Aqueous Se Research Articles

Reactions at Oxide Surfaces. 2. Oxidation of Se(IV) by Synthetic Birnessite

The rates and mechanisms of the reactions between aqueous Se(IV) and synthetic birnessite (δ-MnO2) particles were studied. The experimental results at pH 4 and 25 °C indicate that the initial disappearance of Se(IV)(aq) from solution is rapid with a time scale of minutes. The oxidation product Se(VI)(aq) appears in detectable concentrations after 12 h of reaction and is slowly produced at a constant rate throughout the duration of the experiments (28 days). The extended reaction period results in most of the product Mn(II) being adsorbed by the oxide surface. The lack of oxidation of Se(IV)(aq) in an oxygenated homogeneous solution and the appearance of Se(VI)(aq) in an O_2-free birnessite suspension is evidence for the redox reaction between Se(IV) and Mn(IV). Increasing the pH of the particle suspension from 4 to 7 decreases the initial uptake of Se(IV) and the production of Se(VI). Increasing the temperature of the particle suspension from 25 to 35 °C has little effect on the rate of disappearance of aqueous Se(IV) during the initial 100 h, but after this time period, higher temperatures increase the rate of Se(IV) depletion. Increasing the temperature of the suspension does increase the rate and amount of Se(VI) that is released into solution. These observations suggest the following: (i) birnessite directly oxidizes Se(IV) through a surface mechanism, (ii) the rate-limiting step in the production of Se(VI) is most likely the electron transfer step, and (iii) the reaction products Se(VI) and Mn(II) are released by different steps. The rates and energetics of Se(IV) oxidation by inorganic redox reactions with manganese dioxide are compared to those previously reported for As(III) and Cr(III).

The solubility product of crystalline ferric selenite hexahydrate and the complexation constant of FeSeO 3 +

The aqueous solubility of Fe2(SeO3)3·6H2O(c) was studied in deionized water adjusted to a range in pH values from 0.77 to 5.1 and in Na2SeO3 solutions ranging in concentrations from 0.0002 to 0.02 mol-dm−3. The studies were conducted from both the undersaturation and oversaturation directions, with equilibration periods ranging from 7 to 1725 days. Stoichiometric dissolution of the solid was observed in solutions with pH values up to nearly 4. In general, concentrations of both Se and Fe decreased as pH increased from 1 to 4. Analyses of the equilibrated suspensions confirmed the equilibrium solid to be Fe2(SeO3)3·6H2O(c) and the aqueous Se to be selenite. Pitzer's ion-interaction model was used with selected ion pairs to interpret the solubility data. The logarithm of the solubility product of ferric selenite $$Fe_2 (SeO_3 )_3 .6H_2 O(c) \begin{array}{*{20}c} \to \\ \leftarrow \\ \end{array} 2Fe^{3 + } + 3SeO_3^{2 - } + 6H_2 O$$ was found to be −41.58±0.11. This value is less than any reported in the literature for a ferric selenite by more than 10 orders of magnitude. The solubility data and calculations show an extremely strong interaction between aqueous Fe3+ and SeO32−; interpretation of these data requires the inclusion of FeSeO3+ i.e. $$Fe^{3 + } + SeO_3^{2 - } \begin{array}{*{20}c} \to \\ \leftarrow \\ \end{array} FeSeO_3^ + , log K = 11.15 \pm 0.11$$

Structural studies of selenate incorporation into calcite crystals

The mobility and concentration of aqueous Se species in soil water and ground water are strongly affected by coprecipitation reactions with minerals such as calcite, CaCO 3. Here we present the results of an XAFS investigation to determine how the tetrahedral SeO 4 2− species is accommodated in the bulk calcite structure. Our results show that the SeO 4 2− substitutes for trigonal CO 3 2− groups in octahedral sites.

A combined voltammetry and electrochemical quartz crystal microgravimetry study of the reduction of aqueous Se(IV) at gold

The reduction of Se(IV) in 0.5 M Na 2SO 4 is complex and does not proceed via the Se(IV) → Se(0) → Se(II-) scheme considered by most previous authors. A combined use of voltammetry and electrochemical quartz crystal microgravimetry techniques shows that the direct 6e − Se(IV) → Se(II-) reduction pathway competes with the initial four-electron process. Coupling with a subsequent (fast) chemical reaction between Se(II-) and Se(IV) results in the further deposition of Se(0) at the electrode surface. The electrochemical behavior at more negative potentials reflects a complex interplay of the four-electron and six-electron reduction processes and the chemical reaction along with the effect of Se(II-) ions stripped from the initial selenium layer. Thus the delicate balance between these is influenced by two variables, namely the potential and the Se(II-) concentration in the electrolyte. Further data in support of our mechanistic scheme are furnished by measurements with a Au disk electrode oriented “face-down” in the cell as well as by hydrodynamic voltammetry experiments.

Groundwater contamination at the Kesterson Reservoir, California: 2. Geochemical parameters influencing selenium mobility

Transport of selenium in groundwater at the Kesterson Reservoir in the Central Valley of California was strongly retarded because of chemical reduction and precipitation mediated by microbial activity. Under such conditions, negative correlations were documented between aqueous Se and Fe2+, Mn, and H2S. Locally, the presence of oxidizing species, notably O2 and NO3, suppressed this reduction, permitting Se mobilization in the shallow aquifer. Selenate, the dominant and most oxidized form of Se, was in electrochemical disequilibrium with subordinate concentrations of selenite. Normally slow inorganic reduction rates were accelerated by microbial activity which utilizes oxidized chemical species including selenate as electron donors during the oxidation of organic matter. Two stratified redox barriers to selenium migration were documented beneath Kesterson: an underlying shallow anoxic zone underlying most of the pond bottom, characterized by high organic content and sulfate reduction, and a deeper dynamic front established by localized O2 infiltration from the overlying ponds and Fe2+ release from aquifer materials. The reducing nature of this deeper aquifer ultimately precludes Se transport to regional groundwater.