Selenium Sorption Research Articles

Sorption of selenite in a multi-component system using the “dialysis membrane” method

79Se is a potentially mobile long-lived fission product, which may make a dominant contribution to the long-term radiation exposure resulting from deep geological disposal of radioactive waste. Its mobility is affected by sorption on minerals. Selenium sorption processes have been studied mainly by considering interaction with a single mineral surface. In the case of multi-component systems (e.g. soils), it is difficult to predict the radioelement behaviour only from the mineral constituents. This study contributes to the understanding of multi-component controls of Se concentrations towards predicting Se behaviour in soils after migration from a disposal site. This goal was approached by measuring selenite sorption on mono and multi-phase systems physically separated by dialysis membranes. To the best of the authors’ knowledge, very few studies have used dialysis membranes to study the sorption competition of selenite between several mineral phases. Other workers have used this method to study the sorption of pesticides on montmorillonite in the presence of dissolved organic matter. Indeed, this method allows measurement of individual Kd in a system composed of several mineral phases. Dialysis membranes allowed (i) determination of the competition of two mineral phases for selenite sorption (ii) and determination of the role of humic acids (HAs) on selenite sorption in oxidising conditions. Experimental results at pH 7.0 show an average Se(IV) sorption distribution coefficient (Kd) of approximately 125 and 9410 L kg−1 for bentonite and goethite, respectively. The average Kd for goethite decreases to 613 L kg−1 or 3215 L kg−1 in the presence of bentonite or HA, respectively. For bentonite, the average Kd decreases slightly in the presence of goethite (60 L kg−1) and remains unchanged in the presence of HA. The experimental data were successfully modelled with a surface complexation model using the PHREEQC geochemical code. The drastic decrease in Se(IV) sorption on goethite in a multi-phase system is attributed to competition with dissolved silica released by bentonite. As with Si the HA compete with Se for sorption sites on goethite.

Sorption and Desorption Processes of Selenium (VI) Using Non-Living Biomasses of Aquatic Weeds in Horizontal Flow

The sorption and desorption processes of Se(VI) onto non-living Eichhornia crassipes (E. crassipes) and Lemna minor (L. minor) were evaluated. Different pH values of the initial Se solution (20 μg L−1) were tested at static conditions. At dynamic conditions of horizontal flow, biomass-packed columns (BPC) were estimated as prepared (pH 4) and unprepared (pH 6–7) and at different flow rates. The desorption process was tested using HCl (0.1 M) as the eluent. The maximum Se uptake took place at a pH of 4 for both biomasses. The lowest flow rate improves major Se removal due to the increase in contact time. The Se was desorbed from the biomass with elution efficiencies of 5 and 18 % for E. crassipes and L. minor, respectively. Nevertheless, more time was needed to increase these efficiencies and reach desaturation times. The breakthrough curves showed that unprepared E. crassipes and L. minor BPC at horizontal flow, with a flow rate of 6 and 4 mL min−1 respectively, had a biomass removal capacity of 0.135 and 0.743 μg g−1 correspondingly. The system of E. crassipes is more efficient, suggesting an ion exchange sorption mechanism. This demonstrates that non-living E. crassipes and L. minor have the capacity to remove Se from very dilute solutions.

Sorption of selenium(IV) and selenium(VI) onto synthetic pyrite (FeS2): Spectroscopic and microscopic analyses

Pyrite was hydrothermally synthesized and used to remove Se(IV) and Se(VI) selectively from solution. Surface analyses of pyrite before and after contact with Se(IV) and Se(VI) were conducted using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). All solid samples were acquired by allowing 3.1mmol/L of Se(IV) or Se(VI) to react with 1g/L of pyrite for 1, 15, or 30days. The XPS spectra were fitted using the XPSPEAK program that applies a Gaussian Lorentzian function. The fitted spectra indicate that Se(IV) more strongly reacts with the surface-bound S than with the surface-bound Fe of pyrite. However, there is no apparent evidence of surface reaction with Se(VI). Specifically, fitted XPS spectra showed the presence of sulfide and tetrathionate on the surface, indicating that sulfur (S22-) at the surface of pyrite can be both oxidized and reduced after contact with Se(IV). This occurs via surface disproportionation, possibly resulting in the formation of surface precipitates. Evidence for the formation of precipitates was seen in SEM and AFM images that showed rod-like particles and a phase image with higher voltage. In contrast, there were no important changes in the pyrite after contact with Se(VI) over a period of 30days.

