Aqueous Se Research Articles

A detailed practical and theoretical study of the adsorptive potential of synthesised ceria impregnated activated graphite for aqueous Se(IV) and Te(IV) removal: Process optimisation, characterisation and mechanistic approach

A cost-effective, simple, and earth friendly nanocomposite adsorbent was synthesized by in - situ penetration of ceria nanocrystals on graphite to remove aqueous selenium (IV) and tellurium (IV). Batch experiments optimize parameters such as pH, temperature, sorbent dosage and equilibration time of 30 minutes. The adsorbent exhibited the maximum adsorption capacities of 144.5 mg/g for Te(IV) and 29.3 mg/g for Se(IV). BET analysis revealed a specific surface area of 16.44 m²/g, an average pore diameter of 4.24 nm, and a high pore volume of 0.0519 cm³/g. The adsorbent was characterized before and after adsorption by SEM-EDX, XRD and FTIR. The sorption data could be well interpreted by the Langmuir model for both Se and Te and by the DR isotherm for Te(IV) at ambient temperature. Kinetic empirical data correlated positively with pseudosecond order for both Se and Te and the Elovich model for Te at room temperature. Kinetics and equilibrium isotherms suggested that the adsorption occurred through surface adsorption using electrostatic bonding along with physisorption involving steady diffusion and accommodation of analyte ions into mesopores and defects of the adsorbent. Chemisorption by electron transfer of the oxyanions of Se/Te with Ce3+/Ce4+ of the cerium impregnated graphite (CIAG) and ion exchange with functional groups on the activated graphite edges were other probable mechanisms. The adsorbent demonstrated the ability to be regenerated and reused over multiple cycles, with minimal interference from coexisting ions emphasizing its selectivity. All measurements were conducted in triplicate with the relative standard deviation (RSD) below 5 %. The optimised method was successfully applied in natural water samples from various sources, demonstrating strong potential for scaling up to field applications.

Selenite Stable Isotope Fractionation during Abiotic Reduction by Sodium Sulfide.

Reduction of Se(IV) by sulfur reducing bacteria (SRB) can remove Se from groundwater either by direct respiration or the production of H2S(g) and subsequent abiotic reduction. This study examined abiotic Se(IV) reduction by H2S(g) to determine the associated Se isotope fractionation. The extent of fractionation was compared to the results with studies of Se(IV) reduction in systems containing microorganisms to assess whether these processes could be distinguished. A solution containing Na2S was added in increasing concentrations to solutions containing Se(IV) as SeO32- to reduce and precipitate Se. Precipitates with three distinct colors were observed. Powder X-ray diffraction (PXRD) results yielded three distinct spectra for each of the three colors of precipitate, which corresponded to SenS8-n (orange) or Se(0) (red) and S(0) (yellow). The δ82Se values of the residual dissolved Se increased as the aqueous Se concentration decreased. The S/Se in solution affected the isotopic fractionation, with an 82ε of 10.1 ± 0.6‰ observed for solutions with S/Se < 1.7, and of 7.9 ± 0.3‰ for solutions with S/Se > 1.7.

