The reaction of selenium (IV) with ascorbic acid: its relevance in aqueous and soil systems

Abstract

Abiotic processes able to reduce oxidized Se species may have a strong influence on the environmental behavior of selenium since Se toxicity, bioavailability and mobility follow the order Se(-II)<Se(0)<Se(IV)<Se(VI). Ascorbic acid, which is exudated from plant roots, aquatic macrophytes and phytoplankton and also derives from decomposition of organic matter (e.g. litter, dead roots, algal cells), is present either in soil solution or in the water phase and may interact with Se affecting its speciation and environmental behavior. The rates of the reduction of Se(IV) by ascorbic acid (C(6)H(8)O(6)=H(2)A) were measured in NaCl and NaClO(4) solutions with 1 μM Se(IV) and 100-1000 μM H(2)A as a function of pH, temperature and ionic strength and in the presence of possible interfering metals. The rates of the reaction decreased abruptly with increasing of pH in the range 2-5.5, while slowly at lower pH. The rates showed a small influence of temperature in the range 10-40°C and were independent of ionic strength from 0.01 M to 1M. The values of the second-order rate constant (k) calculated from the values of k(1)/[H(2)A](T) can be determined from the equation: log k = -0.92 × pH - 3368.4/T + 0.24 × I(0.5) + 16.94 for the pH range 2-5.5 (σ=±0.23), from 10 to 40 °C and from 0.01 to 1M ionic strength. The effect of pH and ionic strength on the reaction suggests that the second-order rate expression over the entire pH range investigated can be determined from (H(2)A=C(6)H(8)O(6); HA=C(6)H(7)O(6)(-); H(2)B=H(2)SeO(3); HB=HSeO(3)(-)) [formula, see text] where K(HA), K(A), K(HB), K(B) are the dissociation constants of selenous acid and ascorbic acid, k(H2A-H2B)=5577±78 and k(H2A-HB)=812±102 M(-1)h(-1). The presence of Cu(II), which is a strong catalyzer for the oxidation of H(2)A, decreases the rates of Se(IV) reduction by H(2)A in oxygenated waters. Mn(IV) causes an oxidation of Se(IV) to Se(VI) at high Mn(IV)/H(2)A molar ratios (>0.3), while does not affect significantly the reduction of Se(IV) by H(2)A at low ratios (<0.1). Fe(III) also catalyzes the oxidation of H(2)A but in this case the possible diminution of the reduction rates of Se(IV) by H(2)A are masked by additional processes of adsorption on and coprecipitation by ferric oxyhydroxides, which lower the concentrations of Se(IV).