FACTORS AFFECTING MOLYBDENUM ADSORPTION BY SOILS AND MINERALS

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Abstract
Molybdenum adsorption behavior was investigated on a variety of crystalline and X-ray amorphous aluminum and iron oxide minerals, clay minerals, and arid-zone soils as a function of solution pH, molybdenum concentration, ionic strength, particle concentration, competing anion concentrations, and temperature. All of these factors influenced the extent of the Mo adsorption reaction. Adsorption on all materials showed pronounced pH dependence in the pH range 3 to 10.5. Maximum adsorption occurred at low pH and then decreased rapidly with increasing pH until adsorption was virtually zero near pH 8. Adsorption as a function of pH was studied at two initial molybdenum concentrations. Adsorption on a percentage basis was greater for the smaller Mo concentration. Adsorption as a function of pH was studied at two particle concentrations. At the larger particle concentration, adsorption reached 100% over most of the pH range. At the smaller particle concentration, 100% adsorption did not occur, allowing better definition of the shape of the adsorption envelopes. The effect of ionic strength was studied in background electrolyte concentrations of 0.01, 0.1, and 1.0 M NaCl. On all materials, Mo adsorption was smallest for the largest solution ionic strength. The adsorbents exhibited diverse behavior in ionic strength dependence. Molybdenum adsorption was investigated as a function of temperature at 10, 25, and 40°C and was found to be endothermic, increasing with increasing temperature. In competitive systems containing arsenate or phosphate in concentrations equimolar or twice equimolar to molybdate, Mo adsorption was affected very little by the presence of the competing ion. Molybdenum adsorption on a variety of adsorbents showed dependence on solution Mo concentration and solution pH. Adsorption was affected little by other factors, including particle concentration, solution ionic strength, temperature, and competing ion concentration. These results will facilitate the incorporation of Mo adsorption reactions into chemical speciation and transport codes because only a few factors affecting Mo adsorption will have to be considered.