Insoluble Cr Research Articles

Cysteine-Facilitated Cr(VI) reduction by Fe(II/III)-bearing phyllosilicates: Enhancement from In-Situ Fe(II) generation

Structural Fe in phyllosilicates represents a crucial and potentially renewable reservoir of electron equivalents for contaminants reduction in aquatic and soil systems. However, it remains unclear how in-situ modification of Fe redox states within Fe-bearing phyllosilicates, induced by electron shuttles such as naturally occurring organics, influences the fate of contaminants. Herein, this study investigated the processes and mechanism of Cr(VI) reduction on two typical Fe(II/III)-bearing phyllosilicates, biotite and chlorite, in the presence of cysteine (Cys) at circumneutral pH. The experimental results demonstrated that Cys markedly enhanced the rate and extent of Cr(VI) reduction by biotite/chlorite, likely because of the formation of Cr(V)-organic complexes and consequent electron transfer from Cys to Cr(V). The concomitant production of non-structural Fe(II) (including aqueous Fe(II), surface bound Fe(II), and Cys-Fe(II) complex) cascaded transferring electrons from Cys to surface Fe(III), which further contributed to Cr(VI) reduction. Notably, structural Fe(II) in phyllosilicates also facilitated Cr(VI) reduction by mediating electron transfer from Cys to structural Fe(III) and then to edge-sorbed Cr(VI). 57Fe Mössbauer analysis revealed that cis-coordinated Fe(II) in biotite and chlorite exhibits higher reductivity compared to trans-coordinated Fe(II). The Cr end-products were insoluble Cr(III)-organic complex and sub-nanometer Cr2O3/Cr(OH)3, associated with residual minerals as micro-aggregates. These findings highlight the significance of in-situ produced Fe(II) from Fe(II/III)-bearing phyllosilicates in the cycling of redox-sensitive contaminants in the environment.

Re-yellowing of chromium-contaminated soil after reduction-based remediation: Effects and mechanisms of extreme natural conditions

Chromium (VI) in soil poses a significant threat to the environment and human health. Despite efforts to remediate Cr contaminated soil (Cr-soil), instances of re-yellowing have been observed over time. To understand the causes of re-yellowing as well as the influence of overdosed chemical reductant in remediating Cr-soil, experiments on excess reducing agent interference and soil re-yellowing mechanisms under different extreme conditions were conducted. The results show that the USEPA method 3060A & 7196A combined with K2S2O8 oxidation is an effective approach to eliminate interference from excess FeSO4 reducing agents. The main causes of re-yellowing include the failure of reducing agents, disruption of soil lattice, and interactions between manganese oxides and microorganisms. Under various extreme conditions simulated across the four seasons, high temperature and drought significantly accelerated the failure of reducing agents, resulting in the poorest remediation effectiveness for Cr-soil (91.75 %). Dry-wet cycles promoted the formation of soil aggregates, negatively affecting Cr(VI) removal. While these extreme conditions caused relatively mild re-yellowing (9.46 %–16.79 %) due to minimal soil lattice damage, the potential risk of re-yellowing increases with the failure of reducing agents and the release of Cr(VI) within the lattice. Prolonged exposure to acid rain leaching and freeze-thaw cycles disrupted soil structure, leading to substantial leaching and reduction of insoluble Cr, resulting in optimal remediation effectiveness (94.37 %–97.73 %). As reducing agents gradually and the involvement of the water medium, significant re-yellowing occurred in the remediated soil (51.52 %). Mn(II) in soil enriched relevant microorganisms, and the Mn(IV)-mediated biological oxidation process was also one of the reasons for soil re-yellowing.

Simultaneous nitrate and chromium removal mechanism in a pyrite-involved mixotrophic biofilter.