HTO and selenate diffusion through compacted Na-, Na–Ca-, and Ca-montmorillonite

Radionuclide transport in smectite clay barrier systems used for nuclear waste disposal is controlled by diffusion, with adsorption significantly retarding transport rates. While a relatively minor component of spent nuclear fuel, 79Se is a major driver of the safety case for spent fuel disposal due to its long half-life (3.3 × 105 yr) and its low adsorption to clay (KD < 10 L/kg), thus a thorough understanding of Se diffusion through clay is critical for understanding the long-term safety of spent fuel disposal systems. Through-diffusion experiments with tritiated water (HTO, conservative tracer) and Se(VI) were conducted with a well-characterized, purified montmorillonite source clay (SWy-2) under a constant ionic strength (0.1 M) and three different electrolyte compositions: Na+, Ca2+, and a Na + -Ca2+ mixture at pH 6.5 in order to probe the effects of electrolyte composition and interlayer cation composition on clay microstructure, Se(VI) aqueous speciation, and ultimately diffusion. The results were modeled using a reactive transport modeling approach to determine values of porosity (ε), De (effective diffusion coefficient), and KD (distribution coefficient for adsorption). HTO diffusive flux was higher in Ca-montmorillonite (De = 1.68 × 10−10 m2 s−1) compared to Na-montmorillonite (De = 7.83 × 10−11 m2 s−1). This increase in flux is likely due to a greater degree of clay layer stacking in the presence of Ca2+ compared to Na+, which leads to larger inter-particle pores. Overall, the Se(VI) flux was much lower than the HTO flux due to anion exclusion, with Se(VI) flux following the order Ca (De = 1.03 × 10−11 m2 s−1) > Na–Ca (De = 2.12 × 10−12 m2 s−1) > Na (De = 1.28 × 10−12 m2 s−1). These differences in Se(VI) flux are due to a combination of factors, including (1) larger accessible porosity in Ca-montmorillonite due to clay layer stacking and smaller electrostatic effects compared to Na-montmorillonite, (2) larger accessible porosity for neutral-charge CaSeO4 species which makes up 32% of aqueous Se(VI) in the pure Ca system, and (3) possibly higher Se(VI) adsorption for Ca-montmorillonite. Through a combination of experimental and modeling work, this study highlights the compounding effects that electrolyte and counterion compositions can have on radionuclide transport through clay. Diffusion models that neglect these effects are not transferable from laboratory experimental conditions to in situ repository conditions.

Direct immobilization of Se(IV) from acidic Se(IV)-rich wastewater via ferric selenite Co-precipitation

The attenuation of acidic Se(IV)-rich wastewater, including those associated with acid mine drainage (AMD) and nonferrous metallurgical wastewater (NMW), presents a serious environmental challenge. This study investigates the effects of diverse factors from pH values to Se(IV)/Fe(III) molar ratios, initial Se(IV) concentrations, and alkali neutralization agents on the direct co-precipitation of ferric selenites in AMD and NMW systems involving different orders of Fe(III) and alkali addition. Our results show that amorphous sulfate-substituted ferric (hydrogen) selenite and Se(IV)-bearing ferrihydrite-schwertmannite are the major Se(IV)-attenuation solids except that gypsum is an additional phase in the NMW system with Ca(OH)2 neutralization. Produced ferric selenites achieve 98–99.8% of Se(IV) immobilization under optimal conditions of pH 4.5, Se(IV)/Fe(III) molar ratios of 0.0625–0.5, and initial Se(IV) concentrations of 0.15–1.3 mmol·L-1. Moreover, completing FeSO4+ and FeHSeO32+/FeSeO3+ complexes as well as different ferric selenite co-precipitates are shown to collectively control aqueous Se(IV) remaining. Specifically, three distinct trends of aqueous Se(IV) concentrations separately correspond to changes in the four factors. The co-precipitation in the NMW system via pH adjustment followed by Fe(III) addition is more efficient for Se(IV) fixation than that in the AMD system because of minimal complexation, concurrent Fe(III) hydrolysis, and enhanced ferric selenite co-precipitation in the former.

Comparative study on the reductive immobilization of Se(IV) by Beishan granite and Tamusu claystone

Understanding the retardation of 79Se by the host rock is essential for the safety assessment of the high-level radioactive waste (HLRW) repository and thus the final site-selection process. In this work, we performed a comparative study on the interaction of Se(IV) with Beishan granite and Tamusu claystone, and multiple characterization techniques were used to identify the speciation of Se and to unravel the underlying mechanism. Results showed that Se(IV) can be reduced to Se(0) by the Beishan granite from borehole 16 that contains Fe(II)-bearing fluorannite, and the removal efficiency of Se(IV) is more favorable at acidic condition. We proposed that release of the structure Fe2+ in granite, followed by surface adsorption and reaction could be responsible for the Se(IV) reduction by this granite. In addition to Se(0) product, FeSe2 was also observed for Se(IV) reacted with the granite from borehole 28. This is the first time observed the formation of FeSe2 on natural Fe(II)-containing minerals, which is mainly ascribed to the simultaneous occurrence of aqueous sulfide dissolved from the ground granite powder. The reactive sulfide might be generated from the breakage of Fe(S)–S bonds of iron sulfides (e.g., pyrite) contained in the granite during the anoxic grinding process, and it could mainly account for the Se(IV) reduction by the granite from borehole 28. Due to the presence of pyrite, the Tamusu claystone from borehole Tzk2 was effective to remove aqueous Se(IV) via the formation of Se(0). Conversely, the Se(IV) removal efficiency was much lower for the claystone from borehole Tzk1 that did not contain pyrite, although there was a considerable amount of Fe(II)-bearing fluorannite. Hence, the Fe2+ contained in Tamusu claystone was less active for Se(IV) reduction in comparison with that in Beishan granite from borehole 16. The findings of this study provide important data for the safety assessment of China's future HLRW repository, but also improve our understanding of the geochemical behavior of selenium in the presence of Fe(II)- and sulfide-bearing minerals in the relevant anoxic environments.

