Reduction Of Se Research Articles

Selenium-mediated Cr(VI) reduction and SeNPs synthesis accelerated Bacillus cereus SES to remediate Cr contamination

Microbial biotransformation of Cr(VI) is a sustainable approach to reduce Cr(VI) toxicity and remediate Cr(VI) contamination. In this study, Bacillus cereus SES with the capability of reducing both Cr(VI) and Se(IV) was isolated, and the effect of Se supplementation on Cr(VI) reduction by Bacillus cereus SES was investigated. Se(IV) addition enabled 2.6-fold faster Cr(VI) reduction, while B. cereus SES reduced 96.96% Se(IV) and produced more selenium nanoparticles (SeNPs) in the presence of Cr(VI). Co-reduction products of B. cereus SES on Cr(VI) and Se(IV) were SeNPs adsorbed with Cr(III). The relevant mechanisms were further revealed by proteomics. Se(IV) supplementation mediated the synthesis of Cr(VI) reductants and stress-resistant substances, thus enhancing Cr(VI) resistance and promoting Cr(VI) reduction. Meanwhile, high Se(IV) reduction rate was associated with Cr(VI)-induced electron transport processes, and Cr(VI) mediated the up-regulation of flagellar assembly, protein export and ABC transporters pathways to synthesis and export more SeNPs. Furthermore, Se combined with B. cereus SES had the potential to reduce the toxicity of Cr(VI) via reducing the bioavailability of Cr and improving the bioavailability of Se in soil. Results suggested that Se could be an efficient strategy to enhance the remediation of B. cereus SES on Cr contamination.

Se(IV) reduction and extracellular biosynthesis of Nano-Se(0) by Shewanella oneidensis MR-1 and Shewanella putrefaciens

Microbial reduction of Se(IV) to Se(0) is extremely important for the detoxification of selenium oxyanion, but the reductive mechanism is still controversial. In this study, the reduction of Se(IV) by Shewanella oneidensis MR-1 and Shewanella putrefaciens was systematically investigated. The results showed that excessive Se(IV) severely inhibited the growth of microorganisms and Se(IV) reduction. The extracellular accumulation of Nano-Se(0) particles was shown to be approximately 250–650 nm in diameter. Riboflavin facilitated Se(IV) reduction, while rotenone, as an electron inhibitor, could completely inhibit Se(IV) reduction, validating the extracellular electron transport process for biosynthesized Nano-Se(0). Importantly, the amorphous Nano-Se(0) synthesized by S. oneidensis MR-1 were mixtures of helical chains and Se8 rings, while those synthesized by S. putrefaciens were mainly composed of helical chains. FTIR results demonstrated that Nano-Se(0) particles were wrapped by some proteins, organic substances and lipids, exhibiting superior dispersibility and biocompatibility. Biogenic Nano-Se(0) particles can also be considered for technological applications in various fields.

Construction of a Dual-Functional, Reversible, Ratiometric, Golgi-Targeting Fluorescent Probe for Real-time Monitoring of Dynamics of Intracellular Redox Homeostasis.

Intracellular redox homeostasis is highly important for the physiological processes of living organisms. Real-time monitoring of the dynamics of this intracellular redox process is pivotal but challenging because the biological redox reactions involved in the process are reversible and require at least one pair of oxidizing and reducing species. Thus, biosensors for investigating intracellular redox homeostasis need to be dual-functional, reversible, and, ideally, ratiometric in order for them to have real-time monitoring capacity and to provide accurate imaging information. In light of the importance of the redox pair between ClO- and GSH in living organisms, herein, we used the phenoselenazine (PSeZ) moiety as an electron donor and a reaction site to design a coumarin-based fluorescent probe, PSeZ-Cou-Golgi. After successive treatment with ClO- and GSH, the probe PSeZ-Cou-Golgi experienced an oxidation of selenium (Se) to selenoxide (Se═O) by ClO- and a subsequent reduction of Se═O to Se by GSH. The redox reactions alternatively changed the electron-donating strength of the donor in the probe PSeZ-Cou-Golgi, in turn affecting the intramolecular charge transfer process that resulted in the reversible, ratiometric change of fluorescence from red to green. After four cycles of reversible ClO-/GSH detection during in vitro experiments, the probe PSeZ-Cou-Golgi still had good performance. With the Golgi-targeting group, the probe PSeZ-Cou-Golgi was able to monitor the dynamic change of the ClO-/GSH-mediated redox state during Golgi oxidative stress, making it a versatile molecular tool. More importantly, the probe PSeZ-Cou-Golgi could facilitate the imaging of the dynamic redox state during acute lung injury progression.

