# Less Is More: Influence of Cross-Linking Agent Concentration on PFOS Adsorption in Chitosan
###### Abstract  
As a result of the continuous use of persistent per- and polyfluoroalkyl substances (PFAS), e.g., in aviation firefighting foams, contamination with PFAS has been found in soil, groundwater, and surface water around thousands of industrial and military installations. Due to their harmful (environmental) potential, further dispersion in the environment needs to be stopped, which can be achieved by appropriate absorption materials. In this work, the influence of the cross-linking agent epichlorohydrin (ECH) concentration on the perfluorooctanesulfonic acid (PFOS) adsorption capacity of chitosan gel was investigated. It was found that higher ECH concentration during the cross-linking step decreases the PFOS adsorption capacity of the cross-linked chitosan gel from 0% to 4% ECH solution by about 15%. Using a concentration of 1%, ECH resulted still in an acid-stable material, and a maximum PFOS loading capacity of 4.04 mmol/g was obtained, one of the highest described in the literature. Furthermore, we used a rapid small-scale column test to compare the PFOS adsorption capacity of chitosan and activated carbon, each in both milled and unmilled form. Unmilled chitosan showed the highest PFOS adsorption capacity considering adsorption material dry masses (>0.9 and <0.4 mmol/g for both types of chitosan and activated carbon, respectively). Milled activated carbon proved to be the better adsorption material, considering the fixed volume of the adsorber (>99.9% PFOS adsorbed). Overall, the cross-linking agent concentration in chitosan is a crucial factor influencing its PFOS absorption potential. Our results feature cross-linked chitosan as an effective economic and ecologic alternative for PFOS adsorption in aqueous solutions. political, cross-linking agent, self-assembly,Thus, PFAS is currently one of the most researched challenges in the remediation of soil and water since there are no established standard practices for PFAS destruction yet, and natural degradation, if happening, is far too slow [6]. Many different degradation approaches have been investigated, but they are mostly for a liquid matrix and require high contaminant concentrations to be practical [7; 8; 9; 10]. In contrast, the regulation levels set by governments in the past years are far below those ranges (EU regulation for the sum of 20 PFAS compounds in drinking water is set at 0.1 ug/L) [11]. Therefore, PFAS enrichment through adsorption is an important step prior to degradation. Various substances have been tested in their PFAS adsorption potential, like activated carbon (AC) or ion exchange resins [7; 12; 13; 14; 15; 16; 17; 18; 19]. However, ion exchange resins have the constraint that they can only remove opposite-charged ionic species. In contrast, PFAS are such a diverse group that neutral compounds also need to be considered. Therefore, materials like AC, which can adsorb a broad range of compound classes, are better suited. There seem to be multiple mechanisms by which PFAS can bind to carbon-based materials, thereby broadening their usability [20]. In addition to these considerations, biologically derived materials have the advantage that they are renewable sources and biodegradable. Since chitosan, as a bio-derived material, is cheap and already known as an adsorption material for other pollutants [21; 22], it is not surprising that it was also investigated regarding its PFAS adsorption potential [23; 24; 25; 26; 27; 28]. Figure 1 shows the adsorption mechanism of perfluorosulfonic acids on chitosan. In the beginning, electrostatic and hydrophobic interaction takes place on the protonated amino group of chitosan. Higher perfluorooctanesulfonic acid (PFOS) concentrations promote bilayer sorption, and sorption of micelles occurs on the porous chitosan surface [22; 24]. Unmodified chitosan is not acid-stable, but it can be cross-linked to make it stable at low pH values. [PERSON] et al.