Recent bioreduction of hexavalent chromium in wastewater treatment: A review

Exploration on the Cr(VI) resistance mechanism of a novel thermophilic Cr(VI)-reducing bacteria Anoxybacillus flavithermus ABF1 isolated from Tengchong geothermal region, China

Hexavalent chromium resistance and reduction mechanisms of microorganism provide a critical guidance for Cr(VI) bioremediation. However, related researches are limited in mesophiles and deficient for thermophiles. In this work, a novel alkaline Cr(VI)-reducing thermophile Anoxybacillus flavithermus ABF1 was isolated from geothermal region. The mechanisms of Cr(VI) resistance and reduction were investigated. The results demonstrated that A. flavithermus ABF1 could survive in a wide temperature range from 50°C to 70°C and in pH range of 7.0-9.0. Strain ABF1 showed excellent growth activity and Cr(VI) removal performance when initial Cr(VI) concentration was lower than 200 mg L^-1 . 93.71% of Cr(VI) was removed at initial concentration of 20 mg L^-1 after 72 h. The majority of Cr(VI) was found to be reduced extracellularly by enzymes secreted by cells. XPS and Raman analysis further manifested that Cr₂ O₃ was the product of Cr(VI) reduction. Moreover, the Cr(VI) transportation-related gene cysP and Cr(VI) reduction-related gene azoR of A. flavithermus ABF1 played key roles in inhibiting Cr(VI) entering cells and promoting extracellular Cr(VI) reduction respectively. This work provides novel insight into the mechanisms of Cr(VI) resistance and detoxication of thermophiles, which leads to a promising alternative strategy for heavy metal bioremediation in areas with elevated temperature.

Enhancement of hexavalent chromium reduction by Shewanella oneidensis MR-1 in presence of copper nanoparticles via stimulating bacterial extracellular electron transfer and environmental adaptability

The bioreduction of hexavalent chromium (Cr(VI)) depends highly on bacterial activity, while the release of copper nanoparticles (Cu NPs) poses threats to microorganisms in the environment. This work demonstrated that Cr(VI) reduction efficiency of Shewanella oneidensis MR-1 was remarkably enhanced by 83.7% under 20 mg/L Cu NPs exposure. Cu NPs improved the electron migration capacity of Shewanella oneidensis MR-1 by enhancing bioelectrochemical performance and flavin mononucleotide secretion. Moreover, key genes related to extracellular electron transfer pathways, including direct electron transfer through outer-membrane proteins, flavin-mediated electron transfer, and conductive flagella, were generally upregulated under Cu NPs exposure. In addition, environmental adaptability of Shewanella oneidensis MR-1 was enhanced under Cu NPs exposure by improving environmental information processing and energy and reducing power production, promoting Cr(VI) reduction by Shewanella oneidensis MR-1. This work indicated that Cu NPs could enhance Cr(VI) reduction by Shewanella oneidensis MR-1 through regulating extracellular electron transfer and environmental adaptability.

Interaction of peracetic acid with chromium(III): Understanding degradation of coexisting organic pollutants in water

Peracetic acid (PAA, CH₃C(O)OOH) has gained significant attention for its use in wastewater disinfection. Wastewater usually contains both metal ions and organic pollutants and understanding reactions after adding PAA to such contaminated water is needed. This paper presents results regarding the effect of interactions between chromium(III) (Cr(III)) and PAA on the degradation of selected pharmaceuticals, mainly trimethoprim (TMP). The degradation of pharmaceuticals by PAA, PAA-Cr(III), and H₂O₂-Cr(III) under different conditions was examined (pH = 6.0-10.0 and molar ratios of PAA to Cr(III)). The degradation rate of TMP by PAA-Cr(III) was greater than by PAA and H₂O₂-Cr(III) under alkaline conditions. Degradation studies using quenching agents and probing molecules, and spectroscopic measurements (UV-visible and electron paramagnetic resonance) suggest ^•OH as the major radical species and Cr(IV)/Cr(V) as additional reactive species. The oxidized products of TMP by PAA-Cr(III) were identified and possible pathways proposed. Degradation of other pharmaceuticals having different molecular structures by PAA-Cr(III) and H₂O₂-Cr(III) systems were also investigated. Most of the pharmaceuticals degraded at faster rates by PAA-Cr(III) and H₂O₂-Cr(III) than by PAA alone, suggesting that co-present metal ions may play a significant role in PAA oxidation in water treatment.

Bromate reduction by Shewanella oneidensis MR-1 is mediated by dimethylsulfoxide reductase

Microbial bromate reduction plays an important role in remediating bromate-contaminated waters as well as biogeochemical cycling of bromine. However, little is known about the molecular mechanism of microbial bromate reduction so far. Since the model strain Shewanella oneidensis MR-1 is capable of reducing a variety of oxyanions such as iodate, which has a high similarity to bromate, we hypothesize that S. oneidensis MR-1 can reduce bromate. Here, we conducted an experiment to investigate whether S. oneidensis MR-1 can reduce bromate, and report bromate reduction mediated by a dimethylsulfoxide reductase encoded with dmsA. S. oneidensis MR-1 is not a bromate-respiring bacterium but can reduce bromate to bromide under microaerobic conditions. When exposed to 0.15, 0.2, 0.25, 0.5, and 1 mM bromate, S. oneidensis MR-1 reduced bromate by around 100, 75, 64, 48, and 23%, respectively, within 12 h. In vivo evidence from gene deletion mutants and complemented strains of S. oneidensis MR-1 indicates that MtrB, MtrC, CymA, GspD, and DmsA are involved in bromate reduction, but not NapA, FccA, or SYE4. Based on our results as well as previous findings, a proposed molecular mechanism for bromate reduction is presented in this study. Moreover, a genomic survey indicates that 9 of the other 56 reported Shewanella species encode proteins highly homologous to CymA, GspD, and DmsA of S. oneidensis MR-1 by sequence alignment. The results of this study contribute to understanding a pathway for microbial bromate reduction.

Reviewing variables and their implications affecting adsorption of Cr(VI) onto activated carbon: an in-depth statistical case study

Removal of Cr(VI) from the aqueous phase using numerous activated carbons (AC) has been broadly studied in the last decades. Nevertheless, the diversity of activation methods, AC properties, and adsorption conditions precludes the standardization of specific characteristics required to achieve better adsorption results. This work reviewed the pertinent literature on Cr(VI) adsorption onto AC published over the past four decades. Pearson's correlation matrix and principal component analysis (PCA) assisted in identifying the parameters and AC characteristics that have the greatest influence on the maximum adsorption capacity (q_m). Two hundred thirty-six adsorption assays were found reporting data on 110 ACs and different parameters. Of these, 39.8% of the studies contemplated the variables q_m, pH, temperature (T), surface area (S_BET), micropore volume (V_micro), and mesopore volume (V_meso), and only 19.5% reported the point of zero charge (pH_PZC). Statistical analysis disclosed that S_BET and V_micro have a strong positive correlation with q_m, while V_meso, T, and pH show little or no correlation. The difference between pH and pH_PZC (PZC_diff) indicated a significant anticorrelation with q_m, thus evidencing that lower PZC_diff values enhance adsorption. The findings are useful for all researchers that work with Cr(VI) adsorption on AC since they provide a start point concerning the required adsorbent characteristics and process conditions to be employed.

