Combination of X-ray photoelectron and solid-state 13C nuclear magnetic resonance spectroscopy in the structural characterisation of humic acids

Abstract

Leonardite humic acid (LHA) and soil humic acid (SHA), purchased from the IHSS, as well as purified humic acid from Aldrich (HA) are characterised by cross-polarisation magic angle spinning (CP-MAS) and Bloch decay 13C NMR spectroscopy and X-ray photoelectron spectroscopy (XPS). For the 13C CP-MAS NMR experiments, the effects of both the sample spinning sidebands and the influence of the cross-polarisation contact time are investigated. The functional groups distribution obtained by CP-MAS NMR is compared to the one resulting from chemical analyses. It is found that CP-MAS NMR overestimates the concentration of CO 2H groups for certain humic compounds, whereas the concentration of phenolic groups is in reasonable agreement with the values determined by chemical methods. XPS was used as a complementary technique to understand the overestimation of carboxylic groups by NMR. The results from XPS, a surface characterisation technique, are in good agreement with those measured by elemental analyses of the bulk structure. High resolution C 1s and N 1s spectra show that the overestimation of the CO 2H content by NMR is due to the presence of amide groups which give overlapping signals in the CO 2H region of the NMR spectra.