『Abstract
　The biogeochemistry of trace elements (TE) is largely dependent upon their interaction with heterogeneous ligands including metal oxides and hydrous oxides of iron. The modeling of TE interactions with iron oxides has been pursued using a variety of chemical models. The objective of this work is to show that it is possible to model the adsorption of protons and TE on a crystallized oxide (i.e., goethite) and on an amorphous oxide (HFO) in an identical way. Here, we use the CD-MUSIC approach in combination with valuable and reliable surface spectroscopy information about the nature of surface complexes of the TE. The other objective of this work is to obtain generic parameters to describe the binding of the following elements (Cd, Co, Cu, Ni, Pb, and Zn) onto both iron oxides for the CD-MUSIC approach. The results show that a consistent description of proton and metal ion binding is possible for goethite and HFO with the same set of model parameters. In general a good prediction of almost all the collected experimental data sets corresponding to metal ion binding to HFO is obtained. Moreover, dominant surface species are in agreement with the recently published surface complexes derived from X-ray absorption spectroscopy (XAS) data. Until more detailed information on the structure of the two iron oxides is available, the present option seems a reasonable approximation and can be used to describe complex geochemical systems. To improve our understanding and modeling of multi-component systems we need more data obtained at much lower metal ion to iron oxide ratios in order to be able to account eventually for sites that are not always characterized in spectroscopic studies.』

1. Introduction
2. Methods
　2.1. The CD-MUSIC model (Hiemstra and Van Riemsdijk, 1996)
　2.2. Model parameters
　　2.2.1. Oxide structure: Goethite and HFO
　　2.2.2. Proton affinities, sites density, and capacitance
　　2.2.3. Surface complexes
　　　2.2.3.1. Cadmium
　　　2.2.3.2. Copper
　　　2.2.3.3. Lead
　　　2.2.3.4. Zinc, a special case
　　2.2.4. Metal affinities and charge distribution
　　2.2.5. From goethite to HFO
3. Results
　3.1. Proton adsorption
　　3.1.1. Goethite
　　3.1.2. HFO
　3.2. Metal adsorption
　　3.2.1. Cadmium binding data
　　　3.2.1.1. Sorption onto goethite
　　　3.2.1.2. Sorption onto HFO
　　3.2.2. Lead binding data
　　　3.2.2.1. Sorption onto goethite
　　　3.2.2.2. Sorption onto HFO
　　3.2.3. Copper binding data
　　　3.2.3.1. Sorption to goethite
　　　3.2.3.2. Sorption onto HFO
　　3.2.4. Zinc binding data
　　　3.2.4.1. Goethite
　　　3.2.4.2. Sorption to HFO
4. Discussion
　4.1. Limitations of the proposed approach
　4.2. Extrapolation to other metal ions: Co, Ni
　　4.2.1. Cobalt
　　4.2.2. Nickel
Acknowledgments
Appendix A. Surface species, site densities, and proton binding constants for goethite are taken from Venema et al. (1998)
Appendix B. Summary details of selected sets of experimental data for metal ion binding by goethite
Appendix C. Optimized proportions of the crystallographic plane [110] compared to the total goethite surface area
Appendix D. Summary details of selected sets of experimental data for metal ion binding by hydrous ferric oxides (HFO)
References