Metabolic pathway of Cr(VI) reduction by bacteria: A review

Mitigating chromate toxicity through concurrent denitrification in the H2-based membrane biofilm reactor

High concentrations of hexavalent chromium (Cr(VI)) in industrial wastewaters pose significant environmental and health hazards. Biotranformation is a viable means to lower Cr(VI) toxicity, but research to date has focused on wastewaters with low concentrations (e.g., 2-5 mg/L Cr(VI)). This study evaluated the dynamics of biosorption and biotransformation of higher-concentration Cr(VI) by biofilms in the H2-based membrane biofilm reactor (MBfR). While the biofilm in an MBfR receiving Cr(VI) alone had limited capacity to remove Cr(VI) and Cr(VI) removal ceased in 30 days, an autotrophic denitrifying biofilms achieved 99 % reduction of over 20 mg/L Cr(VI) to less-toxic trivalent chromium (Cr(III)) in continuous long-term operation system over 4 months. Increasing the H2 pressure from 3 psig to 10 psig improved Cr(VI) removal from 87 % to 99 %, which occurred in parallel with over 95 % NO3- reduction to N2. Metagenomic analyses revealed the mechanisms of Cr(VI) bioreduction and highlighted the beneficial role of nitrate (NO3-) as the primary electron acceptor. For example, nitrite reductase NrfA could reduce Cr(VI), which lowered Cr(VI) caused oxidative stress. This research demonstrates the MBfR's effectiveness in reducing elevated levels of Cr(VI) and provides mechanistic understanding of the roles of denitrification in accelerating Cr(VI) reduction and detoxification.

Potential application of Staphylococcus devriesei MS as a biosorbent agent for manganase, chromium, and cadmium heavy metals in contaminated water

This study identified one bacterial isolate as Staphylococcus devriesei, which is resistant to cadmium (Cd), manganese (Mn), and chromium (Cr) using 16S rRNA gene sequencing. Following that, the strain sequence was submitted to GenBank under accession number PQ013181. In this investigation, the biosorption potential of Staphylococcus devriesei was evaluated for the biosorption of chrmoium, cadmium, and manganese ions. The effects of pH, contact time, and initial concentration were examined in a batch-mode study. According to our findings, after 6 h at the ideal pH, Staphylococcus devriesei's maximal biosorption capabilities of Cr and Cd were 98 and 81.2%, respectively. The maximum biosorption of Mn was 95.6% after 24 h at pH 6. SEM micrographs showed that, Staphylococcus devriesei were irregular and cracked with wrinkles on the surface after absorbing the studied Cr metal ions. It was observed that the alterations in cell size occurred when the bacterium was exposed to a dose of Mn and the aggregation of cells was seen. Bacterial cells treated with Cd exhibited irregularities, featuring depressions on their surfaces, and surface wrinkles. FTIR analysis showed obvious alterations in peak positions and intensities before and after the biosorption process. Energy dispersive X-ray analysis showed extra metal depositions on the treated cell surface compared to the control. At the ultrastructural level, TEM imaging demonstrates the involvement of extracellular and intracellular precipitates and accumulated metals on the cell walls. Thus, the results of this study indicated that Staphylococcus devriesei can effectively aid in the remediation of contaminated water with moderate to light levels of Cd, Cr, and Mn.

Enhancing hexavalent chromium stable reduction via sodium alginate encapsulation of newly isolated fungal and bacterial consortia

Chromium [Cr(VI)]-induced soil pollution is a serious environmental threat. Bioremediation utilizes specific microbes capable of transforming Cr(VI) into the less toxic Cr(III), however, microbial efficacy can be inhibited by elevated pollutant concentrations and competition from indigenous microbial communities. Thus, this study explored the potential of single and multi-domain microbial consortia encapsulated in alginate to overcome these shortcomings. The results revealed that (i) fungal treatments demonstrated an elevated tolerance and reduction ability for Cr(VI) compared to bacterial treatments; (ii) combined application of fungi and bacteria was more effective in degrading Cr(VI) in soil compared to the individual treatments; (iii) microbial encapsulation improved microbial response to Cr(VI) toxicity thereby increasing their lifespan and Cr(VI) degrading ability; (iv) microbial consortia significantly decreased soil pH, electrical conductivity, and redox potential while simultaneously increasing soil enzyme activities (urease, sucrase, phosphatase, catalase, and laccase); and (v) The improved tolerance in the inoculated treatment resulted in increased microbial diversity and a substantial variation in microbial community structures, with 10,753 bacterial and 2697 fungal amplicon sequence variants identified across the treatment groups. This study underscores the critical importance of microbial diversity in bioremediation, emphasizing that encapsulation with the right material could improve the effectiveness of environmental remediation strategies.

