Biological Enhancement of Tetrachloroethene Dissolution and Associated Microbial Community Changes

Abstract

A bench-scale study was performed to evaluate the enhancement of tetrachloroethene (PCE) dissolution from a dense nonaqueous phase liquid (DNAPL) source zone due to reductive dechlorination. The study was conducted in a pair of two-dimensional bench-scale aquifer systems using soil and groundwater from Dover Air Force Base, DE. After establishment of PCE source zones in each aquifer system, one was biostimulated (addition of electron donor) while the other was biostimulated and then bioaugmented with the KB1 dechlorinating culture. Biostimulation resulted in the growth of iron-reducing bacteria (Geobacter) in both systems as a result of the high iron content of the Dover soil. After prolonged electron donor addition methanogenesis dominated, but no dechlorination was observed. Following bioaugmentation of one system, dechlorination to ethene was achieved, coincident with growth of introduced Dehalococcoides and other microbes in the vicinity and downgradient of the PCE DNAPL (detected using DGGE and qPCR). Dechlorination was not detected in the nonbioaugmented system over the course of the study, indicating that the native microbial community, although containing a member of the Dehalococcoides group, was not able to dechlorinate PCE. Over 890 days, 65% of the initial emplaced PCE was removed in the bioaugmented, dechlorinating system, in comparison to 39% removal by dissolution from the nondechlorinating system. The maximum total ethenes concentration (3 mM) in the bioaugmented system occurred approximately 100 days after bioaugmentation, indicating that there was at least a 3-fold enhancement of PCE dissolution atthis time. Removal rates decreased substantially beyond this time, particularly during the last 200 days of the study, when the maximum concentrations of total ethenes were only about 0.5 mM. However, PCE removal rates in the dechlorinating system remained more than twice the removal rates of the nondechlorinating system. The reductions in removal rates over time are attributed to both a shrinking DNAPL source area, and reduced flow through the DNAPL source area due to bioclogging and pore blockage from methane gas generation.

Experimental Evaluation and Mathematical Modeling of Microbially Enhanced Tetrachloroethene (PCE) Dissolution

“…the aqueous phase, and (ii) the transformation of PCE to compounds with higher aqueous phase solubilities (e.g., DCEs) (1). Bioenhancement of NAPL dissolution has been observed experimentally in sand column and box experiments (19,21,25). Further experiments with batch cultures have demonstrated the ability of some dechlorinating cultures to transform a mixed (PCE/tridecane) NAPL (17,26) and to achieve complete PCE-DNAPL dissolution (20).…”

“…Several studies with pure cultures (see Table S1, Supporting Information) and with mixed dechlorinating consortia (e.g., [17][18][19][20][21][22][23][24] have shown that bacterial PCE dechlorination occurs at or near saturated PCE concentrations. For instance, Desulfuromonas michiganensis strain BB1 was reported to dechlorinate at saturated PCE concentrations and to grow in the presence of PCE DNAPL (4).…”

“…Several mixed culture studies suggest that PCE dechlorination occurs at high, even saturated, concentrations of PCE (e.g., [17][18][19][20][21][22][23][24]. The ability of these mixed cultures to dechlorinate at elevated PCE concentrations may be due to (i) the presence of dechlorinating strains acclimated to high PCE concentrations, or (ii) dechlorinators that exhibit increased PCE tolerance as members of mixed microbial communities (e.g., biofilms).…”

Transcriptome of Geobacter uraniireducens growing in uranium-contaminated subsurface sediments

“…Molecular and culturing studies have demonstrated that Geobacter species are the predominant microorganisms in a wide diversity of subsurface environments in which dissimilatory Fe(III) reduction is an important process for the degradation of both natural organic matter and organic contaminants or for the stimulation of in situ bioremediation of metal-contaminated subsurface environments (Rooney-Varga et al, 1999;Anderson et al, 2003;Lovley et al, 2004;North et al, 2004;Coates et al, 2005;Lin et al, 2005;Sleep et al, 2006;Holmes et al, 2007;Winderl et al, 2007). By designing an isolation medium that mimics subsurface conditions, it has become possible to isolate Geobacter species that have 16S rRNA gene sequences that are identical or highly similar to the 16S rRNA sequences that predominate in Fe(III)-reducing subsurface environments Shelobolina et al, 2008;Holmes et al, 2008a).…”

Microbially enhanced dissolution and reductive dechlorination of PCE by a mixed culture: Model validation and sensitivity analysis

“…In the past two decades, numerous remediation technologies have been developed to treat DNAPL source zones; however, the ability of a single technology to completely remove or destroy all DNAPL mass and reduce dissolved-phase contaminant concentrations below drinking water standards is limited. Among potential in situ remediation technologies, microbial reductive dechlorination has emerged as an attractive DNAPL source zone remedy (Da Silva et al, 2006;Sleep et al, 2006;Schaefer et al, 2010), and as a source zone polishing step to control residual contaminant concentrations following aggressive physicochemical treatment (Mravik et al, 2003;Ramsburg et al, 2004;Christ et al, 2005). During source zone bioremediation, microbial activity lowers dissolved-phase contaminant concentrations, thereby increasing the driving force for contaminant dissolution from the DNAPL to the aqueous phase, a process commonly referred to as bioenhanced dissolution (Yang and McCarty, 2000;Cope and Hughes, 2001;Yang and McCarty, 2002;Adamson et al, 2003;Sleep et al, 2006;Glover et al, 2007;4 Amos et al, 2008).…”