Potential of stable isotope analysis to deduce anaerobic biodegradation of ethyl tert-butyl ether (ETBE) and tert-butyl alcohol (TBA) in groundwater: a review

Abstract

Understanding anaerobic biodegradation of ether oxygenates beyond MTBE in groundwater is important, given that it is replaced by ETBE as a gasoline additive in several regions. The lack of studies demonstrating anaerobic biodegradation of ETBE, and its product TBA, reflects the relative resistance of ethers and alcohols with a tertiary carbon atom to enzymatic attack under anoxic conditions. Anaerobic ETBE- or TBA-degrading microorganisms have not been characterized. Only one field study suggested anaerobic ETBE biodegradation. Anaerobic (co)metabolism of ETBE or TBA was reported in anoxic microcosms, indicating their biodegradation potential in anoxic groundwater systems. Non-isotopic methods, such as the detection of contaminant loss, metabolites, or ETBE- and TBA-degrading bacteria are not sufficiently sensitive to track anaerobic biodegradation in situ. Compound- and position-specific stable isotope analysis provides a means to study MTBE biodegradation, but isotopic fractionation of ETBE has only been studied with a few aerobic bacteria (εC -0.7 to -1.7‰, εH -11 to -73‰) and at one anoxic field site (δ²H-ETBE +14‰). Similarly, stable carbon isotope enrichment (δ¹³C-TBA +6.5‰) indicated TBA biodegradation at an anoxic field site. CSIA and PSIA are promising methods to detect anaerobic ETBE and TBA biodegradation but need to be investigated further to assess their full potential at field scale.

Keywords: Compound-specific stable isotope analysis; Gasoline ether oxygenates; Isotopic fractionation; Natural attenuation; Position-specific stable isotope analysis; Tertiary alcohols.

Fig. 1

Summary of non-isotopic and isotopic techniques to detect ETBE and TBA biodegradation. The blue line in the figure indicates the groundwater level. The orange ovals indicate the contaminated area

Fig. 2

Stable isotope approaches to assess anaerobic biodegradation at ETBE- and/or TBA-contaminated sites (adapted from Vogt et al. 2016). The red rectangles indicate the knowledge gaps in the use of ETBE and TBA stable isotope approaches to determine biodegradation. Biodegradation of ETBE and TBA can be determined using CSIA and PSIA. PSIA gives additional information on the behaviour of atoms within the molecule

Fig. 3

Rayleigh equation and corresponding ¹³C MTBE degradation graph