Formation of perfluorocarboxylic acids from photodegradation of tetrahydroperfluorocarboxylic acids in water

Abstract

Tetrahydroperfluorocarboxylic acids (2H,2H,3H,3H-PFCAs) have aroused the interest of scholars worldwide due to their potential to generate perfluorinated compounds. In this work, we systematically examined the photodegradation kinetics and mechanisms of typical 2H,2H,3H,3H-PFCAs (CnF2n+1C2H4COOH, n = 6, 7, 8) in aqueous solution by a 500 W Hg lamp. The photodecomposition of 2H,2H,3H,3H-PFCAs all followed pseudo-first-order kinetics, and the photolysis rate coefficients increased with the increasing carbon chain length. Under the same reaction condition, 2H,2H,3H,3H-PFCAs degraded much faster than the corresponding PFCAs. The photodecomposition rate coefficient of C8F17CH2CH2COOH was accelerated by low pH and Fe3+ addition, but decreased by the existence of humic acid, carbonate and bicarbonate. Compared with ultrapure water, a decreased removal of 2H,2H,3H,3H-PFCAs was observed in four types of natural waters, i.e., tap water, Jiuxiang river water, primary effluent and secondary effluent. According to mass analysis, C8F17CH2CH2COOH was mainly decomposed into 8:2 fluorotelomer acid (C8F17CH2COOH), shorter-chain perfluorocarboxylic acids (PFCAs), perfluoro-1-enes (CnF2n) and perfluoroketenes (CnF2n+1CF = C = O). Thus, α-oxidation, decarboxylation and elimination reaction were proposed as reaction pathways. ECOSAR predictions showed that photolysis generally decreased the aquatic toxicity of C8F17CH2CH2COOH.