Reduction Of Selenite Research Articles

Reduction of selenite by cysteine in ionic media

This paper focuses on the abiotic reduction of selenite (Se(IV)) by cysteine (Cys, NH3+CH(CH2SH)COOH), which is a representative thiol produced by aquatic organism under oxidative stress. The rates of reduction of Se(IV) by cysteine were measured in deaerated NaCl solutions and natural waters as a function of pH (4.0–9.0), temperature (10–40°C), and ionic strength (0.01–1.0M). The rates showed a complex dependence on pH with similar values from pH 4.0–5.0, increasing values from pH 5.0–7.0 and then decreasing values at pH higher than 7.0. An apparent energy of activation obtained was 31±6kJmol−1, which was independent of ionic strength.The reaction is due to the following interactionHSeO3-+H2Cys0⇄k-STkST[ST]→kSe(0)Productswhere the selenotrisulfide [ST] is the complex intermediate RS–Se–SR with R=NH3+CH(CH2)COO−. The intermediate ST then decomposes in the final products Se(0) and cystine. The pseudo-first-order rate constant is written ask1=kST×1-k-STk-ST+kSe(0)×αHSeO3-×αH2Cys0×[Cys]Twhere the second-order rate constant (kST) was calculated as (2.0±0.2)×103M−1min−1 and was compared with the second-order rate constants of other reductants (hydrogen sulfide, ascorbic acid). In neutrality and alkaline solution, [ST] undergoes rapid decomposition and the term (1−k−ST/k−ST+kSe(0)) approximates 1 and may be neglected.k1=kST×αHSeO3-×αH2Cys0×[Cys]TEnvironmental significance of the results and the importance of abiotic vs biotic reactions are also briefly discussed.

Sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions

The sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions were investigated as a function of pH, Se(IV) concentration, and Fe(II) concentration under an anoxic condition. The sorption of Se(IV) onto chlorite surfaces followed the Langmuir isotherm regardless of the presence of Fe(II) ions in the solution. The Se(IV) sorption was observed to be very low at all pH values when the solution was Fe(II)-free or the concentration of Fe(II) ions was as low as 0.5 mg/L. However, the Se(IV) sorption was enhanced at a pH > 6.5 when the Fe(II) concentration was higher than 5 mg/L because of the increased sorption of Fe(II) onto the chlorite surfaces. XANES (X-ray absorption near edge structure) spectra of the Se K-edge showed that most of the sorbed Se(IV) was reduced to Se(0) by Fe(II) sorbed onto the chlorite surfaces, especially at pH > 9. The combined results of field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) also showed that elemental selenium and goethite were formed and precipitated on the chlorite surfaces during the sorption of selenite. Consequently it can be concluded that Se(IV) can be reduced to Se(0) in the presence of Fe(II) ions by the surface catalytic oxidation of Fe(II) into Fe(III) and the formation of goethite at neutral and particularly alkaline conditions. Thus the mobility of selenite in groundwater is expected to be reduced by the presence of a relatively higher concentration of Fe(II) in subsurface environments.

The reduction of selenite by Fe(II)-containing minerals

The radioactive isotope 79Se, with a half-life of 2.95×105 years, is presently considered as the key mobile fission product for the disposal of spent fuel and high-level radioactive waste. The solubility of selenium is significantly controlled by its oxidation state, which depends on the redox conditions of the surrounding environment. Tetravalent and hexavalent Se are very soluble and mobile aqueous oxyanions, while Se species with oxidation states 0, -I, and -II are low solubility solids. Due to the weak adsorption of Se(IV) and Se(VI) on natural minerals, in particular on granite or claystone minerals, which are considered as host rocks for nuclear waste disposal, reductive precipitation is considered to be the most effective way to immobilize 79Se. On the other hand, Fe(II)-bearing minerals (such as pyrite, pyrrhotite, magnetite, etc.) are ubiquitous in geological environment. Due to the reducing capacity, reduction of selenite by Fe(II)-containing minerals has received extensive attention worldwide. In this paper, we gave a critical review on the research progress in this field, along with the thermodynamic calculations on the reaction feasibility and the controlling-factor of the reaction products.

