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Direct electrochemical reduction provides a novel strategy for selenium removal from complex wastewaters. While electrochemical Se(IV) reduction is thermodynamically favorable, anion structure reorganization hinders process kinetics and the phase of reduced Se(0) determines process performance. This study evaluates the thermodynamic and kinetic performance of Se(IV) removal via direct electrochemical reduction (SeDER) and proposes moderate heating to promote efficient and continuous process operation. We find that SeDER is a robust process that can handle 0.001–10 mM Se(IV) in a weakly acidic solution (pH 4–7). Se(IV) can be electrochemically removed from the aqueous phase through either a four- or six-electron pathway, with the former generating Se(0) directly attached to the electrode surface and the latter producing Se(-II) that is subsequently converted to Se(0). The four-electron pathway is a surface-limited process below 70 °C and terminates when the cathode is fully covered with the insulative amorphous Se(0). We demonstrate that raising the solution temperature to 80 °C deposits Se(0) in a conductive crystalline form and enables continuous reduction on the electrode surface. In a simple batch process design, we observe Se(IV) removal rates of up to 89 mg h–1 m–2 of electrode surface area, up to 10% Faradaic efficiency, and up to 95% removal, although we observe moderate trade-offs between these metrics depending on the electron pathway and the initial concentration of Se(IV). Our results suggest value in future work to enhance Faradaic efficiency via better reactor and electrode design, investigate parasitic reactions among competing ions, and select cost-effective electrodes for an economically competitive SeDER process.
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... To understand factors affecting DER performance for Se , 2021a)). recovery, a three-electrode electrochemical system with gold as a working electrode was employed to evaluate Se reduction's thermodynamic and kinetic performance (Zou and Mauter, 2021a). This work found that Se reduction via DER is a robust process that can deal with weakly acidic solutions (pH 4 − 7) containing 0.001 − 10 mM Se(IV). ...
... Reducing SeO 4 2to SeO 3 2is a critical challenge in applying DER in wastewater or natural water treatment due to the necessity of anion structure change and the high activation energy required to break the Se--O double bond. Other oxyanions, e.g., SO 4 , and metal oxyanions that complex water matrices can trigger cathodic parasitic reactions to compete with Se removal via DER or lead to codeposition with Se (Zou and Mauter, 2021a). However, DER approaches offer several advantages over indirect electrochemical Se removal, including selective Se removal when the cathode potential is precisely controlled, less solids generation, and direct Se recovery on the cathode. ...
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.
... The developed analytical protocol was applied to 60 mL of synthetic FGD wastewater, in which different concentrations of Se species were spiked. The Se concentrations in the synthetic wastewater were set at 5.0 and 10 µmol L -1 (0.39 and 0.78 mg L -1 ) because Se concentrations in contaminated FGD wastewater fall within 0.10-10 mg L -1 (Zou and Mauter, 2021). The spiked Se IV was 100-fold concentrated through the adsorption-desorption procedure, which enabled the detection and quantification of Se via LEP-OES. ...
Speciation of selenium (Se) is typically carried out using a sophisticated technique such as ICP-MS after preconcentration using an adsorbent; however, the separation and preconcentration of inorganic Se has not been realized in the solutions containing high concentrations of SO42-. A dithiocarbamate-modified cellulose (DMC) was used in this study for the selective extraction and preconcentration of inorganic Se in wastewater, with a portable liquid electrode plasma-optical emission spectrometry (LEP-OES) being employed for quantification. DMC was found to selectively and quantitatively adsorb selenite (SeIV) over a wide range of pH (1.0-8.0); however, less than 3.0% of selenate (SeVI) was adsorbed in a pH range of 3.0-11. Quantitative extraction of SeIV was achieved even in the presence of 3.5 mol L-1 SO42-. The maximum sample volume from which 10 mg of DMC could quantitatively extract SeIV was found to be 500 mL. KOH (0.60 mL, 1.5 mol L-1) was found to quantitatively desorb SeIV retained on the adsorbent and yielded an enrichment factor of 833. The recovery of Se species from synthetic flue-gas desulfurization wastewater containing SeIV and SeVI at concentrations of 5.0 µmol L-1 was 96.2 ± 1.8% and 105.8 ± 1.8%, respectively.
... We have recently reported an alternative strategy for Se direct electrochemical reduction (SeDER) that requires no chemical additives and significantly reduces solids generation. 14 SeDER is thermodynamically favored due to a very positive standard reduction potential for both Se(VI) and Se(IV) oxyanions, but Se(VI) reduction is significantly hindered by molecular-level structure reorganization and high activation energy. On the other hand, Se(IV) is readily electrochemically reduced and separated from the aqueous solution through a four-or six-electron pathway, with the former plating Se(0) directly onto the electrode surface and the latter producing Se(-II) that is chemically converted to Se(0). ...
Removing dissolved selenium (i.e., selenate and selenite) from wastewater is a challenging issue for a range of industries. Iron electrocoagulation can produce Fe(II)-containing solids that can adsorb and chemically reduce dissolved Se. In a series of bench-scale experiments we investigated the effects of dissolved oxygen (fully oxic, partially oxic, and strictly anoxic) and pH (6 and 8) on the rate and extent of dissolved selenate and selenite removal by iron electrocoagulation. These studies combined measurements of the aqueous phase with the direct characterization of the resulting solids. Among the conditions studied the rate and extent of dissolved selenium (Se) removal were highest at pH 8 and strictly anoxic conditions. X-ray absorption spectroscopy demonstrated that in the absence of oxygen, Se was primarily transformed to elemental selenium (Se⁰) and selenide. Green rust that formed in the suspension during electrocoagulation played a key role as a reductant and sorbent of Se. At pH 6 dissolved oxygen did not affect the rates and extents of dissolved Se removal. Under all the conditions studied, dissolved Se removal was more effective with iron electrocoagulation than with the direct addition of pre-synthesized green rust or ferrous hydroxide. The most rapid and substantial dissolved Se removal was achieved by freshly-formed green rust and ferrous hydroxide, which are both Fe(II)-bearing solids. With an improved understanding of the products and mechanisms of the process, iron electrocoagulation can be optimized for removal of Se from wastewater.
