# Notes on Common Misconceptions in Microplastics Removal from Water
## 3 Microplastics Removal from Water
### Degradation
#### 3.2.1 Photochemical Degradation  
Photochemical degradation is a process in which the material undergoes degradation induced by UV light. UV radiation can cause photooxidative breaking of molecule chains and production of radical species, leading to change in mechanical and chemical properties of the irradiated material [66].  
Photochemical degradation of plastic materials is a well described process [49; 67; 68]. It is considered to be one of the most efficient ways of degrading polymers. It takes place under the influence of irradiation of different wavelengths, where each polymer is most sensitive to a different wavelength [69]. During degradation, polymers change their mechanical and optical properties, they become more brittle and less elastic. There is also often a change in colour, when plastics begin to turn yellow.  
Some authors investigated the effect of UV radiation on the thermoplastic polyester elastomer in an aqueous environment and in a water-free environment. Wavelengths shorter than 310 nm proved to be the most effective, because these wavelengths have enough energy to cause scission of polymer chains, forming esters and aldehydes. The presence of water caused the formation of a gel mass from the original polymers [70]. Furthermore, changes in the behaviour of low-density polyethylene (LDPE) under UV radiation were studied. Changes in the extensibility of LDPE in two environments--seawater and air--were monitored for one year. The study showed that photodegradation in the aquatic environment is much slower, which was explained by lower average temperatures [71].  
[PERSON] and [PERSON] investigated the effect of LDPE pre-oxidation in soil induced by UV radiation in order to promote its biodegradation. It has been shown that UV-treated LDPE revealed better degradation efficiency than the ones that were not UV-treated [72]. The problematic comparison of different environments influencing the degradation of plastics is insufficient--there are just two environments to be compared (freshwater-air or seawater-air). Directly comparable data are unavailable and a proper comparison study should contain seawater- and freshwater-air environments. [PERSON] et al. observed the degradation of PE, PP and PS in seawater upon simulated sunlight (see Figure 2) [73]. The degradation was proved via surface changes (fragmentation, surface changes and alteration of the colour) and the increase in dissolved organic carbon (DOC) concentration in treated seawater. DOC is quantified as the organic carbon that passes through a sub-micron filter [74]. Releasing DOC consequently affected microbial activity in seawater.  
Figure 2: Pictures of investigated microparticles ([PERSON] et al. (2020)). Copyright available.  
Other studies mostly dealt with the degradation of PVC [75; 76], which constitutes the most commonly used plastics in everyday life. The aim of these studies was to find out how polymers were degraded upon external influences (temperature, radiation, air and humidity) and whether these influences shorten their durability. The authors concluded that better reproduction of natural weathering in laboratory conditions is achieved during experiments where water is periodically sprayed on exposed material (this simulates natural air humidity).  
[PERSON] et al. investigated PVC granules (diameter 4 \(\pm\) 2 mm) degradation by Fenton reaction. It was demonstrated that Fenton treatment enhanced the formation of PVC degradation products (lower molecular weight fragments) that were further degraded by anaerobic digestion [77]. The effects of UV radiation are not much studied in the aquatic environment, but rather in the atmospheric environment. The future approach must be focused on the aquatic environment.