Montmorillonite metal adsorption

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... 

Boufatit, M, Ait-Amar, H. McWhinnie, W. R. (2007). Development of an Algerian material montmorillonite clay. Adsorption of phenol, 2-dichlorophenol and 2,4,6-trichlorophenol from aqueous solutions onto montmorillonite exchanged with transition metal complexes. Desalination, vol. 206, pp. 394-406. ISSN 0011-9164. Burns, S. E., Bartelt-Hunt, S. L. Smith, J. (2003). Sorption and permeability of gasoline hydrocarbons in organobentonite porous media. Journal of Hazardous Materials, vol. 96, pp. 91-97. ISSN 0304-3894. 

An extension of the relative simple formulation used in SCMs for surfaces with permanent eharges (see Section II.B.l) has been published recently [84]. A fictitious surface species (X ) was defined and hypothetical complexation reactions on site X were written, and thus cation-exchange reactions of permanent negative layer charges were easily incorporated into such model. The model showed not only to fit satisfactorily all of the experimental data of transition metal adsorption on montmorillonite but also to explain specific features of adsorption on clays compared to oxides. 

The clay mineral bentonite (sodium montmorillonite) has an excellent ion exchange and adsorption capacity. Films can be applied to electrode surfaces from colloidal clay solutions by simple dip or spin coating that become electroactive after incorporation of electroactive cations or metal particles 136-143)... 

As noted above, adsorption isotherms are largely derived empirically and give no information on the types of adsorption that may be involved. Scrivner and colleagues39 have developed an adsorption model for montmorillonite clay that can predict the exchange of binary and ternary ions in solution (two and three ions in the chemical system). This model would be more relevant for modeling the behavior of heavy metals that actively participate in ion-exchange reactions than for organics, in which physical adsorption is more important. 

The presence of hydroxyaluminum- and hydroxyaluminosilicate polymer in interlayered montmorillonite greatly promotes the adsorption of Cd, Zn, and Pb (Saha et al., 2001). The adsorption selectivity sequences of montmorillonite (Pb > Zn > Cd) and interlayered montmorillonite (Pb Zn Cd) resemble the metal selectivity on amorphous Fe and Al hydroxides (Saha et al., 2001). On montmorillonite, the metals are predominantly adsorbed on the permanent charge sites in an easily replaceable state. However, a substantial involvement of the edge OH" groups of montmorillonite in specific adsorption of the metals is also observed, especially at higher pH (Saha et al., 2001). 

A theoretical model for the adsorption of metals on to clay particles (<0.5 pm) of sodium montmorillonite, has been proposed, and experimental data on the adsorption of nickel and zinc have been discussed in terms of fitting the model and comparison with the Gouy-Chapman theory [10]. In clays, two processes occur. The first is a pH-independent process involving cation exchange in the interlayers and electrostatic interactions. The second is a pH-dependent process involving the formation of surface complexes. The data generally fitted the clay model and were seen as an extension to the Gouy-Chapman model from the surface reactivity to the interior of the hydrated clay particle. 

As the figure shows the exchange of Sr2+ on Na+-montmorillonite fits the ion exchange theory very well. But the adsorption of heavy metals cannot be accounted for by this theory. Co(II) behaves as if it were monovalent Kd for americium is independent of [Na+] (americium occurs at pH = 6.5 as a hydroxo complex). 

Exchange of complex cations. Complexation of transition metal cations with uncharged ligands such as with amines and with amino acids results in a selectivity enhancement compared to the selectivity of the aqueous metal cation (27, 65-72). Fig. 3 shows an example for the Cu(ethylenediamine) adsorption in montmorillonites of different charge density. Standard thermodynamic data for other cases are given in table IV. In all cases the free ligand concentration in equilibrium solution was... 

Extremely high selectivities are frequently interpreted as "ion fixation", which suggests an irreversible phenomenon. This is the case for exchanges of Cs, Rb and K in illite clay minerals (95-96) as well as for Cu(NHj) exchange in fluorhectorite (66). However, reversibility was verified from the Hess law for adsorption of Cs, Rb and K on the high affinity sites in illite (91) and modified montmorillonites (101) as well as for the exchange of transition metal complexes (29, 75). 

