Selenate Removal Research Articles

Reductive removal of selenate by zero-valent iron: The roles of aqueous Fe2+ and corrosion products, and selenate removal mechanisms

Batch tests were conducted to investigate the roles of dissolved Fe2+ and corrosion products, and the involved mechanisms in selenate (SeVI) removal by zero-valent iron (ZVI). The results showed that insignificant SeVI removal (4–7.5%) was observed in the presence of ZVI or Fe2+ alone. However, external supply of dissolved ferrous ion dramatically enhanced SeVI removal in the presence of ZVI. Selenate removal efficiency increased with increasing Fe2+ concentration. Selenate removal sustained only if Fe2+ was supplied continuously. Both sequential extraction experiments and XPS analysis showed that selenate was reduced step by step, with elemental selenium and adsorbed selenite as the dominant reductive products. Selenite and elemental selenium could be further reduced to selenide, with continuous Fe2+ supply and sufficient reaction time. In the ZVI-SeVI-Fe2+ system, ZVI was the major electron donor for selenate reduction. Fe2+ functioned as electron donor as well and was consumed with a Fe2+:Se stoichiometry of ∼1:1. It also facilitated the transformation of the passive layer of iron coatings to a medium (e.g., magnetite) favoring electron transfer and thus enhanced selenate reduction. Iron corrosion products were media for electron transfer and reactive interfaces for selenium adsorption and reduction. These findings provided a new approach to overcome ZVI surface passivation for long-term application.

Promotion effect of Mn2+ and Co2+ on selenate reduction by zero-valent iron

Our previous work demonstrated that Fe2+ could overcome the passivation of zero-valent iron (ZVI or Fe0) and sustain iron reactivity for rapid selenate reduction. Due to the similar chemical properties to Fe2+ and co-existence in ZVI material as impurities, Mn2+ and Co2+ were evaluated as an alternative in selenate removal by ZVI. The results of batch experiments showed that Co2+ and Mn2+, especially the former, significantly enhanced selenate removal by ZVI. Co2+ promoted Fe2+ release and the formation of a bimetallic (Co0–Fe0) catalytic system by redox reaction. Co2+ and Mn2+ provided around 69% and 30% electron for selenate reduction, respectively. Selenite (SeIV) was the predominant reductive product in the presence of Co2+; however, selenite and elemental Se (Se0) were the main reductive products in the presence of Mn2+. Adsorbed selenate and selenite could be further reduced to elemental Se and even selenide by ZVI in the presence of Co2+ and Mn2+, identified by XPS analysis and sequential extraction test. The roles and evolution of Co2+ and Mn2+, and selenate removal mechanism were also comprehensively investigated by spectroscopic and chemical analysis. This study indicates that the co-existent of Co2+ and Mn2+ would be positive for selenate removal.

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.

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/cm2 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.

Understanding the positive effects of low pH and limited aeration on selenate removal from water by elemental iron

Selenate (SeO42-) is a toxic soluble compound and its presence in the environment should be minimized. Reduction of selenate ions by elemental iron is a promising approach for treating contaminated waters. However, uncertainty exists regarding both the favorable conditions required and the associated mechanisms. In this study, the kinetics and mechanisms of selenate removal using Fe0 at different pH and aeration rates was investigated using quantitative measurement of reactants and products. Under acidic anaerobic conditions, selenate was reduced directly by Fe0. Limited aeration increased removal rates by up to four times due to formation of reactive Fe(II)-bearing solid intermediates on the iron surface. Further increase in aeration rate decreased removal rates but also decreased iron release and acid consumption. Under basic conditions, selenate by Fe0 was negligible under anaerobic conditions and low under limited aeration. Selenate removal rates were shown to be dependent on Se concentration, on available reducing solid surface and on competition with oxygen over electrons.

