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 (⁷F₀ -> ⁵D₀ 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.