Montmorillonite surface complex

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. 

Sorption depends on Sorption Sites. The sorption of alkaline and earth-alkaline cations on expandable three layer clays - smectites (montmorillonites) - can usually be interpreted as stoichiometric exchange of interlayer ions. Heavy metals however are sorbed by surface complex formation to the OH-functional groups of the outer surface (the so-called broken bonds). The non-swellable three-layer silicates, micas such as illite, can usually not exchange their interlayer ions but the outside of these minerals and the weathered crystal edges ("frayed edges") participate in ion exchange reactions. 

The H+ and NH forms of homoionic montmorillonite promote the hydrolysis of chloro-s-triazines to the hydroxy analogs (hydroxy-s-triazines) (73). Apparently, the surface acidity of these clays was extremely high, since no degradation was observed in control experiments conducted at pH 3.5 in homogeneous aqueous solution. Russell et al. (73) suggested that the hydroxy-s-triazine products were stabilized in the protonated form at the silicate surface. The IR spectra of these surface complexes agreed with the spectra obtained in 6N HC1, and it was inferred that the pH at the clay surface was 3 to 4 units lower than that measured in suspension. 

Simulations of three representative Cs-smectites revealed interlayer Cs+ to be strongly bound as inner sphere surface complexes, in agreement with published bulk diffusion coefficients [78]. Spectroscopic and surface chemistry methods have provided data suggesting that in stable 12.4 A Cs-smectite hydrates the interlayer water content is less than one-half monolayer. However, Smith [81] showed using molecular simulations of dry and hydrated Cs-montmorillonite that a 12.4 A simulation layer spacing was predicted at about one full water monolayer. The results of MD computer simulations of Na-, Cs-and Sr-substituted montmorillonites also provide evidence for a constant water content swelling transition between one-layer and two-layer spacings [82]. 

Upon reaction with an adsorptive in aqueous solution (which then becomes an adsorbate), surface functional groups can engage in adsorption complexes, which are immobilized molecular entities comprising the adsorbate and the surface functional group to which it is bound closely [18]. A further classification of adsorption complexes can be made into inner-sphere and outer-sphere surface complexes [19]. An inner-sphere surface complex has no water molecule interposed between the surface functional group and the small ion or molecule it binds, whereas an outer-sphere surface complex has at least one such interposed water molecule. Outer-sphere surface complexes always contain solvated adsorbate ions or molecules. Ions adsorbed in surface complexes are to be distinguished from those adsorbed in the diffuse layer [18] because the former species remain immobilized on a clay mineral surface over time scales that are long when compared, e.g., with the 4-10 ps required for a diffusive step by a solvated free ion in aqueous solution [20]. Outer-sphere surface complexes formed in the interlayers of montmorillonite by Ca2+ or Mg2+ are immobile on the molecular time scale... 

These speciation concepts are illustrated in Fig. 3 for the idealized basal-plane surface of a smectite, such as montmorillonite. Also shown are the characteristic residence-time scales for a water molecule diffusing in the bulk liquid (L) for an ion in the diffuse swarm (DI) for an outer-sphere surface complex (OSQ and for an inner-sphere surface complex (ISC). These time scales, ranging from picosecond to nanosecond [20,21], can be compared with the molecular time scales that are probed by conventional optical, magnetic resonance, and neutron scattering spectroscopies (Fig. 3). For example, all three surface species remain immobile while being probed by optical spectroscopy, whereas only the surface complexes may remain immobile while being probed by electron spin resonance (ESR) spectroscopy [21-23]. 

The ESR spectra of monolayer hydrates of Cu-doped Ca-beidellite and Ca-montmorillonite, with the samples comprising layers oriented perpendicularly to the direction of the applied magnetic field, is essentially of the four-peak type, whereas that when the sample layers are oriented parallel to the magnetic field is of the one-peak type. These results indicate that the symmetry axis of the Cu2+ complex is perpendicular to the smectite siloxane surface [39,41]. The ESEM modulation pattern [41] shows further that the number of nearest-neighbor water molecules is 4, with a Cu2+-D+ distance of 0.29 nm, which is consistent with the 0.198-nm Cu—O distance observed for solvated Cu2+ in aqueous solution [20]. These results and the ESR data are consistent with the inner-sphere surface complex illustrated on the left in Fig. 8. The bivalent cation is coordinated to siloxane surface oxygen ions along the symmetry axis of the complex and to four... 

