## 6 Adsorption Mechanism
### Adsorption Affinity  
The adsorption affinity of resin for pollutants was one of the main mechanisms for studying the properties of microplastics. In the adsorption process of microplastics, they often showed remarkable adsorption capacity for some contaminants. As in the process of chloromethane adsorption of plastic adsorbent, the low solubility of chloromethane was more readily adsorbed [85]. However, with the adsorption of styrene-DVB resin for homocarboxylic acids, carboxylic acids with high molecular weights were more easily adsorbed. The adsorption capacity of mercury was always higher than that of lead in the adsorption capacity of poly(ethylamine) to lead and mercury [57].  
In the adsorption of mercury by a polyethyleneimine-modified porous cellulose carrier, the mercury amine complex was formed by a polyethyleneimine-modified porous cellulose carrier; thus, mercury had high stability, which made microplastics have a high mercury adsorption efficiency [80]. Additionally, the folate-polyaniline hybrid hydrogel can selectively adsorb anionic dyes and has a higher affinity for anionic pollutants, such as chromium (VI), eosin yellow, rose red, and methyl orange [54]. Likewise, polyacrylamide-2-aminothiazole resin also had a high adsorption efficiency for mercury-, cadmium-, lead-, copper-, zinc-, and nickel-polluted wastewater [59].  
Under the condition of the coexistence of Hg(II), Ag(I), Fe(III), Pb(II), Co(II), Cu(II), Ni(II), Cd(II), and Zn(II) ions, 3-aminopyrine hydrophilic spacer arm chelating resin showed strong adsorption performance for Hg(II) [91]. Interestingly, polyvinylchloride microplastics had a higher affinity for TCEP in the adsorption of trin-butyl phosphate and tris (2-chloroethyl) phosphate [94]. When microplastics (MPs) were used to remove fungicides, the adsorption capacity of pyrrole cyclospor was the highest, followed by microoxygenion and dioxygenin [82]. In general, polyethylene exhibited greater adsorption capacity than other types of plastics [125].  
The role of functional groups, such as the adsorption effect of terlomycin on MPs, was in the order of PE (polyethylene) \(<\) PP (polypropylene) \(<\) PS (polystyrene) \(<\) PVC (polyvinyl chloride). In addition to its strength, the adsorption affinity sometimes needed the help of other substances. For example, silica played a vital role in treating oil-containing wastewater by porous waste plastic superhydrophobic foams, and silica had a strong affinity for both porous waste plastic superhydrophobic foams and oil-containing wastewater [125].  
In the adsorption of iminodiacetic acid chelate resin for trivalent metal ions, Sc(III) \(>\) Ga(III) \(\approx\) In(III) \(\approx\) Fe(III) \(>\) Y(III) \(>\) La(III) \(>\) Al(III); however, in aqueous solution, Ga(III)  
Figure 5: The adsorption machines in the process of adsorbing pollutants with microplastics.  
\(>\) Fe(III) \(>\) In(III) \(>\) Sc(III) \(>\) Al(III) \(>\) Y(III) \(>\) La(III) [55]. In the pH range of 2-8, metal ion-impregnated polystyrene resins had a strong adsorption capacity for antibiotic contaminants in water [116]. The adsorption of Cd and Ni ions on polystyrene nanofibers in an aqueous solution can achieve the best effect [58]. Adsorption was the main mechanism of oil pollution treatment for the porous structure of electrospun polyvinyl chloride/polystyrene fiber [118].  
In addition, there was adsorption selectivity, as low concentrations of polystyrene enhance the migration of nonpolar (pyrene) and weakly polar (2,2,4,4-tetrabromodiphenyl ether) compounds but had little effect on the migration of three polar compounds (Bisphenol A, bisphenol F, and 4-nonylphenol) [95]. Moreover, when Co(II), Ni(II), and Cr(III) ions coexist in a solution, Cr(III) replaced them even if Co(II) and Ni(II) were adsorbed on the resin [89].