On-site Formation of Layered Double Hydroxide Compounds and Application to Water Treatment of Toxic Elements

Removal of toxic elements in effluent at low concentration is difficult technique and an effective method has not been established. This paper describes a novel water treatment process that composeds of the following processes ; addition of divalent and trivalent metal ions, conditioning of the slurry with alkaline reagent, neutralizing reaction, returning the settled slurry and gravitational settling of the solids. This neutralization-sedimentation system with slurry returning process allows the formation of so-called Layered Double Hydroxide (LDH) precipitates, which play key roles in removing toxic elements. In the selenate removal process, Green Rust (GR), Fe(II)-Fe(III) type LDH compound, reduce selenate to elemental selenium in anoxic condition, while GR is consequently oxidized to Magnetite (Fe3O4). The black-colored magnetic slurry containing GR and Magnetite contributes to the removal of selenate.In the fluorine removal process, Hydrotalcite, Mg(II)-Al(III) type LDH, surrounds the neutralizer MgO particle, thereby forming a core-shell structure. Optimum fluorine removal ratio reaches 95%, where initial fluorine concentration 15 mg/L decreases below the national environmental standard, 0.8 mg/L. This proposed processe serves as alternative treatment technique for low concentrated toxic elements.

Electrochemical removal of selenate from aqueous solutions

Removal of selenate from solution is investigated in batch electrochemical systems using reactive iron anodes and copper plate cathode in a bicarbonate medium. Iron anodes produce ferrous hydroxide, which is a major factor in the removal of selenate from solution. Iron anodes also generate a significant decrease in the oxidation–reduction potential (ORP) of the solution because it prevents generation of oxygen gas at the anode by electrolysis. The removal rates varied from 45.1% to 97.4%, depending on current density and selenate concentration. The transformation of selenate by the process is modeled based on a heterogeneous reaction coupled with electrochemical generation of ferrous and hydroxide. The rates are optimized at lower initial concentrations, higher electrical currents, and the presence of anions. Presence of dissolved oxygen does not cause any significant effects the removal of selenate.

No.2 湿式排煙脱硫液に含まれるセレン除去法の開発(研究発表)

No.2 湿式排煙脱硫液に含まれるセレン除去法の開発(研究発表)

Effect of pH on nitrate and selenate reduction in flue gas desulfurization brine using the H2-based membrane biofilm reactor (MBfR)

Increased tightening of air regulations is leading more electric utilities to install flue gas desulfurization (FGD) systems. These systems produce brine containing high concentrations of nitrate, nitrite, and selenate which must be removed before discharge. The H2-based membrane biofilm reactor (MBfR) was shown to consistently remove nitrate, nitrite, and selenate at high efficiencies. The maximum selenate removal flux reached 362 mgSe m(-2)d(-1) and was higher than that observed in earlier research, which shows continual improvement of the biofilm for selenate reduction. A low pH of 6.8 inhibited precipitation when treating actual FGD brine, yet did not inhibit removal. SO4(2-) was not removed and therefore did not compete with nitrate, nitrite, and selenate reduction for the available H2.

The removal of selenate to low ppb levels from flue gas desulfurization brine using the H 2-based membrane biofilm reactor (MBfR)

The H 2-based membrane biofilm reactor (MBfR) was shown to consistently remove nitrate, nitrite, and selenate at high efficiencies from flue-gas desulfurization brine. Selenate was removed to <50 ppb which is the National Pollutant Discharge Elimination System (NPDES) criteria for the brine to be released into the environment. When selenate was removed to <50 ppb, nitrate and nitrite were still present in the mg/L range which suggests that selenate is able to be secondarily reduced to low levels when nitrate and nitrite serve as the main electron acceptors for bacterial growth. SO 4 2 - was not removed and therefore did not compete with nitrate and selenate reduction for the available H 2.

