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 <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 (t1/2 ≥ 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.