XPS analysis of sorption of selenium(IV) and selenium(VI) to mackinawite (FeS)

The Se 3d XPS spectra showed evidence of reduction of Se(IV) by contact with the mackinawite surface when certain assumptions were made, but there was no clear evidence for reduction of Se(VI). A continuous oxidation of surface Fe(II) was observed for FeS in contact with Se(IV), as indicated by a decrease of intensities of Fe(II)‐S XPS peaks. S 2p spectra showed some evidence of oxidation of surface S(‐II) associated with Se reduction that resulted in formation of polysulfides. However, the apparent change in S 2p spectra could be the result of overlap between S 2p and Se 3p peaks. FeS particles became red and attached to the wall of reaction vessels within 3–4 days after contact with Se(IV), suggesting that a new solid‐phase, possibly Se(0), may have been produced. The low signal‐to‐noise of the S 2p peaks observed after 15 and 30 days contact with Se indicates increased amounts of sulfur in solution. This supports the existence of a reaction of Se with FeS in which discrete precipitates are formed and sulfide is released to solution. These observations support the conclusion that Se(IV) sorbs onto the surface of FeS and oxidizes surface species of iron and sulfur. © 2011 American Institute of Chemical Engineers Environ Prog, 32: 84–93, 2013.

Infrared spectroscopy study of the sorption of selenium(IV) on natural zeolites

The sorption of selenium(IV) ions on single crystals of natural shabazite, analcime, mesolite, stilbite, and on clinoptilolite- and mordenite-containing tuffs in dependence on the concentration and pH of a solution of sodium selenite was studied by infrared spectroscopy. It was assumed that sorption on clinoptilolite and mordenite tuffs from a 0.1 M solution of sodium selenite with pH 9 occurs in the form of selenite and pyroselenite anions forming a hydrogen bond with the zeolite water molecules. It was established that water molecules in analcime, unlike stilbite, shabazite and mesolite, also form hydrogen bonds with the selenite ion in an alkaline medium. No hydrogen bonds are formed in diluted solutions or an acid medium. The partial desilylation and dealumination of zeolites in alkaline and acid media respectively, were observed.

Sorption of selenium(VI) onto anatase: Macroscopic and microscopic characterization

The sorption of selenium(VI) onto pure anatase, a polymorph of titanium dioxide, was investigated. At the macroscopic level, batch experiments and electrophoretic mobility measurements were performed. Selenium(VI) retention was found to be pH-dependent, i.e. sorption of selenium(VI) decreases with increasing pH (pH range 3.5–11). Selenium(VI) sorption dependence on the ionic strength was also evidenced, i.e. sorption increases while the ionic strength decreases. Electrophoretic mobility measurements showed that selenium(VI) sorption had no effect on the isoelectric point of anatase. At the microscopic level, XPS (X-ray Photoelectron Spectroscopy) measurements evidenced the absence of reduction of selenium(VI) during the sorption process. Furthermore, the nature of the sorbed surface species at the anatase/liquid interface was elucidated using Attenuated Total Reflection Fourier-Transform Infrared (ATR FT-IR) spectroscopy. The spectroscopic results strongly suggested the formation of outer-sphere complexes on the whole pH range, in agreement with batch sorption experiments and electrophoretic mobility findings.

