# Preparation of Freeze-Dried Porous Chitosan Microspheres for the Removal of Hexavalent Chromium
## 3 Results
### The Adsorption Properties of Porous Chitosan Microspheres
#### 3.3.2 Effect of Freezing Time  
According to Section 3.2.4, freezing time played an important role in the structure and porosity of porous chitosan microspheres, which would influence the adsorption property of porous chitosan microspheres for metal ions. Figure 6 shows the adsorption kinetic curve of microspheres prepared with different freezing times (24 h, 48 h and 72 h) and the initial Cr(VI) concentration was kept at 235 mg/L. It can be seen that the adsorption capacity of the microspheres increased with the increase in freezing time. When the freezing time is 72 h, the Cr(VI) adsorption capacity of the porous chitosan microspheres reached 945.2 mg g\({}^{-1}\) under adsorption equilibrium, which was higher than that of a 24-h freezing time with a maximum adsorption capacity of 590.6 mg g\({}^{-1}\) and a 48-h freezing time with a maximum adsorption capacity of 723.6 mg g\({}^{-1}\). These might be attributed to the increase in freezing time resulting in thinner pore walls, higher porosities, and larger specific surface areas of the microspheres, and more functional groups exposed on the surface and in the interior of the chitosan microspheres, which would participate in Cr(VI) adsorption. Compared with the chitosan-based adsorbents in the literature, the maximum adsorption capacity of porous chitosan microspheres (945.2 mg g\({}^{-1}\)) in present study was significantly higher than that of other chitosan-based adsorbents, such as xanthated chitosan (625 mg g\({}^{-1}\)) [37], thiocarbamoyl clitosan (434.8 mg g\({}^{-1}\)) [38], alkyl-substituted polyaniline/chitosan composites (229.8 mg g\({}^{-1}\)) [39], and wet chitosan beads (only 1.292 mg g\({}^{-1}\)) [36]. The reason for this might be that the dry and porous chitosan microspheres provided a lighter denominator in the formula calculation, and the special preparation method facilitated active functional groups exposure and not being consumed by other materials, such as crosslinking agents or plasticizers.  
On the other hand, adsorption time is an important factor that could reflect the adsorption rate of an adsorbent at a certain adsorbate concentration [40]. As shown in Figure 6, quick Cr(VI) adsorption processes took place in first 120, 120, and 180 min of contact time for freezing times of 72, 48, and 24 h, respectively; the adsorption curves then gradually reached equilibrium. This was related to the presence of more vacant active sites of the chitosan microspheres and more free Cr(VI) ions in the beginning of the adsorption process, which gave them more opportunities to combine with each other. Otherwise, after a number of active sites and free Cr(VI) ions were combined, the ion  
Figure 6: The adsorption kinetic curves of porous chitosan microspheres prepared at \(-20\)\({}^{\circ}\)C for different freezing times at a chitosan solution concentration of 3%, (_w/v_) using a 500-\(\upmu\)m syringe needle.  
concentrations and the amount of active sites decreased significantly and the intraparticle diffusion of Cr(VI) limited the adsorption rate, resulting in a slow adsorption. When the freezing times were 24 h and 48 h, the slopes of the adsorption kinetic curves of the chitosan microspheres were smaller than those of the 72-h freezing time within 120 min of contact time, but they were bigger after 120 min of contact time. These indicated that the chitosan microspheres prepared by 72 h of freezing time were quicker to reach a stable value relative to the maximum and had an excellent adsorption rates. All in all, preparation of the porous chitosan microspheres for a freezing time of 72 h had a higher adsorption capacity and adsorption rate for Cr(VI) than those of other freezing times. The swelling degree of the porous chitosan microspheres prepared for 24 h, 48 h and 72 h freezing times were 5.68 \(\pm\) 0.11, 5.71 \(\pm\) 0.13, and 5.72 \(\pm\) 0.09, respectively. The porosity might be the main influence on the adsorption properties of porous chitosan microspheres [25].