The relationship between the molecular structure and ion adsorption on goethite

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

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Ich habe eine Untersuchung angestellt besonders nach der Lebensweise der Ichneumonide, Nemeritis canescens (Gravenhorst), als eines Entoparasiten der Larve von Ephestia kuehniella Zeller. ... Zie: Zusammenfassun

De effecten van baggeren op emissies naar water en lucht

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Surface Complexation of Selenite on Goethite: MO/DFT Geometry and Charge Distribution

The adsorption of selenite on goethite (a-FeOOH) has been analyzed with the charge distribution (CD) and the multi-site surface complexation (MUSIC) model being combined with an extended Stern (ES) layer model option. The geometry of a set of different types of hydrated iron-selenite complexes has been calculated using Molecular Orbital / Density Functional Theory (MO/DFT). The optimized geometries have been interpreted with the Brown bond valence approach resulting in a set of ionic charge distribution values. After correction for dipole orientation effects, it results in the interfacial charge distribution coefficients that can be applied to the analysis of adsorption data. The use of theoretical CD values has the practical advantage of a reduction of the number of adjustable parameters. From a theoretical perspective, the CD values can constrain the model, revealing a surface speciation that can be tested experimentally. Modeling of the adsorption of SeO3 in (pseudo-) monocomponent goethite systems, using the calculated CD values, has revealed the dominant presence of a bidentate surface species º(FeO)2SeO. The dominance of this surface species agrees with the interpretation of EXAFS measurements given in literature. The agreement supports the validity of the approach. To describe the adsorption at very low pH and a high loading, formation of an additional surface species is required in the modeling. The maximum contribution is about 20 % or less. In case of anion competition, as found in the PO4-SeO3 goethite system, the relative contribution increases. Analysis of the adsorption behavior in the PO4-SeO3 goethite systems revealed the probable nature of the additional surface complex, which is found to be a protonated monodentate surface complex ºFeOSeOOH. With the affinity constants derived, the CD model is able to describe the SeO3 adsorption on goethite over a large range of pH, ionic strength, and loading conditions for a variety of goethite preparations. The CD model correctly predicts the proton co-adsorption of selenite and is able to describe the shift of the IEP upon addition of selenite

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