Contribution of individual pure or mixed-phase mineral particles to metal sorption in soils

Abstract

The contribution of individual minerals and their associations to metal sorption in soils is little known. We therefore determined the concentrations of Cd, Cu, Pb, and Zn in individual mineral particles (clay minerals, Fe-rich clay minerals, clay-Fe oxide associations, Fe-oxyhydroxides, calcite) after equilibration of an acid and an alkaline soil sample with 10 mmol/L of these metals with the help of Transmission Electron Microscopy equipped with Energy-dispersive X-ray Spectroscopy (TEM-EDX). The results of the TEM-EDX measurements were compared with those of batch sorption experiments.The alkaline soil showed a stronger sorption of all studied metals than the acidic soil, as expected. This was also true when the individual mineral (associations) were considered, although the clay mineral and clay mineral-rich particles in the acidic soil sorbed more Cd and Zn than those in the alkaline soil. In line with the literature, we consistently observed a stronger sorption of Cu and Pb than of Cd and Zn both in the bulk soil and on the particles with the exception of Zn that showed the highest sorption on clay particles in the acidic soil among the studied metals. Although Cu and Pb may also have precipitated in the alkaline soil, their higher sorption was found on the particles directly, as well. The Fe concentrations of the individual mineral particles correlated with the sorbed amounts of metal. It could be related to the increasing contribution of Fe-oxyhydroxides within the particle associations in the alkaline soil, and rather to the increasing Fe concentration of clay mineral particles in the acidic one. Our results emphasize the important role of Fe oxides as pure minerals or in mineral associations for the sorption of trace metals in soils depending on the soil pH conditions.