Review of kinetics of chemical and photocatalytical oxidation of Arsenic(III) as influenced by pH

The kinetics and stoichiometry of As(III) oxidation by chemical oxidants is presented. The reactions are first-order with respect to each reactant. The second-order rate constants vary with pH and have a range in magnitude of 10(4)-10(7) M-1 s-1 for free available chlorine (FAC), O3, and FeO4(2-) in a pH range of 6.0-9.0. In this pH range, the reactions of As(III) with chloroamine (NH2Cl) and H2O2 are quite slow with rate constants of 2.9-4.3x10(-1) M-1 s-1 and 2.6x10(-2)-4.5x10(1) M-1 s-1 for chloroamine and H2O2, respectively. The pH dependence of the oxidation reactions can be described using acid-base equilibria of both As(III) species and the oxidant. FAC, O3, and FeO(4)2- oxidize As(III) instantaneously at pH 7.0 with half-lives of milliseconds if 2 mg/L excess dose of oxidant is applied. One major advantage of FeO4(2-) ions over the other oxidants is its ability to remove arsenic in water by two mechanisms; it oxidizes As(III) and also subsequently coagulates As(V) through Fe(III) hydroxide produced from Fe(VI) reduction. Photocatalytic oxidation of As(III) to As(V) follows zero-order kinetics and the oxidation is completed in minutes with no significant pH dependence. The removal of dissolved arsenic to values below the World Health Organization drinking water limit of 10 microg/L can be achieved through photocatalytic oxidation of As(III) to As(V) in acidic solution followed by adsorption of As(V) onto TiO2 surfaces.