Dissolved oxygen has no inhibition on methane oxidation coupled to selenate reduction in a membrane biofilm reactor.

Methane oxidation coupled to selenate reduction has been suggested as a promising technology to bio-remediate selenium contaminated environments. However, the effect of dissolved oxygen (DO) on this process remained unclear. Here, we investigate the feasibility of selenate removal at two distinct DO concentrations. A membrane biofilm reactor (MBfR) was initially fed with ∼5 mg Se/L and then lowered to ∼1 mg Se/L of selenate, under anoxic condition containing ∼0.2 mg/L of influent DO. Selenate removal reached approximately 90% without selenite accumulation after one-month operation. Then 6-7 mg/L of DO was introduced and showed no apparent effect on selenate reduction in the subsequent operation. Electron microscopy suggested elevated oxygen exposure did not affect microbial shapes. 16S rDNA sequencing showed the aerobic methanotroph Methylocystis increased, while possible selenate reducers, Ignavibacterium and Bradyrhizobium, maintained stable after oxygen boost. Gene analysis indicated that nitrate/nitrite reductases positively correlated with selenate removal flux and were not remarkably affected by oxygen addition. Reversely, enzymes related with aerobic methane oxidation were obviously improved. This study provides a potential technology for selenate removal from oxygenated environments in a methane-based MBfR.

Generating cellulose-agar composite hydrogels for uptake-release kinetic studies of selenate and selenomethionine.

Cellulose-agar (CAB) composite hydrogel beads were generated for the uptake-release kinetics studies of Se(VI) and selenomethionine (SeMt) from water medium. The objective of this work is to analyze the surface structure, gel properties, thermal stability and chemical functionalities responsible for the adsorption of Se(VI) and SeMt. We propose here a possible mechanism for the adsorptions. Adsorption isotherms are in good agreement with the Freundlich model, yielding a high adsorption capacity for the CAB composite. Maximum adsorption capacity of Se(VI) and SeMt were found to be 7.083 mg g-1 and 34.639 mg g-1 respectively. The mean free energy of adsorption (E*) value was found to be 0.0423 kJ mol-1 and 0.329 kJ mol-1 of Se(VI) and SeMt respectively. 1 M HCl and 0.1 M HCl were able to desorb Se(VI) and SeMt respectively from CAB. The adsorption of Se(VI) was significantly reduced if As(III), Cr(III) and Hg(II) were present as complementary ions in the medium. Similar studies with pristine cellulose beads (CB) yielded insignificant uptake properties.

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

The mining industry needs to treat large volumes of wastewater highly concentrated in chemical compounds that can adversely affect receiving environments. One promising method of treatment is the use of reverse osmosis to remove most of the dissolved salts. However, the resulting brine reject is a highly saline wastewater that needs further treatment to remove the toxic components, such as selenate, which is a chemical compound of great concern in coal-mining regions. Biological reduction and removal of dissolved selenium from a brine solution was achieved. Microorganisms were enriched from environmental samples collected from two mines, respectively, at different geographic locations through adaptive evolution in the laboratory. Batch treatment of typical brine was tested with two different enrichments with the addition of either of two chemical forms of iron, ferrous chloride or zero valent iron. Successful selenium removal in the presence of high nitrate and sulphate concentrations was achieved with a combination of enriched microorganisms from one particular site and the addition of zero-valent iron. The composition and metabolic potential of the enriched microorganisms revealed Clostridium, Sphaerochaeta, Synergistes and Desulfosporosinus species with the metabolic potential for selenate reduction through the YgfK enzymatic process associated with selenium detoxification.

Kinetics and equilibrium adsorption study of selenium oxyanions onto Al/Si and Fe/Si coprecipitates.

Inappropriate treatments for the effluents from semiconductor plants might cause the releases and wide distributions of selenium (Se) into the ecosystems. In this study, Al/Si and Fe/Si coprecipitates were selected as model adsorbents as they often formed during the wastewater coagulation process, and the removal efficiency of selenite (SeO3) and selenate (SeO4) onto the coprecipitates were systematically examined. The removal efficiency of SeO3 and SeO4 was highly related to surface properties of Al/Si and Fe/Si coprecipitates. The surface-attached Al shell of Al/Si coprecipitates shielded a portion of negative charges from the core SiO2, resulting in a higher point of zero charge than that of Fe/Si coprecipitates. Thus, adsorption of SeO3/SeO4 was favorable on the Al/Si coprecipitates. Adsorptions of both SeO3 and SeO4 on Al/Si coprecipitates were exothermic reactions. On Fe/Si coprecipitates, while SeO3 adsorption also showed the exothermic behavior, SeO4 adsorption occurred as an endothermic reaction. The kinetic adsorption data of SeO3/SeO4 on Al/Si and Fe/Si coprecipitates were described well by the pseudo-second-order kinetic model. SeO4 and SeO3 adsorption on Fe/Si or Al/Si were greatly inhibited by the strong PO4 ligand, whereas the weak ligand such as SO4 only significantly affected SeO4 adsorption. The weakest complex between SeO4 and Al was implied by the essentially SeO4 desorption as SeO4/PO4 molar ratios decreased from 0.5 to 0.2. These results were further confirmed by the less SeO4 desorption (41%) from Fe/Si coprecipitates than that from Al/Si coprecipitates (78%) while PO4 was added sequentially.