Study the Remove and Transport Mechanisms of Seleniums in Membrane(UF and NF) Processes

The removal and transport of ionized and non-ionized selenium’s were systemically investigated in this study. The charge repulsion between the membrane surface and selenium ions was an important mechanism for the rejection of ions by a charged membrane. In addition, selenium removal was significantly affected by the J0/k ratio, but was not affected by the cross-flow velocity. Both diffusion and convection are proven to affect the transport of selenium ions transports. The reflection coefficients of both UF and NF membranes increased with increasing the pH, and with the reflection coefficients of selenate are being higher than those of selenite at under the same operating conditions. The spiral-wound type module exhibited slightly higher selenate removal compared to than the flat-sheet type module at under the same operation conditions.

No.20 湿式排煙脱硫廃水に含まれる難除去性セレン除去法の開発(灰・微量元素(1))

No.20 湿式排煙脱硫廃水に含まれる難除去性セレン除去法の開発(灰・微量元素(1))

TREATMENT OF ASH LEACHATE USING DIFFERENT FUNCTIONAL GROUP ANION EXCHANGE RESINS

Fly ash leachate wastewater containing a wide variety of toxic heavy metals either in cationic or oxyanionic forms. Treatment of such water represents a challenge for all researchers due to the higher concentrations and competitive effect of all the existing cations and anions. The process of treatment was investigated using different functional groups anion exchangers to reach the most suitable materials and conditions for lowering the level of contamination for the wastewater reuse. The effects of most abundant anionic species in leachate wastewater such as sulfate on the adsorption of boron, molybdate, chromate, arsenate and selenate oxyanions were studied at different experimental conditions. The data obtained indicate that LAYNAERT is the most suitable anion exchanger for the removal of arsenate and selenate, whereas, IRA743 and A400 are good for the removal of boron and molybdate respectively.

A study of the removal of selenite and selenate from aqueous solutions using a magnetic iron/manganese oxide nanomaterial and ICP-MS

Selenium (Se) is naturally occurring in the environment and is an essential nutrient in mammals. However, environmental Se can be increased to toxic levels through different industrial practices. The potential adsorption of the Se oxoanions, selenite and selenate, from aqueous solutions onto nanosynthesized MnFe 2O 4 was investigated using batch techniques and DRC-ICP-MS spectroscopy. The nanomaterial (NM) was laboratory synthesized through slow titration of a mixture of Fe 2+ and Mn 2+ ions. X-ray diffraction and Scherrer's equation were used to determine the phase of the material and crystallite size, respectively. The effects of pH, reaction time, competitive anions, and the adsorption capacity of the synthesized NM to bind selenite and selenate were investigated. The Langmuir isotherm was used to determine the binding capacity of the NM. Results showed that the phase of the nanomaterial was similar to Jacobsite with a size of 27.5 nm. Results also showed that the sorption of either 100 ppb of selenite or selenate was pH independent in the pH range 2 to 6 and occurred within 5 min of contact time. The introduction of Cl − and NO 3 − anions individually added to solution had no significant effect on the sorption of either selenite or selenate. However, it was found that the addition of SO 4 2 − had a competitive effect only on the sorption of selenate, first seen at 10 ppm and more pronounced at 100 ppm of SO 4 2 −. In the presence of 100 ppm of PO 4 3 −, the adsorption of selenate decreased to 87% while selenite sorption decreased to 20%. From the Langmuir isotherm equation it was determined that the nano-Jacobsite had a selenite and selenate binding capacity of 6573.76 and 769.23 mg Se/kg of NM, respectively.

Adsorptive Removal of Parts per Million Level Selenate Using Iron-Coated GAC Adsorbents

Selenate removal by adsorption using iron-coated granular activated carbons (Fe-GACs) is reported in this study. Adsorption kinetics and equilibrium experiments with initial selenium concentration of 1 mg/L were conducted under three different ionic strengths to study selenate adsorption behavior. Selenate adsorption reached equilibrium within 48 h with more than 85% of the equilibrium capacities being obtained within the first 6 h. High removal efficiency (i.e., >75% ) was achieved for pH range of 2–5. Acid-base titration experiments showed point of zero charge ( pHpzc ) at pH 7.5 for the tested Fe-GAC. Pseudo-second-order kinetic model characterized selenate adsorption kinetics well ( R2 =0.999 ) and the rate constant decreased with ionic strength. Adsorption capacity decreased significantly with increasing ionic strength, which was not observed in selenite adsorption with the same adsorbent. Competitive adsorption with other four oxyanions ( SiO 3 2− , SO 4 2− , PO 4 3− , and CO 3 2− ) showed that sele...

