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Impact of Microplastics on the Immobilization of Cd in Soil by Biochar
Summary
Biochar is often used to lock toxic cadmium in place in farm soil so it doesn't end up in our food, but this study found that microplastics in the soil can interfere with that process, making cadmium more available for crops to absorb. The effect was worse with higher levels of microplastics, larger plastic particles, and certain plastic types (like polypropylene), suggesting that plastic pollution could be quietly undermining efforts to keep toxic metals out of our food supply.
Microplastics (MPs) and Cd commonly coexist in agricultural soils; however, their interference with biochar (BC)-based in situ immobilization and the underlying mechanisms remain poorly understood. In this study, batch incubation experiments were conducted to systematically evaluate the effects of MPs with varying concentrations (0.5%, 1%, and 10%), particle sizes (48 μm and 150 μm), and polymer types (polyethylene, PE; polypropylene, PP) on the Cd immobilization efficiency of KOH-modified biochar (BC1). The results showed that the presence of MPs significantly weakened the ameliorative effects of BC1 on soil physicochemical properties (pH, EC, and CEC). Moreover, MPs interfered with Cd immobilization through physical obstruction and interfacial competitive adsorption, which in turn increased Cd bioavailability. Notably, the coexistence of 10% PP (150 μm) increased the H₂O-extractable Cd concentration by 144.06% relative to the BC1-only treatment. Simultaneously, the coexistence of MPs increased the environmental risk factor (ERF) of Cd by up to 75.74%. The BCR sequential extraction results further demonstrated that MPs inhibited the transformation of Cd from labile fractions (acid-soluble and reducible) into more stable fractions (oxidizable and residual). Overall, the interference effect showed clear dependencies on dosage, particle size, and polymer type, with higher concentrations, larger particle sizes, and PP showing more pronounced negative impacts. Therefore, MPs, as critical interfering factors, can substantially reduce the remediation efficiency of biochar in Cd-contaminated soils and should be explicitly considered in soil risk assessment and remediation strategy design under co-contamination scenarios.