The kinetics of iodide oxidation by the manganese oxide mineral birnessite

The kinetics of iodide (I⁻) and molecular iodine (I₂) oxidation by the manganese oxide mineral birnessite (δ-MnO₂) was investigated over the pH range 4.5–6.25. I⁻ oxidation to iodate (IO3-)">(IO3-) proceeded as a two-step reaction through an I₂ 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 × 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 I₂, and to a lesser extent, IO3-">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 I₂ and IO3-">IO3-, as well as the biophilic nature of I₂, redox transformations of iodine must be taken into account when predicting I transport in aquifers and watersheds.