Paramagnetic Probing of Humic Acid Supramolecular Structure: Iron (III)-Induced Rearrangements Revealed by...

Area of Science:

• Environmental Chemistry
• Soil Science
• Spectroscopy

Background:

• Humic acids (HAs) are crucial soil organic matter components.
• Metal ion complexation significantly alters HA structure and dynamics.
• Molecular-level understanding of these interactions is limited.

Purpose of the Study:

• To investigate iron (III)-induced structural and dynamic changes in soil-derived humic acids.
• To quantify metal-induced reorganization using NMR relaxation parameters.

Main Methods:

• Solid-state 13C CPMAS NMR spectroscopy was used.
• Cross-polarization time constants (T_CH) and proton rotating-frame relaxation times (T_1ρ[H]) were measured.
• Analysis focused on Fe/C ratios to determine binding hierarchies and dynamic changes.

Main Results:

• Iron (III) binding induced distinct structural rigidification, strongest in carboxyl groups, followed by aromatic domains.
• Aliphatic carbons showed minimal response, indicating their exclusion from metal coordination.
• T_1ρ(H) data confirmed restricted molecular mobility in carboxyl and aromatic regions due to Fe (III).

Conclusions:

• Fe (III) acts as a supramolecular cross-linker, selectively rigidifying humic acid structure via carboxylate and aromatic interactions.
• NMR relaxation parameters (T_CH and T_1ρ[H]) provide quantitative insights into metal-induced reorganization of soil organic matter.
• This study enhances mechanistic understanding of organo-mineral interactions in soils.