Kinetics and mechanistic aspects of As(III) oxidation by aqueous chlorine, chloramines, and ozone: Relevance to drinking water treatment Dodd M.C., Vu N.D., Ammann A., Le V.C., Kissner R., Pham H.V., Cao T.H., Berg M., Von Gunten U. Swiss Federal Institute of Aquatic Science and Technology (EAWAG), 8600 Duebendorf, Switzerland; Center for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, Nguyen Trai Street 334, Hanoi, Viet Nam; Laboratory of Inorganic Chemistry, ETH Zurich, 8093 Zurich, Switzerland Abstract: Kinetics and mechanisms of As(III) oxidation by free available chlorine (FAC-the sum of HOCl and OCl - ), ozone (O 3 ), and monochloramine (NH 2 Cl) 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) × 10 5 M -1 s -1 and 1.5 (±0.1) × 10 6 M -1 s -1 at pH 7, whereas the NH 2 Cl-As(III) reaction was relatively slow (k″ app = 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 Cl 2 (1.4 × 10 -6 M) dose as FAC was sufficient to achieve depletion of As(III) to <1 μg/L As(III) within 10 s of oxidant addition to waters containing negligible NH 3 concentrations and DOC concentrations <2 mg-C/L. Even in a water containing 1 mg-N/L (7.1 × 10 -5 M) as NH 3 , >75% As(III) oxidation could be achieved within 10 s of dosing 1-2 mg/L Cl 2 (1.4-2.8 × 10 -5 M) as FAC. As(III) residuals remaining in NH 3 -containing waters 10 s after dosing FAC were slowly oxidized (t 1/2 ≥ 4 h) in the presence of NH 2 Cl formed by the FAC-NH 3 reaction. Ozonation was sufficient to yield >99% depletion of 50 μg/L As(III) within 10 s of dosing 0.25 mg/L O 3 (5.2 × 10 -6 M) to real waters containing <2 mg-C/L of DOC, while 0.8 mg/L O 3 (1.7 × 10 -5 M) was sufficient for a water containing 5.4 mg-C/L of DOC. NH 3 had negligible effect on the efficiency of As(III) oxidation by O 3 , due to the slow kinetics of the O 3 -NH 3 reaction at circumneutral pH. Time-resolved measurements of As(III) loss during chlorination. © 2006 American Chemical Society. Index Keywords: Concentration (process); Oxidation; Ozone; pH effects; Reaction kinetics; Stoichiometry; Water treatment; Buffered reagent solutions; DOC concentrations; Time resolved measurements; Arsenic; ammonia; arsenic; chloramine derivative; chlorine; drinking water; ozone; Arsenic; Concentration (process); Oxidation; Ozone; pH effects; Reaction kinetics; Stoichiometry; Water treatment; arsenic; chlorine; drinking water; oxidation; ozone; water treatment; aqueous solution; article; chlorination; concentration (parameters); kinetics; molecular model; oxidation; ozonation; pH measurement; stoichiometry; water treatment; Ammonia; Arsenic; Cations; Chloramines; Chlorine; Hydrogen-Ion Concentration; Kinetics; Oxidation- Reduction; Ozone; Water Purification; Water Supply Year: 2006 Source title: Environmental Science and Technology Volume: 40 Issue: 10