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Susceptibility of Cd availability in microplastics contaminated paddy soil: Influence of ferric minerals and sulfate reduction
Summary
When microplastics and cadmium contaminate paddy soil together — a common situation in agricultural areas — microplastics increase the availability of cadmium to plants, raising the risk of cadmium uptake into food crops like rice. The mechanism involves microplastics releasing dissolved organic matter that disrupts iron mineral cycling and promotes sulfate-reducing bacteria, which in turn mobilize cadmium from soil particles. These findings highlight that microplastic pollution in farmland does not act alone — it can amplify the toxicity of co-occurring heavy metal contaminants.
The combined contamination of cadmium (Cd) and microplastics (MPs) in paddy soil always occurred, while its influence on Cd availability remained unclear. This study investigated the Cd availability in Cd-MPs co-contaminated paddy soil in consideration of both ferric minerals and sulfate reduction under flooding conditions. The presence of MPs resulted in a higher Cd releasing risk, as represented by the increase in the available Cd and decrease in Fe-Mn oxide-bound Cd contents, especially on the 7th and 14th days based on the sequential extraction results. MPs facilitated the formation of Fe-organic ligands, which accelerated the reductive dissolution of iron minerals but decreased the amounts of amorphous iron minerals due to the release of dissolved organic substances into pore water. Furthermore, MPs promoted the relative abundance of sulfate-reducing bacteria (such as Streptomyces and Desulfovibrio genera), thus increasing the contents of reductive S species, which was advantageous to the co-precipitation of Fe, S, and Cd on the surface of MPs based on our experimental and statistical results. Taken together, both iron and sulfate reduction under anaerobic conditions played a critical role in Cd mobilization in Cd-MPs co-contaminated paddy fields.