Hematite enhances the removal of Cr(VI) by Bacillus subtilis BSn5 from aquatic environment

Characterization and Molecular Insights of a Chromium-Reducing Bacterium Bacillus tropicus

Environmental pollution from metal toxicity is a widespread concern. Certain bacteria hold promise for bioremediation via the conversion of toxic chromium compounds into less harmful forms, promoting environmental cleanup. In this study, we report the isolation and detailed characterization of a highly chromium-tolerant bacterium, Bacillus tropicus CRB14. The isolate is capable of growing on 5000 mg/L Cr (VI) in an LB (Luria Bertani) agar plate while on 900 mg/L Cr (VI) in LB broth. It shows an 86.57% reduction ability in 96 h of culture. It can also tolerate high levels of As, Cd, Co, Fe, Zn, and Pb. The isolate also shows plant growth-promoting potential as demonstrated by a significant activity of nitrogen fixation, phosphate solubilization, IAA (indole acetic acid), and siderophore production. Whole-genome sequencing revealed that the isolate lacks Cr resistance genes in their plasmids and are located on its chromosome. The presence of the chrA gene points towards Cr(VI) transport, while the absence of ycnD suggests alternative reduction pathways. The genome harbors features like genomic islands and CRISPR-Cas systems, potentially aiding adaptation and defense. Analysis suggests robust metabolic pathways, potentially involved in Cr detoxification. Notably, genes for siderophore and NRP-metallophore production were identified. Whole-genome sequencing data also provides the basis for molecular validation of various genes. Findings from this study highlight the potential application of Bacillus tropicus CRB14 for bioremediation while plant growth promotion can be utilized as an added benefit.

A novel approach to modify Stenotrophomonas sp. D6 by regulating the salt composition in the growth medium: Enhanced removal performance of Cr(VI)

In this study, a novel and effective modified microbial reducing agent was developed to detoxify Cr(VI) from aqueous solutions. This was achieved by carefully controlling specific salt components in the growth medium. Based on the single-salt modification, several effective modified salts were selected and added to the medium for synergistic modification. The results showed that the synergistic modification with NH4Cl and KH2PO4 had the best detoxification effect on Cr(VI), reaching 98.5% at 100 mg/L Cr(VI), which was much higher than the 43.7% of the control (original Luria-Bertani medium). This enhancement was ascribed to the ability of NH4Cl and KH2PO4 to stimulate the growth of Stenotrophomonas sp. D6 promoted chromate reductase secretion. The protein content of the modified medium supernatant was significantly increased by 10.76% compared to that before modification. Based on the micro-characterization, the main process for the elimination of Cr(VI) is microbial reduction rather than biosorption. Most of the reduced Cr was found in the extracellular suspension, thereby suggesting that the primary reduction occurred outside the cells, whereas only a small fraction was detected intracellularly. Overall, this study provides a simple and effective method for microbial treatment of heavy metals in aqueous solutions.

Mechanism of Cr(VI) removal by efficient Cr(VI)-resistant Bacillus mobilis CR3

Cr(VI) is a hazardous environmental pollutant that poses significant risks to ecosystems and human health. We successfully isolated a novel strain of Bacillus mobilis, strain CR3, from Cr(VI)-contaminated soil. Strain CR3 showed 86.70% removal capacity at 200 mg/L Cr(VI), and a good Cr(VI) removal capacity at different pH, temperature, coexisting ions, and electron donor conditions. Different concentrations of Cr(VI) affected the activity of CR3 cells and the removal rate of Cr(VI), and approximately 3.46% of total Cr was immobilized at the end of the reaction. The combination of SEM-EDS and TEM-EDS analysis showed that Cr accumulated both on the cell surface and inside the cells after treatment with Cr(VI). XPS analysis showed that both Cr(III) and Cr(VI) were present on the cell surface, and FTIR results indicated that the presence of Cr on the cell surface was mainly related to functional groups, such as O-H, phosphate, and -COOH. The removal of Cr(VI) was mainly achieved through bioreduction, which primarily occurred outside the cell. Metabolomics analysis revealed the upregulation of five metabolites, including phenol and L-carnosine, was closely associated with Cr(VI) reduction, heavy metal chelation, and detoxification mechanisms. In addition, numerous metabolites were linked to cellular homeostasis exhibited differential expression. Cr(VI) exerted inhibitory effects on the division rate and influenced critical pathways, including energy metabolism, nucleotide metabolism, and amino acid synthesis and catabolism. These findings reveal the molecular mechanism of Cr(VI) removal by strain CR3 and provide valuable insights to guide the remediation of Cr(VI)-contaminated sites.

