Mineral and chemical changes of sediments after Cu sorption and then desorption induced by synthetic root exudate

Abstract

Understanding the fate of anthropogenically introduced copper in sediments is important to comprehend the biogeochemical processes; consequently, beneficial utilization of Cu-rich materials can be proposed (e.g. soil amendment). Therefore, we address the behavior of copper and other metals at the liquid-solid interface of different grain sizes in lake sediments. Initially, the sediment fractions were characterized for mineralogy (XRD), chemical structure (FTIR), physicochemical parameters (mainly pH, cation exchange capacity, and electric conductivity), organic content, and chemical composition (AAS). Then, solutions of varying Cu concentrations were added to the fractions; the Cu concentrations of the sorption experiment were chosen according to the exchangeable cations of each fraction. A desorption experiment by synthetic root exudate was followed. The physicochemical parameters, functional groups, and mineralogy were noted before and after the two experiments. The sorption and desorption of Cu, Ca, Mg, K, and Na were also studied. The sediment fractions had similar mineralogy and chemical structure, yet the physicochemical composition and metal contents were different. The Cu sorption experiment showed that surface Ca and embedded Mg were the main cations that were exchanged with Cu, as shown by linear and logarithmic trends, respectively. The copper-sediment interaction mainly occurred at the organic interface. Finally, synthetic root exudate was able to restore part of the initial chemical structure of the sediments, indicating exchangeable Cu sorption on the organic part of the sediments. The various grain sizes had an insignificant influence on the behavior of metal sorption and desorption.