Microbially mediated NO3--N and Cr(VI) reduction is being recognized as an eco-friendly and cost-effective remediation strategy. Iron sulfide mineral, as a natural inorganic electron donor, has a strong influence on NO3--N and Cr(VI) transformation, respectively. However, little is known about the simultaneous nitrate and chromium removal performance and underlying mechanism in an iron sulfide mineral-involved mixotrophic biofilter. This study demonstrated that the NO3--N and Cr(VI) removal efficiencies were stable at 62 ± 8% and 56 ± 10%, and most of them were eliminated in the 0-100-mm region of the biofilter. Cr(VI) was reduced to insoluble Cr(III) via microbial and chemical pathways, which was confirmed by the SEM-EDS morphology and the XPS spectra of biofilm and pyrite particles. SO42- was as a main byproduct of pyrite oxidation; however, the bacterial SO42- reduction synchronously occurred, evidenced by the variations of TOC and SO42- concentrations. These results suggested that there were complicated and intertwined biochemical relations between NO3--N/Cr(VI)/SO42-/DO (electron acceptors) and pyrite/organics (electron donors). Further investigation indicated that both the maximal biomass and greatest denitrifiers' relative abundances in microbial sample S1 well explained why the pollutants were removed in the 0-100-mm region. A variety of denitrifiers such as Pseudoxanthomona, Acidovorax, and Simplicispira were enriched, which probably were responsible for both NO3--N and Cr(VI) removal. Our findings advance the understanding of simultaneous nitrate and chromium removal in pyrite-involved mixotrophic systems and facilitate the new strategy development for nitrate and chromium remediation.

Ultrasensitive fluorescent carbon dot sensor for quantification of soluble and insoluble Cr(VI) in particulate matter

This study investigates advanced functional materials to address the need for practical and affordable analytical techniques for monitoring large amounts of insoluble Cr(VI). N,S-doped fluorescent carbon dots (f-CDs) were fabricated through microwave synthesis, with an average diameter of 10 nm. These f-CDs were explored as potential sensors for detecting Cr(VI) in ambient particulate matter (PM). Laboratory experiments yielded positive results, showing average recoveries of 106.0%, 102.3%, 96.4%, and 101.7% for PbCrO4, BaCrO4, CaCrO4, and (NH4)2CrO4, respectively. Applying the fluorescence method to field PM samples, a method detection limit (MDL) of 0.32 ng/m3 for total Cr(VI) quantification was achieved. The fluorescence decay of carbon dots remained stable over time, suggesting that Cr(VI) quenching primarily occurred due to the internal filter effect through a static quenching mechanism. These f-CDs exhibited advantageous properties, including affordability, solubility, luminescence, and sensitivity, positioning them as a promising alternative for Cr(VI) detection in ambient air particulates. This study contributes to further developing carbon-based functional materials for detecting metallic air pollutants.

Effects of abiotic mineral transformation of FeS on the dynamic immobilization of Cr(VI) in oxic aquatic environments

Iron sulfide (FeS) can reductively convert soluble Cr(VI) into insoluble Cr(III) under anoxic conditions. However, the fate and transformation of FeS and the stability of immobilized Cr under various oxic environmental conditions are poorly understood. The results show that FeS transforms into pyrrhotite and pyrite intermediates principally and finally lepidocrocite and elemental sulfur, accordingly accounting for 66.1% and 33.9%. Temperature, fulvic acid as natural organic matter and coexisted ions of nitrate, bicarbonate, and calcium affect the evolution of FeS insignificantly. Transformation of FeS involves surface-mediated oxidation of FeS solids, and minor proton-promoted dissolution and oxidation, accompanying synergistic oxidation of Fe(II) and S(-II). Cr(VI) removal performances of oxygenated FeS with increasing duration showed a rise-fall trend. Reduction dominates Cr(VI) uptake first and finally, sorption prevails with the gradual FeS oxygenation. Cr(VI) removal correlates linearly with Cr(VI) reduction, and the reduced Cr species can be predicted based on the known Cr(VI) removal performance. As the FeS oxygenation time increases, newly generated pyrite improves Cr(VI) reduction and removal, and then a decreasing ability to reduce Cr(VI) causes a drop in Cr(VI) removal. These findings provide new insight into the oxidative transformation of FeS in oxic aquatic environments and its impact on Cr(VI) levels.

Leaching and geochemical evaluation of oxyanion partitioning within an active coal ash management unit

