BACKGROUND AND OBJECTIVES: Polychlorinated biphenyls are pervasive contaminants that are receiving attention worldwide. Due to their well-known propensity to have harmful impacts on both humans and the environment, polychlorinated biphenyls have been internationally banned for use. In this study, dechlorination of five polychlorinated biphenyl congeners, 2,2′,5,5′-tetrachlorobiphenyl, 2,2′,4,5,5′-pentachlorobiphenyl, 2,2′,3,4,4′,5′-hexachlorobiphenyl, 2,2′,3,4,4′,5′-hexachlorobiphenyl, 2,2′,4,4′,5,5′-hexachlorobiphenyl, and 2,2′,3,4,4′,5,5′-heptachlorobiphenyl, are evaluated. The chlorines from polychlorinated biphenyl congeners were removed using a heterogeneous catalyst synthesized via microwave-assisted impregnation of zinc metal onto pulverized shrimp shell waste.
METHODS: The five polychlorinated biphenyl congeners were dechlorinated through treatments combination of time (1–4 hours), heat (150–250 degree celsius), and catalyst proportion (1–5 percent weight/weight basis). The dechlorination trials followed the Box–Behnken experimental design and then analyzed using response surface methodology. Levels of the remaining polychlorinated biphenyl congeners were monitored by using a gas chromatograph equipped with an electron capture detector.
FINDINGS: The results of the trials demonstrated that among the five polychlorinated biphenyl congeners, only 2,2′,3,4,4′,5,5′-heptachlorobiphenyl did not respond to the provided treatments. Three congeners, namely, 2,2′,5,5′-tetrachlorobiphenyl, 2,2′,4,5,5′-pentachlorobiphenyl, and 2,2′,4,4′,5,5′-hexachlorobiphenyl, showed positive response, and one congener 2,2′,3,4,4′,5′-hexachlorobiphenyl showed negative response to the provided treatments. These findings suggested that chlorine attached to the para position of the biphenyls ring was easier to remove. The efficiency calculation of total polychlorinated biphenyl concentrations after treatments was approximately 25 percent. Such a low degree of effectiveness may be caused by the catalyst becoming inactive, either chemically through the deposition of chlorines that have been removed from the biphenyl ring or mechanically by the leaching of zinc from the surface of the pulverized shrimp shell due to insufficient mechanical strength. Optimization via response surface methodology produced optimal results for dechlorination at 150 degree celcius for 2.4 hours with 5 percent additional catalyst.
CONCLUSION: The total amount of polychlorinated biphenyls that remained after dechlorination was not significantly impacted by the treatment combination of temperature, duration, and weight of the catalyst. However, the treatments had significant effects on the chlorine removal at the para positions of the biphenyl ring. In this case 2,2′,5,5′-tetrachlorobiphenyl, 2,2′,4,5,5′-pentachlorobiphenyl, and 2,2′,4,4′,5,5′-hexachlorobiphenyl congeners have positive responses and 2,2′,3,4,4′,5′-hexachlorobiphenyl congener has a negative response. For polychlorinated biphenyl congeners having more than six chlorines, no chlorine removal was observed.
- Quenched pulverized shrimp shell waste can be used as a catalyst template for the dechlorination of polychlorinated biphenyls;
- Microwave-assissted metal impregnation can be used as a way for impregnating zinc metal onto the surface of quenched pulverized shrimp shells for heterogeneous catalyst synthesis;
- Polychlorinated biphenyls dechlorination can be triggered by zinc impregnated quenched pulverized shrimp shells to produce less chlorinated PCBs.
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