Diffuse sorption modeling: Apparent H/Na, or the same, Al/Na exchange on clays

Clay minerals are specified by permanent negative surface charge. In solutions of sodium salts, the surface of clay is covered by exchangeable sodium ions. In an acidic field (pH < 4–6), sodium ions are displaced from the surface. This apparent H/Na exchange is conditioned by dissolution of alumina, followed by Al/Na exchange. Two kinds of published experimental data were considered in order to follow Al/Na exchange: the first is direct measurement of exchangeable sodium and aluminum in clay, and the second is exchange sorption of trace metal. Because of the equivalency of ionic exchange, trace metal acts as a probe, indicating the sodium content in clay. These experimental data were successfully modeled with use of the Poisson–Boltzmann equation, with the assumption that all exchange cations are located in the diffuse layer.

Chemical composition of the diffuse layer on a clay surface may be calculated with use of the Poisson–Boltzmann equation, known value of permanent surface charge, and the solubility products of alumina.Figure optionsDownload as PowerPoint slide