Solubility and dissolution rate of colloidal γ-FeOOH in seawater

Abstract

Solubilities and dissolution rates of colloidal γ-FeOOH (lepidocrocite), one of the typical iron oxyhydroxides, in seawater over a pH range 5.5–8.2 at 20°C were experimentally determined by dialysis techniques involving γ-activity measurements of 59Fe. The Fe(III) dissolution rate was closely in proportion to the concentration of particulate Fe(III) in seawater and was defined as a first-order reaction. The solubilities and dissolution rate constants within the pH range 7.0–8.2 were independent of pH with values of approx. 1 × 10 −9 mol l −1 and 0.005 day −1, respectively. This result probably indicates the existence of Fe(OH) 3 o as well as Fe(OH) 2 + in the normal pH range of seawater. The solubility and dissolution rate constants were one order of magnitude and one-third, respectively, lower than those of amorphous hydrous ferric oxide determined in a previous study. The Fe(III) dissolution rate of colloidal γ-FeOOH at the normal pH of seawater can be estimated from 0.005 × [Fe(III)particulate] (conc. day −1) by the concentration of particulate Fe(III) as γ-FeOOH in seawater. At lower pHs of 5.5–7.0, the logarithmic solubilities and dissolution rate constants increased linearly with decreasing pH with a slope of −1.0 and −0.95, respectively, indicating that Fe(OH) 2 + is the dominant dissolved ferric species.