Simultaneous Cu-EDTA oxidation decomplexation and Cr(VI) reduction in water by persulfate/formate system: Reaction process and mechanisms

The heavy metal-organic complexes and heavy metal ions pose great threats to human health, and their elimination from water environment is quite urgent. Simultaneous decomplexation of heavy metal complex Cu-EDTA and reduction of Cr(VI) was performed in a persulfate (PS)/formate (FA) system. Oxidizing species and reductive species were simultaneously produced in this PS/FA system, which realized 96.6% of Cu-EDTA decomplexation and 100% of Cr(VI) reduction within 90 min treatment under the solution pH 3.60. Higher solution temperature and acidic conditions favored Cu-EDTA decomplexation and Cr(VI) reduction. There existed an appropriate molar ratio of FA and PS to simultaneously realize high-efficient Cu-EDTA decomplexation and Cr(VI) reduction. SO4•-, •OH, and 1O2 drove the Cu-EDTA decomplexation, which destroyed the chelating sites of Cu and EDTA, and finally it was oxidized to small molecular alcohols, organic acids, and Cu2+. Reductive species CO2•− was produced by the reaction of SO4•− and FA, as well as •OH and FA, which dominated the reduction of Cr(VI) to Cr3+. Carbonates, hydroxides, and oxides including Cu(OH)2, CuCO3, Cu2CO3(OH)2, CuO, Cr(OH)3, and Cr2O3 were the main composites in precipitates after Cu2+ and Cr3+ were precipitated. Furthermore, Cu-EDTA decomplexation and Cr(VI) reduction processes induced by PS/FA were put forward.