Low-pressure reverse osmosis membrane separation of non-fluorinated and perfluorinated organic compounds in water

The purpose of this study is to compare retention characteristics of perfluorinated organic compounds (PFCs) and similar-structured non-fluorinated organic compounds (NFCs) by new generation low-pressure reverse osmosis (LPRO) membranes based on physicochemical properties of the compounds and the results of bench-scale membrane efficiency batch tests as the first-step screening. Molecular weight (MW), molecular size, and hydrophobicity of the compounds greatly influence their retentions by loose and low-desalting membranes than those by tight and high-desalting membranes. The retentions in general increased with increasing MW and molecular length (ML). The retentions are explained more reliably by using both MW and molecular width/height parameters. The roles of MW and ML on the retentions could not be differentiated, since MW increases almost linearly with ML. The NFCs are more easily rejected than the PFCs by the membranes most probably due to hydrophobic adsorption of the PFCs to membranes leading to their slippage through membrane pores owing to their slender molecular structures. Though retentions of NFCs and PFCs in their undissociated states are governed by steric/size exclusion mechanism, hydrophobic adsorption to membrane is a major event for PFCs while it is not for NFCs. Unlike NFCs, retentions of PFCs in their dissociated states are greatly enhanced due to overwhelmingly greater efficacy of electrostatic exclusion. This is the first study comparing retentions of NFCs and PFCs by LPRO membranes and is expected to serve as an important reference for further works.