## 3 Results and Discussion
### Adsorption of Reactive Orange 16 (RO 16) as an Indicator for Fabric Sorption Properties
#### 3.3.5 Kinetic Experiments  
Since the establishment of an adsorption equilibrium is one of the most important aspects while designing an adsorption process, the effect of contact time on the \(q\) of J1.0% CS was investigated. The UV-V absorption spectra of dye solutions were acquired at time  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Fabric**} & \multirow{2}{*}{**Temperature, K**} & \multicolumn{4}{c}{**Thermodynamic Parameters**} \\ \cline{3-5}  & & \(\Delta G^{\circ}\), kJ/mol & \(\Delta H^{\circ}\), kJ/mol & \(\Delta S^{\circ}\), J/mol K \\ \hline \multirow{3}{*}{J} & 298.15 & \(-\)10.17 & \multirow{3}{*}{\(-\)25.81} \\  & 308.15 & \(-\)9.73 & & \\  & 318.15 & \(-\)9.59 & & \\  & 328.15 & \(-\)9.37 & & \\ \hline \multirow{3}{*}{J0.5\% CS} & 298.15 & \(-\)11.39 & \multirow{3}{*}{\(-\)17.16} & \multirow{3}{*}{\(-\)19.40} \\  & 308.15 & \(-\)11.12 & & \\  & 318.15 & \(-\)11.02 & & \\  & 328.15 & \(-\)10.78 & & \\ \hline \multirow{3}{*}{J1.0\% CS} & 298.15 & \(-\)12.71 & \multirow{3}{*}{\(-\)16.43} & \multirow{3}{*}{\(-\)12.58} \\  & 308.15 & \(-\)12.47 & & \\ \cline{1-1}  & 318.15 & \(-\)12.52 & & \\ \cline{1-1}  & 328.15 & \(-\)12.27 & & \\ \hline \multirow{3}{*}{J2.0\% CS} & 298.15 & \(-\)8.53 & \multirow{3}{*}{32.14} & \multirow{3}{*}{136.52} \\  & 308.15 & \(-\)9.75 & & \\ \cline{1-1}  & 318.15 & \(-\)11.82 & & \\ \cline{1-1}  & 328.15 & \(-\)12.37 & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Thermodynamic parameters for RO 16 adsorption onto investigated adsorbents.  
intervals of 15 min up to 3 h and 30 min up to 7 h of contact time, as shown in Figure 11. It is quite evident that the adsorption of RO 16 can be divided into three distinct phases:  
1. rapid dye adsorption during the first 2 h, adsorption process shows 79% of the equilibrium adsorption capacity of J1.0% CS which is ascribed to a high number of free available sites capable of binding dye molecules;
2. a slower rate of dye adsorption from 2 to 4.5 h reaching about 95% of the adsorbent equilibrium adsorption capacity, an increased competition of dye molecules present in the aqueous phase occurred as a consequence of the fewer available binding sites on the adsorbent surface;
3. dye adsorption becomes negligible in the last stage from 4.5 to 7 h, \(\Delta q_{t}\) is 4.8%, so it can be stated that equilibrium was reached after 4.5 h of contact time.  
The time required for attaining adsorption equilibrium (270 min) is quite similar or shorter when compared to other adsorbents, such as phosphoric acid activated carbon from _Melia azedarach_ waste sawdust [29] and waste sunflower seed shells [32].  
To explore the kinetics involved in RO 16 adsorption, the experimental data were fitted with pseudo-first-order and pseudo-second-order kinetic models, Figure 1. Higher deviation of \(q_{c,val}\) from \(q_{c,exp}\) and lower R\({}^{2}\) values were observed for the pseudo-first-order model compared to the pseudo-second-order model (\(\Delta q_{t}\) = 11.7% vs. 12.1%, R\({}^{2}\) = 0.9783 vs. 0.9987), inset of Figure 11. It thus appears that the system under study is more appropriately described by the pseudo-second-order model, which is based on the assumption that the rate-limiting step may be chemical sorption or chemisorption involving valency forces through sharing or exchange of electrons between adsorbent and dye [53].  
The findings of this section prove that the adsorption of selected dye can be used as an indicator for polysaccharide-based fabrics' sorption properties, as well as their capability to interact with different molecules participating in different interaction mechanisms.