## 5 Factors Affecting the Adsorption of Microl plastics
### Effect of pH  
In the process of microplastic adsorption of pollutants, pH was the most significant influencing factor. It had the most crucial effect shown in the adsorption of organic pollutants by microplastics. One of the most important aspects was the effect of pH on the adsorption properties of microplastics, such as microplastics for sulfamethoxazole [41] and three triazole fungicides [50]. There were also some other organic pollutants or compounds adsorbed, and the amount of adsorption increased as the pH value increased. Of course, the pH value also inhibited the adsorption capacity of microplastics, such as the adsorption capacity of 2,4-hexadiene (sorbic acid) [51], adsorption of perfluorooctane sulfonic acid on polyethylene and polystyrene [52], and adsorption of phenol on polymer resin decreased with increasing pH value [53]. In addition, the inhibition effect of pH value was also reflected in the adsorption of inorganic pollutants by microplastics. Under the influence of pH value, folate-polyaniline hybrid hydrogels tend to adsorb anionic dyes, and a low pH value was more conducive to improving the adsorption capacity of folate-polyaniline hybrid hydrogels [54]. Under basic conditions, iminodiacetic acid chelate resin can form a complex with metal ions [55]. However, alkaline conditions can sometimes produce fatal obstacles to the adsorption of plastics. For instance, the adsorption of bisphenol by polythiocylic acid-based coagulants can directly decrease the adsorption efficiency from 90% to 10% under alkaline conditions [55]. In the study of the adsorption of Cr(III) and Fe(III) by modified polyethyleneimine, the alkaline state would precipitate Cr(III) and Fe(III). In contrast, under acidic conditions, modified polyethyleneimine combines more easily with hydrogen ions, and the most favorable condition was a pH of 5 [56]. In addition, sulfamethoxazole mainly existed in the form of anions after ionization in an aqueous solution, and a suitable pH value was conducive to enhance the adsorption affinity of microplastics for organic pollutants [34]. When the pH was 5, the adsorption electrostatic repulsion of nano and microplastics were the lowest. The effect of microplastic pH on pollutant adsorption did not follow the law of increasing or decreasing. In the case of inorganic pollutants, the impact of adsorption will be the highest under the influence of pH value. This was because pH can affect the group, hydrolysis, and ionization of substances in the solution and jeopardize the adsorption performance [31]. In general, microplastics had an optimal range for the adsorption of metal ion pollutants. The maximum adsorption value would appear in the adsorption process of styrene-DVB resin for homocarboxylic acid. For example, in a weakly acidic environment with a pH value of 4-6, poly (ethylimide) adsorbents were effective in decontamination [57]. The effect of pH on removing hexavalent chromium was carried out in the pH range of 2-9 [54]. When the pH was 5, the adsorption capacity of Cd and Ni ions on polystyrene nanofibers reached the highest value [58]. The most favorable adsorption conditions of polyacrylamide-2-aminothiazole resin for mercury, cadmium, lead, copper, zinc, and nickel are 2.50-6.50 [59]. When the pH was 5.95, D152 resin adsorbed cadmium(II). For the adsorption of cadmium and nickel ions on polystyrene nanofibers, the adsorption of Er(III) on acrylic grafted polytetrafluoroethylene fiber was the highest [60]. The positive charge of polystyrene microplastics was favorable for the attraction of HA and FA when the pH was less than 2.75. At pH 5, IDA-chelating resin adsorption of bivalent heavy metal ions follows the same principle. In the pH range of 5-9, the content of Cd adsorbed on MPs first increased and then decreased.