Influences of microplastics types and size on soil properties and cadmium adsorption in paddy soil after one rice season

Microplastics (MPs) were considered as emerging pollutants in soil. Cadmium (Cd) is the typical heavy metal contaminant in paddy soil in China. It was not unraveled that the effects of the existence of MPs on soil properties and Cd adsorption after rice growth in paddy soil, one of the most important soil types in China. In this study, several typical MPs in dosage of 0.5% (w/w) with different particle sizes, including polyethylene (PE, $200\mu m$), polyacrylonitrile (PAN, $200\mu m$), and polyethylene terephthalate (PET, 200 and $10\mu m$), were selected and aged for a rice season ($\sim$6 months) in paddy soil by column experiments. Some essential physicochemical properties of paddy soil collected from different treatments and the adsorption performance of typical metal contaminants (cadmium, Cd) in paddy soil with MPs coexisting were compared. The results indicated that the 6-month existence of $200\mu m$ MPs increased water content (15.94% to 26.08-30.30%), pH value (7.05 to 7.16–7.33), organic matter (OM) content (15.1 g$$kg ⁻¹ to 18.2–22.9 g$$kg ⁻¹), the percentage of soil sand and Cd adsorption capacity of paddy soil, whereas decreased soil cation exchange capacity (CEC), the percentage of soil clay and silt. The adsorption experiments of Cd in soil showed that the adsorption behavior was consistent with the pseudo-second order and Langmuir isotherm model, and the addition of MPs would increase the adsorption capacity of Cd in soil. Besides, the particle size of MPs was found to be one of the dominant factors, in which MPs with smaller particle size ($10\mu m$) favored the increase of water content, pH, OM content, CEC, and adsorption performance with Cd. The results of this study could provide a supplemental understanding of the effect of MPs input on the paddy soil ecosystem, including the soil properties and the bioavailability and transport of metal in paddy soil.