Kinetics and Mechanistic Aspects of As(III) Oxidation by Aqueous Chlorine, Chloramines, and Ozone:  Relevance to Drinking Water Treatment

Kinetics and mechanisms of As(III) oxidation by free available chlorine (FACthe sum of HOCl and OCl-), ozone (O3), and monochloramine (NH2Cl) were investigated in buffered reagent solutions. Each reaction was found to be first order in oxidant and in As(III), with 1:1 stoichiometry. FAC−As(III) and O3−As(III) reactions were extremely fast, with pH-dependent, apparent second-order rate constants, , of 2.6 (±0.1) × 105 M-1 s-1 and 1.5 (±0.1) × 106 M-1 s-1 at pH 7, whereas the NH2Cl−As(III) reaction was relatively slow ( = 4.3 (±1.7) × 10-1 M-1 s-1 at pH 7). Experiments conducted in real water samples spiked with 50 μg/L As(III) (6.7 × 10-7 M) showed that a 0.1 mg/L Cl2 (1.4 × 10-6 M) dose as FAC was sufficient to achieve depletion of As(III) to 99% depletion of 50 μg/L As(III) within 10 s of dosing 0.25 mg/L O3 (5.2 × 10-6 M) to real waters containing