# Microplastics in Water Resources: Threats and Challenges
## 4 Methods for Separation and Extraction of MPs from Environmental Matrices
### Chemical and Enzymatic Digestion  
The general good chemical resistance of plastics allows the application of chemical digestion methods to the sample. This procedure involves exposing the sample to an aggressive environment to break down organic substances and disintegrate agglomerates of mineral substances [114]. Depending on the type of sample, different environments are used. Soil rich in plant matter, animal tissue, sewage, or mineral suspensions may require different protocols. Most often, samples are treated with concentrated potassium hydroxide and hydrogen peroxide combined with iron ions (Fe\({}^{2+}\)). Other substances used include nitric acid, citric acid, acetone, and ethanol [87]. The main reason for the use of chemical preparation is to release MPs particles trapped in the sample matrix [87]. This facilitates subsequent steps of their extraction. Many researchers often use chemical digestion of the sample as a preliminary step before density separation [12]. The unpredictable effect is the influence of chemicals on the MPs themselves [115]. Due to their small size and often developed surfaces, they may be more susceptible to erosion or even more than the larger fragments on which reference studies are usually conducted. This can generate errors in particle size determination. Fragmentation of MPs during chemical preparation leads to an increase in their total quantity. In the study by [PERSON] et al. (2021) [116], it was demonstrated that PET (polyethylene terephthalate) is the polymer most susceptible to chemical damage. During alkaline digestion, PET lost 3.1% of its mass, while oxidative digestion resulted in a loss of mass of 0.7%. In contrast, other polymers such as LDPE (low-density polyethylene), HDPE (high-density polyethylene), PP (polypropylene), PS (polystyrene), PVC (polyvinyl chloride) and PC (polycarbonate) showed greater resistance to chemical degradation [116]. In the study by [PERSON] et al., PA (polyamide), PE (polyethylene), PET (polyethylene terephthalate), PP (polypropylene), and PS (polystyrene) were exposed to acid solutions, alkaline solutions, enzymatic digestion, and oxidation, with changes in particle size being observed. The most significant changes were observed in polyamide (PA), which almost completely decomposed in HNO\({}_{3}\) (nitric acid), while in other solutions, its size decreased by 0.5-3%. An interesting observation is that every polymer type tested, regardless of the chemical digestion method, showed a change in size of at least 0.1% [117].  
An alternative and complement to oxidative methods based on H\({}_{2}\)O\({}_{2}\) are enzymatic methods. They seem to be a \"gentle\" and optimal way to preserve the integrity of MPs extracted from the sample. However, these methods are time-consuming and costly. Therefore, efforts should be made to simplify them as much as possible and combine them with other methods. Good results are obtained by using enzymes followed by digestion with H\({}_{2}\)O\({}_{2}\). Both reagents complement each other [118; 119; 120].