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%) almost-equal-to montmorillonitic soil (116%) almost-equal-to 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.