# Wastewater Treatment by Polymeric Microspheres: A Review
## 3 Natural Polymer
### Cellulose  
Cellulose is abundant in nature, has strong mechanical strength and abundant surface functional groups (hydroxyl groups), and is easy to modify. It is also biodegradable, being non-toxic (as are most natural polymers). Due to its beneficial properties, it is an adequate candidate for adsorbent of MB. The efficiency of its use as adsorbents and their functional groups is summarized in Table 5.  
Millimeter-size hollow microspheres were fabricated from carboxymethyl cellulose microspheres (CMC) and PEI with glutaraldehyde as a crosslinking agent [53]. The synthesis process is described in Figure 9.  
The hydroxide ion in PEI in aqueous solution forms precipitate with Al(III), which can break the coordination bonds between the carboxyl and Al(III) in carboxymethyl cellulose microspheres, forming the hollow structures. SEM analysis was used to investigate the effect of usage and treatment time of PEI, and the results showed that they can make difference in shell thickness and size. The original spherical structure was completely deviated to form a powder-like structure when the PEI usage exceeded 3% and the internal network of composites were loosened as PEI treatment time increased. Thus, the adsorption of CMC-Al@PEI-1.75 was studied. The obtained hollow microsphere showed a superior performance in adsorption of MB of 235 mg/g. The large accessible amount of -NH\({}_{2}\) groups and its unique hollow structure allowed the obtained microsphere to be applied in the treatment of wastewater. Another cellulose microsphere, palladium nanoparticles/cellulose microspheres (Pd NPs/CMs) were fabricated as catalyst for MB degradation [54]. The microsphere was prepared by in-situ reduction method based on the cellulosed micro \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**S. No** & **Polymer** & **Asorbate** & **Adsorbent Capacity** & **Functional Group** & **Reference** \\ \hline
1 & CMC-Al@PEI-1.75 & MB dye & 235 mg/g & Amino & [53] \\ \hline
2 & Pd NP/cellulose & MB dye & 99.8\% & Hydroxyl & [54] \\ \hline
3 & CNC/MnO\({}_{2}\)/SA & MB dye & 95.4\%, 114 mg/g & Carboxyl & [55] \\ \hline \hline \end{tabular} CMC: carboxymethyl cellulose, NP: nano particle, CNC: cellulose nanocrystal.  
\end{table} 
Table 5: Summary efficiency of cellulose as adsorbent. spheres. Pd nanoparticles grew on cellulose microspheres, with a Pd conversion rate of 33%. The spherical structure helped the cellulose microspheres function as suitable carriers for catalysts. The rate of decolorization was determined by UV-vis spectrophotometer. The decolorization efficiency of MB in presence of NaBH\({}_{4}\) was over 99.8% and the microsphere showed excellent reusability for five cycles. The decolorization was conducted through the destruction of the chromophores of MB. Zeta potential was measured to investigate the effect of pH. Solutions below pH 3.5 showed a low decolorization rate. The catalytic performance was better under an alkaline condition than in an acidic condition. First, the reactants were adsorbed on the surface of the catalyst ahead of the reaction, then the BH\({}_{4}^{-}\) transferred to metal from the surface of the catalyst. The reactions continuously relayed to destroy the chromophores of MB, consequently decolorizing it. A cellulose nanocrystal (CNC)/MnO\({}_{2}\)-based porous microsphere is also used for MB decolorization. It was fabricated to solve the problem of CNC-based microsphere, namely its low porosity and specific surface area that limits the adsorption ability in wastewater treatment despite its beneficial properties such as simultaneous adsorption, degradation ability, facile separation, and recyclability [55]. SEM imaging showed its porous, honeycomb-like structure which was formed by freeze-drying the air bubble templated emulsion and ionic crosslinking method that used SA as a crosslinked matrix. SA added carboxyl groups that provided negative charge for interaction with cationic MB dye. The CNC/MnO\({}_{2}\)/SA microsphere showed low density of 0.027 g cm\({}^{3}\) and high porosity of 98.23%.