Optimizing Single-Step Foam Fractionation Enhanced Soil Washing for the Remediation of Per- and Polyfluoroalkyl Substances

This study investigates the remediation of PFAS-contaminated soil by single-step foam fractionation enhanced soil washing, with special focus on compounds with the highest Relative Potency Factors among those listed in the upcoming EU legislation. The experimental setup involved a small-scale reactor filled with water, PFAS-contaminated soil, and a diluted mixture of surfactants. Mechanical stirring and air injection were provided by a vertical propeller. Design of experiments (DOE) and Response Surface Methodology (RSM) were applied. Liquid to Solid ratio (L/S), stirring rate, and the amount of surfactant were varied simultaneously over a set of planned experiments. Stirring rate was observed to play the dominant role in the process. The lowest removal efficiencies increased with decreasing chain length for both Perfluoroalkyl Carboxylic Acids and Perfluoroalkane Sulfonic Acids, consistent with observations in soil washing processes. Since aeration was linked to the propeller rotation, foam fractionation was enhanced by increasing the stirring rate, resulting in removal efficiencies reaching similar levels. This led to some response surfaces overlapping, reflecting the higher sorption affinity of longer chain compounds to the air-water interface of bubbles compared to short chain PFAS. Final trials successfully validated the suggested optimum operating conditions, achieving 90% PFAS removal at L/S 7. These findings confirm the promising potential of foam fraction for the remediation of PFAS contaminated soil applied as a single-step treatment. Modeling and optimization are suggested as precious tools to explore operative conditions, with the aim to maximize treatment performance according to multiple targets in view of future scale-up.