) species, which in turn transformed the formed Cr(VI) back to Cr(III). The free metal ions, including Cr(III), Fe(II) and Fe(III) were removed by the subsequent alkaline precipitation. Also, the combined process is applicable to other Cr(III) complexes with EDTA, tartrate, oxalate, acetate. The applicability of the combined process was further demonstrated by treating two real tanning effluents, resulting in the residual Cr(III) below 1.5 mg/L (the discharge standard of China) and negligible formation of Cr(VI) (<0.004 mg/L) as well. In general, the combined process has a great potential for efficient removal of Cr(III) complexes from contaminated waters.

Rates of Cr(VI) Generation from CrxFe1-x(OH)3 Solids upon Reaction with Manganese Oxide

The reaction of manganese oxides with Cr(III)-bearing solids in soils and sediments can lead to the natural production of Cr(VI) in groundwater. Building on previous knowledge of MnO₂ as an oxidant for Cr(III)-containing solids, this study systematically evaluated the rates and mechanisms of the oxidation of Cr(III) in iron oxides by δ-MnO₂. The Fe/Cr ratio (x = 0.055-0.23 in Cr_xFe_1-x(OH)₃) and pH (5-9) greatly influenced the Cr(VI) production rates by controlling the solubility of Cr(III) in iron oxides. We established a quantitative relationship between Cr(VI) production rates and Cr(III) solubility of Cr_xFe_1-x(OH)₃, which can help predict Cr(VI) production rates at different conditions. The adsorption of Cr(VI) and Mn(II) on solids shows a typical pH dependence for anions and cations. A multichamber reactor was used to assess the role of solid-solid contact in Cr_xFe_1-x(OH)₃-MnO₂ interactions, which eliminates the contact of the two solids while still allowing aqueous species transport across a permeable membrane. Cr(VI) production rates were much lower in multichamber than in completely mixed batch experiments, indicating that the redox interaction is accelerated by mixing of the solids. Our results suggest that soluble Cr(III) released from Cr_xFe_1-x(OH)₃ solids to aqueous solution can migrate to MnO₂ surfaces where it is oxidized.

Reoxidation of Chromium(III) Products Formed under Different Biogeochemical Regimes

Hexavalent chromium, Cr(VI), is a widespread and toxic groundwater contaminant. Reductive immobilization to Cr(III) is a treatment option, but its success depends on the long-term potential for reduced chromium precipitates to remain immobilized under oxidizing conditions. In this unique long-term study, aquifer sediments subjected to reductive Cr(VI) immobilization under different biogeochemical regimes were tested for their susceptibility to reoxidation. After reductive treatment for 1 year, sediments were exposed to oxygenated conditions for another 2 years in flow-through, laboratory columns. Under oxidizing conditions, immobilized chromium reduced under predominantly denitrifying conditions was mobilized at low concentrations (≪1 μM Cr(VI); ∼ 3% of Cr(III) deposited) that declined over time. A conceptual model of a limited pool of more soluble Cr(III), and a larger pool of relatively insoluble Cr(III), is proposed. In contrast, almost no chromium was mobilized from columns reduced under predominantly fermentative conditions, and where reducing conditions persisted for several months after introduction of oxidizing conditions, presumably due to the presence of a reservoir of reduced species generated during reductive treatment. The results from this 3-year study demonstrate that biogeochemical conditions present during reductive treatment, and the potential for buildup of reducing species, will impact the long-term sustainability of the remediation effort.

Remediation of hexavalent chromium contamination in chromite ore processing residue by sodium dithionite and sodium phosphate addition and its mechanism

