Assessment of aerobic biodegradation of lower-chlorinated benzenes in contaminated groundwater using field-derived microcosms and compound-specific carbon isotope fractionation

Biodegradation of lower chlorinated benzenes(tri-, di-and monochlorobenzene) was assessed at a coastal aquifer contaminated with multiple chlorinated aromatic hydrocarbons. Field-derived microcosms, established with groundwater from the source zone and amended with a mixture of lower chlorinated benzenes, evidenced biodegradation of monochlorobenzene(MCB) and 1,4-dichlorobenzene(1,4-DCB) in aerobic microcosms,whereas the addition of lactate in anaerobic microcosms did not enhance anaerobic reductive dechlorination. Aerobic microcosms established with groundwater from the plume consumed several doses of MCB and concomitantly degraded the three isomers of dichlorobenzene with no observable inhibitory effect. In the light of these results, we assessed the applicability of compound stable isotope analysis to monitor a potential aerobic remediation treatment of MCB and 1,4-DCB in this site. The carbon isotopic fractionation factors(ε) obtained from field-derived microcosms were-0.7‰ ± 0.1 ‰ and-1.0‰ ± 0.2 ‰ for MCB and1,4-DCB, respectively. For 1,4-DCB, the carbon isotope fractionation during aerobic biodegradation was reported for the first time. The weak carbon isotope fractionation values for the aerobic pathway would only allow tracing of in situ degradation in aquifer parts with high extent of biodegradation. However, based on the carbon isotope effects measured in this and previous studies, relatively high carbon isotope shifts(i.e., Δδ13C > 4.0 ‰) of MCB or 1,4-DCB in contaminated groundwater would suggest that their biodegradation is controlled by anaerobic reductive dechlorination.

Bioelectrochemically-assisted degradation of chloroform by a coculture of Dehalobacter and Dehalobacterium

Using bioelectrochemical systems(BESs)to provide electrochemically generated hydrogen is a promising technology to provide electron donors for reductive dechlorination by organohalide-respiring bacteria.In this study,we inoculated two syntrophic dechlorinating cultures containing Dehalobacter and Dehalobacterium to sequentially transform chloroform(CF)to acetate in a BES using a graphite fiber brush as the electrode.In this co-culture,Dehalobacter transformed CF to stoichiometric amounts of dichloromethane(DCM)via organohalide respiration,whereas the Dehalobacterium-containing culture converted DCM to acetate via fermentation.BES were initially inoculated with Dehalobacter,and sequential cathodic potentials of-0.6,-0.7,and -0.8 V were poised after consuming three CF doses(500 μM)per each potential during a time-span of 83 days.At the end of this period,the accumulated DCM was degraded in the following seven days after the inoculation of Dehalobacterium.At this point,four consecutive amendments of CF at increasing concentrations of 200,400,600,and 800 μM were sequentially transformed by the combined degradation activity of Dehalobacter and Dehalobacterium.The Dehalobacter 16S rRNA gene copies increased four orders of magnitude during the whole period.The coulombic efficiencies associated with the degradation of CF reached values>60% at a cathodic potential of -0.8 V when the degradation rate of CF achieved the highest values.This study shows the advantages of combining syntrophic bacteria to fully detoxify chlorinated compounds in BESs and further expands the use of this technology for treating water bodies impacted with pollutants.