Effect of mono- and di-valent cations on PFAS removal from water using foam fractionation – A modelling and experimental study

Per- and poly-fluoroalkyl substances (PFAS) are a group of recalcitrant compounds whose widespread use in a variety of consumer products has led to contamination of groundwater and surface water systems. Foam fractionation is a potential remediation technology for treatment of PFAS contaminated water, which takes advantage of the high surface activity imparted by the fluorocarbon chain to remove them from solution by adsorption to the surface of air bubbles. In this study, the effect of mono- and di-valent cations on the performance of a PFAS foam fractionation process where sodium dodecyl sulphate (SDS) is used as a co-foaming agent has been evaluated. The results indicated that the separation of PFAS was improved in an order that followed the charge density of the salts with Mg2+ > Na+ > K+. It was also observed that at salt concentrations above 100 mM for Na+, above 10 mM for K+ and Mg2+ but between 0.1 and 10 mM for Ca2+ in the presence of greater than 4 ppm of SDS, the cations can complex with the SDS in the system and suppress foam formation due to the surfactant precipitation. Foam fractionation was able to remove perfluorohexane sulphonic acid (PFHxS), perfluorooctanoic acid (PFOA) and perfluorooctane sulphonic acid (PFOS) from a sample of Australian groundwater to below the analytical detection limit of 0.1 ppb within 60 min with SDS being used as the co-foaming agent, but was unable to remove the short chain perfluorobutanoic acid (PFBA).