Unraveling the influence of pH on uranium adsorption by polystyrene microplastics: An integrated experimental-density functional theory analysis

Microplastics (MPs) are emerging pollutants that play an important role in the spread of highly toxic heavy metals. Although the pH has been shown to exert a substantial impact on the adsorption capacity of MPs towards heavy metals, the intricate mechanisms underlying how pH functions in the adsorption of radioactive heavy metals by MPs remain largely elusive. Here, we combined batch experiments and density functional theory calculations to investigate the adsorption behavior of radioactive uranium (U) on virgin and aged (modified with –OH or –COOH groups) polystyrene (PS) MPs at pH ranging from 3.0 to 9.0. The kinetic data showed that the U adsorption capacities of virgin PS and PS–COOH MPs were in the order of pH 7.0 > pH 8.2 > pH 4.5 > pH 9.0 > pH 3.0, whereas U adsorption on PS–OH MPs displayed a trend of pH 8.2 > pH 9.0 > pH 7.0 > pH 4.5 > pH 3.0. These outcomes were confirmed by X-ray photoelectron spectroscopy and binding energy analysis. Compared to virgin PS MPs, the higher U adsorption on aged MPs might be attributed to electrostatic attraction between deprotonated –COOH and (UO₂)₃(OH)₅⁺ at pH 7.0 for the PS–COOH MPs and between –OH and (UO₂)₄(OH)₇⁺ or (UO₂)₃(OH)₇⁻ at pH 8.2 for the PS–OH MPs. Intermolecular interaction calculations demonstrated that hydrogen bonding interaction driven by electrostatic and polarization energies contributed to the improved U adsorption amount of the PS–COOH MPs at pH 7.0 and the PS–OH MPs at pH 8.2. Our understanding of the mechanisms by which pH regulates U adsorption by MPs is critical for evaluating the fate and hazard of co-existing radionuclides and MPs in aquatic environments.