As(III) adsorption onto different-sized polystyrene microplastic particles and its mechanism

We systematically investigated the surface characteristics of polystyrene microplastic particles (PSMPs) prepared by ball milling to impart a porous surface structure and special surface characteristics, and studied the mechanism of adsorption of As(III) onto PSMPs. The sizes of the PSMPs prepared by ball milling for 2, 4, and 8 h were in the ranges of 0.1-1, 1-10, and 10-100 μm, respectively. That is, the longer the milling time is, the larger the specific surface area of the particles is. Moreover, the higher the point of zero charge is, the higher the adsorbed amount of As(III) is. The highest adsorption rate of As(III) onto PSMPs was found to be 1.12 mg g^-1. After 1200 min, the adsorption reached equilibrium, and a pseudo-second-order model better fitted the As(III) adsorption kinetics. The Langmuir and Freundlich models could well describe the adsorption isotherms. Furthermore, hydrogen bonds between As(III) and PSMPs were broken at high temperatures, resulting in a decrease in As(III) adsorption onto PSMP, which indicated that the adsorption process was exothermic. Increases in the pH and concentrations of interfering nitrate and phosphate ions in the solution led to inhibited As(III) adsorption of PSMPs. The electrostatic potential of most areas of the PSMP surface was positive, and the H atom on the carboxyl group exhibited a very large positive potential (+56.6 kcal/mol), and thus attracted arsenic oxyanions. Thus, it was determined that As(III) adsorbed to the surface of PSMPs through hydrogen bonding with the carboxyl group. Electrostatic forces and non-covalent interactions are the key mechanisms affecting the adsorption of As(III) onto PSMPs. This work provides a clear theoretical basis for the behavior of the PSMP as an arsenic carrier and might aid to improve the environmental toxicity of arsenic.