Sorption of selenium(IV) and selenium(VI) to mackinawite (FeS): Effect of contact time, extent of removal, sorption envelopes

Higher concentrations (127, 253μM) of Se(IV) at pH 8 were completely removed by 0.5g/L FeS within 120min. Removal of Se(VI) by FeS at pH 8 was less extensive than removal of Se(IV). Only 10% of the Se(VI) was removed by 1g/L FeS within 1h. Removal patterns for Se by FeS depend on pH. Removal patterns of Se at pH 7 and pH 8 were best described by BET models for Se(IV) and Freundlich models for Se(VI), while removal patterns of both at pH 9 and 10 were best described by Langmuir models. Sulfate at 1 and 10mM had negligible effect on removal of Se(IV) by FeS, while sulfate had little effect on removal of Se(VI) by FeS, but there was some indication that sulfate promoted removal of Se(VI) at intermediate concentrations. The test for the effect of pH on sorption of Se(IV) by FeS showed nearly complete removal at all but the high initial pH. When pH was raised back to initial value, greater removals were observed than initially. Mixtures of Se(VI) and FeS showed moderate removal at low pH, a minimum removal near pH 6 and nearly complete removal at high pH. Very high stability was observed with negligible release as pH decreased.

Evaluation of sorption and diffusion behavior of selenium in crushed granite by through-diffusion column tests

Distribution coefficients (Kd), apparent diffusion coefficients (Da) and retardation factor (Rf) in this work obtained by batch and through-diffusion experiments have been performed, respectively. The accumulative concentration method developed by Crank (The mathematics of diffusion, 12) was applied to realize apparent and effective diffusion coefficient (Da and De) of Se. Besides, a non-reactive radionuclide, HTO, was initially conducted in through-diffusion experiment for assessing the ability of radionuclide retardation. The distribution coefficients (Kd) obtained by batch tests in 14 days under aerobic and anaerobic systems were 6.98 ± 0.35 and 5.21 ± 0.25 mL/g. Moreover, Rfcal and Kdcal of Se obtained from accumulative concentration’s method in through-diffusion test showed an obvious discrepancy with the increase of length/diameter (L/D) ratio. However, it presented an agreement of RfH/Se and KdH/Se in a various L/D ratio by comparison of apparent diffusion coefficient’s (Da) between HTO and Se. It appears that the RfH/Se and KdH/Se obtained from the through-diffusion experiments are lower than those derived from the batch experiments. Therefore, it demonstrates that reliable Rf and Kd of Se by through-diffusion experiments could be achieved at a non-reactive radiotracer (HTO) prior to tests and will be more confident in long-term performance assessment of disposal repository.

Synthesis of a Mg-Cd-Al layered double hydroxide and sorption of selenium

AbstractLeaching experiments with metallic uranium-aluminum research-reactor fuel elements in repository-relevant MgCl2-rich salt brine (brine 2) were performed. A Mg-Al layered double hydroxide (LDH) with chloride as the interlayer anion was identified as a crystalline secondary-phase component. In the present study, the incorporation of Cd into the structure of the Mg-Al-Cl LDH was investigated. Synthesis by a coprecipitation method was performed and the Mg-Cd-Al-Cl LDH obtained was characterized. The sorption behavior of selenium on the LDH was investigated in water, clay pore-water (Mont-Terri-type), and brine 2. Using a LDH concentration of 10 g/L, the sorption kinetics were rapid and equilibrium was reached within 12 h. The sorption of selenium decreased with increasing amount of chloride anions in the solutions. The chloride anions acted as competing anions for the sorption of selenium. The effect of pH on selenium sorption was investigated and a large buffer capacity of the LDH was observed. For the range of selenium concentration used, linear sorption isotherms were obtained which obeyed the Freundlich and Dubinin-Radushkevich models. From these, the energies of selenium sorption were calculated to be in the range of ion-exchange processes.

The role of uranium peroxide studtite on the retention of Cs, Sr and Se(VI)

