REACTIONS AT OXIDE SURFACES .2. OXIDATION OF SE(IV) BY SYNTHETIC BIRNESSITE

The rates and mechanisms of the reactions between aqueous Se(IV) and s ynthetic birnessite (delta-MnO2) particles were studied. The experimen tal results at pH 4 and 25 degrees C indicate that the initial disappe arance of Se(IV)(aq) from solution is rapid with a time scale of minut es. The oxidation product Se(VI)(aq) appears in detectable concentrati ons after 12 h of reaction and is slowly produced at a constant rate t hroughout the duration of the experiments (28 days). The extended reac tion period results in most of the product Mn(II) being adsorbed by th e 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 b irnessite 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). I ncreasing the temperature of the particle suspension from 25 to 35 deg rees C has little effect on the rate of disappearance of aqueous Se(IV ) during the initial 100 h, but after this time period, higher tempera tures increase the rate of Se(IV) depletion. Increasing the temperatur e of the suspension does increase the rate and amount of Se(VI) that i s 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 th e electron transfer step, and (iii) the reaction products Se(VI) and M n(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).