Selenium is an indispensable trace element for humans and other organisms; however, excessive selenium in water can jeopardize the aquatic environment. Investigations on the biogeochemical cycle of selenium have shown that anthropogenic activities such as mining, refinery, and coal combustion mainly contributes to aquatic selenium pollution, imposing tremendous risks on ecosystems and human beings. Various technologies thus have been developed recently to treat selenium contaminated water to reduce its environmental impacts. This work provides a critical review on the applications, characteristics, and latest developments of current treatment technologies for selenium polluted water. It first outlines the present status of the characteristics, sources, and toxicity of selenium in water. Selenium treatment technologies are then classified into three categories: 1) physicochemical separation including membrane filtration, adsorption, coagulation/precipitation, 2) redox decontamination including chemical reduction and catalysis, and 3) biological transformation including microbial treatment and constructed wetland. Details of these methods including their overall efficiencies, applicability, advantages and drawbacks, and latest developments are systematically analyzed and compared. Although all these methods are promising in treating selenium in water, further studies are still needed to develop sustainable strategies based on existing and new technologies. Perspectives on future research directions are laid out at the end.
Advanced redox-polymer materials offer a powerful platform for integrating electroseparations and electrocatalysis, especially for water purification and environmental remediation applications. The selective capture and remediation of trivalent arsenic (As(III)) is a central challenge for water purification due to its high toxicity and difficulty to remove at ultra-dilute concentrations. Current methods present low ion selectivity, and require multistep processes to transform arsenic to the less harmful As(V) state. The tandem selective capture and conversion of As(III) to As(V) is achieved using an asymmetric design of two redox-active polymers, poly(vinyl)ferrocene (PVF) and poly-TEMPO-methacrylate (PTMA). During capture, PVF selectively removes As(III) with exceptional uptake (>100 mg As/g adsorbent), and during release, synergistic electrocatalytic oxidation of As(III) to As(V) with >90% efficiency can be achieved by PTMA, a radical-based redox polymer. The system demonstrates >90% removal efficiencies with real wastewater and concentrations of arsenic as low as 10 ppb. By integrating electron-transfer through the judicious design of asymmetric redox-materials, an order-of-magnitude energy efficiency increase can be achieved compared to non-faradaic, carbon-based materials. The study demonstrates for the first time the effectiveness of asymmetric redox-active polymers for integrated reactive separations and electrochemically mediated process intensification for environmental remediation.
The removal of highly toxic, ultra-dilute contaminants of concern has been a primary challenge for clean water technologies. Chromium and arsenic are among the most prevalent heavy metal pollutants in urban and agricultural waters, with current separation processes having severe limitations due to lack of molecular selectivity. Here, we report redox-active metallopolymer electrodes for the selective electrochemical removal of chromium and arsenic. An uptake greater than 100 mg Cr/g adsorbent can be achieved electrochemically, with a 99% reversible working capacity, with the bound chromium ions released in the less harmful trivalent form. Furthermore, we study the metallopolymer response during electrochemical modulation by in situ transmission electron microscopy. The underlying mechanisms for molecular selectivity are investigated through electronic structure calculations, indicating a strong charge transfer to the heavy metal oxyanions. Finally, chromium and arsenic are remediated efficiently at concentrations as low as 100 ppb, in the presence of over 200-fold excess competing salts.
The remediation of toxic metals from water with high concentrations of salt has been an emerging area for membrane separation. Cost-effective nanomaterials such as iron and iron oxide nanoparticles have been widely used in reductive and oxidative degradation of toxic organics. Similar procedures can be used for redox transformations of metal species (e.g. metal oxyanions to elemental metal), and/or adsorption of species on iron oxide surface. In this study, iron-functionalized membranes were developed for reduction and adsorption of selenium from coal-fired power plant scrubber water. Iron-functionalized membranes have advantages over iron suspension as the membrane prevents particle aggregation and dissolution. Both lab-scale and full-scale membranes were prepared first by coating polyvinylidene fluoride (PVDF) membranes with polyacrylic acid (PAA), followed by ion exchange of ferrous ions and subsequent reduction to zero-valent iron nanoparticles. Water permeability of membranes decreased as the percent PAA functionalization increased, and the highest ion exchange capacity (IEC) was obtained at 20% PAA with highly pH responsive pores. Although high concentrations of sulfate and chloride in scrubber water decreased the reaction rate of selenium reduction, this was shown to be overcome by integration of nanofiltration (NF) and iron-functionalized membranes, and selenium concentration below 10 µg/L was achieved.
A flow-through wetland system was established in the Tulare Lake Drainage District (TLDD) in California to determine if selenium (Se) from saline irrigation drainage can be removed prior to impoundment in evaporation basins to reduce potential toxicity to waterbirds. The objective of this research was to evaluate Se speciation, accumulation, and fractionation in the waters and sediments of the newly developed wetland system. The inlet water was dominated by selenate [Se(VI), 92%], with smaller percentages of selenite [Se(IV), 5%] and organic Se [org-Se(-II), 3%]. For the outflow water, the average percentage of Se(VI) was 72% in November 1997 and 59% in February 1999. This change may be due to an increase in either residence time and/or accumulation of organic detrital matter, which may enhance Se(VI) reduction processes. Selenium accumulation, transformation, and incorporation with the solid phase were all intensified in the surface sediment (<20 cm). The highest total Se concentrations in the sediments were found in the top 5 cm and concentrations dramatically decreased with depth. Elemental Se [Se(0)], as extracted by Na2SO3, was the largest fraction (average of 46%) of the total sediment Se, followed by organic matter-associated Se (OM-Se) extracted by NaOH (average of 34%). Soluble, adsorbed, and carbonate-associated Se, as extracted by KCl, K2HPO4 (pH 8.0), and NaOAc (pH 5.0), were about 3, 10, and 3% of the total sediment Se, respectively. After establishing the wetland for 2 yr, significant Se removal from the flowing water was observed. The major sink mechanisms in the sediment are reduction to Se(0) and immobilization into the organic phase.
