Formatted Title
Novel Method for Surface Partitioning Mediated Removal of Poly- and Perfluoroalkyl Substances from AFFF-Impacted Waters
Background/Objectives
Background/Objectives. A key challenge in the removal of PFAS from contaminated waters, such as those affected by aqueous film forming foam (AFFF) applications, is the wide array of concentrations detected (ng/L - mg/L) and concurrently, the very large volumes of media typically requiring treatment. These conditions generally preclude destruction in the field, instead requiring an initial separating/concentrating process, or alternatively, amendment with adsorbent materials to immobilize contaminants. The associated cost in either case is often prohibitive. It is therefore clear that any new process for PFAS separation must be scalable, economical, and with sustained efficacy across a wide concentration gradient. One of the most promising separation processes so far reported is foam fractionation (FF), where the preferential air/water interface partitioning of many target PFAS is exploited to adsorb contaminants onto a harvestable foam generated by circulating gas bubbles through a solution. FF systems have shown promising results at the laboratory and pilot scale, are low impact physical processes that do not interfere with the chemistry of the water to be treated, and have underlined the scope for further improvement in remediation processes tied to the surface activity of target PFAS compounds. Therefore, the objective of this project was to design and evaluate a novel method for generating a stable, high surface area foam (microfoam) to harvest PFAS from simulated AFFF-contaminated waters, and to determine whether this novel process offers an improvement on partitioning-based processes such as batch FF.
Approach/Activities
Approach/Activities. This project comprised a series of laboratory-scale batch experiments where a high-speed rotational blade was used to generate emulsions from PFAS-spiked waters. These emulsions then settled out into a harvestable foam layer comprised of a dense “microfoam” and a “treated solution”. Solutions were prepared with either a blend of AFFF products, or with a mix of long and short-chain PFAS, in concentrations between 500 µg and 1 mg/L ∑PFAS. Additionally, several amendment treatment conditions were incorporated to evaluate the effect of treatment time, electrolyte concentration, surfactant amendment (anionic, cationic and non-ionic) selection and concentration, PFAS species selectivity, and cyclic versus sustained treatment regimes. The experiment was performed with different numbers of cycles, with each cycle being 30 seconds. The efficiency of PFAS removal was quantified by liquid chromatography-tandem mass spectroscopy of collected foam and water samples, and the structure of the formed foams characterised by microscopy to investigate the effect of foam size on PFAS adsorption.
Results/Lessons Learned
Results/Lessons Learned. The novel process removed between 30 and 86% of ∑PFAS mass in one treatment cycle, and >90% after up to five treatment cycles, with considerable variation depending on the experiment conditions. Specific removal efficiencies varied between PFAS compounds and generally reduced as the PFAS carbon chain tail length decreased, as attributable to falling surface activity. Increasing the ionic strength of the solution improved PFAS adsorption to the foam layer, and the choice of cosurfactant strongly influenced the removal efficiency. It appears that for each cosurfactant tested, a different adsorption mechanism is dominant, and that these differences are most important for short-chain perfluoroalkyl acids, and zwitterionic precursor compounds. Removal efficiencies were sustained when ∑PFAS reduced from 1 mg/L to 500 µg/L, and with no more than 100 µg/L of any specific PFAS species, indicating that the process may be applicable across a wide concentration gradient. Critically, the active treatment time required per cycle was only 30 seconds, representing a considerable reduction compared to FF systems currently reported. This short treatment time, in conjunction with the adsorption mechanisms in the solution, may offer a path to a continuous or semi-batch configuration for the treatment of AFFF-contaminated groundwater. The proposed process has low energy consumption, thus improving the overall economic feasibility compared to other PFAS-removal technologies based on FF.