Photoreduction of Fe(III) mediated by structurally different plant-related organic compounds: An EPR study

Plant-related organic compound (PROC) may interact with redox-active metals like iron while they are present in soil or aquatic environment, but their effects on the photoreduction of Fe(III) remain largely unexplored. This study investigates the photochemical behavior of Fe(III)-PROC complexes using alkaline lignin (AL), betaine hydrochloride (BH), and phytic acid (PA) as representative proxies for PROC. The reductive agent AL demonstrated the ability to directly reduce Fe(III) to Fe(II). In contrast, BH, being unable to form strong complexes with Fe(III), was able to quench ^•OH, thereby resulting in a shift of the redox equilibrium towards Fe(II). PA exhibited a strong binding affinity for Fe(III), effectively inhibiting its photoreduction. Electron paramagnetic resonance (EPR) analysis, utilizing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap, revealed that the DMPO-OH signal detected in photolyzed Fe(III)-PROC solutions originated from various pathways. Specifically, uncomplexed Fe(III) in AL or BH solutions was shown to oxidize DMPO directly, leading to the formation of a false DMPO-OH adduct. The addition of ethanol to the photolyzed Fe(III)-AL and Fe(III)-BH systems resulted in the generation of the DMPO-CH(CH₃)OH adduct, thereby confirming the presence of authentic ^•OH in these systems. The photolysis of the Fe(III)-PA complex may proceed via a photodissociation mechanism, where the resulting loosely bound Fe(III) can oxidize DMPO, followed by a nucleophilic attack from water. This research highlights the multifaceted roles of PROC in facilitating the redox cycling of iron within soil and aquatic ecosystems.