Time-Resolved Examination of Fungal Selenium Redox Transformations.

Selenium (Se) is both a micronutrient required for most life and an element of environmental concern due to its toxicity at high concentrations, and both bioavailability and toxicity are largely influenced by the Se oxidation state. Environmentally relevant fungi have been shown to aerobically reduce Se(IV) and Se(VI), the generally more toxic and bioavailable Se forms. The goal of this study was to shed light on fungal Se(IV) reduction pathways and biotransformation products over time and fungal growth stages. Two Ascomycete fungi were grown with moderate (0.1 mM) and high (0.5 mM) Se(IV) concentrations in batch culture over 1 month. Fungal growth was measured throughout the experiments, and aqueous and biomass-associated Se was quantified and speciated using analytical geochemistry, transmission electron microscopy (TEM), and synchrotron-based X-ray absorption spectroscopy (XAS) approaches. The results show that Se transformation products were largely Se(0) nanoparticles, with a smaller proportion of volatile, methylated Se compounds and Se-containing amino acids. Interestingly, the relative proportions of these products were consistent throughout all fungal growth stages, and the products appeared stable over time even as growth and Se(IV) concentration declined. This time-series experiment showing different biotransformation products throughout the different growth phases suggests that multiple mechanisms are responsible for Se detoxification, but some of these mechanisms might be independent of Se presence and serve other cellular functions. Knowing and predicting fungal Se transformation products has important implications for environmental and biological health as well as for biotechnology applications such as bioremediation, nanobiosensors, and chemotherapeutic agents.

Coupling of selenate reduction and pyrrhotite oxidation by indigenous microbial consortium in natural aquifer

Pyrrhotite is ubiquitously found in natural environment and involved in diverse (bio)processes. However, the pyrrhotite-driven bioreduction of toxic selenate [Se(VI)] remains largely unknown. This study demonstrates that Se(VI) is successfully bioreduced under anaerobic condition with the participation of pyrrhotite for the first time. Completely removal of Se(VI) was achieved at initial concentration of 10 mg/L Se(VI) and 0.56 mL/min flow rate in continuous column experiment with indigenous microbial consortium and pyrrhotite. Variation in hydrochemistry and hydrodynamics affected Se(VI) removal performance. Se(VI) was reduced to insoluble Se(0) while elements in pyrrhotite were oxidized to Fe(III) and SO42−. Breakthrough study indicated that biotic activity contributed 81.4 ± 1.07% to Se(VI) transformation. Microbial community analysis suggested that chemoautotrophic genera (e.g., Thiobacillus) could realize pyrrhotite oxidation and Se(VI) reduction independently, while heterotrophic genera (e.g., Bacillus, Pseudomonas) contributed to Se(VI) detoxification by utilizing metabolic intermediates generated through Fe(II) and S(-II) oxidation, which were further verified by pure culture tests. Metagenomic and qPCR analyses indicated genes encoding enzymes for Se(VI) reduction (e.g., serA, napA and srdBAC), S oxidation (e.g., soxB) and Fe oxidation (e.g., mtrA) were upregulated. The elevated electron transporters (e.g., nicotinamide adenine dinucleotide, cytochrome c) promoted electron transfer from pyrrhotite to Se(VI). This study gains insights into Se biogeochemistry under the effect of Fe(II)-bearing minerals and provides a sustainable strategy for Se(VI) bioremediation in natural aquifer.

Understanding the positive effects of dissolved Fe2+ and limited aeration on selenate removal from water by nano zero-valent iron supported on biochar