Effective removal of selenate from aqueous solutions by the Friedel phase

This research has demonstrated that the Friedel phase, e.g. a chloride-containing hydrocalumite (Ca 2Al(OH) 6Cl(H 2O) 2· mH 2O), can rapidly adsorb large amounts of SeO 4 2− (up to 1.37 mmol/g). SeO 4 2− is removed via anionic exchange, as evidenced by the expansion of the d-spacing from 0.78 nm of Cl-hydrocalumite to 0.97–0.98 nm of SeO 4-hydrocalumite. The newly formed SeO 4-adsorbed hydrocalumite is stable in water at pH 4–13, indicating the strong fixation of selenate within the phase. In contrast, intercalated selenate in the Freidel phase can be recovered by desorbing in the NaCl solution, which can also regenerate and recycle the used adsorbent. The findings in this research strongly suggest that the Friedel phase is a new, environmentally friendly and cost-effective adsorbent to adsorb selenate from wastewater streams and dilute solutions.

Integration of H2-Based Membrane Biofilm Reactor with RO and NF Membranes for Removal of Chromate and Selenate

A new hybrid process has been proposed and evaluated the feasibility for complete removal of chromate and selenate at high level. The process consists of a H2-based membrane biofilm reactor (MBfR) and reverse osmosis (RO)/nanofiltration (NF) stages. The essential feature of the process is the recycling of the RO and NF concentrate into the membrane biofilm reactor. First, two different H2-based denitrifying MBfR initially reduced selenate (Se (VI)) or chromate (Cr (III)) stably to Se° or Cr (III) to limited levels (approximately 70–85% removal for selenate and approximately 40–65% removal for chromate). In order to achieve more stable and lower levels, two different membrane (NF and RO) filtration technologies as sequential process were combined. Two wastewaters produced from two MBfRs having similar amounts of target toxic ions (C o = 366 μg-Cr L−1 and C o = 326 μg-Se L−1), pH, and conductivity were tested to determine the solute rejection and the membrane flux for one RO and one NF membranes at varying recovery conditions (10–90%). The results show that the rejection of solutes decreases with increasing the recovery due to the increase in osmotic pressure. The rejections by the RO membrane were >99–98% for chromate and 99–94% for selenate, while slightly lower rejections (<20%) were observed for the NF membrane at the recovery conditions.

PH-dependence of selenate removal from liquid phase by reductive Fe(II)–Fe(III) hydroxysulfate compound, green rust

Removal of selenate ion from the liquid phase by hydroxysulfate green rust (GR SO4) was investigated in the pH range from 7.5 to 10.0. Batch tests showed that the total selenium concentration decreased more rapidly with increasing pH. However, the amount of selenium removed from the liquid phase increased as the pH decreased. X-ray absorption spectrometric analysis demonstrated that the Se(VI) in the selenate ion was reduced to elemental selenium Se(0) at pH 9.0, whereas the existence of small amount of intermediate Se(IV) was detected at pH 7.5. Comparing the mass balance of the amount of consumed ferrous iron in GR SO4, [ΔFe(II)], and the amount of Se(VI) removed from the liquid phase, [ΔSe(VI)], [ΔFe(II)] is approximately six times larger than [ΔSe(VI)] in mol units. This is also indirect but convincing evidence that the oxidation of Fe(II) in GR SO4 leads to the simultaneous reduction of Se(VI) to Se(0). Powder X-ray diffraction analysis showed that the end product of GR SO4 depends on the pH: magnetite for pH > 9.0, goethite for pH < 8.0, and their mixture at pH 8.5. These results indicate that the solution pH has a significant effect on the reaction path of selenate removal by GR SO4.

