Dechlorination of PCE

Tetrachoroethylene (perchloroethylene, PCE) is the only chlorinated ethene that resists aerobic biodegradation. This compound can be dechlorinated to less- or nonchlorinated ethenes only under anaerobic conditions. This process, known as reductive dehalogenation, was initially thought to be a co-metabolic activity. Recently, however, it was shown that some bacteria species can use PCE as terminal electron acceptor in their basic metabolism i.e., they couple their growth with the reductive dechlorination of PCE.35 Reductive dehalogenation is a promising method for the remediation of PCE-contaminated sites, provided that the process is well controlled to prevent the buildup of even more toxic intermediates, such as the vinyl chloride, a proven carcinogen. 

Zhang W, Wang C. (1997). Nanoscale metal particles for dechlorination of PCE and PCBs. Environmental Science and Technology 31 2154-2156. 

In this reaction, H2 is the electron donor which is oxidized and the chlorinated solvent is the electron acceptor which is reduced. In addition to other electron donors (also called fermentation products) that have been identified, the importance of hydrogen as an electron donor in the reductive dechlorination of PCE and TCE was recognized in studies by Holliger et al. (1993), Gossett and Zinder (1996), Smatlak et al. (1996), and Ballapra-gadaetfit/. (1997). 

Okeke BC, YC Chang, M Hatsu, T Suzuki, K Takamizawa (2001) Purification, cloning, and sequencing of an enzyme mediating the reductive dechlorination of tetrachloroethene (PCE) from Clostridium bifermen-tans DPH-1. Can J Microbiol AT. 448-456. 

A comparison had been made of fractionation during the dechlorination of tetrachlo-roethene by Sulfurospirillum multivorans and Desulfitobacterium sp. strain PCE-S in laboratory experiments (Nijenhuis et al. 2005). Isotope fractionation in growing cultures was 1.0052 for Desulfitobacterium sp. and only 1.00042 for Sulfurospirillum multivorans, whereas fractionation was greater in crude cell extracts from both strains. It was concluded that caution should therefore be exercised in applying fractionation factors to the evaluation of in situ bioremediation. 

Vitamin B12 catalyzed also the dechlorination of tetrachloroethene (PCE) to tri-chloroethene (TCE) and 1,2-dichloroethene (DCE) in the presence of dithiothreitol or Ti(III) citrate [137-141], but zero-valent metals have also been used as bulk electron donors [142, 143]. With vitamin B12, carbon mass recoveries were 81-84% for PCE reduction and 89% for TCE reduction cis-l,2-DCE, ethene, and ethyne were the main products [138, 139]. Using Ni(II) humic acid complexes, TCE reduction was more rapid, leading to ethane and ethene as the primary products [144, 145]. Angst, Schwarzenbach and colleagues [140, 141] have shown that the corrinoid-catalyzed dechlorinations of the DCE isomers and vinyl chloride (VC) to ethene and ethyne were pH-dependent, and showed the reactivity order 1,1-DCE>VC> trans-DCE>cis-DCE. Similar results have been obtained by Lesage and colleagues [146]. Dror and Schlautmann [147, 148] have demonstrated the importance of specific core metals and their solubility for the reactivity of a porphyrin complex. 

The measurement technique was the crux of a paper by Acha et al.27 discussing the process of the dechlorination of aliphatic hydrocarbons. An ATR-FTIR sensor was developed to monitor parts per million (ppm) of trichloroethylene (TCE), tetrachloroethylene (PCE), and carbon tetrachloride (CT) in the aqueous effluent of a fixed-bed dechlorinating bioreactor. It was found that the best extracting polymer was polyisobutylene (PIB) as a 5.8 pm film. This afforded detection limits of 2, 3, and 2.5 mg/1 for TCE, PCE, and CT, respectively. The construction and operation of the measurement system are detailed in the paper. 

Dechlorination of 1,1,2,2-TCA can follow one of the three pathways in Figure 3 elimination and formation of a partially chlorinated ethylene sequential dechlorination or direct transformation to ethane. The first pathway is unlikely, given the lack of chlorinated ethylene intermediates and the fact that the transformation rate of 1,1,2,2-TCA is an order of magnitude lower than that of PCE, which has a similar rate to TCE, the DCEs and VC. (Schreier 1996) The recalcitrance of the DCA isomers to transformation (Lowry and Reinhard 1999 McNab and Ruiz 1998) implies that sequential dechlorination through DCA does not occur, but this pathway cannot be ruled out because the DCA tests were conducted using Pd/alumina catalysts, rather than the Pd/C used for the 1,1,2,2-TCA and 1,1,2-TCA tests. However, the available data (lack of chlorinated intermediates and the low reactivity of the DCA isomers) suggest that direct transformation is the most probable pathway. 