Large amounts of chromite ore processing residue (COPR) wastes have been deposited in many countries worldwide, generating significant contamination issues from the highly mobile and toxic hexavalent chromium species (Cr(VI)). In this study, sodium dithionite (Na₂S₂O₄) was used to reduce Cr(VI) to Cr(III) in COPR containing high available Fe, and then sodium phosphate (Na₃PO₄) was utilized to further immobilize Cr(III), via a two-step procedure (TSP). Remediation and immobilization processes and mechanisms were systematically investigated using batch experiments, sequential extraction studies, X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). Results showed that Na₂S₂O₄ effectively reduced Cr(VI) to Cr(III), catalyzed by Fe(III). The subsequent addition of Na₃PO₄ further immobilized Cr(III) by the formation of crystalline CrPO₄·6H₂O. However, addition of Na₃PO₄ simultaneously with Na₂S₂O₄ (via a one-step procedure, OSP) impeded Cr(VI) reduction due to the competitive reaction of Na₃PO₄ and Na₂S₂O₄ with Fe(III). Thus, the remediation efficiency of the TSP was much higher than the corresponding OSP. Using an optimal dosage in the two-step procedure (Na₂S₂O₄ at a dosage of 12× the stoichiometric requirement for 15 days, and then Na₃PO₄ in a molar ratio (i.e. Na₃PO₄: initial Cr(VI)) of 4:1 for another 15 days), the total dissolved Cr in the leachate determined via Toxicity Characteristic Leaching Procedure (TCLP Cr) testing of our samples was reduced to 3.8 mg/L (from an initial TCLP Cr of 112.2 mg/L, i.e. at >96% efficiency).

Siderophore and Organic Acid Promoted Dissolution and Transformation of Cr(III)-Fe(III)-(oxy)hydroxides

The role of microbial activities on the transformation of chromium (Cr) remediation products has generally been overlooked. This study investigated the stability of Cr(III)-Fe(III)-(oxy)hydroxides, common Cr(VI) remediation products, with a range of compositions in the presence of common microbial exudates, siderophores and small organic acids. In the presence of a representative siderophore, desferrioxamine B (DFOB), iron (Fe) was released at higher rates and to greater extents relative to Cr from all solid phases. The presence of oxalate alone caused the release of Cr, but not of Fe, from all solid phases. In the presence of both DFOB and oxalate, oxalate acted synergistically with DFOB to increase the Fe, but not the Cr, release rate. Upon reaction with DFOB or DFOB + oxalate, the remaining solids became enriched in Cr relative to Fe. Such incongruent dissolution led to solid phases with different compositions and increased solubility relative to the initial solid phases. Thus, the presence of microbial exudates can promote the release of Cr(III) from remediation products via both ligand complexation and increased solid solubility. Understanding the potential reaction kinetics and pathways of Cr(VI) remediation products in the presence of microbial activities is necessary to assess their long-term stability.

Hexavalent Chromium Generation within Naturally Structured Soils and Sediments

Chromium(VI) produced from the oxidation of indigenous Cr(III) minerals is increasingly being recognized as a threat to groundwater quality. A critical determinant of Cr(VI) generation within soils and sediments is the necessary interaction of two low-solubility phases-Cr(III) silicates or (hydr)oxides and Mn(III/IV) oxides-that lead to its production. Here we investigate the potential for Cr(III) oxidation by Mn oxides within fixed solid matrices common to soils and sediments. Artificial aggregates were constructed from Cr(OH)₃- and Cr_0.25Fe_0.75(OH)₃-coated quartz grains and either mixed with synthetic birnessite or inoculated with the Mn(II)-oxidizing bacterium Leptothrix cholodnii. In aggregates simulating low organic carbon environments, we observe Cr(VI) concentrations within advecting solutes at levels more than twenty-times the California drinking water standard. Chromium(VI) production is highly dependent on Cr-mineral solubility; increasing Fe-substitution (x = 0 to x = 0.75) decreases the solubility of the solid and concomitantly decreases total Cr(VI) generation by 37%. In environments with high organic carbon, reducing conditions within aggregate cores (microbially) generate sufficient Fe(II) to suppress Cr(VI) efflux. Our results illustrate Cr(VI) generation from reaction with Mn oxides within structured media simulating soils and sediments and provide insight into how fluctuating hydrologic and redox conditions impact coupled processes controlling Cr and Mn cycling.

