Laboratory-scale continuous reactor for soluble selenium removal using selenate-reducing bacterium, Bacillus sp. SF-1

Zero-valent iron (ZVI) facilitated in-situ selenium (Se) immobilization and its recovery by magnetic separation: Mechanisms and implications for microbial ecology

Selenium (Se(VI)) is environmentally toxic. One of the most popular reducing agents for Se(VI) remediation is zero-valent iron (ZVI). However, most ZVI studies were carried out in water matrices, and the recovery of reduced Se has not been investigated. A water-sediment system constructed using natural sediment was employed here to study in-situ Se remediation and recovery. A combined effect of ZVI and unacclimated microorganisms from natural sediment was found in Se(VI) removal in the water phase with a removal efficiency of 92.7 ± 1.1% within 7 d when 10 mg L-1 Se(VI) was present. Soluble Se(VI) was removed from the water and precipitated to the sediment phase (74.8 ± 0.1%), which was enhanced by the addition of ZVI (83.3 ± 0.3%). The recovery proportion of the immobilized Se was 34.2 ± 0.1% and 92.5 ± 0.2% through wet and dry magnetic separation with 1 g L-1 ZVI added, respectively. The 16 s rRNA sequencing revealed the variations in the microbial communities in response to ZVI and Se, which the magnetic separation could potentially mitigate in the long term. This study provides a novel technique to achieve in-situ Se remediation and recovery by combining ZVI reduction and magnetic separation.

Developing a versatile tool for studying kinetics of Selenate-Se removal from aqueous solution using a chemostat bioreactor

Understanding the impact of various parameters on the kinetics of dissolved selenium (Se) removal in bioreactors can be a challenging task, primarily due to the mass transfer limitations inherent in bioreactors employing attached growth configurations. This study successfully established a proof-of-concept for the efficient removal of Se from aqueous solutions using a chemostat bioreactor that relies solely on suspended growth. The research investigated the effect of selenate-Se feed concentrations under two distinct Se concentration conditions. One experiment was conducted at a considerably elevated concentration of 25 mg/L to impose stress on the system and evaluate its response. Another experiment replicated an environmentally relevant concentration of 1 mg/L, mirroring the typical Se concentrations in mine water. The bioreactor, featuring a working volume of 0.35 L, was operated as an anaerobic, fully mixed chemostat with hydraulic retention times (HRTs) ranging from 5 to 0.25 days. The outcomes revealed the chemostat's capacity to remove up to 25 mg/L of dissolved Se from water for all HRTs exceeding 1 day, under otherwise optimal conditions encompassing temperature, pH, and salinity. The research's significance lies in the development of a versatile tool designed to examine Se removal kinetics within a system devoid of mass transfer limitations. Furthermore, this study verified the ability of the bacterial consortium, obtained from a mine-influenced environment and enriched in the laboratory, to grow and sustain Se removal activities within a chemostat operating with HRTs as short as 1 day.

Selenite bioreduction with concomitant green synthesis of selenium nanoparticles by a selenite resistant EPS and siderophore producing terrestrial bacterium

Environmental bacterial isolates play a very important role in bioremediation of metals and toxic metalloids. A bacterial strain with high selenite (SeO32-) tolerance and reducing capability was isolated from electronic waste dump site in Banaras Hindu University, Varanasi, India. Based on 16 S rRNA sequencing and BLAST search, this bacterial isolate was identified as Bacillus paramycoides and designated as strain MF-14. It tolerated Sodium selenite up to 110 mM when grown aerobically in LB broth and reduced selenite into elemental selenium (Se0) significantly within 24 h with concomitant biosynthesis of selenium nanoparticles as clearly revealed by brick red precipitate and specific surface plasmon resonance peak at 210 nm using UV-Visible spectrophotometer. Scanning electron microscopy (SEM) analysis of this bacterial strain exposed to 1mM and 5 mM selenite also demonstrated morphological alterations as cell enlargement due to accumulation and bioprecipitation of elemental selenium (Se0). The FTIR analysis clearly demonstrated that functional groups present on the surface of biogenic selenium nanoparticles (SeNPs) play a significant role in the stabilization and capping of SeNPs. Furthermore, these SeNPs were characterized using spectroscopic analysis involving Dynamic light scattering, zeta potential, XPS, FTIR, XRD and Raman spectroscopy which clearly revealed particle size 10-700 nm, amorphous nature, stability as well as it's oxidation state. The biochemical studies have demonstrated that membrane bound reductase enzyme may be responsible for significant reduction of selenite into elemental selenium. Therefore, we may employ Bacillus paramycoides strain MF-14 successfully for bioremediation of selenite contaminated environmental sites with concomitant green synthesis of SeNPs.

Selenite and tellurite reduction by Aspergillus niger fungal pellets using lignocellulosic hydrolysate

The performance of Aspergillus niger pellets to remove selenite and tellurite from wastewater using batch and continuous fungal pelleted bioreactors was investigated. The acid hydrolysate of brewer's spent grain (BSG) was utilized by A. niger as the electron donor for selenite and tellurite reduction. The dilution of BSG hydrolysate using mineral medium had a positive effect on the selenite and tellurite removal efficiency with a 1:3 ratio giving the best efficiency. However, selenite and tellurite inhibited fungal growth with a 40.9% and 27.3% decrease in the A. niger biomass yield in the presence of 50 mg/L selenite and tellurite, respectively. The maximum selenite and tellurite removal efficiency using 25% BSG hydrolysate in batch incubations amounted to 72.8% and 99.5% Two fungal pelleted bioreactors were operated in continuous mode using BSG hydrolysate as the substrate. Both the selenite and tellurite removal efficiencies during steady state operation were > 80% with tellurite showing a maximum removal efficiency of 98.5% at 10 mg/L influent concentration. Elemental Se nanospheres for selenite and both Te nanospheres and nanorods for tellurite were formed within the fungal pellets. This study demonstrates the suitability BSG hydrolysate as a low cost carbon source for removal of selenite and tellurite using fungal pellet bioreactors.

