Complete lab-scale detoxification of groundwater containing 1,2-dichloroethane.

The suspected carcinogenic solvent 1,2-dichloroethane (1,2-DCA) is the most abundant chlorinated C(2) groundwater pollutant on earth. However, an efficient reductive in situ detoxification technology for this compound is not known. Detoxification results of 1,2-DCA with the recently isolated anaerobic bacterium Desulfitobacterium dichloroeliminans strain DCA1 are presented. First, it was verified that strain DCA1 could compete for nutrients in the presence of fast-growing Enterococcus faecalis; the latter was observed in the enrichment culture from which strain DCA1 was isolated. Subsequently, lab-scale bioaugmentation of the strain to groundwater containing 40 mg 1,2-DCA/l indicated that the bacterium has strong metabolic activity under prevailing environmental conditions, converting the pollutant into ethene. During exponential growth, the maximum 1,2-DCA dechlorination rate exceeded 350 nmol chloride released per min per mg total bacterial protein. Growth and dechlorination within the community with autochthonous bacteria indicated a high competitive strength of strain DCA1. Interestingly this dechlorination process does not produce any toxic byproducts, such as vinyl chloride. Furthermore, complete groundwater detoxification happens within a short time-frame (days) and is robust in terms of bacterial competition, oxygen tolerance, high ionic strength, and pH range.

The quest for microbial reductive dechlorination of C (2) to C (4) chloroalkanes is warranted.

C (2) to C (4) chloroalkanes have been used for a wide range of industrial applications. Consequently, numerous leaks to the environment have occurred. It is generally observed that the lower chlorinated members of the group, containing 1-3 chlorine atoms, accumulate in environments where reductive conditions prevail. Their half-lives under these conditions often exceed several decades. To date, successes in rapid and complete in situ reductive dechlorination have only been obtained with tetrachloroethene (PCE) and trichloroethene (TCE), but not with chloroalkanes. Since the key-player PCE- and TCE-dechlorinating bacteria involved have been studied, these organisms could be used as very efficient tools for low-cost in situ bioremediation. Except for one 1,2-dichloroethane-dehalorespiring bacterium with limited application possibilities and a recent isolate which partly dechlorinates some polychloroethanes, all bacterial reductive conversions of C (2) to C (4) chloroalkanes are based on slow, mostly incomplete and poorly controllable cometabolic dechlorinations. Furthermore, metals such as Fe(0) cannot dechlorinate most lower-chlorinated C (2) to C (4) alkanes. Hence, pump and treat, or aerobic degradation are the applied technologies, although they are expensive and time-intensive. However, energetic consideration of chloroalkane dechlorination suggests that metabolizing anaerobes may exist. Isolation and characterization of these organisms is warranted in order to develop cost-efficient, controlled, fast and complete in situ remediation technologies.

Reductive biodegradation of 1,2-dichloroethane by methanogenic granular sludge: perspectives for in situ remediation.

Granular methanogenic sludge was able to dechlorinate 1,2-dichloroethane (1,2-DCA) to ethene in UASB reactors. Ethanol served as the sole carbon and energy source. The average dechlorination rate measured on the basis of ethene production varied between 1.7 and 2.1 micromol 1,2-DCA/(h.gVSS) (46.7 and 57.4 mg/L.d). In order to elucidate the microbial origin of this bioconversion, enrichment cultures of the methanogenic sludge were prepared with different carbon and electron sources: pyruvate, lactate, H2/CO2, ethanol and formate. Dithiothreitol (DTT) was the strong reductant in order to increase the negative redox potential in the media. A homo-acetogenic gram-positive strain could be isolated in the presence of formate. 16S rRNA of the isolated strain showed that the bacterium was closely related (99.7%) to Acetobacterium wieringae. The strain also grew on pyruvate, lactate, H2/CO2 and ethanol, although dechlorination rates of 1,2-DCA were at least 5 times higher when formate was the (only) electron source. Average conversion rates reached 3 micromol/(h.g(dry cells)) and appeared t relate to cometabolic biocatalysis on the corrinoid centers of the homo-acetogenic strain. Some perspectives of anaerobic in situ bioremediation of groundwater polluted with chloroethanes are presented.