ESR Analysis of Fe3+ and Mn2+ Complexes in Lignite Fulvic Acids

Samples of humic materials—humic acids and fulvic acids—derived from Tisul lignite (Kansk-Achinsk Basin) are investigated by ESR (electron spin resonance) spectroscopy. The concentration of free radicals declines in the following sequence: lignite, humic acids, fulvic acids. The proportion of aliphatic radicals increases in the same sequence. By simulation of the ESR spectra of the fulvic acids and comparison with the experimental spectra, natural complexes of fulvic acids with Fe³⁺ and Mn²⁺ ions are detected. By determining the splitting parameters in zero field (D and E), it is established that the complexes with Fe³⁺ have axial symmetry, with a complex multilevel structure. The internal coordination sphere of the complex consists of water molecules, whereas the external coordination sphere consists of oxygen-bearing fulvic acid groups. The calculated hyperfine structure constant of the experimental ESR spectra for the complexes with Mn²⁺ (79.7 G) is lower than the typical value for the octahedral Mn²⁺ complex [Mn(H₂O)₆]²⁺. That indicates partial replacement of the water molecules by oxygen-bearing functional groups of fulvic acids.