Kinetics of Oxidation of Arsenite by Various Manganese Dioxides

Abstract Three Mn dioxides—birnessite, cryptomelane, and pyrolusite—were examined for their ability to deplete the concentration of As(III), a highly toxic pollutant, in solution. The depletion [oxidation of As(III) to As(V) and sorption of As(III)] of As(III) by all three Mn dioxides follows first‐order kinetics. The rate constants for the depletion of As(III) by birnessite and cryptomelane at 298 K are 0.267 and 0.189 h −1 , respectively. On the other hand, the depletion rate of As(III) by pyrolusite is much slower: the rate constant at 298 K is 0.44 × 10 −3 h −1 . This difference in the rate of depletion is largely attributed to the crystallinity and specific surfaces of the Mn dioxides. Pyrolusite is highly ordered and has a low specific surface of 0.8 hm 2 /kg (7.9 m 2 /g); conversely, birnessite and cryptomelane are poorly crystalline and have relatively high specific surfaces of 27.7 and 34.6 hm 2 /kg (277 and 346 m 2 /g), respectively. The energies of activation for the depletion of As(III) by the Mn dioxides range from 26.0 to 32.3 kJ/mol. The reaction appears to be predominantly diffusion‐controlled. The ability of the Mn dioxides to sorb As(III) and As(V) appears to be related to the specific surface and the point‐of‐zero charge of the oxides. The data indicate that, after the systems have reached equilibrium with respect to the sorption of total As, the depletion of As(III) by the oxides is still progressing. This is apparently because of the one‐to‐one relationship between the amount of As(III) depleted and the amount of As(V) appearing in solution.