Black soldier fly larvae mediate Zinc and Chromium transformation through the ZnuCBA and citric acid cycle system

Intestinal microbiota and metal regulatory proteins (MRPs) underlie the transformation of heavy metals (HMs) by the black soldier fly larvae (BSFL), but the mechanisms involved are still not fully defined. Here, using 16S rRNA and metagenomics-assisted tracing, we found that zinc (Zn) and chromium (Cr) stress led to enrichment of Proteobacteria in the BSFL intestine. Support of Proteobacteria also led to increased levels of the Zn transporter proteins ZnuC/B/A and the Zn efflux proteins zntR/A. Meanwhile, the genes MltE, CitT, and SLT, which mediate the citric acid cycle, were also significantly up-regulated and involved in the cellular uptake of Cr. Although Zn and Cr stress affected the expression of antibiotic resistance genes and pathogenic genes, the BSFL intestine tended to form stable microbial communities (MCs) to transform HMs through a mechanism driven by ZupT and chrA. In addition, the expression of SCARB1 and LdcA was significantly down-regulated by acute HMs stimulation, but BSFL were still able to complete the life cycle. Therefore, we determined the protective role of MCs and MRPs on BSFL during the transformation of HMs.

Single-Chamber Microbial Fuel Cell with an Innovative Sensing Component for Real-Time Continual Monitoring of a Wide Range of Cr(VI) Concentrations in Wastewater

Hexavalent chromium (Cr(VI)) is toxic, carcinogenic, and harmful to biological systems. Common detection methods, such as colorimetry, atomic absorption spectrometry, ion chromatography, and biological systems, can only be used in the laboratory and do not provide real-time feedback. To address these limitations, the current study cloned the ChrB gene, which exhibits high specificity in detecting Cr(VI), and the ChrA gene, which exhibits high Cr(VI) tolerance, into Escherichia coli. This recombinant strain, ChrA-ChrB-E. coli, was integrated into a single-chamber microbial fuel cell for accurate continual monitoring over a wide range of Cr(VI) concentrations. ChrA-ChrB-E. coli thrived in temperatures from 25 °C to 45 °C and pH levels between 5 and 8. Its ability to reduce Cr(VI) remained consistent across Cr(VI) forms, carbon sources, and oxyanions. Cyclic voltammetry was employed to verify the electrical activity of the biosensor. The biosensor exhibited a detection limit of 0.0075 mg/L. Under conditions simulating the regulatory emission limit for Cr(VI) of 0.5 mg/L in industrial wastewater, the biosensor achieved a response time of 20 s during continual operation. When tested with synthetic wastewater containing Cr(VI) concentrations from 0.02 to 150 mg/L, the system exhibited high adaptability and facilitated stable monitoring (relative standard deviation ≤ 2.7%). Additionally, the biosensor's accuracy (-1.73% to 2.5%) matched that of traditional batch methods, highlighting its suitability for real-time Cr(VI) monitoring in aquatic environments.

Adsorption and catalytic reduction of hexavalent chromium based on nanomaterials: A review on metal, metallic oxide, metallic sulfide and carbon-based catalyst

Chromium (Cr) is widely recognized as a significant environmental contaminant and a major contributor to global pollution. As a result, there is a strong emphasis on developing effective methods for the removal and reduction of Cr(VI). This review examines various applications of nanomaterial catalysts, including metallic oxides, metals, metallic sulfides, and carbon-based materials. These materials encompass naturally occurring substances, synthetically produced compounds, and artificially modified forms, all of which typically exhibit favorable adsorption properties and catalytic activity. We systematically summarize the mechanisms of adsorption and catalytic reduction associated with these nanomaterials, including photocatalysis, electrocatalysis, and direct catalysis. Finally, we explore the future directions and prospects of nanomaterials in environmental remediation, highlighting the key challenges that must be addressed in this field.

