Effect of residual vanadyl on the spectroscopic analysis of humic acids

Abstract

The effect of residual vanadyl (VO2+) on data derived from the analysis of humic acids (HAs) by means of fluorescence spectroscopy, solid-state 13C nuclear magnetic resonance spectroscopy (NMR) and UV–Vis spectroscopy was examined in order to reveal potential organic groups involved in metal complexation. The HAs were extracted from agricultural soils from an experimental station in Campinas, Brazil. Their paramagnetic ion content was determined using electron paramagnetic resonance (EPR). Because of the well-resolved EPR signal, the VO2+ ion content was used for the correlation studies. The latter clearly demonstrated that residual VO2+ affects the spectroscopic data and that, besides the intrinsic structural characteristic of HAs, the impact of paramagnetic ions must be considered for careful interpretation of such data. On the other hand, analysis of these effects can give additional structural information. With respect to the UV–Vis spectra, the positive correlation between VO2+ content and the absorption at 665 nm (E6) was attributed to the presence of VO2+ complexes. Further support for metal–humic substance binding was extracted by way of a reduction in the fluorescence intensity with increasing VO2+ concentration for both excitation and emission. A relationship between the suppression of the intensity of the O-alkyl, di-O-alkyl and carboxyl C signals in the solid-state 13C NMR spectra and the VO2+ content demonstrated the involvement of carbohydrates in metal complexation, probably in the uronic acid form. No clear correlation was observed with respect to the O-aryl C signal, which may be due to a low accessibility of the metallic ion to methyl- substituted O-aryl groups as they occur in lignin derivatives.