Remedial extraction and catalytic hydrodehalogenation for treatment of soils contaminated by halogenated hydrophobic organic compounds

摘要

The overall objective of this research was to develop and assess a new method, namedremedial extraction and catalytic hydrodehalogenation (REACH), for removing anddestroying soil contaminants. In particular, I considered hydrophobic halogenatedorganic compounds (HHOCs). In this research, I developed a closed-loop treatmentprocess that catalytically destroys the contaminants of concern, and does not generate asecondary waste stream. Mixtures of water and ethanol appear to be good candidates forthe extraction of 1,2,4,5-tetrachlorobenzne (TeCB) or pentachlorophenol (PCP) fromcontaminated soil. Palladium-catalyzed hydrodehalogenation (HDH) was applied fordestroying TeCB or PCP in mixtures of water and ethanol in a batch mode.The experimental results are all consistent with a Langmuir-Hinshelwood modelfor heterogeneous catalysis. Major findings that can be interpreted within the Langmuir-Hinshelwood framework are as follows: the rate of HDH depends strongly on the solventcomposition, increasing as the water fraction of the solvent increases; the kinetics of the HDH reaction are apparently first-order with respect to the concentration of TeCB in thesolvent; and the HDH rate increases as the catalyst concentration in the reactorincreases. Also, TeCB is converted rapidly and quantitatively to benzene, with onlytrace concentrations of 1,2,4-trichlorobenzene appearing as a reactive intermediate. PCPis transformed to phenol by sequential reductive dehalogenation to tetrachlorophenols,then to trichlorophenols, then to phenol. The degradation of PCP does not follow firstorderkinetics, probably because of competitive reactions of intermediate products thatare generated during PCP degradation. Following the batch studies, the REACHtechnology was applied in continuous mode under baseline conditions for a span of 7weeks to treat soils that had been synthetically contaminated by HHOCs in thelaboratory. Extraction of TeCB and PCP from soils was almost completed within twodays by a 50:50 mixture of water and ethanol. Higher reaction rates were observed forTeCB than for PCP. The activity of the catalyst was slowly lost as contaminant masswas removed from the soil. The deactivated catalyst was successfully regenerated with adilute sodium hypochlorite solution. The results of this research suggest that REACHcould be a viable technology for some contaminated soils.