Reaction of vanadate with aquatic humic substances: An ESR and super(51)V NMR study

• Published in: Environmental science & technology Vol. 32; no. 15; pp. 2257 - 2263
• Main Authors: Lu, Xiaoqiao, Johnson, W D, Hook, J
• Format: Journal Article
• Language: English
• Published: 01.08.1998
• ISSN: 0013-936X

Electron spin resonance (ESR) spectroscopy and super(51)V nuclear magnetic resonance (NMR) spectroscopy have been used to study the interaction of vanadate with aqueous solutions of humic substances (HS) at different pH values and at different concentrations. Under acidic pH conditions, ESR spectra show that humic substances reduce vanadium(V) to vanadium(IV) without further reduction to vanadium(III). The reduced vanadium(IV) ion is bound to oxygen donor atoms, probably at carboxylic acid sites in the humic substances. super(51)V NMR spectra show that the VO sub(2) super(+) cation is immediately reduced and that the decavanadate cation decomposes to the VO sub(2) super(+) cation prior to reduction. The overall rate of reduction depends on both concentration and pH. There is no reduction above pH 6, which suggests that the standard reduction potential of humic substances is about +0.65 V. Near pH 7, vanadate is stabilized by binding to humic substances. As the concentration of humic substances increases, the total vanadium NMR signal intensity decreases. This is due to the quadrupolar nature of the super(51)V nucleus that, when bound to humic substances, is invisible in NMR measurements. Quantitative models applied to intensity changes show that the vanadate monomer forms HS-V(V) complexes. The formation equilibrium constant is estimated to be 108 M super(-1). At pH above 9, NMR signals appear at -623.6 and at -763.2 ppm when humic substances are added to vanadate solution. The intensities of the signals increase with increasing pH and with increasing concentration of humic substances. These signals appear to be associated with peroxyvanadate anions, which are not bound to humic substances.