Release and Control Strategies of Hexavalent Chromium in Drinking Water Distribution Systems

The occurrence of hexavalent chromium Cr(VI) as a carcinogen in drinking water is widely reported. Cr can accumulate in corrosion scales in drinking water distribution systems (DWDSs) and act as the potential source for Cr(VI) release. The goal of this dissertation is to elucidate the mechanisms of Cr(VI) formation through the oxidation of Cr solids by disinfectant free chlorine, and to provide control strategies for Cr(VI) formation in DWDSs. First, X-Ray Absorption Spectroscopy (XAS) analysis discovered that zerovalent Cr(0) coexisted with trivalent Cr(III) in the corrosion scales. The oxidation of the corrosion scales by residual disinfectant chlorine released Cr(VI) and exhibited a three-phase kinetics behavior: an initial 2-hour fast reaction phase, a subsequent 2-to-12-hour transitional phase and a final 7-day slow reaction phase approximately two orders of magnitude slower than the initial phase. Because Cr(0)(s) exhibited a much higher reactivity than Cr(III)(s), the oxidation of Cr(0)(s) in the iron corrosion scales was the dominant reaction for the Cr(VI) formation in drinking water. Second, the corrosion control strategies through adjustments of chemical water parameters were investigated. The results showed an increase in pH, silicate, alkalinity, and calcium suppressed Cr(VI) formation that was mainly attributed to in situ surface precipitation of new Cr(III) solids on the surface of Cr(0)(s), including Cr(OH)3(s), Cr2(SiO3)3(s), CrPO4(s), Cr2(CO3)3(s) and Cr10Ca(CO3)16(s). The Cr(III) surface precipitates were much less reactive with chlorine than Cr(0)(s) and suppressed the Cr redox reactivity. Adding phosphate either promoted or inhibited the Cr(VI) formation, depending on the concentration. Third, it is investigated that the effects of bromide and manganese on Cr(VI) formation through the oxidation of Cr(0)(s) by free chlorine. The present of Br- and Mn2+ significantly enhanced the Cr (VI) formation. Br- and Mn2+ can react with free chlorine to form free bromine and manganese dioxide. Free bromine had faster reaction kinetics with Cr(0)(s) than free chlorine. Although manganese dioxide had slower reaction kinetics with Cr(0)(s) than free chlorine, it can accelerate the reaction between Cr(0)(s) and free chlorine. The outcome of this study aimed to provide control strategies to minimize Cr(VI) formation by inhibiting Cr(0) reactivity in drinking water distribution systems