## 3 Results and Discussion
### Adsorption of Reactive Orange 16 (RO 16) as an Indicator for Fabric Sorption Properties
#### 3.3.3 Isotherm Experiments  
After adopting the previously optimized pH and adsorbent dose, isotherm studies were carried out to gain insights into the distribution of adsorbate at the adsorbent-solution interface [47]. For that purpose, 100 mL of RO 16 aqueous solution of defined concentration (25, 50, 75, or 100 mg/L) was agitated with studied polysaccharide-based fabrics for 21 h. Thereafter, the \(q\) of each fabric was calculated and presented in Figure 8. Obtained results revealed that with the increase in initial RO 16 concentration from 25 to 100 mg/L, the amount of dye adsorbed per gram J, J0.5% CS, or J1.0% CS, increased from 3.49 to 5.22, 4.09 to 7.89, and 6.15 to 11.77 mg/g, respectively. This trend is attributed to the intensified collision among the dye molecules and adsorbent and lower mass transfer resistance at high dye concentrations, both contributing to an increased fabrics' \(q\)[33; 48; 49]. Unexpectedly, fabric J2.0% CS showed the opposite trend. Its \(q\) decreased from 5.08 down to 2.85 mg/g with increasing the initial RO 16 concentration (Figure 8), which can be attributed to a high mass of chitosan per mass of fabric and governing binding mechanisms other than electrostatic interactions, as it was explained above. It is good to mention that at the initial dye concentration of 50-100 mg/L, among all tested fabrics, the lowest \(q\) is observed for fabric J, which was improved after the treatment with 0.5 or 1.0% chitosan. When the initial dye's concentration was set at 100 mg/L, by increasing the chitosan percentage from 0 to 0.5 and 1.0, \(q\) of J0.5% CS and J1.0% CS increased by 1.5 and 2.2 times, respectively. For both J0.5% CS and J1.0% CS, favorable concave-shaped isotherms without strict plateau were observed, indicating their higher affinity for RO 16 and a saturation trend at high dye concentrations [32]. The data presented in Figure 8 were modeled using linearized forms of Langmuir and Freundlich isotherm models (Figure 1), and the isotherm parameters calculated from the intercepts and slopes of the corresponding linear plots (Figure 9) are listed in Table 2. The values of the correlation coefficients (\(R^{2}\)) were compared, and it was found that the adsorption of RO 16 onto studied fabrics was better represented by the Langmuir than by the Freundlich isotherm (\(R^{2}\) = 0.996-0.999 vs. 0.914-0.986). The applicability of the  
Figure 8: The effect of initial RO 16 concentration (\(c_{0}\)) on the adsorption potential (\(q\)) of raw and chitosan-treated fabrics (\(c_{0}\) = 25, 50, 75, or 100 mg/L, \(t\) = 21 h, \(T\) = 25 \({}^{\circ}\)C, pH of 2.0 (for J, J0.5% CS, and J1.0% CS) and 4.0 (for J2.0% CS), the mass of adsorbent 0.25 g). Langmuir isotherm model means that the adsorption occurs at specific, energetically equivalent sites of the fabric with monolayer coverage of the dye over a homogeneous adsorbent surface [36]. This model was also found suitable for describing the adsorption of RO 16 onto corn cob [32], ethylenediamine modified rice hull [50], cotton fabric loaded with metal-organic frame material UiO-66 [48], phosphoric acid activated carbon from _Melia azedarach_ waste sawdust [29], jute stick activated carbon [33], and waste sunflower seed shells [32]. It should not be neglected that the Langmuir model allows the calculation of the Langmuir constant, \(K_{L}\), and adsorbent maximum adsorption capacity, \(q_{m}\) (Table 2). Much higher \(K_{L}\) values observed for J1.0% CS than for other fabrics suggest a stronger interaction between that fabric and RO 16 than between other fabrics and dye. Furthermore, based on the results listed in Table 2, it is evident that fabrics treated with 0.5 or 1.0% chitosan are characterized by 1.7 or 2.3 times higher \(q_{m}\) than untreated fabric. The situation is different in the case of J2.0% CS.