Metal Sorption Effects on Arsenite(III) Oxidation Kinetics at the Birnessite-water Interface

Kirk, Laura E.

MS

University Of Delaware

2003

Arsenic(III) is more toxic and mobile than As(V) in soil and sediment environments, and thus it is advantageous to explore factors that can enhance the oxidation of As(III) to As(V). Previous studies have shown that manganese oxides, such as birnessite, enhance As(III) oxidation. However, these studies did not explore the role that competing cations have on As(III) oxidation. Accordingly, in this research we investigated the effects of Zn2+ and Ca2+ sorption on As(III) oxidation kinetics at the birnessite-water interface using a stirred-batch method and two reaction sequences: (1) metal sorption before As(III) addition, and (2) adding the metal and As(III simultaneously to birnessite suspensions. The effect of pH, initial As(III) concentration, and changes in electrophoretic mobility (EM) were examined. Zinc2+ and Ca2+ sorption onto birnessite was greater at pH 6.0 than at pH 4.5 Zinc2+ and Ca2+ sorption onto birnessite increased as a function of initial aqueous concentration, and increased the EM or birnessite. As the pH increased from 4.5 to 6.0, the As(III) oxidation rate in the absence of Zn2+ and Ca2+ decreased, and less Mn2+ was released to solution. The redox reaction of As(III)/Mn(IV) was not affected by CO2(g). The reaction rate was controlled by increasing As(III) concentrations, which decreased the reaction rates. Zinc2+ decreased the As(III) oxidation rates under all reaction conditions, inhibiting the reaction more when Zn2+ was pre-sorbed to the surface as opposed to added simultaneously with As(III). Zinc2+ inhibited As(III) depletion from solution more at pH 6.0. Calcium2+ decreased as the As(III) oxidation rates, but less than Zn2+. This study provides further understanding of As(III) oxidation kinetics under environmentally applicable conditions where metal oxides and metal cations are both present.