We have measured the sorption of Ni as a function of surface loading and pH with a series of laboratory experiments. We fit the data using a multisite surface complexation model incorporating the basic Stern model to describe the surface electrostatics. At 0.75 wt% Ni and above, however, we cannot model the sorption data in terms of simple surface complexation but must invoke polynuclear complexes (or surface precipitates) to fit the sorption edges. In addition to surface complexation and surface precipitates, Ni is associated with goethite via structural incorporation. After 21 days of sorption, about 90% of the sample with the highest Ni concentration is reversible while 60% of the sample with the lowest concentration (0.03 wt% Ni) is irreversible. In contrast, after 365 days of sorption about 50–60% of the sorbed Ni is irreversible in all the grades (0.003–2.56 wt% Ni) suggesting almost uniform structural incorporation in the lattice of goethite.
- Thermodynamic constants