Modelling molybdate and tungstate adsorption to ferrihydrite

Abstract

The environmental geochemistry of molybdenum and tungsten is not well known. To enable predictions of Mo and W concentrations in the presence of ferrihydrite (hydrous ferric oxide), batch equilibrations were made with MoO 4 2−, WO 4 2−, o-phosphate (PO 4 3−) and freshly prepared ferrihydrite suspensions in 0.01 M NaNO 3 in the pH range from 3 to 10 at 25 °C. The results showed that WO 4 2− is adsorbed more strongly than MoO 4 2−, and that both ions are able to displace PO 4 3− from adsorption sites at low pH. Two models, the Diffuse Layer Model (DLM) and the CD-MUSIC Model (CDM), were tested in an effort to describe the data. In both models, the adsorption of MoO 4 2− and WO 4 2− could be described with the use of two monodentate complexes. One of these was a fully protonated complex, equivalent to adsorbed molybdic or tungstic acid, which was required to fit the data at low pH. This was found to be the case also for a data set with goethite. In competitive systems with PO 4 3−, the models did not always provide satisfactory predictions. It was suggested that this may be partly due to the uncertainty in the PO 4 3− complexation constants.