Hydrated electron: a destroyer of perfluorinated carboxylates?

As a class, perfluorinated carboxylate (PFCA) was ranked among the most prominent organohalogen contaminants in environment with respect to thermal, chemical and biological inertness. Hydrated electron (eaq-), a highly reactive and strongly reductive species, has been reported to readily decompose perfluoroaromatic compounds via intermolecular electron transfer process in aqueous solution. Question then arose: what would happen if perfluorinated carboxylates encountered with hydrated electron? Original laboratory trial on the interaction between F(CF2)nCOO- (n=1, 3, 7) and hydrated electron was attempted by using laser flash photolysis technique in this research work. Abundant hydrated electron (eaq-) could be produced by photolysis of 1.25 x 10-4 M K4Fe(CN)6 in nitrogen saturated water. In the presence of F(CF2)nCOO- (n=1, 3, 7), the decay of eaq- was observed to enhance dramatically, indicating eaq- was able to attack PFCAs. On addition of perfluorinated carboxylates, the loss of eaq- was mainly due to the following channels. By mixing the solution of K4Fe(CN)6 with excess K3Fe(CN)6 and PFCAs, eaq- turned to decayed corresponding to mixed first- and second-order kinetics. Rate constants for the reactions of eaq- with PFCAs could be then easily determined by monitoring the decay of eaq- absorption at 690 nm. Since perfluorinated carboxylates were salts, the influence of ionic strength on k3 was examined systematically by carrying out experiments of varying ionic strength ranging from 0.009 up to 0.102 M by adding NaClO4. In this manner, the second order rate constants for e-aq with CF3COO-, C3F7COO-, C7F15COO- were derived to be (1.9±0.2) x 106 M-1 S-1 (μ=0), (7.1±0.2) x 106 M-1S-1 (μ=0) and (1.7±0.5) x107 M-1S-1 (μ=0.009 M) respectively. Apparently, the length of F(CF2)n group exerted substantial influence on the rate constant. Further study on byproducts analysis by ion chromatography showed that fluorine ion was generated in the mixed solution of K4Fe(CN)6 and PFCAs after irradiation. This phenomenon also proved that hydrated electron could attach to PFCAs and followed by efficient dissociation of PFCAs via heterolytic cleavage of C-F bond. Considering that a majority of hydrated electron are generated in the course of radiation and non-thermal plasma, such technologies are believed to be capable of serving to PFCAs abatement. . (authors)