Water-to-air transfer of branched and linear PFOA: Influence of pH, concentration and water type

The volatilisation of perfluorooctanoic acid (PFOA) was measured experimentally at a range of pH values using a previously published laboratory method. Water-to-air transfer was studied for five structural isomers, namely: the linear isomer (n-PFOA) and the four most commonly occurring branched isomers (3-, 4-, 5- and 6-PFOA). The influence of water concentration and water type on the pH-dependent water-to-air transfer was also investigated for n-PFOA. The water-to-air transfer was studied over the course of 48 h at pH values ranging from 0.2 to 5.5. Under all experimental conditions tested, the volatilisation of PFOA was negligible at pH > 2.5. In experiments performed with MilliQ water, volatilisation increased with decreasing water pH. In experiments performed with tap water and lake water, maximum volatilisation was observed at pH 1. The concentration of PFOA in water had no influence on the pH value at which water-to-air transfer was observed (i.e. at pH < 2.5) for the concentration range tested (0.1-50 μg/L PFOA in deionised water). Although the percentage of PFOA volatilised was significantly different for the four branched isomers at low pH, volatilisation was not observed above pH 2.5 for any branched isomer suggesting that all PFOA isomers have a low pKa. Overall, these laboratory results demonstrate that volatilisation of any structural isomer of PFOA from water is negligible at environmentally-relevant conditions. It is unlikely that PFOA isomers will be fractionated in the environment as a result of volatilisation because it is a process of negligible environmental relevance.