Selenate reduction and adsorption in littoral sediments from a hypersaline California lake, the Salton Sea

The Salton Sea, a hypersaline lake located in Southern California, is a major habitat for migratory waterfowl, including endangered species, recently threatened by selenium toxicity. Selenium is both an essential micronutrient and a contaminant and its speciation and cycling are driven by microbial activity. In the absence of oxygen, microorganisms can couple the oxidation of organic matter with the reduction of soluble selenate and selenite to elemental selenium. In order to better understand and quantify selenium cycling and selenium transfer between water and underlying sediments in the Salton Sea, we measured the maximum potential selenate reduction rates (Rmax) and selenate adsorption isotherms in sediments collected from seven littoral locations in July 2011. We also measured salinity, organic carbon, nitrogen, and elemental selenium content and the abundance of selenate-reducing prokaryotes at each site. Our results showed a high potential for selenate reduction and limited selenate adsorption in all studied sites. Maximum potential selenate reduction rates were affected by sediment Corg content. We showed that selenate reduction potential of Salton Sea sediments far outweighs current dissolved inputs to the lake. Selenate reduction is thus a likely driver for selenium removal from the lake’s water and selenate retention in littoral sediments of the Salton Sea.

Synthesis of Anatase Se/Te‐TiO2 Nanorods with Dominant {100} Facets: Photocatalytic and Antibacterial Activity Induced by Visible Light

AbstractA facile method for the preparation of Te‐ and Se/Te‐doped anatase TiO2 nanorods (NRs) with exposed {100} facets, which provides high photocatalytic and antibacterial activity under irradiation with visible light, is developed. Sodium titanate nanotubes are formed from commercial TiO2 nanoparticles (P25) in NaOH through a hydrothermal reaction. The as‐prepared Na‐titanate nanotubes are then used to form Te‐TiO2 nanotubes in the presence of tellurite (TeO32−) ions and NaBH4. Upon further hydrothermal reaction, the Te‐TiO2 nanotubes are transformed to form Te‐TiO2 NRs that further form Sen/Te‐TiO2 NRs through a reduction of selenite (SeO32−) ions with NaBH4. The Te‐TiO2 NRs and Sen/Te‐TiO2 NRs have highly active {100} facets and thus provide the photocatalytic activities for the generation of ⋅OH at 2.5 and 4.5 times higher than that of commercial P25, respectively. The Te‐TiO2 and Sen/Te‐TiO2 NRs exhibit higher antibacterial activities against E. coli and S. aureus than P25 when activated by visible light. These stable and biocompatible Te‐TiO2 and Sen/Te‐TiO2 NRs hold great potential as potent antibacterial agents.

Impact of temperature, CO2fixation and nitrate reduction on selenium reduction, by a paddy soilClostridiumstrain

To elucidate the impact of CO(2) fixation, nitrate reduction and temperature on selenium reduction by a newly identified acetogenic bacterium, Clostridium sp. BXM. A series of culture experiments were designed to evaluate the impact of temperature, CO(2) fixation and nitrate reduction on the rate and extent of selenium reduction by strain BXM. The products of selenium reduction, CO(2) fixation and nitrate reduction were determined. Molecular analysis was performed to identify the functional genes involved in the selenium reduction process. CO(2) may have enhanced the activity of hydrogenase I and/or the level of cytochrome b, thus increasing selenium reduction. Nitrate may inhibit selenium reduction due to its higher reduction potential and/or by decreasing selenite/selenate reductase activity. The suitable temperature was 37 and 30 °C for selenite reduction under anaerobic and aerobic conditions, respectively. The optimum temperature was 30 °C for selenate reduction under both anaerobic and aerobic conditions. CO(2) fixation and nitrate reduction by Clostridium sp. BXM stimulated each other. Clostridium sp. BXM was capable of reducing up to 36-94% of 1 mmol l(-1) selenate and selenite under anaerobic or aerobic conditions over 15 days. The strain might be used for the precipitation of Se from highly selenium-contaminated water or sediments. The findings contribute to the current understanding about the role that micro-organisms play in the detoxification of toxic selenium compounds in paddy soils. Micro-organisms in paddy soils can influence selenium accumulation in rice grain and hence human selenium intake.