Extracellular electron transfer-dependent Cr(VI)/sulfate reduction mediated by iron sulfide nanoparticles

The slow electron transfer rate is a bottleneck to the biological wastewater treatment. This study evaluated the concomitant biotransformation and nonenzymatic reduction of Cr(VI) mediated by sulfate reducing bacteria (SRB), especially for the reinforcing Cr(VI) reduction via accelerating the electron transfer by the in-situ biosynthesized iron sulfide nanoparticles (FeS NPs). The kinetic results showed that 10 mg/L Cr(VI) was completely removed by pre-cultured FeS NPs within 7 h with k_Cr(VI) of 2.6 × 10^-4 s^-1, one magnitude higher than that without FeS NPs. Despite its competing electron to postpone sulfate reduction, the reduction of Cr(VI) was markedly improved via nonenzymatic reactions by the sulfide, the product of sulfate reduction. In the reinforcing system (bio-FeS NP@SRB), the bio-FeS NPs served as an electronic bypass conduit for CoQ could significantly amplify the electron flux, and switch the Cr(VI) reduction from intracellular space to extracellular environment, which had a great detoxification effect on the microorganisms, eventually markedly promoted electron transfer extracellularly and the reduction of Cr(VI). After the long-term acclimatization, Desulfovibrio became the dominant bacteria at the genus level and accounted for the relative abundance of 32%. This study provides an alternative to use biogenic FeS NPs for Cr(VI) remediation.

Integrated approaches to mitigate threats from emerging potentially toxic elements: A way forward for sustainable environmental management

Potentially toxic elements (PTEs) such as toxic metal (loid)s and other emerging hazardous contaminants, exist in the environment and poses a serious threat. A large amount of wastewater containing PTEs such as cadmium, chromium, copper, nickel, arsenic, lead, zinc, etc. Release from industries during production process. Besides these, chemical-based fertilizers used in soils during crop production have become one of the crucial sources of PTEs. Various techniques are being employed for the mitigation of PTEs like chemical precipitation, ion exchange, coagulation, activated carbon, adsorption, membrane filtration, and bioremediation. Among these mitigation strategies, biological processes such as bioremediation, phytoremediation etc. Are extensively used, as they are economic have high-efficiency rate and are eco-friendly. This review intends to provide information on PTEs contamination through various sources; along with the toxicity of metal (loid)s with respect to their patterns of transmission and risks in the changing environment. Various remediation methods for the management of these pollutants along with their techno-economic perspective are also summarized in this review.

Removal of hexavalent chromium from water by Z-scheme photocatalysis using TiO2 (rutile) nanorods loaded with Au core-Cu2O shell particles

All-solid-state Z-scheme photocatalysts, containing Cu₂O, TiO₂ (rutile), and Au as the electron mediator, were prepared and applied to the reduction of Cr(VI) in aqueous solutions. The Cu₂O-Au-TiO₂ composites were prepared by loading Au core-Cu₂O shell hemisphere particles on TiO₂ (rutile) nanorods using a two-step photocatalytic deposition process. Under ultraviolet-visible (UV-vis) light illumination, the Cu₂O-Au-TiO₂ composites exhibited higher photocatalytic Cr(VI) reduction activities than those exhibited by single TiO₂ (rutile) and Cu₂O. In this reaction, a precipitate containing Cr, which was considered to be Cr(OH)₃, was deposited site-selectively on the Au core-Cu₂O shell particles of the composites, indicating that the reduction site of the composite was Cu₂O, and the reaction proceeded according to the Z-scheme. The Cu₂O-Au-TiO₂ composites also exhibited photocatalytic activity under visible light illumination. The oxidation state of Cu in the Cu₂O-Au-TiO₂ composite gradually changed from Cu(I) to Cu(II) during the photocatalytic Cr(VI) reduction. However the composite maintained its high photocatalytic performance even after oxidation. The role of Au in the Cu₂O-Au-TiO₂ composite was examined by comparing the properties of the Cu₂O-Au-TiO₂ composite with those of the Cu₂O-TiO₂ composite prepared via direct Cu₂O deposition on TiO₂.

Practical limitations of bioaugmentation in treating heavy metal contaminated soil and role of plant growth promoting bacteria in phytoremediation as a promising alternative approach

Bioaugmentation, the addition of cultured microorganisms to enhance the currently existing microbial community, is an option to remediate contaminated areas. Several studies reported the success of the bioaugmentation method in treating heavy metal contaminated soil, but concerns related to the applicability of this method in real-scale application were raised. A comprehensive analysis of the mechanisms of heavy metal treatment by microbes (especially bacteria) and the concerns related to the possible application in the real scale were juxtaposed to show the weakness of the claim. This review proposes the use of bioaugmentation-assisted phytoremediation in treating heavy metal contaminated soil. The performance of bioaugmentation-assisted phytoremediation in treating heavy metal contaminated soil as well as the mechanisms of removal and interactions between plants and microbes are also discussed in detail. Bioaugmentation-assisted phytoremediation shows greater efficiencies and performs complete metal removal from soil compared with only bioaugmentation. Research related to selection of hyperaccumulator species, potential microbial species, analysis of interaction mechanisms, and potential usage of treating plant biomass after treatment are suggested as future research directions to enhance this currently proposed topic.

Remediation of Chromium (VI) from Groundwater by Metal-Based Biochar under Anaerobic Conditions

Iron salt-modified biochar has been widely used to remove Cr(VI) pollution due to the combination of the generated iron oxides and biochar, which can bring positive charge and rich redox activity. However, there are few comprehensive studies on the methods of modifying biochar with different iron salts. In this study, two iron salt (FeCl3 and Fe(NO3)3) modification methods were used to prepare two Fe-modified biochar materials for removing Cr(VI) in simulated groundwater environment. It was revealed by systematic characterization that FeCl3@BC prepared via the FeCl3 modification method, has larger pore size, higher zeta potential and iron oxide content, and has higher Cr(VI) adsorption-reduction performance efficiency as compared to Fe(NO3)3@BC prepared via Fe(NO3)3 modification method. Combined with XRD and XPS analyses, Fe3O4 is the key active component for the reduction of Cr(VI) to Cr(III). The experimental results have shown that acidic conditions promoted Cr(VI) removal, while competing ions (SO42− and PO43−) inhibited Cr(VI) removal by FeCl3@BC. The Elovich model and intra-particle diffusion model of FeCl3@BC can describe the adsorption behavior of Cr(VI) well, indicating that both the high activation energy adsorption process and intra-particle diffusion control the removal process of Cr(VI). The Freundlich model (R2 > 0.999) indicated that there were unevenly distributed chemisorptions centers on the FeCl3@BC surface. Stability experiments exposed that FeCl3@BC was stable under neutral, acidic, and alkaline conditions. Furthermore, the main mechanisms of FeCl3@BC removal of Cr(VI) include electrostatic adsorption, chemical reduction, ion exchange, and co-precipitation. In conclusion, our findings provide a new insight for the selection of iron salt-modified biochar methods, and will also be beneficial for the preparation of more efficient Fe-modified biochars in the future.

