Quantification of perfluorooctanoic acid decomposition mechanism applying negative voltage to anode during photoelectrochemical process

Perfluorooctanoic acid (PFOA) is a carcinogen with a high binding energy between fluorine and carbon and is symmetrically linked, making it difficult to treat. In this study, a self-doped TiO₂ nanotube array (TNTA) was used as the anode and platinum as the cathode to quantify the PFOA removal mechanism using a photoelectrochemical (PEC) system. The external voltage was negative compared to that of the anode. In addition, NO₃⁻ and t-BuOH were used as scavengers to quantify the PFOA oxidation/reduction mechanism in the PEC system. As a result of the study, TNTA crystals are TiO₂ anatase, and the band gap energy was 3.42. The synergy index of PEC was 1.25, and the best electrolyte was SO₄²⁻. The PFOA decomposition activation energy corresponds to 70.84 kJ mol⁻¹. Moreover, ΔH^# and ΔS^# correspond to 68.34 kJ mol⁻¹ and 0.190 kJ mol⁻¹ K⁻¹, respectively. When the external negative voltage was 1 V, the contributions of the oxidation/reduction reaction during PFOA decomposition were 60% and 40%, and when the external negative voltage was 5 V, the contributions of the redox reaction were 45% and 55%. As the external negative voltage increased, the contribution of the reduction reaction increased as the number of electrons applied to the anode increased. When PFOA was decomposed, the by-products were C₇F₁₃O₂H, C₆F₁₁O₂H, C₅F₉O₂H, and C₄F₇O₂H, respectively. This study is expected to be used as basic data for research on the effects of other factors on the oxidation/reduction as well as the selection of anode and cathode materials on the decomposition of pollutants other than PFOA when using a PEC system.