# Preparation of Freeze-Dried Porous Chitosan Microspheres for the Removal of Hexavalent Chromium
## 3 Results
### Adsorption Mechanism  
Figure 9 shows the FTIR spectra of powder chitosan, and porous chitosan microspheres, before and after the adsorption of Cr(VI), to demonstrate the adsorption mechanisms. The major bands of the chitosan powder were assigned as follows (as shown in Figure 9a): (1) an obvious and strong peak at the wavenumber region of 3370 cm\({}^{-1}\) was attributed to inter-molecular hydrogen bonding of chitosan, which was due to overlapping between -NH\({}_{2}\) and -OH stretching vibrations in the region of 3300-3500 cm\({}^{-1}\)[48]; (2) the peaks at 1660 cm\({}^{-1}\),1600 cm\({}^{-1}\) and 1380 cm\({}^{-1}\) presented -CONH bending vibrations of amide I, N-H bending vibration of amide I and -CH symmetric bending vibrations in -CHOH- of amide III, respectively [40; 49]. (3) The peak at 1080 cm\({}^{-1}\) presented -CO stretching vibration in -COH [49]. After the chitosan powder was prepared as porous chitosan microspheres, the intensity of the two peaks at 3370 cm\({}^{-1}\) and 1080 cm\({}^{-1}\) increased, which was associated with forms of intermolecular and intra-molecular hydrogen bonds between -OH, -COH and -NH\({}_{2}\) due to the chitosan chains stretching for dissolution. In addition, no other significant differences existed between the FTIR spectra of chitosan powder and porous chitosan microspheres. However, significant changes in the FTIR spectra took place for the porous chitosan microspheres before and after the adsorption of Cr(VI). Obviously, lower wavenumbers at 1640 cm\({}^{-1}\) and 1550 cm\({}^{-1}\) appeared in the FTIR spectra of the porous chitosan microspheres \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Intra-Particle Diffusion**} & \multirow{2}{*}{**Parameters**} & \multicolumn{3}{c}{**Freezing Time of the Chitosan Microspheres**} \\ \cline{3-5}  & & **24 h** & **48 h** & **72 h** \\ \hline first step & \(k_{\text{zem}}\) (mg g\({}^{-1}\) min\({}^{-0.5}\)) & 40.85 & 46.67 & 80.46 \\  & \(C\) (mg g\({}^{-1}\)) & 31.39 & 6.98 & 74.45 \\ second step & \(k_{\text{zem}}\) (mg/g min\({}^{-0.5}\)) & 21.4 & 26.65 & 16.83 \\  & \(C\) (mg g\({}^{-1}\)) & 188.01 & 229.87 & 631.87 \\  & \(r^{2}\) & 0.996 & 0.977 & 0.967 \\ third step & \(k_{\text{zem}}\) (mg g\({}^{-1}\) min\({}^{-0.5}\)) & 0.62 & 4.31 & 1.55 \\  & \(C\) (mg g\({}^{-1}\)) & 578.9 & 641.9 & 915.8 \\  & \(r^{2}\) & 1 & 1 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Intra-particle diffusion model for Cr(VI) adsorption by the chitosan microspheres prepared by different freezing times at a solution pH of 4. Figure 9: FTIR spectra of (**a**) chitosan powder and porous chitosan microspheres prepared with 3% (\(w/v\)) chitosan solution at \(-20\)\({}^{\circ}\)C for 24 h; (**b**) before adsorption; and (**c**) after adsorption of Cr(VI). after the adsorption of Cr(VI), different from the peak at 1650 cm\({}^{-1}\) assigned to -CONH bending vibration and 1590 cm\({}^{-1}\) assigned to N-H bending vibration, respectively. After the adsorption of Cr(VI), the intensity and width of the peaks at 3400 cm\({}^{-1}\) and 1080 cm\({}^{-1}\) of the porous chitosan microspheres decreased and increased, respectively, which indicated that the amino groups (-NH\({}_{2}\)), -COH, and hydroxyl groups (-OH) were adsorption active sites participating in Cr(VI) adsorption [50]. In fact, the amine and hydroxy groups were protonated to -NH\({}_{3}\)\({}^{+}\) and -OH\({}_{2}\)\({}^{+}\) with a positive charge in an acidic medium through electrostatic attraction and complexation reactions to remove Cr(VI) ions [36]. In addition, the presence of two new peaks at 935 cm\({}^{-1}\) and 775 cm\({}^{-1}\) appearing in the FTIR spectra of the porous chitosan microspheres after Cr(VI) adsorption suggested that Cr(VI) ions were adsorbed by the porous microspheres, possibly attributed to forms of Cr(OH)\({}_{3}\). The porous chitosan microspheres in this work can be regenerated.