Application of autonomous sensor technology to estimate selenium exposure and a site-specific selenium threshold in a Canadian boreal lake.

There is an increasing trend in the use of real-time sensor technology to remotely monitor aquatic ecosystems. Commercially available probes, however, are currently not able to measure aqueous selenium (Se) concentrations. Because of the well-described bioaccumulation potential and associated toxicity of Se in oviparous vertebrates, it is crucial to monitor Se concentrations at sites receiving continuous effluent Se input. This study aimed to estimate Se concentrations in a boreal lake (McClean Lake) downstream from a Saskatchewan uranium mill using real-time electrical conductivity (EC) data measured by autonomous sensors. Additionally, this study aimed to derive a site-specific total aqueous Se (TSe) threshold based on Se concentrations in periphyton and benthic macroinvertebrates sampled from the same lake. To characterize effluent distribution within the lake, eight Smart Water (Libelium) sensor units were programmed to report EC and temperature for five and seven consecutive weeks in 2018 and 2019, respectively. In parallel, periphyton and benthic macroinvertebrates were sampled with Hester-Dendy's artificial substrate samplers (n = 4) at the same sites and subsequently analyzed for Se concentrations. Electrical conductivity was measured with a handheld field meter for sensor data validation and adjusted to the median lake water temperature (13 °C) registered for the deployment periods. Results demonstrated good accuracy of sensor readings relative to handheld field meter readings and the successful use of real-time EC in estimating TSe exposure (r = 0.87; r2 = 0.84). Linear regression equations derived for Se in detritivores versus Se in periphyton and Se in periphyton versus sensor-estimated TSe were used to estimate a site-specific TSe threshold of 0.7 µg/L (±0.2). Moreover, mean Se concentrations in periphyton (16.7 ± 4.4 µg/g dry weight [d.w.]) and benthic detritivores (6.0 ± 0.4 µg/g d.w.) from one of the exposure sites helped identify an area with potential for high Se bioaccumulation and toxicity in aquatic organisms in McClean Lake. Integr Environ Assess Manag 2023;19:395-411. © 2022 SETAC.

Differential selenium uptake by periphyton in boreal lake ecosystems

The largest and most variable step of selenium (Se) assimilation into aquatic ecosystems is the rapid uptake of aqueous Se by primary producers. These organisms can transfer more harmful forms of Se to higher trophic levels via dietary pathways, although much uncertainty remains around this step of Se assimilation due to site-specific differences in water chemistry, hydrological and biogeochemical characteristics, and community composition. Thus, predictions of Se accumulation are difficult, and boreal lake systems are relatively understudied. To address these knowledge gaps, five static-renewal field experiments were performed to examine the bioaccumulation of low, environmentally relevant concentrations of Se, as selenite, by naturally grown periphyton from multiple boreal lakes. Periphyton rapidly accumulated Se at low aqueous Se concentrations, with tissue Se concentrations ranging from 8.0 to 24.9 μg/g dry mass (dm) in the 1–2 μg Se/L treatments. Enrichment functions ranged from 2870 to 12 536 L/kg dm in the 4 μg Se/L treatment, to 11 867–22 653 L/kg dm in the 0.5 μg Se/L treatment among lakes. Periphyton Se uptake differed among the five study lakes, with periphyton from mesotrophic lakes generally accumulating more Se than periphyton from oligotrophic lakes. Higher proportions of charophytes and greater dissolved inorganic carbon in more oligotrophic lakes corresponded to less periphyton Se uptake. Conversely, increased proportions of bacillariophytes and total dissolved phosphorus in more mesotrophic lakes corresponded to greater periphyton Se uptake. Periphyton community composition and water chemistry variables were correlated, limiting interpretation of differences in periphyton Se accumulation among lakes. The results of this research provide insight on the biodynamics of Se assimilation at the base of boreal lake food webs at environmentally relevant concentrations, which can potentially inform ecological risk assessments in boreal lake ecosystems in North America.

