Polyethylene microplastics alter pH-dependent leaching and toxicity of imidacloprid in soil columns

Microplastics (MPs) are widespread in agricultural soils and can interact with pesticide residues; however, their combined effects on pesticide fate and toxicity remain poorly understood. This study investigates the influence of polyethylene (PE) MPs on the adsorption, mobility, and ecotoxicity of imidacloprid (IMD) in soil, using a commercial formulation to simulate agriculturally relevant conditions. Two PE MP size fractions (<125 μm and ≥125 μm) were examined using batch adsorption-desorption experiments and soil column studies, with the latter conducted across three pH conditions (4.5, 6.5, and 8.5). Batch experiments demonstrated size-dependent sorption, with the <125 μm fraction adsorbing more IMD than the ≥125 μm fraction, although overall removal remained limited (∼12.6%). The Freundlich model provided a better fit for the sorption data than the Langmuir model, and desorption reached equilibrium rapidly (∼6 h), indicating weak and reversible interactions. In soil columns, MPs delayed IMD breakthrough and reduced cumulative leaching by approximately 21% relative to MP-free controls. Alkaline conditions accelerated IMD leaching in MP-free columns, whereas the presence of MPs reduced early-stage leaching under the same conditions. The median effective concentrations (EC) of IMD + MP column leachates were lower than those of IMD-only leachates, indicating enhanced toxicity; notably, toxic effects persisted into the later leaching stages only in the presence of MPs. These findings suggest that MPs can modify the environmental fate and ecological risks of pesticides through chemical and physical interactions that influence transport in soils. Consequently, the co-occurrence of MPs and pesticides should be explicitly integrated into environmental risk assessment frameworks.