Effect of pH and Ionic Strength on Boron Adsorption by Pyrophyllite

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~' for pH 7 and 9 and ionic strengths of 0.01 and 0.1 mol L" 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, \|»0 = -47 mV) together with the low B(OH)^ concentration and the low affinity of the clay for B(OH)§. The higher capacity of pyrophyllite, in comparison to montmorillonite, to adsorb B at pH 9 where the \|/0 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. B CAN BE ADSORBED by different clay and Al and Fe oxide minerals that vary in their adsorption capacities (Goldberg and Glaubig, 1985; Keren and Bingham, 1985; Goldberg and Glaubig, 1988). Illite adsorbs more B than montmorillonite (Hingston, 1964; Keren and Mezuman, 1981) although montmorillonite has greater surface area. Kaolinite has a low B adsorption capacity on a mass basis, but on a total surface area basis kaolinite is one of the most reactive clays. The effect of pH and clay particle size on B adsorption by clay has been intensively studied (Keren and Talpaz, 1984; Keren and Bingham, 1985). It was suggested that boron is adsorbed on the particle edge surfaces of clay minerals (e.g., montmorillonite or illite) rather than on planar surfaces. The effect of exchangeable ions on B adsorption by clay minerals was dependent on the pH of the clay suspension. Exchangeable cation species (Na vs. Ca) had little effect on B adsorption by montmorillonite at pH =7; there was a significant exchangeable cation effect at pH >8, with the Ca systems adsorbing more B than Na (Keren and Gast, 1981). The effect of adsorbed ion on B adsorption by montmorillonite at pH >7 was explained by the diffuse double layer theory. Secor and Radke (1985), using the Poisson-Boltzmann equation, also showed that, for low indifferent electrolyte R. Keren, Inst. of Soils and Water, The Volcani Center, ARO, P.O. Box 6, Bet Dagan, Israel; and D.L. Sparks, Dep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303. Joint contribution from the ARO and the University of Delaware. Supported by a grant from U.S. Borax Corp. Received 21 June 1993. "Corresponding author (vwrmen @volcani). Published in Soil Sci. Soc. Am. J. 58:1095-1100 (1994). concentrations, the negative electrostatic field emanating from the particle face of montmorillonite spills over into the edge surface region. The negative field may prevent borate from approaching the adsorption sites at the particle edges. This concept is also supported by the observation that by increasing the NaCl concentration from 0.02 to 0.07 or 0.36 mol L", the amount of B adsorbed by Na-montmorillonite is increased by ~ 10 and 50%, respectively (Keren and O'Connor, 1982). The clay mineral pyrophyllite consists of essentially neutral tetrahedral-octahedral-tetrahedral layers. The lack of an electrical field associated with the planar surfaces may allow the evaluation of the role of edge surface reactivity in B adsorption reaction. If the diffuse double layer associated with the planar surfaces influences B adsorption by clays (especially in the pH range 8-10), B adsorption by pyrophyllite should be greater than mat found in montmorillonite. This hypothesis is tested more fully in our study. The objective of our study was to determine the effect of edge surface properties on B adsorption by pyrophyllite. MATERIALS AND METHODS