Sequential removal of selenate, nitrate and sulfate and recovery of elemental selenium in a multi-stage bioreactor process with redox potential feedback control

Bioreduction can facilitate oxyanions removal from wastewater. However, simultaneously removing selenate, nitrate and sulfate and recovering high-purity elemental selenium (Se⁰) from wastewater by a single system is difficult and may lead to carcinogenic selenium monosulfide (SeS) formation. To solve this issue, a two-stage biological fluidized bed (FBR) process with ethanol dosing based on oxidation-reduction potential (ORP) feedback control was developed in this study. FBR1 performance was first evaluated at various ORP setpoints (between -520 and -360 mV vs. Ag/AgCl) and elevated sulfate concentration. Subsequently, ethanol-fed FBR2 was used to reduce sulfate from FBR1 effluent, followed by an aerated sulfide oxidation reactor (SOR). At - 520 mV≤ ORPs≤ -480 mV, FBR1 removed 100 ± 0.1% nitrate and 99.7 ± 0.3% selenate without sulfate reduction. At ORPs ≥ -440 mV, selenate reduction was incomplete, whereas nitrate removal remained stable. Se⁰ recovery efficiency from FBR1 effluent was 37.5% with 71% Se purity. FBR2 converted 86% of the remaining sulfate in FBR1 effluent to hydrogen sulfide, but the over-oxidation of dissolved sulfide in SOR decreased the overall sulfate removal efficiency to ~46.3%. Overall, the two-stage FBR process with ORP feedback dosing of ethanol was effective for sequentially removing selenate, nitrate and sulfate and recovering Se⁰ from wastewater.

NAD(P)H-dependent thioredoxin-disulfide reductase TrxR is essential for tellurite and selenite reduction and resistance in Bacillus sp. Y3

Microbial reduction of selenite [Se(IV)] and tellurite [Te(IV)] to Se(0) and Te(0) can function as a detoxification mechanism and serve in energy conservation. In this study, Bacillus sp. Y3 was isolated and demonstrated to have an ability of simultaneous reduction of Se(IV) and Te(IV) during aerobic cultivation, with reduction efficiencies of 100% and 90%, respectively. Proteomics analysis revealed that the putative thioredoxin disulfide reductase (TrxR) and sulfate and energy metabolic pathway proteins were significantly upregulated after the addition of Se(IV) and Te(IV). qRT-PCR also showed an increased trxR transcription level in the presence of Se(IV) and Te(IV). Compared with a wild-type Escherichia coli strain, the TrxR-overexpressed E. coli strain showed higher Se(IV) and Te(IV) resistance levels and reduction efficiencies. Additionally, the TrxR showed in vitro Se(IV) and Te(IV) reduction activities when NADPH or NADH were present. When NADPH was used as the electron donor, the optimum conditions for enzyme activities were pH 8.0 and 37°C. The Km values of Te(IV) and Se(IV) were 16.31 and 2.91 mM, and the Vmax values of Te(IV) and Se(IV) were 12.23 and 11.20 µM min-1 mg-1, respectively. The discovery of the new reductive enzyme TrxR enriches the repertoire of the bacterial Se(IV) and Te(IV) resistance and reduction mechanisms. Bacillus sp. Y3 can efficiently reduce Se(IV) and Te(IV) simultaneously. Strain Y3 provides potential applications for selenite and tellurite bioremediation. The TrxR enzyme shows high catalytic activity for reducing Se(IV) and Te(IV). The discovery of TrxR enriches the bacterial Se(IV) and Te(IV) reduction mechanisms.

Optimization of nitrate and selenate reduction in an ethanol-fed fluidized bed reactor via redox potential feedback control

Electron donors are a major cost-factor in biological removal of oxyanions, such as nitrate and selenate from wastewater. In this study, an online ethanol dosing strategy based on feedback control of oxidation-reduction potential (ORP) was designed to optimize the performance of a lab-scale fluidized bed reactor (FBR) in treating selenate and nitrate (5 mM each) containing wastewater. The FBR performance was evaluated at various ORP setpoints ranging between -520 mV and -240 mV (vs. Ag/AgCl). Results suggested that both nitrate and selenate were completely removed at ORPs between -520 mV and -360 mV, with methylseleninic acid, selenocyanate, selenosulfate and ammonia being produced at low ORPs between -520 mV and -480 mV, likely due to overdosing of ethanol. At ORPs between -300 mV and -240 mV, limited ethanol dosing resulted in an apparent decline in selenate removal whereas nitrate removal remained stable. Resuming the ORP to -520 mV successfully restored complete selenate reduction. An optimal ORP of -400 mV was identified for the FBR, whereby selenate and nitrate were nearly completely removed with a minimal ethanol consumption. Overall, controlling ORP via feedback-dosing of the electron donor was an effective strategy to optimize FBR performance for reducing selenate and nitrate in wastewater.

