Simultaneous removal of chromate and arsenite by the immobilized Enterobacter bacterium in combination with chemical reagents

A PadR family transcriptional repressor regulates the transcription of chromate efflux transporter in Enterobacter sp. Z1

Chromium is a prevalent toxic heavy metal, and chromate [Cr(VI)] exhibits high mutagenicity and carcinogenicity. The presence of the Cr(VI) efflux protein ChrA has been identified in strains exhibiting resistance to Cr(VI). Nevertheless, certain strains of bacteria that are resistant to Cr(VI) lack the presence of ChrB, a known regulatory factor. Here, a PadR family transcriptional repressor, ChrN, has been identified as a regulator in the response of Enterobacter sp. Z1(CCTCC NO: M 2019147) to Cr(VI). The chrN gene is cotranscribed with the chrA gene, and the transcriptional expression of this operon is induced by Cr(VI). The binding capacity of the ChrN protein to Cr(VI) was demonstrated by both the tryptophan fluorescence assay and Ni-NTA purification assay. The interaction between ChrN and the chrAN operon promoter was validated by reporter gene assay and electrophoretic mobility shift assay. Mutation of the conserved histidine residues His14 and His50 resulted in loss of ChrN binding with the promoter of the chrAN operon. This observation implies that these residues are crucial for establishing a DNA-binding site. These findings demonstrate that ChrN functions as a transcriptional repressor, modulating the cellular response of strain Z1 to Cr(VI) exposure.

Application of manganese oxide-based materials for arsenic removal: A review

In the context of growing arsenic (As) contamination in the world, there is an urgent need for an effective treatment approach to remove As from the environment. Industrial wastewater is one of the primary sources of As contamination, which poses significant risks to both microorganisms and human health, as the presence of As can disrupt the vital processes and synthesis of crucial macromolecules in living organisms. The global apprehension regarding As presence in aquatic environments persists as a key environmental issue. This review summarizes the recent advances and progress in the design, strategy, and synthesis method of various manganese-based adsorbent materials for As removal. Occurrence, removal, oxidation mechanism of As(III), As adsorption on manganese oxide (MnOx)-based materials, and influence of co-existing solutes are also discussed. Furthermore, the existing knowledge gaps of MnOx-based adsorbent materials and future research directions are proposed. This review provides a reference for the application of MnOx-based adsorbent materials to As removal.

Arsenite Mediates Selenite Resistance and Reduction in Enterobacter sp. Z1, Thereby Enhancing Bacterial Survival in Selenium Environments

Arsenic (As) is widely present in the environment, and virtually all bacteria possess a conserved ars operon to resist As toxicity. High selenium (Se) concentrations tend to be cytotoxic. Se has an uneven regional distribution and is added to mitigate As contamination in Se-deficient areas. However, the bacterial response to exogenous Se remains poorly understood. Herein, we found that As(III) presence was crucial for Enterobacter sp. Z1 to develop resistance against Se(IV). Se(IV) reduction served as a detoxification mechanism in bacteria, and our results demonstrated an increase in the production of Se nanoparticles (SeNPs) in the presence of As(III). Tandem mass tag proteomics analysis revealed that the induction of As(III) activated the inositol phosphate, butanoyl-CoA/dodecanoyl-CoA, TCA cycle, and tyrosine metabolism pathways, thereby enhancing bacterial metabolism to resist Se(IV). Additionally, arsHRBC, sdr-mdr, purHD, and grxA were activated to participate in the reduction of Se(IV) into SeNPs. Our findings provide innovative perspectives for exploring As-induced Se biotransformation in prokaryotes.

Chromate Removal by Enterobacter cloacae Strain UT25 from Tannery Effluent and Its Potential Role in Cr (VI) Remediation

An indigenous chromate-resistant bacterial strain isolated from tannery effluent was identified based on morphological, biochemical, and 16S rRNA gene sequencing, as Enterobacter cloacae UT25. It was found to resist heavy metal ions such as Cr (VI), Pb (II), Cu (II), Co (II), Ni (II), Hg (II), and Zn (II) and antibiotics. The strain was able to remove 89 and 86% chromate, after 24 h of incubation in a Luria-Bertani (LB) medium at an initial Cr (VI) concentration of 1000 and 1500 µg/ml, respectively. Minimum inhibitory concentration (MIC) was observed for chromate to be 80,000 and 1850 µg/ml, after 48 h of incubation in LB and acetate minimal media (AMM), respectively. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analysis showed discrete cells with intact and smooth cell walls and homogenous cytoplasm in the absence of metal stress, whereas chromate stress caused cell lysis and reduction in size, which was a characteristic response to Cr (VI) toxicity. Energy Dispersive X-Ray Spectroscopy (EDX) confirmed the adsorption of oxyanions to the cell wall which was one of the Cr (VI) removal mechanisms by the bacterium. Atomic Force Microscopy (AFM) micrographs of chromate-untreated and treated cells revealed Root Mean Square roughness (Rq) values of 16.25 and 11.26 nm, respectively, indicating less roughness in the presence of stress. The partial gene sequence of class 1 integrons (intI1) of strain UT25 showed 94% homology with intI1 gene of strain Enterobacter hormaechei strain ECC59 plasmid pECC59-1. The present analysis highlighted the potential of E. cloacae UT25 as a promissory bacterium that could be applied in removing chromate from polluted environments.