Synchronous Cr(VI) Remediation and Energy Production Using Microbial Fuel Cell from a Subsurface Environment: A Review

Applying microbial fuel cell (MFC) technology for eco-remediation of Cr(VI) pollution from a subsurface environment has great scientific value and practical significance due to its promising advantages of pollutant remediation and renewable energy generation. The aim of the current review is to summarize the migration characteristics of Cr(VI) in a subsurface soil/water environment and investigate the factors affecting the MFC performance for synchronous Cr(VI) remediation and power generation, and sequentially highlight diverse challenges of MFC technology for in situ remediation of subsurface groundwater and soils. The critical review put forward that Cr(VI) removal efficiency and energy production of MFC can be improved by enhancing the adjustability of cathode pH, setting potential, modifying electrode, and incorporating other technologies into MFC. It was recommended that designing typical large-scale, long-term continuous flow MFC systems, adding electron shuttle media or constructing artificial electron according to actual groundwater/soil and Cr(VI) pollution characteristics, site geology, and the hydrogeology condition (hydrochemical conditions, colloid type, and medium) are essential to overcome the limitations of the small size of the laboratory experiments and improve the application of technology to in situ Cr(VI) remediation. This review provided reference and ideas for future research of MFC-mediated onsite Cr(VI) remediation.

Biosorption of hazardous waste from the municipal wastewater by marine algal biomass

Lead is one of the highly toxic heavy metals causes various diseases even at very lower concentrations to human and affects eco-system. It is mainly released into the water through industrial activities. Phytoremediation is useful to degrade, reduce, metabolize and assimilate lead from wastewater. In this study, Turbinaria ornata was collected from the sea and dried biomass was used for biosorption of heavy metals. Adsorption of heavy metal was maximum after 100 min incubation with alga powder at acidic pH (4.5). The interactive effects of lead concentration, contact times, pH, biomass concentration and agitation speed was evaluated by a two-level full factorial design. Initial lead concentration, agitation speed and biomass concentration were the most important variables affecting lead removal (p < 0.001) were selected for optimization using central composite rotatable design. Lead removal was found to be maximum (99.8%) in optimized conditions: initial lead 99.8 mg/L, 250 rpm agitation speed and 16.2 g/L biomass concentrations. Municipal wastewater was collected and lead concentration (0.013 mg/L) and physiochemical factors were analyzed. Algal biomass removed >98.5% lead form the wastewater within 10 min in an optimized condition. The present study confirmed the potential application of T. ornata for the removal of lead from contaminated environment.

Process optimization and modeling of Cd2+ biosorption onto the free and immobilized Turbinaria ornata using Box-Behnken experimental design

The release of effluents containing cadmium ions into aquatic ecosystems is hazardous to humans and marine organisms. In the current investigation, biosorption of Cd2+ ions from aqueous solutions by freely suspended and immobilized Turbinaria ornata biomasses was studied. Compared to free cells (94.34%), the maximum Cd2+ removal efficiency reached 98.65% for immobilized cells obtained via Box-Behnken design under optimized conditions comprising algal doses of 5.04 g L-1 and 4.96 g L-1, pH values of 5.06 and 6.84, and initial cadmium concentrations of 25.2 mg L-1 and 26.19 mg L-1, respectively. Langmuir, Freundlich, and Temkin isotherm models were suitably applied, providing the best suit of data for free and immobilized cells, but the Dubinin-Radushkevich model only matched the immobilized algal biomass. The maximum biosorption capacity of Cd2+ ions increased with the immobilized cells (29.6 mg g-1) compared to free cells (23.9 mg g-1). The Cd2+ biosorption data obtained for both biomasses followed pseudo-second-order and Elovich kinetic models. In addition, the biosorption process is controlled by film diffusion followed by intra-particle diffusion. Cd2+ biosorption onto the free and immobilized biomasses was spontaneous, feasible, and endothermic in nature, according to the determined thermodynamic parameters. The algal biomass was further examined via SEM/EDX and FTIR before and after Cd2+ biosorption. SEM/EDX analysis revealed Cd2+ ion binding onto the algal surface. Additionally, FTIR analysis confirmed the presence of numerous functional groups (hydroxyl, carboxyl, amine, phosphate, etc.) participating in Cd2+ biosorption. This study verified that immobilized algal biomasses constitute a cost-effective and favorable biosorbent material for heavy metal removal from ecosystems.

Health hazards of hexavalent chromium (Cr (VI)) and its microbial reduction

Industrial effluents/wastewater are the main sources of hexavalent chromium (Cr (VI)) pollutants in the environment. Cr (VI) pollution has become one of the world’s most serious environmental concerns due to its long persistence in the environment and highly deadly nature in living organisms. To its widespread use in industries Cr (VI) is highly toxic and one of the most common environmental contaminants. Cr (VI) is frequently non-biodegradable in nature, which means it stays in the environment for a long time, pollutes the soil and water, and poses substantial health risks to humans and wildlife. In living things, the hexavalent form of Cr is carcinogenic, genotoxic, and mutagenic. Physico-chemical techniques currently used for Cr (VI) removal are not environmentally friendly and use a large number of chemicals. Microbes have many natural or acquired mechanisms to combat chromium toxicity, such as biosorption, reduction, subsequent efflux, or bioaccumulation. This review focuses on microbial responses to chromium toxicity and the potential for their use in environmental remediation. Moreover, the research problem and prospects for the future are discussed in order to fill these gaps and overcome the problem associated with bacterial bioremediation’s real-time applicability.

Highly efficient Cr(VI) removal from industrial electroplating wastewater over Bi2S3 nanostructures prepared by dual sulfur-precursors: Insights on the promotion effect of sulfate ions

In this work, different Bi₂S₃ nanostructures were prepared from various single and dual sulfide precursors via a solvothermal method. It was found that Bi₂S₃ nanostructures prepared from dual sulfur precursors of L-cysteine and ammonium sulfide exhibited highest Cr(VI) removal ability with maximum Cr(VI) removal capacity of 148.95 mg/g in Cr(VI) solution (pH = 2). More importantly, the removal capacity strikingly increased to 223.33 and 240.25 mg/g in two kinds of actual industrial electroplating wastewater. By analyzing the components of actual electroplating wastewater and the results of control experiments in the absence and presence of different ions in Cr(VI) solution, it was found that SO₄^2- played a critical role in the Cr(VI) removal over Bi₂S₃. The addition of SO₄^2- could promote the conversion of Cr(VI) to Cr(III) on the surface of Bi₂S₃, thus leading to the enhanced Cr(VI) removal ability in actual electroplating wastewater. The Bi₂S₃ maintained its original Cr(VI) removal ability after four cycles in the electroplating wastewater, indicating the moderate reuse ability of the sample. This work not only demonstrated an highly efficient nanomaterials for the Cr(VI) removal in industrial electroplating wastewater, but also provided an insight on the influence of the components in wastewater on Cr(VI) removal.

