Modelling molybdate and tungstate adsorption to ferrihydrite

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 MoO42−, WO42−, o-phosphate (PO43−) and freshly prepared ferrihydrite suspensions in 0.01 M NaNO3 in the pH range from 3 to 10 at 25 °C. The results showed that WO42− is adsorbed more strongly than MoO42−, and that both ions are able to displace PO43− 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 MoO42− and WO42− 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 PO43−, the models did not always provide satisfactory predictions. It was suggested that this may be partly due to the uncertainty in the PO43− complexation constants.