## 3 Results and Discussion
### Adsorption of Reactive Orange 16 (RO 16) as an Indicator for Fabric Sorption Properties
#### 3.3.1 Optimization of the Initial Solution's pH to Achieve Maximum RO 16 Removal  
The influence of the initial pH value on the removal/adsorption of RO 16 by different adsorbents was already noted by many authors [29; 30; 31; 32; 33; 34; 35]. Therefore, to infer more information about the nature of the adsorption process, the removal (in further text: \(R\)) of RO 16 by polysaccharide-based fabrics was studied in the pH range of 2-7 by adding appropriate amounts of 1 M HCl, or 0.1 M NaOH. As Figure 6 demonstrates, \(R\) of RO 16 by J, J0.5% CS, J1.0% CS is the highest (accounted by about 51, 60, and 76%, respectively) at a pH 2. Moreover, for fabric J2.0% CS, it can be stated that an optimal solution pH is 4 since there is no significant difference between the \(R\) of dye at a pH 3 and 4 (89.6 vs. 87.6%, Figure 6), and, additionally, the solution's pH 4 is easier to adjust since the lower amount of HCl needs to be added. Since RO 16 belongs to the class of anionic dyes and is negatively charged in the investigated pH range (2-7), it should be emphasized that different adsorption patterns of raw and chitosan-treated jute fabrics originate solely from the changes on the adsorbent surface caused either by the pH change or chitosan deposition. Firstly, the adsorption of the untreated jute fabric should be taken into account. According to the zeta potential measurements, the isoelectric point (IEP) of J is 2.80, indicating that at pH \(<\) 2.80 its surface is positively charged, while above this value, the adsorbent surface converts to negative. As the maximal adsorption for J is attained at pH = 2, it could be suggested that the  
Figure 5: Moisture sorption of raw and chitosan-treated fabrics. adsorption is mainly governed by the electrostatic attraction between negatively charged dye molecules and the positive jute surface. Considering that the molecular structure of RO 16 bears numerous functional groups (Figure 1), it can be supposed that electrostatic interactions are strengthened by multiple hydrogen bonds between dye and fabric surface functional groups, wherein dye can act both as a donor (secondary amino or hydroxyl groups) or acceptor (oxygen of the sulfone, sulfonyl, and carbonyl groups or nitrogen of azo bridge) of hydrogen bonds. Nevertheless, interactions with the lignin portion of jute fabric should not be ruled out since this polymer bears aromatic rings able to interact with the electron-rich phenyl and naphthyl groups of RO 16 via \(\pi\)-\(\pi\) stacking interactions [36; 37]. On the other hand, after the chitosan treatment (i.e., chitosan deposition on the jute fabric surface), it is obvious that the \(R\) of RO 16 by the jute fabric is reinforced. For all samples, as discussed above, the optimal \(R\) of RO 16 by J0.5% CS, and J1.0% CS, is also obtained at pH 2, as in the case of J, Figure 6. Promoted adsorption could be related to the increased number of NH\({}_{2}\) groups introduced with chitosan deposition [38]. These groups are protonated in a highly acidic medium, appearing as ammonium groups (NH\({}_{3}\)\({}^{+}\)), increasing the electro-positivity of the adsorbent at pH 2, thus offering more active sites for capturing negatively charged dye molecules. Further increase in chitosan concentration (-NH\({}_{3}\)\({}^{+}\) groups) does not bring the enhancement of the dye removal by J1.0% CS and J2.0% CS at pH 2, indicating fabrics' saturation with RO 16 molecules, i.e., there is a definite number of the fabric active sites that could be occupied by large RO 16 molecules. According to Figure 6, it is obvious that in the case of J2.0% CS, governing mechanism that participates in the \(R\) of RO 16 changes, as the optimal \(R\) is achieved approximately at the fabric IEP (between pH 3 and 4, Figure 6). As IEP refers to the pH value for which a number of positively and negatively charged fabric surface groups equals providing a formally neutral surface, the optimal \(R\) of RO 16 by J2.0% CS in a neutral environment is due to a governing mechanism other than electrostatic interactions.