摘要
Quinones are present in trace amounts in natural organic matter. The addition of thiol compounds to quinones produces reactive electron-transfer species that may be important for the transformation of chlorinated hydrocarbons under sulfate-reducing conditions. This study systematically investigated the transformation of carbon tetrachloride (CCl <sub>4</sub> ) in homogeneous aqueous solutions containing quinones as electron-transfer mediators and thiol compounds as bulk reductants. The thiol compounds, including sodium hydrosulfide (NaHS) and cysteine, were found to effectively transform CCl <sub>4</sub> . The transformation of CCl <sub>4</sub> followed pseudofirst-order kinetics, and the pseudo-first-order rate constants (k <sub>obs</sub> ) were (3.24 ± 0.46) × 10 <sup>-7</sup> and 1.04 × 10 <sup>-7</sup> s <sup>-1</sup> , respectively, when solutions contained NaHS and cysteine alone. Addition of quinone compounds, including anthraquinone-2,6-disulfonate (AQDS), benzoquinone (BQ), juglone (JQ), naphthoquinone (NQ), lawsone (LQ), and menadione (MQ), increased the transformation rate and efficiency of CCl <sub>4</sub> . The k <sub>0bs</sub> values for CCl <sub>4</sub> transformation in the presence of quinones were 2.6-71 times higher than those for the thiol compounds alone. The enhancement efficiency followed the order JQ > NQ > BQ ≫ AQDS > LQ > MQ. Spectroscopic studies indicated that the quinone compounds generated various active electron-transfer mediators to transfer electrons from the bulk reductants to CCl <sub>4</sub> . BQ and NQ produced mercaptoquinones as active redox mediators that significantly enhanced the transformation rate of CCl <sub>4</sub> in the presence of NaHS. The addition of thiol reductants produced large amounts of AQDS semiquinone radical as the electron shuttle. In addition, MQ and LQ were reduced by NaHS to give hydroquinone, which slightly enhanced the transformation efficiency of CCI4. These results clearly indicate that the enhanced efficiency of quinones for the transformation of chlorinated hydrocarbons is specifically related to the produced reactive species. Mercaptoquinone is a more active mediator than either semiquinone or hydroquinone for transferring electrons in a reducing environment containing thiol reductants. © 2005 American Chemical Society.