Remediation of contaminated soil by solvent extraction and catalytic hydrodehalogenation: Semicontinuous process with solvent recycle



Halogenated hydrophobic organic contaminants (HHOCs), such as chlorinated benzenes and polychlorinated biphenyls, are common soil contaminants. We propose a new method for treating soils contaminated by HHOCs by extracting the contaminants with a solvent, then destroying the contaminants catalytically, enabling the solvent to be reused. Here, we report on the assessment of this technology at the lab scale, operating in a semicontinuous mode where the solvent is recycled in a closed loop. The solvent employed was a mixture of water and ethanol, the catalyst was 1% palladium (Pd) on porous alumina, and hydrogen gas was used as the reductant. We tested the process on two soils: Texas sandy loam contaminated in the laboratory, and Florida sandy clay loam collected from a contaminated field site. The technology worked successfully on the Texas soil: over 90% of the contamination (pentachlorophenol and 1,2,4,5-tetrachlorobenzene) could be extracted and destroyed within 1 week using a solvent flow rate of 0.005 L/(min-kg soil) and a residence time of ∼3.5 min in the catalytic reactor. Seven batches of soil were treated (one week each) without needing to replace the solvent. Catalyst activity decreased over time but was recovered by cleaning the catalyst with a dilute solution of hypochlorite. However, the process was unsuccessful at treating the Florida soil because of rapid catalyst deactivation. We believe that the proposed technology (called remedial extraction and catalytic hydrodehalogenation, or REACH) is a promising “green” technology if we can protect the catalyst from deactivation; possible methods for achieving this are discussed. © 2010 American Institute of Chemical Engineers Environ Prog, 2010.