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Equilibrium and Kinetics of Borate Adsorption-Desorption on Pyrophyllite in Aqueous Suspensions

Inst. of Soils and Water, Agricultural Research Organization (ARO), Volcani Center, P.O. Box 6, Bet Dagan, Israel

P. R. Grossl and D. L. Sparks

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

*Corresponding author (vwrmen{at}volcani).

ABSTRACT

This study was conducted to elucidate the dynamic aspects of the adsorption-desorption of borate ions on edge surfaces of 2:1 clay minerals. A pressurve-jump relaxation method was used to evaluate the elementary processes involved in the adsorption-desorption of borate ions by pyrophyllite in aqueous media at pH 9 and ionic strength of 0.01 (NaNO₃). This clay was selected because of the small deviation from the ideal structural formula of the dioctahedral 2:1 clay minerals. At pH 9, 37% of the total B in solution is in the B(OH)^–₄ form, whereas of the total adsorbed B, the fraction of the adsorbed B(OH)^–₄ is assumed to be 0.99 at all levels of adsorbed B studied. This high fraction is probably due to the absence of repulsive forces associated with the planar surfaces. A linear correlation (R² = 0.94) between the reciprocal value of the relaxation time, ^–1, and the sum of concentrations of the free adsorption sites and borate ions in solution at equilibrium was determined. The forward rate constant, k₁, for the adsorption was 10^4.26 L mol^–1 and the backward rate constant, k_-1, for the desorption was 10^1.11 s^–1. The desorption rate constant was three orders of magnitudes smaller than the adsorption rate constant. The intrinsic equilibrium constant obtained from the kinetic measurements (log₁₀ K_kinetic = 3.15) agreed relatively well with that calculated from the static studies (log₁₀ K_static = 3.51). The suggested reaction scheme for the B-pyrophyllite interaction is:
or

Joint contribution from the ARO, Volcani Center, and the University of Delaware.

Received for publication July 26, 1993.

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