Field monitoring combined with laboratory leaching characterization and geochemical speciation modeling were used to identify important geochemical parameters and controlling mechanisms for leaching of oxyanions from coal ash in a disposal site. The site consisted of dry stacked ash on top of a former surface impoundment. Constituents leached from the dry stacked ash served as sources for the soluble fractions of constituents in underlying impoundment ash. Weathering of field ash was evidenced by the identification of calcite and ettringite. Porewater concentrations of As, B, and V in the field pH range (9–11) and liquid-to-solid ratio (L/S) condition (∼0.6 L/kg-dry) were primarily controlled by Ca-bearing precipitates (Ca-arsenate, B-substituted carbonate solid solution, Ca-vanadate, and V-substituted ettringite solid solution). The leachable fraction of Mo as a function of pH was limited by the available content in the solid, with aqueous concentrations of Mo being a function of L/S. Leaching behavior of Cr and Se was sensitive to the redox changes (from suboxic to oxic conditions) during field ash collection and laboratory leaching tests, including oxidative dissolution of insoluble Cr(III)-oxides and conversion of Se(0) in the solid to dissolved Se(IV)/Se(VI). Oxidation resulted in increased Cr concentrations from < 0.00005 (in porewater) to 0.001–0.1 mg/L (in laboratory eluates). The identified geochemical parameters and leaching-controlling mechanisms provide the bases for understanding and estimating the partitioning of oxyanions in other disposal sites with alkaline and suboxic conditions. This case study also provides a typical example of laboratory test results interpretation and field leaching behavior estimation for other studies.

Selective and Efficient Photoextraction of Aqueous Cr(VI) as a Solid-State Polyhydroxy Cr(V) Complex for Environmental Remediation and Resource Recovery

Aqueous hexavalent chromium (Cr(VI)) treatment and chromium resource recovery toward Cr-containing wastes are of significant importance and necessity to both wastewater remediation and resource recovery. Herein, via mild photoreaction conditions with isopropanol (IPA) as an electron donor, a catalyst-free strategy for aqueous Cr(VI) extraction to form an insoluble polyhydroxy Cr(V) complex is developed for the first time. Aqueous Cr(VI) with concentration from 5 to 150 ppm can be efficiently extracted with high selectivity even in the presence of coexisting ions, and the total Cr concentration in residue solution can be as low as 0.5 ppm. The Cr resource could be efficiently recovered as pure Cr2O3 by calcinating the resulting Cr(V) precipitate. Outstanding extraction efficiency could be realized with various IPA concentrations (1.3-12.0 mol/L) by coordinately tuning the pH value to promote the formation of Cr(VI)-IPA ester. The formed ester undergoes intramolecular electron transition under visible light irradiation, resulting in a polyhydroxy solid-state Cr(V) intermediate complex. The controlled pH value blocks further reduction of Cr(V) to soluble Cr(III); thus the insoluble Cr(V) intermediate complex is stabilized thermodynamically under ambient conditions. Because of its electric neutrality property and the strong intermolecule interaction via hydrogen bonds, a dioxo-bridged di-nuclear Cr(V) complex {Cr2(μ-O)2(OH)4[OCH(CH3)2]2} is finally precipitated as the main product. Satisfactory extraction and recovery of Cr from chromium-plating wastewater and discarded stainless steel verify that this approach is ideal for both one-step purification of Cr(VI)-containing wastewater and selective resource recovery from Cr-containing solid wastes in practical application.

Quantification and the sources identification of total and insoluble hexavalent chromium in ambient PM: A case study of Aktobe, Kazakhstan

Hexavalent chromium (Cr(VI)), a known carcinogen, emanates from both anthropogenic and natural sources. A pilot study of the ambient Cr(VI) concentrations was conducted at the center of Aktobe which is a few kilometers away from major industrial chromium plants. Total Cr(VI) concentrations were measured in the fall and winter seasons with mean values (S.D) of 5.30 (2.16) ng/m3 and 2.26 (1.80) ng/m3, respectively. Insoluble Cr(VI) levels were 4.80 (1.96) and 2.19 (1.75) ng/m3 for the fall and winter, respectively. The total and insoluble Cr(VI) concentrations in the fall season were significantly higher than in winter, likely due to the higher rate of Cr(III) oxidation in the presence of ozone and ROS in fall compared to the rate of Cr(VI) reduction in the presence of VOCs at higher temperatures. On average, total Cr(VI) constituted 34.49% of the total Cr concentrations suggesting that the dominant valence state of Cr in the atmosphere is Cr(III). The previous reference values of exposure to Cr(VI) must be revisited by taking into account the insoluble Cr(VI) concentration since it is more prevalent in the atmosphere compared to soluble Cr(VI). The influence of the chromium plants as potential sources was not obvious in this study.

