Photochemical formation of Fe(II) in humic acid solutions - effects of pH and wavelengths

Humic acid is a collection of organic compounds that exist in nature but whose structures are not well known. They are expected to influence the oxidations and reductions of transient species, e.g. active oxygen species and various transition metals. Commercially available humic acid as received from the manufacturer contains residual iron. Using this residual Fe, we investigated the photochemical behavior of Fe(II) in aqueous solutions to elucidate the photochemical cycles of Fe(III)/Fe(II) in the environment. We investigated the effects of pH and wavelengths using monochromatic radiations at 313, 334, 366, and 405 nm. Photochemically formed Fe(II) concentrations were determined by ferrozine-HPLC technique, and the apparent quantum yields were determined based on total absorbance of the humic acid solutions. Apparent quantum yields of Fe(II) photo- formation were the highest at pH = 4.1 between pH = 3 and 9. The apparent quantum yields of Fe(II) decreased to almost zero at pH = 9. Wavelength-dependence of Fe(II) photoformation showed that Fe(III) is efficiently reduced in shorter wavelengths, and the highest yield was observed at 313 nm among the 4 wavelengths studied. The apparent quantum yields decreased systematically from 313 nm to 405 nm. The results of this study suggested that photochemical formation of Fe(II) in humic acid solutions is significantly influenced by pH of the solutions and wavelengths.