Bioaugmentation of UASB reactors with immobilized Sulfurospirillum barnesii for simultaneous selenate and nitrate removal

Whole-cell immobilization of selenate-respiring Sulfurospirillum barnesii in polyacrylamide gels was investigated to allow the treatment of selenate contaminated (790 µg Se × L−1) synthetic wastewater with a high molar excess of nitrate (1,500 times) and sulfate (200 times). Gel-immobilized S. barnesii cells were used to inoculate a mesophilic (30°C) bioreactor fed with lactate as electron donor at an organic loading rate of 5 g chemical oxygen demand (COD) × L−1 day−1. Selenate was reduced efficiently (>97%) in the nitrate and sulfate fed bioreactor, and a minimal effluent concentration of 39 µg Se × L−1 was obtained. Scanning electron microscopy with energy dispersive X-ray (SEM–EDX) analysis revealed spherical bioprecipitates of ≤2 µm diameter mostly on the gel surface, consisting of selenium with a minor contribution of sulfur. To validate the bioaugmentation success under microbial competition, gel cubes with immobilized S. barnesii cells were added to an Upflow Anaerobic Sludge Bed (UASB) reactor, resulting in earlier selenate (24 hydraulic retention times (HRTs)) and sulfate (44 HRTs) removal and higher nitrate/nitrite removal efficiencies compared to a non-bioaugmented control reactor. S. barnesii was efficiently immobilized inside the UASB bioreactors as the selenate-reducing activity was maintained during long-term operation (58 days), and molecular analysis showed that S. barnesii was present in both the sludge bed and the effluent. This demonstrates that gel immobilization of specialized bacterial strains can supersede wash-out and out-competition of newly introduced strains in continuous bioaugmented systems. Eventually, proliferation of a selenium-respiring specialist occurred in the non-bioaugmented control reactor, resulting in simultaneous nitrate and selenate removal during a later phase of operation.Electronic supplementary materialThe online version of this article (doi:10.1007/s00253-009-1915-x) contains supplementary material, which is available to authorized users.

Biological Alkylation and Colloid Formation of Selenium in Methanogenic UASB Reactors

Bioalkylation and colloid formation of selenium during selenate removal in upflow anaerobic sludge bed (UASB) bioreactors was investigated. The mesophilic (30 degrees C) UASB reactor (pH = 7.0) was operated for 175 d with lactate as electron donor at an organic loading rate of 2 g COD L(-1) d(-1) and a selenium loading rate of 3.16 mg Se L(-1) d(-1). Combining sequential filtration with ion chromatographic analysis for selenium oxyanions and solid phase micro extraction gas chromatography mass spectrometry (SPME-GC-MS) for alkylated selenium compounds allowed to entirely close the selenium mass balance in the liquid phase for most of the UASB operational runtime. Although selenate was removed to more than 98.6% from the liquid phase, a less efficient removal of dissolved selenium was observed due to the presence of dissolved alkylated selenium species (dimethylselenide and dimethyldiselenide) and colloidal selenium particles in the effluent. The alkylated and the colloidal fractions contributed up to 15 and 31%, respectively, to the dissolved selenium concentration. The size fractions of the colloidal dispersion were: 4 to 0.45 mum: up to 21%, 0.45 to 0.2 mum: up to 11%, and particles smaller than 0.2 mum: up to 8%. Particles of 4 to 0.45 mum were formed in the external settler, but did not settle. SEM-EDX analysis showed that microorganisms form these selenium containing colloidal particles extracellularly on their surface. Lowering the temperature by 10 degrees C for 6 h resulted in drastically reduced selenate removal efficiencies (after a delay of 1.5 d), accompanied by the temporary formation of an unknown, soluble, organic selenium species. This study shows that a careful process control is a prerequisite for selenium treatment in UASB bioreactors, as disturbances in the operational conditions induce elevated selenium effluent concentrations by alkylation and colloid formation.