# Wastewater Treatment by Polymeric Microspheres: A Review
## 4 Photocatalytic Degradation
### Alginate  
Alginate beads are often used as a network for photocatalysts. Light can readily penetrate through an alginate bead from outside of the bead. It can be used as photo-bioreactor removal of nutrients (not just dyes) from wastewater [79]. PVA/SA hydrogel beads were fabricated with a across-linking agent of \(\mathrm{FeCl_{3}}\) and boric acid as the catalyst for the photo-Fenton process of TC [80]. Hydrogels rose as a promising 3D material as a heterogenous photo-Fenton catalyst. The optical image of PVA/SA-\(\mathrm{FeCl_{3}}\) bead showed that it had a diameter around 5 mm, and SEM imaging showed that average pore size was around 1.16 \(\upmu\)m. The removal rate reached a maximum of 90.5% within 60 min for TC, at an \(\mathrm{H_{2}O_{2}}\) concentration of 6 mM under visible-light irradiation. The degradation increased when \(\mathrm{H_{2}O_{2}}\) concentration increased from 2 to 6 mM, but decreased after it reached 6 mM, which may be attributed to excessive secondary reactive species that can compete with reactive species for photocatalysis such as \(\cdot\)OH or \(\mathrm{h^{+}}\). While the TC reached 90.5%, the degradation efficiency was 28.6% for TOC. Through free-radical and hole-trapping experiments, the main species for TC degradation was revealed as \(\mathrm{h^{+}}\), with \(\cdot\)O\({}_{2}\)\({}^{-}\) and \(\cdot\)OH also contributing. Inductively coupled plasma atomic emission spectroscopy (ICP measurement) results suggest that the PVA/SA-\(\mathrm{FeCl_{3}}\) beads have good stability through repeated degradation cycle, but after five cycles, Fe could leach out from the catalyst, therefore requiring an enhancement in stability. Other catalysts for dye removal were made using HAP. Three catalysts were made from SA beads, SA-magnetic HAP (Alg-mHAP), Alg-m, Alg-HAP, for removal MO [81]. HAP supports and strengthens the catalyst performance due to its high ion-exchange and adsorption capacity. SEM imaging showed that the  
Figure 18: UV–vis spectra (**a**), GC–MS (**b**) analysis and possible degradation pathways (**c**) of degrading MO during photo-Fenton reaction (Reproduced with permission from [PERSON] et al. [78], Copyright 2020, Elsevier). morphology of sphere SA catalysts changed after the introduction of HAP, Fe\({}_{3}\)O\({}_{4}\) in that the surface became rougher. FT-IR spectra indicate that the phosphate group in HAP and the carboxyl group in alginate are the functional groups, and they remain stable through the photo-Fenton process. Those non-ionized functional groups limit the catalysis to operating in acidic pH in order to minimize the electrostatic repulsion between the anionic MO and the catalyst. The Alg-mHAP catalyst showed the best discoloration among three, 84.28% within 90 min at pH 3, which was the optimal pH. In same condition, Alg-m reached 79.41%, and Alg-HAP reached 71.42%. The removal rate increased as H\({}_{2}\)O\({}_{2}\) concentration increased from 0.5 to 3 mM; however, the rate decreased over 3 mM. The MO degradation fit first-order kinetic model for all three catalysts. Aside from pure SA beads, SA can be modified with other bivalent cations such as Ca\({}^{2+}\), Ba\({}^{2+}\), and Zn\({}^{2+}\) for structural change. BiOCl-calcium alginate (BOC-CA) hydrogel beads were fabricated for the removal of RhB dye [82]. CA can be fabricated by cross-linking Ca\({}^{2+}\) with SA. The cross-linked structure has an \"egg-box\"-like 3D network. BiOCl, which is used as photocatalyst in BOC-CA, can produce oxygen vacancies (OVs) on the surface under UV irradiation, providing a path for the transport of photogenerated carriers. OVs prevent the recombination of holes and electrons by serving as centers for capturing electrons. SEM and optical microsphere images show that BOC-CA has spherical and porous structure, which allow the beads to float on the surface of water, a beneficial property for the absorbance of light. The decolorization efficiency reached 100% within 75 min under UV light, and 90 min under visible light. The efficiency was even high after reuse.