High-rate microbial selenate reduction in an up-flow anaerobic fluidized bed reactor (FBR)

Wastewater contaminated with high concentrations of selenium oxyanions requires treatment prior to discharge. Biological fluidized bed reactors (FBRs) can be an option for removing selenium oxyanions from wastewater by converting them into elemental selenium, which can be separated from the treated effluent. In this study, a lab-scale FBR was constructed with granular activated carbon as biofilm carrier and inoculated with a consortium of selenate reducing bacteria enriched from environmental samples. The FBR was loaded with an influent containing ethanol (10 mM) and selenate (10 mM) as the microbial electron donor and acceptor, respectively. The performance of the FBR in reducing selenate was evaluated under various hydraulic retention times (HRTs) (120 h, 72 h, 48 h, 24 h, 12 h, 6 h, 3 h, 1 h and 20 min). After process acclimatization, selenate was completely removed with no notable selenite produced when the HRT was stepwise decreased from 120 h to 6 h. However, decreasing the HRT to 3 h resulted in selenite accumulation (0.17 ± 0.023 mM) in the effluent although selenate removal efficiency remained at 99.8 ± 0.20%. At 1 h HRT, the FBR removed 90.8 ± 1.4% of the selenate at a rate of 9.6 ± 0.15 mM h^-1, which is the highest selenate reduction rate reported in the literature so far. However, 1 h HRT resulted in notable selenite accumulation (up to 2.4 ± 0.27 mM). Further decreasing the HRT to 20 min resulted in a notable decline in selenate reduction. Selenate reduction recovered from the "shock loading" after the HRT was increased back to 3 h. However, selenite still accumulated until the FBR was operated in batch mode for 6 days. This study affirmed that FBR is a promising treatment option for selenate-rich wastewater, and the process can be efficiently operated at low HRTs.

Bacilli-Mediated Degradation of Xenobiotic Compounds and Heavy Metals

Xenobiotic compounds are man-made compounds and widely used in dyes, drugs, pesticides, herbicides, insecticides, explosives, and other industrial chemicals. These compounds have been released into our soil and water due to anthropogenic activities and improper waste disposal practices and cause serious damage to aquatic and terrestrial ecosystems due to their toxic nature. The United States Environmental Protection Agency (USEPA) has listed several toxic substances as priority pollutants. Bacterial remediation is identified as an emerging technique to remove these substances from the environment. Many bacterial genera are actively involved in the degradation of toxic substances. Among the bacterial genera, the members of the genus Bacillus have a great potential to degrade or transform various toxic substances. Many Bacilli have been isolated and characterized by their ability to degrade or transform a wide range of compounds including both naturally occurring substances and xenobiotic compounds. This review describes the biodegradation potentials of Bacilli toward various toxic substances, including 4-chloro-2-nitrophenol, insecticides, pesticides, herbicides, explosives, drugs, polycyclic aromatic compounds, heavy metals, azo dyes, and aromatic acids. Besides, the advanced technologies used for bioremediation of environmental pollutants using Bacilli are also briefly described. This review will increase our understanding of Bacilli-mediated degradation of xenobiotic compounds and heavy metals.

Effect of Nitrate and Perchlorate on Selenate Reduction in a Sequencing Batch Reactor

Selenate removal from a water body is being vigorously debated owing to severe health impact, but inhibitions of coexisting anions have been reported. To suggest a viable treatment option, this study investigates the effect of nitrate and perchlorate on selenate reduction in a laboratory-scale sequencing batch reactor. The experimental design tests how competing electron acceptors (NO3− and ClO4−) and electron donor (acetate) limitations affect selenate reduction in the reactor. Results show that the reactor achieves almost complete selenate reduction within the initial concentration ranges of 0.1–1 mM by enriching selenate-reducing bacteria with appropriate temperature (30 °C) and acclimation period (50 days). We monitored simultaneous selenate and nitrate reduction in the reactor without specific inhibition due to a difference in microbial growth strategy related to electron donor status. Lack of perchlorate-reducing bacteria makes perchlorate addition (0.2 mM) not to be closely associated with dissimilative perchlorate reduction. These results provide information that can help us to understand the effect of competing electron acceptors on selenate reduction and the kinetics of potential parallel reactions in the reactor.

Dissolved oxygen has no inhibition on methane oxidation coupled to selenate reduction in a membrane biofilm reactor

Methane oxidation coupled to selenate reduction has been suggested as a promising technology to bio-remediate selenium contaminated environments. However, the effect of dissolved oxygen (DO) on this process remained unclear. Here, we investigate the feasibility of selenate removal at two distinct DO concentrations. A membrane biofilm reactor (MBfR) was initially fed with ∼5 mg Se/L and then lowered to ∼1 mg Se/L of selenate, under anoxic condition containing ∼0.2 mg/L of influent DO. Selenate removal reached approximately 90% without selenite accumulation after one-month operation. Then 6-7 mg/L of DO was introduced and showed no apparent effect on selenate reduction in the subsequent operation. Electron microscopy suggested elevated oxygen exposure did not affect microbial shapes. 16S rDNA sequencing showed the aerobic methanotroph Methylocystis increased, while possible selenate reducers, Ignavibacterium and Bradyrhizobium, maintained stable after oxygen boost. Gene analysis indicated that nitrate/nitrite reductases positively correlated with selenate removal flux and were not remarkably affected by oxygen addition. Reversely, enzymes related with aerobic methane oxidation were obviously improved. This study provides a potential technology for selenate removal from oxygenated environments in a methane-based MBfR.

