Se Removal Efficiency Research Articles

Regulating the interlayer SO42--induced rebound of SeO42- in green rust coupled with iron nanoparticles for groundwater remediation

Green rust (GR) is an interlayer anion-containing Fe(II)/Fe(III) mineral material that is versatile in removing a series of ionic contaminants in water. Taking SeO42- (Se(VI)) as the target contaminant, this study identified that the removal processes of Se(VI) by GR could be divided into three stages: initial rapid interlayer exchange, followed by a rebound, and finally slow removal. In addition, as the percentage of SO42- in GR interlayer increased, the Se(VI) removal by GR gradually decreased. To mediate the SO42--induced rebound of Se(VI), the coupling of GR with iron nanoparticles (nFe0@GR) was proposed in this study and it was found that the removal efficiency of Se(VI) by nFe0@GR was 3.53 folds greater than that of GR. This study further revealed that the enhanced reactivity of nFe0@GR with Se(VI) could be attributed to the re-equilibration of SO42- driven by the formed GR in situ. Since it had a weaker electrostatic repulsion with interlayer SeO42- than pristine GR, the Se(VI) could be quickly removed by nFe0@GR without the rebound. Moreover, the nFe0@GR was demonstrated to be effective in immobilizing Se(VI) from simulated groundwater and has a great potential to reduce the risk of Se(VI) re-release into the environment.

Effects of three exogenous phytohormones on the growth, selenium absorption, and metabolism of the microalga Parachlorella kessleri under sodium selenite stress

Treatment of selenium (Se)-laden wastewater using microalgae has gained attention as it is environmentally friendly and cost effective; however, most microalgae are intolerant to Se. This study aimed to investigate the feasibility of alleviating Se stress in Parachlorella kessleri exposed to 8 mg/L Na2SeO3 using indoleacetic acid (IAA), gibberellin (GA3), and melatonin (MT) and to explore the effects of their concentrations (5–80 μM) on the growth adaptability and Se biotransformation efficiency of P. kessleri. The results showed that the alleviation of Se4+ stress depended on the type and concentration of phytohormone used; MT conferred the best protection against Se4+ stress, followed by GA3 and IAA. The 20-μM MT treatment produced the highest algal biomass and chlorophyll a content, i.e., the algal biomass and chlorophyll a content increased by 102.68 % and 232.68 %, respectively, compared with that in the negative control (Se4+ stress). This increase was attributed to the enhancement of MT-induced antioxidant capacity of P. kessleri, which increased the endogenous ascorbic acid and glutathione content and decreased the superoxide anion content, thereby reducing the damage caused by membrane peroxidation. Most importantly, MT effectively promoted the absorption of Se4+ by P. kessleri while enhancing Se volatilization metabolism, alleviating Se toxicity by reducing the excessive accumulation of Se in cells, and increasing the removal efficiency of Se in the medium up to 84.63 %. Although MT reduced Se accumulation in biomass, the organic Se proportion remained >97 % and predominantly in the form of selenomethionine. These results demonstrated that MT has immense potential to alleviate Se4+-induced stress in P. kessleri and can be used as an effective regulatory strategy to promote microalgae-based resourceful treatment of Se-laden wastewater and gain insights into the mechanisms underlying the alleviation of Se toxicity.

Selenite (IV) and selenate (VI) uptake and accumulation capacity of Lemna minor L. from an aquatic medium

ABSTRACT The uptake of sodium selenite (Se(IV)) and sodium selenate (Se(VI)) from aqueous medium by Lemna minor L. and the influence of different Se concentrations on its growth, morphological and ultrastructural characteristics were studied. L. minor was grown at different concentrations (1, 3, 5 and 10 mg L−1) of Se(IV) and Se(IV). The Se(IV) concentration in the plant tissue ranged between 77.7 (± 4.3) to 453 (± 0) mg kg−1 DW. The Se(VI) concentration in plant tissues ranged between 117 (± 11) to 417 (± 2) mg kg−1 DW. The highest bioconcentration factor for Se(VI) was 127 (± 7) at 3 mg/L, with a Se removal efficiency of 44%. For Se(IV), the highest bioconcentration factor was 77.7 (± 4.3) at 1 mg L−1, which had a Se removal efficiency of 23%. Growth of L. minor was suppressed at 10 mg L−1 Se in both forms. The addition of Se promoted the formation of starch granules in L. minor which occupied a chloroplast area of 74% for Se(IV) and 77% for Se(VI). The efficient uptake of both Se forms by L. minor indicates the potential application of this species for phytoremediation of Se laden wastewaters and its use as an alternative feedstock in biofuel production.

