Fate of iron oxalates in aqueous solution: The role of temperature, iron species and dissolved oxygen

The effect of temperature (up to 120 °C) and the iron species (Fe2+ or Fe3+) as well as dissolved oxygen on the degradation of oxalic acid in aqueous solutions was studied. The results obtained showed that the fate of oxalate depends on the concentration of ferric ions, which are involved in the formation of oxalate complexes, and the presence of oxygen which oxidizes iron species. By increasing the temperature (120 °C), the ferric oxalate complexes undergo a ligand-to-metal electron-transfer reaction, leading to the homolytic cleavage of a FeIII–O coordination bond of the oxalate anion ligand giving rise to ferrous ion, oxalate anion and oxalate radical. The presence of dissolved oxygen enhances the degradation of oxalic acid by oxidizing Fe2+ to Fe3+. This explains the fact that oxalic acid is easily mineralized by Fenton oxidation at high temperature (above 100 °C), whereas it proves refractory at lower temperatures. Experiments with a radical scavenger were also carried out to learn about the role of OH radicals.