Concentration (process) Ozone Oxidation pH effects Water Purification Water Supply Hydrogen-Ion Concentration Kinetics pH measurement stoichiometry
Issue Date:
2006
Publisher:
Environmental Science and Technology
Citation:
Volume: 40, Issue: 10, Page : 3285-3292
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) ?? 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 (k??app = 4.3 (?1.7) ?? 10-1 M-1 s-1 at pH 7). Experiments conducted in real water samples
spiked with 50 ?g/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 ?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 ?? 10-5 M) as NH 3, >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 NH 2Cl formed by the FAC-NH3 reaction.
Ozonation was sufficient to yield >99% depletion of 50 ?g/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. ?? 2006 American Chemical Society.