# Wastewater Treatment by Polymeric Microspheres: A Review
## 3 Natural Polymer  
Natural polymers such as lignin, cellulose, or chitosan are often used, or synthetic microspheres are modified using a bio-based polymer such as vanillin. Natural polymers meet one of the requirements to minimize the secondary pollution in that they are mostly eco-friendly and biodegradable. Besides this, natural polymers also exhibit great performance as adsorbents (which is shown in Table 4). Natural polymers require extraction from their natural state. Thus, a new technique to directly extract lignin microsphere from highly alkaline black liquor was developed by lowering the pH under 4 and hydrothermal treatment [47]. Native lignin has the tendency to aggregate in solvents and has low adsorption capacities for metal ion or drugs. However, nano/microstructured lignin shows a much better performance due to increased surface area. Due to the strong adhesive property of lignin to cellulose, delignification and purification processes were required to isolate lignin. Field-Emission SEM (FESEM) imaging showed that the obtained microsphere had uniform poly-disperse morphology, a smooth surface, a relatively uniform size, the typical thermal stability of lignin, and great carbon  
Figure 5: Schematic illustration of the possible interaction mechanism between the H-PDA-MCs and MB (Reproduced with permission from [PERSON] et al. [41], Copyright 2021, Elsevier Ltd.). content around 50-60%. The method resolved the problems of extraction; it was upscalable and was low cost, the extraction efficiency was 15.87 to 21.62 g/L, and the average size of microspheres was around 1 \(\upmu\)m. The DTG profile revealed that highest weight loss rate for cellulose happened at 330 \({}^{\circ}\)C. The direct extraction of lignin microspheres from black liquor in large scale can be applied as agricultural drug delivery agents, heavy metal adsorbent, and UV absorbers for sun block. After extraction, natural polymers can be used as adsorbents. A polymer derived from vanillin methacrylate (VMA) monomer named poly vanillin methacrylate (PVMA) microsphere was fabricated for the adsorption of Cu\({}^{2+}\) ions. Figure 6 explains the preparation process. The microsphere was fabricated via suspension polymerization of VMA and the monomer provided by methacryloyl chloride reacted with vanillin, a biomass. The polymerization was conducted in aqueous media and had a high yield of a microsphere over 90% [48]. The microsphere was endowed with surface pores by controlling the nitrogen bubbling mode and optimizing the cosolvent for dissolving the solid monomer. The reactive aldehyde group of microspheres reacted with glycine, being a Schiff-base chelating material. The Cu\({}^{2+}\) concentration was measured via atomic absorption spectrophotometer and the microsphere showed a great adsorption capacity of Cu\({}^{2+}\) at a maximum of 135 mg/g. The \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
1 & Lignin & Lignin extraction efficiency & 21.62 g/L & - & - & [47] \\ \hline
2 & PVMA & Cu(II) & 135 mg/g & PSO & Freundlich & [48] \\ \hline
3 & HAP & U(VI) & 2659 mg/g & PSO & Freundlich & [49] \\ \hline
4 & NH\({}_{2}\)-SA/PNIPA & Cu(II), Cd(II) & 57.5 mg/g, 100.5 mg/g & PSO & Langmuir & [50] \\ \hline
5 & PEI/HMPCR & Cd(II) & 143.6 mg/g & PSO & Langmuir & [51] \\ \hline
6 & PAM, MBA, pine pollen & MB, MV dye & 668.96, 749.69 mg/g & PSO & Langmuir & [52] \\ \hline \hline \end{tabular}
\begin{tabular}{c c} PVMA: poly (vanillin methacrylate), HAP: hydroxyapatite, SA: sodium alginate, PNIPA: poly-N-isopropyl acrylamide, PEI: polyethyleneimine, HMPCR: cassava residue, MBA: N-N’-methylene-bis-acrylamide, MV: methyl violet. \\ \end{tabular}
\end{table} 
Table 4: Summary efficiency of natural polymer as adsorbent.