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Effect of pH and Ionic Strength on Boron Adsorption by Pyrophyllite

Inst. of Soils and Water, The Volcani Center, ARO, P.O. Box 6, Bet Dagan, Israel

D. L. Sparks

Dep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303

*Corresponding author (vwrmen{at}volcani).

ABSTRACT

The negative electrical field around clay particles whose edge thickness is small relative to the Debye length of the diffuse double layer associated with the planar surfaces may spill over into the edge region. Such a spillover may affect B adsorption by the clay. This study was conducted to determine the effect of edge surface properties of 2:1 clay on B adsorption. Pyrophyllite, which shows little deviation from the ideal formula of dioctahedral structure of 2:1 clay type, was used for that purpose. Boron adsorption by pyrophyllite was studied at 25.0 ± 0.5°C at ionic strengths of 0.005, 0.01, and 0.1 mol L^–1 for pH 7 and 9 and ionic strengths of 0.01 and 0.1 mol L^–1 for pH 5. It was suggested that B is specifically adsorbed via ligand exchange to the structural Al located on the edge surfaces. Boron adsorption increased with increasing ionic strength. The effect of ionic strength on B adsorption was smaller at pH 7 than at pH 9, consistent with the decrease in B(OH)^–₄ activity fraction in equilibrium solution at pH 7 and the low affinity of the clay to B(OH)₃. The effect of ionic strength on B adsorption was small at pH 5, attributable to the weak electrical field at this pH (electrical potential, _o = –47 mV) together with the low B(OH)^–₄ concentration and the low affinity of the clay for B(OH)⁰₃. The higher capacity of pyrophyllite, in comparison to montmorillonite, to adsorb B at pH 9 where the _o at the edge surfaces of the clay is -285 mV, supports the hypothesis that the negative electrical field associated with this planar surfaces affects B adsorption.

Joint contribution from the ARO and the University of Delaware. Supported by a grant from U.S. Borax Corp.

Received for publication June 21, 1993.

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