Photochemical removal of hexavalent chromium and nitrate from ion-exchange brine waste using carbon-centered radicals

Abstract

Ion-exchange (IX) is a promising technology to remove hexavalent chromium Cr(VI) and nitrate from groundwater; however, its wide application is challenged by a costly disposal of spent brine. This study developed a novel photochemical process utilizing reductive carbon-centered radicals to remove Cr(VI) and nitrate in IX spent brine. In the presence of formate or alcohol additives, photolysis of nitrate in spent brine generated highly reductive carbon-centered radicals including ·CO2−, ·CH2OH, ·CH(OH)CH3, and ·C(OH)(CH3)2. Among them, ·CO2− exhibited the highest efficiency to reduce Cr(VI) to non-toxic Cr(III) solids. Increasing the dosage of formate additive promoted ·CO2− production and accelerated Cr(VI) removal. Decreasing pH enhanced Cr(VI) removal by increasing the yield of ·CO2− and thermodynamic favorability of Cr(VI) reduction while lowering total chromium removal because of the formation of soluble Cr(III) species. High levels of chloride in spent brine scavenged HO·, suppressed ·CO2− production, and inhibited Cr(VI) reduction. With an extended reaction time, nitrate was photochemically converted to gaseous nitrogen species. The denitrification process was favored at acidic pH due to the suppressed formation of nitrite. Findings from this study provides a new photochemical treatment technology for hazardous IX spent brine management.