Adsorption of short-chain perfluoroalkyl acids (PFAAs) from water/wastewater

Abstract

Since the regulations for the manufacture and use of long-chain PFAAs have become increasingly stringent, there is a tendency to replace them with short-chain homologs. Adsorption is considered the most cost-effective treatment for thermally, chemically, and biologically stable PFAAs. Many types of adsorbents have been synthesized for the removal of short-chain PFAAs. This review therefore provides a summary of adsorbents (synthesis, capacity, and kinetics), mechanisms (uptake mechanisms and influencing factors), and applications, including large-scale setups and point-of-use/point-of-entry (POU/POE) units, for the removal of short-chain PFAAs. Although short-chain PFAAs with their relatively high intraparticle diffusion rates can be more easily removed at the initial stage than the long-chain ones, their low hydrophobicity makes it difficult for them to compete in the long-term with the more hydrophobic organics in the solution (including the long-chain compounds) for binding sites, not to mention the fact that the already adsorbed short-chain PFAAs can be replaced by the long-chain ones. The most commonly applied adsorbents for PFAA removal have been carbonaceous adsorbents. Yet, significant changes are required to alter or modify the main adsorptive mechanism of hydrophobic interaction of carbonaceous adsorbents in order to improve their low removal capacity for short-chain PFAAs in the presence of long-chain compounds. Also, polymeric adsorbents, particularly amine-functionalized ones, are of limitless tunability for selective PFAA removal, which makes them promising for future applications. Future research should also focus on innovative treatment strategies for simultaneous long-chain and short-chain PFAA removal, development of novel adsorbents with better selectivity for short-chain PFAAs, and evaluation under environmentally-relevant conditions.