Oxygen-induced efficient mineralization of perfluoroalkylether sulfonates in subcritical water

Abstract

The decomposition of perfluoroalkylether sulfonates C 2 F 5 OC 2 F 4 SO 3 - and C 3 F 7 OC 2 F 4 SO 3 - in subcritical water was investigated, and the results were compared with those for perfluorobutanesulfonate ( C 4 F 9 SO 3 - ), which has no ether linkage. This is the first report on the use of subcritical water to decompose perfluoroalkylether sulfonates, which are being developed as alternative surfactants to environmentally persistent and bioaccumulative perfluoroalkylsulfonates. Whereas C 4 F 9 SO 3 - showed little reactivity in subcritical water, C 2 F 5 OC 2 F 4 SO 3 - decomposed efficiently in subcritical water (∼350 °C) in the presence of oxygen gas to form F − and SO 4 2 - in the aqueous phase and CO 2 in the gas phase as major products. Trifluoroacetic acid (CF 3COOH, TFA) and trifluoromethane (CF 3H) were also detected as minor products in the aqueous and gas phases, respectively. Similar decomposition behavior was observed for C 3 F 7 OC 2 F 4 SO 3 - , which decomposed at a rate that was somewhat higher than that of C 2 F 5 OC 2 F 4 SO 3 - . When argon was used in place of oxygen gas, the time profile of the decrease in the amount of C 2 F 5 OC 2 F 4 SO 3 - remained almost unchanged, but the product distribution changed markedly: the amounts of F −, SO 4 2 - , and CO 2 dramatically decreased, the amounts of TFA and CF 3H increased, and a new product, HCF 2 SO 3 - , was detected. These results clearly indicate that treatment with subcritical water in the presence of oxygen gas effectively mineralized perfluoroalkylether sulfonates to F −, SO 4 2 - , and CO 2. On the basis of a product analysis, we propose a decomposition mechanism.