High energy radiation - Induced cooperative reductive/oxidative mechanism of perfluorooctanoate anion (PFOA) decomposition in aqueous solution

Abstract

The mechanism of high-energy radiation induced degradation of perfluorooctanoate anion (PFOA, C₇F₁₅COO^-) was investigated in aqueous solutions. Identification and quantification of transient species was performed by pulse radiolysis and of final products by gas and ion chromatography, electrochemical method using fluoride ion-selective electrode and ESI-MS after γ-radiolysis. Experimental data were further supported by kinetic simulations and quantum mechanical calculations. Radiation induced degradation of PFOA includes as a primary step one-electron reduction of PFOA by hydrated electrons (e^-_aq) resulting in formation of [C₇F₁₅COO^-]^●-. The rate constants of this reaction were found to be in the range 7.7 × 10⁷-1.3 × 10⁸ M^-1s^-1 for ionic strength of the solutions in the range 0.01-0.1 M and were independent of pH of the solutions. At pH > 11 [C₇F₁₅COO^-]^●- tends to defluorination whereas at lower pH undergoes protonation forming [C₇F₁₅COOH]^•-. A sequence of consecutive reactions involving [C₇F₁₅COOH]^•- leads to PFOA regeneration what explains a high radiation resistance of PFOA at moderately acidic solutions. A simultaneous presence of oxidizing transient species (^●OH) in the irradiated system enhanced decomposition of (C₇F₁₄)·COO^- as well as [C₇F₁₅COOH]^•-. The key steps in this complex radical mechanism are the reactions of both these radical anions with ^●OH leading to semi-stable products which further undergo consecutive thermal reactions. On the other hand, direct reactions of PFOA with ^●OH and ^●H were found to be relatively slow (7 × 10³ and <4 × 10⁷ M^-1s^-1, respectively) and do not play relevant role in PFOA degradation. Collected for the first time results, such as dependence of selected reaction rate constants and selected products radiation chemical yields on pH as well as finding of several semi-stable products, missing in previous studies, indicate incompleteness of published earlier reaction pathways of PFOA degradation. The presented overall mechanism explains experimental results and verifies previously suggested mechanisms found in the literature.

Keywords: Advanced oxidation/reduction processes; Gamma-radiolysis; Mechanism of radiation degradation; PFOA; Perfluorooctanoate anion; Pulse radiolysis.