Removal of dissolved selenium by siderite, pyrite, and sphalerite: Implications to selenium sequestration in water-saturated, anoxic coal waste rock

The mechanisms of abiotic sequestration of Se(VI) and Se(IV) on a sample of coal waste rock collected from the Elk Valley, Canada and on three pure mineral species (i.e., siderite, pyrite, and sphalerite) present in coal waste rock were assessed using sterile batch testing under water-saturated, anoxic conditions. Only siderite removed measurable Se(VI) from the test solutions with ~90% removal after 100 d attributed to initial adsorption to the siderite surface as Se(VI) and subsequent reduction to Se(IV) and Se(0). In contrast to Se(VI), all samples removed Se(IV) from the aqueous solutions. The rate of Se(IV) removal was pyrite > siderite > waste rock > sphalerite. The waste rock sample removed Se(IV) from solution in two stages: up to ~40% of the aqueous Se(IV) was rapidly removed (by day 1) followed by slower removal of Se(IV) with up to ~97% removal after 99 d. Se(IV) removal is attributed to the adsorption of Se(IV) and subsequent reduction to Se(0) onto the siderite and pyrite phases of the waste rock. The initial (1 d) removal of Se(IV) by waste rock was characterized using a distribution coefficient (Kd) of 15.5 L/kg. Longer-term Se(IV) removal was fitted with zero and first order removal rates. The removal of Se(IV) by sphalerite was minor and deemed to have a minimal effect on Se sequestration in the waste rock. Desorption tests on waste rock showed that the fraction of sequestered Se susceptible to desorption decreased with time as adsorbed Se(IV) was reduced to insoluble Se(0). These findings show that abiotic Se sequestration can occur in saturated, anoxic zones in coal waste rock dumps.

Aqueous selenium removal and distribution in cattail and duckweed in constructed wetland microcosms

AbstractSelenium (Se)‐affected stormwater runoff raises concerns about potential downstream impact of Se on aquatic ecosystems. Unplanted (UNP) detention ponds augmented with cattail (Typha angustifolia L.; CAT) and duckweed (Lemna minor L.; DWD) may provide a solution for continuous, rapid abatement of Se‐affected runoff. This research was conducted to evaluate the efficacy of CAT or DWD and determining the distribution of Se within a continually flooded detain and drain system. Microcosms containing 3 kg of soil planted to either one CAT, 25 g fresh DWD, or left unplanted (UNP, control) were flooded with a 3‐L solution at 40 μg Se L−1, as sodium selenate (Na2SeO4), or a zero Se control. Over two 10‐d flood–discharge cycles (FDCs), plants in microcosms were evaluated in growth chambers maintained at 30 °C under a 12‐h photoperiod with 400 μmol m−2 s−1 irradiance. Initial and final water, soil, plant, and granular activated charcoal (GAC) were analyzed for total [Se] with inductively coupled plasma–mass spectroscopy (ICP–MS). Data were analyzed with PROC GLM (SAS EG 7.1) at α = .05. Within 10 d after Se application, CAT and DWD decreased aqueous Se from 40 μg Se L−1 to below the 11.8 μg Se L−1 threshold. Selenium recovery ranged between 75 and 100% of the applied Se. In continually flooded systems, the primary elimination pathway appears to be associated with the soil solid phase. Cattail and DWD are suitable species for constructed wetland phytoremediation of Se‐affected runoff. The microcosm design presented may be useful for future evaluations.