AbstractThe formation of uranyl secondary solid phases onto the spent nuclear fuel surface might influence the radionuclide concentration in solution via, among others, sorption processes. In this work, the incorporation of some radionuclides onto the uranium peroxide studtite, UO2O2·4H2O, has been tested.The study was carried out in batch experiments where a known amount of studtite (0.05 g) was put in contact with 20 cm3 of radionuclide solution. Once equilibrium was reached, radionuclide concentrations in solution were determined by ICP-MS. The radionuclide amount attached to the solid was calculated from the mass balance. The S/V values of the experiments were also determined from BET specific solid surface area measurements.In this work, data on sorption of caesium, strontium, and selenium as a function of pH are presented. The behaviour of caesium and strontium are similar: a relatively high amount of radionuclide is sorbed at neutral to alkaline pH while there is almost no sorption at acidic pH. On the other hand, in the case of selenium, the sorption maximum occurs at acidic pH and there is almost no sorption at alkaline pH. The different behavior of the radionuclides is related to the element speciation in solution and the surface charge of the solid. Strontium and caesium are sorbed at alkaline pH because they are positively charged in solution and the surface of the studtite is negatively charged (>O- groups) while selenium(VI) sorbs at acidic pH because the surface of the studtite is positively charged, and the predominant selenium(VI) species in solution is anionic.These preliminary data indicate that the sorption capacity of uranyl secondary solid phases such as studtite is an important process to be considered when establishing the migration of different radionuclides released from spent nuclear fuel.

Competition between selenium (IV) and silicic acid on the hematite surface

Competition between selenium (IV) and silicic acid for the hematite (α-Fe 2O 3) surface has been studied during this work. Single batch experiments have been performed to study separately the sorption of selenium (IV) and silicic acid as a function of the pH. With the help of the 2-pK surface complexation model, experimental data have been fitted using the FITEQL 4.0 program. Two monodentate inner-sphere surface complexes have been used to fit selenite ions retention, FeSeO 3 - and FeHSeO 3. In order to fit sorption of silicic acid, the two following surface complexes, namely FeH 3SiO 4, and FeH 2 SiO 4 - , have been used. Using the surface complexation constants coming from these two binary systems, prediction curves of the effect of silicic acid on the retention of selenium (IV) onto hematite have been obtained. Finally, performed experiments showed a competition between selenium (IV) and silicic acid for the surface sites of hematite. Experimental data matched DDLM predictions, confirming the ability of the surface complexation model to predict quantitatively and qualitatively the ternary system selenium (IV)/H 4SiO 4/hematite.

Sorption of selenium on Mg-Al and Mg-Al-Eu layered double hydroxides

Abstract Salt domes represent deep geological formations which are under consideration as final repositories for irradiated research reactor fuel elements. For long-term safety aspects the mobilisation of the radionuclides due to a water ingress is intensively investigated. At the Institute of Energy Research (IEF-6), leaching experiments were performed in a hot cell facility with UAl x -Al and U3Si2-Al dispersed research reactor fuel elements in repository-relevant MgCl2-rich salt brines under anaerobic conditions. The fuel plates corroded completely within one year and a Mg-Al-layered double hydroxide (LDH) with chloride as interlayer anion was identified as one crystalline phase component of the corrosion products (secondary phases). This Mg-Al-LDH was synthesized, characterized, and the ability to retard europium by an incorporation process was investigated. Europium, as a representative for lanthanides, was identified to be one of the radionuclides which were found in the corrosion products. We could show that europium was incorporated in the lattice structure. LDHs have high anion exchange capacities that enhance their potential to remove anionic contaminants from aqueous systems. In this work the sorption behaviour of selenium in the chemical form as selenite (SeO3 2−) on Mg-Al-LDH and on Mg-Al-Eu-LDH was investigated. Especially the influence of the larger europium-III ion was of interest. It represents in the Mg-Al-Eu-LDH about 10% of the molar aluminium amount. The sorption has been experimentally studied in a wide range of pH, ionic strength, radionuclide and sorbent concentration. Both LDHs with chloride as interlayer anion were synthesized by a coprecipitation method under controlled conditions, and their main physico-chemical properties were analyzed prior to the sorption experiments. The sorption kinetics of selenite on the LDHs in water and in MgCl2-rich brine were rapid using a LDH concentration of 10 g/L. Equilibrium, indicated by stable pH values, was obtained within 5 h for an initial selenite concentration of 4.28×10−13 mol/L. Selenite in water sorbed quantitatively on both LDHs. In MgCl2-rich brine a different behaviour to water was observed. The sorption was not quantitatively, due to the presence of high amounts of chloride anions. Chloride, as competing anion, affected the sorption behaviour of selenite. Using a LDH concentration of 0.1 g/L, the sorption of selenite was negligible. The effect of pH on selenite sorption was investigated, too. The sorption was relatively constant when the initial pH values were adjusted between 4 and 8, because both LDHs exhibit a high pH buffering capacity. For a selenite concentration range between 5.43×10−14 and 3.24×10−12 mol/L sorption isotherms were obtained and the data could be fitted to Freundlich and Dubinin–Radushkevich equations. The sorption capacities of the LDHs and the energies of sorption were determined. The sorption of selenite occurred via an ion exchange mechanism and the sorption behaviour was similar for both LDHs.

