Hydroxyl radical generation in the photo-Fenton process: Effects of carboxylic acids on iron redox cycling

• OH radical generation in the photo-Fenton process was discussed.
• Effects of carboxylic acids on iron redox cycling were examined.
• Oxalic acid and citric acid accelerated the iron redox cycle and the OH radical generation.
• Malonic acid inhibited the iron redox and the OH radical generation.
• A kinetic model reasonably simulated the OH radical generation and iron redox cycle.

From the viewpoint of iron redox cycle promoting OH radical generation in the photo-Fenton process, effects of carboxylic acids formed in the proceeding of the degradation of organic pollutants on the photo-Fenton reactions were discussed. To elucidate the contributions of carboxylic acids forming complexes with iron ions to the photo-Fenton process, the Fenton reaction, the photo-Fenton reaction and the Fenton-like reaction composing the photo-Fenton process were individually examined. A kinetic model simulating the dynamic behaviors of iron redox cycle and OH radical generation was developed to quantify the contributions of carboxylic acids, i.e., citric acid, formic acid, malonic acid and oxalic acid. The FeII–oxalic acid complex and FeII–citric acid complex accelerated the rate of iron redox cycle or OH radical generation in the Fenton reaction by 18% and 8%, respectively. The additions of formic acid and malonic acid slightly affected the OH radical generation rate by the Fenton reaction. The photo-Fenton reaction, which is the controlling-step of the photo-Fenton process, was enhanced by oxalic acid and citric acid but largely suppressed by malonic acid. The effect of formic acid on the photo-Fenton reaction was insignificant. These results in the photo-Fenton reaction could be explained by considering the changes in the UV light absorbance with formed FeIII–carboxylic acid complexes. The effects of carboxylic acids on the Fenton-like reaction were little. In the photo-Fenton process, the additions of oxalic acid and citric acid accelerated the iron redox cycle and as a result the OH radical generation by 26% and 11%, respectively. Formic acid insignificantly affected the iron redox cycle and the addition of malonic acid decelerated the iron redox and OH radical generation by approximately 39%.

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