Quantifying Cr(VI) Production and Export from Serpentine Soil of the California Coast Range

Hexavalent chromium (Cr(VI)) is generated in serpentine soils and exported to surface and groundwaters at levels above health-based drinking water standards. Although Cr(VI) concentrations are elevated in serpentine soil pore water, few studies have reported field evidence documenting Cr(VI) production rates and fluxes that govern Cr(VI) transport from soil to water sources. We report Cr speciation (i) in four serpentine soil depth profiles derived from the California Coast Range serpentinite belt and (ii) in local surface waters. Within soils, we detected Cr(VI) in the same horizons where Cr(III)-minerals are colocated with biogenic Mn(III/IV)-oxides, suggesting Cr(VI) generation through oxidation by Mn-oxides. Water-extractable Cr(VI) concentrations increase with depth constituting a 7.8 to 12 kg/km² reservoir of Cr(VI) in soil. Here, Cr(VI) is produced at a rate of 0.3 to 4.8 kg Cr(VI)/km²/yr and subsequently flushed from soil during water infiltration, exporting 0.01 to 3.9 kg Cr(VI)/km²/yr at concentrations ranging from 25 to 172 μg/L. Although soil-derived Cr(VI) is leached from soil at concentrations exceeding 10 μg/L, due to reduction and dilution during transport to streams, Cr(VI) levels measured in local surface waters largely remain below California's drinking water limit.

Evaluation of layered and mixed passive treatment systems for acid mine drainage

Laboratory column tests for passive treatment systems for mine drainage from a waste rock storage area were conducted to evaluate suitable reactive mixture, system configuration, effects of influent water chemistry, and required residence time. Five columns containing straw, chicken manure, mushroom compost, and limestone (LS), in either layered or mixed configurations, were set up to simulate the treatment system. The results showed that all of the five columns removed metals of concern (i.e. Al, Cd, Co, Cu, Fe, Ni, and Zn) with a residence time of 15 h and greater. Reaction mechanisms responsible for the removal of metals may include sulfate reduction and subsequent sulfide precipitation, precipitation of secondary carbonates and hydroxides, co-precipitation, and sorption on organic substrates and secondary precipitates. The results suggest that the mixed systems containing organic materials and LS perform better than the layered systems, sequentially treated by organic and LS layers, due to the enhanced pH adjustment, which is beneficial to bacterial activity and precipitation of secondary minerals. The column tests provide a basis for the design of a field-scale passive treatment system, such as a reducing and alkalinity producing system or a permeable reactive barrier.

Aerosol-Assisted Self-Assembly of Reticulated N-Doped Carbonaceous Submicron Spheres for Effective Removal of Hexavalent Chromium

This Research Article described a facile one-step method to prepare reticulated N-doped carbonaceous submicron spheres. Through a simple aerosol-assisted technology, glucosamine sulfate used as a carbon source was aerosolized and carbonized to functionalized carbonaceous submicron spheres. The electrostatic attraction between protonated amino groups and sulfate in the aerosol droplets induced a self-assembly and led to the formation of reticular structure, avoiding the use of templates. Compared to bare carbonaceous materials produced from glucose, reticulated N-doped carbonaceous spheres exhibit higher efficiency in the removal of Cr(VI), where the doping of element nitrogen led to electrostatic attraction between protonated nitrogen and chromium ions, and reticulated structure created relatively higher surface area and pore volume, facilitating materials to contact with Cr(VI) ions. XPS characterization proved these novel N-doped carbonaceous materials could effectively transform Cr(VI) to less toxic Cr(III) because of the surface reducing groups. For the practical application, several factors including the initial pH, materials dosage and recycle numbers on the removal performance were studied.

Influence factors for the oxidation of pyrite by oxygen and birnessite in aqueous systems

The oxidation of exposed pyrite causes acid mine drainage, soil acidification, and the release of toxic metal ions. As the important abiotic oxidants in supergene environments, oxygen and manganese oxides participate in the oxidation of pyrite. In this work, the oxidation processes of natural pyrite by oxygen and birnessite were studied in simulated systems, and the influence of pH, Fe(II) and Cr(III) on the intermediates and redox rate was investigated. SO4(2-) and elemental S were formed as the major and minor products, respectively, during the oxidation processes. Ferric (hydr) oxides including Fe(OH)3 and goethite were formed with low degree of crystallinity. Low pH and long-term reaction facilitated the formation of goethite and ferric hydroxide, respectively. The rate of pyrite oxidation by birnessite was enhanced in the presence of air (oxygen), and Fe(II) ions played a key role in the redox process. The addition of Fe(II) ions to the reaction system significantly enhanced the oxidation rate of pyrite; however, the presence of Cr(III) ions remarkably decreased the pyrite oxidation rate in aqueous systems. The introduction of Fe(II) ions to form a Fe(III)/Fe(II) redox couple facilitated the electron transfer and accelerated the oxidation rate of pyrite. The present work suggests that isolation from air and decreasing the concentration of Fe(II) ions in aqueous solutions might be effective strategies to reduce the oxidation rate of pyrite in mining soils.

