Microplastics-Mediated Behavior of Potentially Toxic Elements in Plant–Soil Systems: Adsorption, Bioavailability, and Phytotoxicity

Microplastics (MPs) and potentially toxic elements (PTEs) increasingly co-occur in agricultural and peri-urban soils, yet their combined effects on adsorption, mobility, bioavailability, and phytotoxicity are highly context-dependent. This review synthesizes plant–soil evidence by focusing on the interacting roles of MP polymer type, particle size and shape, aging/weathering state, soil geochemistry, dissolved organic matter, and rhizosphere processes. Across the reported studies, MP-PTE interactions show several major directions of changes: MPs may reduce PTE lability by promoting adsorption, aggregation, or sequestration within coated surfaces and soil aggregates; conversely, they may increase PTE mobility and plant exposure when reversible binding, dissolved organic ligands, pH shifts, or particle transport deliver labile PTEs to root-active zones. Dose-dependent and biphasic responses are also common, with low MP additions sometimes attenuating stress while higher doses intensify toxicity. Quantitatively, available crop studies show that intensified co-exposure can reduce plant biomass by approximately 10.2–29.3%, depending on crop species, plant organ, MP type, dose, and PTE identity, whereas antagonistic or neutral responses are also reported under other exposure conditions. The strongest evidence currently exists for Cd and As, but this review also considers Pb, Cu, Zn, Ni, Cr, and Hg to represent chemically distinct cationic, metalloid, and redox-sensitive PTEs. Overall, MPs should not be treated only as passive contaminant carriers; they act as dynamic reactivity modifiers that can function as sinks, vectors, or indirect regulators of PTE bioavailability depending on soil and rhizosphere boundary conditions.