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

Abstract

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 Fe and Mn complexation sites in fulvic acid (FA) extracted from lignite. EPR signals of FA-Fe and FA-Mn 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 ≈ 10-3) range EPR signal by assuming discrete distributions in the axial ( D) and rhombic (λ) zero field splitting (ZFS) parameters associated with Fe complexation sites. The results refer to the presence of FA-Fe complex structures with either low (| D| ≈ 0.26 cm) or high (| D| ≥ 1.0 cm) axial ZFS parameters along with a broad distribution in λ. Outer-sphere, [Fe(OH)] 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.

The solvothermal synthesis of a 3D rod-like Fe–Al bimetallic metal–organic-framework for efficient fluoride adsorption and photodegradation of water-soluble carcinogenic dyes

“…Signal II consists of a quartet signal as clearly shown in Fig. 4-inset indicating the paramagnetic nature arisen due to Fe 3+ -d 5 [39] and Al 3+ -d 0 [40] metal ions present in Fe-Al BDC which is also supported by the XPS analysis as shown in Fig. 9A-a.…”

The Combined Application of Urea and Fulvic Acid Solution Improved Maize Carbon and Nitrogen Metabolism

“…On the other hand, the adsorption of FA on the adsorption plane creates a repulsive negative electric potential and a steric hindrance on the mineral surface, which further inhibit P binding on the soil surface [57]. Furthermore, the carboxyl, hydroxyl, and alcoholic hydroxyl groups in FA can complex with such metal ions as Ca 2+ and Mg 2+ , decreasing the fixation of phosphorus in soil and thereby increasing its mobility and availability [58][59][60]. Compared with the U treatment, the soil available phosphorus in the U+FA treatment was significantly higher at the seedling stage (Table 3), which would improve the vegetative growth and contribute to the later reproductive growth of maize in this treatment.…”

Fe microenvironments in heat treated rare-earth exchanged montmorillonites

“…In accordance with previous results [10], this signal may be associated with Fe 3+ ions at octahedral sites with various degree of rhombic distortion. The shape of the EMR signal in this range refers to the presence of a distribution in the rhombicity, and consequently to a corresponding distribution in the EFG asymmetry parameter (η) associated with octahedral Fe 3+ ligand field environments [11]. This is expected to lead to a moderate quadrupole splitting distribution in corresponding 57 Fe Mössbauer spectra, possibly appearing as an additional broadening of the absorption peaks of the associated quadrupole doublet(s).…”