Submitted to: Book Chapter
Publication Type: Book / chapter
Publication Acceptance Date: 1/19/2007
Publication Date: 2/15/2008
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2162.pdf
Citation: Goldberg, S.R., Johnston, C.T., Suarez, D.L., Lesch, S.M. 2008. Mechanism of molybdenum adsorption on soils and soil minerals evaluated using vibrational spectroscopy and surface complexation modeling. In: M.O. Barnett and D.B. Kent (eds) Adsorption of Metals by Geomedia II. Variables, Mechanisms, and Model Applications. Elsevier Press. Chapter 9 p: 235-266. 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 36 soil samples was evaluated and predicted using a chemical model and easily measured soil chemical characteristics. Our results will benefit scientists who are developing models of molybdenum movement in 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 on amorphous aluminum and iron oxides was investigated as a function of solution pH and solution ionic strength. In this study in situ Raman and Fourier transform infrared (FTIR) spectroscopic methods were combined with sorption techniques, electrophoretic mobility measurements, and surface complexation modeling to study the interaction of Mo with amorphous oxide surfaces. The speciation of Mo in aqueous solution was examined using Raman and attenuated total reflectance ATR-FTIR methods as a function of solution pH. Good agreement was found between the vibrational spectra of Mo in aqueous solution and those of Mo sorbed to amorphous Al oxide. The mechanisms of Mo sorption to these surfaces based on the spectroscopic, sorption, and electrophoretic mobility measurements are as follows: Mo forms predominantly inner-sphere surface complexes at low pH and predominantly outer-sphere surface complexes at high pH. These surface configurations were used to constrain the input parameters of the triple layer surface complexation model to describe Mo adsorption on soils. After applying the triple layer model to Mo adsorption on 36 soils, a general regression model was developed for predicting Mo surface complexation constants from five independently measured soil chemical characteristics: cation exchange capacity, organic carbon content, inorganic carbon content, aluminum oxide content, and iron oxide content. The triple layer model was well able to predict Mo adsorption on all the soils at all pH values. The surface speciation predicted by the model for soil was in agreement with that predicted from spectroscopy for Mo adsorption on amorphous Al oxide.