title: Kinetics and mechanistic aspects of As(III) oxidation by aqueous chlorine, chloramines, and ozone: relevance to drinking water treatment.
abstract: 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, 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 microg/L As(III) (6.7 x 10(-7) M) showed that a 0.1 mg/L Cl2 (1.4 x 10-6 M) dose as FAC was sufficient to achieve depletion of As(III) to <1 microg/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 Cl2 (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 (t1/2 > or = 4 h) in the presence of NH2Cl formed by the FAC-NH3 reaction. Ozonation was sufficient to yield >99% depletion of 50 microg/L As(III) within 10 s of dosing 0.25 mg/L O3 (5.2 x 10(-6) M) to real waters containing <2 mg-C/L of DOC, while 0.8 mg/L O3 (1.7 x 10(-5) M) was sufficientfor 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.
authors.affiliation: Swiss Federal Institute of Aquatic Science and Technology (EAWAG), 8600 Duebendorf, Switzerland.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Dodd
authors.forename: Ngoc Duy
authors.initials: ND
authors.lastname: Vu
authors.forename: Adrian
authors.initials: A
authors.lastname: Ammann
authors.forename: Van Chieu
authors.initials: VC
authors.lastname: Le
authors.forename: Reinhard
authors.initials: R
authors.lastname: Kissner
authors.forename: Hung Viet
authors.initials: HV
authors.lastname: Pham
authors.forename: The Ha
authors.initials: TH
authors.lastname: Cao
authors.forename: Michael
authors.initials: M
authors.lastname: Berg
authors.forename: Urs
authors.initials: U
authors.lastname: Von Gunten
mesh_terms: D000641:Ammonia / Q000032:analysis / Q000737:chemistry; D001151:Arsenic / Q000737:chemistry* / Q000302:isolation & purification; D002412:Cations; D002700:Chloramines / Q000737:chemistry*; D002713:Chlorine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010084:Oxidation-Reduction; D010126:Ozone / Q000737:chemistry*; D018508:Water Purification / Q000379:methods*; D014881:Water Supply
chemical_list: D002412:Cations; D002700:Chloramines; D002713:Chlorine; D010126:Ozone; D000641:Ammonia; C030816:chloramine; D001151:Arsenic