Sorption-desorption of selenite and selenate on Mg-Al layered double hydroxide in competition with nitrate, sulfate and phosphate.

Selenate and selenite are considered emerging contaminants and pose a risk to living organisms. Since selenium anion species are at low concentration in aquatic environments, materials for its retention are required to enable monitoring. Herein, hydrotalcite was calcined and characterised to investigate sorption and desorption of selenite and selenate in competition with nitrate, sulfate and phosphate. Sorption experiments were carried out in batch system and desorption by sequential dilution. Selenite and selenate concentration remaining after N desorption steps was determined by mass balance. The isotherms were adjusted to the dual-mode Langmuir-Freundlich model (R2 > 0.99). Maximum sorption capacity ranged from 494 to 563 meq kg-1 for selenite and from 609 to 659 meq kg-1 for selenate. Sulfate and phosphate ions showed greater competitive effect on the sorption of selenate and selenite, respectively. Low mobilization factors and high sorption efficiency (MF<3%; SE ≈ 100%) indicated that calcined hydrotalcite has the wanted characteristics for retention of relevant selenium anion species in aqueous media.

Biological removal of selenate and ammonium by activated sludge in a sequencing batch reactor.

Wastewaters contaminated by both selenium and ammonium need to be treated prior to discharge into natural water bodies, but there are no studies on the simultaneous removal of selenium and ammonium. A sequencing batch reactor (SBR) was inoculated with activated sludge and operated for 90days. The highest ammonium removal efficiency achieved was 98%, while the total nitrogen removal was 75%. Nearly a complete chemical oxygen demand removal efficiency was attained after 16days of operation, whereas complete selenate removal was achieved only after 66days. The highest total Se removal efficiency was 97%. Batch experiments showed that the total Se in the aqueous phase decreased by 21% with increasing initial ammonium concentration from 50 to 100mgL-1. This study showed that SBR can remove both selenate and ammonium via, respectively, bioreduction and partial nitrification-denitrification and thus offer possibilities for treating selenium and ammonium contaminated effluents.

Reductive Removal of Selenate in Water Using Stabilized Zero-Valent Iron Nanoparticles.

Polysaccharide-stabilized zero-valent iron (ZVI) nanoparticles were synthesized using sodium carboxymethyl cellulose (CMC) or starch as stabilizer, and tested for reductive removal of selenate in water. Batch kinetic tests showed that the stabilized ZVI nanoparticles offer much faster selenate removal than bare ZVI particles at both pH 6.0 and pH 8.4. X-ray photoelectron spectroscopy (XPS) analyses confirmed Se(VI) was transformed to Se(IV) and Se(0), which are removed along with the nanoparticles. Neutral pH (~7) was found to be most favorable for the reductive removal. Decreasing pH to 5.0 or increasing it to 8.0 reduced the removal rate of CMC-stabilized ZVI by a factor of 4.6 or 1.3, respectively, based on the observed first-order-rate constant. Dissolved organic matter (DOM) at 5 mg/L as total organic carbon (TOC) had modest inhibitive effect, but DOM at 25 mg/L TOC decreased selenate removal by 25%. The stabilized nanoparticles hold the potential to facilitate in situ remediation of selenate-contaminated soil and groundwater.

Mechanisms of Chromate, Selenate, and Sulfate Adsorption on Al-Substituted Ferrihydrite: Implications for Ferrihydrite Surface Structure and Reactivity.

