Relevance of dissolved organic matter components on the adsorption behaviors of cadmium onto non-biodegradable and biodegradable microplastics in an agricultural soil

Purpose The role of microplastics (MPs), as vectors for coexisting contaminants, in affecting the adsorption and desorption behavior of cadmium (Cd) by regulating soil properties such as dissolved organic matter (DOM) is still under debate. Methods A 2-month incubation of soil contaminated with different sizes and concentrations of non-biodegradable polyethylene (PE) and three types of biodegradable MPs were conducted. Subsequently, three-dimensional excitation-emission matrix (3D-EEM) with fluorescence region integration (FRI) analysis of DOM and a batch experiment of Cd behavior on MP-contaminated soils were further conducted. Results and discussion Soil properties such as pH, ammonium nitrogen, and urease activity were significantly affected by MP properties. 3D-EEM analysis indicated that the microbial by-product substances and the humic acid–like substances were the major components of DOM in MP-contaminated soil, accounting for approximately 50%. Overall, Cd adsorption was decreased by higher MP concentration and larger size, and PE-contaminated soils had similar or higher adsorption capacities than soil with biodegradable MPs. MPs can decrease in Cd adsorption and increase in Cd desorption when compared to the control. Both the Langmuir and Freundlich models fitted well with Cd adsorption and desorption isotherms. Correlation analysis confirmed that the characteristics of DOM, such as the humification index values and major components, rather than other soil properties, contributed more to the adsorption behaviors of Cd on MP-contaminated soils. Conclusion Addition of MPs could mitigate soil Cd adsorption capacity, mainly due to the changes of soil DOM characteristics, which will enhance the mobility and risks of Cd to agricultural soil.