Reductive Dechlorination of Carbon Tetrachloride in Homogenous Solutions using Quinone Moiety as Electron Mediator

• Main Authors: Huai-chih Chiang, 姜懷之
• Other Authors: Ruey-an Doong
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/47051492692235067196

碩士 === 國立清華大學 === 原子科學系 === 91 === The dechlorination of carbon tetrachloride (CT) in homogeneous aqueous solutions containing quinone compounds as electron mediators and thiol compounds as bulk reductants was investigated. The use of thiol compounds including sodium hydrosulfide and cysteine as bulk reductant can effectively dechlorinate CT. The dechlorination of CT followed pseudo first-order kinetics and the pseudo first-order rate constant (kobs) was 0.051 d-1 with 5 mM NaHs and 0.009 d-1 with cysteine. Addition of quinone compounds including AQDS, benzoquinone (BQ), naphthoquinone (NQ), lawsone (LQ) and menadione (MQ) could significantly increase the rate and efficiency of CT dechlorination. The enhanced effect followed the order of NQ > BQ >> AQDS > MQ~LQ. A 2- to 40-fold increase in kobs relative to the solutions in the absence of quinone compounds was observed. Spectroscopic results including EPR, FTIR, UV-Vis and LC-MS showed that the selected quinone compounds can form various active electron mediators for electron transfer and can be categorized into three groups. The first group included BQ and NQ that could produce mercaptoquinone as active redox mediators. The second one included AQDS and the quinone moiety can be reduced to semiquinones. Moreover, lawsone and menadione can be reduced to hydroquinone by thiol compound to slightly enhance the dechlorination efficiency of CT. Environmental factors such as pH value, concentrations of thiol and quinone compounds can significantly influence the dechlorination efficiency and the rate of CT and exhibited positive correlations between the kobs for CT dechlorination and environmental factors. Moreover, two humic acids, Aldrich and Yang-ming mountain humic acid were shown to be reduced to hydroquinone moieties as active electron mediators by hydrosulfide ion and the kobs for CT dechlorination were 1.6 — 2.4 times higher than that without humic acid. Since humic substances contain large quinone moiety and sulfur compounds and are abundant under anaerobic conditions, results obtained in this study indicate the potentials of utilizing humic substances for effective electron mediators.