Photo-stability of iron-phenolic complexes derived from peatland upon irradiation in waters under simulated sunlight

Phenolics from peatlands have been proven to enhance the solubility of iron, even under alkali or saline conditions, by forming iron-phenolic complexes. However, the photo-stability of these complexes has received little attention thus far. Based on the investigation of water and peat samples in Jinchuan peatland, in northern China, simulated experiments - using Gallic acid (GA) as a structural analog of peat-derived phenolic acid - were carried out to clarify the mechanisms of photo-stability of iron-phenolic complexes. Our research confirmed that when the molar ratio of GA to Fe reaches 0.1:1, photolysis led to some Fe deposition. By comparing GA concentrations under aerobic/anaerobic conditions, we demonstrated that dissolved oxygen plays a key role, not only in GA autoxidation but also in photo-degradation of GA through direct and indirect photolysis. Besides, adding coexisting phenolic acids into GA/iron systems showed various effects, depending on the different functional groups, while the dominant species of phenolic acids in peatlands, such as p-cumaric acid and p-hydroxybenzoic acid, may still act as reactive oxygen species scavengers, protecting over 99% of the dissolved iron under solar irradiation. Our results indicate that by forming iron-organic complexes with certain phenolics from peatlands, dissolved iron can be effectively protected from precipitation during freshwater transport under solar irradiation, which has profound implications for controlling the limited primary production in certain marine areas.