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

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

A review on recent advances in electrodeionization for various environmental applications

The growing contamination of ecosystems necessitates the development of long-term pollution-removal technologies. Electrodeionization, in notably, has newly proven as an efficient method for removing ionic chemicals from polluted waterways. The fact that continuous electrodeionization is a greener technique is most probably the biggest cause for its success. It replaces the toxic chemicals typically required to replenish resins with electric power, therefore eliminating the wastewater involved with resin renewal. In water treatment, electrodeionization solves some of the drawbacks of ion exchange resin beds, particularly ion dumping as beds expire. This comprehensive review explores the theory, principles, and mechanisms of ion movement and separation in an electrodeionization unit. Also, it investigated the construction and usage, notably in removing heavy metal and its current developments in electrodeionization unit. Recent advances in Electrodeionization like polarity reversal, Resin wafer Electrodeionization, membrane free Electrodeionization, and electrostatic shielding with novel materials and hybrid process along with Electrodeionization were addressed. Further advancements are expected in electrodeionization systems that exhibit better efficacy while running at lower costs due to decreased energy usage, rendering them appealing for industrial scale up across a wide range of applications across the world.

Bioreduction and bioremoval of hexavalent chromium by genetically engineered strains (Escherichia coli MT2A and Escherichia coli MT3)

The number of studies on the removal of hazardous metals from water using genetic engineering technologies is growing. A high rate of metal ion removal from the environment is ensured, particularly through the expression of cysteine and thiol-rich proteins such as metallothioneins in bacterial cells. In this study, we used recombinant strains created by cloning the human metallothioneins MT2A and MT3 into Escherichia coli Jm109 to assess the removal and reduction of hexavalent chromium (Cr(VI)) from aqueous solutions. MT2A was the most effective strain in both Cr(VI) removal (89% in 25 mg/L Cr(VI)) and Cr(VI) reduction (76% in 25 mg/L Cr(VI)). The amount of Cr adsorbed per dry cell by the MT2A strain was 22 mg/g. The biosorption of total Cr was consistent with the Langmuir isotherm model. Scanning electron microscope (SEM) images revealed that the morphological structures of Cr(VI)-treated cells were significantly damaged when compared to control cells. Scanning transmission electron microscope (STEM) images showed black spots in the cytoplasm of cells treated with Cr(VI). Shifts in the Fourier transform infrared spectroscopy analysis (FTIR) spectra of the cells treated with Cr(VI) showed that the groups interacting with Cr were hydroxyl, amine, amide I, amide II, phosphoryl and carbonyl. When all of the experimental data was combined, it was determined that both MT2A and MT3 were effective in removing Cr(VI) from aqueous solutions, but MT2A was more effective, indicating that MT2A may be employed as a biotechnological tool.

Constructed wetlands as a sustainable technology for wastewater treatment with emphasis on chromium-rich tannery wastewater

Water scarcity is a major threat to agriculture and humans due to over abstraction of groundwater, rapid urbanization and improper use in industrial processes. Industrial consumption of water is lower than the abstraction rate, which ultimately produces large amounts of wastewater such as from tannery industry containing high concentration of chromium (Cr). Chromium-contaminated tannery industry wastewater is used for irrigation of food crops, resulting in food safety and public health issues globally. In contrast to conventional treatment technologies, constructed wetlands (CWs) are considered as an eco-friendly technique to treat various types of wastewaters, although their application and potential have not been discussed and elaborated for Cr treatment of tannery wastewater. This review briefly describes Cr occurrence, distribution and speciation in aquatic ecosystems. The significance of wetland plant species, microorganisms, various bedding media and adsorbents have been discussed with a particular emphasis on the removal and detoxification of Cr in CWs. Also, the efficiency of various types of CWs is elaborated for advancing our understanding on Cr removal efficiency and Cr partitioning in various compartments of the CWs. The review covers important aspects to use CWs for treatment of Cr-rich tannery wastewater that are key to meet UN's Sustainable Development Goals.

Model-based analyses of chromate, selenate and sulfate reduction in a methane-based membrane biofilm reactor

Selenate (SeO₄^2-) and sulfate (SO₄^2-) are frequently present together with chromate (CrO₄^2-) in certain industrial wastewaters. SeO₄^2- and CrO₄^2- are required to be reduced while SO₄^2- reduction should be minimized to avoid the production of toxic sulfide. In this study, a modified biofilm model was employed to investigate the interactions between CrO₄^2-, SeO₄^2- and SO₄^2- bioreduction in a methane (CH₄)-based membrane biofilm reactor (MBfR). The model was calibrated using steady-state experimental data of two reported CH₄-based MBfRs reducing these oxyanions. The modeling results suggested that the majority of methanotrophs (>80%) were located in the outer layer of the biofilm, while the oxyanions-reducing bacteria preferred to grow close to the membrane. The introduction of SeO₄^2- or SO₄^2- enriched selenate/sulfate-reducing bacteria (SeRB/SRB) but decreased the abundance of chromate-reducing bacteria (CRB). A biofilm thickness of >300 μm, an HRT of higher than 4 h and an influent dissolved oxygen concentration of 0.3 mg /L were favorable for simultaneous high-level CrO₄^2- and SeO₄^2- removal. A two-stage MBfR system with optimized operational conditions showed promise in retaining high-purity (>98%) selenium nanoparticles when treating both CrO₄^2- and SeO₄^2- impacted wastewaters. Moreover, the model indicated that efficient CrO₄^2- removal (>90%) along with minor SO₄^2- reduction (<10%) could be realized via maintaining appropriate biofilm thickness (200-250 μm) and influent dissolved oxygen (0.7-0.8 mg /L) in a single MBfR. These findings offer insights for the design and operation of CH₄-based technology for remediating CrO₄^2- contaminated industrial wastewaters.

Bioreduction of hexavalent chromium by a novel haloalkaliphilic Salipaludibacillus agaradhaerens strain NRC-R isolated from hypersaline soda lakes

A novel Cr(VI)-resistant haloalkaliphilic bacterial strain NRC-R, identified as Salipaludibacillus agaradhaerens, was isolated from hypersaline soda lakes and characterized for its Cr(VI) bioreduction efficiency. Strain NRC-R grew well and effectively reduced Cr(VI) under a wide range of sodium chloride, pH, shaking velocity and temperature, showing maximum Cr(VI) reduction at 8% NaCl, pH 10, 150 rpm and 35 °C, respectively. Strain NRC-R was able to grow and reduce Cr(VI) effectively in the presence of different heavy metals and oxyanions (Pb²⁺, Zn²⁺, Co²⁺, Mn²⁺, Ni²⁺, Mo²⁺, HPO₄ ^-, NO₃ ^-, SO₄ ^2- and HCO₃ ^-). Furthermore, Fe³⁺ and Cu²⁺ significantly enhanced the Cr(VI) removal by about 1.5 fold. Strain NRC-R could reduce Cr(VI) using a variety of electron donors, exhibiting a maximum reduction in the presence of NADH and fructose. The bioremoval of Cr(VI) using strain NRC-R was due to direct enzymatic reduction and the chromate reductase activity was mainly detected in the bacterial cell membrane. Under the optimized conditions, strain NRC-R showed a remarkable Cr(VI) bioreduction with highest reduction rate of 240 uM/h. Cr(VI) concentrations of up to 3 mM (888.5 mg/L) and 4 mM (1177 mg/L) were completely reduced within 16 h and 32 h, respectively. TEM and SEM-EDX analyses confirmed the biosorption of chromium species into the cells. To the best of our knowledge, this is the first report about Cr(VI) reduction by S. agaradhaerens. In conclusion, S. agaradhaerens NRC-R was a highly efficient Cr(VI) reducing haloalkaliphilic bacterium that has a significant potential in the bioremediation of Cr(VI)-contaminated environments.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03082-2.

