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

• Published in: Environmental science & technology Vol. 40; no. 10; pp. 3285 - 3292
• Main Authors: Dodd, Michael C, Vu, Ngoc Duy, Ammann, Adrian, Le, Van Chieu, Kissner, Reinhard, Pham, Hung Viet, Cao, The Ha, Berg, Michael, von Gunten, Urs
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 15.05.2006
• Subjects: Ammonia - analysis; Ammonia - chemistry; Analytical chemistry; Applied sciences; Arsenic - chemistry; Arsenic - isolation & purification; Cations; Chloramines - chemistry; Chlorine; Chlorine - chemistry; Drinking water; Drinking water and swimming-pool water. Desalination; Exact sciences and technology; Hydrogen-Ion Concentration; Kinetics; Oxidation; Oxidation-Reduction; Ozone - chemistry; Pollution; Water Purification - methods; Water Supply; Water treatment; Water treatment and pollution; Drinking water treatment; Chlorination; Chlorine; Tosylchloramide sodium; Disinfection; Ozone; Chemical reaction kinetics; Arsenites; Trace element; Ozonization; Reaction mechanism; Arsenic III; Oxidation
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es0524999

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 <1 μg/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 × 10-5 M) as NH3, >75% As(III) oxidation could be achieved within 10 s of dosing 1−2 mg/L Cl2 (1.4−2.8 × 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 μg/L As(III) within 10 s of dosing 0.25 mg/L O3 (5.2 × 10-6 M) to real waters containing <2 mg-C/L of DOC, while 0.8 mg/L O3 (1.7 × 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 O3, due to the slow kinetics of the O3−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.