Formation of perfluorocarboxylic acids from photodegradation of tetrahydroperfluorocarboxylic acids in water.

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 (CFCHCOOH, 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 CFCHCHCOOH was accelerated by low pH and Fe 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, CFCHCHCOOH was mainly decomposed into 8:2 fluorotelomer acid (CFCHCOOH), shorter-chain perfluorocarboxylic acids (PFCAs), perfluoro-1-enes (CF) and perfluoroketenes (CFCF = 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 CFCHCHCOOH.