ABSTRACT Batch test was conducted to investigate Se(VI) removal by nano zero-valent iron supported on biochar (BC@nZVI) prepared by a one-step carbothermal synthesis in the presence of ferrous ions and limited aeration. The presence of ZVI and biochar was confirmed in the synthesised material by X-ray diffraction and Raman spectroscopy. TEM analysis revealed that nZVI with a diameter of about 50 nm was dispersed well on the biochar. The Effects of BC@nZVI dosage, initial solution pH, Fe2+ concentration, limited aeration, and coexisting anions on Se(Ⅵ) removal were investigated. The role of Fe2+ on Se(VI) removal have been examined by sequential Fe2+ and Se(VI) dosing tests. More than 98% of Se(VI) (10 mg/L) was removed with BC@nZVI dosage of 1.0 g/L at pH 4.0. The removal percentage of Se(VI) decreased with an increasing in pH from 4.0 to 9.0. The strong electrostatic attraction between Se(VI) and nZVI and the high nZVI activity at low pH are responsible for the significant selenium removal. The removal efficiency of Se(VI) increased with increasing Fe2+ concentration from 0 to 0.5 mmol/L. Fe2+ promotes a shift in the passive layer on nZVI surface towards a medium that favours electron transfer. Se(VI) reduction by ZVI is thought to be accelerated under limited aeration. The positive effect of O2 on selenate reduction can be attributed either to mechanical removal of the oxide layer on the ZVI particles, or to a difference in electron transfer rates, or to a reactive Fe(II)-bearing solid formed when ZVI was oxidised by O2. Cl−, NO3 −, SO4 2− and HA can inhibit the removal of Se(VI) on BC@nZVI in different degrees. The results of XRD, Raman, and XPS characterisation showed that the removal mechanism of Se(VI) include the adsorption and reduction ed of Se(VI) on BC@nZVI. Fe2+ and limited aeration would play a critical role in Se(VI) removal by BC@nZVI.

The mechanism of Se(IV) multisystem resistance in Stenotrophomonas sp. EGS12 and its prospect in selenium-contaminated environment remediation

Human activities have led to elevated levels of selenium (Se) in the environment, which poses a threat to ecosystems and human health. Stenotrophomonas sp. EGS12 (EGS12) has been identified as a potential candidate for the bioremediation of repair selenium-contaminated environment because of its ability to efficiently reduce Se(IV) to form selenium nanospheres (SeNPs). To better understand the molecular mechanism of EGS12 in response to Se(IV) stress, a combination of transmission electron microscopy (TEM), genome sequencing techniques, metabolomics and transcriptomics were employed. The results indicated that under 2 mM Se(IV) stress, 132 differential metabolites (DEMs) were identified, and they were significantly enriched in metabolic pathways such as glutathione metabolism and amino acid metabolism. Under the Se(IV) stress of 2 mM, 662 differential genes (DEGs) involved in heavy metal transport, stress response, and toxin synthesis were identified in EGS12. These findings suggest that EGS12 may respond to Se(IV) stress by engaging various mechanisms such as forming biofilms, repairing damaged cell walls/cell membranes, reducing Se(IV) translocation into cells, increasing Se(IV) efflux, multiplying Se(IV) reduction pathways and expelling SeNPs through cell lysis and vesicular transport. The study also discusses the potential of EGS12 to repair Se contamination alone and co-repair with Se-tolerant plants (e.g. Cardamine enshiensis). Our work provides new insights into microbial tolerance to heavy metals and offers valuable information for bio-remediation techniques on Se(IV) contamination.

Long-term safety and efficacy, including anhedonia, of vortioxetine for major depressive disorder: findings from two open-label studies

Objective Evaluate the long-term safety and efficacy of vortioxetine in the management of major depressive disorder (MDD) in two open-label one-year studies, including a post-hoc analysis of its effects on symptoms related to anhedonia. Methods Both studies were 52-week, open-label, flexible-dose extension studies to evaluate the safety and efficacy of vortioxetine in adult patients with MDD following prior double-blind studies. Patients in the first study (NCT00761306) were flexibly treated with vortioxetine 5 or 10 mg/day (N = 74), and patients in the second study (NCT01323478) received vortioxetine 15 or 20 mg/day (N = 71). Results The safety and tolerability profile of vortioxetine was similar between the two studies; treatment-emergent adverse events with the highest incidence were nausea, dizziness, headache, and nasopharyngitis. Across both studies, improvements achieved during the preceding double-blind studies period were maintained, and additional improvements were observed with open-label treatment. Patients showed a mean ± SD reduction (improvement) in Montgomery and Åsberg Depression Rating Scale (MADRS) total score from open-label baseline to Week 52 of 4.3 ± 9.2 points in the 5–10 mg study, and 10.9 ± 10.0 in the 15–20 mg study. Post-hoc MMRM analyses of MADRS anhedonia factor scores also showed continued improvements over long-term treatment; patients showed a mean ± SE reduction from an open-label baseline to Week 52 of 3.10 ± 0.57 points in the 5–10 mg study, and 5.62 ± 0.60 in the 15–20 mg study. Conclusions Data from both studies confirm the safety and efficacy of flexibly dosed vortioxetine over 52 weeks of treatment and demonstrate that MADRS anhedonia factor scores continue to improve with long-term maintenance treatment.

