The relationship between the molecular structure and ion adsorption on goethite

Abstract

Keywords:Ion adsorption modeling, goethite, iron oxide, CD-MUSIC, phosphate, arsenate, vanadate, molybdate, tungstate, sulfate, selenate.A study is presented on the adsorption of inorganic ions on goethite with emphasis on the adsorption of oxyanions. Experimental results for a range of oxyanions (PO4, AsO4, VO4, WO4, MoO4, CrO4, SeO3, SeO4, SO4, Cl, NO3, ClO4) and Ca are presented and interpreted using the CD-MUSIC model. For some of these ions the coordination and structure of the adsorbed ions on goethite are known from spectroscopy (SO4, SeO4, PO4, AsO4, SeO3). Ideally, surface complexes derived from spectroscopy correspond with those resulting from the modeling of macroscopic adsorption data. This would assure that the mechanistic description of ion binding scales from the microscopic molecular structure to the macroscopic adsorption behavior. In the CD-MUSIC model it is assumed that the charge of the adsorbed ions is distributed at the interface as a function of the coordination and structure of the adsorbed ions and that this distribution of charge can be estimated using the bond valence concept of Pauling. In this study it is found that the macroscopic proton-ion adsorption stoichiometry is almost solely determined by the interfacial charge distribution of adsorbed complexes. It is shown that the experimentally determined proton-ion adsorption stoichiometry can be predicted on the basis of the spectroscopically identified structures of sulfate, selenite, phosphate and arsenate on goethite. By doing so a direct relationship is demonstrated between the molecular structure of adsorbed ions and macroscopic adsorption phenomena. By using this knowledge it is in principle possible to identify the structure and coordination of adsorbed complexes from the macroscopic adsorption data and vice versa. It is found that the spectroscopically suggested differentiation between inner- and outersphere complexes of sulfate and selenate, and the differentiation between bidenate and monodentate phosphate can be modeled satisfactory with the CD-MUSIC approach although the differentiation cannot be established solely from the available adsorption data. It is also found that the proton adsorption on goethite decreases in electrolyte solutions of NaCl, NaNO3 and NaClO4 (below the PZC) in the order Cl>NO3>ClO4 while sulfate and phosphate adsorption is lower in the order Cl<NO3<ClO4. These results can be explained well by assuming outersphere complexes of the electrolyte anions on the goethite surface with different intrinsic affinities.</p