Treatment technologies for selenium contaminated water: A critical review

Metal-sulfide/polysulfide functionalized layered double hydroxides - recent progress in the removal of heavy metal ions and oxoanionic species from aqueous solutions

Water constitutes an indispensable resource for global life but remains susceptible to pollution from diverse human activities. To mitigate this issue, researchers are committed to purifying water using a variety of materials to remove harmful chemicals, such as heavy metals. Layered double hydroxides (LDHs), with their intriguing, layered structure and chemical behavior, have attained substantial attention for their effectiveness in removing heavy metal cations and various inorganic oxoanions from water. To enhance the efficiency, considerable endeavors have focused on functionalizing LDHs with different chemical species. Intercalation with metal sulfides has proven to be particularly effective, facilitating heavy metal absorption through multiple mechanisms, including ion-exchange, reductive precipitation, and surface sorption. This review concentrates on the synthesis and performance of polysulfide (Sx, x = 2-5), Mo-S, and Sn-S anion intercalated LDHs for heavy metal cations and inorganic oxoanion sorption, along with their mechanisms. Furthermore, the discussion includes prospects for expanding the chemistry of metal sulfide intercalated LDHs, with existing challenges and future outlooks.

Molybdenum-Oxysulfide-Functionalized MgAl-Layered Double Hydroxides─A Sorbent for Selenium Oxoanions

Effective removal of chemically toxic selenium oxoanions at high-capacity and trace levels from contaminated water remains a challenge in current scientific pursuits. Here, we report the functionalization of the MgAl layered double hydroxide with molybdenum-oxysulfide (MoO2S2) anion, referred to as LDH-MoO2S2, and its potential to sequester SeVIO42- and SeIVO32- from aqueous solution. LDH-MoO2S2 nanosheets were synthesized by an ion exchange method in solution. Synchrotron X-ray pair distribution function (PDF) and extended X-ray absorption fine structure (EXAFS) revealed an unexpected transformation of the MoO2S22- to Mo2O2S62- like species during the intercalation process. LDH-MoO2S2 is remarkably efficient in removing SeO42- and SeO32- ions from the ppm to trace level (≤10 ppb), with distribution constant (Kd) ranging from 104 to 105 mL/g. This material showed exceptionally high sorption capacities of 237 and 358 mg/g for SeO42- and SeO32-, respectively. Furthermore, LDH-MoO2S2 demonstrates substantial affinity and efficiency to remove SeO32-/SeO42- even in the presence of competitive ions from contaminated water. Hence, the removal of selenium (VI/IV) oxoanions collectively occurs through reductive precipitation and ion exchange mechanisms. This work provides significant insights into the chemical structure of the MoO2S2 anion into LDH and emphasizes its exceptional potential for high-capacity selenium removal and positioning it as a premier sorbent for selenium oxoanions.

Continuous selenite biotransformation and biofuel production by marine diatom in the presence of fulvic acid

In this study, the biochemical response of Phaeodactylum tricornutum to varying concentrations of inorganic selenium (Se) was investigated. It was observed that, when combined with fulvic acid, P. tricornutum exhibited enhanced uptake and biotransformation of inorganic Se, as well as increased microalgal lipid biosynthesis. Notably, when subjected to moderate (5 and 10 mg/L) and high (20 and 40 mg/L) concentrations of selenite under fulvic acid treatment, there was a discernible redirection of carbon flux towards lipogenesis and protein biosynthesis from carbohydrates. In addition, the key parameters of microalgae-based biofuels aligned with the necessary criteria outlined in biofuel regulations. Furthermore, the Se removal capabilities of P. tricornutum, assisted by fulvic acid, were coupled with the accumulation of substantial amounts of organic Se, specifically SeCys. These findings present a viable and successful approach to establish a microalgae-based system for Se uptake and biotransformation.