Boron-dependent autoinducer-2-mediated quorum sensing stimulates the Cr(VI) reduction of Leucobacter chromiireducens CD49

Traditionally, abiotic factors such as pH, temperature, and initial Cr(VI) concentration have been undoubtedly recognized as the external driving forces that dramatically affect the microbial-mediated remediation of Cr(VI) pollutants. However, concentrating on whether and how the biological behaviors and metabolic activities drive the microbial-mediated Cr(VI) detoxification is a study-worthy but little-known issue. In this study, Leucobacter chromiireducens CD49 isolated from heavy-metal-contaminated soil was identified to tolerate 8000.0 mg/L Cr(VI), and reduce 92.7% of 100.0 mg/L Cr(VI) within 66 h. Kinetic models were developed to determine the arithmetic relationships between Cr(VI) concentration and reaction time, and X-ray photoelectron spectroscopy exhibited the co-occurrence of Cr(III) and Cr(VI) on the bacterial cell surface. Furthermore, an integrated genomic-transcriptomic study was employed to explore the genetic-level response of strain CD49 to Cr(VI) stress, and most differentially expressed genes in the Cr(VI)-treatment group were enriched in biological process-related pathways, especially in quorum sensing (QS). Under the optimal conditions based on Box-Behnken Design experiments, intriguingly, boron-dependent autoinducer-2 (AI-2)-mediated QS was stimulated after H3BO3 introduction to further improve the biofilm production, biomass, and Cr(VI) reduction efficiency of strain CD49. Additionally, significantly up-regulated expression of genes chrR, chrA, and luxS further indicated the positive effect of AI-2-mediated QS on Cr(VI) reduction. Collectively, the findings pioneeringly present a chain of evidence for QS-stimulated Cr(VI) reduction, which may provide a theoretical basis for future improvement of microbial-mediated Cr(VI) remediation.

Review of mayflies (Insecta Ephemeroptera) as a bioindicator of heavy metals and microplastics in freshwater

Heavy metal and microplastic pollutions are prevalent in freshwater ecosystems, with many freshwater bodies being contaminated by one or both of these pollutants. Recent studies reported extreme detections of Cd, Pb and Zn, high concentrations of Cr, Pb and Cu and microplastics acting as vectors of pollutants, including heavy metals. Mayflies can serve as bioindicators of heavy metal contamination in freshwater ecosystems because changes in their community structure, physiology, and behaviour can reflect and help predict the concentrations of metals in these environments. This review discusses the ecological alterations induced by tissue metal concentration in mayflies and other macroinvertebrates. As sensitive taxa to heavy metal contamination, mayflies can reflect the impacts of this pollution through their ethology and relationship to the substrate, highlighting issues such as eutrophication, alterations in community structure, inhibitory effects and sediment toxicity. Mayflies are also highly affected by microplastic exposure, which leads to ingestion, bioaccumulation, biomagnification, habitat and community alteration, behavioural changes, physiology alteration and toxicity. Mayflies bioindication metrics for assessing the impact of heavy metals and microplastics include the examination of community alteration, functional feeding behaviour, molecular structure, dietary and toxicity impacts, bioaccumulation and biomagnification and biomarkers. Current challenges for the utilization of mayflies as bioindicators include temporal variations in sensitivity, lack of universally recognised protocols and need for standardised protocols for microplastic analysis. Additionally, the applicability of mayflies as bioindicators may vary across different ecosystems, emphasising the need for selecting suitable indicators that align with the unique characteristics of the ecosystem.

Anchoring spinel cobalt and zinc ferrites on zeolite for highly synergic photocatalytic reduction of chromium (VI)

To tackle the challenges of increasing the efficiency of photocatalysts, a ternary magnetic heterojunction photocatalyst containing spinel cobalt and zinc ferrites, and zeolite (CZZ) was designed and fabricated. The physicochemical properties of the novel photocatalyst were verified using characterization techniques such as XRD, FT-IR, FE-SEM, EDS mapping, N2 adsorption-desorption, VSM, PL, and UV-Vis DRS. The CZZ photocatalyst exhibited a significant Cr (VI) reduction rate of 0.1535 min-1, which was 9.27, 5.37 and 3.58 times higher than those of single ZnFe2O4 nanoparticles (0.0166 min-1), CoFe2O4 nanoparticles (0.0286 min-1), and CoFe2O4-ZnFe2O4 (0.0428 min-1) respectively. CZZ showed an excellent reusability after three consecutive cycles of Cr(VI) reduction. The results from the experiments in different aqueous environments displayed that CZZ could be a promising photocatalyst to reduce Cr(VI) in the treatment of actual aqueous matrices. The present study not only provides a highly active catalytic system for the practical removal of Cr(VI) but also paves the way for the fabrication of high-performance heterojunction photocatalysts.

