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Selenium is an essential trace element inevitably present in almost all the soils. It exists in the various chemical forms at a particular site due to the various features such as pH, organic matter, concentration of cations and anions of the site. An excessive concentration of selenium has been reported in various regions of Punjab. The present research has explicated the mobility of selenium by indigenous bacterial strains isolated from seleniferous soil of Punjab, India. Physiochemical characterization of the soil sample indicated alkaline nature which in turns favors the existence and mobility of selenium oxyanions in the soil. A significant concentration of selenium (2.652 mg/g) has been reported in the soil sample. Total four bacterial strains were isolated from the soil based on their growth in selenium oxyanions supplemented medium. Bacterial isolate SGB-5 was explored for selenium sequestration and transformation studies on the basis of tolerance to selenium oxyanions especially selenate. ICP-MS analysis of biomass indicated significant sequestration of selenium (upto 70%) by the isolate SGB-5. X-ray diffraction spectrum of biomass associated selenium revealed the reduced and crystalline nature of selenium in the biomass. SEM-EDX analysis further confirmed the accumulation of nano-sized elemental selenium around the bacterial cells. Further SGB-5 strain was characterized and identified as Bacillus sp. using 16S rRNA gene sequencing. [ABSTRACT FROM AUTHOR]

Selenium (Se) is an essential micro-nutrient for living organisms, but elevated concentrations in water can adversely affect health. In this research, we investigate the removal of selenium oxyanions (selenate and selenite) in aqueous systems by integration of adsorption on modified zeolites and microbial reduction. Dynamic sorption-reduction experiments were conducted using two sets of zeolite columns for the removal of selenite and selenate oxyanions, respectively. In each case, one column was fully packed with natural, unmodified zeolites, while the other column was composed of 80% natural and 20% iron-coated zeolites, by mass. The initial selenium concentration, selenite (SeIV) or selenate (SeVI), was 790 μg/L, the pH was 7.5, and the flow rate was 3 mL/min. Initially, as expected, the higher selenate removal (34%) was observed with coated zeolite, twice as high compared to the results with unmodified zeolite. Maximum selenite removal was 89% in the column with modified zeolite. Within approximately 14 days, as the biofilm developed inside the columns, selenium reduction in all four columns reached approximately 99%. Biofilm microbial community composition, assessed by 16S rRNA sequencing, is consistent with the presence of mainly selenium-reducing bacteria (Veillonella, Bacteroides, and Escherichia). Selenium oxyanions were reduced to elemental selenium, visible within the bioreactors as red-color aggregates. [ABSTRACT FROM AUTHOR]

Selenium is an essential micro-nutrient for living organisms, but elevated concentrations of it in water can adversely affect health. Nitrate is often found in selenium-contaminated water and negatively correlates with selenium removal. In this study, we investigate the effect of nitrate co-existence on selenium bioremediation in chemically modified zeolite columns. Dynamic sorption-reduction experiments were conducted using natural and iron-coated zeolite columns to remove selenite and selenate oxyanions separately, with and without nitrate anions. Anaerobic sludge was included as microbial inoculum, while lactate was the sole electron donor. The initial selenium concentration (SeIV or SeVI) was 790 µg/L, the nitrate concentration was 620 mg/L, the pH was 7.5, and the flow rate was 3 mL/min. Before introducing nitrate ions, selenium reduction in all four columns reached approximately 99%. However, after introducing nitrate ions, selenate and selenite reduction efficiencies were reduced to approximately 93% and 60%, respectively. Biofilm microbial community composition, assessed by 16S rRNA sequencing, was distinct between the communities with and without nitrate anions. Specifically, in the absence of nitrate, biofilm communities are mainly composed of selenium-reducing bacteria (Veillonella, Bacteroides and Escherichia). In contrast, the presence of nitrate led to mostly denitrifying bacteria (Anaeromusa-Anaeroarcus, Lentimicrobium, Azospirillum and Endomicrobium). Further, comparison of diversity indices (Shannon index, Faith PD and Pielou's) shows alteration in all indices in the presence of nitrate. [ABSTRACT FROM AUTHOR]