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.

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.

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.

Biological selenate and selenite reduction by waste activated sludge using hydrogen as electron donor

Biological reduction of selenium oxyanions is widely used for selenium removal from wastewater. The process is, however, limited by the availability of a suitable, efficient and low cost electron donor. In this study, selenite and selenate reduction by waste activated sludge using hydrogen as the electron donor was investigated. Both selenite and selenate (80 mg/L) were completely removed using H2 within 8 days of incubation. In the presence of sulfate in the medium, the Se removal efficiency decreased to 77.8–95.4% (for selenite) and 88.2–99.4% (for selenate) at different temperatures and initial sulfate concentrations. Thermophilic conditions (50 °C) were better suited for both selenite and selenate reduction using H2 as electron donor with a 0.8–13.5% increase in overall Se removal. Similarly, sulfate reduction also increased from 69.1– 88% at 30 °C to 72–94.6% at 50 °C. Most of the H2 utilized was diverted towards Se and sulfate reduction with minimal production of byproducts such as methane (<0.32 mM) or volatile fatty acids (<0.92 mg/L). The elemental Se produced from selenite and selenate reduction ranged between 33.9 and 52.1 mg/L. The elemental selenium nanoparticles produced as a result of selenite and selenate reduction were characterized using transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and dynamic light scattering (DLS) spectroscopy. Furthermore, characterization of the biomass using Fourier-transform infrared spectroscopy (FTIR) and excitation emission matrix (EEM) spectra of the extracellular polymeric substances (EPS) produced by the waste activated sludge were performed to elucidate the mechanism of selenium oxyanion reduction to elemental selenium nanoparticles.

Enhanced simultaneous removal of toxic (SeO4)2− and metals Cr3+ and Cu2+ using polysulfide intercalated Layered double hydroxide

• Simultaneous removal of Se(VI) and metal cations. • More toxic Se(VI) reduced to nontoxic Se(IV) and Se(0) • Leached amount of Mo inhibited in the presence of metal cations. It is usually very difficult to remove (SeO 4 ) 2− (Se(VI) from aqueous solution due to its high stability and mobility. Here, it was found enhanced removal efficiency of Se(VI) in the presence of metals cations such as Cr(III) and Cu(II) using trimetallic LDH (FeMgAl-MoS 4 LDH) intercalated with MoS 4 2− which abbr. as S-LDH. The removal efficiency of Se(VI) was relatively poor (67.9%) when treated alone. However, in the presence of Cr(III) and Cu(II) cations the removal efficiency of Se(VI) was enhanced to 99.32% and 96.02%, respectively under similar experimental conditions. The metal cations were also simultaneously removed (>82%) and (>99.99%) for Cr(III) and Cu(II) cations, respectively. Moreover, secondary water pollution was also inhibited in the presence of the metal cations with Se(VI) since the amount of Mo leached from S-LDH into the solution was decreased from 19.82 to 1.13 ppm when 140 ppm Se(VI) initial sample was treated alone and in the presence of 20 ppm Cr(III), respectively. Similarly, the amount of Mo leached was decreased from 21.34 to 2.37 ppm when 140 ppm Se(VI) was treated alone and with 20 ppm Cu(II), respectively. This could be due to co-precipitation reaction of the leached MoS 4 2− with the cations in the solution. The pre- and post-adsorption characterization of S-LDH was conducted using XRD, FTIR, SEM-EDX, CHNS and XPS to determine the chemical structure and composition of the adsorbent as well as the reaction mechanisms. The reaction kinetics followed pseudo-second order which related with chemisorption and Langmuir reaction model was better fitted the isotherm data that indicated monolayer adsorption dominate in the reaction process. The evaluation of thermodynamic parameters with negative values of free energy, enthalpy and entropy indicated a spontaneous, exothermic and the solid/solution interface became less random and enthalpy driven processes. Based on the XPS result, the more toxic Se(VI) was mainly reduced to less toxic Se(IV) and Se(0) species due to the (-S-) 2− in MoS 4 2− and Fe(II) in the LDH while themselves oxidized to SO 4 2− and Fe(III), respectively. We believe that this strategy provides a new direction to find enhanced removal of the reluctant Se(VI) and retarding secondary water pollution when MoS 4 based LDHs used as sorbent materials.