A new and practical Se(IV) removal method using Fe3+ type cation exchange resin

An effective method for removing selenium (Se) from water is required from the viewpoint of environmental preservation. To establish this method, a cation exchange resin that adsorbed ferric ions was applied as an adsorbent. In this study, the adsorption behavior of Se to the adsorbent was examined by both batch and column methods. The batch experiment confirmed that selenite ions (Se(IV)) are effectively adsorbed but selenate ions (Se(VI)) are hardly adsorbed. To elucidate the adsorption mechanism, the Fe in the adsorbent and the Fe in the adsorbent after the adsorption of Se(IV) were characterized by Fe K-edge X-ray absorption spectroscopy and ⁵⁷Fe Mӧssbauer spectroscopy. The analytical result of Se K-edge EXAFS spectra for the Se(IV) adsorbed on the adsorbent suggests that Se(IV) are adsorbed specifically to the adsorbent through the formation of Fe-O-Se bonds. The breakthrough curve obtained by the column experiment showed that Se(IV) in 3 tons of synthetic solution containing 0.1 ppm Se can be efficiently removed using a column in which 12.8 g (10.4 cm³) of the adsorbent was packed.

Biosynthesis of selenium nanoparticles and effects of selenite, selenate, and selenomethionine on cell growth and morphology in Rahnella aquatilis HX2

Rahnella aquatilis HX2 (proteobacteria) shows tolerance to selenium (Se). The minimum inhibitory concentrations of selenomethionine (Se-Met), selenite [Se (IV)], and selenate [Se (VI)] to HX2 are 4.0, 85.0, and 590.0 mM, respectively. HX2 shows the ability to reduce Se (IV) and Se (VI) to elemental Se nanoparticles (SeNPs). The maximum production of SeNPs by HX2 strain is 1.99 and 3.85 mM in Luria-Bertani (LB) broth with 5 mM Se (IV) and 10 mM Se (VI), respectively. The morphology of SeNPs and cells were observed by transmission electron microscope, environmental scanning electron microscope, and selected area electric diffraction detector. Spherical SeNPs with amorphous structure were found in the cytoplasm, membrane, and exterior of cells. Morphological variations of the cell membrane were further confirmed by the release of cellular materials absorbed at 260 nm. Flagella were inhibited and cell sizes were 1.8-, 1.6-, and 1.2-fold increases with the Se-Met, Se (VI), and Se (IV) treatments, respectively. The real-time quantitative PCR analysis indicated that some of the genes controlling Se metabolism or cell morphology, including cysA, cysP, rodA, ZntA, and ada, were significantly upregulated, while grxA, fliO, flgE, and fliC genes were significantly downregulated in those Se treatments. This study provided novel valuable information concerning the cell morphology along with biological synthesis process of SeNPs in R. aquatilis and demonstrated that the strain HX2 could be applied in both biosynthesis of SeNPs and in management of environmental Se pollution.

Probiotic and antioxidant properties of selenium-enriched Lactobacillus brevis LSe isolated from an Iranian traditional dairy product

The present study was designed to isolate a highly selenium-tolerant lactobacillus strain from an Iranian traditional dairy product named as Spar. Different criteria such as tolerance to the low pH, simulated gastric juice (SGJ), simulated intestinal juice (SIJ) and bile salts tolerance as well as Caco-2 cell adhesion assay were examined to evaluate the probiotic potentials of the selected isolate. Furthermore, the antioxidant properties of the isolate cultivated in medium containing and free of SeO₃^2- ions were evaluated using DPPH scavenging and reducing power assays. The isolate was identified using conventional identification and 16S rDNA gene sequencing methods as Lactobacillus brevis LSe. The obtained results showed that the isolate was able to tolerate high concentration of sodium selenite (3.16mM). By decreasing the pH of the SGJ from 6 to 3, the survival percent of L. brevis LSe was not significantly changed over the time (p>0.05). In addition, the survival percent of the isolate in the SIJ (pH 6 and pH 8) was not statistically altered after 3h, 6h and 24h of incubation (p>0.05). In the presence of bile salts (0.3% and 0.6%) the survival rate of L. brevis LSe was not significantly decreased (p>0.05).L. brevis LSe also demonstrated the satisfactory ability to adhere to Caco-2 cells which were similar to that of the reference strain L. plantarum. The obtained results of antioxidant evaluation showed that L. brevis LSe containing elemental Se exhibited significantly higher radical scavenging ability (36.5±1.31%) and reducing power (OD₇₀₀, 0.14) than L. brevis LSe cultured in selenite-free medium (p<0.05). To sum up, further investigations should be conducted to merit the probable potential health benefit of Se-enriched L. brevis LSe and its application as Se-containing supplements or fermented foods.