Physiological, biochemical, and genomic elucidation of the Ensifer adhaerens M8 strain with simultaneous arsenic oxidation and chromium reduction

This study reports the simultaneous oxidation of As(III) and reduction of the Cr(VI) strain Ensifer adhaerens M8 screened from soils around abandoned gold tailings contaminated with highly complex metals (loids). Physiological, biochemical, and genomic techniques were used to explore the mechanism. The strain M8 could simultaneously oxidize 1 mM As(III) and reduce 45.3 % 0.1 mM Cr(VI) in 16 h, and the Cr(VI) reduction rate was increased by 5.8 % compared with the addition of Cr(VI) alone. Cellular debris was the main site of M8 arsenic oxidation. Chromium reduction was dominated by the reduction of extracellular hexavalent chromium (23.80-35.67 %). The genome of M8 included one chromosome and four plasmids, and a comparison of the genomes showed that M8 had two more plasmids than strains of the same genus, which may be related to strong environmental adaptations. M8 had 10 heavy metal resistance genes (HMRs), and plasmid D had a complete cluster of arsenic resistance-oxidation-transport genes (arsOHBCCR-aioSR-aioBA-cytCmoeA-phoBBU-PstBACS-phnCDEE). The genes involved in Cr(VI) detoxification include DNA repair (RecG, ruvABC, and UvrD), Cr(VI) transport (chrA, TonB, and CysAPTW) and Cr(VI) reduction. In summary, this study provides a molecular basis for As (III) and Cr (VI) remediation.

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.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.

Long-term effects of phytoextraction by a poplar clone on the concentration, fractionation, and transportation of heavy metals in mine tailings

Mine tailings are one of main causes of diffused heavy metal pollution since the heavy metals in there may acquire mobility. The current knowledge of the processes at work in long-term phytoremediation by woody species remains insufficient. Through a 4-year field study, we evaluated the phytoextraction efficiency of Populus deltoides CL. 'Xianglin 90' grown on a mine tailing co-polluted by Cd, Cu, Cr, Ni, Pb, and Zn. The concentrations of Cd, Cu, Ni, Pb, and Zn in the rhizospheric soil were reduced by amounts ranging from 12.86 to 42.19% during the study period. Bioconcentration factors and translocation factors showed that the accumulation of Cd and Zn occurring in the shoots was the most effective. Combined with the considerable biomass produced by poplar, the extracted amounts of Cd and Zn could reach 0.61 g and 10.66 g plant^-1, respectively, in which the shoots account for 77.3% (Cd) and 89.0% (Zn) of the overall extraction amounts. Acid-soluble Cd and Zn increased by 5.49% and 4.29%, respectively, in the rhizosphere compared to the bulk soil, indicating that poplar enhanced the mobility of Cd and Zn in the rhizosphere, which explained its ability for bioaccumulation and root-shoot translocation. Moreover, calculated time required to address the issue of Cd and Zn pollution was theoretically shortened by more than half from 2015 to 2019. This study brings new insights into the long-term effects of phytoextraction on the concentration, fractionation, and transportation of heavy metals and confirms the potential of poplar as a Cd and Zn remediation species.

Polysaccharides as Support for Microbial Biomass-Based Adsorbents with Applications in Removal of Heavy Metals and Dyes

The use of biosorbents for the decontamination of industrial effluent (e.g., wastewater treatment) by retaining non-biodegradable pollutants (antibiotics, dyes, and heavy metals) has been investigated in order to develop inexpensive and effective techniques. The exacerbated water pollution crisis is a huge threat to the global economy, especially in association with the rapid development of industry; thus, the sustainable reuse of different treated water resources has become a worldwide necessity. This review investigates the use of different natural (living and non-living) microbial biomass types containing polysaccharides, proteins, and lipids (natural polymers) as biosorbents in free and immobilized forms. Microbial biomass immobilization performed by using polymeric support (i.e., polysaccharides) would ensure the production of efficient biosorbents, with good mechanical resistance and easy separation ability, utilized in different effluents' depollution. Biomass-based biosorbents, due to their outstanding biosorption abilities and good efficiency for effluent treatment (concentrated or diluted solutions of residuals/contaminants), need to be used in industrial environmental applications, to improve environmental sustainability of the economic activities. This review presents the most recent advances related the main polymers such as polysaccharides and microbial cells used for biosorbents production; a detailed analysis of the biosorption capability of algal, bacterial and fungal biomass; as well as a series of specific applications for retaining metal ions and organic dyes. Even if biosorption offers many advantages, the complexity of operation increased by the presence of multiple pollutants in real wastewater combined with insufficient knowledge on desorption and regeneration capacity of biosorbents (mostly used in laboratory scale) requires more large-scale biosorption experiments in order to adequately choose a type of biomass but also a polymeric support for an efficient treatment process.

