Transport of polyethylene and polypropylene microplastics under the action of agricultural chemicals: Role of pesticide adjuvants and neonicotinoid active ingredients

Understanding the impact of various agricultural chemical components on the fate and transport of microplastics (MPs) in the subsurface is essential. In this study, column experiments on saturated porous media were conducted to explore the influence of the coexistence environment of pesticide adjuvants (surfactants) and active ingredients (neonicotinoids) on the transport of polyethylene (PE) and polypropylene (PP) MPs. An anionic surfactant (sodium dodecyl sulfate (SDS)), a nonionic surfactant (nonylphenol ethoxylate (NP-40)), and three neonicotinoid insecticides (acetamiprid, dinotefuran, and nitenpyram) could independently increase MP migration by 9.31%-61.01% by improving the hydrophilicity. Acetamiprid or dinotefuran reduced the adhesion work of the binary system by competing with SDS for adsorption sites, thereby inhibiting PE mobility. However, nitenpyram in the mixture was not easily adsorbed on the surface of PE MPs together with SDS because of nitenpyram's high hydrophilicity. Neonicotinoid molecules could not reduce the hydrophilic modification of SDS on PP MPs by competing for adsorption sites. Owing to their weak charge and adhesion work of nonionic surfactants (-4.80 mV and 28.45 kT for PE and -8.21 mV and 17.64 kT for PP), neonicotinoids tended to occupy the adsorption sites originally belonging to NP-40. The long molecular chain of NP-40 made it difficult for high-concentration neonicotinoids to affect the adhesion on MPs. In addition, NP-40 was harder to peel off from the MP surface than SDS, leading to a larger MP transport ability in the sand column.