The Influence of Aqueous Se(IV) on the Stability of Different CaCO3 Polymorphs Precipitated under Ambient Conditions

Selenium is an essential bio-element, but because of its bioaccumulation potential, it can become toxic and is an important pollutant. The ubiquitous mineral calcite (CaCO3) has the ability to immobilize anions as SeO32− by different sorption or coprecipitation processes. Experimental studies have found that SeO32− can incorporate in the crystal structure of calcite by substituting CO32−. The presence of foreign ions in aqueous solution strongly affects CaCO3 precipitation, helping stabilize less stable polymorphs such as vaterite and aragonite or hydrated phases. In this work, we studied the aging process of calcium carbonates precipitated from aqueous solutions highly supersaturated with respect to CaCO3 and slightly supersaturated with respect to CaSeO3·H2O under ambient conditions, for times up to 30 days in which solids were kept in the remaining aqueous solution. Under these conditions, CaCO3 precipitated mainly as low crystallinity vaterite aggregates that hosted up to 16% atomic ratio Se:C. Vaterite purified and increased its crystallinity with aging time, but the vaterite–calcite transformation was strongly inhibited. The incorporation of Se(IV) in vaterite did not significantly affect the cell parameters or the external morphology of the aggregates. The precipitation of selenite as CaSeO3·H2O was conditioned by the availability of free Ca2+ and SeO32− that was not previously incorporated into precipitated carbonates.

Trophic dynamics of selenium in a boreal lake food web.

Selenium (Se) is both an essential micronutrient and a contaminant of concern that is of particular interest in mining-influenced waterbodies in Canada. The objective of this research was to characterize the trophic dynamics of selenium along a gradient of exposure concentrations in a Canadian boreal lake ecosystem. From June 20 to August 22, 2018, six limnocorrals (littoral, ∼3000L enclosures) were spiked with mean measured concentrations of 0.4, 0.8, 1.6, 3.4, 5.6 and 7.9μg Se/L as selenite, and three limnocorrals served as untreated controls (background aqueous Se=0.08-0.09μg/L). Total Se (TSe) concentrations in water, periphyton, phytoplankton, sediment, benthic macroinvertebrates, zooplankton and female finescale dace (Phoxinus neogaeus; added on day 21 of the experiment) were measured throughout and at the end of the experiment. Total Se bioaccumulation by organisms was generally non-linear. Greater uptake by phytoplankton than periphyton was observed. Taxonomic differences in accumulation of TSe by invertebrates (Heptageniidae=Chironomidae>zooplankton) were observed as well. Fish muscle and ovary tissue TSe bioaccumulation was more variable than that at lower trophic levels and uptake patterns indicated that fish did not reach steady state concentrations. This research provides field-derived models for the uptake of Se by algae and invertebrates, and contributes to a better understanding of the dynamics of TSe bioaccumulation over a gradient of exposure concentrations in cold-water lentic systems.

Equilibrium fractionation and isotope exchange kinetics between aqueous Se(IV) and Se(VI)

The selenium (Se) isotope system has been proposed as a redox proxy in environmental and paleoceanographic studies. However, Se isotope exchange among various Se species can potentially interfere with redox-related isotope signatures, and is still poorly understood. In this work, we investigated Se isotope exchange kinetics and equilibrium fractionations between aqueous Se(IV) and Se(VI) under various experimental conditions. At pH = 7, low-Se concentration experiments (0.026 mM Se(IV) and 0.026 mM Se(VI)) at 25 °C, 38 °C and 60 °C were conducted for 900 days, while high-Se concentrations (0.13 mM Se (IV) and 0.14 mM Se(VI); 1.3 mM Se(IV) and 1.4 mM Se(VI)) at 60 °C were conducted for 1547 days. All experiments did not reach isotopic equilibrium, with observed Se isotope fractionations <0.20‰. Adding an electronic shuttle (Anthraquinone-2, 6-disulfonate) did not increase the isotope exchange rate. These results show that under the experimental conditions examined, the isotope exchange reaction between aqueous Se(IV) and Se(VI) is extremely slow.The exchange kinetics between Se(IV) and Se(VI) were also investigated using a 82Se tracer. The exchange rates (R) at 0.13 mM Se(IV) and 0.13 mM Se(VI) at 25 °C, 38 °C and 60 °C were determined to be ≤6.34 × 10−10 M day−1, ≤1.12 × 10−09 M day−1 and ≤1.17 × 10−09 M day−1, respectively. Using the upper bound for the isotope exchange rate at 25 °C and theoretically calculated equilibrium fractionations, and assuming a first order isotope exchange reaction between Se(IV) and Se(VI) by analogy to the sulfur system, the timescale of isotope exchange between aqueous Se (IV) and Se (VI) in a natural lake (Sweitzer Lake, Colorado, USA) was estimated. The minimum half-time (t1/2, time to reach 50% isotopic equilibrium) and the minimum time for detectable isotope exchange (tmin) are ≥440,000 and ≥18,000 years, respectively. In the modern oceans, t1/2 and tmin are ≥51 million and ≥3.6 million years, respectively. These timescales are much longer than the residence time of Se in Sweizer Lake (2.4 years) and the modern ocean (26,000 years). Therefore, when using Se isotopes to trace the biogeochemical cycle of Se in lakes and oceans, the effect caused by isotope exchange between aqueous Se(IV) -Se(VI) systems is insignificant.