Simultaneous removal of lead and selenium through biomineralization as lead selenide by anaerobic granular sludge

This study demonstrated the simultaneous removal of lead (Pb) and selenium (Se) as lead selenide biomineralization using anaerobic granular sludge. The microbial community of the granular sludge was first enriched for 140 days in the presence of Pb(II) only, selenate and selenite only, Pb(II)+selenate, and Pb(II)+selenite. In the absence of Se, removal of Pb(II) mainly occurred via biosorption and deposited on the biomass as lead oxide and lead carbonate. The Pb removal efficiency (94% of initial 50 mg L-1) was reduced to 90% and 86% in the presence of selenate and selenite, respectively, due to biosorption. Addition of Pb(II) didn’t exert any toxic effect on the Se-reducing microbial community, on the contrary: Pb(II) addition improved the Se removal efficiency for selenate from 85% to 90%, but did not affect selenite removal after 14 d of incubation. The bioreduction of the Se-oxyanions produced elemental Se (Se(0)) and selenide, which later interacted with Pb(II) to produce lead selenide (PbSe). Adsorption of Pb(II) onto the Se(0) nanoparticles and precipitation as the Se(0)-Pb complex might also have contributed to the simultaneous removal of Pb and Se. XPS and XRD analysis further confirmed the immobilization of Pb as PbSe, PbO and PbCO3 in the biomass.

Se transformation and removal by a cattail litter treatment system inoculated with sulfur-based denitrification sludge: Role of the microbial community composition under various temperature and aeration conditions

With a narrow margin between deficiency and toxicity, rising levels of selenium (Se) are threatening aquatic ecosystems. To investigate the role of microorganisms in Se bioremediation, a cattail litter system inoculated with the sulfur-based denitrification sludge was conducted. The results show the litter, as a carrier and nutrient source for bacteria, efficiently removed Se by ~ 97.0% during a 12-d treatment with water circulating. As the major removal pathways, immobilization rates of selenite were ~ 2.9-fold higher than selenate, and the volatilization, contributing to ~ 87.7% of the total Se removal, was significantly correlated with temperature (positively) and oxidation-reduction potential (ORP; negatively). Using X-ray absorption spectroscopy to speciate litter-borne Se, more Se0 formed without aeration due to abundant Se-reducing bacteria, among which Azospira and Azospirillum were highly related to the removal of both Se oxyanions, while Desulfovibrio, Azoarcus, Sulfurospirillum, Thauera, Geobacter, Clostridium, and Pediococcus were the major contributors to selenate removal. Overall, our study suggests microbial Se metabolism in the litter system was significantly affected by temperature and ORP, which could be manipulated to enhance Se removal efficiency and the transformation of selenate/selenite into low toxic Se0 and volatile Se, reducing risks posed by the residual Se in the system.

Modeling Study of Selenium Migration Behavior in Wet Flue Gas Desulfurization Spray Towers.

