Adsorption from electrolyte solutions clays

The intrinsic surface charge density reflects particle charge developed from either isomorphic substitutions or adsorption involving H+ or OH-. A widely used technique for measuring intrinsic surface charge density is the Schofield method. In this method [3], clay mineral particles are reacted with an electrolyte solution (e.g., NaCl) at a given pH value and ionic strength the specific surface excess of the cation and the anion adsorbed from the electrolyte is determined and the value of is calculated with the equation... 

If the sole mechanism of ion adsorption is via the diffuse-ion swarm, the anions in an electrolyte solution in which clay mineral particles are suspended will, in general, be excluded from a portion of the suspension volume near the particle surface [23,27]. If q- is the specific adsorbed charge of anions resulting from this exclusion and c is their bulk concentration in a 1 1 electrolyte... 

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. 

See, e.g., D.M.C. MacEwan and M. J. Wilson, Interlayer and intercalation compounds of clay minerals, in G. W. Brindley and G. Brown, op cit. That 1 1 electrolytes cannot be dissolved completely in adsorbed water on smectites and illitic micas has been shown by A. M. Posner and J. P. Quirk, The adsorption of water from concentrated electrolyte solutions by montmorillo-nite and illite, Proc. Royal Soc. (London) 278A 35 (1964). 

Equation 3.108 predicts a higher local concentration of cations near a negatively charged clay surface than in bulk solution, and a lower concentration of anions near the surface than in solution. Figure 3.24 shows this predicted distribution of monovalent cations and anions near the clay surface for two different concentrations of electrolyte in solution. More modem statistical mechanical models of this clay interfacial region have predicted that ion-ion correlation (electrostatic) effects should cause deviations from this classical picture, such as the positive adsorption of anions at intermediate distances from the surface when the cation is divalent or multivalent. 

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