Cr(VI)-bioremediation mechanism of a novel strain Bacillus paramycoides Cr6 with the powerful ability to remove Cr(VI) from contaminated water

This research provided an excellent novel hexavalent chromium (Cr(VI))-removal bacterium, Bacillus paramycoides Cr6, and investigated its removal mechanism from the perspective of molecular biology. Cr6 could resist up to 2500 mg/L Cr(VI), and the removal rate of 2000 mg/L Cr(VI) reached 67.3% under the optimal culture conditions of 220 r/min, pH 8 and 31 ℃. When the initial concentration of Cr(VI) was 200 mg/L, Cr6 had a removal rate of 100% within 18 h. The differential transcriptome analysis identified two key structural genes named bcr005 and bcb765 of Cr6, which were upregulated by Cr(VI). Their functions were predicted and further confirmed by bioinformatic analyses and in vitro experiments. bcr005 encodes Cr(VI)-reductase BCR005, and bcb765 encodes Cr(VI)-binding protein BCB765. Real-time fluorescent quantitative PCRs were performed, and the data illustrated a parallel pathway (one is Cr(VI) reduction, and the other is Cr(VI) immobilisation) of Cr6 to remove Cr(VI), which relies on the synergistic expression of the genes bcr005 and bcb765 induced by different concentrations of Cr(VI). In summary, a deeper molecular mechanism of Cr(VI) microorganism removal was elaborated; Bacillus paramycoides Cr6 was an exceptional novel Cr(VI)-removed bacterial resource, while BCR005 and BCB765 were two new-found efficient enzymes that have potential practical applications for sustainable microbial remediation of Cr-contaminated water.

Melatonin improves the removal and the reduction of Cr(VI) and alleviates the chromium toxicity by antioxidative machinery in Rhodobacter sphaeroides

Bioremediation with photosynthetic bacteria (PSB) is thought to be a promising removal method for hexavalent chromium [Cr(VI)]-containing wastewater. In the present study, Rhodobacter sphaeroides (R. sphaeroides) SC01 was used for the investigation of Cr(VI) removal in Cr(VI)-contaminated solution in the presence of melatonin. It was found that exogenous melatonin alleviated oxidative damage to R. sphaeroides SC01, increased Cr (VI) absorption capacity of cell membrane, and improved the reduction efficiency of Cr(VI) via the activation of chromate reductants. The results showed that melatonin could further promote the increase in Cr(VI) removal efficiency, reaching up to 97.8%. Furthermore, melatonin application resulted in 296.9%, 44.4%, and 69.7% upregulation of ascorbic acid (AsA), glutathione (GSH), and cysteine (Cys) relative to non-melatioin treated R. sphaeroides SC01 at 48 h. In addition, the resting cells, cell-free supernatants (CFS), and cell-free extracts (CFE) with melatonin had a higher Cr(VI) removal rate of 18.6%, 82.0%, and 15.2% compared with non-melatonin treated R. sphaeroides SC01. Fourier transform infrared spectroscopy (FTIR) revealed that melatonin increased the binding of Cr(III) with PO4^3- and CO groups on cell membrane of R. sphaeroides SC01. X-ray diffractometer (XRD) analysis demonstrated that melatonin remarkably bioprecipitated the production of CrPO₄·6H₂O in R. sphaeroides SC01. Hence, these results indicated that melatonin plays the important role in the reduction and uptake of Cr(VI), demonstrating it is a great promising strategy for the management of Cr(VI) contaminated wastewater in photosynthetic bacteria.

