Microplastics in soils with contrasting texture, organic carbon and mineralogy: changes in cadmium adsorption forms...

Environmental contamination with high-density polyethylene microplastics (MPs) can alter the dynamics of Cd adsorption in soil. The objective of the present study was to identify changes induced by MPs in the Cd²⁺ adsorption on soils with contrasting texture, organic matter, and clay mineralogy. Leaching columns (three sections of 0.05 × 0.15 m) were filled with disturbed samples (< 2 mm) of the A and Bi horizons of Udic Inceptisol (UI), and Humic Inceptisol (HI) and A, E and Bh of Orthod Spodosol (OS). The first layer of soil (0-0.05 m) was mixed with 2% (w/w) of MPs (two sizes: 150 µm and 40-48 µm). The soil columns were incubated for 90 days, with 10 periodic Cd applications (1 L each) (92, 185, 231, 527, 1186 mg L^-1) in column surface. The pseudo-total, water-soluble, exchangeable (outer sphere), and inner sphere Cd fractions were extracted by selective and sequential treatments and determined by ICP-OES. MPs did not result in impacts on pseudo-total Cd in soils layers. The pseudo-total Cd for the sandy soil (OS) (clay < 100 g kg^-1) accumulated in the first layer due to its higher organic carbon (18.2 g kg^-1). A negative effect was observed for the smaller MPs on the outer sphere adsorption of Cd²⁺ in the most electronegative first layer (A horizon) of HI and OS. There was difference among soil class on total Cd²⁺ collected in leaching solution collected in the column base (mg column^-1): OS-695.1 > UI-367.8 > HI-214.1. In practical case of double environmental contamination, the addition of < 2% (w/w) of MPs did not interfere in the cationic heavy metal dynamics, independent of the texture, organic matter content and mineralogy of the clay fraction of the used soils.