Enhanced reduction of uranium (VI) in groundwater via regulation of heat-activated persulfate: The role of formate and its mechanisms

Hexavalent uranium (U(VI)) in groundwater may cause human health problems. Therefore, the effectively removal of U(VI) from groundwater has become a urgent issue. Advanced reduction processes based on persulfate (PS) have been used to reduce pollutants. However, low efficiency limited its application. In this study, the U(VI) reduction in groundwater by heat-activated persulfate was systematically examined in the presence of formate (FA). The results indicated that FA enhanced U(VI) reduction compared to heat-persulfate system. The U(VI) reduction efficiency reached 86% in the presence of 5 mgL-1, 0.5 M persulfate and 0.3 M formate at acidic pH (4.6) and 70 degrees C within 60 min. Coexisting HCO3-, acetic acid and oxalic acid influenced U(VI) reduction, while NO3- and Cl- had no effect on U(VI) reduction by heat-persulfate-formate (H-PS-FA) process. According to the electron spin resonance experiment (ESR) results, CO2- was verified to be largely responsible for U(VI) reduction in H-PS-FA system, meanwhile, S2O8- and O-2(-) radicals also play a non-negligible role in reduction. More importantly, the test of U(VI) reduction in real groundwater validated that H-PS-FA system has significant potential and can be applied for reduction of U(VI) contaminated groundwater in uranium mining areas.