Wet flue gas desulfurization (WFGD) system is the core equipment for removing SO2 from coal-fired power plants, and it also has an important synergistic effect on the removal of selenium. However, the removal efficiency of Se across WFGD systems is not as expected, and it varies greatly in different coal-fired units (12.5-96%). In this study, a mathematical model was established to quantitatively describe the selenium migration behavior in WFGD spray towers, including the conversion of gaseous selenium to particulate selenium and the capture of gaseous SeO2 and particles by droplets. The calculation results show that the behavior of selenium in the spray tower can be divided into three stages: preparation, condensation, and removal. The condensation stage significantly affected the selenium distribution and its total removal efficiency. Furthermore, five factors which may affect the selenium behavior were investigated. Among them, the inlet particle size distribution and the droplet temperature had great impacts on the outlet selenium concentration, which may be the reason for the unstable selenium removal efficiencies. This study can help in understanding the migration process of selenium in WFGD spray towers and provide some guidance for the development of specific selenium control technologies.

Highly efficient removal of selenite by electrolysis-assisted nano-zerovalent iron (nZVI): Implication for corrosion and reduction

In present work, an efficient purification of selenite-contaminated water using electrolysis-assisted nano zerovalent iron (E-nZVI), which achieved a removal capacity of 15.83 mg Se(IV)/g nZVI and exceeded the none-assisted nZVI system by 135% was tested. Although Se(IV) removal dramatically increased with applied voltages over 2.0 V in E-nZVI system, the kinetics indicate that higher applied voltage (~4.0 V or more) and lower resistance could not result in apparent promotion on the reaction rate constant. Furthermore, the results from separate anode-cathode tests and surface-sensitive quantitative characterization reveal the synergistic effects of nZVI and cathodic reduction on elemental selenium precipitation, and nZVI corrosion aggravated by electrochemical oxidization improved removal performance in the anode chamber. In conclusion, the electrolysis-assisted nZVI showed good removal efficiency of Se(IV) and is a potential water treatment method for removing reducible contaminants.

Selenium removal by clam shells and gravels amended with cattail and reed litter

Increasing selenium (Se) levels in aquatic environments raise concerns all over the world. This study investigated effects of organic amendments (cattail and reed litter) and porous media (gravels and clam shells) on Se removal efficiency of horizontal subsurface flow constructed wetlands. Our results show clam shells reduced Se (by mass) up to 2.4-fold faster than gravels within 19 days. Using clam shells as the sole substrate, 96.3% removal efficiency was obtained for cattail litter as an amendment, compared to 88.7% for reed litter over 10 days, although the latter released carbon and nitrogen at least 1.4-fold faster than the former. Meanwhile, speciation analysis suggests Se0 (~75%) and organo-Se (~94%) dominated the biofilms on shells and plant litter, respectively, as substrates. Overall, this study suggests clam shells and cattail litter as an effective medium and carbon source, respectively, can enhance microbial Se removal without posing risks to wildlife health.

The Effect of Gypsum on the Fixation of Selenium in the Iron/Calcium-Selenium Coprecipitation Process.

The coprecipitation of selenium(IV) (Se) with iron(III) (Fe) is a widely practiced method for the removal of Se from mineral processing effluents, but the effect of gypsum as a major secondary mineral on the iron-selenium coprecipitation process is still of concern. In our work we first investigated the effects of pH, Fe/Se molar ratio and the neutralizing agent on the removal efficiency of Se by iron-selenium coprecipitation method. The developed two-step Fe-Se coprecipitation method (Fe/Se molar ratio of 4) was superior to the one-step Fe-Se coprecipitation method at pH 4 using CaO as base in terms of the stability of the generated Fe-Se coprecipitates. Raman experimental results indicated the iron-selenium coprecipitates had the by-product of calcium selenite. We then investigated the effect of incorporation of Se into gypsum on the coprecipitation process at different pHs. The fourier transform infrared spectroscopy (FTIR), x-ray diffraction (XRD), and scanning electron microscopy (SEM) of the calcium-selenium coprecipitates showed that the Se incorporated into the structure of gypsum at pH 8-10. Therefore, this work has important implications for the development of new technologies for efficient Se removal.