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.

Advances in mechanism for the microbial transformation of heavy metals: implications for bioremediation strategies

Heavy metals are extensively discharged through various anthropogenic activities, resulting in an environmental risk on a global scale. In this case, microorganisms can survive in an extreme heavy metal-contaminated environment via detoxification or resistance, playing a pivotal role in the speciation, bioavailability, and mobility of heavy metals. Therefore, studies on the mechanism for the microbial transformation of heavy metals are of great importance and can provide guidance for heavy metal bioremediation. Current research studies on the microbial transformation of heavy metals mainly focus on the single oxidation, reduction and methylation pathways. However, complex microbial transformation processes and corresponding bioremediation strategies have never been clarified, which may involve the inherent physicochemical properties of heavy metals. To uncover the underlying mechanism, we reclassified heavy metals into three categories based on their biological transformation pathways, namely, metals that can be chelated, reduced or oxidized, and methylated. Firstly, we comprehensively characterized the difference in transmembrane pathways between heavy metal cations and anions. Further, biotransformation based on chelation by low-molecular-weight organic complexes is thoroughly discussed. Moreover, the progress and knowledge gaps in the microbial redox and (de)methylation mechanisms are discussed to establish a connection linking theoretical advancements with solutions to the heavy metal contamination problem. Finally, several efficient bioremediation strategies for heavy metals and the limitations of bioremediation are proposed. This review presents a solid contribution to the design of efficient microbial remediation strategies applied in the real environment.

Dissecting the Simultaneous Extracellular/Intracellular Contributions to Cr(VI) Reduction under Aerobic and Anaerobic Conditions Using the Newly Isolating Cr(VI)-Reducing Bacterium of Pseudomonas sp. HGB10

Quantifying extracellular and intracellular contributions to Cr(VI) reduction is crucial for understanding bacterial Cr(VI)-reduction mechanisms. However, this contribution under different oxygen conditions remains largely unexplored. This study quantified the extracellular/intracellular contribution to aerobic and anaerobic Cr(VI) reduction using Pseudomonas sp. HGB10, an isolated Cr(VI)-reducing bacterium, as the experimental model. Interestingly, it was found that the lower anaerobic minimum inhibitory concentration (MIC) does not necessarily imply a lower anaerobic Cr(VI)-reduction rate for HGB10. For the initial Cr(VI) concentration of 20 mg L-1, the maximum anaerobic Cr(VI)-reducing rate reached 100%, while the aerobic counterpart was only 75%, even though the value of the aerobic MIC (400 mg L-1) is twice that of the anaerobic (200 mg L-1). Additionally, the calculated extracellular contributions to aerobic and anaerobic Cr(VI) reduction were 10.76% and 55.71%, respectively, while the intracellular contributions were 68.29% and 40.38%. The sum of extracellular and intracellular contributions to Cr(VI) reduction (79.05% and 96.09%) under aerobic and anaerobic conditions was nearly balanced with the corresponding maximum values despite minor relative errors. These results indicated that anaerobic Cr(VI) reduction mainly occurred extracellularly rather than intracellularly, which differs from the existing result. Overall, our findings provide new insights into bacterial Cr(VI) reduction.

The influence of urban environmental effects on the orchard soil microbial community structure and function: a case study in Zhejiang, China

The urban environmental effects can have multifaceted impacts on the orchard soil microbial community structure and function. To specifically study these effects, we investigated the soil bacterial and fungal community in the laxly managed citrus orchards using amplicon sequencing. Ascomycota demonstrated significant dominance within the citrus orchard soils. The increased presence of beneficial Trichoderma spp. (0.3%) could help suppress plant pathogens, while the elevated abundance of potential pathogenic fungi, such as Fusarium spp. (0.4%), might raise the likelihood of disorders like root rot, thereby hindering plant growth and resulting in reduced yield. Moreover, we observed significant differences in the alpha and beta diversity of bacterial communities between urban and rural soils (p < 0.001). Environmental surveys and functional prediction of bacterial communities suggested that urban transportation factors and rural waste pollution were likely contributing to these disparities. When comparing bacterial species in urban and rural soils, Bacillus spp. exhibited notable increases in urban areas. Bacillus spp. possess heavy metal tolerance attributed to the presence of chromium reductase and nitroreductase enzymes involved in the chromium (VI) reduction pathway. Our findings have shed light on the intricate interplay of urban environmental effects and root systems, both of which exert influence on the soil microbiota. Apart from the removal of specific pollutants, the application of Bacillus spp. to alleviate traffic pollution, and the use of Trichoderma spp. for plant pathogen suppression were considered viable solutions. The knowledge acquired from this study can be employed to optimize agricultural practices, augment citrus productivity, and foster sustainable agriculture.

