Oxidation of Chromium(III) by Free Chlorine in Tap Water during the Chlorination Process Studied by an Improved Solid-Phase Spectrometry

The oxidation of Cr(III) at naturally-occurring concentration levels, i.e. , μg dm −3 or lower levels, by free chlorine during the chlorination process of tap water was studied using an improved solid-phase spectrophotometric method, which can be directly applicable to the specific determination of Cr(VI) at μg dm −3 or lower levels. The effect of the pH on the oxidation kinetics was investigated under three different pH conditions. The results showed that free chlorine oxidized the Cr(III) to Cr(VI), following the pseudo-first-order kinetics with half lifetimes of 3.0, 3.3 and 14.4 h at pH 5.0, 7.0 and 8.0, respectively, if the hypochlorite concentration was maintained at 4 mg Cl dm −3 .