Equilibrium, kinetics and molecular dynamic modeling of Sr2+ sorption onto microplastics.

Microplastics (MPs) are becoming ubiquitous pollutants in the global environments, which can potentially sorb metals ions in aquatic environments, causing adverse consequences. The interaction between Sr2+ and MPs, and the involved mechanisms have not been studied. Here we investigated the sorption behaviors of Sr2+ by polyamide (PA), polystyrene (PS), and polypropylene (PP). Three phenomenological mathematical models were developed and applied to describe the rate-limiting step in the sorption process. The molecular dynamic (MD) simulation was also conducted to investigate the sorption mechanism. The results showed that the optimum isotherm was presented by the nonlinear Temkin model. The maximum sorption capacities of Sr2+ by PA, PS and PP were 31.8, 51.4 and 52.4 μg g-1, respectively, with the initial Sr2+concentration of 3400 μg L-1. The phenomenological models adequately described the sorption kinetics data, concluding that the internal diffusion was the limiting step for Sr2+ sorption onto PS; while the external and internal diffusion were the slowest steps in the case of PA and PP. The MD study revealed that the main sorption mechanism was electrostatic interaction. The interaction energies of PA-SrCl2, PS-SrCl2, and PP-SrCl2 were -5.638, -6.418, and -13.05 kcal mol-1.