Reductive ethylenediamine group immobilized in hybrid Pd-based nanocomposite for efficiently sequestrating selenate

Reductive sequestration of selenate [Se(VI)] has emerged as an effective and promising approach in consideration of the poor adsorptive selectivity toward Se(VI) by regular adsorption processes, where the adsorption efficiency for Se(VI) would be typically compromised by the coexisting anions, especially sulfate anion, which has similar structure and property as that of selenate. Herein, we propose a synergistic strategy of ion-exchange and catalytic reduction for this purpose; that is, a Pd-embedded hybrid nanocomposite (Pd-EDA@CMPS) was subtly assembled by in situ nucleating Pd-nanoparticle inside a porous polystyrene adsorbent possessing ethylenediamine (EDA) functional group, which is a reversible redox unit and served as electron donator for Se(VI) reduction, and the embedded Pd nanoparticle exhibited superior catalytic reduction activity and stability for sustainable use. The reduction product, i.e., Se(IV), was immediately in-situ sequestrated inside the hybrid nanocomposite. Macroscopic batch experiments verified the outstanding anti-interference of Se(VI) removal by Pd-EDA@CMPS against the ubiquitous competing anions such as sulfate, chloride, bicarbonate, and phosphate even with 100-times higher concentration. Furthermore, the notable applicability of Pd-EDA@CMPS in a wide pH range (i.e., 3.0 ∼ 11.0) was confirmed, and an interesting pH-buffering effect may be responsible for such a satisfied performance. Besides the excellent hydraulic properties and mechanical strength, the exhausted Pd-EDA@CMPS can be regenerated by successive treatment with NaOH and NaBH4 solution for reuse without obvious loss in capacity.

Aerobic reduction of selenite and tellurite to elemental selenium and tellurium nanostructures by Alteromonas sp. under saline conditions

Selenium (Se) and tellurium (Te) wastewaters are typically characterized with higher salinities. Limited studies have determined microbial reduction of Se/Te oxyanions under saline conditions. This study reports bacterial reduction of Se(IV)/Te(IV) oxyanions at different salinities by a bacterial strain isolated from selenite-reducing halophilic aerobic granular sludge. Among several isolates, ORB-2 was consistent and efficient in reducing selenite and tellurite under saline conditions. This strain showed higher tolerance and grew rapidly in the presence of 10 mM selenite (790 mg L−1 Se(IV)) and 7.5 mM tellurite (957 mg L−1 Te(IV)). Growth was accompanied by Se(IV)/Te(IV) removal and conversion to cell-associated red/black elemental forms. Selenite (0.25–2 mM; 20–158 mg L−1 Se(IV)) removal efficiencies ranged between 82 and 96% with maximum removal by 48 h. For tellurite (0.25–2 mM; 32–254 mg L−1 Te(IV)), the removal efficiencies were 75–92% in 72 h. Growth of ORB-2 and concomitant removal of selenite or tellurite was observed in the presence of 2–8% NaCl. Biogenesis of Se(0) and Te(0) nanostructures was confirmed by micro-Raman spectroscopy. Extracted Se(0) and Te(0) nanostructures had a hydrodynamic diameter of 180 and 111 nm, respectively. Based on 16S rRNA gene sequencing, ORB-2 was identified as Alteromonas sp. Efficient conversion and association of biogenic Se(0)/Te(0) nanostructures with the cells under saline conditions shows promising use in bioremediation and scarce metal(loid) recovery.

Riboflavin secreted by Shewanella sp. FDL-2 facilitates its reduction of Se(iv) and Te(iv) by promoting electron transfer.

