Comparison of mineral and soluble iron Fenton's catalysts for the treatment of trichloroethylene

Contaminant degradation, stoichiometry, and role of hydroxyl radicals (OH) in four Fenton's systems were investigated using trichloroethylene (TCE as a model contaminant. A standard Fenton's system, a modified soluble iron sy stem with a pulse input of hydrogen peroxide, and two modified mineral-cata lyzed systems (pH 3 and 7) were studied. In the standard Fenton's system, w hich had the most efficient reaction stoichiometry, 78% of the TCE was degr aded; however, chloride analysis indicated that no more than two of the thr ee chlorines were displaced per TCE molecule degraded. Although the modifie d soluble iron system was characterized by 91% TCE degradation, chloride an alysis also indicated that no more than two of the chlorines were lost from the TCE. In the goethite system of pH 3. > 99% of the TCE was degraded. Ne ar-complete release of chloride suggested that the TCE may have been minera lized. Only 22% degradation of TCE was achieved in the pH 7 goethite system , and there was minimal release of chloride. The mineral-catalyzed reaction s exhibited the least efficient reaction stoichiometry of the four systems. Experiments using hydroxyl radical scavengers showed that the standard Fen ton's system degraded TCE entirely by hydroxyl radical mechanisms, while ap proximately 10-15% of the degradation achieved in the modified soluble iron and goethite-catalyzed systems at pH 3 was mediated by non-hydroxyl radica l mechanisms. In the goethite system at pH 7, only non-hydroxyl radical mec hanisms were found. The goethite-catalyzed system at pH 3 effectively degra ded the parent compound and may have the potential to mineralize contaminan ts when used for in situ soil and groundwater remediation and ex situ waste stream treatment in packed-bed reactors. (C) 2001 Published by Elsevier Sc ience Ltd.