Precipitate coating on cellulose fibre as sorption medium for selenium preconcentration and speciation with hydride generation atomic fluorescence spectrometry

Lanthanum hydroxide precipitate is for the first time coated onto cellulose fibre and serves as a novel sorption medium for separation and speciation of inorganic selenium. A micro-column packed with precipitate-layer-coated cellulose fibre is incorporated into a sequential injection system for selenite retention from a neutral aqueous solution, which is afterwards stripped with a NaBH 4–NaOH solution as eluent. The hydride generation is actuated by merging the eluate and hydrochloric acid downstream, followed by the detection with atomic fluorescence spectrometry. Total inorganic selenium is derived by pre-reduction of selenate and speciation is estimated by difference. The coated precipitate layer can be used for 150 runs for selenium sorption, offering a clear advantage over the conventional precipitation protocols where a large amount of precipitate is dissolved into a small volume of eluent which might interfere with the detection. With a sample volume of 1.0 mL, an enrichment factor of 9.7 and a detection limit of 9 ng L −1 are obtained in a linear range of 0.05–2.5 μg L −1. A sampling frequency of 24 h −1 is achieved along with a R.S.D. of 1.7% at 0.5 μg L −1 Se(IV). The procedure is validated by analyzing selenium in a reference material GBW 10010 (rice) and a human hair sample. It is further demonstrated by speciation of inorganic selenium in surface water samples by pre-reduction of selenate.

Sorption of selenium(IV) onto magnetite in the presence of silicic acid

Sorption of selenium(IV) and silicic acid onto magnetite (Fe 3O 4) was investigated in binary systems, with concentrations of silicic acid under the solubility limit of amorphous silica. Using the double diffuse layer model (DDLM), surface complexation constants of selenium(IV) and H 4SiO 4 onto magnetite were extracted using Fiteql 4.0. Then, prediction curves of the sorption of selenium(IV) in the presence of silicic acid onto magnetite were obtained, using the calculated surface complexation constants. Finally, laboratory experiments were performed and showed a competition between selenium(IV) and silicic acid for the surface sites of magnetite. Experimental results matched the model predictions, confirming its ability to model qualitatively and quantitatively the ternary system.

Enhanced Antioxidant Formula Based on a Selenium‐Supplemented Carotenoid‐Producing Yeast Biomass

Carotenoid-producing yeast species such as Rhodotorula glutinis and Sporobolomyces roseus efficiently accumulated selenium from the growth medium. It was observed that incorporation of selenium into yeast cells during the growth inhibited production of beta-carotenoid and other carotenoid precursors (torularhodin and torulene). The yeasts with high content of the carotenoid pigments and selenium may be used for the preparation of a new type of antioxidant formula that could be directly applied for various human and animal diets. We have demonstrated that such a formula can only be produced by separate processes of the cultivation of red yeasts and a subsequent sorption of selenium into the cells.

Sorption behavior of selenium on humic acid under increasing selenium concentration or increasing solid/liquid ratio

The sorption of selenium (Se) on humic acid (HA) was investigated in order to better understand the fate of stable and radioactive Se in soils and sediments. An ultrafiltration technique was used to determine size distributions of HA-sorbed-Se when increasing Se concentration and solid/liquid ratio. The results showed that the Se sorption onto HA followed the Freundlich isotherm. No solid/liquid ratio-dependence was observed especially when <3 kDa molecular size fraction was used from solid/liquid separation. The Freundlich isotherm parameters K F and n obtained using the <3 kDa molecular size fraction for solid/liquid separation were 3.7 × 10 2 and 0.82, respectively. In addition, since dissolved HA increased with decreased ionic strength in the HA suspension, ionic strength could promote aggregation of HA. The conformational change of HA could affect the sorption behavior of Se on HA.