External sodium acetate improved Cr(VI) stabilization in a Cr-spiked soil during chemical-microbial reduction processes: Insights into Cr(VI) reduction performance, microbial community and metabolic functions

Interest combined chemical and microbial reduction for Cr(VI) remediation in contaminated sites has greatly increased. However, the effect of external carbon sources on Cr(VI) reduction during chemical-microbial reduction processes has not been studied. Therefore, in this study, the role of external sodium acetate (SA) in improving Cr(VI) reduction and stabilization in a representative Cr(VI)-spiked soils was systemically investigated. The results of batch experiments suggested that the soil Cr(VI) content declined from 1000 mg/kg to 2.6-5.1 mg/kg at 1-5 g C/kg SA supplemented within 15 days of reaction. The external addition of SA resulted in a significant increase in the relative abundances of Cr(VI)-reducing microorganisms, such as Tissierella, Proteiniclasticum and Proteiniclasticum. The relative abundance of Tissierella increased from 9.1% to 29.8% with the SA treatment at 5 g C/kg soil, which was the main contributors to microbial Cr(VI) reduction. Redundancy analysis indicated that pH and SA were the predominant factors affecting the microbial community in the SA treatments at 2 g C/kg soil and 5 g C/kg soil. Functional prediction suggested that the addition of SA had a positive effect on the metabolism of key substances involved in Cr(VI) microbial reduction. This work provides new insightful guidance on Cr(VI) remediation in contaminated soils.

Enhancement of Cr(VI) reduction by indigenous bacterial consortia using natural pyrite: A detailed study to elucidate the mechanisms involved in the highly efficient and possible sustainable system

Pyrite was applied to Cr(VI) bioremediation as an inorganic electron donor due to the ability to provide electrons, while the role of pyrite in Cr(VI) bioremediation where organics as electron donors remains unknown. Herein a pyrite-based Cr(VI) bioreduction process in the sediment system containing lactate was demonstrated to be effective to detoxify Cr(VI): over 2200 mg L^-1 Cr(VI) was continuously removed within 210 h with high reactivity (10.5 mg/(L·h)) all along. High-throughput 16S rDNA gene sequencing indicated that the pyrite could shape a functioning community that electrochemically active bacteria dominated (such as Fusibacter sp. and Rhodobacteraceae) instead of iron-oxidizing bacteria and sulfur-oxidizing bacteria. Mineralogy analysis results indicated that Fe(III), S₂^2- and S⁰ formed on the pyrite surface after the oxidation of Cr(VI) might serve as the electron acceptor of microflora, then the S^2- and Fe(II) with strong Cr(VI) reduction ability were formed by microbial reduction to enhance the removal of Cr(VI). This study provides new insights into thoroughly understanding the role of pyrite in the practical application of Cr(VI) bioreduction.

Isotherm and kinetics modeling of biosorption and bioreduction of the Cr(VI) by Brachybacterium paraconglomeratum ER41

Chromium is one of the most widely used metals in industry. Hexavalent form [Cr(VI)], which is found in industrial discharges, is very toxic and very soluble in water. From soil taken from an abandoned lead and iron mine, a bacterial strain capable of reducing Cr(VI) was isolated and identified as Brachybacterium paraconglomeratum ER41. Objective of this work was to evaluate the power of this bacterium to reduce Cr(VI). Results obtained showed that this bacterium is capable of eliminating 100 mg/L of Cr(VI) after 48 h (pH 8 and temperature 30 °C). For modeling biosorption kinetics, pseudo-first-order and intraparticle diffusion models gave a better fit. Furthermore, the adsorption mechanism conformed well to Langmuir's isothermal model indicating monolayer type sorption. Biomass analysis of this bacterium before and after contact with chromium by scanning electron microscopy-energy-dispersive X-ray and by Fourier transform infrared spectroscopy showed that the surface ligands of bacterial wall are probably responsible for biosorption and bioreduction process. These results suggest a potential application of B. paraconglomeratum ER41 in bioremediation of polluted discharges.

Efficient removal of mercury and chromium from wastewater via biochar fabricated with steel slag: Performance and mechanisms

Biochar derived from biomass is regarded as a promising adsorbent for wastewater treatment, but the high cost of modification is still a challenge for its large-scale practical applications. In this study, we employed steel slag as a low-cost fabricant and synthesized hydrothermally carbonized steel slag (HCSS), as a stable environmentally functional material for heavy metal removal. Typically, positively and negatively charged heavy metal contaminants of Hg²⁺ and Cr₂O₇²⁻ were employed to testify the performance of HCSS as an adsorbent, and good capacities [(283.24 mg/g for Hg (II) and 323.16 mg/g for Cr (VI)] were found. The feasibility of HCSS on real wastewater purification was also evaluated, as the removal efficiency was 94.11% and 88.65% for Hg (II) and Cr (VI), respectively. Mechanism studies revealed that the modification of steel slag on bio-adsorbents offered copious active sites for pollutants. As expected, oxygen-containing functional groups in HCSS acted as the main contributor to adsorption capacity. Moreover, some reactive iron species (i.e., Fe²⁺) played an essential role in chemical reduction of Cr (VI). The adsorptive reactions were pH-dependent, owing to other more mechanisms, such as coprecipitation, ion-exchange, and electrostatic attraction. This promising recycling approach of biomass waste and the design of agro-industrial byproducts can be highly suggestive of the issues of resource recovery in the application of solid waste-derived environmentally functional materials for heavy metal remediation.

