Pathway-Dependent Isotope Fractionation during Aerobic and Anaerobic Degradation of Monochlorobenzene and 1,2,4-Trichlorobenzene

Abstract

Stable carbon isotope fractionation is a valuable tool for monitoring natural attenuation and to establish the fate of groundwater contaminants. In this study, we measured carbon isotope fractionation during aerobic and anaerobic degradation of two chlorinated benzenes: monochlorobenzene (MCB) and 1,2,4-trichlorobenzene (1,2,4-TCB). MCB isotope fractionation was measured in anaerobic methanogenic microcosms, while 1,2,4-TCB isotope experiments were carried out in both aerobic and anaerobic microcosms. Large isotope fractionation was observed in both the anaerobic microcosm experiments. Enrichment factors (ε) for anaerobic reductive dechlorination of MCB and 1,2,4-TCB were -5.0‰ ± 0.2‰ and -3.0‰ ± 0.4‰, respectively. In contrast, no significant isotope fractionation was found during aerobic microbial degradation of 1,2,4-TCB. The cleavage of a C-Cl σ bond occurs during anaerobic reductive dechlorination of MCB and 1,2,4-TCB, while no σ bond cleavage is involved during aerobic degradation via dioxygenase. The difference in isotope fractionation for aerobic versus anaerobic biodegradation of MCB and 1,2,4-TCB can be explained by the difference in the initial step of aerobic versus anaerobic biodegradation pathways.

Compound-Specific Isotope Analysis (CSIA) Application for Source Apportionment and Natural Attenuation Assessment of Chlorinated Benzenes

“…Kaschl et al [24,27] demonstrated MCB anaerobic biodegradation at two contaminated sites, while [28] investigated anaerobic biodegradation of dichlorobenzenes (DCBs) and MCB by interpreting CSIA results on the basis of an isotope mass balance approach. On the contrary, almost no isotope carbon fractionation for MCB and 1,2,4 TCB during aerobic biodegradation was demonstrated [23,24] -results that were confirmed by Liang et al [25]. Thus, the lack of carbon isotope fractionation limits the use of carbon-CSIA for MNA applications in contaminated environments under oxic conditions.…”

“…Although a reactive transport model is needed for a conclusive δ 13 C MCB data interpretation, some further reasoning can be inferred. An enriched δ 13 C MCB value of −28.3‰ compared to δ 13 C DCBs values ranging from −29.3‰ to −31.0‰ leads to hypothesizing further anaerobic MCB degradation to benzene in the vicinity of S70T [25]. On the contrary, where δ 13 C MCB values remained depleted compared to δ 13 C DCBs values (S80 and S67T), local anaerobic biodegradation of MCB is not likely to be occurring.…”

“…Yet, if contaminants undergo a preferential transport along the plumes and/or when biodegradation processes are taking place, the δ 13 C sum interpretation is not an easy task [31]. However, assuming that the carbon pool is conservative during anaerobic biodegradation (supposing DCBs → MCB → benzene) [32,33] and that aerobic processes are not inducing significant fractionations, particularly for CBs [23][24][25], δ 13 C sum values should retain their signature along the flowpath, regardless of the occurrence of degradation. Eventually, with further degradation and mineralization of benzene to CO 2 , δ 13 C sum could show an enrichment along the plume [32,33].…”

“…In contrast to compounds such as chlorinated ethenes, for which there is a well-developed literature on CSIA applications, only few studies are available for CBs. Significant carbon enrichment factors were obtained in laboratory experiments under anaerobic conditions, ranging from −0.8‰ to −6.3‰ for several CBs [22][23][24][25][26], which allows for the use of CSIA for tracing in situ biodegradation of CBs in contaminated anoxic aquifers. Kaschl et al [24,27] demonstrated MCB anaerobic biodegradation at two contaminated sites, while [28] investigated anaerobic biodegradation of dichlorobenzenes (DCBs) and MCB by interpreting CSIA results on the basis of an isotope mass balance approach.…”

Ecophysiology and environmental distribution of organohalide-respiring bacteria

“…0.2 ml headspace samples were used for benzene measurement as described before [345], and 1 ml liquid samples were used for the detection of CBs. Liquid samples were extracted with 400 µl hexane by overnight shaking at 600 rpm followed by 100 analysis was performed with NG-Tax, an in-house pipeline [346] in which operational taxonomic units (OTUs) were assigned to taxonomy using uclust [347] in an open reference approach against the SILVA 16S rRNA gene reference database [348].…”