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

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 pressure-jump relaxation method was used to evaluate the elementary processes involved in the adsorption-desorption of borate ions hy 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 almost-equal-to 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, tau-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: S(OH)2 + B(OH)4 half arrow left over half arrow right SO2-B(OH)2 + 2H2O or S(OH)2 + B(OH)4 half arrow left over half arrow right SO2H-B(OH)3 + H2O