Sustainable enhancement of Cr(VI) bioreduction by the isolated Cr(VI)-resistant bacteria

Bioreduction of mobile Cr(VI) to sparingly soluble Cr(III) is an effective strategy for in situ remediations of Cr contaminated sites. The key of this technology is to screen Cr(VI)-resistant bacteria and further explore the sustainable enhancement approaches towards their Cr(VI) reduction performance. In this study, a total of ten Cr(VI)-resistant bacteria were isolated from a Cr(VI) contaminated site. All of them could reduce Cr(VI), and the greatest extent of Cr(VI) reduction (98%) was obtained by the isolated CRB6 strain. The isolated CRB6 was able to reduce structural Fe(III) in Nontronite NAu-2 to structural Fe(II). Compared with the slow bioreduction process, the produced structural Fe(II) can rapidly enhance Cr(VI) reduction. The resist dissolution characteristics of NAu-2 in the redox cycling may provide sustainable enhancement of Cr(VI) reduction. However, no enhancement on Cr(VI) bioreduction by the isolated CRB6 was observed in the presence of NAu-2, which was attributed to the inhibition of Cr(VI) on the electron transfer between the isolated CRB6 and NAu-2. AQDS can accelerate the electron transfer between the isolated CRB6 and NAu-2 as an electron shuttle in the presence of Cr(VI). Therefore, the combination of NAu-2 and AQDS generated a synergistic enhancement on Cr(VI) bioreduction compared with the enhancement obtained by NAu-2 and AQDS individually. Our results highlight that structural Fe(III) and electron shuttle can provide a sustainable enhancement of Cr(VI) reduction by Cr(VI)-reducing bacteria, which has great potential for the effective Cr(VI) in-situ remediation.

A Bacillus and Lysinibacillus sp. bio-augmented Festuca arundinacea phytoremediation system for the rapid decontamination of chromium influenced soil

Phytoremediation as an efficient and eco-friendly soil detoxification method has received widespread attention. In this study, two newly screened Chromium (Cr) reducing strains (Bacillus sp. AK-1 and Lysinibacillus sp. AK-5) were used to remediate Cr contaminated soil in conjunction with the application of hyperaccumulator tall fescue (Festuca arundinacea), thus establishing a soil Cr decontamination system. In this system, soil urease and dehydrogenase activities were increased, the malondialdehyde (MDA) contents in leaves of tall fescue were significantly decreased, while glutathione (GSH) contents increased. In terms of Cr fractions, the proportion of acetic acid extractable Cr decreased by 12.82-20.00% in treatment groups, respectively, compared with CK, while residual Cr increased by 9.41-22.37%. Moreover, biomass, root length and shoot length of tall fescue in treatment groups increased by 80.77-139.74%, 60.85-68.04%, 7.06-27.10%, respectively. In addition, the root system of tall fescue accumulated 303.887-372.167 mg kg^-1 of Cr, and the aboveground part accumulated 16.289-19.289 mg kg^-1 of Cr. Therefore, the application of strains AK-1 and AK-5 reduced the toxicity of Cr to plants and greatly increased plant accumulation potential, which indicated that AK-1 and AK-5 could improve removal efficiency of phytoremediation in Cr contaminated soil by reducing its bio-toxicity and promoting growth of tall fescue growth.