Se-Bearing Colloidal Particles Produced by Sulfate-Reducing Bacteria and Sulfide-Oxidizing Bacteria: TEM Study

As determined by transmission electron microscopy (TEM), the reduction of selenate and selenite by Desulfovibrio desulfuricans, a sulfate-reducing bacterium, produces spherical (Se, S) sub-micro particles outside the cell. The particles are crystalline or amorphous, depending on medium composition. Amorphous-like Se-rich spherical particles may also occur inside the bacterial cells. The bacteria are more active in the reduction of selenite than selenate. The Desulfovibrio desulfuricans bacterium is able to extract S in the (S, Se) solid solution particles and transform S-rich particles into Se-rich and Se crystals. Photoautotrophs, such as Chromatium spp., are able to oxidize sulfide (S2-). When the bacteria grow in sulfide- and selenide-bearing environments, they produce amorphous-like (S, Se) globules inside the cells. TEM results show that compositional zonation in the (S, Se) globules occur in Chromatium spp. collected from a top sediment layer of a Se-contaminated pond. S2- may be from the products of sulfate-reducing bacteria. Both the sulfate-reducing bacteria and photosynthetic Chromatium metabolize S preferentially over Se. It is proposed that the S-rich zones are formed during photosynthesis (day) period, and the Se-rich zones are formed during respiration active (night) period. The results indicate that both Desulfovibrio desulfuricans and Chromatium spp. are able to immobilize the oxidized selenium (selenate and/or selenite) in the forms of elemental selenium and (Se, S) solid solutions. The bacteria reduce S in the (Se, S) particles and further enrich Se in the crystalline particles. The reduced S combines with Fe2+ to form amorphous FeS.

In Situ Time-Resolved X-ray Near-Edge Absorption Spectroscopy of Selenite Reduction by Siderite

The reduction-oxidation reaction between aqueous selenite (SeO(3)(2-)) and siderite (FeCO(3(s))) was monitored by in situ, time-resolved X-ray absorption near-edge structure (XANES) spectroscopy at the selenium K edge in a controlled electrochemical environment. Spectral evolutions showed that more than 60% of selenite was reduced at the siderite surface after 20 h of experiment, at which time the reaction was still incomplete. Fitting of XANES spectra by linear combination of reference spectra showed that selenite reaction with siderite is essentially a two-step process, selenite ions being immobilized on siderite surface prior to their reduction. A kinetic model of the reduction step is proposed, allowing to identify the specific contribution of surface reduction. These results have strong implications for the retention of selenite by corrosion products in nuclear waste repositories and in a larger extent for the fate of selenium in the environment.

Formation of Se (0) Nanoparticles by Duganella sp. and Agrobacterium sp. isolated from Se-laden soil of North-East Punjab, India

BackgroundSelenium (Se) is an essential trace element, but is toxic at high concentrations. Depending upon the geological background, the land use or on anthropogenic pollution, different amounts of Se may be present in soil. Its toxicity is related to the oxyanions selenate and selenite as they are water soluble and bioavailable. Microorganisms play an important role in Se transformations in soil and its cycling in the environment by transforming water-soluble oxyanions into water insoluble, non-toxic elemental Se (0). For this study, soil samples were collected from selenium-contaminated agricultural soils of Punjab/India to enrich and isolate microbes that interacted with the Se cycle.ResultsA mixed microbial culture enriched from the arable soil of Punjab could reduce 230 mg/l of water soluble selenite to spherical Se (0) nanoparticles during aerobic growth as confirmed by SEM-EDX. Four pure cultures (C 1, C 4, C 6, C 7) of Gram negative, oxidase and catalase positive, aerobic bacteria were isolated from this mixed microbial consortium and identified by 16 S rDNA gene sequence alignment as two strains of Duganella sp. (C 1, C 4) and two strains of Agrobacterium sp.(C 6, C 7). SEM/TEM-EDX analyses of the culture broth of the four strains revealed excretion of uniformly round sharply contoured Se (0) nanoparticles by all cultures. Their size ranged from 140–200 nm in cultures of strains C 1 and C 4, and from 185–190 nm in cultures of strains C 6 and C 7. Both Duganella sp. revealed better selenite reduction efficiencies than the two Agrobacterium sp.ConclusionsThis is the first study reporting the capability of newly isolated, aerobically growing Duganella sp. and Agrobacterium sp. from soils of Punjab/India to form spherical, regularly formed Se (0) nanoparticles from water soluble selenite. Among others, the four strains may significantly contribute to the biogeochemical cycling of Se in soil. Bioconversion of toxic selenite to non-toxic Se (0) nanoparticles under aerobic conditions in general may be useful for detoxification of agricultural soil, since elemental Se may not be taken up by the roots of plants and thus allow non-dangerous fodder and food production on Se-containing soil.

