Removal of perfluoroalkyl substances (PFAS) from different water types by techniques based on anion exchange (AIX), powdered activated carbon (PAC), iron(III) chloride and nanofiltration (NF) membrane – A systematic comparison

Abstract

Presence of per- and polyfluoroalkyl substances (PFAS) in groundwater and surface water used for drinking water production is a major concern, due to possible adverse effects of PFAS on human health. Stricter guidelines on PFAS levels in drinking water currently being implemented on global scale typically require use of advanced techniques for water treatment. The aim of this study was to systematically compare four different treatment techniques for removal of PFAS and to evaluate the impact of water type on the removal efficiency. We hypothesized that the water type has a significant influence on the removal efficiency for the tested treatment techniques. The four different treatment techniques included i) anion exchange (AIX) MIEX®, ii) powdered activated carbon (PAC), iii) coagulation with ferric chloride (FeCl₃), and iv) nanofiltration (NF) membrane. Mean ∑PFAS removal was found to be highest for NF membrane (48 ± 7.6 %), followed by AIX (30 ± 7.7 %), PAC (18 ± 3.7 %) and FeCl₃ (8.8 ± 8.9 %). For NF membrane, observed removal efficiency of PFAS was best described by a sigmoid curve centred around 400 Da, with low removal (25–35 %) of low-molecular-weight PFAS (<400 Da) and higher removal (47–75 %) of PFAS with greater molecular weight (>400 Da). For AIX and PAC, PFAS removal depended on perfluorocarbon chain length and functional group, e.g. mean ∑PFAS removal efficiency significantly increased (p < 0.05) from 12 % using a PAC dose of 20 mg L⁻¹ to 46 % using a PAC dose of 100 mg L⁻¹. Significant correlations were observed between removal of individual PFAS and dissolved organic carbon (DOC) removal and DOC characterisation parameters (specific ultra-violet absorbance (SUVA), humification index (HIX), freshness index (FI), absorbance at 254 nm (UV254)). This illustrates the importance of considering DOC characteristics and their seasonal variations when choosing PFAS removal technique and indicates potential of these parameters as predictors of PFAS removal efficiency.