Selenate removal by zero-valent iron under anoxic conditions: effects of nitrate and sulfate

Batch experiments were conducted to examine aqueous Se(VI) removal by zero-valent iron (ZVI) under anoxic conditions in the presence and absence of NO3− and SO42−. Initial concentrations for Se(VI), SO4, and NO3–N of 5 mg L−1, 1800 mg L−1, and 13 mg L−1, respectively, were employed to mimic mine waters. In the control experiment, 90% Se(VI) removal occurred within 1.5 h without SO42− and NO3− (B1). This removal threshold was reached after 3 h with NO3− added (B3) and after 33 h with SO42− added (B2). Removal reached 90% after 42 h with both SO42− and NO3− added (B4). Modeled Se(VI) removal rates consistently followed first-order kinetics and revealed that the presence of SO42− and, to a lesser extent, NO3− inhibited Se(VI) removal. Increases in pH and Fe coupled with decreasing Eh are consistent with ZVI corrosion under anoxic conditions. Transmission electron microscopy, Raman spectroscopy, and X-ray diffraction revealed magnetite [Fe3O4] and lepidocrocite [γ-FeOOH] formed at ZVI surfaces during the experiments. X-ray absorption near edge structure spectroscopy indicated that Se(VI) was predominantly reduced to Se(0) (70–80%), but Se(IV) (10–13%) and Se(-II) (2–13%) were also detected at reacted ZVI surfaces. Overall, the results show that although SO42− and NO3− present in mine wastes can reduce reaction rates, Se(VI) removal by ZVI under anoxic conditions is associated with extensive reduction to insoluble Se(0).

Distribution of Experimentally Added Selenium in a Boreal Lake Ecosystem.

Human activities have increased the release of selenium (Se) to aquatic environments, but information about the trophic transfer dynamics of Se in Canadian boreal lake systems is limited. In the present study, Se was added as selenite to limnocorrals (2-m-diameter, 3000-L in situ enclosures) in a boreal lake in northwestern Ontario to reach nominal concentrations of 1 and 10 µg Se/L in triplicate each for 77 d, and 3 additional limnocorrals were controls with no Se added. Total Se concentrations were determined in water, sediment, periphyton, benthic macroinvertebrates, zooplankton, and reproductively mature female fathead minnows (Pimephales promelas; added on day 33) collected throughout (and at the end of) the exposure period. Mean measured water Se concentrations in the control, 1-, and 10-µg/L treatments were 0.12, 1.0, and 8.9 µg/L. At the end of exposure (day 77), enrichment functions ranged from 7772 L/kg dry mass in the 8.9-µg/L treatment to 23495 L/kg dry mass in the 0.12-µg/L treatment, and trophic transfer factors for benthic macroinvertebrates ranged from 0.49 for Gammaridae to 2.3 for Chironomidae. Selenium accumulated in fathead minnow ovaries to concentrations near or above the current US Environmental Protection Agency criterion (15.1 µg/g dry mass for fish ovary/egg) in the 1.0- and 8.9-µg/L treatments, suggesting that, depending on aqueous Se speciation, such exposures have the potential to cause Se accumulation in fish to levels of concern in cold-water, boreal lake systems. Environ Toxicol Chem 2019;38:1954-1966. © 2019 SETAC.

