Reductive Dechlorination of α-, β-, δ-, and γ-Hexachlorocyclohexane Isomers by Hydroxocobalamin in the Presence of Either Dithiothreitol or Titanium(lll) Citrate as Reducing Agents

摘要

The effect of the reducing potential on the reductive dehalogenation of the different HCH (hexachlorocyclohexane) isomers has not yet been studied. In the present study, the potential for dehalogenation of α-, β-, δ-, and γ-HCH isomers by the dithiothreitol (DTT) and titanium(lll) citrate (reducing potential at pH 7, -0.33 and -0.48 V, respectively), with and without the addition of hydroxocobalamin was investigated. In the presence of DTT without catalyst, there was no disappearance of any of the HCH isomers studied after 1 h of treatment. However, disappearance of the γ- and a-HCH isomers was observed during the same time period when titanium(lll) citrate was used as the reductant in the absence of catalyst (62.9 and 16.6% disappearance, respectively). Addition of the hydroxocobalamin to the DTT system favored mainly the disappearance of γ- and a-HCH (92.9 and 30.8% disappearance after 1 h, respectively); disappearance of δ-HCH and β-HCH was small (11.9%) or negligible, respectively. Addition of the hydroxocobalamin to the titanium(lll) citrate system favored the degradation of all HCH isomers under study: γ- and α-HCH completely disappeared to undetectable levels (<0.1%) after 1 and 2 min, respectively; degradation of (δ-HCH and β-HCH was slower than that of the other two isomers, although they had almost completely disappeared (99.9 and 99.6% disappearance, respectively) after 10 and 60 min, respectively. The order of disappearance, γ-HCH > α-HCH > δ-HCH > β-HCH, coincided with a decreasing order of the axially positioned Cl atoms of these isomers (considering their thermodynamically most stable configuration). This study is the first description of the rapid degradation of δ- and β-HCH under abiotic conditions, and the results demonstrate the effect of the reducing potential on the reductive dehalogenation of HCH isomers.