Effects of adsorption and oxidation of dissolved organic matter by ferrihydrite and/or birnessite on its binding characteristics to copper

Dissolved organic matter (DOM) is a natural ligand for heavy metal binding. Adsorption and oxidation of DOM by iron and/or manganese oxides may change its molecular composition and affect its binding with heavy metals, but the underlying mechanisms are still poorly understood. In this study, the effects of adsorption and oxidation of DOM by ferrihydrite and/or birnessite on its binding characteristics with copper ions (Cu²⁺) were investigated using fluorescence spectroscopy and Fourier transform infrared spectroscopies combined with two-dimensional correlation analysis. Results showed that the adsorption and oxidation of DOM on birnessite significantly weakened the Cu²⁺ binding ability of DOM especially in the presence ferrihydrite, which was ascribed to the preferentially adsorption/oxidation of phenolic compounds and the adsorption of carboxylic compounds. Specifically, the reactions of DOM with birnessite enhanced the sensitivity of fulvic-like fraction to Cu²⁺ binding, being less favorable in the presence of a high concentration of ferrihydrite. Despite the unchanged binding order of Cu²⁺ with fluorescence components after adsorption and oxidation of DOM on birnessite, the binding of Cu²⁺ with carboxylic group became later compared with carbohydrate C–O, phenolic and aryl groups and aliphatic C–H. However, with the increase in the content of ferrihydrite, the binding of Cu²⁺ with carboxylic groups was more advanced. Our results highlighted the importance of the ratio of birnessite and ferrihydrite in controlling the binding characteristics of Cu²⁺ with DOM fractions, which helps to understand the reactions of heavy metals with natural organic matter under the impacts of complex mineral assemblages.