Polystyrene-nickel interactions in soil: Implications for metal mobility, plant uptake, and human health

The coexistence of microplastics (MPs) and metals is ubiquitous in terrestrial ecosystems. However, their coupled effects on soil biota and human health remain unknown. The present study investigated the interactive impacts of polystyrene (PS)-nickel (Ni) contamination on Ni mobility in the soil-medicinal plant system and the associated health risks. A soil pot experiment was conducted with Capsella bursa-pastoris under six treatments: control (CK), PS alone (1 % w/w, PS), Ni at 50 and 500 mg kg^-1 (Ni50 and Ni500), and their combinations (Ni50 +PS, Ni500 +PS). The introduction of PS into soil promoted Ni transformation to the reducible soil fraction (F2) and increased its bioavailability by up to 81.2 % compared to treatments without MPs. The mechanism underlying Ni redistribution involved PS-soil associations that mask mineral binding sites and induce a "dilution effect," whereby Ni loosely associated with PS surfaces became more mobile and bioavailable in the soil matrix. The most prominent increase was recorded in control soil (CK) and soil with 50 mg kg^-1 Ni (Ni50). PS addition to CK and Ni50 also stimulated Ni uptake by C. bursa-pastoris and increased the carcinogenic risk by 62.5 % compared to CK and by 28.6 % in Ni50 +PS compared to Ni50. Structural Equation Modeling (SEM) and Principal Component Analysis (PCA) confirmed that Ni bioavailability and transfer to and within the plant were strongly influenced by the PS presence. The findings of this study provided valuable insights into the toxic effects of PS-Ni exposure on the food safety of medicinal plants.