Biosorption of chromium by live and dead cells of Bacillus nitratireducens isolated from textile effluent

Bacillus nitratireducens was isolated from textile effluent and showed high tolerance to chromium (Cr), reaching up to a 1000 mg/L MIC value. This research was aimed at utilizing biosorbents from live and dead cells of B. nitratireducens to remove Cr from an aqueous solution. A batch biosorption test was performed, and mechanisms analysis was approached by an adsorption-desorption test, SEM-EDS, and FTIR analysis. Cr removal by dead cells in 25, 50, and 100 mg/L of Cr were 58.99 ± 0.7%, 69.8 ± 0.2%, and 82.87 ± 0.11%, respectively, while that by live cells was 73.08 ± 1.9%, 80.27 ± 6.33%, and 86.17 ± 1.93%, respectively. Live cells showed significantly higher Cr removal and adsorption capacities as compared to dead cells. In all concentrations, absorption contributed more than adsorption to the Cr removal by both live and dead cells. Absorption of Cr was subjected to occur due to passive mechanisms in dead cells while involving some active mechanisms in live cells. SEM-EDS confirmed the detection of Cr on the cell surface, while FTIR revealed the shifting of some peaks after the biosorption test, suggesting interactions between Cr and functional groups. Further TEM analysis is suggested to be conducted as a future approach to reveal the inner structure of cells and confirm the involvement of absorption mechanisms.

Microbial innovations in chromium remediation: mechanistic insights and diverse applications

The ubiquity of hexavalent chromium (Cr(VI)) from industrial activities poses a critical environmental threat due to its persistence, toxicity and mutagenic potential. Traditional physico-chemical methods for its removal often entail significant environmental drawbacks. Recent advancements in remediation strategies have emphasized nano and bioremediation techniques as promising avenues for cost-effective and efficient Cr(VI) mitigation. Bioremediation harnesses the capabilities of biological agents like microorganisms, and algae to mitigate heavy metal contamination, while nano-remediation employs nanoparticles for adsorption purposes. Various microorganisms, including E. coli, Byssochlamys sp., Pannonibacter phragmitetus, Bacillus, Aspergillus, Trichoderma, Fusarium, and Chlorella utilize bioreduction, biotransformation, biosorption and bioaccumulation mechanisms to convert Cr(VI) to Cr(III). Their adaptability to different environments and integration with nanomaterials enhance microbial activity, offering eco-friendly solutions. The study provides a brief overview of metabolic pathways involved in Cr(VI) bioreduction facilitated by diverse microbial species. Nitroreductase and chromate reductase enzymes play key roles in nitrogen and chromium removal, with nitroreductase requiring nitrate and NADPH/NADH, while the chromium reductase pathway relies solely on NADPH/NADH. This review investigates the various anthropogenic activities contributing to Cr(VI) emissions and evaluates the efficacy of conventional, nano-remediation, and bioremediation approaches in curbing Cr(VI) concentrations. Additionally, it scrutinizes the mechanisms underlying nano-remediation techniques for a deeper understanding of the remediation process. It identifies research gaps and offers insights into future directions aimed at enhancing the real-time applicability of bioremediation methods for mitigating with Cr(VI) pollution and pave the way for sustainable remediation solutions.