Microstructure and properties of Cu-Cr and Cu-Cr-Mg alloys under different processing conditions

Cu-0.5Cr (wt%) and Cu-0.5Cr-0.1Mg (wt%) alloys were manufactured in this study to analyse the relationship between the physical properties and microstructure under different states. Optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) were used to observe the microstructures of the alloys following homogenization, hot-rolled, solid solution, cold-rolled and aging. The results showed that insoluble Cr phases existed in copper chromium and copper chromium magnesium alloys after holding at 925°C for 12 h for homogenization and hot rolling deformation of 80%, and Mg elements prevented the segregation of Cr phases at grain boundaries. After solution treatment by holding at 1000°C for 1 h, the mean grain size of both alloys were 299 nm and 81 nm, respectively, and the volume fraction of undissolved Cr phase was 0.4% and 0.2%, respectively. After 60 % cold deformation treatment, the hardness of the two alloys increased significantly as a result of the work hardening caused by the generation of dislocation. In the aging stage of 450°C for 1 h, lots of Cr precipitates were found in Cu-0.5Cr and Cu-0.5Cr-0.1Mg alloys, with a corresponding hardness of 162.8 HV and 170.1 HV and electrical conductivity of 82.9% IACS and 77.8% IACS, respectively.

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.

Key Cr species controlling Cr stability in contaminated soils before and chemical stabilization at a remediation engineering site

Linking chromium (Cr) speciation with its stability in soils is vital because insoluble Cr(VI) and chemically adsorbed Cr(VI) could hinder the remediation efficiency and release Cr(VI) for a prolonged period of time. In this study, we investigated key Cr species to probe the mechanisms controlling the release of insoluble Cr(VI) at Cr-contaminated sites using synchrotron-based X-ray absorption near-edge structure (XANES) for the first time. Chromite, stichtite and Cr-silicate were predominant forms of Cr(III). Insoluble Cr(VI) was hosted by layered double hydroxides (LDHs) such as brownmilerite and hydrotalcite. Anion competition tests documented a substitution of absorbed Cr(VI) by SO42- and NO3-. Acid extraction released 6.7–25.7% more Cr(VI) than anion extraction, possibly attributing to the erosion of LDH and CaCrO4 in calcite rather than Cr-bearing minerals. Brown and red soils released maximally 62% and 44% of total Cr(VI) by 10 mol/(kg soil) and 2 mol/(kg soil) of H+, respectively. SO42-, H2O and H+ contributed to more release of total Cr(VI) in brown soils (22%, 33% and 7%) than red soils (25%, 17% and 2%). More crystalline Cr structures were found after chemical stabilization, indicating a higher Cr stability in chemically stabilized soils. Cr and Mn exhibited an overlapped distribution pattern in both contaminated and chemically stabilized soils, hinting at the re-oxidation of Cr(III). Insoluble Cr(VI) could be released by acidic rainfalls and soil organic matters, posing potential threats to Cr long-term stability in field-scale remediation.

Highly efficient removal of Cr(VI) from aqueous solution by pinecone biochar supported nanoscale zero-valent iron coupling with Shewanella oneidensis MR-1

Nanoscale zero-valent iron (nZVI) has been extensively used to remove various pollutants. However, the rapid deactivation due to aggregation and surface passivation severely limits its practical application. In this study, a novel composite with nZVI supported by pinecone biochar (nZVI-PBC) was successfully synthesized and used for the removal of high concentration Cr(VI) from aqueous solution in the presence of Shewanella oneidensis MR-1 (MR-1). The results showed that the nZVI-PBC coupling with MR-1 (nZVI-PBC/MR-1) exhibited an excellent removal performance for high concentration Cr(VI) compared to the nZVI-PBC alone. Under optimal conditions, 100 mg/L Cr(VI) could be removed completely by nZVI-PBC/MR-1 within 48 h, while only 39.50% of Cr(VI) was removed by nZVI-PBC alone. The improvement of Cr(VI) removal is due to the dissolution of the surface passivation layer of nZVI-PBC, formation of sorbed Fe(II) in the presence of MR-1, and an important role of extracellular polymeric substance (EPS) derived from MR-1. X-ray photoelectron spectroscopy (XPS) and Cr K-edge X-ray absorption near-edge structure spectra (XANES) confirmed that most Cr(VI) was reduced to insoluble Cr(III) and formed Cr2O3, CrxFe1-x(OH)3 and FeCr2O4 precipitates, and a small amount of unreduced Cr(VI) was immobilized through adsorption and complexation. The results suggest that nZVI-PBC/MR-1 can effectively overcome the limitations of nZVI and achieve highly efficient removal of high concentration Cr(VI).

