Oxyanion Adsorption

The ionic potentials of the nonmetal elements in Groups IIIA to VIA exceed 100 nm for their common oxidation states, and therefore these elements form oxyanions instead of hydrolytic species in soil solutions. The same tendency is observed for the Group VIB metals chromium and molybdenum. Examples of inorganic oxyanions commonly found in the aqueous phases of soil are B(OH)J, CO, NOJ, H3Si04, POj , SOj , AsOl , SeOf , MoOl , and, when the oxyanion is multivalent, some of the protonated forms. The qualitative and mechanistic features of the adsorption of these oxyanions—a topic on which there is an abundant literature —are the principal concerns of the present section. Quantitative models of inorganic oxyanion adsorption are described in Chap. 5. 

A consensus exists that inorganic oxyanion adsorption by soil minerals involves almost universally the two-step ligand exchange reaction 

Much experimental evidence has been adduced in support of the ligand exchange mechanism for oxyanion adsorption. The principal lines of reasoning that lead to this mechanism can be summarized as follows. 

Strengite (FeP04 2H2O) Metastrengite (FeP04 2H2O) H2P04-(aq) 

The cation exchange mechanism in Table 4.3 can be represented by the reaction 

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S. L. Cumberland and G. F. Strouse, Analysis of the Nature of Oxyanion Adsorption on Gold Nanomaterial Surfaces, Langmuir 18, 269-276 (2002). 

Suarez, D. L, Goldberg, S., and Su, C., 1999, Evaluation of oxyanion adsorption mechanisms on oxides using FTIR spectroscopy and electrophoretic mobility in Sparks, D. L., and Grundl, T. J., eds., Mineral-Water Interfacial Reactions, American Chemical Society, p. 137-178. 

Rietra, R.P., Hiemstra, T.. and van Riemsdijk, W.H., Electrolyte anion affinity and its effect on oxyanion adsorption on goethite, J. Colloid Interf. Sci., 229, 199, 2000. Rietra, R.P., Hiemstra, T, and van Riemsdijk, W.H., The relationship between molecular structure and ion adsorption on variable charge minerals, Geochim. Cosmochim. Acta, 63. 3009, 1999. 

Oxyanion adsorption has a tendency to maximize at a pH near the pK value of the protonated form of the adsorbing anion. 

Evaluation of Oxyanion Adsorption Mechanisms on Oxides Using FTIR Spectroscopy and Electrophoretic Mobility... 

The preceding examples consistently show that oxyanion adsorption on common oxides (of Al, Fe, Mn, Ti) leads to strong specific binding, commonly through bidentate covalent complexes and in some cases with monodentate binding as well. 

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