Selenium volatilization in plants, microalgae, and microorganisms

The augmented prevalence of Se (Se) pollution can be attributed to various human activities, such as mining, coal combustion, oil extraction and refining, and agricultural irrigation. Although Se is vital for animals, humans, and microorganisms, excessive concentrations of this element can give rise to potential hazards. Consequently, numerous approaches have been devised to mitigate Se pollution, encompassing physicochemical techniques and bioremediation. The recognition of Se volatilization as a potential strategy for mitigating Se pollution in contaminated environments is underscored in this review. This study delves into the volatilization mechanisms in various organisms, including plants, microalgae, and microorganisms. By assessing the efficacy of Se removal and identifying the rate-limiting steps associated with volatilization, this paper provides insightful recommendations for Se mitigation. Constructed wetlands are a cost-effective and environmentally friendly alternative in the treatment of Se volatilization. The fate, behavior, bioavailability, and toxicity of Se within complex environmental systems are comprehensively reviewed. This knowledge forms the basis for developing management plans that aimed at mitigating Se contamination in wetlands and protecting the associated ecosystems.

Defect Engineering in GO Membranes - Tailoring Size and Oxidation Degree of Nanosheet for Enhanced Pore Channels

Graphene Oxide (GO) membrane has been extensively applied in the field of water purification and membrane separation processes. While the solute molecule transport in GO membranes encompasses interlayer channels, edge defects, and in-plane crack-like holes, the significance of edge defects or crack-like pores in ultrathin membranes is often overlooked. In our study, we focused on the construction of short-range channel GO membranes with varied defect structures by modulating the transverse size of the porous nanosheets. GO nanosheets with different sizes were procured through high-energy γ-irradiation combined with centrifugation. Notably, the large-sized porous GO nanosheets (L-pGO) exhibit a consistent structure, and numerous in-plane defects. In contrast, the smaller counterparts (S-pGO) present a fewer in-plane defects. The performance metrics revealed that L-pGO exhibited a water flux of 849.25 L m-2  h-1  bar-1 , while S-pGO demonstrated nearly 100 % dye rejection capacity. These findings underscore the potential of defect engineering as a powerful strategy to enhance the efficiency of two-dimensional membranes.

Removal and recycling of aqueous selenite anions using cobalt-based metal-organic-framework coated on multi-walled carbon nanotubes composite membrane

The utilization of selenium as a novel functional material is rapidly expanding, and the retrieval of selenium from waste containing selenium is gaining recognition in the industry. This study prepared a novel composite membrane coated with the cobalt-based metal-organic framework coated on multi-walled carbon nanotubes (Co-MOF@MWCNTs). The MWCNTs served as the skeleton to support the active components of Co-MOF, which enabled efficient removal and resource utilization of liquid selenite (SeO32-). The morphology, structure, and composition of the prepared membrane were characterized using field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), etc.. Applying a permeate flux of 67.08 L m-2 h-1, the SeO32- removal efficiency of the composite membrane reached up to 92.2%. The composite membrane containing CoSeO4 can be used as an electrocatalytic oxygen evolution catalyst. Density functional theory calculations and electrochemical analysis showed that the conversion from O* to OOH* was a rate-determining step. Under 1.0 M KOH conditions, the lowest overpotential for Co-MOF@MWCNTs-40 at 10 mA cm-2 was 360 mV. In this study, the process of selenium resource utilization and the mechanism of SeO32- removal by Co-MOF@MWCNTs are revealed. It demonstrates that membrane-based sequestration of SeO32- can provide a viable approach for SeO32- removal and utilization in wastewater.

Unraveling the molecular mechanisms of selenite reduction: transcriptomic analysis of Bacillus reveals the key role of sulfur assimilation

Selenite biotransformation by microorganisms is an effective detoxification and assimilation process. However, current knowledge of the molecular mechanisms of selenite reduction remains circumscribed. Here, the reduction of Se(IV) by a highly selenite-resistant Bacillus sp. SL (up to 50 mM) was systematically analyzed, and the molecular mechanisms of selenite reduction were investigated. Remarkably, 10 mM selenite was entirely transformed by the strain SL within 20 h, demonstrating a faster conversion rate compared to other microorganisms. Furthermore, glutathione (GSH) and exopolysaccharides (EPS) changes were also monitored during the process. Transcriptomic analysis revealed that the genes of ferredoxin-sulfite oxidoreductase (6.82) and sulfate adenylyltransferase (6.32) were significantly upregulated, indicating that the sulfur assimilation pathway is the primary reducing pathway involved in selenite reduction by strain SL. Moreover, key genes associated with NAD(P)/FAD-dependent oxidoreductases and thioredoxin were significantly upregulated. The reduction of Se(IV) was mediated by multiple pathways in strain SL. To our knowledge, this is the initial report to identify the involvement of sulfur assimilation pathway in selenite reduction for bacillus, which is rare in aerobic bacteria.