Se changed the component of organic chemicals and Cr bioavailability in pak choi rhizosphere soil.

Rhizosphere organic chemicals response and its role on Cr/Se adsorption are of great importance to understand Cr/Se bioavailability in Cr-contaminated soil with the application of Se. In the current work, the processes were carried out using rhizobox experiment (Brassica campestris L. ssp. chinensis Makino). The results showed that in soil contaminated by 200 mg kg-1 Cr(III), Se(IV) complexed with Cr(III) and carboxylic acid (cis-9,10-Epoxystearic acid, hexadecanedioic acid) reduced Cr(VI) to Cr(III), thus increasing of Cr adsorption, furtherly decreasing Cr bioavailability. While in soil contaminated by 120 mg kg-1 Cr(VI), Se(VI) competed for adsorption sites with Cr(VI) and salicylic acid activated insoluble Cr(III), thus decreasing Cr adsorption, finally increasing Cr bioavailability. Moreover, with Cr contamination, Se bioavailability in soil was enhanced by the secretion of carboxylic acid, which can reduce Se to lower valent state and compete the adsorption sites and complex with Se oxyanion. These results yielded a better understanding of rhizosphere dynamics regulating by Se application in Cr-contaminated soil. Moreover, the current study supplemented the theoretical basis for beneficial elements application as an environment-friendly resource to facilitate cleaner production in heavy metal contaminated soil.

A Comparison in Metal Leachability Between Phosphoric Acid and Ascorbic Acid Stabilised Ferrochrome Slag

Ferrochrome (FeCr) slag was milled and stabilised with either ascorbic acid or phosphoric acid. The stabilised FeCr was then geopolymerised with 1 M KOH in order to obtain a monolith with at least an unconfined compressive strength of 1 MPa. The leachability of metals of the stabilised geopolymerised monoliths were then compared with the unstabilised geopolymerised monolith. Ascorbic acid stabilisation was only effective in Cr leaching reduction by 99.45% but was not effective on immobilisation of Fe, Zn, Ni and Mn. Ascorbic acid stabilisation was thought to proceed via the reduction of Cr(VI) species to insoluble Cr (III) species. Phosphate stabilisation reduced the leachability of Cr, Ni, Zn, Mn, Fe by 99.5%, 67.1%, 71.1%, 96.8% and 85.4% respectively. Phosphate stabilisation was thought to proceed via the formation of phosphate compounds of the metal ions in question. The phosphate stabilised FeCr slag leachability was within the allowable Toxicity Characteristic Leaching Procedure (TCLP) limits and its use is not detrimental to the environment.

Is Urinary Chromium Specific to Hexavalent Chromium Exposure in the Presence of Co-exposure to Other Chromium Compounds? A Biomonitoring Study in the Electroplating Industry.

Electroplating processes are widely used in metal industries to improve the resistance properties of manufactured metal parts. Workers in this industry are potentially exposed both to hexavalent chromium (Cr(VI)) and to other chromium compounds [mostly trivalent chromium (Cr(III))], due to the use of chromic acid baths. The goal of this study was to validate urinary chromium as a Cr(VI) exposure biomarker in the presence of exposure to other chromium compounds. A biomonitoring study consisted in monitoring airborne chromium exposure and urinary chromium for one working week in 93 workers from nine electroplating companies. Chromium concentrations were measured in all urinations of each volunteer for the working week. Individual airborne soluble and insoluble Cr(VI) as well as Cr(III) concentrations were measured for all of the shifts of the week. The main statistical analysis consisted in modelling, in a Bayesian framework, the pre- and post-shift urinary chromium as a function of airborne Cr(III) and airborne Cr(VI), taking into account the day of the week and the time of collection of the urines (pre- or post-shift). Preliminary descriptions showed an increase in pre-shift urinary chromium during the working week. The model showed an increase in urinary chromium over the shift related to the shift-specific airborne Cr(VI) concentration as well as an increasing trend over the week and a relationship with the mean weekly Cr(VI) thought to reflect chronic exposure. Taking into account the Cr(VI) exposure, there was no evidence of an effect of Cr(III) exposure on urinary chromium. A biological limit value (BLV) was derived from the French occupational exposure limit for Cr(VI) of 1 µg m-3 and was estimated at between 1.9 and 2.6 µg g-1 creatinine for a urinary sample collected at the end of the shift on the last working day of the week. In the present context of mixed exposure to Cr(III) and Cr(VI) in electroplating, this study showed that urinary chromium depended only on airborne Cr(VI) concentrations, which justifies using a BLV for assessing workers' exposure. The estimated BLV was close to the recommended French BLV, which is 1.8 µg g-1 creatinine, in the electroplating industry.

