Oxygen-induced efficient mineralization of perfluoroalkylether sulfonates in subcritical water

The decomposition of perfluoroalkylether sulfonates C2F5OC2F4SO3- and C3F7OC2F4SO3- in subcritical water was investigated, and the results were compared with those for perfluorobutanesulfonate (C4F9SO3-), 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 C4F9SO3- showed little reactivity in subcritical water, C2F5OC2F4SO3- decomposed efficiently in subcritical water (∼350 °C) in the presence of oxygen gas to form F− and SO42- in the aqueous phase and CO2 in the gas phase as major products. Trifluoroacetic acid (CF3COOH, TFA) and trifluoromethane (CF3H) were also detected as minor products in the aqueous and gas phases, respectively. Similar decomposition behavior was observed for C3F7OC2F4SO3-, which decomposed at a rate that was somewhat higher than that of C2F5OC2F4SO3-. When argon was used in place of oxygen gas, the time profile of the decrease in the amount of C2F5OC2F4SO3- remained almost unchanged, but the product distribution changed markedly: the amounts of F−, SO42-, and CO2 dramatically decreased, the amounts of TFA and CF3H increased, and a new product, HCF2SO3-, was detected. These results clearly indicate that treatment with subcritical water in the presence of oxygen gas effectively mineralized perfluoroalkylether sulfonates to F−, SO42-, and CO2. On the basis of a product analysis, we propose a decomposition mechanism.