Evaluation of Cr(VI) Reduction Using Indigenous Bacterial Consortium Isolated from a Municipal Wastewater Sludge: Batch and Kinetic Studies

Hexavalent Chromium (Cr(VI)) has long been known to be highly mobile and toxic when compared with the other stable oxidation state, Cr(III). Cr(VI)-soluble environmental pollutants have been detected in soils and water bodies receiving industrial and agricultural waste. The reduction of Cr(VI) by microbial organisms is considered to be an environmentally compatible, less expensive and sustainable remediation alternative when compared to conventional treatment methods, such as chemical neutralization and chemical precipitation of Cr. This study aims to isolate and identify the composition of the microbial consortium culture isolated from waste activated sludge and digested sludge from a local wastewater treatment plant receiving high loads of Cr(VI) from an abandoned chrome foundry in Brits (North Waste Province, South Africa). Furthermore, the Cr(VI) reduction capability and efficiency by the isolated bacteria were investigated under a range of operational conditions, i.e., pH, temperature and Cr(VI) loading. The culture showed great efficiency in reduction capability, with 100% removal in less than 4 h at a nominal loading concentration of 50 mg Cr(VI)/L. The culture showed resilience by achieving total removal at concentrations as high as 400 mg Cr(VI)/L. The consortia exhibited considerable Cr(VI) removal efficiency in the pH range from 2 to 11, with 100% removal being achieved at a pH value of 7 at a 37 ± 1 °C incubation temperature. The time course reduction data fitted well on both first and second-order exponential rate equation yielding first-order rate constants in the range 0.615 to 0.011 h−1 and second order rate constants 0.0532 to 5 × 10−5 L·mg−1·h−1 for Cr(VI) concentration of 50–400 mg/L. This study demonstrated the bacterial consortium from municipal wastewater sludge has a high tolerance and reduction ability over a wide range of experimental conditions. Thus, show promise that bacteria could be used for hexavalent chromium remediate in contaminated sites.

Differential transformation mechanisms of exotic Cr(VI) in agricultural soils with contrasting physio-chemical and biological properties

The transformation mechanisms of Cr(VI) in agricultural soils at the molecular level remain largely unknown due to the multitude of abiotic and biotic factors. In this study, the different speciation and distribution of Cr in two types of agricultural soil (Ultisol and Fluvo-aquic soils) after two weeks of aging was investigated using synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy, microfocused X-ray fluorescence (μ-XRF) and X-ray transmission microscopy (STXM). The microbial community structure of the two soils was also analyzed via high-throughput sequencing of 16S rRNA. Cr(VI) availability was relatively lower in the Ultisol than in the Fluvo-aquic soil after aging. Cr K-edge bulk XANES and STXM analysis indicated that Cr(VI) was reduced to Cr(III) in both soils. μ-XRF analysis and STXM analysis indicated the predominant association of Cr with Mn/Fe oxides and/or organo-Fe oxides in both soils. Additionally, STXM-coupled imaging and multiedge XANES analyses demonstrated that carboxylic groups were involved in the reduction of Cr(VI) and subsequent retention of Cr(III). 16S rRNA analysis showed considerably different bacterial communities across the two soils. Redundancy analysis (RDA) suggested that soil properties, including the total carbon content, Fe oxide component and pH, were closely linked to Cr(VI)-reducing functional bacteria in the Ultisol, including chromium-reducing bacteria (CRB) (e.g., Bacillus sp.) and dissimilatory iron-reducing (DIRB) (e.g., Shewanella sp.) bacteria, which possibly promoted Cr(VI) reduction. These findings shed light on the molecular-level transformation mechanisms of Cr(VI) in agricultural soils, which facilitates the effective management of Cr-enriched farmland.

The shuttling effects and associated mechanisms of different types of iron oxide nanoparticles for Cu(II) reduction by Geobacter sulfurreducens

Iron oxide nanoparticles (IONPs), commonly occurring in soils, aquifers and subsurface sediments, may serve as important electron shuttles for the biotransformation of coexisting toxic metals. Here, we explored the impact of different IONPs (low-crystallinity goethite and ferrihydrite, high-crystallinity magnetite and hematite) on the reduction of Cu(II) by Geobacter sulfurreducens and the associated electron shuttle mechanisms. All four IONPs tested can function as electron shuttles to enhance long distance electron transfer from bacteria to Cu(II). Upon IONPs addition, the rate of Cu(II) reduction increased from 14.9 to 65.0-83.8 % in solution after 7 days of incubation. Formation of both Cu(I) and Cu(0) on the iron oxide nanoparticles was revealed by the X-ray absorption near-edge spectroscopy. The IONPs can be utilized as conduits by bacteria to directly transfer electrons and they can also reversibly accept and donate electrons as batteries through a charging-discharging cycle to transfer electron. The latter mechanism (geo-battery) played an important role in all four types of IONPs while the former one (geo-conductor) can only be found in the magnetite and hematite treatments due to the higher crystallinity. Our results shed new light on the biogeochemically mediated electron flux in microbe-IONPs-metal networks under anaerobic iron-reduction conditions.

