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A Comparative Electron Spin Resonance Study of VO²⁺ Complexation in Synthetic Molecules and Soil Organics¹

ABSTRACT

The vanadyl ion, VO²⁺, has been used as an ESR (electron spin resonance) probe of adsorption sites in pure organic complexing agents and natural peat to obtain information on the nature of the metal-organic bond in soils. Oxygen ligands of soil organics displace on the average about two ligand H₂O molecules from VO(H₂O)₅²⁺ in forming an inner-sphere complex with the metal. The bonding mechanism of natural organics is very similar to that of polycarboxylic acids such as uronic acid. Unlike soluble low molecular weight carboxylic acids, the solid polycarboxylates are not able to bond to all four equatorial ligand positions of VO(H₂O)₅²⁺. Evidence is presented that Fe and Al in the soil organics occupy complexation sites and partially block VO²⁺-carboxylate bond formation. The heterogeneous nature of natural organic complexing sites is demonstrated by the broadened nature of VO²⁺ ESR spectra when the metal is adsorbed on soil organics in comparison with pure polycarboxylic acids.

¹ Contribution of the Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853. Agronomy Paper no. 1318.

² Assistant Professor of Soil Chemistry.

Received for publication November 5, 1979. Accepted for publication February 20, 1980.