## 5 Factors Affecting the Adsorption of Microl plastics
### Effects of Functional Groups  
Functional groups are also an essential factor that affects the adsorption of microplastics. The existence of functional groups endowed materials with different properties. As these functional groups showed unique properties, they exhibited excellent adsorption effects on pollutants. In addition to the functional groups of the resin itself, aging [73] and weathering yielded more surface area and produced all kinds of functional groups. This increased their polarity, charge, roughness, and porosity to a certain extent, which affected the adsorption capacity of microplastics [74]. It was proven that the adsorption capacity of PE for chlorotetracycline and amoxicillin increased by 1.08-14.24 times after aging [75]. Furthermore, different functional groups may produce different results in acid-base, hydrogen bonding, complexation, etc.  
Functional groups in the adsorption of microplastics generally promoted the experiment. For example, in the adsorption experiment of resin for phenol, the amine group on the resin, quaternary amine, primary amine, and secondary amine groups were conducive to the adsorption of resin for phenol, while the tertiary amine group is not [76]. The tert-butyl ester group in glass fiber-reinforced plastics also promoted the adsorption of polyacrylic acid [77]. When coal and waste polyvinyl chloride adsorbed mercury, the carbon chloride functional group could be converted to ionic chloride to realize the adsorption of mercury [78].  
Calcium ion polymers can effectively adsorb organic micropollutants because of the \(\pi\)-rich network in porous polycalixarenes [79]. Similar to the adsorption of mercury by polyethyleneimine-modified porous cellulose carriers, the adsorption of metal pollutants by microplastics was free from the influence of pH value due to the action of the amine group [80]. Polyacrylonitrile-thiosemicarbazone resin contained triazolthione functional groups, Pd(II) adsorption required one active functional group, while Ru(IV) and Ir(IV) adsorption required two functional groups [64].  
However, under exceptional circumstances, the functional groups inhibited the adsorption of microplastics. For example, alcohol groups in cellulose can be acetyl, which changed the structure of cellulose, and cellulose triacetate is more acetyl than cellulose acetate, which reduced its ability to adsorb 2,4-hexadiene (sorbic acid) [51]. Furthermore, with the increasing in the polarity of aromatic amines, the adsorption of polyethylene to aromatic amines was hindered [20]. In contrast to the adsorption experiment on polyethylene and nylon fibers, there was no obstruction of hydrophilic group amide, and polyethylene fiber had a decisive advantage in the adsorption capacity and adsorption speed; however, the other structure of the resin was unable to avoid the effect of inhibition [81].  
In addition, in the adsorption experiment of Cu(II) by cross-linked polystyrene-supported low-generation diethanolamine dendritic polymer, too many functional groups caused a large area of blockage, and Cu(II) could not adsorb well on the resin [69]. To a certain extent, the adsorption efficiency of coal and waste PVC on mercury gradually increased with increasing PVC content. However, with the addition of abundant PVC, the adsorption efficiency of coal and waste PVC on mercury decreased because too much PVC easily blocked the pore size [78].