Marine microbial Mn(II) oxidation mediates Cr(III) oxidation and isotope fractionation

The stable chromium (Cr) isotope system has been used extensively over the past decade as a paleoredox proxy. The prevailing view has been that Cr oxidation is essentially limited to terrestrial settings and that the Cr isotope system is thus ideally suited to track atmospheric oxygenation. Further, although manganese(III,IV) oxide minerals (MnOx) are generally accepted to be the primary naturally occurring oxidants for the oxidation of insoluble Cr(III) to Cr(VI) and may fractionate Cr isotopes, there are conflicting reports in the literature regarding the direction and magnitude of Cr isotope fractionation by MnOx. We investigated Cr isotope fractionation during coupled Mn(II) and Cr(III) oxidation by laboratory cultures of marine Mn(II)-oxidizing bacteria and by natural microbial communities in seawater samples collected from the seasonally low-oxygen zone of the Chesapeake Bay, Maryland, USA. We found evidence for rapid marine Cr(III) oxidation linked to microbial Mn(II) oxidation. We also show that the initial partial oxidation of Cr(III) to Cr(VI) can produce an isotope fractionation of 0.8 ± 0.1‰. Further, given that Cr(VI) reduction and sequestration into marine sediments can lead to large isotopic fractionations (>1‰), evidence for rapid Cr oxidation in marine environments contradicts the idea that Cr isotope variations in the sedimentary record must be linked to subaerial oxidative weathering. As oxygen can accumulate in the surface ocean even under an anoxic atmosphere, these results call for a reevaluation of how we can link Cr isotope values to atmospheric oxygenation.

In Situ Biostimulation of Cr(VI) Reduction in a Fast-Flowing Oxic Aquifer

Hexavalent chromium is among the most common and hazardous inorganic contaminants in soils and groundwater. A promising in situ bioremediation approach is the delivery of electron donors to stimulate microbially mediated Cr(VI) reduction, producing relatively insoluble Cr(III) precipitates. Even though this strategy has been implemented successfully in the past, it was primarily under favorable hydrological conditions, such as relatively slow groundwater flow velocities. To evaluate whether microbially mediated Cr(VI) reduction can be sustained at high groundwater flow velocities and under nearly oxygen-saturated conditions, we have conducted laboratory-scale column experiments and a field-scale pilot study using molasses as the electron donor. Despite the unfavorable conditions, both experiments provided clear evidence for microbially mediated Cr(VI) reduction. In particular, the well used for injection in the field experiment became anoxic after two months of injection and the Cr(VI) concentration decreased by a factor of six. After stopping the injection, these conditions prevailed for at least four months. In the column experiments, Cr(VI) reduction was accompanied by distinct Cr isotopic fractionation characterized by an enrichment factor of −1.25‰. In contrast, Cr isotope data collected from the field experiment were ambiguous, which was because of the complexation of Cr(III) with organic carbon and the heterogeneous distribution of molasses due to the presence of narrow preferential flow paths. A substantial injectivity decrease likely caused by the formation of biofilm and the precipitation of Fe sulfides formed an additional challenge during the field experiment. Our data, however, also suggest that most of the identified challenges could be addressed by limiting the targeted dissolved organic carbon concentrations to about 10 mg/L. In conclusion, the injection of molasses constitutes a promising bioremediation strategy for the long-term treatment of Cr(VI) contamination even at high flow rates and under oxic conditions.

