Biologically Enhanced Dissolution of Tetrachloroethene DNAPL

Abstract

One major problem with tetrachloroethene (PCE) contamination of aquifers is its ability to form dense, nonaqueous-phase liquids (DNAPL), which can act as a persistent contamination source for decades. Batch studies were performed to determine the potential for biological reductive PCE dehalogenation at high concentration and the effect on competing microorganisms, including methanogens and homoacetogens. Results show that PCE dehalogenation can be obtained at saturation concentration (>0.9 mM). Also, trichloroethene was dehalogenated up to 2.26 mM, and no apparent inhibitory effect on dehalogenation was found with cis-1,2-dichloroethene (cDCE) and ethene at the highest tested levels of 0.66 and 1.05 mM, respectively. However, such high concentrations of PCE, cDCE, and ethene were inhibitory to methanogens, and high concentrations of PCE were inhibitory to homoacetogens. Such inhibition is highly beneficial as it greatly diminished the competition by methanogens and homoacetogens for added electron donors, including hydrogen, resulting in highly efficient substrate utilization for dehalogenation. PCE DNAPL dehalogenation in a column study required less than 1 g of the electron donor pentanol to dehalogenate 1 g of PCE to cDCE (<2 mol of pentanol/mol of PCE). Additionally, DNAPL dissolution rate was significantly enhanced when directly coupled with biological dehalogenation.

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

“…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). Efforts to model metabolic reductive dechlorination have included substrate fermentation, nonlinear dechlorination kinetics (with and without competitor, product, and selfinhibition), and growth of competitor populations (e.g., refs 22, 27-30).…”

“…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).…”

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

“…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). While dissolution enhancements may lead to short term increases in dissolved phase cis-DCE concentrations, the overall source longevity and associated cleanup times decrease, potentially resulting in reduced long-term risk.…”

“…Carr et al, 2000;Yang and McCarty, 2000;Cope and Hughes, 2001;Sleep et al, 2006;Amos et al, 2008Amos et al, , 2009Schaefer et al, 2010;Philips et al, 2011). This variability in dissolution enhancement has been attributed to several factors, including differing microbial populations, electron donor limitations, competitive and threshold inhibitions, and competitor populations (e.g., methanogens), as well as changing flow conditions due to gas formation, microbial growth, and experimental design.…”