Isolation and identification of selenite reducing archaea from Tuz (salt) Lake In Turkey

In this study, Tuz lake brine samples were investigated for isolation and identification of selenite resistant halophilic prokaryotes. Among the 20 strains of extremely halophilic Bacteria and Archaea, a Gram negative rod designated as strain 106, showed high capacity in the resistance to selenite (25 mM) under aerobic conditions. Phenotypic characterizations and phylogenetic analyses based on 16S rDNA sequence comparison indicated that strain 106 was Halorubrum xinjiangense. The ability of strain 106 to deposite selenium-containing particles were investigated by Transmission Electron Microscopy (TEM). Electron micrographs shows intact cells after selenite reduction and large amounts of selenium-containing particles are present in the culture medium indicating that strain 106 is able to efficiently transport elemental selenium out of the cell.

Electrochemical Reduction of Selenite and Selenate Accelerated by Methyl Viologen

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Localization of the Dissimilatory Arsenate Reductase in <I>Sulfurospirillum Barnesi</I> Strain SeS-3

Problem statement: Sulfurospirillim barnesii strain SeS-3 is one of the recently known bacterial isolates that can obtain energy to support growth by respiring the toxic oxyanions of arsenic and selenium under anaerobic conditions. Approach: The ultimate goal of this investigation is to localize the active sites for the reduction of arsenate arsenite as well as selenate selenite in S. barnesii strain SeS-3. Results: The ability of the type strain Sulfurospirillium barnesii strain SES-3 (ATCC 700032) to reduce selenate and arsenate were tested using cell grown anaerobically with 20 mM lactate as the carbon source and either 10 mM selenate or 5 mM arsenate as the terminal electron acceptors were harvested after the turbidity reached 0.3, 0.4 absorption units at 600 nm, cell density 4gm wet cells. The results of this study showed that the mobilization of the toxic oxyanions of arsenic and selenium by Sulfurospirillum barnesii strain SES-3 is linked to the membrane. The enzyme is specific for the reduction of arsenate, selenate, selenite, nitrite, thiosulfate and phosphate. Whereas, no specificity was detected for arsenite nitrate, fumarate, when they served as the final electron acceptor. Conclusion/Recommendations: This study concluded that the mechanism of arsenate reduction by S.barnesii strain SeS-3 is connected into the membrane. The environmental significance of this bacterium and its impact to the bioremediation potential in the underground water and sedimentary environment is also discussed.