A series of compressed powder samples of a double-phase selenium mixture were prepared with various fractional volumes of amorphous (a) and crystalline (c) phases. The direct current properties of the samples were measured and are discussed. The activation energy was found to decrease from 1.8 eV (for the 90% amorphous-10% crystalline sample) to 0.67 eV (for the 100% crystalline sample). For the same range of fractional volumes, the conductivity σm of the mixture at room temperatures was found to increase from ,and the pre-exponential factor σ0 increased from . An empirical formula to fit the measured conductivity data over the entire range of the double-phase mixture is proposed. This formula was also tested for massive selenium samples of different degrees of crystallinity (defined from X-ray measurements) and very satisfactory results were obtained.
The reduction of dissolved selenium oxyanions from mine wastewater utilizing an elemental iron cementation technology has been studied on a laboratory scale at Montana Tech of The University of Montana and on a pilot scale at MSE-Technology Applications as a water treatment process potentially capable of removing dissolved selenium to <50 μg/L. Laboratory and pilot scale studies using elemental iron have demonstrated effective removal of dissolved selenium. Enhanced selenium reduction rates have been demonstrated when galvanically coupled iron/copper and iron/nickel are utilized. The laboratory and pilot scale results are presented and discussed.
Previously, we isolated a selenate- and arsenate-reducing bacterium, designated strain SF-1, from selenium-contaminated sediment
and identified it as a novel species, Bacillus selenatarsenatis. B. selenatarsenatis strain SF-1 independently reduces selenate to selenite, arsenate to arsenite, and nitrate to nitrite by anaerobic respiration.
To identify the genes involved in selenate reduction, 17 selenate reduction-defective mutant strains were isolated from a
mutant library generated by random insertion of transposon Tn916. Tn916 was inserted into the same genome position in eight mutants, and the representative strain SF-1AM4 did not reduce selenate
but did reduce nitrate and arsenate to the same extent as the wild-type strain. The disrupted gene was located in an operon
composed of three genes designated srdBCA, which were predicted to encode a putative oxidoreductase complex by the BLASTX program. The plasmid vector pGEMsrdBCA, containing
the srdBCA operon with its own promoter, conferred the phenotype of selenate reduction in Escherichia coli DH5α, although E. coli strains containing plasmids lacking any one or two of the open reading frames from srdBCA did not exhibit the selenate-reducing phenotype. Domain structure analysis of the deduced amino acid sequence revealed that
SrdBCA had typical features of membrane-bound and molybdopterin-containing oxidoreductases. It was therefore proposed that
the srdBCA operon encoded a respiratory selenate reductase complex. This is the first report of genes encoding selenate reductase in
Capacitive deionization (CDI) technologies have gained intense attention for water purification and desalination in recent years. Inexpensive and widely available porous carbon materials have enabled the fast growth of electrosorption research, highlighting the promise of CDI as a potentially cost-effective technology to remove ions. Whereas the main focus of CDI has been on bulk desalination, there has been a recent shift towards electrosorption for selective ion separations. Heavy metals are pollutants that can have severe health impacts and are present in both industrial wastewater and groundwater leachates. Heavy metal ions, such as chromium, cadmium, or arsenic, are of great concern to traditional treatment technologies, due to their low concentration and the presence of competing species. The modification/functionalization of porous carbon and recent developments of faradaic and redox-active materials have offered a new avenue for selective ion-binding of heavy metal contaminants. Here, we review the progress in electrosorptive technologies for heavy metal separations. We provide an overview of the wide applicability of carbon-based electrodes for heavy metal removal. In parallel, we highlight the trend toward modification of carbon materials, new developments in faradaic interfaces, and the underlying physico-chemical mechanisms that promote selective heavy metal separations.
Selenium (Se) contamination as a result of anthropogenic activity (i.e. mining, power generation, and oil & gas refining) is becoming a global concern due to its associated aquatic toxicity concerns. Herein, heterogeneous nanoscale photocatalysts were synthesized by depositing noble metal nanoparticles (gold (Au), silver (Ag), platinum (Pt) and palladium (Pd)) onto titanium dioxide (TiO2), which demonstrated work-function dependent bimodal selectivity of final products during the photocatalytic reduction of selenate to elemental Se (Se0) or hydrogen selenide gas (H2Se). The Se-noble metal-TiO2 (Se-NM-TiO2) photocatalytic system is structured in a direct Z-scheme arrangement when Au, Ag or Pt are used, allowing for high selectivity towards H2Se. In contrast, Pd acted as an electron sink which decreased reducibility of the photogenerated electrons, ultimately causing a higher selectivity towards Se0. Au–TiO2 offers the largest H2Se selectivity of all catalysts tested, while Pd–TiO2 offers the highest selectivity to solid Se0 generation. This study elucidates electron transport mechanisms and Fermi level equilibration via quantized double-layer charging effects of the Se-NM-TiO2 system and sheds light on advanced reduction processes using nanoscale heterogeneous catalysts. Finally, tunability of the Se reduction product is key to designing a sustainable treatment approach with a potential for Se capture and reuse.
