Molybdenum adsorption behavior was investigated on various crystalline and x-ray amorphous Al and Fe oxide minerals, clay minerals, CaC@, and arid-zone calcareous and noncalcareous s soils. Molybdenum adsorption on botb Al and Fe oxides exhibited a maximum at low pH extending to about pH 4 to 5. Above pH 5 adsorption decreased rapidly, with little adsorption occurring above pH 8. Molybdenum adsorption was higher for the oxide minerals having higher specific surface area and lower crystallinity. Molybdenum adsorption on the clay minerals exhibited a peak near pH 3 and then decreased rapidly with increasing pH until adsorption was virtually zero near pH 7. The magnitude of MO adsorption on clays increased in the order: kaolinite < illite < montmorillonite. Shifts in point of zero charge were observed on Al and Fe oxides and kaolinite following M O adsorption, indicating an inner-sphere adsorption mechanism for MO on these surfaces. Molybdenum adsorption behavior on three arid-zone noncalcareous soils resembled that on clays, exhibiting a peak near pH 3 to 4 and decreasing with increasing pH up to pH 7. This behavior is expected since the oxide content of these soils is low. Molybdenum adsorption on calcite and two calcareous arid-zone soils was low, indicating that CaCOJ is not a significant sink for MO in soils.
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.
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