Adsorption on amorphous oxides

Goldberg, S. and Johnston, C.T. (2001) Mechanisms of arsenic adsorption on amorphous oxides evaluated using macroscopic measurements, vibrational spectroscopy, and surface complexation modeling. Journal of Colloid and Interface Science, 234(1), 204-16. 

Computer simulation of adsorption on amorphous oxide surfaces... 

As soon as the expressions and constants in Eq. (4) are fixed one may proceed with calculation of the adsorption energy at a given point of adsorption space. However, to make such a calculation possible one has to know the positions of all the atoms of adsorbent relative to the given point. In other words, one has to know exactly the atomic structure of the adsorbent. This is what is in fact unknown for amorphous oxides. Although one can simulate the atomic structure at the surface of an amorphous oxide as described above, the reliability of the result can at present only be checked by comparison of prediction of adsorption properties with experimental data. But the calculation of adsorption properties (described below) includes, generally speaking, two unknowns the atomic structure of an adsorbent and the adsorption potential. This is the reason why the computer simulation of physical adsorption on amorphous oxides should be preceded by similar simulations on oxides with well defined crystalline structures. 

Thus, the surface of this amorphous carbon (which is a model of the surfaces of non-graphitized carbon blacks [23]) differs considerably from the surface of amorphous oxide and the main structural characteristics such as the C-C and 0-0 coordination numbers are also drastically different. Nevertheless, the adsorption properties of heterogeneous surfaces of various nongraphitized carbon blacks with respect to an inert adsorbate such as argon are not that drastically different and actually have many common features. We discuss these properties in the next section. Here we only use this fact to show that subtle structural differences of various models of amorphous oxide surfaces discussed above may be not that important for their adsorption properties in comparison to other factors such as indefiniteness of adsorption potential on oxide surfaces (see below). Because of its generality and in spite of its approximate character, the BS appears to be a convenient model for the computer simulation of adsorption on amorphous, and even more general (see Introduction) heterogeneous oxide surfaces. 

Physical adsorption on the (001) face of MgO also attracted considerable attention in recent years (see short review and references in Ref. [26]). It provides another opportunity to test methods of adsorption potential calculation which can be used later to simulate adsorption on adsorbents with less reliable atomic structure of surfaces like amorphous oxide. There is a large and rapidly changing electric field near the surface of MgO which should be much stronger than in silicalite due to small cations of Mg " " and larger ionicity of MgO in comparison to Si02. Thus calculations with polar and quadrupole molecules which were carried out on that surface (see Ref. [26] and references therein) necessarily employ methods which may useful for computer simulations on amorphous oxides. 

Van Riemsdijk et al. [53] were the first to show that electrostatic effects could explain non-stoichiometric exchange ratios. Predictions with the one-pKn SCG model and the two-pKn SGC model were both in a good agreement with experimentally observed proton/M ratios and metal ion isotherms at a series of pH values for rutile, hematite and amorphous iron oxide. In contrast with Benjamin and Leckie [86], Van Riemsdijk et al. [53] concluded that incorporation of surface heterogeneity is not required to describe cadmium adsorption on amorphous iron oxide. 

The full set of equations can be solved with a computer program using the mathematical approach outlined by Westall (1980). Caution must be used to ensure that the computer code does not implement the standard and reference states proposed by Hayes and Leckie (1987). A fit of the triple layer model to silver adsorption on amorphous iron oxide is presented in Figure 6.7. 

Use of surface speciation models for prediction of adsorption and transport requires specification of the mode of bonding and speciation of oxyanions on oxide surfaces. FTIR spectroscopy (especially ATR and DRIFT) offers the potential to establish symmetry of surface species, protonation, and determination of monodentate or bidentate bonding. Determination of surface speciation is greatly enhanced when the spectroscopic information is combined with measurements of electrophoretic mobility (EM), calculation of point of zero charge and proton balance measurements before and after adsorption. We review adsorption of phosphate, carbonate, boron, selenate and selenite on Fe and A1 oxides. New preliminary spectra and EM and proton balance information for arsenate and arsenite adsorption on amorphous Fe and A1 oxide suggest that HASO4 and H2ASO3 are the dominant surface species. 

The same type of porphyrin-Ru complex was immobilized by coordina-tive adsorption on aminopropylsilicas (Fig. 26) as either amorphous or crystalline supports [79]. Mesoporous crystalline MCM-48 was the best support, as shown by the improved results obtained in the epoxidation of styrene with 2,6-dichloropyridine N-oxide (TON > 13 000 and 74% ee). The versatility of this catalyst was demonstrated in the intramolecular cyclopropanation of frans-cinnamyl diazoacetate. TON was ten times higher than that obtained in solution and 85% ee was observed. The solid was recycled and reused, although partial loss of selectivity occurred. 

Cavallaro and McBride (1984a) observed that the removal of Fe oxides from two clay soils reduced Zn adsorption. Shuman (1976) reported that the removal of Fe oxides resulted in an increase or decrease in Zn adsorption, but later in another similar study (1988) he found that the removal of either amorphous or crystalline Fe oxides increased Zn adsorption capacity and decreased Zn-bonding energy. The author explained that adsorption sites on the Fe oxide coatings were not as numerous as those released when the coatings were removed. Elliott et al. (1986) observed that DCB extraction of Fe oxides from two subsoils of the Atlantic Coastal Plain increased heavy metal adsorption. Wu et al. (1999) found that Cu adsorption on the fine clay fraction increased after dithionite treatment with possible exposure of much more high-affinity sites for Cu on the fine clay. 

Benjamin, M.M. Leckie, J.O. (1981) Multiple-site adsorption of Cd, Cu, Zn, and Pb on amorphous iron oxyhydroxide. J. Colloid Interface Sci. 79 209-221 Benjamin, M.M. Leckie, J.O. (1981a) Competitive adsorption of Cd, Zn, Cu and Pb on amorphous iron oxyhydroxide. J. Colloid Interface Sci. 83 410-419 Benjamin, M.M. Leckie, J.O. (1982) Effects of complexation by Cl, SO4, and S2O3 on the adsorption behavior of cadmium on oxide surfaces. Environ. Sci. Tech. 16 162-170 Benjamin, M.M. (1978) Effects of competing metals and complexing ligands on trace metal adsorption. Ph.D. Thesis Benjamin, M.M. Hayes, K.E. Leckie, K.O. 

Kanai, H., Navarrete, R.C., Macisko, C.W. Scriven, L.E. (1992) Rheol. Acta 31 333 Kandori, K. Ishikawa,T. (1991) Selective adsorption of water on amorphous ferric oxide hydroxide. Langmuir 7 2213-2218 Kandori, K. Aoki,Y. Yasukawa, A. Ishikawa, T. (1998) Effects of metal ions on the morphology and structure of hematite particles produced from forced hydrolysis reaction. 

Comparisons with the vibrational spectroscopic studies of the adsorption and dehydrogenation of ethene on single-crystal Pt surfaces (Section X.B.l) show that the di-cr-C2H4 to ethylidyne conversion occurs on (111) facets of the Pt crystallites of the catalysts. It is considered that the di-cr -C2H4 species occur on metal sites on which this conversion is not allowed, perhaps on (100), (110), or (210) facets. It is not clear whether the labile it-C2H4 species is formed on amorphous areas of the clean Pt particles or whether it occurs on sites which are affected by proximity of the metal oxide support (408) we favor the former possibility. 

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