Electrocatalytic nitrate reduction into recyclable ammonium under benign conditions is significant. However, the development of such a process has been retarded by the lack of efficient electrocatalysts for high selective synthesis of ammonia from nitrate electroreduction. In this work, TiO2 nanotubes with rich oxygen vacancies (TiO2-x) are reported to exhibit high Faradaic efficiency (85.0%) and selectivity (87.1%) toward the ammonium synthesis from nitrate electroreduction. 15N isotope labeling experiments prove that ammonium originates from nitrate reduction. Both the 1H nuclear magnetic resonance (NMR) spectra and colorimetric methods are performed to quantify ammonia. Online differential electrochemical mass spectrometry (DEMS) and density functional theory calculations reveal the function of oxygen vacancy in nitrate electroreduction, that is, the oxygen atom in nitrate fill in oxygen vacancies of TiO2-x to weaken the N-O bonding and restrain the formation of by-products, resulting in high Faradaic efficiency and ammonium selectivity. This strategy may open a paradigm for the development of rationally designed nanostructures as the electrocatalysts for selective nitrate electroreduction to ammonium.
Severe effects of selenium (Se) occurred among birds feeding and nesting at Kesterson Reservoir (San Joaquin Valley, California) in 1983‐1985. This paper describes the integration of site monitoring, risk assessment, and management actions conducted after the effects of Se were discovered. Selenium contamination of the site occurred over just a few years, but actions to resolve the contamination issues required >20 years. The Reservoir, a series of 12 ponds totaling about 1,280 acres (518 hectares), served for storage and evaporation of subsurface agricultural drainage. Selenium concentrations in Reservoir inflow in 1983 were about 300 µg/L, primarily as selenate; within the ponds it was biogeochemically reduced to other inorganic and organic forms and bioaccumulated by biota or deposited to sediments. An estimated 9000 kg of Se were delivered to Kesterson in 1981‐1986. A 1985 order required cleanup and abatement of the Reservoir, so Reclamation and the US Department of the Interior undertook actions and studies to reduce hazards to birds. In 1988, about one million cubic yards (764,500 cubic meters) of soil were used to fill portions of the Reservoir, transforming it into terrestrial habitat. Intensive monitoring began in 1989 to assess the impact of the Reservoir on wildlife, provide a basis for adjusting site management, verify the effectiveness of cleanup actions, and provide a basis for modifying future monitoring. Monitoring continued until 2014, with modifications and management actions based on results of two risk assessments (1993 and 2000). Monitoring results in 2013‐2014 showed that Se concentrations were relatively stable over time and risks to wildlife were low. From the initial problem discovery to the conclusion of actions taken to remediate the site, combining responsive, reactive, and adaptive monitoring; modeling; risk assessment, and mitigation actions proved effective in solving the problem so that risks to wildlife were reduced to minimal levels. This article is protected by copyright. All rights reserved. Because severe effects of selenium (Se) occurred in birds at Kesterson Reservoir, portions of the Reservoir were filled with soil, transforming it into terrestrial habitat; we describe the integration of subsequent site monitoring, ecological risk assessment, and management actions. Monitoring in 1989-2014 and two risk assessments (1993 and 2000) assessed impacts on wildlife and provided a basis for adjusting site management and modifying future monitoring; final results showed that Se concentrations were relatively stable over time and risks to wildlife were low. From the initial problem discovery to the conclusion of actions taken to remediate the site, combining responsive, reactive, and adaptive monitoring; modeling; and mitigation actions proved effective in solving the problem so that risks to wildlife were reduced to minimal levels. Selenium contamination of the site occurred over just a few years, but actions to resolve the contamination issues required more than 20 years.
In the present work selenium films were electrodeposited onto fluorine-doped SnO 2 substrates, departing from a Se ⁴⁺ aqueous electrolyte. The temperature and deposition potential were varied; the structure, morphology and band gap were studied by Atomic Force Microscopy, Raman Spectroscopy and optical absorption. The voltammetry study showed two reduction peaks that shift towards positive potentials as the deposition temperature increases from 25 to 75 °C: the first one shifts between -925 mV and -1120 mV versus Pt and the second between -1060 mV to -1250 mV versus Pt A black/greyish selenium film of a-Se and trigonal Se was deposited at -925 [email protected] °C and a deep red monoclinic Se one at -960 [email protected] °C; in the other conditions the film color indicate a mixture of the red a-Se and trigonal phases confirmed by Raman and optical spectroscopy. Trigonal phase has a calculated band gap of 1.6 eV, red and black a-Se band gap value of 1.83 eV, and monoclinic Se of 2.0 eV, respectively. From Electrochemical Impedance Spectroscopy, the films were found to be n-type, with carrier densities around 10 ¹⁸ -10 ¹⁹ cm ⁻³ , and Flat Band Potentials between 0.6-1.0 V in dependence of the deposition conditions. The results point out the feasibility of controlling the phase of Se layers using a single aqueous bath and mild temperature conditions.
In November 2015, the United States Environmental Protection Agency (EPA) promulgated the Effluent Limitations Guidelines (ELGs) for the Steam Electric Power Generating Sector. These guidelines either eliminate or lower permissible discharge limits for six wastewater streams produced at coal fired power plants (CFPPs), including flue gas desulfurization (FGD) wastewater. This paper summarizes the state of the art, describes fundamental challenges, and highlights critical research needs in FGD wastewater treatment. We begin by describing the processes that influence FGD wastewater production and composition. We then critically evaluate the best available technologies for treating FGD wastewater identified by the EPA during the regulatory process. Finally, we identify four critical challenges and research needs in complying with the 2015 ELGs including (1) removing selenium species from FGD wastewater, (2) achieving zero liquid discharge of pollutants from FGD wastewater while enabling water reuse at CFPPs, (3) developing water treatment systems that can respond to short-term fluctuations in CFPP electricity generation and wastewater production, and (4) optimizing the balance of capital and operational costs for FGD treatment when the power plant lifespan is uncertain.