Microbial Transformations of Selenium Species of Relevance to Bioremediation

Selenium species, particularly the oxyanions selenite (SeO3 (2-)) and selenate (SeO4 (2-)), are significant pollutants in the environment that leach from rocks and are released by anthropogenic activities. Selenium is also an essential micronutrient for organisms across the tree of life, including microorganisms and human beings, particularly because of its presence in the 21st genetically encoded amino acid, selenocysteine. Environmental microorganisms are known to be capable of a range of transformations of selenium species, including reduction, methylation, oxidation, and demethylation. Assimilatory reduction of selenium species is necessary for the synthesis of selenoproteins. Dissimilatory reduction of selenate is known to support the anaerobic respiration of a number of microorganisms, and the dissimilatory reduction of soluble selenate and selenite to nanoparticulate elemental selenium greatly reduces the toxicity and bioavailability of selenium and has a major role in bioremediation and potentially in the production of selenium nanospheres for technological applications. Also, microbial methylation after reduction of Se oxyanions is another potentially effective detoxification process if limitations with low reaction rates and capture of the volatile methylated selenium species can be overcome. This review discusses microbial transformations of different forms of Se in an environmental context, with special emphasis on bioremediation of Se pollution.

Selenium: environmental significance, pollution, and biological treatment technologies

Selenium is an essential trace element needed for all living organisms. Despite its essentiality, selenium is a potential toxic element to natural ecosystems due to its bioaccumulation potential. Though selenium is found naturally in the earth's crust, especially in carbonate rocks and volcanic and sedimentary soils, about 40% of the selenium emissions to atmospheric and aquatic environments are caused by various industrial activities such as mining-related operations. In recent years, advances in water quality and pollution monitoring have shown that selenium is a contaminant of potential environmental concern. This has practical implications on industry to achieve the stringent selenium regulatory discharge limit of 5μgSeL(-1) for selenium containing wastewaters set by the United States Environmental Protection Agency. Over the last few decades, various technologies have been developed for the treatment of selenium-containing wastewaters. Biological selenium reduction has emerged as the leading technology for removing selenium from wastewaters since it offers a cheaper alternative compared to physico-chemical treatments and is suitable for treating dilute and variable selenium-laden wastewaters. Moreover, biological treatment has the advantage of forming elemental selenium nanospheres which exhibit unique optical and spectral properties for various industrial applications, i.e. medical, electrical, and manufacturing processes. However, despite the advances in biotechnology employing selenium reduction, there are still several challenges, particularly in achieving stringent discharge limits, the long-term stability of biogenic selenium and predicting the fate of bioreduced selenium in the environment. This review highlights the significance of selenium in the environment, health, and industry and biotechnological advances made in the treatment of selenium contaminated wastewaters. The challenges and future perspectives are overviewed considering recent biotechnological advances in the management of these selenium-laden wastewaters.

Kinetics of selenate sorption in soil as influenced by biotic and abiotic conditions: a stirred flow-through reactor study

This study (i) quantified the kinetics of selenate sorption and (ii) measured the influence of biotic processes in soil selenate stabilisation. Stirred flow-through reactor experiments were conducted on samples of a silty clay soil (pH = 8, Eh = 240-300 mV) from Bure (France) in both non-sterile and sterile conditions. Parameters of the proposed two-site sorption model (EK), adapted from van Genuchten and Wagenet (1989), were estimated by nonlinear regression. Fast selenate sorption on type-1 sites was moderate, with an equilibrium constant of 25.5 and 39.1 L/kg for non-sterile and sterile conditions. Rate-limited sorption on type-2 sites increased with time, and was predominant for longer periods of time in non-sterile conditions. At equilibrium, it would represent over 96% of the sorbed inventory, with mean sorption times of 17 h and 191 h for non-sterile and sterile conditions. Our results showed for Bure soil that (i) selenate sorption in flowing and mildly-oxidising conditions was strongly kinetically controlled, especially in non-sterile conditions, (ii) selenate desorption was much slower than sorption, which suggests its pseudo-irreversible stabilisation, and (iii) microbial activity increased the contribution of rate-limited sorption on type-2 sites, for which it increased sorption rate by a factor 7 but also facilitated its reversibility. This work stresses the limits of the Kd approach to represent selenate sorption in flowing conditions and supports an alternative formulation like the EK model, but also points out that biotic conditions are significant sources of variability for sorption parameters.

Colloidal properties of nanoparticular biogenic selenium govern environmental fate and bioremediation effectiveness

Microbial selenium (Se) bioremediation is based on conversion of water soluble, toxic Se oxyanions to water insoluble, elemental Se. Formed biogenic elemental Se is of nanometer size, hampering straightforward separation from the aqueous phase. This study represents the first systematic investigation on colloidal properties of pure biogenic Se suspensions, linking electrophoretic mobility (ζ-potential) to column settling behavior. It was demonstrated that circumneutral pH, commonly applied in bioremediation, is not appropriate for gravitational separation due to the negative ζ-potential preventing agglomeration. Mono/di/trivalent counter cations and acidity (protons) were used to screen efficiently the intrinsic negative charge of biogenic Se suspensions at circumneutral pH. Fast settling was induced by La(3+) addition in the micromolar range (86.2 ± 3.5% within 0.5 h), whereas considerably higher concentrations were needed when Ca(2+) or Na(+) was used. Colloidal stability was furthermore studied in different model waters. It was demonstrated that surface waters as such represent a fragile system regarding colloidal stability of biogenic Se suspensions (ζ-potential ∼ -30 mV), whereas dissolved organic matter increases colloidal stability. In marine waters, biogenic Se is colloidally destabilized and is thus expected to settle, representing a potential sink for Se during transport in the aquatic environment.

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.

