Clay Mineral Surface Charge

Interaction 3 shows metal-humic substances interacting through water bridging. It is a weak interaction involving metals with high hydration energy (Bohn et al., 1985). 

There are two types of charges at the surface of mineral particles (1) permanent charge and (2) variable charge. Permanent charge is due to isomorphous substitutions, whereas variable charge is caused by dissociation of mineral-edge hydroxyls. 

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Mineral segregation in industry relies heavily on the selective adsorption of macromolecules onto the surfaces of those minerals that have particular industrial applications. This selectivity is governed mainly by the surface chemistry of the mineral and the type of polymer used as a flocculant. " Effectiveness of flocculation depends upon the charge, concentration and molecular weight of the polymer, and also the pH and salt concentration of the clay suspension. The bonding between the anionic flocculant polyacrylamide (PAM) and clay mineral surfaces has been effectively reviewed recently by Hocking et al and the reader is referred to this should they require an in-depth literature review. For more information on general colloidal chemistry of clay suspensions the reader is referred to the review of Luckham and Rossi." ... 

In natural soils which commonly contain illite and smectite, there can be a significant charge imbalance between Si4+ and Al3+ in the structures of these clay minerals. This results in a net negative charge on the clay mineral surfaces, resulting in more adsorption of mobile cations. When an acid front encounters these adsorbed mobile cations, they are very easily displaced by the H ion, which by virtue of its small size is strongly adsorbed to the clay surface. As a result, the measured CEC of such clay-bearing soils is predicted to increase, as we have observed in our experiments. 

MC and MD studies of hydrated smectites with monovalent counterions Li+, Na+, K+, Cs+ were also performed [62, 63, 69, 70, 72, 77-80], An increase of the simulation cell size of 2 1 Na-saturated clay or alternation of its shape from rectangular did not have a significant effect on the calculated interlayer properties [70]. It has been revealed that the mechanism of swelling and hydration depends upon the interlayer ion charge. Also the greater role of the clay mineral surface in organizing interlayer water in the case of K-montmorillonite with a weakly solvating counterion was concluded [64, 68]. 

Name the various functional groups of (a) clay mineral surfaces and (b) soil organic matter. Explain which of these functional groups exhibits constant charge or variable charge behavior and discuss the practical significance of this behavior. 

The methods described above for the study of TCE are representative of the methods we are currently using to study the adsorption of a variety of organic molecules by clay mineral surfaces. We shall briefly summarize the results obtained so far in two other cases, one dealing with the adsorption of methylene blue to clays and the other dealing with a recalibration of the alkyl ammonium ion method for surface charge determination. In both of these studies, essentially the same computational techniques as those described above were used. 

Another important result of the unsatisfied bonds on the clay mineral surfaces is that they adsorb ions to balance the particle s charge. The unsatisfied charges are mostly negative, so mostly cations are adsorbed. Because they are held on the surface and not within the crystal, such ions can be exchanged fed other ions. This slows the loss of ions from soils and retains the ions in states that are available for plant uptake, but ultimately the ions are lost. 

The cations are generally weakly held to the clay mineral surfaces, and can be readily displaced by other cations present in the solution. The cation (base) exchange depends on the ionic strength (charge to ionic radius ratio) and the concentration of fhe solufion. The cation exchange (or adsorption) capacity of a clay mineral reflecfs ifs charge deficiency per unif mass and is a function of the particle composition and of ifs specific surface (Table 7.2). 

These features of the M -0 RDF, which persist as well in the two- and three-layer hydrates (16-18), suggest that the near-neighbor coordination structure of water molecules about interlayer L, Na and is indeed very similar to that found in concentrated aqueous solutions, irrespective of the type of surface complex formed by the counterion. [Both types of surface complex are observed for monovalent counterions on montmorillonite, because the clay mineral has both tetrahedral and octahedral charge sites (4, 16-20).] What effect, then, does the clay mineral surface have The answer to this question comes from an examination of cation mobility, as revealed by MD simulation (17, 18). 

Table I shows self-diffusion coefficients (T = 300 K) for the three interlayer cations in low-order hydrates of montmorillonite, as calculated conventionally from the slopes of graphs of the (three-dimensional) mean-square cation displacement versus time (8, 16-18). Experimental values of the cation self-diffusion coefficients in aqueous solution also are listed (32). It is apparent that monovalent cation mobility in the one-layer hydrate is at best a few percent of that in bulk aqueous solution, and that the mobility increases significantly with increasing water content, to approach about 25% of the bulk-solution value in the three-layer hydrate. The constrained geometry and the charge sites on the clay mineral surface thus act to retard significantly the diffusive motions of interlayer cations through adsorbed water.

The ratio of protonated base to unprotonated base in the interlayer clay spaces is much higher than the ratio in the homogeneous solution (outside interlayer clay space), mainly because of the increased acidity of water in the interlayer space. Protonation is also enhanced by the ability of the negatively charged clay mineral surface to lower the chemical potential of the protonated form of the base relative to the neutral form, and therefore drive the equilibrium toward protonation, as shown in Figure 3.5. 

We have applied the MD/HA method to bentonite, and have uncovered the mechanisms involved in the extremely low permeability, delayed diffusion and secondary consolidation (Ichikawa et al. 2002, 2004) water molecules close to the clay mineral surface are strongly constrained due to the charged state of the mineral, and the clay minerals themselves form a nano-order of stacked structure. 

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