Application of microbial immobilization technology for remediation of Cr(VI) contamination: A review

The discharge of chromium (Cr) contaminated wastewater is creating a serious threat to aquatic environment due to the rapid pace in agricultural and industrial activities. Particularly, the long-term exposure of Cr(VI) polluted wastewater to the environment is causing serious harm to human health. Therefore, the treatment of Cr(VI) contaminated wastewater is demanding widespread attention. Regarding this, the bioremediation is being considered as a reliable and feasible option to handle Cr(VI) contaminated wastewater because of having low technical investment and operating costs. However, certain factors such as loss of microorganisms, toxicity to microorganisms and uneven microbial growth cycle in the presence of high concentrations of Cr(VI) are hindering its commercial applications. Regarding this, microbial immobilization technology (MIT) is getting great research interest because it could overcome the shortcomings of bioremediation technology's poor tolerance against Cr. Therefore, this review is the first attempt to emphases recent research developments in the remediation of Cr(VI) contamination via MIT. Starting from the selection of immobilized carrier, the present review is designed to critically discuss the various microbial immobilizing methods i.e., adsorption, embedding, covalent binding and medium interception. Further, the mechanism of Cr(VI) removal by immobilized microorganism has also been explored, precisely. In addition, three kinds of microorganism immobilization devices have been critically examined. Finally, knowledge gaps/key challenges and future perspectives are also discussed that would be helpful for the experts in improving MIT for the remediation of Cr(VI) contamination.

Self-Assembled Silver Nanoparticles Decorated on Exfoliated Graphitic Carbon Nitride/Carbon Sphere Nanocomposites as a Novel Catalyst for Catalytic Reduction of Cr(VI) to Cr(III) from Wastewater and Reuse for Photocatalytic Applications

Silver nanoparticles decorated on an exfoliated graphitic carbon nitride/carbon sphere (AgNP/Eg-C₃N₄/CS) nanocomposites were synthesized by an adsorption method with a self-assembled process. These nanoparticles were characterized by different techniques like UV-visible (UV-vis) spectroscopy, photoluminescence (PL) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), thermal gravimetric analysis (TGA), Raman spectroscopy, scanning electron spectroscopy (SEM), transmission electron spectroscopy (TEM), electrochemical impedance spectroscopy (EIS), and ζ potential. AgNP/Eg-C₃N₄/CS nanocomposites showed a higher catalytic reduction activity for the conversion of Cr(VI) into Cr(III) with formic acid (FA) at 45 °C when compared to bulk graphitic carbon nitride (Bg-C₃N₄, Eg-C₃N₄, CS, and Eg-C₃N₄/CS). The kinetic rate constants were determined as a function of catalyst dosage, concentration of Cr(VI), pH, and temperature for the AgNP/Eg-C₃N₄/CS nanocomposite. This material showed higher reduction efficiency (98.5%, k = 0.0621 min^-1) with turnover frequency (0.0158 min^-1) for the reduction of Cr(VI) to Cr(III). It also showed great selectivity and high stability after six repeated cycles (98.5%). Further, the reusability of the Cr(III)-AgNP/Eg-C₃N₄/CS nanocomposite was also investigated for the photocatalytic degradation of methylene blue (MB) under visible light irradiation with various time intervals and it showed good degradation efficiency (α = 97.95%). From these results, the AgNP/Eg-C₃N₄/CS nanocomposite demonstrated higher catalytic activity, improved environmental friendliness, lower cost for the conversion of toxic Cr(VI) to Cr(III) in solutions, and also good reusability.

Mechanism of Cr(VI) reduction by Lysinibacillus sp. HST-98, a newly isolated Cr (VI)-reducing strain

Facing the increasingly severe Cr(VI) pollution, bioreduction has proved to be an eco-friendly remediation method. An isolated strain identified as Lysinibacillus can relatively reduce Cr(VI) well. Even if the concentration of Cr(VI) increased to 250mg/L, the strain HST-98 could also grow and remove Cr(VI) well. After optimization of reaction conditions, the optimal pH, temperature, and electron donor are 8~9, 36°C, and sodium lactate, respectively. Coexisting metal ions such as Cu2+, Co2+, and Mn2+ are beneficial to reduce Cr(VI), while Zn2+, Ni2+, and Cd2+ are just the opposite. What is more, the mechanism of the reduction by the strain HST-98 is chiefly mediated by intracellular enzymes. After gene sequence homology blast and analysis, the genes and enzymes related to chromium metabolism in strain HST-98 have been annotated, which helps us to further understand the reduction mechanism of the strain HST-98. In general, Lysinibacillus sp. HST-98 is a potential candidate to repair the Cr(VI)-contaminated sites.

Depolymerization and Demethylation of Kraft Lignin in Molten Salt Hydrate and Applications as an Antioxidant and Metal Ion Scavenger

To improve the reactivity and enrich the functionality of lignin for valorization, kraft lignin was depolymerized and demethylated via cleaving aryl and alkyl ether bonds in acidic lithium bromide trihydrate (∼60% LiBr aqueous solution). It was found that the cleavage of the ether bonds followed the order of β-O-4 ether > aryl alkyl ether in phenylcoumaran > dialkyl ether in resinol > methoxyl (MeO). The depolymerization via β-O-4 cleavage occurred under mild conditions (e.g., <0.5 M HCl at 110 °C), while sufficient demethylation of the lignin needed harsher conditions (>1.5 M HCl). Both depolymerization and demethylation generated new aromatic hydroxyl (ArOH). With 2.4 M HCl, MeO content dropped from 4.85 to 0.95 mmol/g lignin, and ArOH content increased from 2.78 to 5.09 mmol/g lignin. The depolymerized and demethylated kraft lignin showed excellent antioxidant activity and Cr(VI)-scavenging capacity, compared with original kraft lignin and tannins.