Performance of a pilot-scale BDD reactor by numerical analysis of reaction rate parameters and additional numbers for mass transfers

The performance of a pilot-scale boron-doped diamond (BDD) reactor through a numerical analysis of reaction rate parameters and enhanced mass transfer has been investigated. The main objective of this research is to evaluate the efficiency of the reactor in mineralizing and degrading caffeine as an emerging contaminant. Based on the kinetic mechanisms and mass transport correlations reported in the literature, two reaction rate kinetic models for caffeine degradation are proposed and analyzed. The models consider different electrolytes (NaCl and Na2SO4) and applied current densities. The kinetic fitting process utilizes the gradient-maximal electrochemical approach, together with orthogonal placement methods, fourth-order Runge-Kutta (RK4) methods, and Nelder & Mead methods for optimization of kinetic parameters and spatial discretization of the material balance. Experimental data obtained from a factorial design with four factors and two levels (24) validate the proposed kinetic models. Caffeine degradation is achieved with NaCl and Na2SO4 electrolytes at concentrations of 60 ppm and 100 ppm, respectively. The corresponding applied loads are 1.5 AhL-1 and 3 AhL-1. Na2SO4 exhibits superior performance with a total organic carbon (TOC) removal efficiency of 99.13%, while NaCl achieves 31.47% mineralization. The behavior of caffeine degradation under the operational and scale conditions demonstrates that NaCl, as a support electrolyte, enables controlled charge transfer (current density) during the degradation process. In contrast, Na2SO4 as a support electrolyte introduces a mixed control of charge and mass transfer. The pilot-scale kinetic parameters obtained in this study provide valuable insights into the support electrolyte dynamics and current density dynamics in BDD-based Electrooxidation (EO) systems, particularly in complex matrix applications. Furthermore, the observed electrical consumption supports the potential application of EO as a viable technology for industrial-scale tertiary wastewater treatment, specifically for caffeine removal.

Effect of Dietary Selenium on the Growth and Immune Systems of Fish

Dietary selenium (Se) is an essential component that supports fish growth and the immune system. This review attempts to provide insight into the biological impacts of dietary Se, including immunological responses, infection defense, and fish species growth, and it also identifies the routes via which it enters the aquatic environment. Dietary Se is important in fish feed due to its additive, antioxidant, and enzyme properties, which aid in various biological processes. However, excessive intake of it may harm aquatic ecosystems and potentially disrupt the food chain. This review explores the diverse natures of dietary Se, their impact on fish species, and the biological methods for eliminating excesses in aquatic environments. Soil has a potential role in the distribution of Se through erosion from agricultural, industrial, and mine sites. The research on dietary Se's effects on fish immune system and growth can provide knowledge regarding fish health, fish farming strategies, and the health of aquatic ecosystems, promoting the feed industry and sustainable aquaculture. This review provides data and references from various research studies on managing Se levels in aquatic ecosystems, promoting fish conservation, and utilizing Se in farmed fish diets.

Evaluation of selenite reduction under salinity and sulfate stress in anaerobic membrane bioreactor

Current microbial reduction technologies have been proven to be suitable for decontaminating industrial wastewaters containing high concentrations of selenium (Se) oxyanions, however, their application is strictly limited by the elemental Se (Se0) accumulation in the system effluents. In this work, a continuous-flow anaerobic membrane bioreactor (AnMBR) was employed for the first time to treat synthetic wastewater containing 0.2 mM soluble selenite (SeO3 2-). The SeO3 2- removal efficiency by the AnMBR was approachable to 100% in most of the time, regardless of the fluctuation in influent salinity and sulfate (SO4 2-) stress. Se0 particles were always undetectable in the system effluents, owing to their interception by the surface micropores and adhering cake layer of membranes. High salt stress led to the aggravated membrane fouling and diminished content ratio of protein to polysaccharide in the cake layer-contained microbial products. The results of physicochemical characterization suggested that the sludge-attached Se0 particles presented either sphere- or rod-like morphology, hexagonal crystalline structure and were entrapped by the organic capping layer. According to the microbial community analysis, increasing influent salinity led to the diminished population of non-halotolerant Se-reducer (Acinetobacter) and increased abundance of halotolerant sulfate reducing bacteria (Desulfomicrobium). In the absence of Acinetobacter, the efficient SeO3 2- abatement performance of the system could still be maintained, as a result of the abiotic reaction between SeO3 2- and S2- generated by Desulfomicrobium, which then gave rise to the production of Se0 and S0.