Efficient recovery of selenium from the dilute acidic solution is of great importance to the cleaner production of this scattered element. In order to overcome the bottlenecks of low recovery ratio and severe environment issues in conventional processes, an easy-operation electrochemical recovery process of Se was developed using low-cost stainless steel cathodes. It has been demonstrated that Se ions can be successfully electrodeposited, but the recovery ratio of Se is significantly limited by the low Se concentration. Consequently, a cylinder turbulent electrochemical reactor was employed for the extraction of metals from dilute solution (0.3 g/L Se(IV)). 97.6% selenium was successfully recovered within 90 min, nano-sized and mesoporous selenium product was also obtained due to the excellent mass transport effect. This mass transport-enhanced technique may serve as a promising alternative for metal and metalloid electrowinning.
Chemical precipitation method was adopted to remove sulfate from wet flue gas desulfurization (FGD) wastewater and mixtures of Ca(OH)2(CH) and NaAlO2(SA) were used as precipitants. The mechanisms of sulfate removal were explored according to the experimental and simulated results. These showed that three kinds of precipitations, which were gypsum, ettringite and co-precipitation onto aluminum hydroxides, were formed when sulfate in water reacted with CH and SA. The optimum operation condition for removing sulfate was that the molar ratio of CH/SA was 2, the initial pH value 5, the precipitant dosage 15 g/L, the reaction time 20 min, and the reaction temperature 55 °C. The sulfate was reduced from 4,881 mg/L to 784 mg/L under the optimized condition. In addition, the heavy metals and fluoride were also mostly removed. The post treatments of the supernatant illustrated that removal of sulfate from wet FGD wastewater by co-precipitation with CH and SA was a better choice.
Due to recent changes in regulatory discharge limits, selenium removal from electric utility wastewaters is becoming an important issue. In this work, Mg-Al-CO3 layered double hydroxide (LDH) in granular form was evaluated for treatment of selenium-containing groundwater in small scale column tests. The LDH material showed good capacity for removing selenate from groundwaters that contained very high levels of sulfate and total dissolved solids. Column tests were investigated using the granular LDH to treat groundwater containing trace levels of selenium < 2 ppb. Removal of sulfate using chemical pre-treatment of the groundwater resulted in about 3X higher selenium loading onto the granular LDH. The structural changes in the media after exhaustion and analysis of the effluent water from the column test were also studied to better understand the species removed by the LDH. These results show that the LDH is a promising sorbent for removing selenium from wastewaters with high levels of sulfate and background species.
Selenium (Se) removal from synthetic solutions and from real Flue Gas Desulfurization (FGD) wastewater generated by a coal-fired power plant was studied for the first time using a commercial iron oxide impregnated strong base anion exchange resin, Purolite® FerrIX A33E. In synthetic solutions, the resin showed high affinity for selenate and selenite, while sulfate exhibited a strong competition for both oxyanions. The FGD wastewater investigated is a complex system that contains Se (~1200 µg L-1), SO42- (~1.1 g L-1), Cl¯ (~9.5 g L-1), and Ca2+ (~5 g L-1), alongside a broad spectrum of toxic trace metals including Cd, Cr, Hg, Ni, and Zn. The resin performed poorly against Se in the raw FGD wastewater and showed moderate to good removal of several trace elements such as Cd, Cr, Hg, and Zn. In FGD effluent, sulfate was identified as a powerful competing anion for Se, having high affinity for the exchange active sites of the resin. The desulfurization of the FGD effluent using BaCl2 led to the increase in Se removal from 3% (non-desulfurized effluent) to 80% (desulfurized effluent) by combined precipitation and ion exchange treatment. However, complete desulfurization using equimolar BaCl2 could not be achieved due to the presence of bicarbonate that acts as a sulfate competitor for barium. In addition to selenium and sulfate removal, several toxic metals were efficiently removed (Cd: 91%; Cr: 100%; Zn: 99%) by the combined (desulfurization and ion exchange) treatment.
Voltammetric techniques have been employed to show that underpotential deposited Cu on polycrystalline Au electrodes in aqueous 0.1 M HClO4 catalyzes the reduction of purified selenate, SeO4²⁻, to yield a layer of adsorbed copper selenide, CuxSe. Subsequent oxidation of this layer led to the loss of Cu, leaving behind adsorbed, elemental Se, which could be oxidized to selenite, SeO3²⁻, at higher potentials. Application of this method made it possible to detect SeO4²⁻ down to nM levels. Voltammetric features observed on Au(poly) in Cu²⁺-free 0.1 M HClO4 containing SeO4²⁻ reported earlier in the literature could be attributed to the reduction of SeO3²⁻ impurities present in the commercial chemical.
Selenium contaminated wastewater derived from global industrial activities (i.e. coal and mineral mining, metal smelting, oil extraction and refining, and agricultural irrigation) can bioaccumulate in aquatic organisms and presents a source of toxicity for many organisms, including humans. Selenium represents an extremely difficult contaminant to remove from wastewater due to its range of solubility, toxicity, and state of matter over different oxidation states. Recently, the application of nanomaterials for removing selenium from wastewater has received increasing interest from the power generation and industrial mining sectors. Several classes of nanomaterials, such as nanoscale adsorbents, catalysts and reactants, show promising potential in removing selenium in a wide range of oxidation states. This review article provides a summary of current selenium removal technologies, highlights the gaps in these current technologies and focuses on emerging nanomaterials capable of removing selenium oxyanions from wastewater to ultra-low microgram per litre limits. Recent published literature has focused on the modification of different nanomaterials in order to achieve high surface adsorbing activity, high reactivity, selectivity and sustainable treatment capability in efforts to remove selenium oxyanions. The majority of promising nanotechnologies for selenium removal are undergoing intense research and development in efforts to get the technology into wastewater treatment markets. These nanomaterials have the ability to remove selenium contaminants to previously unachievable ultra-low levels, while implementing reliable and sustainable treatment techniques.