During the reductive dechlorination of chloroethylenes such as PCE and TCE, a suite of reduction products have been identified, including trans- and cis- dichloroethylenes, vinyl chloride, methane, ethane, ethylene, acetylene, and small amounts of C3-C6 hydrocarbons (Burris et al., 1995 Orth and Gillham, 1996). Dichloroethylene isomers and vinyl chloride can be further reduced in the ZVI system, forming ethene and ethane (for dichlorothylene) and ethene (for vinyl chloride) as main products (Roberts et al., 19% Alien-King et al., 1997 Deng et al., 1999). Less chlorinated ethylenes are reduced... 

In order to explain the degradation kinetics of TCE and PCE, for which the adsorption onto the nonreactive sites is significant (Burris et al., 1995), a two-site model is developed. The basic assumption for the single-site model, i.e., pre-adsorption equilibrium followed by reductive dechlorination, is still valid here. In addition, the two-site model assumes that there are both reactive and nonreactive sites on the iron surface, and while the adsorption of TCE and coadsorbate can occur on both types of sites, reductive dechlorination of TCE only takes place on the reactive sites. Coadsorbate is not involved in redox reactions. The reaction scheme for this model is ... 

Reductive dechlorination of chlorinated solvents in the ZVI system is a surface-mediated process. Adsorption of the chlorinated compounds takes place prior to the reduction, but the overall rate of reduction is limited by the electron transfer from the surface to the chlorinated compounds. The adsorption can occur on either reactive or nonreactive sites, while the reduction rate is directly proportional to the amount adsorbed onto the reactive sites. The proportion adsorbed onto reactive sites to the nonreactive sites is related to the nature of chlorinated compounds. Higher chlorinated ethylenes such as PCE and TCE are likely to have a larger portion going to the nonreactive sites compared to less chlorinated ethylenes like vinyl chloride. A two-site model incorporating the known observations related to the ZVI system has been developed and such a model can be applied to explain the adsorption and reduction of chlorinated solvents in the presence of competing coadsorbates. 

At the site of chemical laundry, a project with an electrokinetic biofence (EBIS ) started in April 2001. This fence has been installed to disperse nutrients in the ground and groundwater in order to enhance reductive dechlorinization of present perchloroethene (PCE), trichloroethene (TCE), cw-l,2-dichloroethene (C-DCE),... 

VOCs An electrokinetic biofence is designed to remove VOCs in a groundwater plume or to stop their downstream migration. The biofence will stimulate and create optimal conditions for biological activity to dechlorinate VOCs downstream of the fence. In Figures 17.6 and 17.7, the development in concentrations of PCE + TCE and C-DCE + VC is represented. Note that the building of the chemical laundry starts just 1 m south of the nutrient infiltration. 

From Figure 17.6, it can be observed that there is a decrease of PCE and TCE as these compounds are being dechlorinated and their degradation products C-DCE and VC are formed. This effect is shown in Figure 17.7, which depicts the... 

Due to the sequential biodegradation pathway (Fig. 19.1), a sequential electrolytic production of hydrogen and oxygen holds the opportunity to stimulate the reductive dechlorination of the higher chlorinated compounds (PCE, TCE) down-gradient the cathode in the first step and subsequently the oxidative degradation of the lower chlorinated metabolites (cA-DCE, VC) down-gradient the anode. The... 

Figure 19.9. Electrochemically enhanced microbial reductive dechlorination of perchloro-ethene (PCE) and oxidative degradation of vinyl chloride (VC). Stimulated biodegradation is indicated by a chloride increase during electrochemical treatment.

Overall, dechlorination is more rapid for highly chlorinated compounds than for compounds that are less chlorinated. Figure 23.1.5 qualitatively shows the reaction rate and required conditions for halorespiration of PCE to ethene. PCE (four chlorines) degrades the fastest under all anaerobic environments, while VC (a single chlorine) will degrade only under sulfate-reducing and methanogenic conditions, with a relatively slow reaction rate. 

The process that exhibited the highest potential for biodegradation of PCE and TCA was reductive dechlorination with 86% and 84% of the analyzed studies showing... 

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