Selenium (Se) is an essential micro-nutrient for living organisms, but elevated concentrations in water can adversely affect health. In this research, we investigate the removal of selenium oxyanions (selenate and selenite) in aqueous systems by integration of adsorption on modified zeolites and microbial reduction. Dynamic sorption-reduction experiments were conducted using two sets of zeolite columns for the removal of selenite and selenate oxyanions, respectively. In each case, one column was fully packed with natural, unmodified zeolites, while the other column was composed of 80% natural and 20% iron-coated zeolites, by mass. The initial selenium concentration, selenite (SeIV) or selenate (SeVI), was 790 μg/L, the pH was 7.5, and the flow rate was 3 mL/min. Initially, as expected, the higher selenate removal (34%) was observed with coated zeolite, twice as high compared to the results with unmodified zeolite. Maximum selenite removal was 89% in the column with modified zeolite. Within approximately 14 days, as the biofilm developed inside the columns, selenium reduction in all four columns reached approximately 99%. Biofilm microbial community composition, assessed by 16S rRNA sequencing, is consistent with the presence of mainly selenium-reducing bacteria (Veillonella, Bacteroides, and Escherichia). Selenium oxyanions were reduced to elemental selenium, visible within the bioreactors as red-color aggregates.

Selenium is an essential micro-nutrient for living organisms, but elevated concentrations of it in water can adversely affect health. Nitrate is often found in selenium-contaminated water and negatively correlates with selenium removal. In this study, we investigate the effect of nitrate co-existence on selenium bioremediation in chemically modified zeolite columns. Dynamic sorption-reduction experiments were conducted using natural and iron-coated zeolite columns to remove selenite and selenate oxyanions separately, with and without nitrate anions. Anaerobic sludge was included as microbial inoculum, while lactate was the sole electron donor. The initial selenium concentration (SeIV or SeVI) was 790 µg/L, the nitrate concentration was 620 mg/L, the pH was 7.5, and the flow rate was 3 mL/min. Before introducing nitrate ions, selenium reduction in all four columns reached approximately 99%. However, after introducing nitrate ions, selenate and selenite reduction efficiencies were reduced to approximately 93% and 60%, respectively. Biofilm microbial community composition, assessed by 16S rRNA sequencing, was distinct between the communities with and without nitrate anions. Specifically, in the absence of nitrate, biofilm communities are mainly composed of selenium-reducing bacteria (Veillonella, Bacteroides and Escherichia). In contrast, the presence of nitrate led to mostly denitrifying bacteria (Anaeromusa-Anaeroarcus, Lentimicrobium, Azospirillum and Endomicrobium). Further, comparison of diversity indices (Shannon index, Faith PD and Pielou’s) shows alteration in all indices in the presence of nitrate.

As an essential micronutrient, the selenium nanoparticles (SeNPs) are gaining much importance in therapeutics, agriculture, nutrient supplements, and antimicrobial agents. Different techniques can synthesize these SeNPs. Nowadays, biological synthesis is favored because it is environmentally safe, least toxic, more stable, and requires less energy. The whole microbial cells or their extracts with added selenium oxyanions or selenium dioxides as the precursor synthesize SeNPs after incubation. The elemental selenium (Se0) is significantly less toxic than its oxyanions, selenite, and selenate. Selenomethionine and selenocysteine are the primary organic forms of Se, involved in many proteins. Therefore, transforming these toxic materials into a biologically safe atomic structure becomes essential. Many microorganisms convert the environmentally available toxic selenium oxyanions such as selenate or selenite into less toxic elemental Se nanoparticles (Se0). These microbes work as bio-nano-factories that utilize the Se anions into their metabolic pathways and detoxify them by forming the nanomaterial as the byproduct. In respiration, they may even use the selenite /selenate as a final electron acceptor. This review focuses on the biological methods for SeNPs synthesis, using various microbes reported and their applications in different fields.

[Display omitted] • A modified geopolymer with Se oxyanions uptake capacity was synthesized. • The Se oxyanions uptake mechanism of synthetic ettringite depends on the Se concentration. • Ion exchange is the primary Se oxyanions uptake mechanism of in-situ ettringite. • A thermodynamic model was developed to describe the uptake of Se oxyanions. • The basic structure and cation uptake capacity of the modified geopolymer are retained. Studies have demonstrated the remarkable capacity of metakaolin-based geopolymers for the uptake of cationic radionuclides, such as Cs+ and Sr2+. However, the lack of uptake ability towards anionic radionuclides represents a potential avenue for improvement in utilising this material for nuclear waste disposal applications. Additionally, the prevalence of ettringite as an anion exchange agent formed in alkaline environments, such as cement hydration, is widely acknowledged in the field. However, no previous studies have been conducted to explore the utilisation of the advantageous qualities of both metakaolin-based geopolymer and ettringite in developing a comprehensive incorporation process for both cationic and anionic radionuclides. In this study, compositions and sets of preparation conditions were proposed to modify metakaolin-based geopolymer, resulting in the in-situ formation of ettringite, with the aim of enhancing its uptake capabilities for selenium oxyanions. The uptake behaviour and mechanism of ettringite, and geopolymer with in-situ ettringite, were proposed through co-precipitation experiments, binding and structural analysis. The results reveal that the ettringite can uptake SeO 3 2- and SeO 4 2- through co-precipitation; however, its effectiveness is limited to SeO 3 2- in the uptake process, where the mechanism is contingent on the concentration of Se oxyanions present. On the other hand, modified geopolymer with in-situ ettringite retained its capacity to uptake cationic radionuclides such as Cs+ and Sr2+ while evolving to exhibit an ability to uptake SeO 3 2-. Thermodynamic modelling was carried out according to the ion exchange mechanism, which effectively predicts the uptake of SeO 3 2- at low concentrations. The proposed composition and preparation conditions hold the potential for developing an effective incorporation process for cationic and anionic radionuclides. The outcomes of this research enhance the understanding of the uptake behaviour of Se oxyanions by ettringite and provide insight into the potential of metakaolin-based geopolymers to immobilise anions. [ABSTRACT FROM AUTHOR]