The biological reduction of selenite (Se(iv)) or tellurite (Te(iv)) to Se0 or Te0 has received increasing attention, as related studies have favored the development of Se/Te pollution control methods. In the presence of the electron donor, the microbes acquired energy and transferred electrons to Se(iv) or Te(iv) to achieve their detoxication. However, the microbial electron transfer pathways involved in this process are still not fully understood. In this study, we reported that marine Shewanella sp. FDL-2 (FDL-2) was capable of reducing Se(iv) and Te(iv) through a novel riboflavin-involved pathway. The results showed that FDL-2 can effectively reduce 10 mM Se(iv) and 5 mM Te(iv) to Se0 and Te0, which was further confirmed by XPS and XRD analyses. RT-qPCR results indicate the upregulation of genes coding flavin-related proteins, and the production of flavin-related substances by strain FDL-2 during Se(iv)/Te(iv) bioreduction was proven by fluorescence chromatography analysis. In addition, the presence of riboflavin enhanced the electron transfer efficiency, indicating its promoting effect on the bioreduction of Se(iv)/Te(iv). Overall, our results highlight a riboflavin-involved electron transfer pathway during Se(iv)/Te(iv) bioreduction and thus deepen our understanding of the corresponding mechanism.

Selenium Oxoanions Removal from Wastewater by MoS42− Intercalated FeMgAl LDH: Catalytic Roles of Fe and Mechanism Insights

FeMgAl−MoS4 LDH was successfully synthesized by a one-pot hydrothermal process followed by ion-exchange methods, and this novel adsorbent was first conducted for aqueous selenite and selenate elimination. The Fe as a component for metal cation layers of LDHs could modulate the layer charge density, leading to more functional groups inserted into layers, and more importantly, this heterogeneous Fe can catalyze the surface reactions between Se(IV) or Se(VI) with S(-II) for oxoanions sequestration. The mechanisms are ion exchange between functional groups with HSeO3− and SeO32− for Se(IV) or SeO42− for Se(VI), followed by reduction by S(-II) from MoS42− groups. The existence of Fe in LDH cation layers, obviously enhanced the reactions (almost two times more for Se(IV) and three times more for Se(VI), respectively), resulting in satisfying adsorption capacities of 483.9 mg/g and 167.2 mg/g for Se(IV) and Se(VI), respectively. Mechanisms were further revealed by elementary analysis, XRD, FT−IR, SEM−EDX, and XPS, as well as the quantitative study. For sorption kinetics, the calculated values of capacities from the pseudo-second-order model are much closer to the experimental values. For sorption isotherms, Langmuir is better than the Freundlich isotherms model for closer capacities (505 mg/g for selenite and 172 mg/g for selenate). All these results demonstrated that the presence of heterogeneous Fe could catalyze the reduction of Se (IV/VI) for the aqueous system, and maybe other high oxidative states hazardous ions. So FeMgAl−MoS4 is a kind of novel adsorbent that offers a promising multi-functional and highly efficient solution for water selenium purification.

Genetic mechanisms for Se(VI) reduction and synthesis of trigonal 1-D nanostructures in Stenotrophomonas bentonitica: Perspectives in eco-friendly nanomaterial production and bioremediation

Selenate (Se(VI)) is one of the most soluble and toxic species of Se. Microbial Se(VI) reduction is an efficient tool for bioremediation strategies. However, this process is limited to a few microorganisms, and its molecular basis remains unknown. We present detailed Se(VI)-resistance mechanisms under 50 and 200 mM, in Stenotrophomonas bentonitica BII-R7, coupling enzymatic reduction of Se(VI) to formation of less toxic trigonal Se (t-Se). The results reveal a concentration-dependent response. Despite the lack of evidence of Se(VI)-reduction to Se(0) under 50 mM Se(VI), many genes were highly induced, indicating that Se(VI)-resistance could be based on intracellular reduction to Se(IV), mainly through molybdenum-dependent enzymes (e.g. respiratory nitrate reductase), and antioxidant activity by enzymes like glutathione peroxidase. Although exposure to 200 mM provoked a sharp drop in gene expression, a time-dependent process of reduction and formation of amorphous (a), monoclinic (m) and t-Se nanostructures was unravelled: a-Se nanospheres were initially synthesized intracellularly, which would transform into m-Se and finally into t-Se nanostructures during the following phases. This is the first work describing an intracellular Se(VI) reduction and biotransformation process to long-term stable and insoluble t-Se nanomaterials. These results expand the fundamental understanding of Se biogeochemical cycling, and the effectiveness of BII-R7 for bioremediation purposes.

