DISSOLUTION RATE AND SOLUBILITY OF COLLOIDAL HYDROUS FERRIC-OXIDE IN SEAWATER

Dissolution rate constants and solubilities of colloidal hydrous ferric oxide in seawater over the pH range 5.5-8.1 at 20-degrees-C were experimentally determined by dialysis techniques involving gamma-activity measurements of Fe-59. The Fe (III) dissolution rate was defined as a first-order reaction in proportion to the concentration of particulate Fe(III) in seawater. These Tate constants and solubilities within the pH range 6.8-8.1 were independent of pH with values: 0.015+/-0.002 day-1 and 1.05+/-0.05 X 10(-8) mol l-1, respectively. This result probably indicates the existence of Fe(OH)30 as well as Fe(OH)2+ in the normal pH range of seawater. At lower pHs, the logarithmic dissolution rate constants and solubilities increased linearly with decreasing pH with a slope of -0.55 and - 1.O. respectively. The Fe(III) dissolution rate of colloidal hydrous ferric oxide at the normal pH of seawater can be estimated from 0.015x[Fe(III)p] (conc. day-1) by the concentration of particulate Fe(III) in seawater.