| Abstract
| - Quinones are present in trace amounts in natural organicmatter. The addition of thiol compounds to quinonesproduces reactive electron-transfer species that may beimportant for the transformation of chlorinated hydrocarbonsunder sulfate-reducing conditions. This study systematicallyinvestigated the transformation of carbon tetrachloride(CCl4) in homogeneous aqueous solutions containing quinonesas electron-transfer mediators and thiol compounds asbulk reductants. The thiol compounds, including sodiumhydrosulfide (NaHS) and cysteine, were found to effectivelytransform CCl4. The transformation of CCl4 followed pseudo-first-order kinetics, and the pseudo-first-order rateconstants (kobs) were (3.24 ± 0.46) × 10-7 and 1.04 ×10-7 s-1, respectively, when solutions contained NaHS andcysteine alone. Addition of quinone compounds, includinganthraquinone-2,6-disulfonate (AQDS), benzoquinone(BQ), juglone (JQ), naphthoquinone (NQ), lawsone (LQ),and menadione (MQ), increased the transformation rate andefficiency of CCl4. The kobs values for CCl4 transformationin the presence of quinones were 2.6−71 times higher thanthose for the thiol compounds alone. The enhancementefficiency followed the order JQ > NQ > BQ ≫ AQDS >LQ > MQ. Spectroscopic studies indicated that thequinone compounds generated various active electron-transfer mediators to transfer electrons from the bulkreductants to CCl4. BQ and NQ produced mercaptoquinonesas active redox mediators that significantly enhancedthe transformation rate of CCl4 in the presence of NaHS.The addition of thiol reductants produced large amounts ofAQDS semiquinone radical as the electron shuttle. Inaddition, MQ and LQ were reduced by NaHS to givehydroquinone, which slightly enhanced the transformationefficiency of CCl4. These results clearly indicate that theenhanced efficiency of quinones for the transformation ofchlorinated hydrocarbons is specifically related to theproduced reactive species. Mercaptoquinone is a moreactive mediator than either semiquinone or hydroquinonefor transferring electrons in a reducing environmentcontaining thiol reductants.
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