Investigation of the adsorption behavior of Pb(II) onto natural-aged microplastics as affected by salt ions

In this study, the adsorption behavior of Pb(II) on natural-aged and virgin microplastics in different electrolyte solutions was investigated. The results demonstrated that natural-aged microplastics exhibited higher adsorption capacity for Pb(II) compared to virgin ones, and the addition of CaCl2 strongly inhibited the adsorption amount of Pb(II). The adsorption kinetics of Pb(II) adsorption were better fitted by the pseudo-second order model and Elovich equation, and were slowed down greatly at higher ionic strength. The rate-limiting steps of adsorption process were dominated by intra-particle diffusion. The adsorption isotherm of Pb(II) onto microplastics affected by salt ions can be well described by Freundlich model, the greater adsorption efficiency of natural-aged microplastics proved that adsorption process was multilayer and heterogeneous. In addition, pH significantly influenced the adsorption of Pb(II) due to the changes electrostatic interactions. The effect of fulvic acid in the electrolyte solutions was also revealed and attributed to the complexation with Na+ and Ca2+. Furthermore, the higher pH and ionic strength in different environmental water dramatically decreased adsorption capacity onto microplastics. Finally, it's confirmed that the adsorption mechanisms affected by salt ions mainly involve electrostatic interaction, surface complexation, and ionic exchange. These findings indicate that salt ions exert an important influence on the adsorption of heavy metals for MPs, which should be further concerned.