Dechlorination of toxic chlorinated compounds

Abstract

It was discovered in our laboratory that the bimetallic system palladium on iron (palladized-iron, Pd/Fe) rapidly and completely hydrodechlorinates aqueous solutions of ubiquitous chlorinated aliphatic and aromatic environmental contaminants such as TCE (trichloroethylene), chlorophenols and PCBs (polychlorobiphenyls) at room temperature and atmospheric pressure. When TCE comes in contact with the Pd/Fe surface, it forms ethane as the major gaseous product. Chlorophenols are converted to phenol and PCBs are converted to biphenyl. It was also discovered that through a series of redox reactions metallic iron completely reduces perchlorate to chloride. The objectives of this research are: to show that two iron materials can completely dechlorinate aqueous solutions of toxic chlorinated compounds; to begin to understand how these reactions occur, and; to understand the implications of oxidizing iron. In this research, batch and scaled-up systems were used for fundamental and practical studies of the Pd/Fe system. TCE, chlorophenols and PCBs were the model compounds for these studies. The batch reactions were performed in 1 OmL vials containing 2-5g Pd/Fe and 2-5mL of an aqueous solution of a reactant. The scaled-up reactions were performed in a closed-loop column apparatus with 700g of Pd/Fe and approximately 250mL of an aqueous solution of a reactant. The batch reactions were used to: show complete and rapid hydrodechlorination of the reactants to their products; demonstrate the performance of various types of iron; determine the sequential order of the removal of the chlorine substituents from chlorophenols and PCBs; observe the effect of an HCl-treated and an untreated iron surface prior to palladium deposition and the effects these surfaces have on the hydrodechlorination reactions of chlorophenols and PCBs; and determine what implications iron oxide build-up on the Pd/Fe surface may have on scaled-up Pd/Fe systems. The scaled-up column apparatus was used to determine the longevity of Pd/Fe surfaces and to find a suitable regeneration method. Because Pd/Fe was found to be a suitable reductant for chlorinated organic molecules, chlorine-containing ions such as ClO₄⁻ were also investigated. It was discovered that HCl-treated iron is a suitable reductant for the reduction of the perchlorate ion at room temperature and atmospheric pressure. Palladized-iron and zero-valent iron have been found to be suitable materials for the remediation of many ubiquitous environmental contammants. The hydrodechlorination of chlorinated compounds occurs completely and rapidly with Pd/Fe. Iron effectively reduces perchlorate ions to chloride ions. Both Pd/Fe and zero-valent iron surfaces are relatively easy to prepare, and can be used at room temperature and atmospheric pressure for the dechlorination reactions.