# Notes on Common Misconceptions in Microplastics Removal from Water
## 3 Microplastics Removal from Water
### Degradation
#### 3.2.2 Photocatalytic Degradation  
In the context of this communication, photocatalytic degradation is similar to photochemical degradation. Under certain conditions, it may result in the deterioration of some of the physical properties of polymers. However, for an effective process, a photocatalyst has to be employed. A photocatalyst is defined as \"a material which absorbs light to bring it to higher energy level and provides the energy to a reacting substance to make a chemical reaction occur\" [78]. In general, a photocatalyst yields reactive species upon UV irradiation, causing degradation of illuminated material. A study on high-density polyethylene (HDPE) microparticles (0.7-1 mm) degradation using a protein-based porous N-TiO\({}_{2}\) semiconductor was performed and implicated that a potential for microplastic photocatalytic elimination exists. The advantage of protein-based catalysts is the reduction of the TiO\({}_{2}\) bandgap from 3.3 eV to 2.9 eV. This bandgap drop enhances catalyst utilization upon visible light, absorbing photons at 427 nm [79]. This was followed by a study using a protein-derived C,N-TiO\({}_{2}\) semiconductor in form of colloidal nanoparticles in aqueous environment and the combined effect of pH and temperature was investigated, in order to find optimal conditions. A low pH improved the interaction between the colloidal nanoparticles of the catalyst and microplastic particles; a low temperature increased the surface area via fragmentation, enhancing the interaction with catalyst nanoparticles [80]. These results were supported by an upcoming study performed by a similar research group [81]. Another study was focused on the degradation of LDPE in a water environment utilizing visible light and ZnO nanorods as a catalyst. The degradation of microplastic particles was monitored by FTIR and the formation of lower molecular weight compounds was detected [82]. This study provides valuable observations; it shows that such method can be an ideal way to follow the degradation mechanism, due to its high effectivity and reliability. The degradation was indicated by the formation of carbonyl and vinyl groups. [PERSON] et al. also tested a ZnO-based photocatalyst for microplastic polyethylene degradation. They also combined the method with coagulation as a \"mainstream technology\" for microplastics removal [32]. They concluded that none of these methods are capable of complete microplastics removal themselves, however, their combination may become a future development trend. ZnO photocatalytic abilities upon visible light in surface degradation changes of LDPE was confirmed by [PERSON] et al. ZnO was supported by Pt nanorods which were reported as efficient photocatalysts for LDPE degradation in a water environment [83]. A comparative study on degradation and mineralization of C\({}^{14}\)-PS nanoparticles (<1 \(\upmu\)m) in water and air was performed upon UV-C irradiation. The authors found out that mineralization in water was significantly more efficient than mineralization in air. The phenomena were attributed to the water environment that enabled the formation of alkyl radicals at 254 nm and their further transformation towards peroxy- and alkoxy-radicals, due to the presence of dissolved oxygen. They were further transformed via acetophenone and ketones towards ending products (CO\({}_{2}\) and H\({}_{2}\)O). Proposed mechanism is given on Figure 3[84]. This study provides a mechanism of PS degradation based on its chemical nature and, due to lack of such studies, provides one of the first theories of such nanoparticles degradation. It must also be noted that such mechanisms probably would not work for microparticles with units of hundreds and thousands \(\upmu\)m dimensions. It must be also pointed out that such scenario with polystyrene particles is very likely to happen. PS is widely used as insulation of buildings and the use of such material is therefore very broadened.