# Wastewater Treatment by Polymeric Microspheres: A Review
## 3 Natural Polymer
### Chitosan
Similarly, other CMs with BiSe, zinc-bismuth-selenide-chitosan microspheres (ZBiSe-CM), were fabricated as catalysts for photocatalytic degradation of CR dye [64]. First, ZBiSe nanoparticles were synthesized via a solvothermal process and then crosslinked with chitosan for support and to prevent leaching of the catalyst. SEM imaging showed that the average size was 30.9 nm for each nanoparticle and 812 um for the spherical and porous microsphere, and XRD analysis confirmed that crystallite size was 27.04 nm for the nanoparticles. The degradation efficiency of the ZBiSe-NP reached 99.63% for 40 ppm concentration of CR at catalyst dosage 0.225 g, pH 8.0, and  
Figure 11: Schematic representation of the adsorption mechanism (Reproduced with permission from [PERSON] et al. [62], Copyright 2021, Elsevier Ltd.). 36-38 \({}^{\circ}\)C for 2 h of solar light illumination. The photodegradation of microsphere followed PFO kinetics, and the microsphere remained its high decontamination efficiency for five cycles. Both BSCM-CM and ZBiSe-CM shows high potential for photocatalytic degradation of CR dye using clean and renewable energy of solar light irradiation sources, while ZBiSe-CM showed higher degradation efficiency than BSCM-CM. The effects of several factors for the absorption and the relationship between the factors were investigated using response surface methodology (RSM) modeling. A quaternized chitosan@chitosan cationic polyelectrolyte antibacterial microsphere (CCQM) was fabricated via the chemical crosslinking and emulsification method for removal of dyes and heavy metal ions [65]. Figure 12 presents the fabrication process. CCQM has a higher specific surface area, pore volume, and surface charges, lower crystallinity, and displayed higher adsorption capacities than did pristine chitosan microspheres. Through DTG analysis, it was found that the highest decomposition rate was at a lower temperature than pristine chitosan and that the addition of quaternized chitosan increased the thermal stability. The adsorption followed the Langmuir and PSO models and Fickian diffusion laws' equation, indicating that the adsorption was primarily monolayer chemisorption and pore diffusion mainly determined the adsorption rate. The adsorption reached equilibrium within 4 min, and the adsorption capacity was 1500 mg/g for CR, 179.4 mg/g for MO, and 687.6 mg/g for Cu(II), and 398.8 mg/g for Fe(III). The effective absorbance of Cu\({}^{2+}\) is due to its smaller steric hindrance. Cation exchange, chelating effect, proton exchange, and complex formation facilitated effective heavy metal ion removal, and electrostatic interaction and hydrogen bonding facilitated the removal of dyes. CCQM can be used as a filler in adsorption columns to purify wastewater, as it has superb recyclability, antibacterial viability for sterilizing, and biodegradability within 165 days. Other CMs made using the emulsion cross-linking method is chitosan-activated carbon composite  
Figure 12: Schematic diagram for the fabrication process of CCQM, the dissolved mixed solution of chitosan and quaternized chitosan (**a**) the corresponding simulated image (**d**), photograph of cationic polyelectrolyte microsphere preparation (**b**), the simulated cross-linked network image of chitosan and quaternized chitosan (**e**), the photograph of dried CCQM (**c**) and the corresponding simulated image (**f**) (Reproduced with permission from [PERSON] et al. [65], Copyright 2021, Elsevier Ltd.). microspheres, which were prepared for adsorption of MO [66]. A chitosan solution was mixed with activated carbon powder and then chitosan was cross-linked by epichlorohydrin under microwave irradiation, which effectively shortens the cross-linking time. SEM imaging displayed that the obtained microsphere had a diameter of 200-400 \(\upmu\)m and activated carbon powder was dispersed on the surface of chitosan composite microsphere. The maximum adsorption of MO was obtained when the mass ratio of chitosan to activated carbon was 10:4, labeled CA4, and the capacity was 35.4 mg/g.