Mutual influence of copper and paraquat on their adsorption in soil

Abstract

Paraquat and copper (Cu) have been commonly used and detected in soils. Therefore, it is important to understand their mobility in the environment. This study investigated the competitive effects of paraquat and Cu on their adsorption in five representative Chinese soils by performing batch adsorption equilibrium experiments and spectroscopic analysis. The adsorption of paraquat in soils varied with soil types and was positively correlated with both cationic exchange capacity and soil organic matter content. Paraquat exerted a more remarkable suppression effect on the adsorption of Cu than Cu on the adsorption of paraquat. In the presence of 0.12 and 0.19 mmol L−1 paraquat, Cu adsorption decreased by 16% and 22% in Heilongjiang soil and by 24% and 37% in Jiangxi soil, respectively. While in the presence of 0.1 and 0.2 mmol L−1 Cu, paraquat adsorption decreased by 4% and 8% in Heilongjiang soil and by 15% and 19% in Jiangxi soil. Exchange selectivity involving symmetric (Paraquat2+, Cu2+) cation exchange is the probable basis for the suppression effect. An ultraviolet-visible absorption study suggested that formation of Cu–paraquat complexes was unlikely in solution or on the soil surface. Cu K-edge X-ray absorption spectroscopy indicated that Cu in soils may have some water hydration layers as the nearest neighbors, and each Cu was coordinated with five oxygen atoms. The results greatly improve our knowledge about the molecular-scale adsorption mechanisms of paraquat and Cu in soils and will be useful to predict the behavior, transport, and fate of paraquat and Cu in agricultural soils.