From Release to Transformation: Elucidating the Microplastic-Facilitated Fate of Brominated Additives in Soil

Microplastics (MPs) are pervasive vectors in soil, but their quantitative role in additive transport and transformation remains unclear. This study examines the release, transport, and transformation of TBBPA and BDE-209 from polystyrene MPs. Vertical MP migration forms an observed secondary emission front under hotspot-like conditions at 5-7 cm depth, which influences the spatial distribution of the strongly hydrophobic BDE-209, leading to a concentration plateau at this interface. We developed a mechanistic model based on Fick's second law, parameterized with the first-order release rate constant (krelease), colloidal diffusion coefficient (Dsc), and biodegradation rate constant (k) under our experimental conditions. This model successfully captured the distinct migration patterns of the two additives, supported by machine learning (XGB) validation within the scope. Depth and time were the primary experimental controls, while EC and pH were the key soil properties influencing additive fate. Beyond transport, MP-driven soil changes promoted extensive additive transformation into 59 byproducts. The majority of BDE-209 transformation products were predicted to exhibit significant oral toxicity and high persistence in the environment. Our findings propose a novel, comprehensive framework that advances beyond existing approaches by linking MP-mediated transport to hazardous product formation, providing essential insights for the risk assessment of plastic-contaminated soils.