Effects of selenium on benthic macroinvertebrates and fathead minnow (Pimephales promelas) in a boreal lake ecosystem

Selenium (Se) is a contaminant of concern in many aquatic ecosystems due to its narrow range between essentiality and toxicity in oviparous (yolk-bearing) vertebrates. The objective of the present study was to determine the effects of Se, experimentally added to in situ limnocorrals as selenite, on invertebrate communities and fathead minnow (Pimephales promelas) at environmentally realistic Se concentrations. Nine limnocorrals were deployed in a mesotrophic lake at the International Institute for Sustainable Development – Experimental Lakes Area in Ontario, Canada in May 2017. From June 1 to August 17, 2017, selenite was added to six enclosures to attain mean measured aqueous Se concentrations of 1.0 ± 0.10 or 8.9 ± 2.7 μg/L Se (in triplicate) and three limnocorrals were untreated controls (background mean aqueous Se = 0.12 ± 0.03 μg/L). Benthic macroinvertebrates were collected throughout and at the end of the exposure period using artificial substrates to determine density, dry biomass, diversity, and taxa richness at the family level. Reproductively mature female fathead minnows (added on d 33 of the study) were collected throughout and at the end of the exposure period. After 77 d, Chironomidae and Gammaridae densities and biomass were significantly lower in the 8.9 μg/L Se treatment relative to the 1.0 μg/L Se treatment and the control. Invertebrate diversity (measured as Shannon’s and Simpson’s indices) significantly declined in the 1.0 μg/L and 8.9 μg/L Se treatments relative to the control (0.12 μg/L Se group). Fulton’s condition factor for fathead minnow was significantly less in the 8.9 μg/L treatment compared to 0.12 and 1.0 μg/L Se experimental groups. The results of this study indicated that exposure to relatively low aqueous selenite concentrations can negatively affect invertebrate density and biomass, as well as fish condition. More research is necessary to characterize the risk of selenite exposure to aquatic invertebrates under realistic field conditions, and future risk assessments may need to consider reduced food availability as a factor that may impair the health of higher trophic level organisms in areas with elevated selenite.

Получение водного коллоидного раствора селена путем механического диспергирования

Получение водного коллоидного раствора селена путем механического диспергирования

Adsorption of Selenite onto Bacillus subtilis: The Overlooked Role of Cell Envelope Sulfhydryl Sites in the Microbial Conversion of Se(IV).

Microbial activities play a central role in the global cycling of selenium. Microorganisms can reduce, methylate, and assimilate Se, controlling the transport and fate of Se in the environment. However, the mechanisms controlling these microbial activities are still poorly understood. In particular, it is unknown how the negatively charged Se(IV) and Se(VI) oxyanions that dominate the aqueous Se speciation in oxidizing environments bind to negatively charged microbial cell surfaces in order to become bioavailable. Here, we show that the adsorption of selenite onto Bacillus subtilis bacterial cells is controlled by cell envelope sulfhydryl sites. Once adsorbed onto the bacteria, selenite is reduced and forms reduced organo-Se compounds (e.g., R1S-Se-SR2). Because sulfhydryl sites are present within cell envelopes of a wide range of bacterial species, sulfhydryl-controlled adsorption of selenite likely represents a general mechanism adopted by bacteria to make selenite bioavailable. Therefore, sulfhydryl binding of selenite likely occurs in a wide range of oxidized Se-bearing environments, and because it is followed by microbial conversion of selenite to other Se species, the process represents a crucial step in the global cycling of Se.

