Molecular orbital study of Fe(II) and Fe(III) complexation with salicylate and citrate ligands: Implications for soil biogeochemistry

The formation of mineral-associated organic matter (OM), typically with Fe oxy(hydr)oxide minerals, contributes to the long-term storage of soil C. However, the changing climate is predicted to alter precipitation frequency and intensity patterns such that soils may experience greater periods of anoxic conditions leading to the reductive dissolution of the Fe oxy(hydr)oxide minerals. This would subject the released OM to microbial decomposition and the Fe cation to react with soil solution constituents. Using salicylate and citrate as model soil ligands, this study uses density functional theory to provide physical insights into the chemical bonds formed between Fe(II) and Fe(III) cations with the organic acids. The Delta G degrees for the complexation reactions of both salicylate and citrate were negative indicating that the complex formation is energetically favorable. Furthermore, the Delta G(0) was more negative for the salicylate ligand as compared with the citrate ligand suggesting that the involvement of carboxyl-O and phenoxyl-O to form a complex is more thermodynamically favorable. We show from the molecular electron density data that the bonds involved in the complexation are more electrostatic than covalent in nature. Molecular orbital calculations show that the energies of the highest occupied orbitals of Fe(II)-salicylate and Fe(II)-citrate complexes are less negative than that of the uncomplexed Fe(II)-(H2O)(6), suggesting that the complexed Fe(II)-organic species will be preferentially oxidized upon return of the soil to aerobic conditions. Our results provide a more fundamental chemical understanding of the reactions involved in the biogeochemical cycling of soil Fe and C. This knowledge may assist in evaluating the potential role of enhanced soil C sequestration as one approach to mitigate climate change.

Automated summary

Formation of mineral-associated organic matter contributes to soil carbon storage, but changing climate may destabilize minerals. Using model soil ligands, this study examines the chemical bonds formed between Fe cations and organic acids, providing insights into the biogeochemical cycling of soil Fe and C.