Oxidation of Cr(III)–Fe(III) Mixed-Phase Hydroxidesby Chlorine: Implications on the Control of Hexavalent Chromium inDrinking Water

摘要

The occurrence of chromium (Cr) as an inorganic contaminant in drinking water is widely reported. One source of Cr is its accumulation in iron-containing corrosion scales of drinking water distribution systems as Cr(III)–Fe(III) hydroxide, that is, FexCr(1–x)(OH)3(s), where x represents the Fe(III) molar content and typically varies between 0.25 and 0.75. This study investigated the kinetics of inadvertent hexavalent chromium Cr(VI) formation via the oxidation of FexCr(1–x)(OH)3(s) by chlorine as a residual disinfectant in drinking water, and examined the impacts of Fe(III) content and drinking water chemical parameters including pH, bromide and bicarbonate on the rate of Cr(VI) formation. Data showed that an increase in Fe(III) molar content resulted in a significant decrease in the stoichiometric Cr(VI) yield and the rate of Cr(VI) formation, mainly due to chlorine decay induced by Fe(III) surface sites. An increase in bicarbonate enhanced the rate of Cr(VI) formation, likely due to the formation of Fe(III)-carbonato surface complexes that slowed down the scavenging reaction with chlorine. The presence of bromide significantly acceleratedthe oxidation of FexCr(1–x)(OH)3(s) by chlorine, resulting fromthe catalytic effect of bromide acting as an electron shuttle. A highersolution pH between 6 and 8.5 slowed down the oxidation of Cr(III)by chlorine. These findings suggested that the oxidative conversionof chromium-containing iron corrosion products in drinking water distributionsystems can lead to the occurrence of Cr(VI) at the tap, and the abundanceof iron, and a careful control of pH, bicarbonate and bromide levelscan assist the control of Cr(VI) formation.