Mutual influence of copper and paraquat on their adsorption in soil

Paraquat and copper (Cu) are commonly used and detected in soil. Therefore, it is important to understand their mobility in the environment. In this study, the competitive effects of paraquat and Cu on their adsorption in five representative Chinese soils were investigated using batch adsorption equilibrium experiments and spectroscopic analysis. The results showed that the adsorption of paraquat in soil varied with soil type and was positively correlated with both soil cation exchange capacity and 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. 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, respectively. Exchange selectivity involving symmetric cation (paraquat²+ and Cu²+) exchange is the probable basis for the suppression effect. The ultraviolet-visible absorption experiments suggested that the formation of Cu-paraquat complexes was unlikely to happen in a solution or at the soil surface. Copper K-edge X-ray absorption spectroscopy indicated that Cu in soil may have some water as hydration layers as the nearest neighbors, and each Cu atom was coordinated with five oxygen atoms. These findings greatly improve our knowledge of the molecular-scale adsorption mechanisms of paraquat and Cu in soil and can be used to predict the behavior, transport, and fate of paraquat and Cu in agricultural soils.