RETRACTED ARTICLE: Microstructural, morphological behavior and removal of Cr(VI) and Se(IV) from aqueous solutions by magnetite nanoparticles/PVA and cellulose acetate nanofibers

Different contents of magnetite nanoparticles (MNPs)/graphene (G) nanosheets were incorporated into polyvinyl alcohol (PVA) and cellulose acetate (CA) electrospun nanofibers. The obtained compositions were investigated using XRD, TEM, FTIR and FESEM. The microstructural investigation displayed that nanofibers were formed with diameters around 120 nm up to 900 nm in the case of the composition at 0.1MNPs-G/PVA, while morphological features illustrated that the composition at 0.0MNPs-G/PVA fibers was formed with diameters 0.2 µm up to 0.6 µm, besides G, which was scattered through the fibers with dimensions around 17 µm. The removal efficiency of Se(IV) and Cr(VI) from aqueous solutions was evaluated and the optimum trend was obtained for the composition of 0.05MNPs-G@PVA. The high porous nanofibers may be suggested for more investigations for heavy metal removal.

Selenium removal and biotransformation in a floating-leaved macrophyte system

Selenium (Se) is an essential micronutrient for animals with a relatively narrow margin between essentiality and toxicity. To evaluate Se removal efficiency by a constructed wetland treatment system and its potential eco-risk, a floating-leaved macrophyte system was constructed, consisting of three main trophic levels. Over 21-d treatment, water Se concentration was gradually reduced by 40.40%, while 24.03% and 74.41% of the removed Se were found in the plant Nymphoides sp. and sediment, respectively. Among plant tissues, roots accumulated the highest Se level, although the greatest total Se was found in stems, followed by leaves, roots and rhizomes. X-ray absorption spectroscopy revealed that 82.65% of the absorbed selenite by the plants was biotransformed to other forms, as organo-Se species accounted for 45.38% of the Se retained in the sediment, which was primarily responsible for the entry of Se into the detritus food chain. The proportion of organo-Se compounds increased with trophic levels from sediments to fish, indicating, instead of direct uptake of selenite, the food chain transfer and biotransformation of Se may serve as a key exposure route for Se in aquatic organisms. When exposed to organo-Se compounds, i.e., SeCys and SeMet, the plants, shrimp and fish tended to accumulate more Se. However, the greater trophic transfer factor was obtained for selenate, leading to higher Se levels accumulated in fish. Overall, in addition to key mechanisms involved in Se removal, our research also provides a much better understanding of the potential eco-risk that may be posed by the floating-leaved plant system for bioremediation of Se via food chain transfer and biotransformation, paving the way for a low eco-toxic treatment system for Se remediation.

Relationship between selenium removal efficiency and production of lipid and hydrogen by Chlorella vulgaris

In our previous studies, Chlorella vulgaris had proven highly efficient in removing selenium (Se) from water, while the disposal of Se containing in algal biomass was still an issue of concern. Firstly, this research suggests algal Se could be released back to water, posing risks to aquatic wildlife. Thus, we further explored the possibility of using C. vulgaris to remove Se and produce lipid and hydrogen simultaneously. Our results show the higher percentage of saturated fatty acids, especially palmitic acid, was found in the sulfur (S) deprived algae exposed to either selenate or selenite, although the highest lipid content (21.9%) was found in the selenite treated algae in full BG11 medium. In addition, compared with the Se free algae, hydrogen production rate was 2.1- and 4.3-fold higher for the selenate and selenite treated algae, respectively. Se removal efficiency achieved by the selenite treated algae through accumulation and volatilization was 2.3-fold higher than the selenate treatment under hypoxic condition with S deprived, which is in contrast to the results obtained under aerobic conditions.

Removal of selenium containing algae by the bivalve Sinanodonta woodiana and the potential risk to human health

