EQUILIBRIUM AND KINETICS OF BORATE ADSORPTION-DESORPTION ON PYROPHYLLITE IN AQUEOUS SUSPENSIONS

This study was conducted to elucidate the dynamic aspects of the adsor ption-desorption of borate ions on edge surfaces of 2:1 clay minerals. A pressure-jump relaxation method was used to evaluate the elementary processes involved in the adsorption-desorption of borate ions hy pyr ophyllite 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 tot al adsorbed B, the fraction of the adsorbed B(OH)4- is assumed to be a lmost-equal-to 0.99 at all levels of adsorbed B studied. This high fra ction is probably due to the absence of repulsive forces associated wi th the planar surfaces. A linear correlation (R2 = 0.94) between the r eciprocal value of the relaxation time, tau-1, and the sum of concentr ations of the free adsorption sites and borate ions in solution at equ ilibrium was determined. The forward rate constant, k1, for the adsorp tion 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 o rders of magnitudes smaller than the adsorption rate constant. The int rinsic equilibrium constant obtained from the kinetic measurements (lo g10 K(kinetic) = 3.15) agreed relatively well with that calculated fro m the static studies (log10 K(static) = 3.51). The suggested reaction scheme for the B-pyrophyllite interaction is: S(OH)2 + B(OH)4 half arr ow left over half arrow right SO2-B(OH)2 + 2H2O or S(OH)2 + B(OH)4 hal f arrow left over half arrow right SO2H-B(OH)3 + H2O