How do polystyrene microplastics affect the adsorption of copper in soil?

Microplastics (MPs) commonly coexist with heavy metals in the soil environment. MPs can influence the activity of heavy metals, and the specific mechanisms need to be further explored. Here, different contents of polystyrene (PS) MPs were added to soil to explore their effects on the adsorption and desorption characteristics of copper (Cu) in soil. The adsorption process was mainly chemical adsorption and belonged to a spontaneous, endothermic reaction. The hydrophobicity of MPs slowed down the adsorption and desorption rates. The main adsorption mechanisms included complexation by oxygen-containing functional groups, ion exchange (accounting for 33.97-36.04 % of the total adsorption amounts), and electrostatic interactions. MPs lacked oxygen-containing functional groups and were predominantly engaged in ion exchange and electrostatic interactions. MPs diluted, blocked the soil, and covered the active sites of soil, which reduced adsorption (3.56-16.18 %) and increased desorption (0.90-2.07 %) of Cu in soil samples, thus increasing the activity and mobility of Cu. These findings provide new insights into the effects of MPs on the fate and risk of heavy metals in soil. ENVIRONMENTAL IMPLICATION: The existing literature concerning the effects of microplastics on the adsorption of heavy metals in soil is insufficient. Our investigation unveiled that the main adsorption mechanisms of different soil samples included complexation by oxygen-containing functional groups, ion exchange (accounting for 33.97-36.04 % of the total adsorption amounts), and electrostatic interactions. MPs lacked oxygen-containing functional groups and were predominantly engaged in ion exchange and electrostatic interactions. MPs diluted, blocked the soil, and covered the active sites of soil, which reduced adsorption (3.56-16.18 %) and increased desorption (0.90-2.07 %) of Cu in soil samples, thus increasing the activity and mobility of Cu.