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Competing Ion Behavior in Direct Electrochemical Selenite Reduction

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Selenium (Se) is an essential element with application in manufacturing from food to medical industries. Water contamination by Se is of concern due to anthropogenic activities. Recently, Se remediation has received increasing attention. Hence, different types of remediation techniques are listed in this work, and their potential for Se recovery is evaluated. Sorption, co-precipitation, coagulation and precipitation are effective for low-cost Se removal. In photocatalytic, zero-valent iron and electrochemical systems, the above mechanisms occur with reduction as an immobilization and detoxification process. In combination with magnetic separation, the above techniques are promising for Se recovery. Biological Se oxyanions reduction has been widely recognized as a cost-effective method for Se remediation, simultaneously generating biosynthetic Se nanoparticles (BioSeNPs). Increasing the extracellular production of BioSeNPs and controlling their morphology will benefit its recovery. However, the mechanism of the microbial production of BioSeNPs is not well understood. Se containing products from both microbial reduction and abiotic methods need to be refined to obtain pure Se. Eco-friendly and cost-effective Se refinery methods need to be developed. Overall, this review offers insight into the necessity of shifting attention from Se remediation to Se recovery.
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Selenium (mainly in the forms of selenite (Se(IV)) and selenate (Se(VI)) is a regulated drinking water contaminant, but there is little information on the kinetics and mechanisms of Se(IV) oxidation during water treatment. Species-specific and apparent second-order rate constants for the oxidation of Se(IV) at pH 7.0 were determined in buffered solutions and they decrease in the order bromine (5.8 ± 0.3 × 103 M-1 s-1) > ozone (O3, 513.4 ± 10.0 M-1 s-1) > chlorine (61.0 ± 3.6 M-1 s-1) > permanganate (2.1 ± 0.1 M-1 s-1), monochloramine (NH2Cl, (1.3 ± 0.1) × 10-3 M-1 s-1), and hydrogen peroxide (H2O2, (2.3 ± 0.1) × 10-5 M-1 s-1). The reaction stoichiometries for the reactions of Se(IV) with bromine, O3, chlorine, NH2Cl, and H2O2 are 1:1. For Mn(VII), the stoichiometries varied with pH and were 5:2, 3:2, and 1:2 for acidic, neutral, and alkaline conditions, respectively. Based on the reaction orders and stoichiometries, the corresponding Se(IV) oxidation mechanisms for various oxidants are discussed. The role of bromide for Se(IV) oxidation was also investigated during chlorination and ozonation of Se(IV)-containing water. During chlorination, bromide-catalysis enhances the rate of the oxidation of Se(IV) to Se(VI) from 50% to nearly 90% with bromide concentrations of 50 μg L-1 and 200 μg L-1, respectively, at pH 7.0 and a chlorine dose of 2.0 mg L-1 (within 15 min). During ozonation, bromide had no effect on Se(IV) oxidation. Based on the determined second order rate constants, the oxidation of Se(IV) by chlorine and ozone were successfully predicted in a natural water by a kinetic model. The second order rate constants for the same oxidants were also investigated and/or evaluated for other related anions, such as arsenite (As(III)) and sulfite (S(IV)). They decreased in the order S(IV) > As(III) > Se(IV).
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Electrochemical systems are an attractive option for onsite latrine wastewater treatment due to their high efficiency and small footprint. While concerns remain over formation of toxic byproducts during treatment, rigorous studies examining byproduct formation are lacking. Experiments treating authentic latrine wastewater over variable treatment times, current densities, chloride concentrations, and anode materials were conducted to characterize byproducts and identify conditions that minimize their formation. Production of inorganic byproducts (chlorate and perchlorate) and indicator organic byproducts (haloacetic acids and trihalomethanes) during electrolysis dramatically exceeded recommendations for drinking water after one treatment cycle (~10-30,000 times), raising concerns for contamination of downstream water supplies. Stopping the reaction after ammonium was removed (i.e., the chlorination breakpoint) was a promising method to minimize byproduct formation without compromising disinfection and nutrient removal. Though treatment was accelerated at increased chloride concentrations and current densities, byproduct concentrations remained similar near the breakpoint. On TiO2/IrO2 anodes, haloacetic acids (up to ~50 µM) and chlorate (up to ~2 µM) were of most concern. Although boron-doped diamond anodes mineralized haloacetic acids after formation, high production rates of chlorate and perchlorate (up to ~4 and 25 µM) made them inferior to TiO¬2/IrO2 anodes in terms of toxic byproduct formation. Organic byproduct formation was similar during chemical chlorination and electrolysis of wastewater, suggesting that organic byproducts are formed by similar pathways in both cases (i.e., reactions with chloramines and free chlorine).
