Comparing Four Innovative Technologies for Enhanced Leachate Treatment: An Evaluation of PFAS Foam Fractionation Efficacy

Introduction: PFAS, a group of chemicals widely used in various applications due to their water and oil resistance, are persistent, toxic, and accumulate in the environment. Disposal of products containing PFAS in landfills can result in PFAS contamination in the landfill leachate that is discharged to water resource recovery facilities (WRRFs) andwhich can lead to groundwater contamination (REF1). Foam fractionation (FF) is a process that utilizes the surfactant nature of PFASs to separate them into foam, which is collected as a concentrate for further treatment (REF2). FF significantly reduces the volume of PFAS-containing waste, making it an ideal first step in a PFAS treatment process. This study evaluates four different FF technologies and provides insights for their implementation based on bench scale comparison results. Materials and methods: In this study, four technologies were evaluated for PFAS removal. The technologies and their providers are listed as TechA, TechB, TechC, and TechD in Table 1. Equalized volume samples (55 liters) were collected from an anonymous facility's EQ-tank and distributed to different technology providers for testing. Leachate wet chemistry parameters samples were determined in accordance with APHA standard methods. Table 2 lists the analyzed parameters. A total of 40 PFAS compounds were quantified using the EPA 1633 method, the Total Oxidizable Precursor, and the Total Organic Fluorine assay. Table 3 provides a complete list of these compounds and their concentrations before FF. Results and discussion PFAS Removal Efficiency: The study evaluated the removal efficiency of different PFAS into the foam phase for the four FF technology providers. The findings, depicted in Figure 1, align with previous literature: longer fluorocarbon chains and the presence of a more surface-active sulfonic acid head group result in greater PFAS removal compared to its carboxylic counterpart. The study also shows that removal efficiency improves exponentially with longer fluorocarbon chains, and chains longer than six carbons resulted in undetected levels. This supports previous research that suggests chain length has a greater impact than head group. Longer fluorocarbon chains make PFAS less soluble and more hydrophobic, enhancing its ability to adsorb onto air-water interfaces due to their superior surface properties. Impact of Process Configuration: Technology providers employed different strategies in using FF as indicated in Table 1. A notable finding from comparing these technologies is the effectiveness of cationic co-surfactants in removing PFAS. The cationic headgroup of these co-surfactants allows for binding to PFAS at the water-air interface. TechA and TechB tested a wide range of co-surfactants at 100 mg/L and found that CTAB-based surfactants are favorable in reducing the short-chain compounds up to 50% removal. The consistent observation of the use of co-surfactants showed enhanced stability and density of the foam that helps accumulate more foam and minimize the extent of interstitial liquid drainage before the foam is harvested (REF4). However, TechC experiments showed no impact of using 8 mg/L co-surfactant compared to the absence of co-surfactant. This can be attributed to reduction of additive performance due to electrostatic uptake by competing contaminants in leachate, suggesting the need for a higher dose. For TechC it was found that a 45-minute contact time is sufficient to achieve non-detection levels of long-chain PFAS in a leachate primarily dominated by PFCAs. The study also assessed the impact of the number of FF units in series, with the consensus being that using two to three treatment units is adequate to achieve the desired removal. TechD's proprietary model approach showed that increasing the treatment stages from 3 to 5 resulted in a 10% increase in the removal of short-chain PFAS, while long-chains remained non-detectable in both cases. TechC's unique two-stage method involves de-foaming the leachate from the first fractionation unit and then re-foaming the primary foamate generated over repeated primary fractionation cycles (Figure 2). This approach significantly reduces generated foam volumes, condensing the volume of foam by up to 180 times in the secondary foamate, minimizing the capital and operating costs associated with downstream destruction technologies. Conclusions 1.Findings of this work are illustrated in Figure 3. 2.Long-chain PFAS have been effectively removed to below detection limit, while short-chain PFAS require a polishing step using adsorptive media due to their low removal rate. However, this poses challenges due to the high fouling potential of the media. Therefore, the implementation of FF and modifications to its configuration strongly depend on treatment objectives and upcoming regulations. 3.For the specific leachate that was tested, a 45-minute contact time of and up to three treatment stages in series were sufficient for removing long-chain PFAS. However, these factors may vary based on the quality of the leachate water being treated. 4.FF is a highly selective method for removing surfactants without altering effluent water quality. This results in an effluent with similar water quality to the influent but significantly reduced concentrations of PFAS, VOCs, total suspended solids, and iron.