# Preparation of Freeze-Dried Porous Chitosan Microspheres for the Removal of Hexavalent Chromium
## 3 Results
### The Adsorption Properties of Porous Chitosan Microspheres  
#### 3.3.1 The Choice of pH of Initial Adsorption Solution  
The pH level of waste water affects the adsorption capacity of an adsorbent in many cases, which is ascribed to the fact that it can affect the surface charge of an adsorbent and the degree of ionization and the nature of ions [33]. The main reason for the adsorption of Cr(VI) ions on chitosan is the protonation of active sites on the surface of the microspheres. This is to say, -NH\({}_{2}\) is cationic into -NH\({}_{3}\)\({}^{+}\) and -NH\({}_{2}\)\({}^{+}\), which results in the differences in Cr(VI) anion ion forms with significant removal differences. When the pH value was less than 2, the chromium ion was converted into neutral H\({}_{2}\)CrO\({}_{4}\), which could not be adsorbed by the protonated sites on the surface of the adsorbent materials. When pH was about 3, a change in the form of Cr(VI) resulted in a decrease in the adsorption capacity [34]. On the contrary, when the pH value was higher than 5, dichromatic ions were transformed into monochromatic ions with the same charge; this is to say that both HCrO\({}_{4}\)\({}^{-}\) and Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\) were transformed into CrO\({}_{4}\)\({}^{2-}\), resulting in a decrease in Cr(VI) absorption [35].  
Figure 4: Effects of microneedle diameters on the morphology of porous chitosan microspheres prepared from 3% (\(w/v\)) chitosan solution at \(-20\)\({}^{\circ}\)C for 24 h.  
Some other heavy metals, such as Pb(II) and Cd(II), may coexist with Cr(VI) in industrial wastewater, and thus it is necessary to investigate the adsorption capacities of chitosan porous microspheres for Cr(VI) in the presence of these coexisting ions. Due to the presence of NH-C=O, -OH, -COH and -NH\({}_{2}\) groups, porous chitosan microspheres showed a higher affinity towards multivalier metals, but the pH of the initial adsorption solution beneficial to the maximum adsorption efficiency for Pb(II), Cd(II) and Cr(VI) was different. The adsorption capacities of Pb(II), Cd(II) and Cr(VI) were maximal at pH values of 6, 6, and 3, respectively. For Cr(VI) ion adsorption, the electrostatic attraction between the positively charged microspheres (protonated amino and carboxylic groups of adsorbent at lower pH values) and negatively charged HCrO4\({}^{-}\) can result in the maximum absorption of Cr(VI) ions.  
In an alkaline medium, OH\({}^{-}\) ions occupied the active cites of the adsorbents, which was not conducive to adsorption. For the removal of Cr(VI), the initial pH value was 4, according to the report of [PERSON] et al. (2016), who reported that the predominant species of chromium in acidic aqueous solutions were HCrO4\({}^{-}\) and Cr2O7\({}^{-}\), and the surfaces of the adsorbents were highly protonated, which benefited the absorption of Cr(VI). Hence, a pH range from 3.0 to 5.0 was beneficial to the maximum adsorption efficiency for Cr(VI), and further adsorption experiments were carried out at pH 4.0. Meanwhile, the acid-base resistance of porous chitosan microspheres at pH 4.0 was better than that at pH 3.0 and  
Figure 5: Effects of freezing time on the morphology of porous chitosan microspheres obtained at \(-\)20 \({}^{\circ}\)C for various freezing times: (a) 24 h, (b) 48 h and (c) 72 h; with a chitosan solution concentration of 3% (\(w/v\)) and syringe needle diameter of 500 \(\upmu\)m.  
5.0. The acid-base resistance at pH 4.0 of porous chitosan microspheres prepared with 3% chitosan solution at \(-20\)\({}^{\circ}\)C for 24 h was 0.96 \(\pm\) 0.01, while those at pH 3.0 and 5.0 were 0.91 \(\pm\) 0.02 and 0.89 \(\pm\) 0.01, respectively.