Laboratory algal cultures exposed to selenate were shown to produce and release selenomethionine, selenomethionine oxide, and several other organic selenium metabolites. Released discrete organic selenium species accounted for 1.6 - 13.1 % of the selenium remaining in the media after culture death, with 1.3 - 6.1 % of the added selenate recovered as organic metabolites. Analysis of water from an industrially-impacted river collected immediately after the death of massive annual algal blooms showed that no selenomethionine or selenomethionine oxide was present. However, other discrete organic selenium species, including a cyclic oxidation product of selenomethionine, were observed, indicating the previous presence of selenomethionine. Industrial biological treatment systems designed for remediation of selenium-contaminated waters were shown to increase both the concentration of organic selenium species in the effluent, relative to influent water, and the fraction of organic selenium to up to 8.7 % of the total selenium in the effluent, from less than 1.1 % in the influent. Production and emission of selenomethionine, selenomethionine oxide, and other discrete organic selenium species were observed. These findings are discussed in the context of potentially increased selenium bioavailability caused by microbial activity in aquatic environments and biological treatment systems, despite overall reductions in total selenium concentration.
Increasing evidences suggest that nanoscale zero-valent iron (nZVI) is an effective agent for treatment and removal of selenium from water. For example, 1.3 mM selenite was quickly removed from water within 3 min with 5 g/L nZVI. In this work, reaction mechanisms of selenite [Se(IV)] in a single core–shell structured nanoscale zero-valent iron (nZVI) particle were studied with the method of spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) integrated with X-ray energy dispersive spectroscopy (XEDS). This method was utilized to visualize solid phase translocation and transformation of Se(IV) such as diffusion, reduction, deposition and the effect of surface defects in a single nanoparticle. Se(IV) was reduced to Se(-II) and Se(0), which then formed a 0.5 nm layer of selenium at the iron oxide-Fe(0) interface at a depth of 6 nm from the surface. The results provided near atomic-resolution proof on the intraparticle diffusion-reduction of Se(IV) induced by nZVI. The STEM mapping also discovered that defects on the surface layer accelerate the diffusion of selenium and increase the capacity of nZVI for selenium sequestration.
Selenium (Se) is one of contaminants required to be regulated during drinking water treatment, however, little information has been collected to date regarding Se removal by coagulation. In this study, the performance of Se removal by coagulation has been evaluated with respect to the dependence on Se species, coagulant type, water pH and interfering ions. The results showed that a Fe-based coagulant was much more efficient than Al-based coagulants in Se removal. The removal of selenite (Se(IV)) by coagulation was much more pronounced than that of selenate (Se(VI)). With an FC dosage of more than 0.4 mM Fe/L, Se(IV) removal efficiency of more than 98% could be achieved when the initial Se(IV) concentration was 250 μg/L. For Al-based coagulants (AlCl3 (AC) and polyaluminum chloride (PACl)) Se removal efficiency was positively correlated with the content of Al13 species during the coagulation process. Adsorption onto hydroxide flocs was the most active coagulation mechanism for Se removal and precipitation also played specific roles at low dosage, especially for Se(IV) removal and with Fe coagulant. High coagulant dosage and weakly acidic pH could enhance the formation of hydroxide flocs having more active adsorption sites and high zeta potential, and thus favored Se removal. These findings are important to understand the efficiency and mechanisms of Se removal by coagulation.
The electrical conductivity of liquid selenium and selenium‐tellurium mixtures was studied in detail. Measurements in the temperature range 230° to 500°C from dc to 1 mc/sec indicated the conductivities to be frequency‐independent at low field strength (<5 v/cm). For larger fields (5 to 250 v/cm) the conductivity increases and higher dc than ac values result. Adding increasing amounts of tellurium to selenium produced a continuous rise in conductivity from 1.26×10−4 ohm−1 cm−1 (pure Se) to 2.08×10−1 ohm−1 cm−1 (50.3 percent Se, 49.7 percent Te) at 480°C and a decrease in activation energy from 1.14 ev to 0.84 ev. Comparatively good reproducibility was obtained in the measurements. An attempt to study ionic migration in liquid selenium by tracer methods has thus far not given satisfactory results. The conductivity values obtained for pure selenium are in good agreement with those presented by Henkels but disagree with those of Borelius. The electrical characteristics of liquid selenium and selenium‐tellurium mixtures indicate that hole conduction as pictured for selenium in the solid state cannot be considered as an adequate explanation of the conduction process in these liquids. A complex mixture of conduction by electrons, holes, and ions is believed to exist.
The effects of various anions on the oxidation of glucose on single crystal gold electrodes were studied in HClO4, CF3SO3H, HNO3, H2SO4, H3PO4 and HCl solutions. Contrary to common belief, glucose can be oxidized on gold in acid solutions, but only in the absence of strongly adsorbed anions such as chlorides, sulfates and phosphates. The reaction is partly or completely inhibited in solutions containing specifically adsorbed anions. The data show some indication of an (OH)ads layer on gold at potential cathodic to +1.2 V vs rhe in HClO4 and CF3SO3H solutions. Based on the rates of oxidation of glucose, the following sequence has been found for the inhibition of the glucose oxidation by adsorbed anions: ClO−4 ≈ CF3SO−3 ⪡ NO−3 ⪡ HSO−4 (SO2−4) < H2PO−4 (HPO2−4 < Cl−.
The electroplating of metallic selenium from acid baths has been achieved and reduced to practice. The paper discusses in succession the polymorphism of selenium; selenium ions in selenious acid solution; cathodic deposition as the amorphous and metallic phase; the nucleation characteristic and crystal growth of the metallic modification; the properties of various types of plating baths; a selenium-carbon anode for the stabilization of the bath composition; and finally a number of influences affecting the crystallization habit of the deposit. The current density of 200 amp./ft.2 attainable in a well-balanced plating bath compares favorably with standard baths for true metals.