Characteristics of iron (hydr)oxides and Cr(VI) retention mechanisms in soils from tropical and subtropical areas of China

Though both iron (hydr)oxides and soil organic matter (SOM) significantly influence heavy metal behaviors in soils, studies on the characteristics of natural minerals and the synergic effects of the two on Cr(VI) transformation are limited. This study investigated Cr(VI) retention mechanisms in four soils from tropical and subtropical regions of China based on a comprehensive characterization of Fe (hydr)oxides. These soils exhibited varying quantities of hematite, ferrihydrite and goethite, with distinct Al substitution levels and varied exposed crystallographic facets. Adsorption experiments revealed a positive correlation between Fe (hydr)oxide content and Cr(VI) fixation amount on colloid, which was influenced by the mineral types, Al substitution levels and facet exposures. Further, Cr(VI) was sequestered on soil by adsorption and reduction. In soils enriched with crystalline Fe (hydr)oxides, Cr(VI) reduction was primarily governed by SOM, while in soils enriched with poorly crystalline Fe (hydr)oxides, mineral-associated Fe(II) also contributed to Cr(VI) reduction. Aging experiments demonstrated that SOM and mineral-associated Fe(II) expedited Cr (VI) passivation and diminished the Cr leaching. These results improve our understanding of natural Fe (hydr)oxide structures and their impact on Cr(VI) behavior in soils, and shed light on complex soil-contaminant interactions and remediation of Cr(VI) polluted soils.

Screening of cadmium-chromium-tolerant strains and synergistic remediation of heavy metal-contaminated soil using king grass combined with highly efficient microbial strains

The present study involved the isolation of two cadmium (Cd) and chromium (Cr) resistant strains, identified as Staphylococcus cohnii L1-N1 and Bacillus cereus CKN12, from heavy metal contaminated soils. S. cohnii L1-N1 exhibited a reduction of 24.4 % in Cr6+ and an adsorption rate of 6.43 % for Cd over a period of 5 days. These results were achieved under optimal conditions of pH (7.0), temperature (35 °C), shaking speed (200 rpm), and inoculum volume (8 %). B. cereus strain CKN12 exhibited complete reduction of Cr6+ within a span of 48 h, while it demonstrated a 57.3 % adsorption capacity for Cd over a period of 120 h. These results were achieved under conditions of optimal pH (8.0), temperature (40 °C), shaking speed (150 rpm), and inoculum volume (2-3 %). Additionally, microcharacterization and ICP-MS analysis revealed that Cr and Cd were accumulated on the cell surface, whereas Cr6+ was mainly reduced extracellularly. Subsequently, a series of pot experiments were conducted to provide evidence that the inclusion of S. cohnii L1-N1 or B. cereus CKN12 into the system resulted in a notable enhancement in both the plant height and biomass of king grass. In particular, it was observed that the presence of S. cohnii L1-N1 or B. cereus CKN12 in king grass led to a significant reduction in the levels of Cd and Cr in the soils (36.0 % and 27.8 %, or 72.9 % and 47.4 %, respectively). Thus, the results of this study indicate that the combined use of two bacterial strains can effectively aid in the remediation of tropical soils contaminated with moderate to light levels of Cd and Cr.

Deciphering the impacts of chromium contamination on soil bacterial communities: A comparative analysis across various soil types

Addressing the widespread concern of chromium (Cr) pollution, this study investigated its impacts on bacterial communities across eight soil types, alongside the potential Cr transformation-related genes. Utilizing real-time PCR, 16S rRNA gene sequencing and gene prediction, we revealed shifts in bacterial community structure and function at three Cr exposure levels. Our results showed that the bacterial abundance in all eight soil types was influenced by Cr to varying extents, with yellow‒brown soil being the most sensitive. The bacterial community composition of different soil types exhibited diverse responses to Cr, with only the relative abundance of Proteobacteria decreasing with increasing Cr concentration across all soil types. Beta diversity analysis revealed that while Cr concentration impacted the assembly process of bacterial communities to a certain extent, the influence on the compositional structure of bacterial communities was primarily driven by soil type rather than Cr concentration. The study also identified biomarkers for each soil type under three Cr levels, offering a basis for monitoring changes in Cr pollution. By predicting crucial functional genes related to Cr transformation, it was observed that the relative abundance of chrA (chromate transporter) in yellow‒brown soil significantly exceeded that in all other soil types, suggesting its potential for Cr adaptation. The study also revealed correlations among soil physicochemical properties, Cr concentration, and these functional genes, providing a foundation for future research aimed at more precise functional analysis and the development of effective soil remediation strategies.

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.

