Swellable Organically Modified Silicas for the removal of PFAS from aqueous solutions

Although water is one of the essential components for life, there is a continuous deterioration of water quality due to numerous anthropogenic activities that result in the introduction of organic, inorganic, and biological pollutants into the various water sources. Of particular concern is fluorinated alkyl substances (PFAs), a class of amphiphilic organic molecules that differ in alkyl carbon chain length and polar functional group at the head of the chain[1]. Since their carbon chain is fully fluorinated, they are very stable and persistent, and they also exhibit hydrophobic and lipophobic properties. The presence of some PFAs in aquatic environment is of greater concern as toxicological data suggests an association to adverse human health effects[2]. For this reason, remediation of PFAs from the environment has rapidly increased; as a matter-of-fact PFAs are recalcitrant to conventional water treatment, adsorption is considered as a better technique due to its cost efficiency, eco-friendliness, and high efficiency[3]. For this purpose, different porous materials, such as silicas, zeolites, clays, resins and active carbon are used. A promising novel material for adsorption of these pollutants is Swellable Organically Modified Silica (SOMS), an organic-inorganic hybrid material[4]. The unique feature of this material is that its structure allows for an expansion of three to five times its volume in the presence of organic compounds, thus enabling their efficient adsorption[5]. During the synthesis, it is possible to functionalize these materials with quaternary amine groups, in order to obtain a positively charged material called QASOMS[6]. In this work, QASOMS was synthesized and used as adsorbent for the removal of three PFAs molecules from water within a concentration range between 50 and 800 ppm. The adsorption property of the hybrid QASOMS material is compared with the performances of commercially available ultra-stable high-silica Y zeolite and granular activated carbon. Perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS) and perfluorobutanesulfonic acid (PFBS) were used as model PFAs molecules. 19F-NMR analysis was used for the identification and quantification of PFAs. The area of the CF3 signal of PFOA was used to prepare a calibration curve that was then used for evaluating the PFAS concentration after 24h contact with the sorbent materials. QASOMS showed high affinity for all the three perfluoroalkyl compounds throughout the whole concentration range tested.