Microbial electrolysis cell produced biogas as sustainable electron donor for microbial chromate reduction

Microbial electrolysis cell (MEC) is an attractive technology to produce biogas containing hydrogen (H2) and methane (CH4). Both gaseous products are effective electron donors for chromate [Cr(VI)] bioreduction when either is applied individually. However, microbial Cr(VI) reduction driven by H2 and CH4 mixture remains poorly understood. This study investigates Cr(VI) biotransformation supported by fluctuant MEC-produced biogas. Synergy between H2 and CH4 promoted Cr(VI) detoxification. Maximum Cr(VI) removal rate reached 1.81 ± 0.17 mg/L·d with 200.5 ± 13.6 μmol H2 and 203.3 ± 9.0 μmol CH4 produced by MEC at 0.6 V in a 4-d cycle. Biogas yield and H2/CH4 mole ratio were mainly regulated by Geobacter and Methanobacterium. The process in which Cr(VI) reduced to insoluble Cr(III) was proposed in two major pathways. A single autohydrogenotrophic genus (e.g., Hydrogenophaga, Thiobacillus) oxidized H2 independently coupled with Cr(VI) reduction. Cr(VI) reduction was also achieved through CH4-metabolizing microorganisms (e.g., Methanobacterium, Methanosaeta), which convert CH4 into organic metabolites to donate electrons for heterotrophic Cr(VI) reducers (e.g., Geobacter, Anaerolineaceae). Genes relevant to Cr(VI) reduction (yieF) and CH4 oxidation (mcrA) increased significantly. Electrons were effectively transferred through extracellular cytochrome c and intracellular nicotinamide adenine dinucleotide. This study offers an innovative route to sustainable bioremediation of Cr(VI)-polluted groundwater and promising direct utilization of mixed biogas from MEC.

Simultaneous Removal of Arsenate and Chromate from Ground- and Surface- Waters by Iron-Based Redox Assisted Coagulation

Arsenic (As) and chromate (Cr(VI)) contamination of ground and surface waters is a major problem worldwide. Given that a new drinking water limit is anticipated for Cr(VI) and that the limit of arsenic in drinking water is quite low (10 μg/L), there is an urgent need for evaluating technologies that could be efficient for removal of both contaminants simultaneously. In this work, the use of Fe(II) redox assisted coagulation was investigated to simultaneously remove the contaminants of interest. The basic principle of this technology is that Fe(II) could react with Cr(VI) and form Fe(III)-hydroxides and insoluble Cr(III) species, while the freshly formed Fe(III) hydroxides are very efficient adsorbents for As(V). The effect of pH, the water matrix composition, Fe(II) dose, initial contaminant concentrations, NOM presence and phosphate concentration were the examined parameters. The results revealed that with a dose of 2 mg/L Fe(II), residual As(V) and Cr(VI) concentrations were both below 10 μg/L, from initial concentrations of 50 μg/L. Though, this is effective only at circumneutral pH values. This is however not a big obstacle, since most natural waters, especially groundwaters, have near neutral pH values. At these pH values, residual iron concentration was far below 200 μg/L. The presence of phosphate anions inhibited As(V) removal but had no effect on Cr(VI) removal. Increasing Fe(II) concentrations eliminated the effect of phosphate and provided simultaneous phosphate removal. Therefore, Fe(II) coagulation can be applied, with secured results, for simultaneous As(V), Cr(VI) and phosphate removal from waters.

Sulfur-based autotrophic biosystem for efficient vanadium (V) and chromium (VI) reductions in groundwater

Vanadium and chromium co-exist commonly with elevated levels in groundwater at vanadium smelting sites. While bioremediation has been recognized promising for this co-contamination treatment in aquifer, interactions during vanadium (V) [V(V)] and chromium (VI) [Cr(VI)] bio-reductions under autotrophic condition remain largely unknown. In this study, efficient reductions of V(V) and Cr(VI) from synthetic groundwater were realized simultaneously in a continuous flow autotrophic sulfur-based biosystem, with more than 85% overall removals under hydrochemical and hydrodynamic fluctuations during 276-d operation. Soluble Cr(VI) was reduced to insoluble Cr(III) preferentially, while reduction of soluble V(V) to insoluble V(IV) was easily inhibited. Elemental sulfur [S(0)] was bio-oxidized to sulfate. Analyses of carbon isotope, microbial community and metabolic pathway revealed the synergetic mechanisms. Autotrophs (e.g., Sulfuricurvum) utilized energy released from S(0) oxidation to synthesize volatile fatty acids (VFAs), which were consumed by heterotrophic V(V) and/or Cr(VI) reducers (e.g., Geobacter). Functional genes responsible for S(0) oxidation and reduction of V(V) and Cr(VI) were detected. V(V) and Cr(VI) reductions were catalyzed by both cytochrome c and nicotinamide adenine dinucleotide. VFAs were also transformed to glycogen in cells to store energy. Robust remediation strategy is thereby proposed for aquifer co-contaminated by V(V) and Cr(VI).