Anaerobic and aerobic sorption of cesium and selenium on mudrock

This study is focused on sorption and determination of distribution ratios (Rd) of cesium and selenium on mudrock, which is the potential host rock for waste disposal in Taiwan. Batch tests including sorption kinetic and isotherms tests have been performed in synthetic groundwater at aerobic and anaerobic conditions which might be found in the deep geologic environment. It is found that Rd for sorption of cesium did not have an obvious difference in both conditions with various contact time. However, Rd in anaerobic condition for sorption of selenium was greater than that in aerobic condition. Selenium is a redox sensitive element and its solubility in reducing conditions is controlled by the formation of metallic or precipitable selenium. It demonstrated variation of Rd with time in both conditions for Se sorption kinetic experiments was equal (10 ml/g) and indicated a part formation (10%) of precipitable selenium (Se 0 , FeSe or FeSe 2 ) in the solution. Moreover, it was not enough to form precipitable selenium completely in reducing condition as to insufficient experimental period (2 weeks) and in the presence of Fe 2+ . The Freundlich and Langmuir isotherm for the concentration ranges (i.e., 10 –3 –10 –7 M) conducted in both conditions seem to be adequate to quantitatively describe the sorption of cesium and selenium, respectively.

Sorption kinetics of selenium on humic acid

This study investigated selenium sorption kinetics on humic acid (HA) as a function of initial Se concentration (10–300 μg·1−1) and solid/liquid ratio (0.01–0.1). From the result, it was clear that the Se sorption kinetics on HA could be expressed by a pseudo-second order equation. This was possible because the sorption mechanism on HA is a multiple sorption process including specific and non specific mechanisms. Additionally, the 3-D empirical equation for the amount of sorbed Se could be determined as a function of the initial Se concentration and solid/liquid ratio.

Interactions of Aqueous Selenium (−II) and (IV) with Metallic Sulfide Surfaces

The present study was initiated to determine the capacity of sulfide minerals (pyrite FeS2 and chalcopyrite CuFeS2) to delay the migration of inorganic selenium species in geological formations. Interactions between Se(IV) and Se(-II) and synthetic and natural sulfide minerals were investigated under anoxic conditions using the batch method. Significant sorption of selenium occurred under acidic conditions. Analysis of the solids after Se sorption using X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) demonstrated the presence of reduced selenium species on the metallic sulfide surfaces, thus suggesting an oxido-reduction process coupled with sorption. Selenium reduction occurred concomitantly with the oxidation of pyritic sulfur, whereas metallic species (Fe, Cu) were not involved in the redox process. Formation of ferroselite (FeSe2) was postulated to take place on the synthetic solid while surface complexation or ionic exchange processes were more likely on the natural solids.

Removal of Se(IV) from aqueous solution using sulphuric acid-treated peanut shell

A carbonaceous sorbent was prepared from peanut shell via sulphuric acid treatment. Se(IV) removal from aqueous solution on the sorbent was studied varying time, pH, Se(IV) concentration, temperature and sorbent status (wet and dry). Se(IV) removal was faster using the wet sorbent than the dry sorbent following a pseudo-first-order model. Se(IV) removal increases at low pH values, and decreases as pH increases until pH 7. Sorption was found to fit the Langmuir equation and sorption capacity for the wet sorbent was higher than that for the dry one. Both sorbents showed an increased selenium sorption by rising the temperature. Redox processes between Se(IV) and the carbon sorbent are involved. Analysis by scanning electron microscope and X-ray powder diffraction for the sorbent after the reaction with acidified Se(IV) confirmed the availability of elemental selenium as particles on the sorbent surface as a result of Se(IV) reduction. Physicochemical tests showed an increase in sorbent acidity, cation exchange capacity (CEC) and surface functionality after the reaction with acidified Se(IV), indicating the oxidation processes occurring on the sorbent surface. Due to its reduction properties, the sorbent seems efficient for Se(IV) removal from aqueous solution.