Highly reductive radical CO2− deriving from a system with SO4− and formate anion: Implication for reduction of Cr(VI) from wastewater

An advanced reduction process (ARP) was established for the highly reductive radical CO2− produced by thermally activated persulfate (TAP) system in the presence of formate anions (FA). A series of control tests suggested that FA converted a strong oxidative condition into a reductive one, with CO2− as the predominant radical for Cr(VI) reduction initiated by SO4− in the presence of FA. This mechanism was confirmed by electron spin resonance experiment. The ability of the system to reduce Cr(VI) mainly depended on the dosage ratio of FA and persulfate (PS), and the system maintained a strong reductive condition at ratios of FA to PS equal to or higher than one. Moreover, the rate of reaction increased with increasing PS dosage and reaction temperature, but decreased with increasing pH. The reduction efficiency of Cr(VI) reached more than 99% within 240 min for an initial Cr(VI) concentration of 50 mg·L−1 under the condition of [FA]0 = 30 mM, [PS]0 = 30 mM, T = 70 °C, and unadjusted pH.