Fabrication of MNPs/rGO/PMMA Composite for the Removal of Hazardous Cr(VI) from Tannery Wastewater through Batch and Continuous Mode Adsorption

Herein, we report the synthesis of magnetic nanoparticle (MNP)-reduced graphene oxide (rGO) and polymethylmethacrylate (PMMA) composite (MNPs/rGO/PMMA) as adsorbent via an in situ fabrication strategy and, in turn, the application for adsorptive removal and recovery of Cr(VI) from tannery wastewater. The composite material was characterized via XRD, FTIR and SEM analyses. Under batch mode experiments, the composite achieved maximum adsorption of the Cr(VI) ion (99.53 ± 1.4%, i.e., 1636.49 mg of Cr(VI)/150 mg of adsorbent) at pH 2, adsorbent dose of 150 mg/10 mL of solution and 30 min of contact time. The adsorption process was endothermic, feasible and spontaneous and followed a pseudo-2nd order kinetic model. The Cr ions were completely desorbed (99.32 ± 2%) from the composite using 30 mL of NaOH solution (2M); hence, the composite exhibited high efficiency for five consecutive cycles without prominent loss in activity. The adsorbent was washed with distilled water and diluted HCl (0.1M), then dried under vacuum at 60 °C for reuse. The XRD analysis confirmed the synthesis and incorporation of magnetic iron oxide at 2θ of 30.38°, 35.5°, 43.22° and 57.36°, respectively, and graphene oxide (GO) at 25.5°. The FTIR analysids revealed that the composite retained the configurations of the individual components, whereas the SEM analysis indicated that the magnetic Fe3O4-NPs (MNPs) dispersed on the surface of the PMMA/rGO sheets. To anticipate the behavior of breakthrough, the Thomas and Yoon-Nelson models were applied to fixed-bed column data, which indicated good agreement with the experimental data. This study evaluates useful reference information for designing a cost-effective and easy-to-use adsorbent for the efficient removal of Cr(VI) from wastewater. Therefore, it can be envisioned as an alternative approach for a variety of unexplored industrial-level operations.

Hexavalent Chromium Removal and Prokaryotic Community Analysis in Glass Column Reactor Packed with Aspen Wood as Solid Organic Substrate

Microbial hexavalent chromium (Cr(VI)) reduction is a promising method for Cr(VI)-laden wastewater treatment. However, the soluble organic substrate required for heterotrophic microbial Cr(VI) reduction necessitates constant supervision, and an excessive supply of soluble organic substrate can result in deterioration of the quality of the effluent. In this study, we evaluated aspen wood, a low-cost lignocellulose biomass, as a solid organic substrate for heterotrophic Cr(VI) reduction. A laboratory-scale aspen wood-packed glass column reactor inoculated with activated sludge was operated for 148 days for evaluation. Following reactor operation, an effective average dissolved Cr(VI) removal rate of 0.75 mg L^-1 h^-1 was confirmed under an average dissolved Cr(VI) loading rate of 0.90 mg L^-1 h^-1. Subsequently, 16S ribosomal ribonucleic acid gene amplicon sequencing analysis revealed that the dominant prokaryotic operational taxonomic units detected in the reactor were associated with prokaryotic lineages with the capacity for lignocellulose biodegradation, Cr compound resistance, and Cr(VI) reduction. Proteobacteria and Chloroflexi were two major prokaryotic phyla in the reactor. Our data indicate that aspen wood is an effective solid organic substrate for the development of simplified, effective, and low-cost microbial Cr(VI)-removing reactors.

Synergistic effect of pistachio shell powder and nano-zerovalent copper for chromium remediation from aqueous solution

Pistachio shell powder supported nano-zerovalent copper (ZVC@PS) material prepared by borohydride reduction was characterized using SEM, FTIR, XRD, TGA/DTA, BET, and XPS. SEM, XRD, and XPS revealed the nano-zerovalent copper to consist of a core-shell structure with CuO shell and Cu(0) core with a particle size of 40-100 nm and spherical morphology aggregated on PS biomass. ZVC@PS was found to contain 39% (w/w %) Cu onto the pistachio shell biomass. Batch sorption of Cr(VI) from the aqueous using ZVC@PS was studied and was optimized for dose (0.1-0.5 g/L), initial Cr(VI) concentration(1-20 mg/L), and pH (2-12). Optimized conditions were 0.1 g/L doses of sorbent and pH=3 for Cr(VI) adsorption. Langmuir and Freundlich adsorption isotherm models fitted well to the adsorption behavior of ZVC@PS for Cr(VI) with a pseudo-second-order kinetic behavior. ZVC@PS (0.1g/L) exhibits q_max for Cr(VI) removal up to 110.9 mg/g. XPS and other spectroscopic evidence suggest the adsorption of Cr(VI) by pistachio shell powder, coupled with reductive conversion of Cr(VI) to Cr(III) by ZVC particles to produce a synergistic effect for the efficient remediation of Cr(VI) from aqueous medium.

Boosted Cr(VI) sorption coupled reduction from aqueous solution using quaternized algal/alginate@PEI beads

APEI beads (algal/alginate-PEI) were quaternized for enhancing the sorption of Cr(VI) (Q-APEI). The readily reduction of Cr(VI) into Cr(III) in acidic solution and in the presence of organic material constitute an additional phenomenon to be taken into account for the removal of Cr(VI) by Q-APEI. The optimal pH value for both the sorption and reduction of Cr(VI) was close to 2. The sorption isotherm was well described by the Sips model in batch system; the experimental maximum Cr(VI) sorption capacity of Q-APEI was 334 mg Cr(VI) g^-1, including a reduction yield close to 25%. The pseudo-second-order kinetic model (PSORE) and the Yan model fit the uptake kinetics and breakthrough curves, in a fixed-bed system with circulation or single-path modes, respectively. The mechanism of reduction-assisted sorption allows boosting the global removal of chromate. Furthermore, the testing of Cr(VI) for three successive sorption and desorption cycles shows the remarkable stability of the sorbent for Cr(VI) removal. The Cr(VI) sorption coupled reduction mechanism and interactions between the sorbent and Cr(VI) were further explained using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS).

Modeling transmission of hexavalent chromium concentration and its health cost with a water quality analysis simulation program

In this study, the Water Quality Analysis Simulation Program (WASP7) was used to evaluate the transmission of hexavalent chromium (Cr(VI)) contamination in a water-sediment system and its flux into cultivated soils. The agricultural areas adjacent to the Wu River in Taiwan were taken as the study area, as these soils were heavily polluted with Cr(VI) concentrations of 2173-3271 μg/kg. The rates of accumulation of Cr(VI) are affected by the distance from the source of contamination and the size and type of cultivated areas. The highest concentrations of Cr(VI) (4.27 mg/kg) were detected in soil from Changhua city and correlated with the greater risk of gastric cancer in residents. Specifically, the risk of gastric cancer due to Cr(VI) contamination of agricultural soil was 3 × 10 - 7 - 15.2 × 10 - 6 in Taichung city (upstream) and 1.3 × 10 - 6 - 76.3 × 10 - 6 in Changhua county (downstream). The values of statistical life-years (VSLYs) were US$6.2-10 million for rice, US$42-60 million for corn, and US$360-580 million for other vegetables, respectively, each year. It is critical that techniques other than source reduction are used to reduce human exposure to Cr(VI), such as chemical oxidation or ion-exchange treatment to remove Cr(VI) from factory wastewaters, prior to their discharge into rivers. PRACTITIONER POINTS: This study evaluated the transmission of hexavalent chromium (Cr(VI)) contamination in a water-sediment-soil system. Maximum concentrations of Cr(VI) most rapidly accumulated in the smallest cultivated areas. The highest concentrations of Cr(VI) (3.3 mg/kg) were correlated with the greater risk of gastric cancer. Young children had a threefold greater risk of gastric cancer than adults. Techniques other than source reduction are prior to their discharge into rivers.

