Photoreduction of Fe(III) by hydroxycarboxylic acids in seawater

Abstract

The photoreduction of Fe(III) in seawater (pH 8.1) has been investigated over a range of natural sunlight intensities at 5, 10 and 15°C in the presence of hydroxycarboxylic acid (HCA), such as sugar acids. The Fe(III) photoreduction abilities of HCA were in the order: glucaric acid-1,4-lactone = glucaric > tartaric > gluconic >> citric > glyceric = malic > glucuronic acids. The order probably depends on the complexation ability of HCA with Fe(III) in seawater and the photo-activity of Fe(III)-HCA complex. Stable soluble Fe(III)-glucaric complex systems indicated a reversible photochemical reaction ▪ The photoreduction rate constant ( k red) was estimated from the oxidation rate constant ( k ox) of Fe(II) in the presence of glucaric form and steady state Fe(II) concentration assuming first-order processes. The photoreduction rate constant increases linearly with increasing light intensity and is independent of temperature. The photoreduction mechanism is probably a photoinduced ligand to metal charge transfer (LMCT) reaction. For colloidal hydrous ferric oxide-HCA systems, the net photoreduction rate probably depends on the surface concentration of adsorbed HCA on colloidal ferric oxide (surface complexation), the back oxidation rate of photoproduced Fe(II), and hydrolytic precipitation rate of reoxidized Fe(III) in seawater remaining HCA. These above interpretations are supported from the observed shift in the Fe(III)HCA absorption spectra measured before and after each photo-experiment. The photoreduction of Fe(III)HCA complex systems is probably promoted by the absorption at longer ultraviolet (u.v.) wavelengths (290–400 nm) of sunlight by the second absorption band centered at 310 nm observed in the spectra of soluble Fe(III)HCA complexes in seawater.