셀레늄-미생물간의 반응 및 셀레늄 광물화 특성

철환원 박테리아인 미시가넨시스를 이용하여 용존 셀레늄을 제거할 때, 물 속의 다른 금속성분들인 철, 황산염, 그리 구리가 미칠 수 있 영향을 살펴보았다. 미시가넨시스 박테리아는 산화수가 4가인 산화 셀레나이트(2 mM)를 셀레나이드로 환원시키고 물속의 셀레늄 농도를 점차 감소시켰다. 환원된 셀레나이드는 용존 2가 철과 결합하여 나노입자 크기의 철-셀레나이드로 침전되었다. 용존 황산염과 구리는 미생물의 셀레나이트 환원작용에 부정적인 영향을 끼쳤는데, 특히 구리 성분은 미생물에 대해 독성으로 작용하여 셀레나이트 제거가 원활히 이뤄지지 못하게 하였다. 이러한 결과로부터 알 수 있는 것은 셀레늄으로 오염된 현장을 미생물로 정화할 때 황산염 혹은 구리의 농도 분포와 양을 충분히 고려해야 한다는 사실이다. 궁극적으로 미생물에 의한 철-셀레나이드 광물형성작용은 지하수를 따라 원거리로 이동할 수 있는 셀레늄의 확산을 억제하는 중요한 수단이라고 볼 수 있다. Removal of dissolved selenium by D. michiganensis, a iron-reducing bacterium, and effects of dissolved metal elements such as iron, sulfate, and copper were investigated. Selenide that was reduced from selenite (2 mM) by D. michiganensis was gradually removed from the aqueous medium. As the reduced selenide was combined with aqueous iron, it was precipitated as a nanoparticulate iron-selenide. Sulfate and copper negatively affected the microbial selenite reduction, and the copper was especially toxic to the bacterium, inhibiting a microbial removal of dissolved selenite. These results show that it should be carefully biotreated for a selenium-contaminated site considering in situ sulfate or copper distribution and concentration. Consequently, the formation of iron-selenide by bacteria will be an important measure for preventing a long-distance migration of selenium in the subsurface environments.

A bacterial process for selenium nanosphere assembly

During selenate respiration by Thauera selenatis, the reduction of selenate results in the formation of intracellular selenium (Se) deposits that are ultimately secreted as Se nanospheres of approximately 150 nm in diameter. We report that the Se nanospheres are associated with a protein of approximately 95 kDa. Subsequent experiments to investigate the expression and secretion profile of this protein have demonstrated that it is up-regulated and secreted in response to increasing selenite concentrations. The protein was purified from Se nanospheres, and peptide fragments from a tryptic digest were used to identify the gene in the draft T. selenatis genome. A matched open reading frame was located, encoding a protein with a calculated mass of 94.5 kDa. N-terminal sequence analysis of the mature protein revealed no cleavable signal peptide, suggesting that the protein is exported directly from the cytoplasm. The protein has been called Se factor A (SefA), and homologues of known function have not been reported previously. The sefA gene was cloned and expressed in Escherichia coli, and the recombinant His-tagged SefA purified. In vivo experiments demonstrate that SefA forms larger (approximately 300 nm) Se nanospheres in E. coli when treated with selenite, and these are retained within the cell. In vitro assays demonstrate that the formation of Se nanospheres upon the reduction of selenite by glutathione are stabilized by the presence of SefA. The role of SefA in selenium nanosphere assembly has potential for exploitation in bionanomaterial fabrication.

Selenite Reduction by the Thioredoxin System: Kinetics and Identification of Protein-Bound Selenide

Selenite (SeO(3)(2-)) assimilation into a bacterial selenoprotein depends on thioredoxin (trx) reductase in Esherichia coli, but the molecular mechanism has not been elucidated. The mineral-oil overlay method made it possible to carry out anaerobic enzyme assay, which demonstrated an initial lag-phase followed by time-dependent steady NADPH consumption with a positive cooperativity toward selenite and trx. SDS-PAGE/autoradiography using (75)Se-labeled selenite as substrate revealed the formation of trx-bound selenium in the reaction mixture. The protein-bound selenium has metabolic significance in being stabilized in the divalent state, and it also produced the selenopersulfide (-S-SeH) form by the catalysis of E. coli trx reductase (TrxB).

Characterization of Pseudomonas stutzeri NT-I capable of removing soluble selenium from the aqueous phase under aerobic conditions

Pseudomonas stutzeri strain NT-I was isolated from the drainage wastewater of a selenium refinery plant. This bacterium efficiently reduced selenate to elemental selenium without prolonged accumulation of selenite under aerobic conditions. Strain NT-I was able to reduce selenate completely at high concentrations (up to 10 mM) and selenite almost completely (up to 9 mM). In addition, higher concentrations of selenate and selenite were substantially reduced. Activity was observed under the following experimental conditions: 20-50°C, pH 7-9, and 0.05-20 g L(-1) NaCl for selenate reduction, and 20-50°C, pH 6-9, and 0.05-50 g L(-1) NaCl for selenite reduction. Under anaerobic conditions, selenate was reduced more rapidly, whereas selenite was not reduced at all. The high selenate- and selenite-reducing capability at high concentrations suggested that strain NT-I is suitable for the removal of selenium from high-strength industrial wastewater.

