The kinetics of iodide oxidation by the manganese oxide mineral birnessite

The kinetics of iodide (I-) and molecular iodine (I-2) oxidation by the manganese oxide mineral birnessite (delta-MnO2) was investigated over the pH range 4.5-6.25. I- oxidation to iodate (IO3-) proceeded as a two-step reaction through an I-2 intermediate. The rate of the reaction varied with both pH and birnessite concentration, with faster oxidation occurring at lower pH and higher birnessite concentration. The disappearance of I- from solution was first order with respect to I- concentration, pH, and birnessite concentration, such that -d[I-]/dt = k[I-][H+][MnO2], where k, the third order rate constant, is equal to 1.08 +/- 0.06 x 107 M-2 h(-1). The data are consistent with the formation of an inner sphere I- surface complex as the first step of the reaction, and the adsorption of I- exhibited significant pH dependence. Both I-2, and to a lesser extent, IO3- sorbed to birnessite. The results indicate that iodine transport in mildly acidic groundwater systems may not be conservative. Because of the higher adsorption of the oxidized I species I-2 and IO3-, as well as the biophilic nature of I-2, redox transformations of iodine must be taken into account when predicting I transport in aquifers and watersheds. Published by Elsevier Ltd.