The electrochemical oxidation of glucose has been studied in phosphate buffer (pH = 7.4) on single crystal and polycrystalline gold electrodes using electrochemical techniques, ex situ NMR, in situ FTIR spectroscopy, and isotope labeling. Under these conditions, the results indicate that the rate determining step for the electro-oxidation of glucose is bond breaking between the hydrogen atom and the C 1 carbon atom. Gluconolactone appears to be an intermediate and sodium gluconate is the reaction product. First order kinetics with respect to glucose were also found. Gold would be a useful electrode for sensor applications, if the inhibition of the glucose oxidation by the adsorption of Cl - could be avoided
Removal of the toxic selenium compounds selenite and selenate from waste water before discharge is becoming increasingly imperative in industrialized countries. Bacteria can reduce selenate to selenite, but also further to elemental selenium, selenide or organic selenium. In this paper, we aim to exclusively bio-reduce selenate to selenite in an open high-rate bioreactor. This conversion could be part of a two-stage process in which the selenite is subsequently reduced by chemical means under optimal conditions to produce a biomass-free selenium product. In the process, yield and reduction rate of biological selenate to selenite should be maximized, while formation of elemental selenium, selenide and organic selenium compounds should be avoided. Fed-batch experiments with a liquid volume of 0.25-0.75 L at different temperatures 20-30-40-50 °C, pH settings 6-7-8-9, initial biomass concentration of 1 or 5 g wet weight granular Eerbeek sludge and various lactic acid concentrations were performed to determine their effect on the biological conversion of selenate to selenite. Furthermore, the effect on selenite losses by further biological reduction or, if present, chemical reduction was investigated as well. Optimal selenate reduction to selenite was found at 30 °C and pH 6 or 7 or 8 with 25 mM selenate and 13.75 mM lactic acid in the influent, with a selenite yield of 79-95%. In all the other conditions, less selenate was reduced to selenite. Also a 5 times higher electron donor concentration resulted in less selenite production, with only 22% of the selenate converted to selenite. The high yield and the high biological reduction rate of at least 741 mg Se/g initial VSS/day detected in the 1 g initial biomass experiment implicate that biological selenate conversion to selenite is a feasible process.
Selenium (Se) is an important element belonging to the chalcogen family. Electrochemistry is of significant importance in various applications of Se and its compounds, ranging from non-vacuum processed solar cells, direct methanol/polymer electrolyte membrane fuel cells, electrocatalysts, batteries, and metallic alloys. However, rich information on Se electrochemistry remains unexplored in the last few decades. Here we made an attempt to assess the reported studies on electrochemical behaviours of Se. Fundamental concepts of Se electrochemistry in aqueous media and ionic liquids at various electrodes, polarographic and cyclic voltammetric analysis, cathodic and anodic stripping analysis, electrodeposition mechanism and electrocatalysis are discussed. Electrochemical atomic layer epitaxy (ECALE) in engineering Se atomic layers and fabrication of Se nanostructures by electrochemical methods are highlighted. Se electrochemistries in application areas are reviewed. Voltammetric studies of Se electrodeposition on molybdenum (Mo) electrode are presented with a view of its applicability in electrodeposited CuInSe2/CuIn(Ga)Se2 (CIS/CIGS) solar cells.
Removal of selenate from solution is investigated in batch electrochemical systems using reactive iron anodes and copper plate cathode in a bicarbonate medium. Iron anodes produce ferrous hydroxide, which is a major factor in the removal of selenate from solution. Iron anodes also generate a significant decrease in the oxidation-reduction potential (ORP) of the solution because it prevents generation of oxygen gas at the anode by electrolysis. The removal rates varied from 45.1 to 97.4%, depending on current density and selenate concentration. The transformation of selenate by the process is modeled based on a heterogeneous reaction coupled with electrochemical generation of ferrous and hydroxide. The rates are optimized at lower initial concentrations, higher electrical currents, and the presence of anions. Presence of dissolved oxygen does not cause any significant effects the removal of selenate.
Studies of the electrodeposition of Se atomic layers on Au(111) and Au(110) are presented. Three electrochemical methods of forming Se atomic layers were investigated: reductive deposition, oxidative stripping of bulk Se, and reductive stripping of bulk Se. The resulting Se atomic layers were studied using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). LEED indicated the formation of Au(111)(√3×√3)R30°-Se and Au(110)(2×3)-Se structures. STM analysis confirmed the presence of those structures along with several others. At low Se coverages on Au(111), a mosaic structure was formed, composed of a large number of small domains of a (√3×√3)R30°-Se structure, separated by areas void of Se. At higher coverages, near 1/3, the (√3×√3)R30° structure covered most of the surface, except for a number of linear phase boundaries. Commensurate with completion of the (√3×√3)R30° structure, some domains of square Se8 rings were usually evident, as well. At still higher coverages, a heterogeneous surface was formed, composed of a complex network of rings, chains, clusters, and pits. This heterogeneity appears to result from slow deposition kinetics, probably the result of both a low exchange current and low Se surface mobility. Some of the kinetic sluggishness may have resulted from the need to translate whole domains of Se atoms from one site to another, in order to remove phase boundaries. STM studies of the Au(110) surface indicated that only the (2×3) structure was formed at coverages much below 1 monolayer and that it was formed homogeneously. At monolayer coverages and above, a honeycomb structure composed of chains of Se atoms was observed, which filled in at still higher coverages to complete a second Se layer.
The electrodeposition of Se(IV) at a Au electrode in 0.1 M HClO4 is concluded to produce Se in three distinct states of activity, and three anodic stripping peaks are observed for large quantities of deposited Se. Approximately a mono-layer is initially deposited which is apparently stabilized by one-dimensional interaction with the Au electrode surface. The formation of a bulk deposit of Se produces a large activity gradient which is the driving force for irreversible diffusional transport of Se into the Au electrode forming a AuSe alloy of unknown stoichiometry. Application of stripping voltammetry for determination of trace Se(IV) in 0.1 M HClO4 is possible if the total deposit does not exceed the equivalent of one monolayer. The detection limit of the technique is approximately 0.04 ppb Se(IV) in 0.1 M HClO4.
