Quantitation of hydroxyl radicals from photolysis of Fe(III)-citrate complexes in aerobic water

Background. For their high photoreactivity, Fe(III)-carboxylate complexes are important sources of H2O2 for some atmospheric and surface waters. Citrate is one kind of carboxylate, which can form complexes with Fe(III). In Our previous study, we have applied Fe(III)-citrate complexes to degrade and decolorize dyes in aqueous solutions both under UV light and sunlight. Results have shown that carboxylic acids can promote the photodegradation efficiency. It is indicated that the photolysis of Fe(III)-citrate complexes may cause the formation of some reactive species (e.g. H2O2, and -OH). This work is attempted to quantify hydroxyl radicals generated in the aqueous solution containing Fe(III)-citrate complexes and to interpret the photoreactivity of Fe(III)-citrate complexes for degrading organic compounds. Methods. By using benzene as the scavenger to produce phenol, the photogeneration of -OH in the aqueous solution containing Fe (III)-citrate complexes was determined by HPLC. Results and Discussion. In the aqueous solution containing 60.0/ 30.0 mu M Fe(III)/citrate and 7.0 mM benzene at pH 3.0, 96.66 mu M -OH was produced after irradiation by a 250 W metal halide light (lambda >= 313 nm) for 160 minutes. Effects of initial pH value and concentrations of Fe(III) and citrate on -OH radical generation were all examined. The results show that the greatest photoproduction of -OH in the aqueous solution (pH ranged from 3.0 to 7.0) was at pH 3.0. The photoproduction of -OH increased with increasing Fe(III) or citrate concentrations. Conclusion. In the aqueous solutions containing Fe(III)-citrate complexes, -OH radicals were produced after irradiation by a 250 W metal halide light. It can be concluded that Fe(III)-citrate complexes are important sources of -OH radicals for some atmospheric and surface waters. Recommendations and Outlook. It is believed that the photolysis of Fe(Ill)-citrate complexes in the presence of oxygen play an important role in producing -OH both in atmospheric waters and surface water where high concentrations of ferric ions and citrate ions exist. The photoproduction of -OH has a high oxidizing potential for the degradation of a wide variety of natural and anthropogenic organic and inorganic substances. We can use this method for toxic organic pollutants such as organic dyes and pesticides.