Photoreductive Dissolution of Colloidal Iron Oxides in Natural Waters

Size-separation (0.1-μm filtration and ultrafiltration) techniques and coulometric procedures have been used to investigate the photoreductive dissolution of iron oxides under conditions typical of natural waters. In the absence of organic agents, iron oxides are solubilized to varying degrees through photodissociation of ferric hydroxy groups at the colloid surface. The degree of dissolution is dependent principally on the chromophore concentration and is highest at low pH and high colloidal surface area. Naturally occurring organic materials significantly increase the initial rate of dissolution. Despite the enhancement by organic agents significant effects of light on net dissolution are only observed at low pH. Irradiation of natural waters produces hydrogen peroxide in quantities sufficient to markedly affect the solution-phase speciation of iron and under certain conditions (e.g., seawater) also affects the net photodissolution of iron oxides.