# Less Is More: Influence of Cross-Linking Agent Concentration on PFOS Adsorption in Chitosan
## 3 Results and Discussion
### Rapid Small-Scale Column (RSSC) Test
The same volume of adsorber was used in all four columns to better compare the performance of the four adsorbent materials under field conditions. The results of the RSSC test are shown in Figure 7.  
Figure 6: Scanning electron microscopy (SEM) image of (**a**) milled chitosan **ECH1** and (**b**) milled GAC. The Supplementary Materials contains these pictures and other SEM images with higher resolution. Figure 7: (**a**) Dependence of the eluent volume on the PFOS effluent concentration (filled symbols; c\({}_{0}\)(PFOS) = 83 mmol/L) and (**b**) dependence of the eluent volume on the PFOS loading q on the adsorber material (dry mass) as calculated from the data shown in (**a**) (hollow symbols). The gap in data points between 600 and 900 mL was due to the weekend, but the effluent was continuously pumped and collected the whole time. The RSSC test was conducted in such a way that all five columns were connected to different channels of the same peristaltic pump, which pumped the PFOS solution from the same reservoir. After percolating the columns, the effluent was collected and then analyzed via CIC. The measured PFOS concentrations in the effluent are shown in Figure 7a, while the loading capacity based on the dry mass of the adsorber is shown in Figure 7b. From a practical point of view, column **C3**, filled with milled GAC, showed the highest PFOS retention, considering the absolute PFOS reduction since no breakthrough was observed. Even after nearly 1400 mL effluent passed the column, the measured PFOS concentration in the effluent was around the LOQ for the CIC. Column **C5**, filled with milled chitosan, on the other hand, showed the fastest increase of PFOS concentration in the effluent after roughly 300 mL passed the column. After 1400 mL passed the column, the PFOS effluent concentration of **C5** was similar to the PFOS effluent concentrations found for the blank column **C1**. The PFOS concentration in the effluent for the GAC in **C2** and the chitosan in string form in **C4** were similar. However, PFOS effluent concentrations for both GAC columns were systematically lower compared to the chitosan columns. While it might be confusing at first glance that the loading capacity of the milled GAC in **C3** is lower than that of the GAC in **C2**, it can be easily explained by the experimental setup. As all four columns were filled with the same volume of material, a higher mass of the milled GAC could be fitted into that volume, thereby decreasing q (for similar amounts of adsorbed PFOS). Results of the RSSC test (Figure 7b) showed that for the applied experimental conditions, the milling did not alter the PFOS loading capacity and behavior in a major way for either chitosan or activated carbon. The results for the milled chitosan in **C5** showed a faster increase of the PFOS loading q in the beginning than for the unmilled chitosan in **C4**. However, the data points of **C5** at the end indicate an asymptotic behavior, which is not observable for **C4**. So, it appears to be, that the maximum PFOS loading capacity of the chitosan is reduced by the milling. The results show that milling improved the adsorption of PFOS for GAC but decreased it for chitosan. One potential reason for this finding might be the different rigidities of the materials. For GAC, milling significantly reduced the particle size, thereby improving the PFOS adsorption kinetics and homogenizing the liquid flow profile without destroying the surface structure necessary for adsorption. In contrast, the unmilled chitosan strings are already permeable for the solution, so the PFOS adsorption kinetics are only slightly improved by the smaller particle size. However, due to the softness of the material, the scaffold structure of the chitosan gel probably gets impaired by the necessary force to mill it, thus lowering the PFOS adsorption capacity of the material.