Identification and determination of selenosulfate and selenocyanate in flue gas desulfurization waters

In this work, 13 selenium species in flue gas desulfurization (FGD) waters from coal-fired power plants were separated and quantified using anion-exchange chromatography coupled to inductively coupled plasma mass spectrometry. For the first time, we identified both selenosulfate (SeSO(3)(2-)) and selenocyanate (SeCN(-)) in such waters, using retention time matching and confirmation by electrospray mass spectrometry. Besides selenite and selenate, selenosulfate was the most frequently occurring selenium species. It occurred in most samples and constituted a major fraction (up to 63%) of the total selenium concentration in waters obtained from plants employing inhibited oxidation scrubbers. Selenocyanate occurred in about half of the tested samples, but was only a minor species (up to 6% of the total selenium concentration). Nine additional Se-containing compounds were found in FGD waters, but they remain unidentified at this point.

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.

The removal of selenate to low ppb levels from flue gas desulfurization brine using the H2-based membrane biofilm reactor (MBfR)

The H(2)-based membrane biofilm reactor (MBfR) was shown to consistently remove nitrate, nitrite, and selenate at high efficiencies from flue-gas desulfurization brine. Selenate was removed to <50 ppb which is the National Pollutant Discharge Elimination System (NPDES) criteria for the brine to be released into the environment. When selenate was removed to <50 ppb, nitrate and nitrite were still present in the mg/L range which suggests that selenate is able to be secondarily reduced to low levels when nitrate and nitrite serve as the main electron acceptors for bacterial growth. SO(4)(2-) was not removed and therefore did not compete with nitrate and selenate reduction for the available H(2).

Combined speciation analysis by X-ray absorption near-edge structure spectroscopy, ion chromatography, and solid-phase microextraction gas chromatography-mass spectrometry to evaluate biotreatment of concentrated selenium wastewaters

In this study we evaluate the potential of anaerobic granular sludge as an inoculum for the bioremediation of selenium-contaminated waters using species-specific analytical methods. Solid species formed by microbial reduction were investigated using X-ray absorption near-edge structure (XANES) spectroscopy at the selenium K-edge. Furthermore, dissolved selenium species were specifically determined by ion chromatography (IC) and solid-phase microextraction gas chromatography-mass spectrometry (SPME-GC-MS). Least-squares linear combination of the XANES spectra for samples incubated with the highest selenate/selenite concentrations (10(-3) M) show the predominance of elemental selenium and a Se(-I) selenide, such as ferroselite, the thermodynamically most stable iron selenide. In contrast, elemental selenium and Se(-II) selenides are the main species detected at the lower selenate/selenite concentrations. In each repeated fed batch incubation, most aqueous selenite anions were converted into solid selenium species, regardless of the type of electron donor used (acetate or H(2)/CO(2)) and the selenium concentration applied. On the other hand, at higher concentrations of selenate (10(-4) and 10(-3) M), significant amounts of the oxyanion remained unconverted after consecutive incubations. SPME-GC-MS demonstrated selenium alkylation with both electron donors investigated, as dimethyl selenide (DMSe) and dimethyl diselenide (DMDSe). Selenite was even more alkylated in the presence of H(2)/CO(2) (maximum 2156 μg of Se/L of DMSe + DMDSe) as compared to acetate (maximum 50 μg of Se/L). In contrast, selenate was less alkylated using both electron donors (maximum 166 and 3 μg of Se/L, respectively). The high alkylation potential for selenite limits its bioremediation in selenium laden waters involving H(2)/CO(2) as the electron donor despite the fact that nontoxic elemental selenium and thermodynamically stable metal selenide species are formed.

The essential toxin: the changing perception of selenium in environmental sciences

During the last decades, the perception of selenium has undergone substantial changes. While its toxic effects were recognized causing hair and hoof loss in animals during the 1930s, its essential role in microbial, animal and human metabolism has been recognized later, i.e. with the discovery of selenium deficiency causing "white muscle disease" in feedstock in the 1950s. Nowadays, the positive effect of systematic selenium supplementation is discussed in manifold topics such as cancer or diabetes prevention and avian influenza susceptibility. Treatment of selenium containing waste streams poses a notable challenge to environmental engineers, and to date no ultimate solution has been found for e.g. the selenium contamination in agricultural areas of the western USA. For the future, selenium contamination carries an imminent danger, if the increasing energy demand is covered by fossil fuel combustion, which will lead to major selenium emission and toxicity. This review presents current knowledge of selenium's role in environmental sciences and outlines potentially feasible treatment options targeting a variety of selenium contaminated waste streams.

Macro- and microscale investigation of selenium speciation in Blackfoot river, Idaho sediments

The transport and bioavailability of selenium in the environment is controlled by its chemical speciation. However, knowledge of the biogeochemistry and speciation of Se in streambed sediment is limited. We investigated the speciation of Se in sediment cores from the Blackfoot River (BFR), Idaho using sequential extractions and synchrotron-based micro-X-ray fluorescence (micro-SXRF). We collected micro-SXRF oxidation state maps of Se in sediments, which had not been done on natural sediment samples. Selective extractions showed that most Se in the sediments is present as either (1) nonextractable Se or (2) base extractable Se. Results from micro-SXRF showed three defined species of Se were present in all four samples: Se(-II,O), Se(IV), and Se(VI). Se(-II,O) was the predominant species in samples from one location, and Se(IV) was the predominant species in samples from a second location. Results from both techniques were consistent, and suggested that the predominant species were Se(-II) species associated with recalcitrant organic matter, and Se(IV) species tightly bound to organic materials. This information can be used to predict the biogeochemical cycling and bioavailability of Se in streambed sediment environments.