A review of the treatment technologies for hexavalent chromium contaminated water

The toxicity of hexavalent chromium (Cr(VI)) present in the environment has exceeded the current limits or standards and thus may lead to biotic and abiotic catastrophes. Accordingly, several treatments, including chemical, biological, and physical approaches, are being used to reduce Cr(VI) waste in the surrounding environment. This study compares the Cr(VI) treatment approaches from several areas of science and their competence in Cr(VI) removal. As an effective combination of physical and chemical approaches, the coagulation-flocculation technique removes more than 98% of Cr(VI) in less than 30 min. Most membrane filtering approaches can remove up to 90% of Cr(VI). Biological approaches that involve the use of plants, fungi, and bacteria also successfully eliminate Cr(VI) but are difficult to scale up. Each of these approaches has its benefits and drawbacks, and their applicability is determined by the research aims. These approaches are also sustainable and environmentally benign, thus limiting their effects on the ecosystem.

Effective Cr(VI) reduction and immobilization in chromite ore processing residue (COPR) contaminated soils by ferrous sulfate and digestate: A comparative investigation with typical reducing agents

Chemical reduction combined with microbial stabilization is a green and efficient method for the remediation of hexavalent chromium (Cr(VI)) contaminated soil. In this study, the combination of ferrous sulfate with kitchen waste digestate was applied to reduce and immobilize Cr(VI) in chromite ore processing residue (COPR) contaminated soils, and systematically evaluated the remediation performance of Cr(VI) compared with several typical reducing agents (i.e., ferrous sulfate, zero valent iron, sodium thiosulfate, ferrous sulfide, and calcium polysulfide). The results showed that the combination of ferrous sulfate and digestate had superior advantages of a lower dosage of reducing agent and a long-term remediation effect compared to other single chemical reductants. Under an Fe(II):Cr(VI) molar ratio of 3:1% and 4% digestate (wt), the content of Cr(VI) in the soil decreased to 5.07 mg/kg after 60 days of remediation. Meanwhile, the leaching concentrations of Cr(VI) were below detection limit, which can meet the hazardous waste toxicity leaching standard. The risk level of Cr pollution was decreased from very high risk to low risk. The X-ray photoelectron spectroscopy (XPS) results further demonstrated that the combined treatments were beneficial to Cr(VI) reduction and stabilization. The abundance of bacteria with Cr(VI) reducing ability was higher than other treatments. Moreover, the high abundance of carbon and nitrogen metabolism in the combined treatments demonstrated that the addition of digestate was beneficial to the recovery and flourishing of Cr(VI)-reducing related microorganisms in COPR contaminated soils. This work provided an alternative way on Cr(VI) remediation in COPR contaminated soils.

Influencing factors and mechanism of Cr(VI) reduction by facultative anaerobic Exiguobacterium sp. PY14

Microbial reduction is an effective way to deal with hexavalent chromium [Cr(VI)] contamination in the environment, which can significantly mitigate the biotoxicity and migration of this pollutant. The present study investigated the influence of environmental factors on aqueous Cr(VI) removal by a newly isolated facultative anaerobic bacterium, Exiguobacterium sp. PY14, and revealed the reduction mechanism. This strain with a minimum inhibitory concentration of 400 mg/L showed the strongest Cr(VI) removal capacity at pH 8.0 because of its basophilic nature, which was obviously depressed by increasing the Cr(VI) initial concentration under both aerobic and anaerobic conditions. In contrast, the removal rate constant for 50 mg/L of Cr(VI) under anaerobic conditions (1.82 × 10-2 h-1) was 3.3 times that under aerobic conditions. The co-existence of Fe(III) and Cu(II) significantly promoted the removal of Cr(VI), while Ag(I), Pb(II), Zn(II), and Cd(II) inhibited it. Electron-shuttling organics such as riboflavin, humic acid, and anthraquinone-2,6-disulfonate promoted the Cr(VI) removal to varying degrees, and the enhancement was more significant under anaerobic conditions. The removal of aqueous Cr(VI) by strain PY14 was demonstrated to be due to cytoplasmic rather than extracellular reduction by analyzing the contributions of different cell components, and the end products existed in the aqueous solution in the form of organo-Cr(III) complexes. Several possible genes involved in Cr(VI) metabolism, including chrR and chrA that encode well-known Chr family proteins responsible for chromate reduction and transport, respectively, were identified in the genome of PY14, which further clarified the Cr(VI) reduction pathway of this strain. The research progress in the influence of crucial environmental factors and biological reduction mechanisms will help promote the potential application of Exiguobacterium sp. PY14 with high adaptability to environmental stress in Cr(VI) removal in the actual environment.