# Wastewater Treatment by Polymeric Microspheres: A Review
## 3 Natural Polymer
Figure 6: Schematic strategy for preparing polymeric microspheres starting from vanillin (**A**,**B**), for preparing Schiff-base chelating microspheres by reaction with glycine (**C**) and for performing adsorption by the microspheres toward Cu\({}^{2+}\) ions (**D**) (Reproduced with permission from [PERSON] et al. [48], Copyright 2016, American Chemical Society). adsorption process was chemical adsorption, and the Schiff-base structures and the carboxylic groups both contributed to adsorption. TGA analysis confirmed that the polymer chains decomposed in the range of 250 to 480 \({}^{\circ}\)C.  
Figure 7 shows successful adsorption from the color change of the microsphere before and after adsorption. The microsphere can be used as green adsorbents for the treatment of wastewater and can also be used as biomaterials for enzyme immobilization. The microsphere can also be prepared using other lignin derivatives instead of vanillin, such as syringaldehyde and p-hydroxy benzaldehyde. For the removal of uranium from wastewater, a hydroxyapatite (HAP) microsphere adsorbent was fabricated using a template-free hydrothermal method [49]. XRD analysis, SEM images, and TEM images as shown in Figure 8 revealed that HAP was crystallized in a hexagonal structure and had a hollow and hierarchical nanostructure. It had a large surface area; the BET surface area was 182.6 m\({}^{2}\)/g and the average pore size was 10.5 nm, indicating efficient removal of U(VI) from wastewater within 5 min ascribed to the abundant active sites that the HAP microspheres provide. The uranium concentration was measured though a uranium analyzer. The XRD peaks showed that the surface chemisorption between U(VI) and HAP formed new uranium-containing compound named autunite (Ca(UO\({}_{2}\))(PO\({}_{4}\))\({}_{2}\).3H\({}_{2}\)O), which is assumed as main sorption mechanism. The adsorption process of both PVMA microsphere and HAP microsphere followed Freundlich isotherm and PSO kinetic model. Figure 7: Typical photographs of pure PVMA microspheres, chelating microspheres, and the microspheres after adsorbing Cu\({}^{2+}\) (Reproduced with permission from [PERSON] et al. [48], Copyright 2016, American Chemical Society). Amine terminated sodium alginate/poly-N-isopropyl acrylamide (NH\({}_{2}\)-SA/PNIPA) thermosensitive hydrogel microsphere was fabricated for treatment of heavy metal ions in wastewater [50]. NH\({}_{2}\)-SA/PNIPA showed high, but different adsorption capacity for copper and cadmium ions, due to their different hydration. The concentrations of metal ions were measured via atomic absorption spectrophotometer. The adsorption capacity for Cu(II) was 57.5 mg/g at 20 \({}^{\circ}\)C and 56.7 mg/g at 40 \({}^{\circ}\)C. The adsorption capacity for Cd(II) was 100.5 mg/g at 20 \({}^{\circ}\)C and 88.6 mg/g at 40 \({}^{\circ}\)C. Higher temperatures facilitated the adsorption process, but the adsorption capacity of NH\({}_{2}\)-SA/PNIPA decreased at temperatures above the LCST because NH\({}_{2}\)-SA/PNIPA became hydrophobic and its volume shrank. Diffusion coefficient D\({}_{\text{s}}\) was obtained by HSDM (homogeneous surface diffusion model), 4.75 \(\times\) 10\({}^{-11}\) m\({}^{2}\)/s at 20 \({}^{\circ}\)C and 3.75 \(\times\) 10\({}^{-11}\) m\({}^{2}\)/s at 40 \({}^{\circ}\)C for Cu(II), and 3.24 \(\times\) 10\({}^{-11}\) m\({}^{2}\)/s at 20 \({}^{\circ}\)C and 1.84 \(\times\) 10\({}^{-11}\) m\({}^{2}\)/s at 40 \({}^{\circ}\)C for Cd(II). The adsorption process followed the PSO model and the Langmuir isotherm model, and the process was mainly controlled by complexation reaction. The microsphere could remain its adsorption performance for five cycles. Polyethyleneimine modified magnetic porous cassava residue microspheres (PEI/HMPCR) was prepared for adsorption removal of Cd(II) from wastewater [51]. PEI/HMPCR was prepared via reverse emulsion polymerization with pretreated cassava residue (PCR) as raw material, modified by linear PEI and PEG2000 as porogen.