When Co-Contaminants Compete: Limits of PFAS sorption in mixed-contaminant groundwater

Per- and polyfluoroalkyl substances (PFAS) are increasingly detected in groundwater co-contaminated with conventional pollutants, such as hydrocarbons, heavy metals and chlorinated solvents. Such mixtures are challenging for groundwater treatment because co-contaminants can strongly compete with PFAS for sorption sites, thereby reducing sorbent performance. Moreover, while ion-exchange resins and activated carbon remain the industry standards for PFAS removal, innovative sorbents may offer new pathways for improved PFAS removal. Understanding the sorption behaviour of PFAS in the presence of co-contaminants is essential for designing effective groundwater treatment strategies for complex contaminated sites.  

In this study we investigate the performance of 17 sorbents for PFAS removal in co-contaminated groundwater. The sorbents included 6 industry standards, ion exchange resins (n=3), activated carbon (n=3), as well as 11 innovative sorbents: surface-modified bentonites (n=2), surface-modified zeolites (n=3), proteins (n=3), a cyclodextrin (n=1), an iron-oxide based material (n=1) and an activated carbon/aluminum hydroxide-based material (n=1). The groundwater tested was pre-treated to remove volatile aromatic hydrocarbons but contained high dissolved organic carbon (20 mg/L) and had elevated ionic strength (0.017 M), along with residual phenols (phenol index: 7.3 µg/L) and mineral oil (C10–C40: 60 µg/L), concentrations typical for residually contaminated groundwater. PFAS concentrations were dominated by PFOA (~830 ng/L) and PFOS (~100 ng/L), with additional short-chain PFAS, e.g. PFBA (24 ng/L) and PFBS (18 ng/L).

All sorbents were initially screened at two sorbent concentrations (0.1 and 1.0 mg/L) and the six best performing sorbents were tested on a range of eight sorbent concentrations (0.01-2.0 mg/L). Finally, column experiments were performed with three sorbents to simulate full-scale treatment plant flow conditions. Despite residual co-contamination, several sorbents achieved high (>96%) PFAS removal at 1.0 g/L. Notably, at low sorbent concentrations albumin, an egg-based protein, showed PFOA removal comparable to activated carbon at the same sorbent concentration (~30%), whereas casein, a bovine milk-based protein, was contaminated and caused PFOA concentrations in the groundwater to increase to 2500 ng/L. Sorption capacities (Langmuir q_max) ranged from 1.24 to 10.45 µg/g, with ion-exchange resins highest. All sorbents were sensitive to interfering co-contaminants, which was especially apparent at low sorbent concentrations (10-50 mg/L). Overall, this study highlights that the presence of co-contaminants can substantially interfere with the sorption of PFAS in groundwater treatment and underscores the need for sorbents capable of maintaining high PFAS removal efficiencies under the complex chemical conditions typical of co-contaminated groundwater.