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.

Enhanced photocatalytic organic pollutant degradation and H2 evolution reaction over carbon nitride nanosheets: N defects abundant materials

Post thermal treatment of bulk graphitic carbon nitride (g-C₃N₄) by ammonia gas acts as a significant structure regulation approach, while pure ammonia-assisted g-C₃N₄ synthesis from precursors like melamine is rarely investigated. Here we prove the synthesis of N-defects abundant carbon nitride nanosheets (ACN) through a one-pot thermal polymerization of melamine in pure ammonia gas, for photocatalytic organic pollutant removal in water and H₂ evolution applications. Compared to bulk g-C₃N₄ (BCN), ACN-550 (ACN prepared at 550 °C) exhibited thin-layered porous morphology with higher surface area and abundant N defects, resulting in wider distribution of active sites. Moreover, the abundant N defects in the heptazine heterocycle structure could change the electronic structure of g-C₃N₄, leading to more efficient transport of photogenerated charge carriers and enhanced photoreduction potential, which gives rise to notable improvement activities in photocatalytic reaction. With superoxide ion radical and photoinduced holes as the predominant reactive species, ACN-550 realized efficient photocatalytic bisphenol A (BPA) degradation, which is 1.6- and 4.7-fold high over commercial TiO₂ (P25) and BCN, respectively. ACN-550 exhibited excellent reusability and stability in five consecutive photocatalytic BPA degradation tests. In photo-reductive H₂ production system by ACN-550, 761.8 ± 4.3 μmol/h/g H₂ was produced, which was 11.6-fold as high as that by BCN.

Selenite elimination via zero-valent iron modified biochar synthesized from tobacco straw and copper slag: Mechanisms and agro-industrial practicality

Cost-effectively improving the performance of biochar is essential for its large-scale practical application. In this work, the agro-industrial by-products copper slag and tobacco straw were employed for the preparation of modified biochar (CSBC). The obtained CSBC exhibited satisfactory capacity on Se(IV) immobilization of 190.53 mg/g, with surface interactions determined by the monolayer and mainly chemisorption. The removal mechanisms included chemical reduction, electrostatic attraction, co-precipitation, and formation of complexations. Interestingly, the existence of Cu2Se structure after adsorption indicated the involvement of Cu species within Se(IV) elimination. Moreover, the industrial agricultural practicality of CSBC was evaluated by regeneration tests, economic assessment, and pot experiments. The results demonstrate that iron species-modified biochar prepared from two agro-industrial by-products is a promising and feasible candidate for selenite removal from wastewater.

Insight into the fraction variations of selenium and their effects on humification during composting

This study investigated the variation of selenium fractions and their effects on humification during composting. Selenite and selenate were added to a mixture of goat manure and wheat straw for composting. The results demonstrated that the bioavailable Se in the selenite added treatment (9.3-13.8%) was lower than in the selenate added treatment (18.1-47.3%). Meanwhile, the HA/FA of selenite and selenate added treatments were higher than in control, indicating that the selenium addition (especially selenite) promoted the humification of composting. Importantly, selenite enriched the abundance of Tepidimicrobium and Virgibacillus which were responsible to improve humification performance. Selenate increased the abundance of Thermobifida and Cellvibrio which facilitated the composting humification. The genes encoding CAZymes involved in the degradation of organic materials were also analyzed, and selenium could contribute to the synthesis of humus. KEGG pathway analysis revealed that the selenite addition promoted amino acids and carbohydrate metabolism compared to the control.

