# Wastewater Treatment by Polymeric Microspheres: A Review
## 4 Photocatalytic Degradation  
An advanced oxidation process (AOP) that involves the production of \(\cdot\)OH is one promising method for removal of organic pollutants [72]. In one method of AOP, Fenton process, \(\cdot\)OH is easily produced by Fe\({}^{2+}\) catalyst and H\({}_{2}\)O\({}_{2}\) is often used as one method, but the conventional method requires large amounts of iron sludge and a high dosage of H\({}_{2}\)O\({}_{2}\)[73]. The combination of heterogenous photo-Fenton and photocatalysis enhances the degradation efficiency of pollutants as well as recyclability and overcomes many problems that conventional methods face [74]. Introducing polymer such as chitosan and PVA can enhance the performance of catalyst through enhanced mechanical properties and adsorption capacity [75]. Immobilization of photocatalysts such as ZnO or TiO\({}_{2}\) can help inhibit the recombination of electrons and holes, increasing \(\cdot\)OH production. Such  
Figure 16: Schematic preparation procedure of CS-MA-DETA microspheres and probable bonding modes and adsorption sites for Cd(II) and Pb(II) on CS-MA-DETA microsphere surface. (CS-MA-DETA, DETA grafted CS-MA; DETA, diethylenetriamine; CS-MA, MA grafted CS; MA, methylacrylate; CS, chitosan.) (Reproduced with permission from [PERSON] et al. [70], Copyright 2017, American Chemical Society). modifications have the ability to scale up for commercial uses [76]. Table 8 summarizes the efficiency of photocatalytic spheres. The photo-Fenton process is also used in the decolorization of dyes. Fe/polymer-based photocatalysts were made for the adsorption of MB [77]. Alginate and chitosan spheres were studied, but since the polymer chain of alginate chitosan degraded under UV irradiation, only the chitosan sphere was investigated further. Figure 17 shows the Fe/Alginate beads and chitosan spheres. Immobilization of Fe\({}^{2+}\) was conducted via ultrasound (US) bath and EDS analysis confirmed that the sorption of iron process took 15 min, substantially shorter than the conventional way, which takes 30 h. The amino group of chitosan captures the Fe\({}^{2+}\) ions. The removal efficiency for MB reached 98.8% within 6 min at pH 3, with 300 mg/L of H\({}_{2}\)O\({}_{2}\) and 2 g of Fe/chitosan/US-15 min sphere. The effects of such factors were investigated using RSM. TG, DTG analysis showed that the main decomposition occurred at 150-350 \({}^{\circ}\)C. The discoloration followed the PFO kinetic model and the L-H isotherm model. For actual industrial wastewater, the catalytic sphere showed 96% for decolorization and a very high efficiency toward real effluents. Another photocatalyst for dye removal is FCF/PDA-PVDF beads (FDPB), which were fabricated by carbothermal reduction to immobilize Fe species/biochar fiber composite (FCF) catalyst for photodegradation of MO [78]. Powdered FCF was mixed with PDA-modified (polyvinylidene fluoride) PVDF through a phase inversion process. SEM imaging shows that FDPB had macropores varying in the range of 1.0 \(\upmu\)m to 100 \(\upmu\)m inside the beads. The structure allows the self-floating property derived from abundant air chambers inside the bead, which is beneficial for collecting light for photodegradation. Electron-rich material such as FCF increases the reduction of Fe\({}^{3+}\) into Fe\({}^{2+}\). In addition, PDA excited under visible light can produce electrons for formation of -OH. Figure 18 shows result of MO degradation by the UV-vis spectra and assumed degradation pathways via LC-MS analysis. The FDPB almost showed perfect decolorization efficiency in 60 min. The adsorption mechanism was described in two stages, surface adsorption and intra-particle diffusion. Figure 19 shows the schematic of the assumed mechanism of photo-Fenton degradation. FDPB has a hierarchical porous structure that allows for the rapid diffusion of MO, and PDA does its role of blocking the leaching of iron by its abundant catechol groups and by enhancing the adsorption and diffusion of MO.