Retention of per- and polyfluoroalkyl substances (PFAS) and influencing factors by conventional and modified constructed wetlands treating municipal wastewater effluent

Abstract

Per- and polyfluoroalkyl substances (PFAS) are a broadly applied group of fluorinated compounds that are characterised by extreme persistence and environmental toxicity. Conventional wastewater treatment plants (WWTPs) are designed to remove oxygen-depleting compounds, as well as nutrients, but lack the specific capabilities to effectively remove PFAS. Nature-based solutions are an established technology to further treat WWTP effluent and are gaining attention for the removal of PFAS. However, significant knowledge gaps remain regarding the ability of constructed wetlands (CWs) to remove PFAS and the mechanisms involved. This study investigated the occurrence of 11 PFAS in a conventional WWTP effluent and the efficacy of PFAS removal in a conventional vertical subsurface flow CW and four different CWs modified by biochar, granular activated carbon (GAC) and natural zeolite. The average concentration of the 11 selected PFAS in the WWTP effluent was 177 ± 83 ng/L, with the highest levels observed for 6:2-fluorotelomersulfonic acid (H4-PFOS) at 72 ± 46 ng/L and perfluorohexanoic acid (PFHxA) at 28 ± 7 ng/L. The results indicate that modified CWs that incorporate biochar, GAC, and natural zeolite are significantly more effective in removing bulk organics, nitrate, phosphate, and PFAS from municipal wastewater. While long-chain PFAS ≥ 8 C-atoms were partly retained in the conventional (5–59 %) and the zeolite-containing wetland (39–76 %), shorter PFAS exhibited negative removal. The combined adsorbent CW demonstrated the highest removal rates below the limit of quantification (LOQ) across all sampling campaigns due to synergistic effects of enhanced water retention, adsorption and ion exchange.