The influence of soil organic matter on the fate of trichloroethylene in soil

Trichloroethylene (TCE) contamination of soil and groundwater is extensive in Canada and the U.S. Under saturated soil conditions, TCE partitions with the organic fraction of soil and under anaerobic conditions is subject to biodegradation to dichloroethylene (DCE) isomers and vinyl chloride (VC). However, biodegradation is slow, and TCE often persists with accumulation of the DCE isomers and VC. The objective of the present study was to determine the effect of soil organic matter (SOM) on the fate of TCE in a saturated soil environment under anaerobic conditions. In natural soil, the presence of inorganic minerals as well as indigenous microorganisms greatly complicates the study of the concurrent processes of sorption and anaerobic biodegradation. To overcome such difficulties, a surrogate soil organic matter (SSOM) was used in conjunction with the pure strain Desulfomonile tiedjei to determine the effects of sorption and desorption on the anaerobic biodegradation of TCE. Composted sphagnum moss, sterilized so as to eliminate indigenous microbial activity, was implemented as a SSOM since it is representative of natural SOM. Desulfomonile tiedjei was utilized as a model of anaerobic consortia that exist in soil since it is a well studied microorganism. Results of the present study indicate that as the initial contact time ( i.e. aging) between the TCE and SSOM increases, TCE becomes increasingly resistant to biodegradation. The extent of biodegradation declined by 75% when the SSOM was aged with TCE for 30 d. This decline was attributed to TCE partitioning with SSOM and possibly due to complexation with dissolved organic matter (DOM). Results of this study indicate that soils contaminated with TCE for prolonged time periods may be less amenable to bioremediation efforts. Resistance of TCE to biodegradation was not paralleled by resistance to desorption for the 30 d aging that was studied. Results of longer term biodegradation tests (i.e. incubation times of up to 24 d) indicate that Desulfomonile tiedjei is capable of sustaining itself on the SSOM. It would appear that the bacteria degraded the SSOM itself to produce a compound with the following molecular formula: C₈H₂ON₄. In the longer term biodegradation tests, dechlorination of TCE was approximately 20 times greater in the presence of the SSOM compared to the case where the SSOM was absent. Since VC was never detected following biodegradation, it would appear that Desulfomonile tiedjei is a potential candidate for bioaugmentation for the purposes of remediation of contaminated soil. However, it should be noted that the major product of reductive dechlorination by Desulfomonile tiedjei was cis-1,2-DCE, and although it is considered to be less toxic than TCE or VC, it was also found to be less amenable to sorption by the SSOM compared to TCE. Hence, depending on the concentration of cis-1,2-DCE, a subsequent aerobic treatment may be necessary for complete conversion of this intermediate compound to CO₂.