Ferrihydrite is a nanocrystalline Fe (hydr)oxide and important sink for environmental contaminants. Although Fe (hydr)oxides are rarely pure in natural systems, little is known about the effects of structural impurities such as Al on the surface properties and reactivity of ferrihydrite. In this study, we characterized the adsorption mechanisms of chromate, selenate, and sulfate on Al-substituted ferrihydrite (0, 6, 12, 18, and 24 mol % Al) using in situ attenuated total reflection Fourier transform infrared spectroscopy. Spectral data sets recorded as a function of pH were processed using a multivariate curve resolution technique to identify which types of surface species form and to generate their concentration profiles as a function of pH and Al content. Results show a significant increase in relative fraction of outer-sphere complexes for all three oxyanions with increasing Al substitution. In addition, the effect of Al substitution is found to be mechanism-specific in the case of chromate, with bidentate complexes disproportionately suppressed over monodentate complexes at higher Al contents. Overall, our findings have important implications for the fate of chromate, selenate, and sulfate in subsurface environments and offer new insight into the surface reactivity of Al-ferrihydrite.

High efficiency adsorption and removal of selenate and selenite from water using metal-organic frameworks.

A series of zirconium-based, metal-organic frameworks (MOFs) were tested for their ability to adsorb and remove selenate and selenite anions from aqueous solutions. MOFs were tested for adsorption capacity and uptake time at different concentrations. NU-1000 was shown to have the highest adsorption capacity, and fastest uptake rates for both selenate and selenite, of all zirconium-based MOFs studied here. Herein, the mechanism of selenate and selenite adsorption on NU-1000 is explored to determine the important features that make NU-1000 a superior adsorbent for this application.

Photocatalytic removal of selenite and selenate species: effect of EDTA and other process variables.

TiO2-assisted photocatalysis was employed for the removal of aqueous phase selenite and selenate species in conjunction with EDTA as a hole (h+) scavenger. Findings from the binary selenite/EDTA and selenate/EDTA systems showed high selenite and selenate removal at pH 4 and pH 6, with faster removal kinetics noted for the selenite species compared with the selenate species that showed a gradual change over the reaction course. The noted removal of selenite and selenate was attributed to their reduction by the conduction band electrons (e-). The effect of pH studies indicated high selenite, selenate, and EDTA removal in the acidic pH range, with the following specific trend: pH 4 > pH 6 > pH 12. Different from the EDTA studies, the use of thiocyanate alone did not initiate reduction of selenium oxyanions, and hence, its role as a hole scavenger in the present systems was not evident. However, the addition of EDTA to respective selenite/selenate/thiocyanate system at pH 4 did yield near complete removal of selenite and selenate species. The marginal role of thiocyanate as a hole scavenger was attributed to its negligible adsorption onto TiO2 surface. Furthermore, at pH 4 and within 3 h reaction time, enhanced selenate removal was noted with an increase in its initial concentration from 20 to 100 ppm, with near complete selenate removal noted for both cases. In general, findings from the present work indicate that both selenite and selenate can be successfully removed from the aqueous phase employing the TiO2-mediated photocatalysis and h(+)-scavenging agent EDTA.

Modeling oxyanion adsorption on ferralic soil, part 2: chromate, selenate, molybdate, and arsenate adsorption.

High levels of oxyanions are found in the soil environment, often as a result of human activity. At high concentrations, oxyanions can be harmful to both humans and wildlife. Information about the interactions between oxyanions and natural samples is essential for understanding the bioavailability, toxicity, and transport of these compounds in the environment. In the present study, the authors investigated the reactivity of different oxyanions (AsO4 , MoO4 , SeO4 , and CrO4 ) at different pH values in 2 horizons of a ferralic soil. By combining available microscopic data on iron oxides with the macroscopic data obtained, the authors were able to use the charge distribution model to accurately describe the adsorption of these 4 oxyanions and thus to determine the surface speciation. The charge distribution model was previously calibrated and evaluated using phosphate adsorption/desorption data. The adsorption behavior on ferralic soil is controlled mainly by the natural iron oxides present, and it is qualitatively analogous to that exhibited by synthetic iron oxides. The highest adsorption was found for arsenate ions, whereas the lowest was found for selenate, with chromate and molybdate ions showing an intermediate behavior.

Adsorption of selenite and selenate by nanocrystalline aluminum oxide, neat and impregnated in chitosan beads.

