Insight into the interactions between microplastics and heavy metals in agricultural soil solution: adsorption performance influenced by microplastic types.

Microplastics (MPs) are widely present in soils, often co-contaminated with heavy metals (HMs), complicating the assessment of their adsorption performance. This study prepared environmental-simulating microplastics (EMPs) and compared their Cd/Cr adsorption-desorption properties with five commercial MPs in artificial soil solutions. Aging treatments altered the physicochemical characteristics of MP surfaces, increasing oxygen-containing functional groups and forming smaller particles. These changes enhanced HM adsorption, with EMPs showing higher adsorption capacities for Cd and Cr than the five single-type commercial MPs. Among the commercial MPs, degradable polylactic acid (PLA) showed the highest maximum adsorption capacities for Cd (4.52 mg g) and Cr (3.78 mg g) at elevated concentrations, indicating its greater potential for HM transport. Adsorption kinetics revealed that surface chemisorption and intraparticle diffusion were the key rate-limiting steps in the MP-Cd/Cr adsorption processes. Desorption of Cd was more pronounced than that of Cr, indicating higher activity of Cd on MP surfaces. Higher HM accumulation factors of aged MPs (Cd: 3.49-8.24%, Cr: 1.95-7.82%) suggest their potential to accumulate and immobilize soil HMs. The EMPs exhibited the highest accumulation factors, implying a greater impact of mixed MPs on soil total and bioavailable Cd/Cr concentrations than single-type MPs. These findings offer new insights into the interactions between pollutants in soils co-contaminated with mixed MPs and HMs.