Biological alkylation and colloid formation of selenium in methanogenic UASB reactors

Bioalkylation and colloid formation of selenium during selenate removal in upflow anaerobic sludge bed (UASB) bioreactors was investigated. The mesophilic (30 degrees C) UASB reactor (pH = 7.0) was operated for 175 d with lactate as electron donor at an organic loading rate of 2 g COD L(-1) d(-1) and a selenium loading rate of 3.16 mg Se L(-1) d(-1). Combining sequential filtration with ion chromatographic analysis for selenium oxyanions and solid phase micro extraction gas chromatography mass spectrometry (SPME-GC-MS) for alkylated selenium compounds allowed to entirely close the selenium mass balance in the liquid phase for most of the UASB operational runtime. Although selenate was removed to more than 98.6% from the liquid phase, a less efficient removal of dissolved selenium was observed due to the presence of dissolved alkylated selenium species (dimethylselenide and dimethyldiselenide) and colloidal selenium particles in the effluent. The alkylated and the colloidal fractions contributed up to 15 and 31%, respectively, to the dissolved selenium concentration. The size fractions of the colloidal dispersion were: 4 to 0.45 mum: up to 21%, 0.45 to 0.2 mum: up to 11%, and particles smaller than 0.2 mum: up to 8%. Particles of 4 to 0.45 mum were formed in the external settler, but did not settle. SEM-EDX analysis showed that microorganisms form these selenium containing colloidal particles extracellularly on their surface. Lowering the temperature by 10 degrees C for 6 h resulted in drastically reduced selenate removal efficiencies (after a delay of 1.5 d), accompanied by the temporary formation of an unknown, soluble, organic selenium species. This study shows that a careful process control is a prerequisite for selenium treatment in UASB bioreactors, as disturbances in the operational conditions induce elevated selenium effluent concentrations by alkylation and colloid formation.

Selenium oxyanion inhibition of hydrogenotrophic and acetoclastic methanogenesis

Inhibitory effects of selenite and selenate towards hydrogenotrophic and acetoclastic methanogenesis were evaluated in anaerobic toxicity assays. The 50% inhibitory concentration (IC50) of both selenium oxyanions was below 6.1x10(-5)M in hydrogenotrophic assays, whereas acetoclastic methanogens were less inhibited: IC50=8.3x10(-5)M and 5.5x10(-4)M for selenite and selenate, respectively. Selenite completely inhibits methanogenesis from both substrates tested at concentrations > or =10(-3)M selenite, while only marginal methanogenic activities occur at equimolar concentrations of selenate. Selenite becomes irreversibly inhibitory upon a single exposure, whereas selenate inhibits methanogens upon repeated exposure. Consequently, methane recovery can be seriously hampered or even impossible during anaerobic treatment of highly selenium contaminated waste streams.

Bacillus selenatarsenatis sp. nov., a selenate- and arsenate-reducing bacterium isolated from the effluent drain of a glass-manufacturing plant

A facultatively anaerobic, selenate- and arsenate-reducing bacterium, designated strain SF-1T, was isolated from a selenium-contaminated sediment obtained from an effluent drain of a glass-manufacturing plant in Japan. The bacterium stained Gram-positive and was a motile, spore-forming rod capable of respiring with selenate, arsenate and nitrate as terminal electron acceptors. The major cellular fatty acids of the strain were iso-C15 : 0, iso-C17 : 1 ω10c and C16 : 1 ω7c alcohol. The G+C content of the genomic DNA was 42.8 mol%. Though the nearest phylogenetic neighbour was Bacillus jeotgali JCM 10885T, with a 16S rRNA gene sequence similarity of 99.6 %, DNA–DNA hybridization studies showed only 14 % relatedness between these strains, a level that is clearly below the value recommended to delimit different species. This, together with the phenotypic differences (utilization of electron acceptors, NaCl tolerance), suggests that strain SF-1T represents a novel species of the genus Bacillus, for which the name Bacillus selenatarsenatis sp. nov. is proposed. The type strain is SF-1T (=JCM 14380T=DSM 18680T).

Bioconversion of selenate in methanogenic anaerobic granular sludge

The capacity of anaerobic granular sludge to remove selenate from contaminated wastewater was investigated. The potential of different types of granular sludge to remove selenate from the liquid phase was compared to that of suspended sludge and contaminated soil and sediment samples. The selenate removal rates ranged from 400 to 1500 microg g VSS(-1) h(-1), depending on the source of biomass, electron donor, and the initial selenate concentration. The granular structure protects the microorganisms when exposed to high selenate concentrations (0.1 to 1 mM). Anaerobic granular sludge "Eerbeek," originating from a UASB reactor treating paper mill wastewater, removed about 90, 50, and 36% of 0.1, 0.5, and 1 mM of Se, respectively, from the liquid phase when incubated with 20 mM lactate at 30 degrees C and pH 7.5. Selenite, elemental Se (Se(o)), and metal selenide precipitates were the conversion products. Enrichments from the anaerobic granular sludge "Eerbeek" were able to convert 90% of the 10-mM selenate to Se(o) at a rate of 1505 microg Se(VI) g cells(-1) h(-1), a specific growth rate of 0.0125 g cells h(-1), and a yield of 0.083 g cells mg Se(-1). Both microbial metabolic processes (e.g dissimilatory reduction) as well as microbially mediated physicochemical mechanisms (adsorption and precipitation) contribute to the removal of selenate from the Se-containing medium.

