Boron Adsorption Mechanisms on Oxides, Clay Minerals, and Soils Inferred from Ionic Strength Effects

Prediction of anion adsorption behavior is enhanced by understanding the adsorption mechanism. This study was conducted to evaluate ionic strength effects on B adsorption and to infer B adsorption mechanisms on various surfaces. Boron adsorption on the Fe oxide goethite, the Al oxide gibbsite, the clay minerals kaolinite and montmorillonite, and two arid-zone soils was investigated as a function of solution pH (3-11) and ionic strength of the background electrolyte (0.01-1.0 M NaCl). Boron adsorption on the oxides and kaolinite increased from pH 3 to 6, exhibited a peak at pH 6 to 8.5, and decreased from pH 8.5 to 11. For B adsorption on montmorillonite and the soils, the adsorption maximum was located near pH 9. Ionic strength dependence, measured as the increase of the B adsorption maximum in 1.0 M NaCl solutions compared with 0.01 M NaCl solutions increased in the order: goethite (3%) < kaolinite (15%) < gibbsite (-30%) < montmorillonite (109%) ~ montmorillonitic soil (116%) = kaolinitic soil (129%). Shifts in zero point of charge were observed on goethite, gibbsite, and kaolinite following B adsorption. Ionic strength effect results suggest an inner-sphere adsorption mechanism for goethite, gibbsite, and kaolinite and an outer-sphere adsorption mechanism for montmorillonite and the soils. These mechanisms are also indicated by zero point of charge determinations, microelectrophoresis measurements, or both. The constant capacitance model, containing an inner-sphere adsorption mechanism, was able to describe B adsorption on goethite, gibbsite, kaolinite, and kaolinitic soil. The model was unable to describe B adsorption on montmorillonite and montmorillonitic soil because the computer optimizations diverged. B is an important element in crop production. Boron deficiency is of concern in areas receiving plentiful rainfall while B toxicity may be a problem in arid areas (Keren and Bingham, 1985). Because the concentration range between plant deficiency and toxicity is narrow and plants respond only to the B activity in soil solution, understanding the mechanism of B adsorption on soil materials is necessary. Although organic matter plays an important role in B sorption, this study focused on soil minerals. Anion exchange with surface hydroxyl groups has been invoked as the mechanism of B adsorption on Al and Fe oxide minerals (Sims and Bingham, 1968; McPhail et al., 1972). This type of ligand exchange is a mechanism by which anions become specifically adsorbed onto oxide mineral surfaces. Specifically adsorbing anions form inner-sphere surface complexes. Inner-sphere surface complexes contain no water molecules between the adsorbate anion and the surface USDA-ARS, U.S. Salinity Lab., 4500 Glenwood Dr., Riverside, CA 92501. Contribution from the U.S. Salinity Lab. Received 17 July 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 57:704-708 (1993). functional group (Sposito, 1984). The zero point of charge (ZPC) of variable-charge minerals is shifted to a more acid value following specific adsorption of anions. Boron adsorption lowered the ZPC of boehmite (Fricke and Leonhardt, 1950), pseudoboehmite (Alwitt, 1972), Al-hydroxide gel (Beyrouty et al., 1984), and magnetite (Blesa et al., 1984), indicating specific adsorption. No information on the effect of B adsorption on the ZPC of clay minerals is available. Previous research has indicated that B adsorption occurs on the edges of the clay minerals illite (Couch and Grim, 1968) and montmorillonite (Keren et al., 1981). Ligand exchange with reactive surface hydroxyl groups on the broken edges has been suggested as the mechanism of B adsorption on clay minerals (Keren and Talpaz, 1984). By studying the effects of ionic strength on ion adsorption, Hayes and Leckie (1987) were able to distinguish between innerand outer-sphere metal ion surface complexes, while Hayes et al. (1988) were able to distinguish between innerand outer-sphere anion surface complexes. Outer-sphere surface complexes contain at least one water molecule between the adsorbate anion and the surface functional group (Sposito, 1984). Studying Se adsorption, Hayes et al. (1988) suggested that SeOI" , showing strong ionic strength dependence in its adsorption behavior, was weakly bonded as an outer-sphere surface complex, while SeO!" , showing little ionic strength dependence in its adsorption behavior, was specifically adsorbed in a strong inner-sphere surface complex. These adsorption mechanisms were verified spectroscopically using extended x-ray absorption fine structure measurements (Hayes et al., 1987). The constant capacitance model is a chemical surface complexation model that uses a ligand exchange mechanism to describe specific anion adsorption (Stumm et al., 1980). The model explicitly defines inner-sphere surface complexes and chemical reactions and considers the charge on both the adsorbate and the adsorbent. The constant capacitance model has been used successfully to describe B adsorption on various Al and Fe oxides via ligand exchange with surface hydroxyl groups (Goldberg and Glaubig, 1985), clay minerals via ligand exchange with aluminol groups (Goldberg and Glaubig, 1986a), and soils (Goldberg and Glaubig, 1986b) as a function of solution pH. The objectives of our study were to: (i) investigate the ionic strength effects on B adsorption behavior by oxides, clay minerals, and soils; (ii) evaluate the nature of the adsorbed B surface complexes using the ionic strength data; and (iii) investigate the ability of