Humic substances (HS), derived from the degradation of organic matter in terrestrial and aquatic systems, play critical roles in nutrient cycling, metal complexation, and soil fertility. Their ability to bind trace elements, including molybdenum, is of particular agricultural importance, as Mo is an essential micronutrient that regulates nitrogen fixation and assimilation. Despite Mo’s pivotal role, its solubility and bioavailability in soils remain limited, often resulting in plant deficiencies. Understanding Mo–HS interactions is therefore crucial for developing sustainable strategies to improve nutrient availability.

HS samples obtained from Greek peaty lignite (leonardite) were studied for their interaction with Mo(VI). Solutions of HS were titrated with Mo(VI) and analyzed. Mo concentrations in supernatants were determined via atomic emission spectroscopy. Solid residues containing Mo–HS complexes were characterized using UV–Vis spectroscopy and FTIR spectroscopy.

The addition of Mo(VI) to HS solutions decreased pH, consistent with proton-releasing complexation reactions. UV–Vis spectra revealed charge-transfer processes indicative of Mo–HS associations without Mo reduction. FTIR analyses confirmed the involvement of functional groups—primarily carboxylic, with additional contributions from phenolic and alcoholic moieties—in Mo binding. Shifts and intensity changes in COO–, C=O, and O–H vibrations substantiated the formation of stable complexes.

This study demonstrates that HS effectively complexes Mo(VI), thereby enhancing its solubility and potential bioavailability in soils, highlighting the agronomic potential of humic-rich materials, such as leonardite, as natural Mo carriers to improve crop nutrition. These findings contribute to the broader understanding of HS-mediated trace element dynamics and their role in sustainable agriculture.

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