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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.

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Nanotechnology has been evolving in the past decades as an alternative to conventional fertilizers. Ferrihydrite nanoparticles that model the available Fe pool of soils are proposed to be used to recover Fe deficiency of plants. Nevertheless, ferrihydrite aqueous suspensions are known to undergo slow transformation to a mixture of goethite and hematite, which may influence its biological availability. Several nanocolloid suspensions differing in the surfactant type were prepared for plant treatment and fully characterized by transmission electron microscopy and 57Fe Mössbauer spectroscopy supported by magnetic measurements. The rate of transformation and the final mineral composition were revealed for all the applied surfactants. Nanomaterials at different stages of transformations were the subject of plant physiological experiments aiming at comparing the behavior and plant accessibility of the manufactured suspensions of nanoscale iron(III) oxide and oxide–hydroxide particles. Graphical abstract