Effect of background ions and physicochemical factors on the cotransport of microplastics with Cu2+ in saturated porous media

Microplastics (MPs) in subsurface environments are migratory and can carry heavy metals, increasing the extent of MP and heavy metal pollution. This study used quartz sand-filled column experiments to investigate the adsorption and cotransport behaviours of PS-MPs, O₃, UV-aged PS-MPs, and Cu²⁺ at different MP concentrations, ionic strengths, and ionic valences in a saturated porous medium. The results showed that when MPs migrate alone in the absence of an ionic background, higher concentrations have increased mobility. In contrast, an increase in the background ion concentration or ion valence inhibits the individual transport capacity of PS-MPs. An increase in the concentration of background ions or elevation in the valence state promotes Cu²⁺ transport because of the action of the double electric layer on the surface of the colloid and the electrostatic repulsive forces combined with the background ions. The adsorption capacity of aged PS-MPs was stronger than that of PS-MPs because of the binding of the aged PS-MPs to Cu²⁺ through complexation and electrostatic attraction. In the binary system of PS-MPs/Cu²⁺, PS-MPs promoted Cu²⁺ transport and the mobility of Cu²⁺ loaded by PS-MPs decreased with increasing background ion concentration. The cotransport results showed that MPs promote Cu²⁺ transport in the following order: O₃-aged Ps > UV-aged Ps > Ps, as the increasing cation concentration in the MPs and Cu²⁺ occupies the PS surface adsorption sites. Overall, PS is an effective carrier for Cu²⁺. These findings offer fresh exploration concepts for the joint migration of MPs and heavy metals in underground settings.