High dechlorination of PCBs catalysed by carbon under mild conditions |
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Authors: | Abdul Ghaffar Masaaki Tabata Farhana Mazher Azhar Mashiatullah |
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Institution: | (1) IAD, PINSTECH, P.O. Nilore, Islamabad, Pakistan;(2) Faculty of Science and Engineering, Saga University, Saga, Japan;(3) Lahore Women College University, Lahore, Pakistan |
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Abstract: | This article reports high dechlorination of toxic polychlorinated biphenyls (PCBs) under mild conditions. PCBs are priority
pollutants acting as endocrine disruptors, human carcinogens and environmental estrogens. Previous remediation methods involving
high temperature and pressure have drawbacks such as high cost, de novo dioxins synthesis and difficult recovery of vaporized
PCBs. On the other hand, dechlorination methods using mild conditions show the problem of catalyst deactivation. Here, activated
carbon was used for the first time as catalyst to dechlorinate 2,4,5 trichlorobiphenyl. High dechlorination, of 87%, was achieved
under mild conditions. 2,4,5 trichlorobiphenyl was treated at 40–150°C with calcium hydroxide, sodium hydroxide and sulfur
in mixed water and organic solvents. Dechlorination products were biphenyl, orthohydroxy biphenyls, 2,4 dihydroxy biphenyls
and biphenyl-2-thiol. Dichlorobiphenyl and orthochlorobiphenyls were found in trace quantities. We found that carbon particles
catalysed dechlorination by substitution reactions and suppressed further chlorination. Dechlorination at biphenyl ortho position
was preceded by substitution reaction by hydroxyl and thiol ions. Moreover, in the absence of carbon, dechlorination was lower
and substituted products were not observed. These findings may be applied at industrial scale to remediate PCB-contaminated
waste. |
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