Microbial selenate sorption and reduction in nutrient limited systems

In this study, batch sorption experiments and X-ray adsorption spectroscopy (XAS) were utilized to investigate selenate sorption onto Shewanella putrefaciens 200R. Selenate sorption was studied as a function of pH (ranging from 3 to 7), ionic strength (ranging from 0.1 to 0.001 M), and initial selenate concentration (ranging from 10 to 5000 microM) in the absence of external electron donors. The results show that the extent of selenate sorption is strongly dependent on pH and ionic strength, with maximum sorption occurring at low pH (pH = 3) and low ionic strength (0.001 M NaCl) conditions. The strong dependence of Se sorption with ionic strength suggests the formation of outersphere complexes with the cell wall functional groups. Langmuir isotherm plots yielded log Kads values from 2.74 to 3.02. Desorption experiments demonstrated thatthe binding of selenate onto S. putrefaciens was not completely reversible. XANES analysis of the cells after sorption experiments revealed the presence of elemental selenium, indicating that S. putrefaciens has a capacity to reduce Se(VI) to Se(0) in the absence of external electron donors. We conclude that Se sorption onto S. putrefaciens cell walls is the result of the combination of outer-sphere complexation and cell surface reduction. This sorption process leads to a complex reservoir of bound Se which is not entirely reversible.

Reduction kinetics of As (V) to As (III) by a dissimilatory arsenate-reducing bacterium, Bacillus sp. SF-1

This study proposes a kinetic model that accounts for the toxicity of both arsenate and arsenite and characterizes the arsenate reduction ability of a dissimilatory arsenate-reducing bacterium, Bacillus sp. SF-1 as a bioremediation agent. The model results correlated well with a series of batch reduction experiments conducted anaerobically in serum bottles with initial arsenate concentrations of 360, 735, and 1,500 mg-As/L. The reduction rate was expressed by the Haldane equation that describes the inhibitory effect of high concentrations of arsenate. The reduction rate constant k(r), half saturation constant K(S), and inhibition constant K(I) were estimated respectively as 1.2 x 10(9) mg-As/cells/h, 1.5 x 10(2) mg-As/L, and 4.2 x 10(2) mg-As/L. Lethal effects of arsenite that is accumulated as the end-product of arsenate reduction were expressed by the first-order term with a lethal constant of 2.7 x 10(-4) L/mg-As/h. The yield for the bacterial cells by arsenate respiration was estimated at 4.0 x 10(8) cells/mg-As.

Selenite reduction by a denitrifying culture: batch- and packed-bed reactor studies

Selenite reduction by a bacterial consortium enriched from an oil refinery waste sludge was studied under denitrifying conditions using acetate as the electron donor. Fed-batch studies with nitrate as the primary electron acceptor showed that accumulation of nitrite led to a decrease in the extent of selenite reduction. Also, when nitrite was added as the primary electron acceptor, rapid selenite reduction was observed only after nitrite was significantly depleted from the medium. These results indicate that selenite reduction was inhibited at high nitrite concentrations. In addition to batch experiments, continuous-flow selenite reduction experiments were performed in packed-bed columns using immobilized enrichment cultures. These experiments were carried out in three phases: in phase I, a continuous nitrate feed with different inlet selenite concentration was applied; in phase II, nitrate was fed in a pulsed fashion; and in phase III, nitrate was fed in a continuous mode but at much lower concentrations than the other two phases. During the phase I experiments, little selenite was removed from the influent. However, when the column was operated in the pulse feed strategy (phase II) or in the continuous mode with low nitrate levels (phase III), significant quantities of selenium were removed from solution and retained in the immobilization matrix in the column. Thus, immobilized denitrifying cultures can be effective in removing selenium from waste streams, but nitrate-limited operating conditions might be required.

Characterizing kinetics of transport and transformation of selenium in water-sediment microcosm free from selenium contamination using a simple mathematical model

This study developed a seven-compartment model for predicting the fate of selenium (Se) in an aquatic environment containing a water-sediment boundary. Speciation of Se in water-sediment microcosms under microaerobic conditions was measured to evaluate first-order kinetics of Se transportation and transformation. The microcosm consisted of a 10-ml solution containing 1mM soluble Se as selenate (Se6+) or selenite (Se4+) and 8 g wet sediment that was free from Se contamination, sampled from the Senri, Yamato, or Yodo Rivers in Osaka, Japan. Stepwise reaction coefficients describing transportation and transformation were determined using an inverse method on this model which includes: selenate (Se(W)6+) and selenite (Se(W)4+) in ponded water; selenate (Se(S)6+) and selenite (Se(S)4+), elemental Se (Se0), organic Se (Se2-) in sediment; and gaseous Se (DMSe). During this 1-month experiment, soluble Se was transported from ponded water to the sediment and Se was transformed sequentially to other Se species through biochemical reactions. Experimental and kinetic analyses indicated quantitatively that the Yamato River microcosm, with its high organic matter content, had a high adsorption rate of soluble Se. The Yodo River microcosm had a low adsorption rate for Se6+ and a low Se reduction rate. The Senri River microcosm had an apparent high volatilization rate of DMSe. The model developed in this study is extremely useful for predicting fate of Se in aquatic environment in the field.