AWWA WQTC64027

Pre-oxidation of arsenite (As(III)) to arsenate (As(V)) is commonly recommended as a means of improving arsenic removal during physicochemical drinking water treatment processes. However, oxidant-scavenging matrix constituents such as dissolved organic matter (DOM) and NH₃/NH₄⁺ which can be present at high concentrations within reduced, As(III)- laden groundwaters (2-4) may impair As(III) oxidation efficiency by competing with As(III) for available oxidant. Quantitative knowledge of the rate constants and mechanisms governing oxidation of As(III) by common drinking water oxidants would greatly facilitate modeling and optimization of As(III) oxidation processes for treatment of such waters. In the present investigation, apparent second-order rate constants, k"_app, were measured for oxidation of As(III) by free available chlorine (FAC - representing bulk HOCl/OCl^-), NH₂Cl, and O₃ within the pH range 2 to 11, to permit evaluation of pH-dependencies for each reaction. Stoichiometries and reaction orders were also measured, to facilitate identification of oxidation mechanisms. Additional experiments were conducted in water samples collected from Lake Zurich, in Switzerland, and from two groundwater treatment facilities in Hanoi, Vietnam, to quantify matrix effects on As(III) oxidation efficiency, and to test the suitability of measured rate constants for modeling As(III) oxidation in real waters. Includes 10 references, tables, figures.