The influence of humic acid on europium–mineral interactions

Abstract

The retardation of metal migration through the Geosphere is in part determined by sorption processes. These sorption interactions may be complicated by the presence of organic matter, such as humic acids. This work examined the sorption of europium onto colloidal goethite and boehmite and the influence of humic acids over a range of concentrations (0 to 20mgl−1) and pH (2 to 12). Humic acid adsorption was also studied, as was its effect on the zeta potential of the colloidal minerals.Results showed that humic acid readily adsorbed to the minerals, with adsorption decreasing with increasing pH. The presence of humic acid at all concentrations and at all pHs decreased the zeta potential of the minerals. In some cases the humic acid was able to cause complete charge reversal on the minerals. The results suggest that the humic acid adsorption mechanism may primarily be one of ligand exchange.Europium adsorption in the absence of humic acid was typical, showing an increase with pH and a distinct adsorption edge. However, in the presence of varying concentrations of humic acid, europium adsorption to the minerals was significantly affected. Generally, humic acid was shown to enhance europium adsorption at low pH, but reduce europium adsorption at intermediate and high pH. Humic acid adsorption data, zeta potential data and europium adsorption data are compared and adsorption mechanisms suggested. The significance of these results with respect to further experimental and modelling efforts to understand the Geosphere are discussed.