Bioreduction and biosorption of Cr(VI) by a novel Bacillus sp. CRB-B1 strain

This study reported an efficient novel chromium reducing bacteria (Bacillus sp. CRB-B1) and investigated its removal mechanism. Bacillus sp. CRB-B1 could effectively reduce high level Cr(VI), under a wide range of shaking velocity (125-200 rpm), temperature (33-41 °C), pH (6-9). The co-existing ions Cd2+ and NO3- inhibited its Cr(VI) reduction capacity, while Cu2+ enhanced the reduction efficiency. In addition, Bacillus sp. CRB-B1 could reduce Cr(VI) using glucose and fructose as an electron donor. Micro-characterization analysis confirmed the Cr(VI) reduction and adsorption ability of Bacillus sp. CRB-B1. Cells degeneration result indicated that Cr(VI) removal was mainly bioreduction rather than biosorption. The cell-free suspension had a Cr(VI) removal rate of 68.5.%, which was significantly higher than that of cell-free extracts and cell debris, indicating Cr(VI) reduction mainly occurs extracellularly, and possibly mediated by extracellular reductase. The reduced Cr was mainly distributed in the extracellular suspension, and a small amount was accumulated in the cells. In conclusion, Bacillus sp. CRB-B1 was a highly efficient Cr(VI) reducing bacteria, which has potential in the remediation of Cr(VI)-containing water and soil.

Remediation of hexavalent chromium contaminated soil by nano-FeS coated humic acid complex in combination with Cr-resistant microflora

A novel nano-composite material (CMC-FeS@HA) combining the advantages of humic acid (HA) and FeS was synthesized to remediate hexavalent chromium (Cr(VI)) contaminated soil along with chromium (Cr) resistant microflora. The characteristic analysis confirmed the successful synthesis of the nano-composite, which provided further mechanism evidence of its detoxification effect on polluted soil. Energy Dispersive System analysis proved the adsorption of the microbe consortium (MC) for Cr. After remediation, Cr(VI) in all treatments was dramatically reduced and the leachable Cr in soil treated by CMC-FeS@HA and MC decreased 89.14% compared with control. The result of BCR sequential extraction showed that Cr was stabilized, whose form changed to oxidizable and residual from HOAC-extractable. Besides, CMC-FeS@HA, as a sustained-release acid with high biocompatibility, could continuously decrease the pH of strongly alkaline soil and created a suitable micro-ecological environment for soil microorganisms. Moreover, CMC-FeS@HA dramatically improved soil physicochemical property, soil microbial activity (dehydrogenase, hydrolase, urease, and invertase activities), and soil microecological diversity. In total, this study provided a useful technology for soil remediation, which innovatively combined chemical remediation and microbial-remediation with a positive effect on soil quality, providing a good approach for the multiple technology combination in the environmental cause.

Feasibility and mechanism of microbial-phosphorus minerals-alginate immobilized particles in bioreduction of hexavalent chromium and synchronous removal of trivalent chromium

Chromium(VI) contaminated groundwater has become an increasingly prominent problem due to its extensive application in industry. Based on the easy-loss defect of microbial in practical application and previous research on the coupling enhancement of Cr(VI) bioreduction by phosphorus minerals, Microbial-Phosphorus minerals-Alginate (MPA) immobilized particles were proposed and investigated in this study. The feasibility of MPA immobilized particles were proved, with the higher reduction efficiency, lower phosphorus surplus, significant 94% of total Cr reduction and 85% of intragranular fixation. These superiorities were also obtained at different pH and initial Cr(VI) concentration conditions. Furthermore, the mechanisms of the enhancement of MPA were investigated from microbial level (microbial biomass, antioxidase, gene expression and microbial community analysis) and physics level (adsorption kinetic and isotherm), where the speculation that the reduction mainly took place outside the particles was proposed. This research provides a new approach for the practical application of Cr(VI)-contaminated groundwater in-situ bioremediation.