Redistribution of Ni in soil containing polystyrene microplastics: evidence from sequential extraction

The co-occurrence of microplastics (MPs) and heavy metals in terrestrial ecosystems represents an emerging environmental concern, yet their combined influence on metal partitioning in soil remains less understood. This study investigated the effect of polystyrene (PS) MPs on nickel (Ni) redistribution and mobility using a controlled soil incubation experiment. Soil was amended with 1% (w/w) PS and spiked with Ni at 50 and 500 mg kg^-1, followed by sequential extraction according to the BCR protocol to assess Ni fractionation. In untreated soil, Ni was predominantly associated with oxidizable (F3) and residual (F4) fractions, with averages of 41.9% and 43.7% of the total content. The introduction of PS significantly altered Ni partitioning, promoting its transformation toward the reducible fraction (F2), which represents weakly bound, potentially mobile forms. In moderately contaminated soil (50 mg kg^-1 of Ni), addition of PS increased the proportion of Ni in F2 by up to 84.6% compared to the corresponding treatment without PS. In highly contaminated soil (500 mg kg^-1 of Ni), the increase was less pronounced (31.4%), indicating a concentration-dependent response. The sum of acid-soluble and reducible fractions (F1+F2), considered phytoavailable Ni, increased significantly in PS-treated soils. Polystyrene enhanced Ni availability by up to 81.2% relative to treatments without MPs. Correspondingly, mobility factors (MFs) were consistently higher under PS exposure, demonstrating reduced Ni stability in the soil matrix. These findings demonstrate that PS-MPs modify geochemical partitioning of Ni, promoting its migration from stable to more labile fractions. The effect was most pronounced under moderate contamination levels, suggesting that MPs may act as facilitators of metal redistribution in environmentally relevant scenarios. This mechanistic evidence highlights the importance of considering MP–metal interactions when assessing metal mobility in terrestrial systems.