Effect of clay colloid - CuO nanoparticles interaction on retention of nanoparticles in different types of soils: role of clay fraction and environmental parameters.

The extensive application of metal oxide nanoparticles (NPs) in various sectors has raised concern about their subsequent release and potentially harmful impacts on the soil system. The present study has addressed the interaction of CuO NPs with bentonite clay colloids (CC) under varying environmental parameters as a model to represent the soil pore water scenario. Based on CuO - CC interaction in model and natural soil solution extracts (SSE), the role of clay fraction and their stability on CuO retention in various types of soils have been evaluated. Results suggested that increasing ionic strength (IS) in the system caused aggregation of CuO NPs, and in the presence of CC, critical coagulation concentration decreased drastically from 27.8 and 17.3 mM to 10.7 and 0.33 mM for NaCl and CaCl2 respectively, due to heteroaggregation in the system. Interestingly, in the SSE, the dominating role of ionic valency, dissolved organic carbon (DOC), and CC was observed in colloidal stabilization over IS. No significant impact of temperature was observed on the stability of CuO NPs both in model and SSE. Further, stability studies in the SSE were correlated with NPs retention behavior in soils. Observations suggest that retention of CuO NPs in soils is a function of binding of the colloidal fraction to the soil, which in turn depends on the colloidal stability. The highest retention was observed in black and laterite soils, whereas lower binding of clay fraction in red soil caused the least retention. A decrease in Kd values after a certain application concentration provided maximum sustainable application concentration of CuO NPs, which may vary with soil properties. Results suggest that the binding of clay and organic matter with a sandy matrix of soil plays a prime role in deciding the overall fate of CuO NPs in the soils.