# Wastewater Treatment by Polymeric Microspheres: A Review
## 3 Natural Polymer
### Chitosan  
Chitosan is obtained by treating chitin, obtained from living organisms. Ascribed to its eco-friendly and abundant functional groups such as amino and hydroxyl groups, it is used as adsorbents for pollutants. Additionally, modification into magnetic microsphere is possible due to its nano particle encapsulation property. Its performances as adsorbents are summarized in Table 7.  
Polyethyleneimine-functionalized chitosan/Span80 microspheres ((CYCTS/Span80)-@-PEI) were fabricated to remove diclofenac sodium (DS) from wastewater effectively [62]. Carboxylated chitosan (CYCTS) was added dropwise into cross-linking agent Zr4+ solution to spherical structure. PEI was grafted on to its surface, and nonionic surfactant Span80 was dissolute from microsphere using an organic solvent for formation of pores. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**S. No** & **Polymer** & **Adsorbate** & **Adsorbent Capacity** & **Kinetic Model** & **Isotherm Model** & **Reference** \\ \hline \hline
1 & (CYCTS/Span80)-@PEI & DS & 572.28 mg/g & PSO & Langmuir, Freundlich & [62] \\ \hline \hline
2 & BSCM-CM & CR dye degradation efficiency & 85\% & first order & - & [63] \\ \hline
3 & ZBiSe-CM & CR dye degradation efficiency & 99.63\% & PFO & - & [64] \\ \hline
4 & CCQM & CR, MO dye, Cu(II), Fe(II) & 1500, 179.4, 687.6, & & & \\ \hline
5 & chitosan-activated carbon composite & MO dye & 35.64 mg/g & PSO & Freundlich & [66] \\ \hline
6 & CTA-CSM & MO dye & 131.9 mg/g, 98.8\% & PSO & Langmuir & [67] \\ \hline
7 & CS-PSSS & MB dye & 854 mg/g & PSO & Langmuir & [68] \\ \hline
8 & CS & Cr(VI) & 945.2 mg/g & PSO & & [69] \\ \hline
9 & CS-MA-DETA & Pb(II), Cd(II) & 239.2, 201.6 mg/g & PSO & Langmuir & [70] \\ \hline
10 & CM with PHEMA brushes & Cu(II) & 299 mg/g & PSO & Langmuir & [71] \\ \hline \hline \end{tabular}
\begin{tabular}{c} CYCTS: carboxylated chitosan, DS: diclofenac sodium, BSCM: bismuth cobalt selenide, CM, CSM: chitosan microsphere, CR: Congo red, CCQM: quaternized chitosan@chitosan cationic microsphere, CTA: trimethyl ammonium chloride, PSS: poly(sodium 4-styrene sulfonate, CS: chitosan, PHEMA: poly(hydroxyethyl) methacrylate), MA: methyl acrylate, DETA: diethylenetriamine. \\ \end{tabular}
\end{table} 
Table 7: Summary efficiency of chitosan as adsorbent. The surface of microsphere, examined through SEM image, had substantial number of NH\({}^{3+}\) and a dense pore structure, which was useful for adsorption of DS. The DS adsorption was measured via UV-vis spectrophotometer, and the maximum adsorption was 572.28 mg/g at pH 5.00 and 27 \({}^{\circ}\)C with adsorbent loading of 15 mg. The adsorption process followed the PSO kinetic model, Langmuir isotherm model, and Freundlich isotherm model, indicating that the adsorption was a multi-layer process. Figure 11 depicts the main adsorption mechanisms (which were electrostatic interactions, physical adsorption, and hydrogen bonding). The removal rate of DS remained over 80% after five cycles with a high recovery rate. A chitosan-bismuth cobalt selenide hybrid microsphere (BSCM-CM) was made to remove organic pollutants from aqueous environment [63]. Bismuth cobalt selenide (BSCN) tri-composite nanoparticles with crystalline structures and a narrow bandgap of 2.48 eV were prepared and were embedded in chitosan microspheres via a solvothermal process to prevent the leaching of the photocatalyst and to enhance the recovery and reuse of the composite. The SEM image, FT-IR, and XRD results confirmed that the obtained BSCM-CM had smooth face morphology with an average nanoparticle size of 30 nm, an average microsphere size of 734 um, and a crystallite size of 21.3 nm. In sunlight radiation, the degradation efficiency for CR dye was 85% of the 90 ppm solution at catalyst amount of 0.6 g, pH 8, for 100 min. The adsorption was well described by first order kinetics, with a constant rate of 1.50 \(\times\) 10\({}^{-2}\) min\({}^{-1}\). The microsphere could be regenerated and reused for up to five cycles.