Ion adsorption to ferrihydrite: A CD-MUSIC model database constrained from X-ray absorption spectroscopic information

Despite a large body of literature on the adsorption of ions to ferrihydrite, there has been only few attempts to compile a database of surface complexation constants, which can be useful in practical applications, for example when predicting ion sorption in soil. Here a database of surface complexation constants for the Ferrihydrite CD-MUSIC model has been compiled, which currently contains entries for 55 ions based on data primarily collected in the form of adsorption edges (> 600), where the adsorption was studied as a function of pH. X-ray spectroscopic evidence and information from molecular orbital density functional theory calculations aided in the development of the database. We include results for single-sorbate systems that have not previously been analyzed with the model, for example Al 3+ , Cr 3+ , Hg 2+ and rare-earth elements, although the constants for the latter should be regarded as highly preliminary. Apart from optimizing surface complexation constants for single-sorbate systems, constants were derived also for ternary surface complexes, where such have been proven using X-ray absorption spectroscopy, e.g. for Pb 2+ -PO 4 , Cd 2+ -PO 4 , UO 2 -CO 3 and UO 2 -PO 4 systems with ferrihydrite. The model was generally successful in predicting adsorption patterns both for single- and multi-sorbate systems. The importance of electrostatic and competitive interactions from CO 3 , PO 4 , SO 4 2− and natural organic matter on ion adsorption was underlined, and the need for additional data, particularly for rare earth elements and organic acids, is emphasized. The database was developed for Visual MINTEQ 4.0 but can be transferred to other software packages after carefully checking for consistency in the solution complexation constants and in the treatment of multidentate complexes.