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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 (O-3), 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 O-3-As(III) reactions were extremely fast, with pH-dependent, apparent second-order rate constants, k"(app), of 2.6 (+/- 0.1) x 10(5) M-1 s(-1) and 1.5 (+/- 0.1) x 10(6) M-1 s(-1) at pH 7, whereas the NH2Cl-As(III) reaction was relatively slow (k "(app)) 4.3 (+/- 1.7) x 10(-1) M-1 s(-1) at pH 7). Experiments conducted in real water samples spiked with 50 mu g/L As(III) (6.7 x 10(-7) M) showed that a 0.1 mg/L Cl-2 (1.4 x 10(-6) M) dose as FAC was sufficient to achieve depletion of As(III) to < 1 mu g/L As(III) within 10 s of oxidant addition to waters containing negligible NH3 concentrations and DOC concentrations < 2 mg-C/L. Even in a water containing 1 mg-N/L (7.1 x 10(-5) M) as NH3, > 75% As(III) oxidation could be achieved within 10 s of dosing 1-2 mg/L Cl-2 (1.4-2.8 x 10(-5) M) as FAC. As(III) residuals remaining in NH3-containing waters 10 s after dosing FAC were slowly oxidized (t(1/2) >= 4 h) in the presence of NH2Cl formed by the FAC-NH3 reaction. Ozonation was sufficient to yield > 99% depletion of 50 mu g/L As(III) within 10 s of dosing 0.25 mg/L O-3 (5.2 x 10(-6) M) to real waters containing < 2 mg-C/L of DOC, while 0.8 mg/LO3 (1.7 x 10(-5) M) was sufficient for a water containing 5.4 mg-C/L of DOC. NH3 had negligible effect on the efficiency of As(III) oxidation by O-3, due to the slow kinetics of the O-3-NH3 reaction at circumneutral pH. Time-resolved measurements of As(III) loss during chlorination and ozonation of real waters were accurately modeled using the rate constants determined in this investigation.

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