# Less Is More: Influence of Cross-Linking Agent Concentration on PFOS Adsorption in Chitosan
## 3 Results and Discussion
### Rapid Small-Scale Column (RSSC) Test  
Finally, we compared the chitosan polymer **ECH1** with activated carbon in a rapid small-scale column (RSSC) test regarding their PFOS adsorption behavior. In order to quickly see a PFOS breakthrough for the investigated materials, a very high PFOS sorbate concentration of around 40 mg/L (~80 mmol/L) was used as artificial wastewater. In preparation for this experiment, various materials were tested regarding their fluorine leaching and PFOS adsorption behavior to determine their suitability as a tube or stopper for the adsorbent material in the columns (see Figure S3). The adsorption tests showed that of the tested materials, only the paper filter adsorbed PFOS. The release of fluorine-containing species was observed for the fluorinated ethylene propylene (FEP) tube and both glass wool types. For the glass wool, it could be shown that inorganic fluoride was released, while the FEP tube released organic fluorine species. After considering all results, polypropylene fleece was chosen as the filter material, and Tygon(r) LMT55 was used for the tubing. To get a better understanding of the used adsorption materials, their BET surface areas were determined, and the results are shown in Table 1. Additionally, the size distribution of the milled GAC was determined by the suspension pressure method (see Table S10), which revealed that according to weight, two-thirds of the material had a particle size between 200-630 um and one-third between 20-200 um. The GAC had an inner surface area of almost 1300 m\({}^{2}\)/g, regardless of whether it was milled or not. The unmilled **ECH1** had roughly one order of magnitude less surface area with 94.1 m\({}^{2}\)/g. Milled **ECH1** had around one order of magnitude less BET area (3.4 m\({}^{2}\)/g) in comparison to unmilled **ECH1**. SEM images of the freeze-dried, milled **ECH1** showed only two-dimensional flakes remaining after the freeze-drying process in contrast to the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & **GAC (C2)** & **GAC Milled (C3)** & **ECH1 String (C4)** & **ECH1 Milled (C5)** \\ \hline BET area [m\({}^{2}\)/g] & 1291 & 1272 & 94.1 & 3.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: BET surface area for the adsorption materials used in rapid small-scale column test. For experimental conditions, see Supporting Information. visible particles for the milled activated carbon (see Figure 6) and thereby confirmed that the three-dimensional scaffold in the chitosan structure got destroyed. For the unmilled **ECH1** in string form, this collapse of the scaffold structure was only visible in the SEM images for the outside layers of the strings, while the inner structure of the unmilled **ECH1** is preserved (see Figure S29). The unmilled **ECH1**, therefore, showed a similar behavior as seen before for the chitosan polymers shown in Figure 3b,c (which was equivalent in synthesis to **ECH2**). Thus, it is likely that the main reason for the surprisingly small BET surface area of milled **ECH1** is the freeze-drying process, and it was not possible to determine the surface area in the wet state. The influence of the drying method is further confirmed by air-dried **ECH1** polymer strings, which are much thinner than freeze-dried ones and show a complete loss of scaffold structure in SEM images (see Figures S30-S32). For the column test, five different columns were filled with almost identical column volumes: **C1:** a blank column with just the stopper material and tubing, but without any adsorption material; **C2:** a column filled with granular activated carbon (GAC); **C3:** a column filled with the same GAC as **C2** but after milling (see Supplementary Materials for milling protocol and particle size distribution); **C4:** a column filled with **ECH1** in string form as obtained from the synthesis protocol described above; **C5:** a column filled with **ECH1** in milled form.