Sorption on oxides

Liitzenkirchen, J. (1997) Ionic strength effects on cation sorption on oxides. Macroscopic observations on their significance in microscopic interpretation. J. Coll. Int. Sci. 195 149-155... 

Sorption on oxides and hydrous oxides has been extensively studied both experimentally and theoretically. In order to define an ideal oxide or hydrous oxide exchanger, we will rely on experiments with well-defined sorbents such as chromatographic alumina (1-4). Briefly, the adsorption characteristics of oxides and hydrous oxides are (1) At high pH, they act as cation exchangers but with less sensitivity to the total salt concentration than clay minerals. This behavior is illustrated in Figure 2, where sorption of strontium on alumina is shown. 

For the data that we have collected so far for sorption on oxides, Equation 4 is a good empirical representation of the results ... 

The calculations show a strong dependence on concentration of the monovalent ion at low pH but a very small dependence at the upper pH values shown in Figure 6. The reason for this is that, because of the strong pH dependence of sorption on oxides, at low pH the clay-like behavior dominates and at the upper pH region, the oxide-like behavior dominates. 

The apparent saturation capacity of an oxide surface for hydrogen adsorption at a given temperature and the large change to a new apparent saturation at another temperature, facts familiar to all who have studied the slow sorption processes on oxides, should be re-studied in reference to Volkenstein s assumption that the sites available for adsorption, the thermal sites, vary with temperature. On this view the measurements of Shou-Chu Liang and the writer would gain new significance. In brief, measurements of slow sorption on oxide surfaces need to be... 

In the presence of mineral phases containing anions that would form sparingly soluble compounds (e.g. POt - and F for the lower oxidation states) an enhanced plutonium uptake due to chemisorption can be expected (57). For plutonium in the higher oxidation states the formation of anionic carbonate complexes would drastically reduce the sorption on e.g oxide and silicate surfaces. 

Stauffer TB, MacIntyre WG. 1986. Sorption of low-polarity organic compounds on oxide minerals and aquifer material. Environ Toxicol Chem 5 949-955. 

Metal sorption on Fe/Al oxides is an inner sphere complexion. The formation of a surface-metal bond releases protons for every metal ion adsorbed. Heavy metal sorbed on Fe oxides can be exchanged only by other metal cations having a similar affinity or by H (McBride, 1989). Metal adsorption on Fe oxides is an initial rapid adsorption reaction, followed by slow diffusion (Barrow et al., 1989). Metal ions (Ni2+, Zn2+ and Cd2+) slowly... 

Zinc adsorption can occur via exchange of Zn2+ and Zn(OH)+ with surface-bound Ca2+ on calcite (Zachara et al., 1988). Zinc and Ni form surface complexes on calcite as hydrate until they are incorporated into the structure via recrystallization (Zachara et al., 1991). The selectivity of metal sorption on calcite is as follows Cd > Zn > Ni (Zachara et al., 1991). The easily reducible oxide bound metals are primarily from Mn oxides (Chao, 1972 Shuman, 1982 and 1985a). At pH > 6, Zn sorption on Mn oxide abruptly increases because of hydroxylation of the ions (Loganathan et al., 1977), and a high soil pH in arid soil may favor Zn sorption on Mn oxides due to a great... 

Goldberg (2002) found no evidence of any competition in sorption of arsenate and arsenite on Al or Fe-oxides and montmorillonite, but only a small and apparent competitive effect of equimolar arsenate on arsenite sorption on kaolinite and illite. The minor competitive effect in this study was due to the small concentrations of arsenic which is very low for saturation site. Competition for sorption sites is evident by increasing the surface coverage of the sorbents. Arsenate prevents arsenite sorption on metal oxides when the surfaces of the sorbents are saturated by the anions (Jain and Loeppert 2000 Violante and Pigna 2002). 

A sample may be characterized by the determination of a number of different analytes. For example, a hydrocarbon mixture can be analysed by use of a series of UV absorption peaks. Alternatively, in a sediment sample a range of trace metals may be determined. Collectively, these data represent patterns characteristic of the samples, and similar samples will have similar patterns. Results may be compared by vectorial presentation of the variables, when the variables for similar samples will form clusters. Hence the term cluster analysis. Where only two variables are studied, clusters are readily recognized in a two-dimensional graphical presentation. For more complex systems with more variables, i.e. //, the clusters will be in -dimensional space. Principal component analysis (PCA) explores the interdependence of pairs of variables in order to reduce the number to certain principal components. A practical example could be drawn from the sediment analysis mentioned above. Trace metals are often attached to sediment particles by sorption on to the hydrous oxides of Al, Fe and Mn that are present. The Al content could be a principal component to which the other metal contents are related. Factor analysis is a more sophisticated form of principal component analysis. 

In addition to the dependence of sorption on the organic fraction of the sorbent, and the KQw of the sorbate, Chiou et al. (13) cite the following observations as support for the hypothesis that the sorptive mechanism is hydrophobic partitioning into the organic (humic) fraction of the sediments (1) the linearity of the isotherms as the concentration approaches solubility, (2) the small effect of temperature on sorption, and (3) the lack of competition between sorbates for the sorbent. These arguments also illustrate the applicability of the approach for modeling sorption on hydro-phobic compounds an approach which has been criticized when used in the context of adsorption of trace metals onto oxides (17). 

The vanadate equilibria are given in Table 7.7. The V02(0H)2 and V03(0H) anions are sorbed on positively charged sites on oxides and silicates at low pH, but sorption decreases with pH as the surface positive charge decreases. Consequently V is quite soluble at high pH and less soluble at low pH. 

The redox and sorption behaviour of Sb is similar, but no volatile forms are produced. The oxidized form of Sb, antimonite, Sb(V), has the anionic form Sb(OH)e at pH > 4, and is sorbed on oxides and sihcate clays. The reduced form, antimonite, Sb(III), is present as the uncharged Sb(OH)3 molecule except at very low or very high pH where the Sb(OH)2 cation and Sb(OH)4 anion form, respectively. The uncharged Sb(OH)3 is little sorbed on soil surfaces. 

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