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Monovalent Ion Adsorption by an Oxisol¹

ABSTRACT

An experimental method was developed to measure simultaneously the net proton surface charge density (_H) and surface excesses of metal (n_M) and ligand (n_L) on an Oxisol suspended in a ML electrolyte solution. The soil sample was first saturated with a LiClO₄ solution whose pH and ionic strength were equal to the maximum values to be used in the adsorption experiments. The ionic strength then was decreased, LiClO₄ was exchanged for ML (ML = NaCl or KNO₃), and a soil paste was prepared to allow reproducible sampling of small quantities of the whole soil. A discontinuous titration followed in which the soil paste was equilibrated with a series of ML electrolyte solutions of specific pH and ionic strength. Values of n_M and n_L were measured at 25°C, and it was shown that the point of zero net charge (PZNC) 4.2, independent of the electrolyte and its ionic strength in the range 4 to 500 mol m^–3. Given a negligible permanent surface charge, typical of Oxisols, PZNC = PZNPC (point of zero net proton charge). The net proton surface charge density then could be calculated from titration data renormalized to the PZNC. The net proton surface charge of the Ha-plustox used in this study vanished at pH 4.2 and showed very little dependence on ionic strength, indicating the dominant role of kaolinitic and organic matter constituents in determining its surface properties. Surface complexation reactions between the soil and a 1:1 electrolyte solution were characterized quantitatively by intrinsic equilibrium constants calculated according to the triple layer model. The average values found were similar to those that have been obtained previously for specimen hydrous oxides suspended in 1:1 electrolyte solutions.

¹ Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521.

² Graduate Research Assistant and Professor of Soil Science, respectively. Present address of the senior author is Swiss Federal Inst. for Water Resources and Water Pollution Control, CH-8600 Dübendorf, Switzerland.

Received for publication December 8, 1986.