Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2010
Publication Date: 3/1/2010
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2304.pdf
Citation: Goldberg, S.R. 2010. Competitive adsorption of molybdenum in the presence of phosphorus or sulfur on gibbsite. Soil Science. 175(3):105-110. 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 the aluminum oxide gibbsite, a common soil mineral, was evaluated as a function of equilibrium solution molybdenum concentration, solution pH, and competing phosphorus and sulfur concentration and described using a chemical model. 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: Anion adsorption on the aluminum oxide, gibbsite was investigated as a function of solution pH (3-11) and equilibrium solution Mo (3.13, 31.3, or 313 µM), P (96.9 µM), or S (156 µM)concentration. Adsorption of all three anions decreased with increasing pH. Electrophoretic mobility measurements indicated a downward shift in point of zero charge, indicative of an inner-sphere adsorption mechanism for all three anions. The constant capacitance model having an inner-sphere adsorption mechanism was able to describe Mo and P adsorption; while the triple layer model with an outer-sphere adsorption mechanism was used to describe S adsorption. Competitive adsorption experiments showed a reduction of Mo adsorption at a Mo:P ratio of 1:30 and 1:300 but no reduction at a Mo:S ratio of 1:52 and 1:520. These concentrations are realistic of natural systems where Mo is found in much lesser concentrations than P or S. Using surface complexation constants from single ion systems, the triple layer model predicted that even elevated S concentrations did not affect Mo adsorption. The constant capacitance model was able to predict the competitive effect of P on Mo adsorption semi-quantitatively.