# Removal of Chromium(VI) by Chitosan Beads Modified with Sodium Dodecyl Sulfate (SDS)
## 3 Results and Discussion
### Adsorption Experiment
#### 3.1.2 Effect of pH on the Adsorption of Cr(VI)  
The solution pH is one of the most important parameters in the adsorption process. Figure 3 shows the effect of the pH on Cr(VI) adsorption by SDS-modified chitosan beads. In this experiment, the shaking time was 24 h, temperature was 25 \({}^{\circ}\)C, dose of adsorbent was 0.02 g/L, and initial Cr(VI) concentration was 1 mg/L. The acidity of the solution had a significant effect on the adsorption of SDS-modified chitosan beads towards Cr(VI), where the amino groups of chitosan were protonated and positively charged. Moreover, the sulfate group is a typical strong acid group. Therefore, even under the acidic condition, which would not easily hydrolyze with water, surfactants could behave fully in their anionic form. It is well-known that chitosan molecules are protonated at amino groups which carry cationic adsorption sites and that they are dissolved in the acidic region [36]. Apart from cationic amino groups, chitosan chain adsorption (leading to interparticle bridging) can also occur through hydrogen bonds [46]. Similar to cationic polyacrylamide, chitosan is, thus, able to aggregate anionic soluble compounds (through electrostatic affinities) and simultaneously flocculate particulate matter through interparticle bridging mechanisms. However, the protonation of amino groups (i.e., the effective charge density of the chitosan polymer chain) can be influenced by the pH [47]. This makes it impossible to conduct adsorption experiments at pH levels of 1-3. The effect of pH on the adsorption of Cr(VI) by SDS-modified chitosan beads was estimated by adjusting the pH in the range of 4 to 10 (Figure 3). The maximum uptake of Cr(VI) ions took place at pH 4-5, which may be attributable to the changes in the surface charge of the adsorbent. With an increase of the pH at above pH 5, the uptake decreased.  
Cr(VI) may be present in the form of HCrO\({}_{4}\)- and CrO\({}_{4}\)2. As shown in Figure 4, HCrO\({}_{4}\)- is dominant in the pH range of 2-4. However, CrO\({}_{4}\)2- becomes increasingly dominant as the solution becomes more basic, and the form of CrO\({}_{4}\)2- becomes stable above the pH of 7 [32]. The following equations indicate the shift process of Cr(VI) species:  
Footnote 2: The concentration of the carbon atoms is 1 mg/L.  
\[\mathrm{HCrO_{4}}^{-}\mathrm{\leftrightarrow CrO_{4}}^{2-}+\mathrm{H^{+}~{}pKa=5.9} \tag{9}\]  
\[\mathrm{H_{2}CrO_{4}}\mathrm{\leftrightarrow HCrO_{4}}^{-}+\mathrm{H^{+}~{}pKa=4.1} \tag{10}\]  
\[\mathrm{Cr_{2}O_{7}}^{2-}+\mathrm{H_{2}O}\mathrm{\leftrightarrow 2 HCrO_{4}}^{-}\mathrm{pKa=2.2} \tag{11}\]  
Figure 3: Effect of pH on the adsorption of Cr(VI) onto the SDS-modified chitosan beads.  
From these results, the functional groups combining with chromate ions may decrease due to the increase in their ion valence. It is supposed that amino acids with a positive charge combine with hydroxyl ions after binding with protons. Therefore, the adsorption capacity can largely depend on the kind of functional group and the existing form of Cr (VI). The surface of the SDS-chitosan beads became positively charged, owing to strong protonation in these pH ranges. This led to a stronger attraction between the positively charged surface and the negatively charged Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\) or HCrO\({}_{4}\)\({}^{-}\).