Removal of selenate from brine using anaerobic bacteria and zero valent iron

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

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

A membrane-based green and low-cost system for ensuring safe drinking water in a selenium-affected region

Towards an efficient, low-cost solution to the problem of contamination of groundwater by selenium leached out from earth's mineral crust, a new system is developed using a novel graphene-based nanocomposite membrane. The system not only purified selenium-contaminated groundwater with high degree but also ensured safe disposal of the rejected selenium through algorithmic chemical stabilization in a mineral matrix. All experiments were conducted with live contaminated water from selenium affected area rather than using synthetic solution in a semi-pilot unit involving a largely fouling-free flat sheet cross-flow membrane module. Pure water flux of up to 190 Lm^-2h^-1(LMH) with 96-97% selenium rejection at an optimum operating pressure of only 14 bar could be achieved. Rejected selenium was stabilized in mineral matrix through chemical coagulation-precipitation using suitable coagulants following prior optimization of the critical operating parameters by Model-based calibration toolbox (MATLAB R2020a). A high degree of stabilization efficiency (99.8%) could be achieved as reflected in an error-index of only 1.13%. For selenium-affected region, the membrane-integrated hybrid treatment system proved to be a potential candidate technology offering safe drinking water at an approximate cost of only 1.77 $/m³ which was found to be affordable to the consumers in subsequent willingness to pay survey.

Selenium (Se) recovery for technological applications from environmental matrices based on biotic and abiotic mechanisms

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.

Recent advances in technologies for removal and recovery of selenium from (waste)water: A systematic review

Selenium (Se) is distributed into different environmental compartments by natural and anthropogenic activities, and generally discharged in the form of selenate [SeO₄^2-] and selenite [SeO₃^2-], which are both toxic. Physical-chemical and biological treatment processes have been reported to exhibit good treatment efficiencies for Se from aqueous streams, only a few demonstrated to achieve effluent concentrations <5 μg/L. Moreover, there are only a few numbers of studies that describe the progress in technological developments over the last decade. Therefore, to unify the state of knowledge, identify ongoing research trends, and determine the challenges associated with available technologies, this systematic review critically analyses the published research on Se treatment. Specific topics covered in this review include (1) Se chemistry, toxicity, sources and legislation, (2) types of Se treatment technologies, (3) development in Se treatment approaches, (4) Se recovery and circular economy and (5) future prospects. The current research has been found to majorly focused on Se removal via adsorption techniques. However, the key challenges facing Se treatment technologies are related to the presence of competing ions in the solution and the persistence of selenate compared to selenite during their reduction.

Effective Se reduction by lactate-stimulated indigenous microbial communities in excavated waste rocks

Waste rocks generated from tunnel excavation contain the metalloid selenium (Se) and its concentration sometimes exceeds the environmental standards. The possibility and effectiveness of dissolved Se removal by the indigenous microorganisms are unknown. Chemical analyses and high-throughput 16S rRNA gene sequencing were implemented to investigate the functional and structural responses of the rock microbial communities to the Se and lactate amendment. During anaerobic incubation of the amended rock slurries from two distinct sites, dissolved Se concentrations decreased significantly, which coincided with lactate degradation to acetate and/or propionate. Sequencing indicated that relative abundances of Desulfosporosinus burensis increased drastically from 0.025 % and 0.022% to 67.584% and 63.716 %, respectively, in the sites. In addition, various Desulfosporosinus spp., Symbiobacterium-related species and Brevibacillus ginsengisoli, as well as the Se(VI)-reducing Desulfitobacterium hafniense, proliferated remarkably. They are capable of incomplete lactate oxidation to acetate as only organic metabolite, strongly suggesting their involvement in dissimilatory Se reduction. Furthermore, predominance of Pelosinus fermentans that ferments lactate to propionate and acetate implied that Se served as the electron sink for its fermentative lactate degradation. These results demonstrated that the indigenous microorganisms played vital roles in the lactate-stimulated Se reduction, leading to the biological Se immobilization treatment of waste rocks.