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This paper presents a general review of the selenium chemistry and fate in wet limestone flue gas desulphurisation (FGD) systems and major factors associated to the operation of wet limestone FGDs affecting Se oxidation. Selenium chemistry in FGD waters is more than the interconversion between selenite and selenate; selenium can form a complex array of chemical aqueous complexes some of them analogous to those of sulphur as well as interact with polyoxosulfur anions. However, most of Se-aqueous complexes that may encounter in FGD waters are still unidentified. FGD water re-circulation, fly ashes (FAs) accumulated in the FGD scrubber, oxi-reduction potential (ORP), water flow quantity, flow variability, and pressure are operating FGD conditions that may affect the implementation of effective strategies in management and control of Se in FGD waters. These operating FGD conditions may have a strong influence in the reaction kinetics and the mass transfer of water and constituents through the water treatment processes for Se. Other influences that will affect selenium removal in FGDs can be the oxidizing and reducing agents complexing or chelating agents and/or competing oxyanions. Development of effective strategies in management and control of Se is also complicated by difficulties in measuring and analysing selenium speciation in FGD wastewaters. The effect of sample handling and preservation methods on selenium speciation is not well understood yet and analytical methods may not achieve accurate total selenium concentrations in complex FGD wastewater matrices.
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In this paper, we reported microbially catalyzed sulfate reduction with polarized electrode (− 400 mV vs. Ag/AgCl) as the sole electron donor. In potentiostatic batch assays, sulfate was reduced to sulfide, at rates falling in the range of 6.70–12.16 equiv./l d. Cyclic voltammetry tests revealed that the sulfate-reducing biofilms could accept electrons from electrodes directly without via electron shuttles or hydrogen production. Scanning electron microscope revealed that the electrode was colonized by several ellipse-shaped and short rod-shaped microorganisms, which closely related to Desulfobulbus propionicus and Geobacter species.
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The literature data on the electroreduction of nitric acid to hydroxylamine on various metals is analyzed as regards the electrocatalytic trends. In general moderate values of MH and MO bond energies, between the metal M and the anchoring group −H or −O (of −NO3), herald higher electrocatalytic activity; too low MH interaction energy or too high MO bond energy is associated with lower activity. Some volcano-type correlations between the MH or MO interaction energies and the electrocatalytic activities are observed for the transition metals whereas the sp metals do not show such relationships. It is further observed that the ability of the metal to electrocatalyze nitric acid reduction is roughly related to its exchange current density for the hydrogen evolution reaction. These correlations are interpreted on the basis of previous kinetic theory and some mechanistic proposals are put forward.
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Leachate derived from unlined coal ash disposal facilities is one of the most significant anthropogenic sources of selenium to the environment. To establish a practical framework for predicting transport of selenium in ash leachate, sorption of Se(IV) and Se(VI) from 1 mM CaSOâ was measured for 18 soils obtained down-gradient from three ash landfill sites and evaluated with respect to several soil properties. Furthermore, soil attenuation from lab-generated ash leachate and the effect of Ca{sup 2+} and SOâ²⁻ concentrations as well as pH on both Se(IV) and Se(VI) was quantified for a subset of soils. For both Se(IV) and Se(VI), pH combined with either percentage clay or dithionite-citrate-bicarbonate (DCB)-extractable Fe described < 80% of the differences in sorption across all soils, yielding an easy approach for making initial predictions regarding site-specific selenium transport to sensitive water bodies. Se(IV) consistently exhibited an order of magnitude greater sorption than Se(VI). Selenium sorption was highest at lower pH values, with Se(IV) sorption decreasing at pH values above 6, whereas Se(VI) decreased over the entire pH range (2.5-10). Using these pH adsorption envelopes, the likely effect of ash leachate-induced changes in soil pore water pH with time on selenium attenuation by down gradient soils can be predicted. Selenium sorption increased with increasing Ca{sup 2+} concentrations while SOâ2- suppressed sorption well above enhancements by Ca{sup 2+}. Soil attenuation of selenium from ash leachates agreed well with sorption measured from 1 mM CaSOâ, indicating that 1 mM CaSOâ is a reasonable synthetic leachate for assessing selenium behavior at ash landfill sites.
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Raman and Se-77 NMR spectroscopy confirm that when selenous acid is reduced by thiosulfate in water selenopentathionate and tetrathionate are formed.Depending upon the stoichiometry and pH, two isomers of the selenopentathionate ion, O- and S-bonded, are formed. Insufficiently acid solutions cause decomposition to selenium and tetrathionate ion.Fresh solutions prepared from crystalline sodium selenopentathionate and water undergo slow decompositon. NMR and Raman spectra show the presence of both the O-bonded and S-bonded linkage isomers. The O-bonded isomer facilitates the formation of tetrathionate. Addition of thiosulfate to selenotrithionate solution or sulfite to selenopentathionate solution yields trithionate with no indication of dithionate or tetrathionate formation. This suggests that simple S—S bond formation at selenium does not occur but that there may be direct attack of the incoming ligand on the attached ligand. Key words: selenite, thiosulfate, selenopentathionate, Se-77 NMR, Raman spectroscopy, linkage isomerism.
