# Wastewater Treatment by Polymeric Microspheres: A Review
## 2 Synthetic Polymer
### Polydopamine
\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 & PDA & MB dye & 191.55 mg/g & PSO & Langmuir, Temkin & [41] \\ \hline
2 & PDA & MB dye & 90.7 mg/g & PSO & Langmuir & [42] \\ \hline
3 & PDA & MB, MG, NR dye & 93.86, 91.98, 88.85 mg/g & PSO & Langmuir & [43] \\ \hline
4 & PAM/PA/PDA & Cu(II) & 231.36 mg/g & - & - & [44] \\ \hline
5 & PDA & Cr(VI) & 199.6 mg/g & PFO & Langmuir & [45] \\ \hline
6 & PDA@ZIF-8 & Cr(VI) & 136.56 mg/g & PSO & Langmuir & [46] \\ \hline \end{tabular}
\end{table} 
Table 3: Summary efficiency of PDA as adsorbent. also used as adsorbents for metal ions. A PAM/PA/PDA hydrogel-based electrochemical sensor for the detection of Cu\({}^{2+}\) ions was prepared through interactions between the DA, acrylamide (AAM), and phytic acid (PA) monomers [44]. They were made into microstructured 3D network to increase the surface area for an enhanced number of immobilized molecules and ions, as well as high conductivity. EIS was used to investigate the interfacial resistances. The PAM/PA/PDA hydrogel had a very low resistance of 0.3 k\(\Omega\), proving its superb electrical conductivity. The PAM/PA/PDA sensor showed a low detection limit of 1 nmol/L and a wide linear range. The hydrogel also showed a high adsorption capacity towards Cu\({}^{2+}\) ions (231.36 \(\pm\) 4.70 mg/g). The adsorption mechanism was inferred using XPS, which indicated that Cu\({}^{2+}\) ions strongly interact with hydroxyl and carboxyl group of the obtained hydrogel. The sensor made from PAM/PA/PDA hydrogel shows a high selectivity toward Cu\({}^{2+}\) ions and high reproducibility. Therefore, it can be used for both the detection and removal of Cu\({}^{2+}\) ions from aqueous condition. Besides incorporating other substances, the adsorption capacity of the PDA microsphere can also be enhanced by controlling the size of microsphere. A PDA microsphere inspired by mussels was prepared into controllable sizes through in situ oxidative polymerization at air condition by adjusting alkaline solutions (such as ammonia [45]). With lower amounts of ammonia, the PDA microsphere is likely to have a larger size. Dynamic light scattering (DLS) analysis was measured for investigation of particle size distribution, which were mostly in the range of 520-570 nm. The adsorption of Cr(VI) reached equilibrium within 8 min, at an optimal pH range of 2.5 to 3.8. The PDA microsphere showed remarkable sorption selectivity while competing ions of SO\({}_{4}\)\({}^{2-}\), supported by XPS results, NO\({}_{3}\)\({}^{-}\) and Cl\({}^{-}\) coexisted at high extent, due to the formation of the strong amine and carbonyl/hydroxyl group complexation. Due to well-dispersed morphology and a strong affinity between Cr(VI) and catechol or amine groups, the adsorption capacity reached 42,000 kg/kg sorbent, and wastewater could be reduced from 2000 ppb to below 50 ppb, meeting the drinking water criterion. One kg sorbent can purify around 100 t Cr(VI) contaminated wastewater, and efficiently regenerated by binary alkaline and salts mixtures. A core-double-shell structured magnetic polydopamine@zeolitic imidazolate framework-8 (MP@ZIF-8) hybrid microsphere was also prepared for the synergistic reduction and adsorptive removal of Cr(VI) [46]. The morphology and structure were confirmed using SEM/TEM images. The MP@ZIF-8 microsphere has a core of magnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles, a PDA layer as an inner shell, and a porous ZIF-8 nanocrystal as an outer shell. Magnetization hysteresis loops were also characterized, and the results showed that the microsphere had a low magnetization saturation value of 59.4 emu/g. It showed an improved Cr(VI) removal capacity of 136.64 mg/g, higher than that of pristine magnetic PDA, which had a Cr(VI) removal capacity of 92.27 mg/g. The removal kinetics followed the PSO kinetic model and were heavily dependent on pH values.