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

Abstract

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⁻¹) and Cr (3.78 mg g⁻¹) 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.