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 by pyrophyllite in aqueous media at pH 9 and ionic strength of 0.01 (NaNOj). 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 (R = 0.94) between the reciprocal value of the relaxation time, T ~ ' , and the sum of concentrations of the free adsorption sites and borate ions in solution at equilibrium was determined. The forward rate constant, k,, for the adsorption was 10L inol"' and the backward rate constant, k-i, for the desorption was 10' " s". The desorption rate constant was three orders of magnitudes smaller than the adsorption rate constant. The intrinsic equilibrium constant obtained from the kinetic measurements (logic Kktotk = 3.15) agreed relatively well with that calculated from the static studies (logic KaMc = 3.51). The suggested reaction scheme for the B-pyrophyllite interaction is: S(OH)2 + B(OH)4 ** SO2-B(OH)2 + 2H2O or S(OH)2 + B(OH)4 ** SO2H-B(OH)3 + H2O I ASSESSING B CONCENTRATION in the soil solution, the physico-chemical characteristics of the soil must be taken into consideration because of the interaction between B and soil constituents. This is particularly important since there is a relatively small range between levels of soil solution B that cause deficiency and toxicity symptoms in plants. Existing criteria, however, make no reference to the properties of the clay minerals; therefore, the interaction of B with surfaces of clay particles dispersed in aqueous solution is a subject of continuing investigation in the agricultural and environmental sciences. R. Keren, 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. Joint contribution from the ARO, Volcani Center, and the University of Delaware. Received 26 July 1993. *Corresponding author (vwrmen@volcani). Published in Soil Sci. Soc. Am. J. 58:1116-1122 (1994). Boron can be effectively leached from soil although the rate of removal is much slower for B than for Cl or SO4 salts (Bingham et al., 1972). Different clay and Al and Fe oxide minerals are very different in their adsorption capacities for B (Goldberg and Glaubig, 1985, 1988; Keren and Bingham, 1985). Adsorption of B by soil constituents has been considered as a possible mechanism controlling B removal from the soil solution. Couch and Grim (1968) concluded that the surface area of the clay is one of the important factors controlling B fixation by illite and proposed a two-step mechanism for B retention. The first step was a rapid chemical adsorption of tetrahedral B(OH),j~ on the clay and the second step was a much slower diffusion of B into the tetrahedral sheet of the crystal. An area in B-clay interactions that has not received enough attention is the kinetics and mechanisms of B adsorption and desorption. Griffin and Burau (1974) studied the kinetics of B desorption from soils and observed two pseudo-first-order reactions and a very slow reaction. They speculated that the fast reactions were due to desorption from hydroxy-Fe, -Mg, and -Al materials in the clay size fraction, whereas the slowest reaction rate was due to diffusion of B from the interior of clay minerals to the solution phase. Boron is adsorbed mainly on the edge surfaces of the 2:1 clay minerals (Keren and Bingham, 1985; Keren and Sparks, 1994; Keren and Talpaz, 1984). Since the negative electrical field (e.g., emanating from the particle face of montmorillonite) extends to the edge surface region (Secor and Radke, 1985), it influences edges, making them less accessible to approaching borate anions. To avoid the complexity associated with the negative electrical field of the planar surfaces, pyrophyllite was selected for this study. Pyrophyllite, in contrast to montmorillonite and illite, shows little deviation from the ideal formula for 2:1 clay minerals. The dioctahedral structure of pyrophyllite consists of essentially neutral tetrahedral-octahedral-tetrahedral layers. The lack of layer charge provides a means of assessing the magnitude and the reactivity of the edge charge of phyllosilicates in B adsorption and desorption reactions. To obtain information about the elementary processes involved in the adsorption-desorption of borate by phyllosilicates, a pressure-jump relaxation study was conKEREN ET AL.: BORATE ADSORPTION-DESORPTION ON PYROPHYLLITE 1117 ducted with pyrophyllite suspensions in aqueous media. The application of relaxation methods to the investigation of rapid adsorption-desorption processes in aqueous suspensions has proven quite successful. For example, the kinetics and mechanisms of the molybdate, sulfate, selenate, and selenite adsorption-desorption on goethite was investigated by Zhang and Sparks (1989, 1990,a,b). The objectives of this study were to elucidate the dynamic aspects of the adsorption-desorption of borate on pyrophyllite surfaces and to test the suitability of the competitive adsorption model (Keren et al., 1981) to describe borate adsorption by pyrophyllite. MATERIALS AND METHODS