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Elemental selenium dissolves in sulfite solution to form selenosulfate ion: Se + SO32− = SeSO32−.The formation constants for this equilibrium at temperatures from 0 to 35 °C are reported for the first time. The isomeric thioselenate anion, SSeO32−, is not, however, produced by the reaction of sulfur with selenite nor is the selenoselenate ion, Se2O32−, formed from selenium and selenite. Selenotrithionate is formed rapidly from the reaction of selenous acid with sulfite and hydrogen sulfite according to: HSeO3− + 3 HSO3− = Se(SO3)22− + SO42− + 2H2O.Two isomers of the selenotrithionate ion are observed by Se-77 NMR and Raman spectroscopy, one with O-bonded Se, Se(OSO2)22−, and the other with S-bonded Se, Se(SO3)22−. Both isomers are formed in reactions with hydrogen sulfite but only the O-bonded isomer is formed in sulfite solutions at ambient temperatures. The Raman and Se-77 NMR spectra of the various sulphur–selenium anions formed are given and the parallel with the reactions of selenous acid and thiols is discussed. Keywords: selenium, sulfite, selenosulfate, selenotrithionate, Se-77 NMR, Raman spectroscopy, equilibria, aqueous solutions.
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Based on a novel strategy for modulating the fluorescence of selenide and selenoxide, we have designed and developed a reversible fluorescent probe for hypochloric acid. And the synthesis, characterization, fluorescence properties, as well as the biological applications in living cells and animals, have all been described.
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We have developed a system for operating, measuring and characterizing reactors for water desalination by a capacitive de-ionization (CDI) method. The system monitors and controls the flow of water through the reactor, which consists of activated carbon electrodes. In-house software controls the reactor, collects and processes the relevant data. In this paper, our system is described in detail along with its advantages for the modular study of various reactors. We describe herein a study of a symmetrical CDI cell based on activated carbon electrodes. Parasitic surface reactions of the carbon, electrodes were observed in spite of the fact that the desalination is occurring within the electrochemical window of water (1.23 V). Over time, these reactions oxidize the carbon's surface area, resulting in diminished conductivity, which detrimentally affect the desalination process. In the present paper, we discuss the effects of the experimental conditions on the surface reactions of the carbon electrodes in CDI cells.Research Highlights► Design and assembly via LabVIEW software of an elaborate experimental CDI system. ► The interpretation of residual current and pH fluctuations upon voltage cycling. ► XPS indications for the formation of surface oxide groups by the CDI Process. ► Controlling the surface reactions by reducing the working voltages. ► Controlling the surface reactions by varying the gaseous environment.
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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.
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Selenium is a natural trace element that is of fundamental importance to human health. The extreme geographical variation in selenium concentrations in soils and food crops has resulted in significant health problems related to deficient or excess levels of selenium in the environment. To deal with these kinds of problems in the future it is essential to get a better understanding of the processes that control the global distribution of selenium. The recent development of analytical techniques and methods enables accurate selenium measurements of environmental concentrations, which will lead to a better understanding of biogeochemical processes. This improved understanding may enable us to predict the distribution of selenium in areas where this is currently unknown. These predictions are essential to prevent future Se health hazards in a world that is increasingly affected by human activities.
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Integrating the chemistry of selenium with its biology and ecotoxicology gives indications on how to regulate its environmental levels.
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Solution chemistry of Se(IV), in particular the acid-base properties, salt and complex formation, chemical reduction and reaction of Se(IV) with organic and inorganic sulfur compounds are briefly summarized. The electrochemical reduction of Se(IV) on dropping and stationary mercury electrodes is dealt with in some detail. The effects of antecedent acid-base equilibria and of consecutive reactions of the reduction product, Se(2-), adsorption of their products, and effects of added metal ions are discussed. The principles and applications of stripping analyses for determination of ultratraces of Se(IV) are summarized. The behavior on unreactive (Au, Pt, carbon) and reactive (Hg, Ag, Cu) electrodes are compared.
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Selenium pollution is a worldwide phenomenon and is associated with a broad spectrum of human activities, ranging from the most basic agricultural practices to the most high-tech industrial processes. Consequently, selenium contamination of aquatic habitats can take place in urban, suburban, and rural settings alike--from mountains to plains, from deserts to rainforests, and from the Arctic to the tropics. Human activities that increase waterborne concentrations of selenium are on the rise and the threat of widespread impacts to aquatic life is greater than ever before. Important sources of selenium contamination in aquatic habitats are often overlooked by environmental biologists and ecological risk assessors due to preoccupation with other, higher priority pollutants, yet selenium may pose the most serious long-term risk to aquatic habitats and fishery resources. Failure to include selenium in the list of constituents measured in contaminant screening/monitoring programs is a major mistake, both from the hazard assessment aspect and from the pollution control aspect. Once selenium contamination begins, a cascade of bioaccumulation events is set into motion which makes meaningful intervention nearly impossible. However, this cascade of events need not happen if adequate foresight and planning are exercised. Early evaluation and action are key. Prudent risk management based on environmentally sound hazard assessment and water quality goals can prevent biological impacts.
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