Reduction of selenite to red elemental selenium by moderately halotolerant Bacillus megaterium strains isolated from Bhitarkanika mangrove soil and characterization of reduced product

Two Gram (+) bacterial strains, BSB6 and BSB12, showing resistance and potential for Se(IV) reduction among 26 moderately halotolerant isolates from the Bhitarkanika mangrove soil were characterized by biochemical and 16S rDNA sequence analyses. Both of them were strictly aerobic and able to grow in a wide range of pH (4–11), temperature (4–40 °C) and salt concentration (4–12%) having an optimum growth at 37 °C, pH ∼7.5 and 7% salt (NaCl). The biochemical characteristics and 16S rDNA sequence analysis of BSB6 and BSB12 showed the closest phylogenetic similarity with the species Bacillus megaterium. Both the strains effectively reduced Se(IV) and complete reduction of selenite (up to 0.25 mM) was achieved within 40 h. SEM with energy dispersive X-ray and TEM analyses revealed the formation of nano size spherical selenium particles in and around the bacterial cells which were also supported by the confocal micrograph study. The UV–Vis diffuse reflectance spectra and XRD of selenium precipitates revealed that the selenium particles are in the nanometric range and crystalline in nature. These bacterial strains may be exploited further for bioremediation process of Se(IV) at relatively high salt concentrations and green synthesis of selenium nanoparticles.

Adsorption of selenite and selenate ions onto thiourea‐formaldehyde resin

AbstractIn the present work, thiourea‐formaldehyde (TUF) chelating resin was synthesized and used in the adsorptions of selenite (SeO) and selenate (SeO) ions. The effects of initial acidity and initial selenium concentrations on the adsorptions were examined by batch technique. The synthesized resin was applied to the elemental analysis to determine its composition. FT‐IR spectra and SEM/EDS were also recorded before and after selenite adsorption. It was found that selenite and selenate ions were adsorbed onto TUF resin at strong acidic conditions (3–5M HCl). The adsorption capacities of the resin were calculated as 833.3 mg g−1 TUF resin for selenite ions and 526.3 mg g−1 TUF resin for selenate. All the adsorption data obtained for both selenite and selenate ions fitted well to the Langmuir isotherm. It was seen that the adsorption mechanisms in the both adsorptions were governed by the reduction of selenite or selenate to elemental selenium, Se0. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.

A novel method for the measurement of elemental selenium produced by bacterial reduction of selenite

The measurement of elemental selenium (Se0) is needed to assess the rate and magnitude of bacteria reduction of selenite or selenate. We have developed a spectrophotometric method for the measurement Se0 that is rapid and can be employed to measure the quantity of Se0 produced by bacterial cultures. This method employs the use of 1M Na2S to convert the insoluble elemental selenium to a red-brown solution and with this method there is a direct correlation between concentration of elemental selenium and the absorption at 500nm. To demonstrate the utility of this assay, we have followed the reduction of selenite to Se0 by Moraxella bovis, and by bacterial consortia in soil and water samples.

Monitoring microbial redox transformations of metal and metalloid elements under high pressure using in situ X‐ray absorption spectroscopy

X-ray absorption spectroscopy is a well-established method for probing local structural and electronic atomic environments in a variety of systems. We used X-ray absorption near-edge structure (XANES) spectroscopy for monitoring in real-time conditions selenium reduction in situ in live cultures of Shewanella oneidensis MR-1 under high hydrostatic pressure. High-quality XANES data show that Shewanella oneidensis MR-1 reduces selenite Se(IV) to red elemental selenium Se(0) up to 150 MPa without any intermediate redox state. MR-1 reduces all selenite provided (5-10 mM) between 0.1 and 60 MPa. Above 60 MPa the selenite reduction yield decreases linearly with pressure and the activity is calculated to stop at 155 ± 5 MPa. The analysis of cultures recovered after in situ measurements showed that the decrease in activity is linked to a decrease in viability. This study emphasizes the promising potential of XANES spectroscopy for real-time probing in situ microbial redox transformations of a broad range of metal and metalloid elements in live samples, including under high hydrostatic pressure.