Nanocrystalline metal oxide impregnated chitosan beads (MICB) were successfully developed with nanocrystalline aluminum oxide (n-Al2O3) to form n-Al2O3 impregnated chitosan beads (AICB). AICB were able to simultaneously adsorb inorganic aqueous selenite and selenate more effectively than n-Al2O3 or chitosan alone. For completeness, adsorption performance was also compared to n-TiO2, a widely studied adsorbent for selenium, and n-TiO2 impregnated chitosan beads (TICB). For the selenite system, n-Al2O3 was the primary active adsorbent responsible for removal as chitosan has a low affinity for selenite. For selenate, however, chitosan was the primary active adsorbent. The association constants for the adsorbent/adsorbate complexes and the relative amounts in which they are present supported this hypothesis. The association constants for selenate binding on n-Al2O3 and chitosan were 1.215 × 10(-2) and 3.048 × 10(-3), respectively, and the association constants for selenite binding on n-Al2O3 and chitosan were 1.349 × 10(-2) and 1.990 × 10(-4), respectively. For systems with coexisting selenite and selenate, AICB is potentially the most robust option as it maintained the most consistent performance regardless of fractionation of the selenium species. Kinetic studies and equilibrium isotherms were completed and effectively modeled using pseudo-second order kinetics and Langmuir adsorption theory, making it the first comprehensive systematic study of neat n-Al2O3 and AICB for selenium adsorption. pH significantly impacted adsorption due to changes in the adsorbent surface charge; increasing pH corresponded with decreasing adsorbent performance, beginning at approximately pH 6.5-7 for AICB. The trend in performance due to the effect of pH indicated that selenate binds to the amine group in chitosan, as suggested by other studies. In addition, increasing background sulfate concentration was found to negatively impact adsorption efficacy for both selenite, and more significantly, selenate, as sulfate is known to compete with selenium oxyanions due to their similar structures. The results indicate that, in order to maintain consistent removal in more realistic systems, a pre-treatment process to manage sulfate will be necessary as indicated for other adsorbents implemented for selenium adsorption in aqueous systems.

Evaluation of selenium species in selenium-enriched pakchoi (Brassica chinensis Jusl var parachinensis (Bailey) Tsen & Lee) using mixed ion-pair reversed phase HPLC-ICP-MS.

HPLC-ICP-MS based on ion-paired reversed phase chromatography for the selenium speciation using the mixture of 1-butanesulfonic acid (BA) and trifluoroacetic acid (TFA) as the mixed ion-pairing reagents was developed and applied to selenium-enriched pakchoi (Brassica chinensis Jusl var parachinensis (Bailey) Tsen & Lee). Several conditions of ion-paired reversed phase HPLC-ICP-MS, such as pH of the mobile phase, concentration of ion pairing reagents, types and length of analytical column, and flow rate of the mobile phase, were optimised for five selenium species; selenate (Se(VI)), Selenite (se(IV)), selenocysteine (SeC), Se-methylselenocysteine (SeMC) and selenomethionine (SeM). The results showed that the optimum conditions for pH, BA and TFA condition, type of separating column and flow rate, were 4.5, 8mM, 4mM, C18 (250 mm length × 4.6mm I.D) and 1.2 mL min(-1), respectively. These conditions archived separation of the organic selenium species. The limits of detection (LOD) and quantitation (LOQ) of each selenium species were lower than 5 and 16 ng Se mL(-1), respectively. Furthermore, the recoveries of most selenium species were good, except for SeC. In this research, selenium-enriched pakchoi was cultivated by supplementing inorganic selenium from selenate into sand. The result showed that inorganic selenium, SeMC, SeM and several unknown species were found in selenium-enriched pakchoi sprouts by using the proposed method. Thereby, the biotransformation of selenate in pakchoi was similar to other Brassicaceae plants such as kale and broccoli.

Iron anode mediated transformation of selenate in sand columns.

Removal of aqueous selenate by iron electrolysis is investigated using sand-packed column experiments under a flowing condition. An iron anode generates ferrous ions, while cathode produces hydroxide, thus producing ferrous hydroxide capable of reducing selenate to elemental selenium. Additionally, siderite could reduce selenate or selenite to elemental selenium. The removal rate of selenate is proportional to the contact time and the yield of ferrous hydroxide or ferrous carbonate. At a sequence of anode-cathode, the transformation of selenate mostly occurs in the zone after cathode. An operation of 48 h electrolysis finally transforms 82.2% of selenate at 0.2 mM of initial concentration, 1.8 m/day of seepage velocity and 1.26 mA/cm(2) of current density. A longer reactive zone after cathode slightly increases the reduction of selenate to 84.1%, in comparison with 82.2% of a shorter residence time in the reactive zone after cathode. With shorter electrode spacing (approximately 27% shorter), the transformation rate of selenate decreased to 73.5%; however, the specific electrical energy consumption was saved by 78%. A sequence of cathode-anode was ineffective in removing selenate because of the lack of reducing agent in the column. The results indicate that the electrochemical system might be effective in removing selenate in a single well.