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United States Department of Agriculture

Agricultural Research Service

Title: Factors Affecting Molybdenum Adsorption by Soils and Minerals

Authors
item Goldberg, Sabine
item Forster, Harold

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 20, 1997
Publication Date: N/A

Interpretive Summary: Molybdenum is a specifically adsorbing anion that can be detrimental to animals at elevated levels. Detrimental levels can occur from ingestion of forage plants grown on soils irrigated with waters containing high concentrations of molybdenum. A better understanding of the adsorption behavior of molybdenum is necessary. Adsorption of molybdenum by aluminum and iron oxides, clay minerals, and soils was investigated under changing conditions of solution molybdenum concentration, pH, ionic strength, particle concentration, temperature, and competing ion concentration. Our results will benefit scientists who are developing models of molybdenum movement in arid zone soils. The results can be used to improve predictions of molybdenum behavior in soils and thus aid action and regulatory agencies in the management of soils and waters which contain elevated concentrations of molybdenum.

Technical Abstract: Molybdenum adsorption behavior was investigated on various crystalline and amorphous Al and Fe oxides, clay minerals, and arid-zone soils as a function of solution pH, Mo concentration, ionic strength, particle concentration, competing anion concentration, and temperature. All of these factors influenced the extent of the Mo adsorption reaction. Adsorption on all materials showed pronounced pH dependence over the pH range 3 to 10.5. Adsorption as a function of pH was studied at two initial Mo concentrations. Adsorption on a percent basis was greater for the lower Mo concentration. Adsorption as a function of pH was studied at two particle concentrations. At the higher particle concentration, adsorption attained 100% over most of the pH range. At the lower particle concentration, adsorption of 100% was avoided, allowing much improved 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 lowest for the highest solution ionic strength. Molybdenum adsorption was investigated as a function of temperature at 10, 25, and 40øC and found to be endothermic increasing with increasing temperature. In competitive systems containing arsenate or phosphate in concentrations equimolar or twice equimolar to Mo, Mo adsorption was affected very little by the presence of the competing ion.

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