Simulation of the ESEM pattern for Cu2+-doped Mg-montmorillonite leads to a coordination number of six and a Cu2+-D distance of 0.29 nm [41]. X-ray diffraction shows that the smectite layers are about 1.04 nm apart when the ESR lineshape becomes isotropic with a single peak. This large basal-plane spacing and the ESEM data suggest a diffuse-layer Cu(H20)g+ species that tumbles sluggishly. Copper-doped Mg-hectorite whose layers are about 0.54 nm apart yields an ESR spectrum like those for beidellite and montmorillonite at low relative humidity, whereas with the layers 1.04 nm apart, the spectrum is again isotropic [39]. Figure 8 illustrates the three Cu2+ surface complexes that appear successively as a smectite... 

FIGURE 2.3 Potentiometric titration curve of copper-montmorillonite in 0.1 mol dm-3 NaC104 solution, m = 50 mg, V = 20 cm3 (upper left). Vs are the experimental points, line is the plotted curve by the surface complexation model. The concentration of surface sites—lower left interlayer cations upper right silanol sites lower right aluminol sites (Nagy and Konya 2004). 

The Concentration of Edge Sites and Intrinsic Stability Constants of Protonation and Deprotonation of Silanol and Aluminol Sites of Montmorillonite Samples Calculated by the Surface Complexation Model... 

The role of sodium ions is predicted to be small according to Schulze-Hardy s rule that is, the effectiveness of counterions in the interfacial phenomena is approximately proportional to the sixth power of the counterion charge. The formation of complexes between Na+-ions and anionic sites on the montmorillonite surface is included here for the sake of completeness, although such complexes are subject to reservations because they are unknown in aqueous solutions, and evidence against their physical reality has been presented (Janssen and Stein 1986). Indeed, as will be seen later, they are not prevalent. 

The publications on amino acid adsorption on clay minerals before 1974 are summarized in the book by Theng (1974). It provides information on the adsorbed quantity of different amino acids on cation-exchange montmorillonites and the characteristic IR bands of amino acid-montmorillonites adsorption compounds. Usually, only the adsorbed quantity of amino acids on montmorillonites is shown, and no adsorption mechanism is usually hypothetized (e.g., Friebele et al. 1981 Rak and Tarasevich 1982), except in Stadler and Schindler (1993a) where the adsorption is evaluated by the surface-complexation model, and the possible surface complexes are given for p-alanine. 

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. 

For the interpretation of the results using the surface-complexation model, reactions 2.47-2.53 have to be taken into account. In addition, the surface acid-base properties and the neutralization reactions of the layer charge have to be included as in Section 2.4.2 the parameters determined there are treated as fixed, input data. In the case of copper- and zinc-montmorillonite, the copper and zinc concentration of the solution and solid also have to be determined, and these data have to be taken into consideration. That is, the quantity of the total sorbed valine and the copper or zinc ion concentrations versus pH function can be fitted, and KH2Valx, KAioH2Vai> and KSi0CuVal stability constants can be computed. The results of the parameter fit for copper- and zinc-montmorillonites as well as the obtained stability constants are shown in Figures 2.17 and 2.18, and in Table 2.12, respectively. 

Metals constitute an important source of pollution in soils. As discussed in Chapters 5 and 6, they can bind to soils through humic substances, surface complex-ation, or ion exchange. In some cases more than one type of interaction can occur, as in clay minerals (e.g., montmorillonite and vermiculite) that bind metals through ion exchange as well as surface com-... 

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. 

Collins et al. (1999a) found that Hg2+ sorbed to goethite as an iimer-sphere bidentate complex. Cheah et al. (1998) found that Cu " " sorbed to amorphous silica and Y-AI2O3 as monomeric and monodentate iimer-sphere surface complexes. However, bidentate complexes may also form on Y-AI2O3. Using polarized EXAFS, Dahn et al. (2003) determined that Ni " " sorbed to montmorillonite edge sites as an inner-sphere mononuclear surface complex. Inner-sphere surface complexes were observed with XAS for Cr " " adsorption on manganese (Manceau and Charlet, 1992) and iron oxides (Charlet and Manceau, 1992). 

