# Wastewater Treatment by Polymeric Microspheres: A Review
## 3 Natural Polymer
### Chitosan
TG curves showed that CA4 loses weight rapidly in the temperature range 200-350 \({}^{\circ}\)C due to the decomposition of chitosan. The adsorption was dependent on temperature, and maximum adsorption occurred at 30 \({}^{\circ}\)C. The adsorption followed the PSO model and the Freundlich isotherm model. The microsphere retained 94.49% of its adsorption capacity for MO after it was reused for three cycles. A quaternary ammonium salt-modified chitosan microsphere (CTA-CSM) was prepared for the treatment of dye in wastewater [67]. The CTA-CSM was fabricated through an emulsion cross-linking reaction between 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CTA) and chitosan. SEM imaging confirmed that the microsphere had smooth surface and was well dispersed. Through laser light scattering particle size analyzer, it was discovered that particle size was distributed in the range of 5 to 125 \(\upmu\)m. The grafting of CTA on the CS microsphere enhanced the adsorption capacity for MO dye from previous reports, as the maximum removal rate was 98.8% and adsorption capacity was 131.9 mg/g under optimal condition. The MO adsorption process well fitted by the Langmuir isotherm model and followed the PSO kinetic model. Additionally, the CTA-CSM retained its high removal rate after it was reused for five cycles over 87.4%. Thus, CTA-CSM is a promising adsorbent for treating wastewater that contains MO dye for its cost-effective, sustainable, and highly reusable properties. For the removal of cationic dyes and MB, a MCS-g-PSSS microsphere was made by grafting poly(sodium 4-styrene sulfonate) onto magnetic chitosan microspheres [68]. Figure 13 shows the preparation process. The magnetic chitosan microspheres were prepared by reverse-phase suspension. Then, SSS was grafted onto MCS microspheres. XRD patterns showed that MCS-g-PSSS microsphere still had Fe\({}_{3}\)O\({}_{4}\) peaks, which implies that the microsphere is still magnetic after grafting of PSSS. Magnetic hysteresis loops show that the saturation magnetization the microsphere was 5.2 emu/g. The adsorption process followed PSO model, and the adsorption isotherms followed the Langmuir model. The saturated adsorption capacity reached 854 mg/g for MB at an initial concentration of 1000 mg/L at pH 7.0 and 0.02 g of feed dosage at 25 \({}^{\circ}\)C. FT-IR and XPS results confirmed that the high adsorption capacity is driven from the strong electrostatic interactions and \(\pi\)-\(\pi\) stacking. The adsorption capacity increased with a rising temperature and decreased with ion addition. The spontaneity of the adsorption increased as temperature increased. Chitosan microspheres are also used as adsorbents of heavy metal ions. A porous chitosan microsphere was produced without other polymers using the freezing-lyophilization drying method for the removal of hexavalent chromium, Cr(VI) [69]. The optimal chitosan microsphere was produced by freezing chitosan hydrogel beads at \(-20\)\({}^{\circ}\)C and subsequently lyophilizing the frozen structure, which allowed for the easy obtaining of some beneficial structures of the chitosan microspheres for efficient removal, rough surfaces, and large pores. The microsphere was characterized using porosity analysis. Microspheres with good sphericity, thinner pore walls, and small pore sizes made with an initial chitosan solution concentration of 3% (\(w/v\)), a syringe diameter of 500 \(\upmu\)m, a pH range from 3.0 to 5.0, a temperature of \(-20\)\({}^{\circ}\)C, and 72 h freezing time showed a maximum adsorption capacity of 945.2 mg/g for Cr(VI). The adsorption capacity increased as freezing time increased. The adsorption rate was governed by multiple steps, and the adsorption was primarily attributed to amine and hydroxy groups of chitosan microspheres. In optimal conditions, the equilibrium adsorption rate was 1.83 \(\times\) 10\({}^{-5}\) g/mg\(\cdot\)min and the adsorption amount at equilibrium was 107.05 mg/g.