The review deals with the principal advances in the electrochemistry of selenium and tellurium. Problems concerning the kinetics and mechanisms of the cathodic reduction of selenium and tellurium to different valence states and their anodic oxidation in aqueous and non-aqueous solutions are discussed.
The practical applications of electrochemistry in the isolation, refining, and determination of small amounts of the two elements are described.
The bibliography includes 178 references.
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.
This work presents a simple, reproducible and low cost method, employing differential pulse anodic stripping voltammetry, for determination of selenium(IV). A gold electrode obtained from recordable compact disks (CD-R) was used to evaluate the voltammetric behavior of the metallic ion in 0.1molL(-1) HClO(4). To evaluate the voltammetric behavior of Se(IV), parameters such as deposition potential and deposition time were optimized. A wide linear response range, from 0.5 to 291ngmL(-1), was obtained using a 5.0mm diameter gold electrode. Recovery tests for Se(IV) utilizing standard reference solutions provided values between 94 and 96%.
The essential trace mineral, selenium, is of fundamental importance to human health. As a constituent of selenoproteins, selenium has structural and enzymic roles, in the latter context being best-known as an antioxidant and catalyst for the production of active thyroid hormone. Selenium is needed for the proper functioning of the immune system, and appears to be a key nutrient in counteracting the development of virulence and inhibiting HIV progression to AIDS. It is required for sperm motility and may reduce the risk of miscarriage. Deficiency has been linked to adverse mood states. Findings have been equivocal in linking selenium to cardiovascular disease risk although other conditions involving oxidative stress and inflammation have shown benefits of a higher selenium status. An elevated selenium intake may be associated with reduced cancer risk. Large clinical trials are now planned to confirm or refute this hypothesis. In the context of these health effects, low or diminishing selenium status in some parts of the world, notably in some European countries, is giving cause for concern.
In many environmental contaminant situations selenium has become the primary element of concern because of its bioaccumulative nature in food webs. Initial concerns about selenium were related to fish kills at Belews Lake, NC, Martin Lake, TX, and Kesterson Reservoir, CA, and to bird deformities at Kesterson Reservoir. Additional concerns were identified under the National Irrigation Water Quality Program at Salton Sea, CA, Kendrick, WY, Stewart Lake, UT, and Grand Valley and Uncompahgre Valley, CO. Recent studies have raised concerns about selenium impacts on aquatic resources in Southeastern Idaho and British Columbia. The growing discomfort among the scientific community with a waterborne criterion has lead the US Environment Protection Agency to consider a tissue-based criterion for selenium. Some aquatic ecosystems have been slow to recover from selenium contamination episodes. In recent years, non-governmental researchers have been proposing relatively high selenium thresholds in diet and tissue relative to those proposed by governmental researchers. This difference in opinions is due in part to the selection of datasets and caveats in selecting scientific literature. In spite of the growing selenium literature, there are needs for additional research on neglected organisms. This review also discusses the interaction of selenium with other elements, inconsistent effects of selenium on survival and growth of fish, and differences in depuration rates and sensitivity among species.
The reduction of Cr(VI) to Cr(III) is achieved in a flow-by, parallel-plate reactor equipped with reticulated vitreous carbon (RVC) electrodes;this reduction can be accomplished by the application of relatively small potentials. Treatment of synthetic samples and field samples (from an electrodeposition plant) results in final Cr(VI) concentrations of 0.1 mg/L (i.e., the detection limit of the UV-vis characterization technique used here) in 25 and 43 min, respectively. Such concentrations comply with typical environmental legislation for wastewaters that regulate industrial effluents (at presenttime = 0.5 mg/L for discharges). The results show the influence of the applied potential, pH, electrode porosity, volumetric flow, and solution concentration on the Cr(VI) reduction percentage and on the required electrolysis time. Values for the mass transfer coefficient and current efficiencies are also obtained. Although current efficiencies are not high, the fast kinetics observed make this proposed treatment an appealing alternative. The lower current efficiency obtained in the case of a field sample is attributed to electrochemical activation of impurities. The required times for the reduction of Cr(VI) are significantly lower than those reported elsewhere.
Enterobacter cloacae SLD1a-1 is capable of reductive detoxification of selenate to elemental selenium under aerobic growth conditions. The initial reductive step is the two-electron reduction of selenate to selenite and is catalyzed by a molybdenum-dependent enzyme demonstrated previously to be located in the cytoplasmic membrane, with its active site facing the periplasmic compartment (C. A. Watts, H. Ridley, K. L. Condie, J. T. Leaver, D. J. Richardson, and C. S. Butler, FEMS Microbiol. Lett. 228:273-279, 2003). This study describes the purification of two distinct membrane-bound enzymes that reduce either nitrate or selenate oxyanions. The nitrate reductase is typical of the NAR-type family, with alpha and beta subunits of 140 kDa and 58 kDa, respectively. It is expressed predominantly under anaerobic conditions in the presence of nitrate, and while it readily reduces chlorate, it displays no selenate reductase activity in vitro. The selenate reductase is expressed under aerobic conditions and expressed poorly during anaerobic growth on nitrate. The enzyme is a heterotrimeric (alphabetagamma) complex with an apparent molecular mass of approximately 600 kDa. The individual subunit sizes are approximately 100 kDa (alpha), approximately 55 kDa (beta), and approximately 36 kDa (gamma), with a predicted overall subunit composition of alpha3beta3gamma3. The selenate reductase contains molybdenum, heme, and nonheme iron as prosthetic constituents. Electronic absorption spectroscopy reveals the presence of a b-type cytochrome in the active complex. The apparent Km for selenate was determined to be approximately 2 mM, with an observed Vmax of 500 nmol SeO4(2-) min(-1) mg(-1) (kcat, approximately 5.0 s(-1)). The enzyme also displays activity towards chlorate and bromate but has no nitrate reductase activity. These studies report the first purification and characterization of a membrane-bound selenate reductase.
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