Electrochemical oxidation coupled with hydrated electron reduction in ultraviolet (UV)-sulfite system for complete defluorination and mineralization of Perfluorooctanoic Acid (PFOA)

Hydrated electron reduction in an ultraviolet (UV)-sulfite system and electrochemical oxidation on a boron doped diamond (BDD) anode both are efficient processes for the degradation of perfluorooctanoic acid (PFOA). However, hydrated electron reduction has inherent deficiency in the mineralization (i.e., converting organic carbon into CO2) of PFOA. Furthermore, toxic byproducts such as partially hydrodefluorinated carboxylic acids (HDCAs) and perfluorosulfonic acids (PFSAs) might give secondary pollution if the wholly hydrodefluorination of PFOA is not achieved. BDD electrochemical oxidation can obtain a high mineralization efficiency, but it leads to a low defluorination (i.e., transforming C−F bonds into F− ions) efficiency due to the generation of toxic volatile fluorinated alkanes. Herein, we report a coupled process that is hydrated electron reduction as pre-treatment and BDD electrochemical oxidation as post-treatment to achieve a complete defluorination and mineralization of PFOA, the results demonstrate that 50 mg/L PFOA can be 100 % deluorinated and mineralized under 5 h UV-sulfite reduction followed by 10 h electrochemical oxidation. This coupled process strongly inhibits the production of the toxic volatile fluorinated pollutants and quickly converts the residual toxic sulfite in UV-sulfite system to the sulfate. Mechanism investigation demonstrates that BDD oxidation is able to transform HDCAs and PFSAs formed by hydrated electron reduction to the perfluorocarboxylic acids (PFCAs) with shorter carbon chain than PFOA, which can be further defluorinated and mineralized.