Quinoline Sorption on Na-Montmorillonite: Contributions of the Protonated and Neutral Species

Authors

Calvin C. Ainsworth, Pacific Northwest National Laboratory
John M. Zachara, Pacific Northwest National LaboratoryFollow
Ron L. Schmidt, Pacific Northwest National Laboratory

Date of this Version

1987

Citation

Clays and Clay Minerals, Vol. 35, No. 2, 121-128, 1987

Abstract

Dilute aqueous solutions of quinoline were contacted with Na-montmorillonite to elucidate the sorption process of the neutral and protonated species. Sorption occurs via a combination of ion exchange and molecular adsorption and yields S-type isotherms. Exchange between the quinolinium ion (QH⁺ and Na can be described by means of Vanselow selectivity coefficients and a thermodynamic exchange constant (K_ex). Due to the apparent adsorption of the neutral species at high mole fractions (x) of the solid phase, the thermodynamic standard state was defined as 0.5 mole fraction. The selectivity at pH ~4.95 of the QH⁺ species over Na (at X_QH+ 0.5) was determined to be K_v = 340. At pH ≥ 5.5 surface mole fractions of 0.5 could not he obtained without adsorption of the neutral species. This study suggests that at dilute solution concentrations quinoline is sorbed preferentially as the cation even at pHs >> pK_a. A critical surface-solution concentration is apparently necessary for adsorption of the neutral species.