Metal binding sites and partial structures of soil fulvic and humic acids compared: aided by Eu(III) luminescence spectroscopy and DEPT/QUAT 13C NMR pulse techniques

The excitation spectra (7Fo → 5Do transition) of Eu(III)-fulvate and -humate were analyzed by a Lorentzian-Gaussian curve fitting method, and they revealed the formation of 1:1 (EuL2+; L = carboxylate: 579.04 nm) and 1:2 (EuL2+: 579.25 nm) complexes. This suggested that soil fulvic and humic acids have metal binding sites in two different chemical environments made up of carboxylate moieties. However, the 1:2 complex (EuL2+) seemed to be preponderant in fulvic acid rather than humic acid. This may be caused by the differences in their partial structures and functional group contents. The differences between soil fulvic and humic acids are discussed with the aid of the 13C NMR DEPT (distortionless enhancement by polarization transfer pulse sequence) and QUAT (quaternary carbon only spectra) subspectra.