Kinetics and Mechanisms of Cr(VI) Formation via the Oxidation of Cr(III) Solid Phases by Chlorine in Drinking Water

Hexavalent chromium Cr(VI), typically existing as the oxyanion form of CrO4(2-), is being considered for more stringent drinking water standards by regulatory agencies. Cr(VI) can be inadvertently produced via the oxidation of trivalent chromium Cr(III) solids. This study investigated the kinetics and mechanisms of Cr(III) solids oxidation by chlorine in drinking water and associated Cr(VI) formation. Batch experiments were carried out with three Cr(III) solids of environmental relevance, i.e., chromium hydroxide Cr(OH)3(s), chromium oxide Cr2O3(s), and copper chromite Cu2Cr2O5(s). Impacts of water chemical parameters including pH (6.0-8.5) and bromide concentration (0-5 mg/L) were examined. Results showed that the rapid oxidation of Cr(III) solid phases by chlorine was accompanied by Cr(VI) formation and an unexpected production of dissolved oxygen. Analysis of reaction stoichiometry indicated the existence of Cr intermediate species that promoted the autocatalytic decay of chlorine. An increase in pH modestly enhanced Cr(VI) formation due to changes of reactive Cr(III) surface hydroxo species. Bromide, a trace chemical constituent in source waters, exhibited a catalytic effect on Cr(VI) formation due to an electron shuttle mechanism between Cr(III) and chlorine and the bypass of Cr intermediate formation. The kinetics data obtained from this study suggest that the oxidation of Cr(III) solids by chlorine in water distribution systems can contribute to Cr(VI) occurrence in tap water, especially in the presence of a trace level of bromide.

Interaction mechanisms and kinetics of ferrous ion and hexagonal birnessite in aqueous systems

BACKGROUND

In soils and sediments, manganese oxides and oxygen usually participate in the oxidation of ferrous ions. There is limited information concerning the interaction process and mechanisms of ferrous ions and manganese oxides. The influence of air (oxygen) on reaction process and kinetics has been seldom studied. Because redox reactions usually occur in open systems, the participation of air needs to be further investigated.

RESULTS

To simulate this process, hexagonal birnessite was prepared and used to oxidize ferrous ions in anoxic and aerobic aqueous systems. The influence of pH, concentration, temperature, and presence of air (oxygen) on the redox rate was studied. The redox reaction of birnessite and ferrous ions was accompanied by the release of Mn²⁺ and K⁺ ions, a significant decrease in Fe²⁺ concentration, and the formation of mixed lepidocrocite and goethite during the initial stage. Lepidocrocite did not completely transform into goethite under anoxic condition with pH about 5.5 within 30 days. Fe²⁺ exhibited much higher catalytic activity than Mn²⁺ during the transformation from amorphous Fe(III)-hydroxide to lepidocrocite and goethite under anoxic conditions. The release rates of Mn²⁺ were compared to estimate the redox rates of birnessite and Fe²⁺ under different conditions.

CONCLUSIONS

Redox rate was found to be controlled by chemical reaction, and increased with increasing Fe²⁺ concentration, pH, and temperature. The formation of ferric (hydr)oxides precipitate inhibited the further reduction of birnessite. The presence of air accelerated the oxidation of Fe²⁺ to ferric oxides and facilitated the chemical stability of birnessite, which was not completely reduced and dissolved after 18 days. As for the oxidation of aqueous ferrous ions by oxygen in air, low and high pHs facilitated the formation of goethite and lepidocrocite, respectively. The experimental results illustrated the single and combined effects of manganese oxide and air on the transformation of Fe²⁺ to ferric oxides. Graphical abstract:Lepidocrocite and goethite were formed during the interaction of ferrous ion and birnessite at pH 4-7. Redox rate was controlled by the adsorption of Fe2+ on the surface of birnessite. The presence of air (oxygen) accelerated the oxidation of Fe2+ to ferric oxides and facilitated the chemical stability of birnessite.