# Wastewater Treatment by Polymeric Microspheres: A Review
## 2 Synthetic Polymer
### Polydopamine  
Polydopamine (PDA) is often chosen as adsorbents due to its abundant functional groups and the fact that it can easily coat various substances. PDA-based microspheres mostly showed high adsorption capacities toward MB dye and Cr(VI) ions, as shown in Table 3.  
H-PDA-MCs (Hollow polydopamine microcapsules) were synthesized via the oxidation self-polymerization of dopamine, using a Fe\({}_{2}\)O\({}_{3}\) nano-cube as a template, to purify organic wastewater by capturing organic dyes [41]. The obtained microcapsule had a hollow, mesoporous structure with a large specific surface area of 24.4 m\({}^{2}\)/g and it had an electronegative surface under neutral or alkaline conditions. The results of a batch adsorption experiment show that it had an adsorption capacity for MB of 191.55 mg/g with a short equilibrium time of 60 min at 25 \({}^{\circ}\)C. The adsorption process follows the Temkin and Langmuir isothermal linear models and was a spontaneous and endothermic process. The effective capture of MB results from multiple interactions of electrostatic interaction, hydrogen bonding, and \(\pi\)-\(\pi\) stack as shown in Figure 5. To investigate the adsorption mechanism, UV-vis spectra was used. The adsorption peak of MB in the microsphere showed a bathochromic shift compared to the pristine MB due to the interaction between the microsphere and the dye. PDA microspheres prepared using a similar oxidative polymerization method was tested for adsorption of MB from aqueous solution [42]. TEM imaging showed that the microsphere had an average diameter of around 600 nm, which results in a large specific surface area. MB adsorption was highly dependent on the initial solution pH, temperature, initial MB concentration, and contact time. The adsorption capacity of the obtained microsphere was 90.7 mg/g at 25 \({}^{\circ}\)C, and its high performance is driven from the \(\pi\)-\(\pi\) stacking, electrostatic interactions, and the structure properties of PDA microspheres (which is supported by the FTIR results). Its monolayer adsorption capacity was 88.89 mg/g. Additionally, the adsorption process took two steps, and the intraparticle diffusion was not the rate-limiting step. Thermodynamic analyses revealed that the adsorption was spontaneous, endothermic, and a physisorption process. Another PDA microsphere was fabricated for the selective adsorption and separation of organic dyes [43]. The PDA microsphere was synthesized via oxidation polymerization under basic conditions. Its adsorption capacity for nine water-soluble dyes was evaluated. It showed high, efficient adsorption ability toward cationic dyes (MB): malachite green (MG) and neutral dye (NR). However, it showed low ability towards cationic dyes (safranine T (ST), and RhB), and almost no adsorption ability for anionic dyes: eosin-Y (EY), eosin-B (EB), acid chrome blue K (ACBK), and MO. UV-vis spectroscopic spectra revealed that the chemical structures of dye molecules are the cause of varying adsorption abilities for different dyes. The adsorption for cationic and neutral dyes followed the PSO model and the Langmuir isotherm model. The selective adsorption of the PDA microsphere has potential in separating mixture of dyes in aqueous solution. The above three PDA microspheres for cationic dye adsorption all followed the PSO model and the Langmuir isotherm model, indicating the monolayer adsorption. In addition, electrostatic interaction and \(\pi\)-\(\pi\) interaction were the main mechanisms of the adsorption. PDA based microspheres are