High energy radiation – Induced cooperative reductive/oxidative mechanism of perfluorooctanoate anion (PFOA) decomposition in aqueous solution. (May 2022)

• Record Type: Journal Article
• Authors: Szreder, Tomasz; Kisała, Joanna; Bojanowska-Czajka, Anna; Kasperkowiak, Małgorzata; Pogocki, Dariusz; Bobrowski, Krzysztof; Trojanowicz, Marek
• Is Part Of: Chemosphere. Volume 295(2022)
• Publication Date: 2022-05
• Subjects: Perfluorooctanoate anion -- PFOA -- Advanced oxidation/reduction processes -- Mechanism of radiation degradation -- Pulse radiolysis -- Gamma-radiolysis; Pollution -- Periodicals; Pollution -- Physiological effect -- Periodicals; Environmental sciences -- Periodicals; Atmospheric chemistry -- Periodicals; 551.511
• DOI: 10.1016/j.chemosphere.2022.133920
• Languages: English
• ISSNs: 0045-6535

Abstract

Abstract: The mechanism of high-energy radiation induced degradation of perfluorooctanoate anion (PFOA, C7 F15 COO − ) 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 [C7 F15 COO − ] ●− . The rate constants of this reaction were found to be in the range 7.7 × 10 7 −1.3 × 10 8 M −1 s −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 [C7 F15 COO − ] ●− tends to defluorination whereas at lower pH undergoes protonation forming [C7 F15 COOH] − . A sequence of consecutive reactions involving [C7 F15 COOH] − 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 (C7 F14 )·COO − as well as [C7 F15 COOH] − . 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 × 10 3 and <4 × 10 7 M −1 s −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. Graphical abstract: Image 1 Highlights: Decomposition mechanism of PFOA, strongly justified by experiments, was elaborated. Transient, semi-stable and stable products of decomposition of PFOA were identified. Radiation chemical yields and rate constants of selected species were measured. Experiments were supported by quantum mechanical calculations. Decomposition paths of PFOA were discussed in respect to technological applications.