The substitution of selenium oxyanions in the hydroxyapatite structure was examined using multinuclear solid-state resonance spectroscopy (ssNMR). The study was supported by powder X-ray diffractometry (PXRD) and wavelength dispersion X-ray fluorescence (WD-XRF). Samples of pure hydroxyapatite (HA300) and selenate (HA300-1.2SeO4) or selenite (HA300-1.2SeO3) substituted hydroxyapatites were synthesized using the standard wet method and heated at 300 °C to remove loosely bonded water. PXRD data showed that all samples are single-phase, nanocrystalline hydroxyapatite. The incorporation of selenite and selenate ions affected the lattice constants. In selenium-containing samples the concentration of Se was very similar and amounted to 9.55% and 9.64%, for HA300-1.2SeO4 and HA300-1.2SeO3, respectively. PXRD and ssNMR data showed that the selenite doping significantly decreases the crystallite size and crystallinity degree. 31P and 1H NMR experiments demonstrated the developed surface hydrated layer in all samples, especially in HA300-1.2SeO3. 1H NMR studies showed the dehydroxylation of HA during the selenium oxyanions substitution and the existence of hydrogen bonding in structural hydroxyl group channels. 1H→77Se cross polarization NMR experiments indicated that selenites and selenates are located in the crystal lattice and on the crystal surface.

Inappropriate treatments for the effluents from semiconductor plants might cause the releases and wide distributions of selenium (Se) into the ecosystems. In this study, Al/Si and Fe/Si coprecipitates were selected as model adsorbents as they often formed during the wastewater coagulation process, and the removal efficiency of selenite (SeO 3 ) and selenate (SeO 4 ) onto the coprecipitates were systematically examined. The removal efficiency of SeO 3 and SeO 4 was highly related to surface properties of Al/Si and Fe/Si coprecipitates. The surface-attached Al shell of Al/Si coprecipitates shielded a portion of negative charges from the core SiO 2 , resulting in a higher point of zero charge than that of Fe/Si coprecipitates. Thus, adsorption of SeO 3 /SeO 4 was favorable on the Al/Si coprecipitates. Adsorptions of both SeO 3 and SeO 4 on Al/Si coprecipitates were exothermic reactions. On Fe/Si coprecipitates, while SeO 3 adsorption also showed the exothermic behavior, SeO 4 adsorption occurred as an endothermic reaction. The kinetic adsorption data of SeO 3 /SeO 4 on Al/Si and Fe/Si coprecipitates were described well by the pseudo-second-order kinetic model. SeO 4 and SeO 3 adsorption on Fe/Si or Al/Si were greatly inhibited by the strong PO 4 ligand, whereas the weak ligand such as SO 4 only significantly affected SeO 4 adsorption. The weakest complex between SeO 4 and Al was implied by the essentially SeO 4 desorption as SeO 4 /PO 4 molar ratios decreased from 0.5 to 0.2. These results were further confirmed by the less SeO 4 desorption (41%) from Fe/Si coprecipitates than that from Al/Si coprecipitates (78%) while PO 4 was added sequentially. [ABSTRACT FROM AUTHOR]

Biogenic formation of nano-sized particles composed of various materials (in particular, selenium) by live microorganisms is widespread in nature. This phenomenon has been increasingly attracting the attention of researchers over the last decade not only owing to a range of diverse applications of such nanoparticles (NPs) in nanobiotechnology, but also because of the specificity of methodologies and mechanisms of NPs formation related to “green synthesis”. In this mini-review, recent data are discussed on the multifaceted role of proteins in the processes of microbial reduction of selenium oxyanions and the formation of Se NPs. Besides the involvement of proteins in reducing selenites and selenates, their participation in the microbially driven growth and stabilisation of Se NPs is analysed, which results in the formation of unique nanostructured materials differing from those obtained chemically. This mini-review is thus focussed on proteins involved in microbial synthesis of Se NPs and on instrumental analysis of these processes and their products (biogenic nanostructured selenium particles functionalised by a surface-capping layer of various biomacromolecules). [ABSTRACT FROM AUTHOR]