Bacterial Resistance against Heavy Metals in Pseudomonas aeruginosa RW9 Involving Hexavalent Chromium Removal

Pseudomonas aeruginosa RW9 is a promising candidate for the bioremediation of chromium hexavalent (Cr(VI)) pollution, as it resists a high concentration of up to 60 mg/L of Cr(VI). Leaving cells exposed to Cr(VI) has large bioreduction potential, implying its capacity to extract the ions from the contaminated medium. In this study, the tolerance for and distribution of Cr(VI) were investigated to identify the cells’ adaptation and removal strategies. Micro-characterization analysis was conducted to assess the effect of Cr(VI) on the cells. The cells’ elongation was observed at higher Cr(VI) concentrations, signifying their adaptation to DNA damage caused by Cr(VI) toxicity. Cr(VI) distribution analysis showed that the strain developed a complex mechanism to adapt to Cr(VI), based on surface-bound (0.46 mg/L), intracellularly accumulated (1.24 mg/L) and extracellular sequestration (6.74 mg/L), which accounted for 85% of the removal efficiency. The extracellular sequestration might be attributable to the production of metabolites, in accordance with the fourier-transform infrared spectroscopy (FTIR) spectra and orcinol analysis that confirmed the presence of a glycolipid biosurfactant, rhamnolipid. Remarkably, the rhamnolipid was slightly induced in the presence of Cr(VI). From the data obtained, it was confirmed that this local strain is well equipped to survive high doses of Cr(VI) and has great potential for application in Cr(VI) bioremediation.

Amino-assisted AHMT anchored on graphene oxide as high performance adsorbent for efficient removal of Cr(VI) and Hg(II) from aqueous solutions under wide pH range

The adsorbents with high adsorption capacity for simultaneously removing Cr(VI) and Hg(II) from aqueous solutions under broad working pH range are highly desirable but still extremely scarce. Here, a novel adsorbent with multidentate ligands was facilely fabricated by covalently bonding 4-amino-3-hydrazino-5-mercapto- 1,2,4-triazole on graphene oxide via the Schiff's base reaction. The maximum adsorption capacities of Cr(VI) and Hg(II) on the current adsorbent were 734.2 and 1091.1 mg/g, which were 14.36 and 5.61 times higher than that of the pure graphene oxide, respectively, exceeding those of most adsorbents previously reported. More interestingly, Cr(VI) and Hg(II) concentrations were decreased from 2 mg/L to 0.0001 mg/L for Hg(II) and 0.004 mg/L for Cr(VI), far below the WHO recommended threshold for drinking water. Moreover, the adsorbent shows an excellent performance for simultaneous removal of Cr(VI) and Hg(II) with more than 99.9% and 98.6% removal efficiencies in aqueous solutions. Finally, the adsorbent was successfully applied in dealing with the real industrial effluent, implying huge potential in industrial application. This work offers a new possibility for the removal of the metallic contaminants by rational designing target groups and ligands.

Modelling and optimization of hexavalent chromium removal from aqueous solution by adsorption on low-cost agricultural waste biomass using response surface methodological approach

In this study, a response surface methodology (RSM) approach using central composite design (CCD) was investigated to develop a mathematical model and to optimize the effects of pH, adsorbent amount and temperature related to the hexavalent chromium removal by biosorption on peanut shells (PSh). The highest removal percentage of 30.28% was found by the predicted model under the optimum conditions (pH of 2.11, 0.73 g of PSh and 37.2 °C) for a 100 mg/L initial Cr(VI) concentration, which was very near to the experimental value (29.92%). The PSh was characterized by SEM, EDX, FTIR, BET, XRD analyses. Moreover, a Langmuir isotherm fitted well (R² = 0.992) with the experimental data, and the maximum adsorption capacity was discovered to be 2.48 and 3.49 mg/g respectively at 25 and 45 °C. Kinetic data were well foreseen by pseudo second order. Thermodynamic study depicted that biosorption of Cr(VI) onto PSh was spontaneous and endothermic. Regeneration of the PSh using NaOH showed a loss <5% in the Cr(VI) removal efficiency up to three recycle runs. In summary, the Cr(VI) removal onto economic, sensitive and selective biosorbent (PSh) was optimized using CCD to study biosorption behaviors.

Polysaccharide-based (nano)materials for Cr(VI) removal

Chromium is a potentially poisonous and carcinogenic species, which originates from human activities and various industries such as leather, steel, iron, and electroplating industries. Chromium is present in various oxidation states, among which hexavalent chromium (Cr(VI)) is highly toxic as a natural contaminant. Therefore, chromium, particularly Cr(VI), must be eliminated from the environment, soil, and water to overcome significant problems due to its accumulation in the environment. There are different approaches such as adsorption, ion exchange, photocatalytic reduction, etc. for removing Cr(VI) from the environment. By converting Cr(VI) to Cr(III), its toxicity is reduced. Cr(III) is essential for the human diet, even in small amounts. Today, biopolymers such as alginate, cellulose, gum, pectin, starch, chitin, and chitosan have received much attention for the removal of environmental pollutants. Biopolymers, particularly polysaccharides, are very useful compounds due to their OH and NH₂ functional groups and some advantages such as biodegradability, biocompatibility, and accessibility. Therefore, they can be widely applied in catalytic applications and as efficient adsorbents for the removal of toxic compounds from the environment. This review briefly investigates the application of polysaccharide-based (nano)materials for efficient Cr(VI) removal from the environment using adsorption/reduction, photocatalytic, and chemical reduction mechanisms.

Bioremediation of Chromium by Microorganisms and Its Mechanisms Related to Functional Groups

Heavy metals generated mainly through many anthropogenic processes, and some natural processes have been a great environmental challenge and continued to be the concern of many researchers and environmental scientists. This is mainly due to their highest toxicity even at a minimum concentration as they are nonbiodegradable and can persist in the aquatic and terrestrial environments for long periods. Chromium ions, especially hexavalent ions (Cr(VI)) generated through the different industrial process such as tanneries, metallurgical, petroleum, refractory, oil well drilling, electroplating, mining, textile, pulp and paper industries, are among toxic heavy metal ions, which pose toxic effects to human, plants, microorganisms, and aquatic lives. This review work is aimed at biosorption of hexavalent chromium (Cr(VI)) through microbial biomass, mainly bacteria, fungi, and microalgae, factors influencing the biosorption of chromium by microorganisms and the mechanism involved in the remediation process and the functional groups participated in the uptake of toxic Cr(VI) from contaminated environments by biosorbents. The biosorption process is relatively more advantageous over conventional remediation technique as it is rapid, economical, requires minimal preparatory steps, efficient, needs no toxic chemicals, and allows regeneration of biosorbent at the end of the process. Also, the presence of multiple functional groups in microbial cell surfaces and more active binding sites allow easy uptake and binding of a greater number of toxic heavy metal ions from polluted samples. This could be useful in creating new insights into the development and advancement of future technologies for future research on the bioremediation of toxic heavy metals at the industrial scale.

