The kinetics of iodide (I
−) and molecular iodine (I
2) oxidation by the manganese oxide mineral birnessite (δ-MnO
2) was investigated over the pH range 4.5-6.25. I
− oxidation to iodate 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
+][MnO
2], where
k, the third order rate constant, is equal to 1.08 ± 0.06 × 10
7 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, 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 , 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.
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