Microplastic interaction with Fe-P coupling pathways induced cadmium immobilization in tropical agricultural soils

Agroecosystems face substantial risks from the combined contamination of microplastics (MPs) and cadmium (Cd). However, how MPs interfere with intrinsic iron (Fe) and phosphorus (P) cycles to affect Cd speciation in tropical soils remains poorly understood. This study investigated the effects of polyethylene (PE) and polypropylene (PP) on Cd transformation in two typical tropical agricultural soils (red soil and paddy soil). Results from the four-week short-term incubation showed that MPs significantly reduced the exchangeable Cd fraction while increasing the reducible fraction across different aggregate sizes, thereby decreasing Cd chemical extractability and environmental mobility. This immobilization was more effective for PE than PP and most pronounced in macroaggregates. Mechanistic pathways were soil-type specific. In red soil, MPs promoted organic matter (OM) accumulation, driving the crystallization of Fe oxides and the mobilization of soil P from Ca·Mg-P, which was subsequently stabilized as Fe·Al-P. The released phosphate was adsorbed onto more stable Fe oxides, facilitating Cd surface association and amorphous Cd-phosphate co-precipitation. In paddy soil, MPs potentially promoted highly localized anoxic microenvironments, leading to an increase in amorphous Fe oxides (Feo). Crucially, while X-ray diffraction (XRD) only detected macroscopic Fe oxide (e.g., Fe₂O₃) alterations, multi-spectroscopic evidence (X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS)) robustly supported the spatial co-location and formation of amorphous Fe-P and Cd-phosphate complexes at micro-interfaces. In conclusion, pristine MPs alter Cd fate by differentially regulating Fe-P biogeochemical cycles, providing a critical baseline for assessing co-contamination risks in tropical agroecosystems.