## 4 Impact of Plastic Particles on Soil Properties
### Interaction of Plastic Particles with Soil Contaminants  
Plastic particles can significantly interact with various soil contaminants, influencing their behaviour, mobility, and bioavailability. Understanding these interactions is crucial for assessing the environmental risks associated with plastic pollution in soils. Plastics often contain additives like phthalates, bisphenols, and flame retardants, which can leach into the soil and interact with other contaminants, potentially exacerbating their effects. However, these effects are not discussed in this review. The influence of MPs on the adsorption process and the potential distribution of metal ions has been widely discussed in scientific studies, as outlined in Table S3 (SI). However, only a few studies [46; 63] have considered pH as a factor influencing metal absorption by MP particles. The adsorption processes of metal ions on MPs are positively correlated with the degradation state and morphology of their surfaces, surface areas, and surface chemical functional groups, primarily represented by charged, polar, or oxygen-containing functional groups [56; 64; 65; 66; 67; 68]. [PERSON] et al. [69] observed that bioplastics generally exhibit a higher adsorption capacity for metal ions compared to conventional plastics, likely due to changes in surface properties during degradation. Organic substances such as pesticides, pharmaceuticals, or flame retardants, which are ubiquitous environmental contaminants, can potentially be adsorbed by MP particles. Thus, MPs can act as vectors, contributing to the environmental distribution and potential bioavailability of these organic contaminants to various organisms, including humans [70]. The sorption of organic molecules to MPs is predominantly influenced by hydrophobic partitioning and hydrophobic interactions, although this does not uniformly apply to all interactions of organic contaminants with MPs and depends on various factors (as listed in Table S4, SI). Potential binding mechanisms of organic contaminants on MP surfaces include interactions such as \(\pi\)-\(\pi\) conjugation [57; 71], polar interactions such as hydrogen bonding [57; 72], and various electrostatic interactions [68] via polar functional groups such as COOH, NH\({}_{2}\) and -SO\({}_{3}\)H [72]. [PERSON] et al. [60] suggest that the sorption of organic pollutants to MPs is more influenced by electrostatic interactions at a pH below 6, while at pH 6 and above, hydrophobic forces may be more dominant [73; 74; 75; 76]. Other  
Figure 3: Summary of 43 studies from Table S2 (SI), related to changes of soil pH by microplastics. A: pH has decreased. B: pH change is inconclusive. C: pH has increased. []: not mentioned. factors that may contribute to the adsorption of organic molecules on MPs are surface morphological features, such as cracks and surface roughness, polymer crystallinity, and particle density [77]. The aging process clearly contributes to the sorption of organic substances on MPs, as indicated by several studies [37; 75; 78]. This suggests that hydrophobic particle formation and hydrophobic interactions might be more significant for the sorption of organic molecules on MP particles, while particle roughness seems to contribute more to metal ion adsorption. However, further investigation is needed to validate these hypotheses. In terms of the aging process of MP particles, both conventional MPs and MBPs undergo aging processes [38]. The adsorption capacity of aged MPs is generally higher compared to pristine MPs, and the presence of oxygen-containing functional groups on aged MPs can increase the adsorption affinity for hydrophilic organic pollutants, including some antibiotics [38]. MBPs may exhibit a higher affinity for hydrophilic compounds and show different sorption capacities compared to conventional MPs. Additionally, MBPs may undergo more significant changes in the aging process due to their faster degradation rates [38].