Kaolinite metal adsorption

Simple ion exchange describes the competitive adsorption onto clays of most metal cations present in solution at concentrations from about 10 to 10 mol/kg. It has most often been used.to describe the sorption of alkaline earth and alkali metal cations onto clays. In the case of minerals having pH-dependent surface charge (e.g., kaolinite, metal oxyhydroxides) simple ion exchange or the power-exchange function (see below) may also fit the adsorption data mea.sured in systems at constant pH. 

The effects of electrolyte concentration. Okazaki et al. [26] reported no effect of electrolyte concentration on adsorption of copper and zinc by iron and aluminium oxides. Similarly, from their own work, and from a survey of the literature, Hayes and Leckie [27] concluded that there was little effect of electrolyte concentration on metal adsorption. However, a wider view of the literature shows that there are indeed effects on surfaces which are probably negatively charged - for example, for adsorption of nickel on kaolinite [31], for zinc on soil [32], for copper, cadmium and lead on kaolinite [33] and for cadmium on soil [34]. This discrepancy probably arises because metal adsorption on oxides often... 

Cygan, R. T., Nagy, K. L., and Brady, P. V. (1998). Molecular models of cesium sorption on kaolinite. In Adsorption of Metals by Geomedia Variables, Mechanisms, and Model Applications, ed. Jenne, E. A., Academic Press, San Diego, CA, 383-399. 

Brady, P.V., Cygan, R.T., and Nagy, K.L., Surface charge and metal sorption to kaolinite, in Adsorption of Metals by Geomedia, Jenne, E.. ed.. Academic Press, New York, 1998, p. 371. 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

Baham, J., and Sposito, G. J. (1994). Adsorption of dissolved organic carbon extracted from sewage sludge on montmorillonite and kaolinite in the presence of metal ions. J. Environ. Qual. 23,147-153. 

HIGH-TEMPERATURE ADSORPTION OF HAZARDOUS METAL CHLORIDES USING ACTIVATED KAOLINITE... 

The fallacy of assigning single values to the sorption of metal cations is evident from Figs. 10.13 and 10.14, which show Pb adsorption by kaolinite and Cd by montmorillonite. Lead adsorption has been measured as a function of pH. Clearly, a sorption isotherm equation is needed to... 

Kent et al. (1986) have tabulated reported TL intrinsic constants and , values for a variety of metal oxyhydroxides of Al, Si, Fe, and Ti. More recently. Smith and Jenne (1988,1991) reevaluated published TL modeling of adsorption by ferric oxyhydroxide solids and by 5-Mn02. Their analysis led to a set of intrinsic constants based on measurement and estimation that have been reproduced in Tables 10.12 and 10.13. The intrinsic constants in the tables were derived independent of values for C[ chosen by others. Few studies have applied TL modeling to adsorption by clays, although James and Parks (1982) and Mahoney and Langmuir (1991) TL-modeled alkali metal and alkaline earth adsorption by clays, including beidellite, illite, kaolinite, and montmorillonite. 

The majority of adsorption studies deal with the interactions between metal ions and either real or model sediment phases. Amorphous FeCOH), hydrous MnO.. clays (bentonite, illite, kaolinite), SiO., Al Oj and organic matter (often humic substances) are the solid phases most frequently studied. 

Specific adsorption on well defined materials has been the subject of many reviews [8-13]. Specific adsorption plays a key role in transport of nutrients and contaminants in the natural environment, and many studies with natural, complex, and ill defined materials have been carried out. Specific adsorption of ions by soils and other materials was reviewed by Barrow [14,15]. The components of complex mineral assemblies can differ in specific surface area and in affinity to certain solutes by many orders of magnitude. For example, in soils and rocks, (hydr)oxides of Fe(IH) and Mn(IV) are the main scavengers of metal cations and certain anions, even when their concentration expressed as mass fraction is very low. Traces of Ti02 present as impurities are responsible for the enhanced uptake of U by some natural kaolinites. In general, complex materials whose chemical composition seems very similar can substantially differ in their sorption properties due to different nature and concentration of impurities , which are dispersed in a relatively inert matrix, and which play a crucial role in the sorption process. In this respect the significance of parameters characterizing overall sorption properties of complex materials is limited. On the other hand the assessment of the contributions of particular components of a complex material to the overall sorption properties would be very tedious. 

FIGURE 7.14 Schematic representation of the adsorption mechanisms for SDS/C[2EOg mixtures on kaolinite. (From Xu, Q. and Somasundaran, R, Miner. Metal. Processing, 9, 29, 1992. With permission.)... 

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