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.

NemA Catalyzes Trivalent Organoarsenical Oxidation and Is Regulated by the Trivalent Organoarsenical-Selective Transcriptional Repressor NemR

Synthetic aromatic arsenicals such as roxarsone (Rox(V)) and nitarsone (Nit(V)) have been used as animal growth enhancers and herbicides. Microbes contribute to redox cycling between the relatively less toxic pentavalent and highly toxic trivalent arsenicals. In this study, we report the identification of nemRA operon from Enterobacter sp. Z1 and show that it is involved in trivalent organoarsenical oxidation. Expression of nemA is induced by chromate (Cr(VI)), Rox(III), and Nit(III). Heterologous expression of NemA in Escherichia coli confers resistance to Cr(VI), methylarsenite (MAs(III)), Rox(III), and Nit(III). Purified NemA catalyzes simultaneous Cr(VI) reduction and MAs(III)/Rox(III)/Nit(III) oxidation, and oxidation was enhanced in the presence of Cr(VI). The results of electrophoretic mobility shift assays and fluorescence assays demonstrate that the transcriptional repressor, NemR, binds to either Rox(III) or Nit(III). NemR has three conserved cysteine residues, Cys21, Cys106, and Cys116. Mutation of any of the three resulted in loss of response to Rox(III)/Nit(III), indicating that they form an Rox(III)/Nit(III) binding site. These results show that NemA is a novel trivalent organoarsenical oxidase that is regulated by the trivalent organoarsenical-selective repressor NemR. This discovery expands our knowledge of the molecular mechanisms of organoarsenical oxidation and provides a basis for studying the redox coupling of environmental toxic compounds.

Chemical-Assisted Microbially Mediated Chromium (Cr) (VI) Reduction Under the Influence of Various Electron Donors, Redox Mediators, and Other Additives: An Outlook on Enhanced Cr(VI) Removal

Chromium (Cr) (VI) is a well-known toxin to all types of biological organisms. Over the past few decades, many investigators have employed numerous bioprocesses to neutralize the toxic effects of Cr(VI). One of the main process for its treatment is bioreduction into Cr(III). Key to this process is the ability of microbial enzymes, which facilitate the transfer of electrons into the high valence state of the metal that acts as an electron acceptor. Many underlying previous efforts have stressed on the use of different external organic and inorganic substances as electron donors to promote Cr(VI) reduction process by different microorganisms. The use of various redox mediators enabled electron transport facility for extracellular Cr(VI) reduction and accelerated the reaction. Also, many chemicals have employed diverse roles to improve the Cr(VI) reduction process in different microorganisms. The application of aforementioned materials at the contaminated systems has offered a variety of influence on Cr(VI) bioremediation by altering microbial community structures and functions and redox environment. The collective insights suggest that the knowledge of appropriate implementation of suitable nutrients can strongly inspire the Cr(VI) reduction rate and efficiency. However, a comprehensive information on such substances and their roles and biochemical pathways in different microorganisms remains elusive. In this regard, our review sheds light on the contributions of various chemicals as electron donors, redox mediators, cofactors, etc., on microbial Cr(VI) reduction for enhanced treatment practices.

Microbial Oxidation of Arsenite: Regulation, Chemotaxis, Phosphate Metabolism and Energy Generation

Arsenic (As) is a metalloid that occurs widely in the environment. The biological oxidation of arsenite [As(III)] to arsenate [As(V)] is considered a strategy to reduce arsenic toxicity and provide energy. In recent years, research interests in microbial As(III) oxidation have been growing, and related new achievements have been revealed. This review focuses on the highlighting of the novel regulatory mechanisms of bacterial As(III) oxidation, the physiological relevance of different arsenic sensing systems and functional relationship between microbial As(III) oxidation and those of chemotaxis, phosphate uptake, carbon metabolism and energy generation. The implication to environmental bioremediation applications of As(III)-oxidizing strains, the knowledge gaps and perspectives are also discussed.