Sorptive removal of short-chain perfluoroalkyl substances (PFAS) during drinking water treatment using activated carbon and anion exchanger

(2023) 35:12 Riegel et al. Environmental Sciences Europe https://doi.org/10.1186/s12302-023-00716-5 Environmental Sciences Europe Open Access RESEARCH Sorptive removal of short‑chain perfluoroalkyl substances (PFAS) during drinking water treatment using activated carbon and anion exchanger Marcel Riegel*, Brigitte Haist‑Gulde and Frank Sacher Abstract Laboratory tests and column tests were carried out in a waterwoks to investigate the removal of short- and longchain PFAS using activated carbon filtration and ion exchange treatment. For all adsorbents, the sorption affinity of short-chain per- and polyfluoroalkyl carboxylic acids (PFCA) was significantly lower than that of long-chain PFAS or short-chain per- and polyfluoroalkyl sulfonic acids (PFSA). In the PFAS-polluted groundwater matrix, the short-chain PFCA PFBA and PFPeA could only be sufficiently removed with activated carbon over short run times of 6000 and 11,000 bed volumes (BV), respectively. Longer PFCA with a chain length of C6 or more were removed over longer run times. The removal of short-chain PFCA using ion exchange media could also only be achieved over relatively short run times of 5000 BV for PFBA, 10,000BV for PFPeA and 18,000 BV for PFHxA. These are sometimes significantly longer than those of activated carbon. Due to the higher material costs for ion exchange media, there are nevertheless no lower operating costs when the ion exchangers are used in single-use mode. However, ion exchangers can be regenerated and then reused which can result in economic advantages compared to activated carbon filtration. However, for the extensive regeneration, especially for the elution of the long-chain PFAS, the additional use of ethanol is needed in the process. In contrast, the short-chain PFBA and PFPeA can be extracted without organic solvent from a weakly basic ion exchanger. Keywords Per- and polyfluoroalkyl substances, PFAS, Removal, Drinking water, Activated carbon, Ion exchange, Regeneration Introduction The group of per- and polyfluoroalkyl substances (PFAS) is a large family of anthropogenic substances. They consist in part of aliphatic, acyclic hydrocarbons in which many (poly) or all (per) of the hydrogen atoms have been *Correspondence: Marcel Riegel DVGW-Technologiezentrum Wasser, Karlsruher Str. 84, 76139 Karlsruhe, Germany replaced by fluorine atoms (Buck et al. [3]). These carbon chains are connected to different functional groups. Due to their hydrophilic and hydrophobic properties, PFAS are as well oil and water repellent. PFAS show a high stability to thermal, biological and chemical processes. Thus, they have been used in a wide range of industrial and household products over several decades. One main application is their use in aqueous film forming foams (AFFF). The group of PFAS can be subdivided into per- and polyfluoroalkyl carboxylic acids (PFCA) and per- and polyfluoroalkyl sulfonic acids (PFSA). In addition to © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Riegel et al. Environmental Sciences Europe (2023) 35:12 these two subgroups, a broad variety of PFAS exist with different chemical structures at the non-fluorinated part of the molecules (Buck et al. [3]). Single perfluorinated substances, like perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), have been well researched and are regulated due to their extreme resistance to degradation and their bioaccumulation potential. Owing to their very high toxicity to humans, the use of PFOS has been forbidden in the EU to a large extend since 2006 (EC [7]), based on agreements in the Stockholm Convention. The use of PFOA is also strongly restricted and in 2019 the use of firefighting foams containing PFOA has also been banned in the EU (Stockholm Convention [28]). Since over 150 countries across all inhabited continents have ratified the Stockholm Convention, the use of designated PFAS is virtually banned worldwide (Brennan et al. [2]). Consequently, alternative PFAS are now used in many applications. These PFAS have either shorter-chain lengths or are only partly fluorinated compounds (such as the fluorotelomers) [1, 24, 26]. The non-fluorinated part of the fluorotelomers with shorter-chain lengths might be degraded microbially in the environment leading to the formation of PFCA or PFSA (Pancras et al. [21]). Short-chain PFAS are defined to have five or less carbon atoms in the case of PFSA and to have seven or less carbon atoms in the case of PFCA ([3], OECD [20]). In general, short-chain PFAS are less toxic than longchain PFAS; however, the short-chain PFAS have been found to be more mobile in groundwater and able to move more rapidly in the case of soil contamination [33]. This is apparent by comparing the drinking water guidance values set from the German Environmental Protection Agency for the C4 compound PFBA (perfluorobutanoic acid) of 10 µg/L and for the C8 compound PFOS of 0.1 µg/L (UBA [34]). Nevertheless, short-chain perfluoroheptanoic acid (PFHpA) has a health-oriented guidance value of 0.3 µg/L and thus exhibits a comparable toxicity like PFOS. In addition, the EC Drinking Water Directive 2020/2184 includes a drinking water limit value of 0.1 µg/L for the sum of 20 PFAS (C4 to C13 of PFCA and PFSA) (EU [8]). Most cases of PFAS contamination of groundwater have resulted from firefighting operations using aqueous film forming foams near airports. These contaminations are often characterised by the occurrence of long-chain PFAS, like PFOS, PFHxS and PFOA. More recent groundwater contaminations are often characterised by contributions from short-chain PFAS, such as PFBA, PFPeA (perfluoropentanoic acid) or PFHxA (perfluorohexanoic acid) [12, 25, 38]. Adsorption onto granular activated carbon (GAC) is a field-proven technology for the removal of long-chain Page 2 of 12 PFAS, like PFOS and PFOA, from contaminated water [14, 27]. Due to the raising concerns of emerging shortchain PFAS, new treatment technologies have recently been developed, investigated and evaluated. Sustainable PFAS treatment technologies ensure a destruction of the substance until full degradation and mineralisation has (...truncated)