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

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

Microbial consortia capable of reducing selenate in the presence of nitrate enriched from coalmining-impacted environments

Biological treatment to remove dissolved selenium from mine-impacted water is often inhibited by the co-contaminant nitrate. In this work, we enriched microbial consortia capable of removing dissolved selenium in the presence of nitrate from native bacteria at sites influenced by coalmine seepages with elevated concentrations of Se, nitrate, and sulfate. Enrichments were collected from sediments in different vegetated or non-vegetated seepage collection ponds, and all demonstrated the potential for dissolved selenium removal. Nitrate inhibited dissolved selenium removal rates in four of these enrichments. However, microorganisms enriched from a mine seepage influenced natural vegetated marsh removed dissolved Se and nitrate simultaneously. Additionally, enrichments from one seepage collection pond achieved enhanced dissolved selenium removal in the presence of nitrate. Based on functional metagenomics, the dominant species with the metabolic capacity for selenate reduction were classified in Orders Enterobacterales and Clostridiales. Most putative selenate reductases identified as either ygfK, associated with selenoprotein synthesis or production of methylated organoselenium compounds, and narG, nitrate reductases with an affinity also for selenate.Key points• Enriched mine influenced sediment bacteria have the capacity for removal of dissolved Se species.• Consortia from a vegetated natural marsh reduced Se without inhibition from nitrate.• Nitrate stimulated the removal of Se by consortia from a disused tailing pond.

Application of Fe-biochar composites for selenium (Se+6) removal from aqueous solution and effect of the presence of competing anions under environmentally relevant conditions

This study's aim was to compare biochar and steam activated biochar functionalized with iron for removal of selenium as selenate from solutions also containing nitrate and sulfate. The Fe-biochar composites were made impregnating iron (ferric nitrate) onto regular biochar (RB) and steam activated biochar (SAB), forming the Fe-biochar composites FeRB and FeSAB. Iron oxyhydroxide deposits were observed on the surface of FeRB using Raman spectroscopy analysis, but not on the FeSAB surface. Unmodified biochar samples did not remove selenium, as Se(VI), from solution, whereas FeRB and FeSAB recovered 8.3 mg-Se⋅g-composite^-1 and 5.9 mg-Se⋅g-composite^-1, respectively. Higher Se uptake was achieved at higher Fe-loads and lower acetone:biochar ratio, to a maximum of 17.3 mg-Se⋅g-composite^-1 (FeRB). Washing after Fe-impregnation using deionized water diminished nitrate and Fe-leaching from the Fe-biochar composites while removing selenium. Se(VI) and sulfate uptake were observed when washed composites were tested in the presence of possible competing ions at environmentally relevant concentrations ([Se]_t=0 = 1.01 ± 0.03 mg⋅L^-1; [N-NO₃^-]_t=0 = 40.2 ± 0.2 mg⋅L^-1; and [SO₄^2-]_t=0 = 496 ± 25 mg⋅L^-1). One of the possible mechanisms of removal might be the complexation of Se with the iron oxyhydroxide deposits (goethite and hematite) found on the FeRB surface.

Strategies to enhance the stability of nanoscale zero-valent iron (NZVI) in continuous BrO3- reduction

The reduction of bromate to bromide was successfully achieved by bimetallic catalysts with NZVI support in continuous-flow reactors. The stability of NZVI-supported bimetallic catalysts was enhanced by decelerating the iron corrosion and sequential rapid passivation of the iron-Cu-Pd ensembles under optimized reaction conditions. Thus >99% bromate removal can be continuously achieved for 11 h. The lifetime of the bimetallic catalyst was further enhanced and tested under different hydraulic retention time, catalyst loading, and initial bromate concentrations. At the optimized operation conditions, the catalyst showed a complete bromate reduction by 24 h and then the reactivity slowly decreased to 20% over the next 100 h. X-ray diffraction and X-ray photoelectron spectroscopy showed that the reactive NZVI support was oxidized to Fe(II) and Fe(III) along with Cu(0) oxidation to CuO, while the oxidation state of Pd did not change. Therefore, bromate reduction occurred on the surface of reactive NZVI support and Cu(0) particle, while Pd played a role as a hydrogenation catalyst that prolonged the lifetime of the bimetallic catalyst.