Rejection of perfluoroalkyl acids by nanofiltration and reverse osmosis in a high-recovery closed-circuit membrane filtration system

Abstract

Poly- and perfluoroalkyl substances (PFASs), especially perfluoroalkyl acids (PFAAs), have garnered broad attention due to their near ubiquitous presence in environmental and biological matrices, recalcitrant nature, and reported negative human health impacts. Processes using high-pressure membranes such as reverse osmosis (RO) and nanofiltration (NF) have been investigated as PFAA treatment technologies due to their ability to separate dissolved solutes such as inorganic ions, and small molecular weight organic compounds. The major drawback associated with high-pressure membranes is management of the concentrate stream produced during treatment. As a result, high recovery membrane system configurations have been developed with the goal of minimizing volume of concentrate requiring disposal. In the context of PFAS treatment, high-recovery membrane applications may be beneficial; PFAS residuals could be concentrated allowing advantages in terms of disposal, e.g., more effective application of PFAS destruction technologies. The objective of this study was to evaluate the PFAA rejection performance of commercially available high-pressure membranes in a pilot-scale closed-circuit membrane filtration (CCMF) system. The rejection of nine PFAAs by four spiral wound membrane products spanning characteristics ranging from loose NF to seawater RO was investigated during and through two closed-circuit sequences each operating up to 97% recovery. Mechanisms of PFAA rejection including steric and electrostatic exclusion were investigated through analysis of generated data, and ionic strength experiments. Additionally, short-term foulant accumulation during CCMF and normalized energy analysis were performed by monitoring the calculated temperature corrected specific flux (TCSF) and design software simulations, respectively. Results from this study demonstrate that tight NF and RO membranes are effective for separating and concentrating PFAAs during high recovery CCMF operation. During CCMF sequences to 97% water recovery, the NF90, CR100, and SW30 membranes evaluated exhibited overall rejection values of > 98.3% for the PFAAs quantified in this study. The loose NF membrane element investigated (NF270) exhibited the lowest PFAA rejection performance during the high recovery experiments, particularly at water recoveries > 90%. Diminished PFAA rejection performance of the NF270 is likely a result of both steric and electrostatic exclusion being significant separation mechanisms. Feed water amended with sodium sulfate yielded lower rejection for PFAAs by the NF270 compared to unamended feed water supporting the hypothesis that that high recovery CCMF operation may negatively impact PFAS rejection by loose NF membranes, particularly membranes that rely on electrostatic exclusion for separation of anions.