Reductive Dechlorination Of Chlorobenzenes Research Articles

A NOVEL CATALYST FOR REDUCTIVE DECHLORINATION OF CHLOROBENZENE IN SUBCRITICAL WATER: BIFUNCTIONAL Fe/ZrO2

Bifunctional Fe / ZrO 2 was prepared by mechanical mixing method, and its bifunctional effect on reductive dechlorination of chlorobenzene in subcritical water was studied. Dechlorination efficiency increased with increasing iron content in catalyst and catalyst amount. Dechlorination efficiency slowed when the iron content in catalyst reached 30%; bifunctional catalyst of Fe / ZrO 2 was more efficient in dechlorination of chlorobenzene than Fe alone. Catalyst of Fe (30%)/ ZrO 2 was characterized by means of X-ray diffraction (XRD), H 2 temperature programmed desorption ( H 2-TPD), and N 2 adsorption. The possible mechanism of dechlorination in subcritical water by this bifunctional catalyst was proposed. H + produced in the water dissociation formed the highly reactive spillover hydrogen on the surface of catalyst, and then reacted with chlorobenzene adsorbed on the catalyst surface by ZrO 2 to form benzene and chloride ions.

Radiolytic and thermal dechlorination of organic chlorides adsorbed on molecular sieve 13X.

Reductive dechlorination of chlorobenzene (PhCl), trichloroethylene (TCE), tetrachloroethylene (PCE), 1- and 2-chlorobutanes, chloroform, carbon tetrachloride, and 1,1,1- and 1,1,2-trichloroethanes adsorbed on molecular sieve 13X was investigated. The molecular sieve adsorbing the organic chlorides was irradiated with gamma-rays, heated, or allowed to stand at room temperature in a sealed ampule and was then soaked in water. The dechlorination yields were determined from the Cl- concentrations of the supernatant aqueous solutions. It was found that the chlorinated alkanes adsorbed on the molecular sieve are readily dechlorinated on standing at room temperature. The dechlorination at room temperature was limited for TCE and PCE. PhCl was quite stable even at 200 degrees C. gamma-Radiolysis was examined for PhCl, TCE, and PCE at room temperature. The radiation chemical yields of the dechlorination, G(Cl-), were 1.9, 40, and 30 for PhCl, TCE, and PCE, respectively. After 5 h of heating at 200 degrees C, the dechlorination yields for TCE and PCE were 24.5 and 4.3%, respectively. TCE is much more reactive than PCE in the thermal dechlorination, whereas their radiolytic dechlorination yields are comparable. The pH of the supernatant solutions decreased along with the dechlorination.

Dechlorination of chlorobenzenes in anaerobic estuarine sediment

The reductive dechlorination of chlorobenzenes (CBs) in an anaerobic estuarine sediment was examined. The sediment was preexposed to various anthropogenic chemicals from the surrounding industries. The sulfate content (20 mmoles/l) in the interstitial waters indicated that the sediment was sulfidogenic. All the CBs were transformed following a first-order reaction kinetics with rate constants ranging from 0.0016 to 0.0389 day−1 or half-lives between 17 and 433 days. Disappearance of CBs in autoclaved sediment were observed but to a much less extent than in the test sediment. Detected intermediates indicated that the removal of chlorine atom occurred at all possible positions on the aromatic ring. The contribution of different dechlorination pathways were estimated using branched chain first-order irreversible reaction kinetics. The results showed that the preferential dechlorination was in the following order: two chlorine atoms on both sides > one of the adjacent chlorine atoms > chlorine with no adjacent chlorine atoms. The major pathway observed for CBs followed a thermodynamically most favorable step that gave the highest energy for dechlorination.

Dechlorination of chlorobenzenes in anaerobic estuarine sediment

The reductive dechlorination of chlorobenzenes (CBs) in an anaerobic estuarine sediment was examined. The sediment was preexposed to various anthropogenic chemicals from the surrounding industries. The sulfate content (20 mmoles/l) in the interstitial waters indicated that the sediment was sulfidogenic. All the CBs were transformed following a first-order reaction kinetics with rate constants ranging from 0.0016 to 0.0389 day-1 or half-lives between 17 and 433 days. Disappearance of CBs in autoclaved sediment were observed but to a much less extent than in the test sediment. Detected intermediates indicated that the removal of chlorine atom occurred at all possible positions on the aromatic ring. The contribution of different dechlorination pathways were estimated using branched chain first-order irreversible reaction kinetics. The results showed that the preferential dechlorination was in the following order: two chlorine atoms on both sides > one of the adjacent chlorine atoms > chlorine with no adjacent chlorine atoms. The major pathway observed for CBs followed a thermodynamically most favorable step that gave the highest energy for dechlorination.

Prediction of Redox Potentials and Isomer Distribution Yields for Reductive Dechlorination of Chlorobenzenes

Abstract Gibbs free energies of reductive dechlorination processes of chlorobenzenes are calculated from thermodynamic data. The results are utilized to predict redox potentials and isomer distribution yields. The model predicts a standard redox potential of 0.680 V for the reduction of hexachlorobenzene to pentachlorobenzene, tapering off to 0.535 V for the reduction of monochlorobenzene to benzene. It is shown that under anaerobic conditions, reductive dechlorination is more likely to occur, while aerobic conditions favour the formation of chlorophenols. An isomer distribution yield is predicted for each of the reductive dechlorination processes of chlorobenzenes. Predicted yields correspond to experimental values within 10%. The model includes a possibility to establish a temperature dependence of the relative isomer yields.