Oxygen-containing functional groups enhance uranium adsorption by aged polystyrene microplastics: Experimental and theoretical perspectives

Although oxygen-containing functional groups generated during unavoidable aging play a key role in improving the adsorption performance of heavy metals by microplastics (MPs), little is known about the contribution of such groups to the interactions between highly toxic radioactive heavy metals and aged MPs. Here, we used a combination of batch experiments and density functional theory calculations to examine the effects and mechanisms of aging and the produced oxygen-containing functional groups, i.e., C═O, –OH, phenolic hydroxyl (–OHm), and –COOH, on the adsorption behavior of representative radionuclide uranium (U) on polystyrene (PS) MPs. The kinetic experimental results showed that the amount of U adsorption by the aged PS MPs was significantly higher than that of virgin PS MPs (P < 0.05). Binding-energy and thermodynamic data indicated that the affinity of PS MPs to a typical U species, i.e., uranyl, followed the order of PS═O–OHm > PS═O > PS–COOH > PS–OH > PS–OHm > virgin PS. Characterization and surface potential analyses demonstrated that the electronegative oxygen atoms in oxygen-containing functional groups in the aged PS MPs increased along with the aging time, and could serve as potential sites for electrostatic interactions with uranyl. Intramolecular interaction calculations confirmed that hydrogen bonding by oxygen-containing functional groups could strengthen the interactions between uranyl and aged PS MPs via electrostatic and polarization effects. Our findings provide an in-depth understanding of the role of aging in U adsorption by MPs, which is critical for assessing the fate of coexisting radionuclides and MPs in the environment, and their associated risks.