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. 

Soma et al. (12) have generalized the trends for aromatic compound polymerization as follows (1) aromatic compounds with ionization potentials lower than approximately 9.7 eV formg radical cations upon adsorption in the interlayer of transition-metal ion-exchanged montmorillonites, (2) parasubstituted benzenes and biphenyls are sorbed as the radical cations and prevented from coupling reactions due to blockage of the para position, (3) monosubstituted benzenes react to 4,4 -substituted biphenyls which are stably sorbed, (4) benzene, biphenyl, and p-terphenyl polymerized, and (5) biphenyl methane, naphthalene, and anthracene are nonreactive due to hindered access to reaction sites. However, they observed a number of exceptions that did not fit this scheme and these were not explained. 

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. 

Here, the adsorption of valine on different cation-exchanged montmorillonites is described (Nagy and Konya 2004). A discussion of the kinds of interactions that are possible in the ternary system of montmorillonite/valine/metal ions will be presented, and a description how the metal ions can affect these interactions. The interlayer cations (calcium, zinc, copper ions) were chosen on the basis of the stability constants of their complexes with valine. The adsorption of valine on montmorillonite is interpreted using a surface-complexation model. 

Abollino, O., M. Aceto, M. Malandrino, C. Sarzanini, and E. Mentasti. 2003. Adsorption of heavy metals on Na-montmorillonite Effect of pH and organic substances. Water Res. 37 1619-1627. 

Natural bleaching clays are aluminum silicates (bentonite, atta-pulgite, and montmorillonite), containing relatively high amounts of Mg, Ca, or Fe. The clays are generally activated by heat treatment. The high metal content, however, limits the adsorptive activity of these... 

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. 

Sakurai, K., and Huang, P. M. (1995). Cadmium adsorption on the hydroxyalu-minium-montmorillonite complex as influenced by oxalate. In Environmental Impact of Soil Componenet Interactions, Vol. Il, Metals, Other Inorganics and Microbial Activities, ed. Huang, P. M., Berthelin. J., Bollag, J.-M., McGill, W. B., and Page, A. L., Lewis Publishers, Boca Raton. FL, 39-46. 

Another standardized database for the diffuse layer model was developed for montmorillonite by Bradbury and Baeyens (2005). Surface complexation constants for strong and weak sites and cation exchange were fit to adsorption data for various metals using constant site densities and protonation-dissociation constants in a nonelectrostatic modeling approach. Linear free energy relationships were developed to predict surface complexation constants for additional metals from their aqueous hydrolysis constants. 

The above sequence has been observed in studies of alkaline earth adsorption on y-Al203 (Huang Stumm, 1973). The trend is also consistent with the expectation based on the expected preference of harder Lewis acids for hard Lewis bases like surface hydroxyls. Limited spectroscopic evidence is available for sorption of alkaline earth metals because many of these metals do not exhibit sufficiently high K-shell fluorescence energies to be studied in the presence of corundum and water using current EXAFS methods. Chen and Hayes (1999) have shown that Sr(II) sorbs to montmorillonite, illite, and hectorite primarily as a weakly associated outer-sphere complex. Similar findings have been reported for sorption of Sr(II) to clay minerals (Parkman et al., 1998 O Day et al., 2000 Sahai et ah, 2000). 

Sometimes, however, a comparison between the adsorption branch and the desorption branch may lead to a conclusion about the shape of the capillaries. An adsorption branch which has no inflexion point and gives a sharp rise only for relative pressures close to unity, combined with a desorption branch showing a definite inflexion point at medium values of relative pressures, indicates fissure-shaped capillaries (68). Hysteresis curves of this form are, for example, found with a lomerations which consist of disk- or plate-shaped particles, such as montmorillonites, and indeed hysteresis curves published by Barrer and MacLeod (59) show this behavior. Similar curves are found with the dehydration products of many well-crystallized metal oxide hydrates, such as those of the aluminum hydrates (gibbsite, bayerite, boehmite, and diaspora) (60). 

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