A comparative study of electrocoagulation treatment with iron, aluminum and zinc electrodes for selenium removal from flour production wastewater

Electrocoagulation (EC) using iron (Fe), zinc (Zn) and aluminum (Al) electrodes was comparatively applied in the treatment of selenium (Se) in flour production (FP) wastewater. It was indicated that EC treatment with Fe anode obtained highest removal efficiency (79.1%) for Se in the 90 min treatment in the comparative study, which could be attributed to the superior adsorption capacity of in-situ generated iron flocs. Removal of Se resulted from electrodeposition and adsorption to in-situ generated flocs in EC treatment, and the operational conditions significantly influenced the Se removal performance in this work. The results showed the acidic condition and higher current density favored EC treatment on Se removal, EC removed up to 97.8% of Se at pH 4 under 15 mA cm^-2, whereas it obtained 83.5% and 50.4% of removal efficiency at pH 7 and 10, respectively. There was competitive adsorption in the process of selenium removal, as the in-situ generated flocs effectively removed 35.6% of humic acid-like (HA-like) substance in FP wastewater after 90 min treatment. The FTIR results showed that HA-like substance mainly contained the protein water hydrogen bond, carboxylate COO antisymmetric stretching and other functional groups. Through the analysis of existence of Se in flocs and wastewater, it can be found that approximately 2.8%-3.92% of Se was removed by electrodeposition process. This study illustrated the Se removal mechanism and provided constructive suggestion for food manufacturing to the metal removal and utilization of advanced treatment.

Hydrogeochemical Evaluation of Groundwater Quality Parameters for Ogallala Aquifer in the Southern High Plains Region, USA

The purposes of this study are to analyze the groundwater quality of the Ogallala Aquifer and evaluate the hydrological characteristics in the southern High Plains region of the Permian Basin, Texas. Levels of chloride, fluoride, nitrate, selenium, pH, and total dissolved solids (TDS) were analyzed for the period 1990–2016. Data concerning a total of 133 wells were collected from the Texas Water Development Board (TWDB), which is an open database provided by the US government. The average levels of contaminants were compared to their respective Maximum Contaminant Levels (MCL) stipulated by the Environmental Protection Agency (EPA). The study area experienced high concentrations of most parameters including chloride, fluoride, nitrate, selenium, and TDS, within the contaminants’ respective MCLs. Borden and Dawson counties experienced the highest overall amounts of groundwater pollutants. Possible sources of each contaminant are discussed, with oil and gas activities, agricultural practices, and other human actions impacting the conditions. This research provides important information about groundwater quality of the Ogallala Aquifer and contributes to understanding the response to development in the Permian Basin, Texas.

Reductive removal of selenate (VI) in aqueous solution using rhodium metal particles supported on TiO2

Selenium and its compounds in high concentration are toxic for humans, especially selenate (VI) is the most toxic due to its high solubility in water. To promote the reductive reaction of Se(vi) to Se(iv) or Se(0), which is relatively easy to remove in water, noble metal particles were added as reaction sites with a reductant. The highest removal performance of selenate in aqueous solution was achieved using rhodium particles supported on TiO₂ (Rh/TiO₂). Selenate was rapidly reduced with hydrazine on the metal particle, leading to a selenium deposition on the particle inhibiting the stable reductive reaction. On the other hand, when a weaker reductant such as formaldehyde was used for the selenate reduction, the selenium deposition was suppressed due to its low reactivity, resulting in a stable reductive reaction of selenate in water.

Light-Activated Hydroxyapatite Photocatalysts: New Environmentally-Friendly Materials to Mitigate Pollutants

This review focuses on a reasoned search for articles to treat contaminated water using hydroxyapatite (HAp)-based compounds. In addition, the fundamentals of heterogeneous photocatalysis were considered, combined with parameters that affect the pollutants’ degradation using hydroxyapatite-based photocatalyst design and strategies of this photocatalyst, and the challenges of and perspectives on the development of these materials. Many critical applications have been analyzed to degrade dyes, drugs, and pesticides using HAp-based photocatalysts. This systematic review highlights the recent state-of-the-art advances that enable new paths and good-quality preparations of HAp-derived photocatalysts for photocatalysis.