LDPE, PP, and PET Microplastics’ Influence on Cd and Cu Behavior in Diverse Soils

The increasing accumulation of microplastics (MPs) in soils necessitates a better understanding of their effects on soil chemistry and trace element behavior. This study examined the influence of three MPs—low-density poly(ethylene) (LDPE), polypropylene (PP), and poly(ethylene terephthalate) (PET)—on cadmium (Cd) and copper (Cu) mobility in four clayey–sandy soils with similar organic matter content but differing pH, representing acidic and alkaline terrestrial matrices. Soils were incubated with 1% (w/w) MPs at 60% water-holding capacity for 30 and 90 days. Total Cd and Cu concentrations remained largely unaffected; however, time-dependent changes in metal availability and distribution were observed. Extractability (CaCl2 and DTPA), sequential BCR fractionation, and environmental risk indices (CF, Igeo, RAC, MF, and PLI) indicated slight increases in Cd availability after 30 days, which became more pronounced after 90 days, particularly in acidic soils (pH 5.5). The magnitude of the MP effect followed the trend PET > PP > LDPE in all cases. Among the two target metals, Cd exhibited substantially higher mobility than Cu, as reflected by RAC and MF values. Specifically, Cd RAC (6–35%) and Cd MF (28–63%) values were considerably higher than those of Cu (1.1–3.8% and 15–28%, respectively). Overall, although the general pollution indices remained relatively stable, MPs altered the extractability and geochemical partitioning of the examined heavy metals—particularly Cd—indicating their potential role as vectors in soil environments. These results demonstrate that incubation time, polymer type, and soil pH jointly regulate MP-induced change in Cd and Cu mobility, with important implications for soil ecosystem risk.