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

Abstract

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 10⁴−10⁷ M⁻¹s⁻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 2.9–4.3 × 10⁻¹ M⁻¹s⁻¹ and 2.6 × 10⁻²–4.5× 10¹ M⁻¹s⁻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 FeO₄ 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 μ g/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.

Keywords:

• Arsenite
• arsenate
• oxidation
• photocatalytic
• ferrate(VI)
• removal

Notes

^aRate constants have a unit of M⁻²s⁻¹.

*Estimated for pseudo–first-order condition of excess oxidant (2 mg/L).

¹for 2 mg Fe in FeO₄ ²⁻.