Reduction of selenite by bacterial exudates

Bacterial reduction of Se(IV) is a significant component of the global selenium cycle, and hence affects the fate and transport of selenium in both natural and contaminated environments. However, it is unknown whether bacterially-produced exudates are capable of reducing selenium independent of bacterial cells. In this study, we measured the rate and extent of Se(IV) reduction by exudates from three bacterial species, and we determined the importance of exudate sulfhydryl sites by conducting parallel experiments after treatment of the exudates with a sulfhydryl-specific blocking molecule. We also conducted experiments with whole cell biomass for each of the three bacterial species to determine the importance of exudate-only reduction relative to whole biomass-promoted reduction. Under our experimental conditions, exudates from Bacillus subtilis and Pseudomonas putida remove Se(IV) from solution after an approximately 20–24 h lag period, but exudates from Shewanella oneidensis are ineffective at Se(IV) removal, except when the cells are grown in the presence of Se(IV). For both B. subtilis and P. putida, pretreatment blocking of the sulfhydryl sites on the exudate molecules dramatically decreases the rate and extent of Se(IV) removal, strongly suggesting that sulfhydryl groups on the exudate molecules play a key role in the Se(IV) reduction mechanism. The behavior of S. oneidensis exudates indicates that up-regulation of the Se(IV) reduction mechanism can occur in response to the Se content of the medium in which the cells grow. Our results demonstrate the capacity of some bacterial exudates to reduce Se(IV), and suggest that the activity of bacterial exudates should be accounted for when modeling selenium cycling in natural and engineered environments.

Comprehensive Evaluation of Affordable Cathode Materials in Direct Electrochemical Selenite Reduction

Selenium (Se) is a naturally occurring metalloid that has been widely sourced for health care, clean energy technology development, and agricultural activities. These activities have accelerated Se release into the aquatic environment, urging engineered solutions to mitigate aquatic Se pollution and their ecological impact. Our lab previously demonstrated a direct electrochemical reduction approach to convert 97% of soluble selenite into elemental Se(0) on a gold cathode. This study builds upon our prior success in selenite separation to further explore alternative cathode materials to gold. Six economically competitive materials (Ni, graphite, Cu, Fe, stainless steel, and Ti) were evaluated for key performance parameters, including Se removal efficiency, Faradaic efficiency, energy consumption, and electrode durability. Preliminary cyclic voltammetry scans revealed that Ni and graphite could sustain Se(IV)/Se(0) reduction in diluted water matrices within their electrochemical window. The subsequent 24-h chronoamperometry (CA) test demonstrated 67% selenite removal using a Ni cathode, while graphite offered a better removal efficiency (92%). Separated selenite ions were primarily plated as elemental Se(0) on the cathode surface (Ni & graphite) or within the electrode structure (graphite). While graphite has better selenite removal than Ni, it demands more energy input and has lower Faradaic efficiency (C-3.7% vs. Ni-12.7%). Switching from a CA mode to a chronopotentiometry (CP) mode did not significantly impact selenite removal performance, and the energy consumption was increased by 15%. Continued exfoliation is observed on the graphite electrode surface, potentially due to the expansion of inner gas bubbles and the lattice destruction caused by Se(0) insertion. Our results suggest graphite offers comparable selenite separation performance to a gold cathode. Still, electrode exfoliation and higher energy input due to parasitic reactions must be appropriately addressed in future research efforts.

Selenium Nanowire Formation by Reacting Selenate with Magnetite.

The mobility of 79Se, a fission product of 235U and long-lived radioisotope, is an important parameter in the safety assessment of radioactive nuclear waste disposal systems. Nonradioactive selenium is also an important contaminant of drainage waters from black shale mountains and coal mines. Highly mobile and soluble in its high oxidation states, selenate (Se(VI)O42-) and selenite (Se(IV)O32-) oxyanions can interact with magnetite, a mineral present in anoxic natural environments and in steel corrosion products, thereby being reduced and consequently immobilized by forming low-solubility solids. Here, we investigated the sorption and reduction capacity of synthetic nanomagnetite toward Se(VI) at neutral and acidic pH, under reducing, oxygen-free conditions. The additional presence of Fe(II)aq, released during magnetite dissolution at pH 5, has an effect on the reduction kinetics. X-ray absorption spectroscopy analyses revealed that, at pH 5, trigonal gray Se(0) formed and that sorbed Se(IV) complexes remained on the nanoparticle surface during longer reaction times. The Se(0) nanowires grew during the reaction, which points to a complex transport mechanism of reduced species or to active reduction sites at the tip of the Se(0) nanowires. The concomitant uptake of aqueous Fe(II) and Se(VI) ions is interpreted as a consequence of small pH oscillations that result from the Se(VI) reduction, leading to a re-adsorption of aqueous Fe(II) onto the magnetite, renewing its reducing capacity. This effect is not observed at pH 7, where we observed only the formation of Se(0) with slow kinetics due to the formation of an oxidized maghemite layer. This indicates that the presence of aqueous Fe(II) may be an important factor to be considered when examining the environmental reactivity of magnetite.

