# Wastewater Treatment by Polymeric Microspheres: A Review
## 3 Natural Polymer
### Chitosan
Cross-linking of CS improves mechanical property, but it consumes the functional groups that are beneficial for adsorption. Thus, a chitosan-methylThe CS-MA-DETA was fabricated using two steps as shown in Figure 14. First, MA was grafted to amino groups on CS microsphere surface. Next, DETA was grafted to terminal ester groups of MA on CS-MA microsphere surface. The fabrication was conducted without cross-linker, confirmed by FTIR and elemental analysis. The SEM image in Figure 15 shows that the microsphere had 3.04 \(\upmu\)m mean diameter and had uniform wrinkle-like topography, which are beneficial to capture metal ions. Elemental analysis showed that addition of MA increases the C/N ratio to 4.76. The specific surface area was 27.806 m\({}^{2}\)/g, the pore diameter was 3.452 nm, and TGA curves revealed that mass loss at the first step was 3%, which is around 10% of CS. Both CM with PHEMA brushes and CS-MA-DETA microsphere followed the PSO model and the Langmuir model, indicating that the adsorption process was primarily monolayer adsorption and chemisorption. Figure 13: The schematic diagram of preparing MCS-g-PSSS microspheres (Reproduced with permission from [PERSON] et al. [68], Copyright 2021, Elsevier Ltd.). Figure 16 shows the possible bonding modes and adsorption sites. The efficiency was 239.2 mg Pb(II)/g, or 201.6 mg Cd(II)/g. The microsphere can be readily regenerated and recycled for five cycles. Cross-linked chitosan (CCS) microspheres tethered with melamine-conjugated poly(hydroxyl methacrylate) (PHEMA) brushes were synthesized for effective Cu(II) uptake [71]. In order to solve the issue of consuming functional groups while cross linking, PHEMA and melamine was incorporated to add functional groups in this case. The microsphere was fabricated via surface-initiated atom transfer radical polymerization (ATRP) of hydroxyethyl methacrylate (HEMA) along with melamine being subsequently covalent immobilized onto the ends of the poly(hydroxylethyl methacrylate (PHEMA) brushes' chain. The water contact angle was measured to investigate the surface  
Figure 14: Schematic Preparation Procedures of CS-MA and CS-MA-DETA Microspheres. (CS-MA-DETA, DETA Grafted CSMA; DETA, Diethylenediamine; CS-MA, MA Grafted CS; MA, Methyl1 Acrylate; CS, Chitosan.) (Reproduced with permission from [PERSON] et al. [70], Copyright 2017, American Chemical Society). wettability. Grafting PHEMA brushes lowers the water contact angle due to the abundant hydroxyl groups of the surface of microsphere. The sufficient nitrogen from triazine heterocyclic structures and oxygen from the PHEMA chains offered increased adsorption sites. Adsorption of Cu(II) reached equilibrium within 20 min, and the adsorption process was controlled by intra-particle diffusion and chemisorption processes. The maximum adsorption capacity was around 4.68 mmol/L (or 299 mg/g) at pH 5, 55 \({}^{\circ}\)C and an initial Cu(II) concentration of 2.21 mmol/L. The adsorption mechanism was mainly ascribed to coordination or chelation interactions of amino or hydroxyl groups with Cu(II) ions. Figure 15: SEM images of CS, CS-MA-DETA, and Pb(II)-loaded CS-MA-DETA microspheres. ((**a–c**): pictures of CS microspheres; (**d–f**): pictures of CS-MA-DETA microspheres; (**g–l**): pictures of CS-MA-DETA microspheres after five-time adsorption for Pb(II); pictures of each group above are in turn taken at magnifications of 3 k, 15 k, and 30 k\(\times\). CS-MA-DETA, DETA grafted CS-MA; DETA, diethylenetriamine; CS-MA, MA grafted CS; MA, methyl acrylate; CS, chitosan.). (Reproduced with permission from [PERSON] et al. [70], Copyright 2017, American Chemical Society).