Strontium adsorption onto soil minerals is an important retardation mechanism for Sr " ". Chen et al. (1998) investigated the adsorption of Sr " " onto kaolinite, illite, hectorite, and montmorillonite over a range of ionic strengths and from two different electrolyte solutions, NaNO3 and CaCb- In all cases, the EXAFS spectra suggested Sr adsorbed to clay minerals as an outer-sphere mononuclear complex. Sahai et al. (2000) also found that on amorphous silica, goethite, and kaolinite substrates, Sr"+ adsorbed as a hydrated surface complex above pH 8.6. On the other hand, Collins et al. (1998) concluded from EXAFS spectra that Sr " " adsorbed as an inner-sphere complex on goethite. 

Bostick et al. (2002) studied Cs+ adsorption onto vermiculite and montmorillonite with EXAFS and found that Cs+ formed both inner-and outer-sphere complexes on both aluminosihcates. The inner-sphere complexes bound to the siloxane groups in the clay structure. Combes et al. (1992) found that NpOj adsorbed onto goethite as a mononuclear surface complex. Waite et al. (1994) were successful in describing uranyl adsorption to ferrihydrite with the diffuse layer model using the inner-sphere, mononuclear, bidentate surface complex observed with EXAFS. 

All surface complexes between counterions and the clay mineral surfaces were inner sphere due to our use of a monolayer of water. When present, water molecules tended to position themselves in such a way as to be in equatorial association with the counterions, i.e., they did not interpose themselves between a counterion and a clay surface. The equilibrium do0, spacings between hydrated montmorillonite layers in these simulations were reasonable [see, e.g., Brindley (1980) for experimental values], varying from 1.209 0.005 nm (K) to 1.214 0.004 nm (Rb) to 1.223 0.005 nm (Cs). 

Other large organic molecules may favourably interact with natural zeolite or clay surfaces. Of interest is the ability of these materials, e.g., clinoptilolite- or montmorillonite-rich rocks, to adsorb on their hydrophilic, negatively charged surfaces complex substances, such as aflatoxins, which arc toxic secondary metabolites of several agricultural products, containing polar functional groups [70,71J. Adsorption, which has been proven either in-vitro or in-vivo, is effective and amounts to some hundred pg per g of adsorber. 

Morton JD, Semrau JD, Hayes KF (2001) An X-ray absorption spectroscopy study of the structure and reversibility of copper adsorbed to montmorillonite clay. Geochim Cosmocliim Acta 65 2709-2722 Muller B, Sigg L (1992) Adsorption of lead(II) on the goethite surface Voltammetric evaluation of surface complexation parameters. J Coll Interf Sci 148 517-532 Neder RB, Burghammer M, Grasl T, Schulz H, Bram A, Fiedler S (1999) Refinement of the kaolinite structure from single-crystal synchrotron data. Clays Clay Miner 47 487-494 Needleman HL (1983) Low level lead exposure and neuropsychological performance In Lead versus health - Sources and effects of low level lead exposure. Rutter M, Russell JR (eds) Wiley, Chichester, p 229-248... 

There appears to be little point in a detailed evaluation of the more ambiguous hypotheses of heavy metal fixation. However, it may be noted that a number of investigators have considered the possibility of either the sorption of complex ions on clay mineral surfaces or reaction of heavy metal cations with clay surfaces in some manner other than simple electrostatic sorption (31, 66, 67, 156, 157, 217), However, the solubility products of Cu(OH)2 and Zn(OH)2 in aqueous suspensions of montmorillonite (19) have been found to be quite similar to those previously found for solutions in contact with only the pure hydroxides. This would indicate that metal ion-clay mineral surface complex formation is not important otherwise the apparent solubility would have been greater in the presence of montmorillonite. 

This expression can reproduce the values of 5 for K- and Cs-mont-morillonite in Table 1.8 if/is given the values 0.80 and 1.2, respectively. The impossibly high value of/for Cs-montmorillonite emphasizes that part of the difference between 5 e and for Cs-montmorillonite and the other clay samples must come from surface complexation of the exchangeable cations. 

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