The kinetics of iodide oxidation by the manganese oxide mineral birnessite

• Published in: Geochimica et cosmochimica acta 2009-05, Vol.73 (10), p.2850-2861
• Main Authors: Fox, Patricia M., Davis, James A., Luther, George W.
• Format: Article
• Language: English
• Subjects: Freshwater
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2009.02.016

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 ( IO 3 - ) 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, IO 3 - 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 IO 3 - , 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.