Selenium (Se) is an essential micronutrient for animals and humans with a relatively narrow margin between nutritional essentiality and potential toxicity. Even though our previous studies have demonstrated algae could efficiently remove Se, mainly through volatilization, concern is raised about eco-risks posed by the remaining Se in algae. Here, Sinanodonta woodiana was investigated as a biofilter for the removal of Se-containing Chlorella vulgaris and for its potential risk to human health. Our results suggest filtration rates of S. woodiana were independent of Se levels in algal biomass, with a removal efficiency of between 60 and 78%. However, Se concentrations accumulated in mussels were significantly correlated with algal-borne Se levels, with a dietary assimilation efficiency ranging from 12% to 46%. Thus, a pilot biofiltration system was set up to assess uptake and depuration processes. The system was found to efficiently remove Se laden algae through the uptake by mussels, while 21% of Se in mussels could be depurated in 6 days. Among tissues, gills accumulated the highest Se concentration after assimilating algal-borne Se but shed Se compounds in the fastest pace during depuration. Health risks posed by consumption of mussels exposed to different sources of Se were further assessed. S. woodiana accumulated the highest Se concentration after exposure to waterborne SeMet, followed by dietary Se, selenite and control. The relatively higher Se levels were found in gills for all the treatments. After boiling, the most common method of cooking mussels, the greatest reduction in Se concentration occurred in mantle for the control and dietary Se groups and in muscle for the SeMet and selenite treatments. Therefore, within the safe limits, Se-containing mussels can be consumed as a dietary supplement. Overall, our research suggests incorporation of mussels into an algal treatment system can improve Se removal efficiency and also provide financial incentives for practitioners.

Phytoremediation of seleniferous soil leachate using the aquatic plants Lemna minor and Egeria densa

Phytoremediation of selenium (Se)-containing Hoagland solution and seleniferous soil leachate using two aquatic plants Lemna minor and Egeria densa was evaluated. L. minor showed the highest Se removal efficiency (97%) in the Hoagland solution with a bioconcentration factor (BCF) of 504.35 (±0.83). In artificial soil leachate with addition of 2 mg L−1 MnSO4, L. minor and E. densa showed a Se removal efficiency of 77% and 60%, respectively. The addition of K2S2O8 decreased the Se uptake by both plants by 40% and the medium pH decreased from 7 to 3, whereas the addition of SO42− decreased the removal efficiency of both aquatic plants by 30%, in which only 3% of Se was taken up by the plants. L. minor was selected to remove Se from a real seleniferous soil leachate which contained 74 μg L−1 Se and a 76% efficiency was achieved, with a Se uptake of 29 µg g−1 dry weight and a BCF of 393.2 (±13.6). This study demonstrates that aquatic plants such as Lemna minor and Egeria densa can be used to remove Se from seleniferous soil leachate and that the phytoremediation efficiency depends on the composition of the extractant used for soil washing.

Selenite removal from groundwater by zero-valent iron (ZVI) in combination with oxidants

We recently demonstrated common oxidants simply coupled with ZVI to continuously drive the accelerated Fe0 corrosion and hence achieve fast and very efficient removal of heavy metals and metalloids from groundwater. In this study, we aimed first to answer a basic question of the oxidant dosage theoretically required to sequester a certain amount of selenite. The specific ratio of oxidant dosage to Se(IV) removal, which reflected the theoretically minimal oxidant dosage required to sequester one mole of Se(IV), was almost independent of the initial Se(IV) concentration but significantly affected by the difference in oxidant species. To sequester one mole of Se(IV), the minimum dosage of the required oxidant was calculated to be 3.94–4.09 for NaClO, 3.90–4.33 for H2O2, and 3.29–3.54 for KMnO4, respectively. Simultaneous aeration increased the removal efficiency of Se(IV) and substantially reduced the required dosage of oxidants. To form a strong contrast with very limited Se(IV) removal by ZVI alone, the coupling of NaClO and H2O2 into the ZVI system remarkably enhanced the performance of Se(IV) removal during the long term fixed-bed experiments with a real groundwater background. The ZVI columns coupled by NaClO and H2O2 steadily treated 10,000 volumes (BV) of real groundwater. X-ray adsorption near edge structure (XANES) demonstrated that more than 85% of the selenium was reduced to Se0 and Se2−, with Se0 as the dominant selenium species sequestered in the solid phase. Synchrotron-based scanning transmission soft X-ray microscopy (STXM) explicitly revealed the ubiquitous spatial distribution of selenium in the corrosion products and hence demonstrated a high accessibility of these corrosion products to selenite.