SOLUBILITY AND DISSOLUTION RATE OF COLLOIDAL GAMMA-FEOOH IN SEAWATER

Solubilities and dissolution rates of colloidal gamma-FeOOH (lepidocrocite), one of the typical iron oxyhydroxides, in seawater over a pH range 5.5-8.2 at 20-degrees-C were experimentally determined by dialysis techniques involving gamma-activity measurements of Fe-59. 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 x 10(-9) mol l-1 and 0.005 day-1, respectively. This result probably indicates the existence of Fe(OH)3-degrees 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 gamma-FeOOH at the normal pH of seawater can be estimated from 0.005 x [Fe(III)particulate] (conc. day-1) by the concentration of particulate Fe(III) as gamma-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.