PHOTOASSISTED DISSOLUTION OF COLLOIDAL IRON-OXIDES BY THIOL-CONTAINING COMPOUNDS .1. DISSOLUTION OF HEMATITE (ALPHA-FE2O3)

Hematite ( alpha -Fe//2O//3) dissolution in the presence of a series of alpha -mercaptocarboxylic acids has been investigated under dark and light conditions and the effects on the dissolution process of a variety of factors including temperature, pH, organic acid concentration, and amount of hematite present have been examined. UV/near-UV light significantly enhances dissolution of the colloidal iron oxide in the presence of mercaptoacetic acid with the rate constant for decomposition of a proposed surface-located ferric thiol complex increasing from 3. 3 multiplied by 10** minus **3 to 10. 0 multiplied by 10** minus **3 min** minus **1 on photolysis. Excitation of either ligand field states associated with free electrons on the S atom or high-energy S( sigma ) yields d(Fe(III)) charge transfer states are proposed to account for this enhancement in dissolution rate. While the dark dissolution rate decreases as the degree of methylation increases, the effectiveness of light in assisting hematite dissolution decreases as methylation decreases.