The substitution of selenium oxyanions in the hydroxyapatite structure was examined using multinuclear solid-state resonance spectroscopy (ssNMR). The study was supported by powder X-ray diffractometry (PXRD) and wavelength dispersion X-ray fluorescence (WD-XRF). Samples of pure hydroxyapatite (HA300) and selenate (HA300-1.2SeO4) or selenite (HA300-1.2SeO3) substituted hydroxyapatites were synthesized using the standard wet method and heated at 300 °C to remove loosely bonded water. PXRD data showed that all samples are single-phase, nanocrystalline hydroxyapatite. The incorporation of selenite and selenate ions affected the lattice constants. In selenium-containing samples the concentration of Se was very similar and amounted to 9.55% and 9.64%, for HA300-1.2SeO4 and HA300-1.2SeO3, respectively. PXRD and ssNMR data showed that the selenite doping significantly decreases the crystallite size and crystallinity degree. 31P and ¹H NMR experiments demonstrated the developed surface hydrated layer in all samples, especially in HA300-1.2SeO3. ¹H NMR studies showed the dehydroxylation of HA during the selenium oxyanions substitution and the existence of hydrogen bonding in structural hydroxyl group channels. ¹H→77Se cross polarization NMR experiments indicated that selenites and selenates are located in the crystal lattice and on the crystal surface. [ABSTRACT FROM AUTHOR]

Abstract: Removal of selenium oxyanions by the binary oxide systems, Al- or Fe-oxides mixed with X-ray noncrystalline SiO2, was previously not well understood. This study evaluates the adsorption capacity and kinetics of selenium oxyanions by different metal hydroxides onto SiO2, and uses X-ray absorption spectroscopy (XAS) to assess the interaction between selenium oxyanions and the sorbents at pH 5.0. The binary oxide systems of Al(III)- or Fe(III)-oxides mixed with SiO2 were prepared, and were characterized for their surface area, point of zero charge (PZC), pH envelopes, X-ray diffraction analysis (XRD), and then macro-scale adsorption isotherm and kinetics of selenite and selenate, micro-scale adsorption XAS. The adsorption capacity of selenite and selenate on Al(III)/SiO2 is greater than on Fe(III)/SiO2. Adsorption isothermal and kinetic data of selenium can be well fitted to the Langmuir isotherm and pseudo-second-order kinetic models. Based on simple geometrical constraints, selenite on both the binary oxide systems forms bidentate inner-sphere surface complexes, and selenate on Fe(III)/SiO2 forms stronger complexes than on Al(III)/SiO2. [Copyright &y& Elsevier]

Brassica juncea was grown in a soil spiked with selenium oxyanions (selenite and selenate) in order to verify the contribution of both plants and rhizospheric bacteria to the abatement of soluble forms of the metalloid. A mass balance of selenium was calculated in pots and the different chemical species of this contaminant were measured. Evidence gained suggests that selenium oxyanions were reduced into less bioavailable forms thank to a marked contribution of the soil bacterial population. Rhizobacteria resulted particularly elicited by the presence of B. juncea which directly participated in selenium decontamination through either phytoextraction or putative volatilisation. Moreover, these microbes colonizing B. juncea root system were monitored by both culture dependent and culture independent methods ( i.e. DGGE analysis). Finally, bacterial isolates were tested in vitro for their resistance to selenium oxyanions. [ABSTRACT FROM AUTHOR]

Abstract: To understand the molecular events involved in the reduction of selenite to non-toxic elemental selenium, 4000 clones of Ralstonia metallidurans CH34 were produced by random Tn5 transposon integration mutagenesis. Eight mutants were able to resist up to 15 mM selenite while the MIC for the wild-type strain was estimated as 4–6 mM selenite. The identification of the disrupted genes was carried out by Southern blot analysis and inverse PCR. The three resistant mutants containing only one insertion were further characterized. Tn5 disrupted a gene that encoded a protein which was closely related to proteins of the DedA family. This family represents a group of integral membrane proteins with completely unknown functions. Phenotypic characterization of the dedA mutants and selenite consumption experiments strongly suggest that DedA is involved in the uptake of selenite. [Copyright &y& Elsevier]