Modeling of sorption processes is essential for the prediction of contamination transport in the environment. Sorption modeling, under varying environmental conditions, can be achieved using surface complexation models (SCM). Unlike empirical distribution models, SCMs provide a more robust description of sorption reactions on mineral surfaces and can predict sorption as a function of pH, concentration and competing ions. The need to distinguish between the different types of complexes on mineral surfaces has been emphasized in the last 20 years and a multitude of studies has been devoted to investigating surface speciation of various compounds. In this study, a SCM is employed to simulate chromate adsorption on ferrihydrite under various environmental conditions. The sensitivity of the model to the type and number of surface complexes taken into account was tested, taking into account the available spectroscopic studies. The CD MUSIC model introduced by Hiemstra and Riemsdijk (1996) was employed and the surface structure of ferrihydrite as presented by Hiemstra (2013) was utilized as the basis for 1-pK Basic Stern SCM. The surface protonation constants of singly and triply coordinated oxygens proposed by Hiemstra (2013) were used, while the equilibrium constants of surface species were fitted using pH-edge data. The sensitivity of the model to the number and type of reactive surface oxygens was tested. The distribution of different surface species (monodentate and bidentate complexes) determined spectroscopically was utilized as additional constraint to assess the success of the model to predict sorption. The results indicated that either approach, with a single bidentate species, or with both monodentate and bidentate species yielded satisfactory macroscopic predictions.

Surface complexation modelling of chromate adsorption on iron oxides

251st National Meeting (2016)

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