Thermal- and photo-induced degradation of perfluorinated carboxylic acids: Kinetics and mechanism.
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Perfluorinated carboxylic acids (PFCAs) of different carbon chain lengths are chemicals of concern to human health and their removal, using conventional remediation technologies, is challenging. The present paper pursuits thermal and photo-induced degradation of PFCAs (F(CF2)nCOOH, n=1-9) under various concentrations of four different acids (HNO3, H2SO4, HCl, and H3PO4) covering a range of strong acidic to basic pH. For thermal-induced experiments, the temperature was set at 40C, 60C, and 80C at acid strengths of 0.04-18.4M. Photo-induced experiments were conducted at pH 0.5, 7.0, and 13.0 under a light intensity of (15010)100W/cm2. The degradation first-order rate constant (k1, h-1) as a function of [H+] was modeled by considering equilibrium of nondissociated (F(CF2)nCOOH, HX) and dissociated (F(CF2)nCOO-, X-) species of PFCAs (HX X-+H+, pKa=-0.1). Species-specific rate constants, k1HX, reasonably described the trend of thermal and photo decay of PFCAs, where k1HX increased with acidity of solution and the carbon chain length of PFCAs. Mechanism of degradation of PFCAs (e.g. perfluorooctanoic acid (PFOA)) involved homolytic breakage of CC bond between alkyl and carboxyl groups, which produced radicals and subsequently decarboxylation to perfluoroheptene-1. Density functional theory (DFT) calculations supported the mechanism. The calculations indicated that a breaking of CC bond is more feasible with nondissociated HX than dissociated X- species of PFCAs and also with increase in chain length. The potential of a combination of thermal- and photo-induced processes under acidic conditions to enhance degradation of PFOA in water is presented.