EPR Analysis of Fe³⁺ and Mn²⁺ Complexation Sites in Fulvic Acid Extracted from Lignite

Lignite is a rich source of humic substances such as humic and fulvic acids that are natural chelating agents with multifold applications in fields ranging from agriculture to biomedicine. Associations of heterogeneous molecular components constitute to their complex and still unresolved structure. In this work we utilize X-band electron paramagnetic resonance (EPR) spectroscopy to characterize Fe³⁺ and Mn²⁺ complexation sites in fulvic acid (FA) extracted from lignite. EPR signals of FA-Fe³⁺ and FA-Mn²⁺ complexes are identified and investigated in detail under various conditions by the means of a newly developed program code and associated analysis method that yields an accurate description of the low-field ( g_eff ≈ 10-3) range EPR signal by assuming discrete distributions in the axial ( D) and rhombic (λ) zero field splitting (ZFS) parameters associated with Fe³⁺ complexation sites. The results refer to the presence of FA-Fe³⁺ complex structures with either low (| D| ≈ 0.26 cm^-1) or high (| D| ≥ 1.0 cm^-1) axial ZFS parameters along with a broad distribution in λ. Outer-sphere, [Fe(OH₂)₆]³⁺ based complexes are found to be characterized with λ = 1/3 along with lower axial ZFS values, in accordance with a distorted octahedral ligand configuration.