Performance and enhancement mechanism of corncob guiding chromium (VI) bioreduction

Chromium-contaminated groundwater has drawn extensive attention due to its high toxicity and wide application. Although bioremediation is considered to be an effective approach for Cr(VI) removal, a better method is still urgently needed. In this study, corncob-guided Cr(VI) reduction achieved the highest removal efficiency due to the highest amount of total carbon and available carbon emissions. After verifying the sustainability and operational feasibility of this approach, the broad-spectrum applicability of corncob to guide Cr(VI) bioreduction was further explored under various operating conditions. In addition, it suggested that the carrier effect, nutrient element release and electron shuttle effect were the main mechanisms enhancing the reduction process, with approximate contribution rates of 12.5%, 7.5% and 75%, respectively. Microbiological analysis demonstrated that the addition of solid-phase carbon sources increased the abundance of microbes related to carbon metabolism and promoted the expression of glycolytic metabolic pathway. Furthermore, the addition of corncob led to an elevation of expression level of the electron transport pathway, which is consistent with the function of the electron shuttle. This study provides theoretical and technical support for the practical application of corncob-mediated Cr(VI) bioreduction.

Efficient Sequestration of Hexavalent Chromium by Graphene-Based Nanoscale Zero-Valent Iron Composite Coupled with Ultrasonic Pretreatment

Nanoscale zero-valent iron (nZVI) has attracted considerable attention for its potential to sequestrate and immobilize heavy metals such as Cr(VI) from an aqueous solution. However, nZVI can be easily oxidized and agglomerate, which strongly affects the removal efficiency. In this study, graphene-based nZVI (nZVI/rGO) composites coupled with ultrasonic (US) pretreatment were studied to solve the above problems and conduct the experiments of Cr(VI) removal from an aqueous solution. SEM-EDS, BET, XRD, and XPS were performed to analyze the morphology and structures of the composites. The findings showed that the removal efficiency of Cr(VI) in 30 min was increased from 45.84% on nZVI to 78.01% on nZVI/rGO and the removal process performed coupled with ultrasonic pretreatment could greatly shorten the reaction time to 15 min. Influencing factors such as the initial pH, temperature, initial Cr(VI) concentration, and co-existing anions were studied. The results showed that the initial pH was a principal factor. The presence of HPO₄²⁻, NO₃⁻, and Cl⁻ had a strong inhibitory effect on this process, while the presence of SO₄²⁻ promoted the reactivity of nZVI/rGO. Combined with the above results, the process of Cr(VI) removal in US-nZVI/rGO system consisted of two phases: (1) The initial stage is dominated by solution reaction. Cr(VI) was reduced in the solution by Fe²⁺ caused by ultrasonic cavitation. (2) In the following processes, adsorption, reduction, and coprecipitation coexisted. The addition of rGO enhanced electron transportability weakened the influence of passivation layers and improved the dispersion of nZVI particles. Ultrasonic cavitation caused pores and corrosion at the passivation layers and fresh Fe⁰ core was exposed, which improved the reactivity of the composites.

Modeling cometabolism of hexavalent chromium by iron reducing bacteria in tertiary substrate system

In this study, a bacterial strain Serratia sp. was employed for the reduction of synthetically prepared different concentration of Cr(VI) solution (10, 25, 40, 50 and 100 mg/L). Cometabolism study have been carried out in the binary substrate system as well as in the tertiary substrate system. The results revealed that when glucose was added as a co-substrate, at low Cr(VI) concentration, complete reduction was achieved followed by increased biomass growth, but when Cr(VI) concentration was increased to 100 mg/L, the reduction decline to 93%. But in presence of high carbon iron filings (HCIF) as co-substrate even at higher Cr(VI) concentration i.e. 100 mg/L, 100% reduction was achieved and the cell growth continued till 124 h. The study was illustrated via Monod growth kinetic model for tertiary substrate system and the kinetic parameters revealed that the HCIF and glucose combination showed least inhibition to hexavalent chromium reduction by Serratia sp.

Disentangling Microbial Syntrophic Mechanisms for Hexavalent Chromium Reduction in Autotrophic Biosystems

Hexavalent chromium [Cr(VI)] is one of the common heavy-metal contaminants in groundwater, and the availability of electron donors is considered to be a key parameter for Cr(VI) biotransformation. During the autotrophic remediation process, however, much remains to be illuminated about how complex syntrophic microbial communities couple Cr(VI) reduction with other elemental cycles. Two series of Cr(VI)-reducing groundwater bioreactors were independently amended by elemental sulfur and iron and inoculated with the same inoculum. After 160 days of incubation, both bioreactors showed similar archaea-dominating microbiota compositions, whereas a higher Cr(VI)-reducing rate and more methane production were detected in the Fe⁰-driven one. Metabolic reconstruction of 23 retrieved genomes revealed complex symbiotic relationships driving distinct elemental cycles coupled with Cr(VI) reduction in bioreactors. In both bioreactors, these Cr(VI) reducers were assumed to live in syntrophy with oxidizers of sulfur, iron, hydrogen, and volatile fatty acids and methane produced by carbon fixers and multitrophic methanogens, respectively. The significant difference in methane production was mainly due to the fact that the yielded sulfate greatly retarded acetoclastic methanogenesis in the S-bioreactor. These findings provide insights into mutualistic symbioses of carbon, sulfur, iron, and chromium metabolisms in groundwater systems and have implications for bioremediation of Cr(VI)-contaminated groundwater.

Hexavalent chromium reducing bacteria: mechanism of reduction and characteristics

As a common heavy metal, chromium and its compounds are widely used in industrial applications, e.g., leather tanning, electroplating, and in stainless steel, paints and fertilizers. Due to the strong toxicity of Cr(VI), chromium is regarded as a major source of pollution with a serious impact on the environment and biological systems. The disposal of Cr(VI) by biological treatment methods is more favorable than traditional treatment methods because the biological processes are environmentally friendly and cost-efficient. This review describes how bacteria tolerate and reduce Cr(VI) and the effects of some physical and chemical factors on the reduction of Cr(IV). The practical applications for Cr(VI) reduction of bacterial cells are also included in this review.

Chromium behavior in a highly urbanized coastal area (Bahía Blanca Estuary, Argentina)

The contamination of estuaries by heavy metals from anthropogenic activities in the industrial, domestic, and agricultural sectors is a global concern. In this study, the Cr, Fe, and Mn levels in the suspended particulate matter (SPM) were analyzed in estuarine waters from Bahia Blanca Estuary, during 2014-2015. The values of particulate Cr ranged from 7.33 to 35.20 μg g^-1, which could be associated to several anthropogenic sources. The positive correlations found between Cr and Chlorophyll-a, and Cr and particulate organic carbon (POC) suggest the strong influence of phytoplankton on the adsorption of this metal and on the increase of particulate Cr. Negative correlations were found between Cr and DO and between Cr and pH, which could indicate an increasing trend in the dissolved form of Cr. This study suggests that the physical-chemical characteristics of the water column as well as phytoplankton and POC dynamics influence the behavior of Cr in this estuary.