Solution and surface chemistry of the Se(IV)-Fe(0) reactions: Effect of initial solution pH

Aspects of solution and solid-phase reactions between selenite (Se(IV)) and nanoscale zero-valent iron (nZVI) were investigated. Experimental results on the effects of initial solution pH, formation and evolution of nZVI corrosion products, and speciation of selenium in nZVI were presented. In general, the rate of Se(IV) removal decreases with increasing initial pH. The observed rate constants of Se(IV) removal decreased from 0.3530 to 0.0364 min−1 as pH increased from 4.0 to 10.0. Composition and morphology of nZVI corrosion products and selenium species were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Results confirmed that Se(IV) was reduced to Se(0) and Se(-II) by nZVI. Lower solution pH favored further reduction of Se(0) to Se(-II). Amorphous FeOOH, magnetite/maghemite (Fe3O4/γ-Fe2O3) and ferrous hydroxide (Fe(OH)2) were identified as the main corrosion products. Under alkaline conditions, the corrosion products were mainly of Fe(OH)2 along with small amounts of Fe3O4, while nZVI in acidic solutions was oxidized to mostly Fe3O4 and amorphous FeOOH. Furthermore, these corrosion products acted as intermediates for electron transfer and reactive/sorptive sites for Se(IV) adsorption and reduction, thus played a crucial role in the removal of aqueous Se(IV).

Investigation of the nature and origin of the geological matrices rich in selenium within the Hydrogeological Experimental Site of Poitiers, France

The selenium (Se) content and the associated release mechanisms in both surface and groundwater have become a major concern worldwide over the past 30 years. Within the Hydrogeological Experimental Site of Poitiers (HESP), a large range of aqueous Se concentrations (from non-detectable to more than 30 ppb) is observed in a limited area (about 10ha), where water flows are highly characterized. This site thus consists of an interesting spot to better understand the release mechanisms of selenium into groundwater. The present study consists of an identification and a characterization of the lithological sources of Se within the HESP. Total rock analyses applied to core samples from different depths and wells demonstrated that selenium is concentrated in argillaceous sediments enriched with organic matter, pyrite and uranium that fulfill a part of the karst cavities developed within the Bajocian host rocks. Mineralogical and petrographic investigations highlighted the heterogeneity of these filling materials and showed the presence of successive deposits under different climatic conditions. Extensive characterizations dated the selected Se-rich samples from the Upper Cretaceous and converged to demonstrate the external and continental origin of the studied filling materials and their transformation after deposition under reduced conditions. Only indirect correlations allow considering an agreement between the Se history and the very mature organic matter identified in the argillaceous samples. This association will be further favored to determine the mechanisms releasing selenium into groundwater.

Selenium attenuation via reductive precipitation in unsaturated waste rock as a control on groundwater impacts in the Idaho phosphate patch

Phosphate mining in southeastern Idaho has historically resulted in the release of dissolved metals and inorganics to groundwater and surface water, primarily due to leachate from waste rock in backfilled pits and overburden storage piles. Selenium (Se) is of particular concern due to its high concentration in leachate and its limited attenuation downgradient of source zones under oxic conditions. Assessments of potential groundwater/surface water impacts from waste rock typically involve laboratory characterization using saturated and unsaturated flow columns packed with waste rock. In this study, we compare the results of saturated and unsaturated column tests with groundwater quality data from the Mountain Fuel, Champ, South and Central Rasmussen Ridge Area (SCRRA), Smoky Canyon, Ballard, Henry, and Enoch Valley Mines, to understand the release and attenuation of Se in different geochemical environments. Column studies and field results demonstrate that the ratio of aqueous Se to aqueous sulfate (Se:SO4 ratio) is a useful metric for understanding Se release and attenuation, where the extent of sulfate reduction is much less than Se reduction. Comparison of dissolved Se and sulfate results suggests that the net leachability of Se from unsaturated waste rock is variable. Overall, Se concentrations in groundwater directly beneath waste rock dumps is not as high as would be predicted from unsaturated columns. Lower Se:SO4 ratios are observed immediately beneath waste rock dumps and backfilled pits relative to areas receiving shallow waste rock runoff. It is hypothesized that Se released in the oxic upper portions of the waste rock is subsequently attenuated via reductive precipitation at depth in unsaturated, low-oxygen portions of the waste rock. This highlights an important mechanism by which Se may be naturally attenuated within waste rock piles prior to discharge to groundwater and surface water. These results have important implications for mining practices in the region. A better understanding of Se dynamics can help drive waste rock management during active mining and capping/water management options during post-mining reclamation.