Hard and Soft Acid-Base Model Verified for Monovalent Cation Selectivity
- Shihe Xu and
- James B. Harsh
Experimental support was obtained for a quantitative ion-exchange model based on the principle of hard and soft acids and bases (HSAB) applied to monovalent cation exchange on clay minerals, oxides, and exchange resins. According to the model, the thermodynamic exchange constant (Kex) can be obtained from the differences in absolute electronegativity (ϰ) and absolute softness (S) between the exchanging cations in binary exchange as follows: where the subscripts M and R denote exchange and reference cations, respectively, and α and β are coefficients dependent only on the exchanger surface properties, temperature, pressure, and ionic strength. This study was conducted to test the applicability of this model to monovalent cation exchange on clay minerals and ionic-exchange resins. Exchange isotherms for Li, K, Rb, Cs, Ag, and Tl+ vs. Na on vermiculite, smectites, hydrous Zr oxides, silica gel, and ion-exchange resins were obtained experimentally or from the literature and α and β were obtained by fitting the data to the above equation. An excellent fit (R2 ≥ 0.954, n = 5) of the alkali metal cation-exchange data to the model resulted on all of the tested exchangers, whereas Ag and Tl+ exchange was successfully fit only on hard surfaces such as a vermiculite and on highly hydrated surfaces such as certain ion exchange resins. According to the HSAB model, coefficients α and β determine the relative contribution to Kex from ionic or covalent bonding in surface complexation. The |α/β| ratio depended on the layer charge of clay minerals and on the acid strength of the exchange resins and hydrous oxides.
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