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.

Oxidation of Dissolved Tetravalent Selenium by Birnessite: Se Isotope Fractionation and the Effects of pH and Birnessite Structure

Redox reactions control the mobility and bioavailability of selenium (Se) in biogeochemical systems, both modern and ancient. Se isotope ratio measurements (e.g., 82Se/76Se) have been developed to enhance understanding of biogeochemical transformations and transport of Se. Stable isotope ratios of many elements are known to be powerful indicators of redox reactions, and shifts in 82Se/76Se have been observed for Se reduction reactions. However, Se isotope shifts caused by naturally relevant oxidation reactions have not been published. Here, we report Se isotope fractionation factors for oxidation of Se(IV) by birnessite. Experiments were conducted at pH = 4.0 and 5.5, with two types of birnessite of contrasting composition at two concentrations of suspended birnessite. The results are consistent with a single 82Se/76Se fractionation factor, for all times during all experiments, of 0.99767 (±0.0035 2 s.d.). Expressed as ε, the fractionation is 2.33‰ (±0.08‰).

Efficient removal of selenite in aqueous solution by MOF-801 and Fe3O4/MOF-801: Adsorptive behavior and mechanism study

Metal-organic framework materials (MOFs) and MOFs-based composite materials have been extensively applied to adsorb heavy metals due to their high specific surface area and high porosity. Herein, the MOF-801 and its complex Fe3O4/MOF-801 were respectively prepared and employed as adsorbents to capture the Se(IV) in an aqueous solution. And the Fe3O4/MOF-801 composite was post-synthesized from MOF-801 by a simple cryogenic hydrothermal process. The experimental results showed that the maximum adsorption capacity of MOF-801 and Fe3O4/MOF-801 was 101.2 and 206.6 mg/g under acidic conditions, respectively. The coexisting anions, except phosphate and sulfate ions, had negligible effect on the Se(IV) adsorption. The mechanism of Se(IV) removal by the two adsorbents was thoroughly analyzed by Zeta potential, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). And the results illustrates that the Se(IV) was captured on the surface hydroxyl sites of the MOF-801, while the Fe3O4/MOF-801 captured Se(IV) through electrostatic attraction, inner-sphere complexation, and the reduction effect. Furthermore, the existence of charge transfer in the Fe3O4/MOF-801, which facilitated the adsorption and reduction of Se(IV), was confirmed by the XPS analysis. This work provides a promising strategy to design high-performance magnetic MOFs composites for the removal of Se(IV) from wastewater.

Electrocatalytic Selenium Redox Reaction for High‐Mass‐Loading Zinc‐Selenium Batteries with Improved Kinetics and Selenium Utilization

AbstractBatteries usually deliver mass loading‐dependent electrochemical performance. Taking the selenium cathode as an example, the Se reaction kinetics, utilization, and cycling lifespan seriously deteriorate with increased Se mass loading. Here, an electrocatalytic Se reduction/oxidation reaction strategy to realize high‐Se‐loading Zn||Se batteries with fast kinetics and high Se utilization is proposed. Specifically, the synergetic effects of Cu and Co transition‐metal species inside the channel structure of the host can effectively immobilize and catalytically convert Sen during cycling, which thus facilitates Se utilization and 6‐electron (Se4+ ↔ Se2–) conversion kinetics. In particular, the Cu[Co(CN)6] host exhibits a remarkably low energy barrier (1.63 kJ mol–1) and low Tafel slope (95.23 mV dec–1) for the Se reduction, and the highest current response for Se oxidation. Accordingly, the Zn battery employing a Se‐in‐Cu[Co(CN)6] cathode delivers a capacity of 664.7 mAh g–1 at 0.2 A g–1, an excellent rate capability with 430.6 mAh g–1 achieved even at 10 A g–1, and long‐cyclic life over 6000 cycles with 90.6% capacity retention. Furthermore, an A‐h‐level (≈1350 mAh) Zn||Se pouch‐type battery with high Se loading (≈12.3 mg(Se) cm–2) shows a high Se utilization of 83.3% and outstanding cyclic stability with 89.4% initial capacity retained after 400 cycles at exceeding 98% Coulombic efficiency.