# Less Is More: Influence of Cross-Linking Agent Concentration on PFOS Adsorption in Chitosan
## 3 Results and Discussion
### Adsorption Isotherms  
Next, we investigated the adsorption isotherms of **ECH1** to see if the Langmuir or Freundlich theory describes the PFOS adsorption process best. To obtain a more robust basis for the fits, three experiments per condition were prepared without averaging them. The results of those experiments are shown in Figure 5.  
The adsorption time was increased compared to the earlier experiments in order to ensure that even at the highest concentrations, the adsorption reached its equilibrium. Fitting of the experimental data to the respective equations was performed using Origin and is explained in more detail in the Supplementary Materials chapter 2.3.  
The sorption behavior in the investigated concentration range is better described by the [PERSON] (R\({}^{2}\) = 0.852) than the [PERSON] (R\({}^{2}\) = 0.737) model, implying a 1:1 stoichiometry  
Figure 5: Sorption Isotherms of PFOS on chitosan and results of modeling according to [PERSON] and [PERSON] models on **ECH1**. The error bars show the standard deviation from 3 CIC measurements per adsorption experiment. If no error bars are shown, they are smaller than the symbols. 108 h adsorption time, 100 mg adsorption material, 100 mL solution.  
between adsorbate and binding site. [PERSON] et al. came to the conclusion that the adsorption of PFOS on chitosan was described slightly better by the [PERSON] model (R\({}^{2}\) = 0.961 vs. 0.937), albeit at roughly double the equilibrium concentration of this work and under acidic conditions [24]. The maximum sorption capacity of PFOS, according to the Langmuir model in the present work, is 4.04 mmol/g, while [PERSON] et al. reported 5.21 mmol/g with a pH below 3. Considering that they reported a sorption capacity of PFOS that was two times higher for the acidic conditions compared to the neutral pH, the maximum sorption capacity reported in this work at a neutral pH value can also be expected to be even higher at the same acidic conditions used in the work of the [PERSON] group. We selected neutral pH adsorption conditions for our experiments to better reflect common real-world applications of PFAS adsorption. In a recent work, the [PERSON] group investigated a mixture of non-cross-linked chitosan with a covalent organic framework (COF) structure [27]. The maximum adsorption capacity reported in their work was 2.8 mmol/g for such a mixture, albeit with a much quicker PFOS uptake. Compared to other materials (activated carbon (0.35-1.0 mmol/g), minerals (0.3 \(\times\) 10\({}^{-3}\)-0.23 mmol/g, biochar (0.27-0.34 mmol/g), ion exchange resins (0.4-5.1 mmol/g)) in the literature, the herein presented chitosan showed one of the highest adsorption capacities for PFOS [7; 12; 13; 16; 39; 40; 41; 42], with the exception of the results discussed above and for selected ion exchange resins, but those were all tested at strongly acidic pH conditions. One notable caveat for the chitosan results is that they are all based on the dry weight of chitosan. However, in practice (typical application), the chitosan polymer contains about 95% (_w/w_) water (see Table S6), which implies that the weight of the wet polymers used in the experiments is approximately 20 times higher.