Mass-Transfer-Limited Biodegradation at Low Concentrations - Evidence from Reactive Transport Modeling of Isotope Profiles in a Bench-Scale Aquifer

Fengchao Sun, Adrian Mellage, Mehdi Gharasoo, Aileen Melsbach, Xin Cao, Ralf Zimmermann, Christian Griebler, Martin Thullner, Olaf A. Cirpka, Martin Elsner

Research output: Contribution to journalArticlepeer-review

22 Scopus citations

Abstract

Organic contaminant degradation by suspended bacteria in chemostats has shown that isotope fractionation decreases dramatically when pollutant concentrations fall below the (half-saturation) Monod constant. This masked isotope fractionation implies that membrane transfer is slow relative to the enzyme turnover at μg L-1 substrate levels. Analogous evidence of mass transfer as a bottleneck for biodegradation in aquifer settings, where microbes are attached to the sediment, is lacking. A quasi-two-dimensional flow-through sediment microcosm/tank system enabled us to study the aerobic degradation of 2,6-dichlorobenzamide (BAM), while collecting sufficient samples at the outlet for compound-specific isotope analysis. By feeding an anoxic BAM solution through the center inlet port and dissolved oxygen (DO) above and below, strong transverse concentration cross-gradients of BAM and DO yielded zones of low (μg L-1) steady-state concentrations. We were able to simulate the profiles of concentrations and isotope ratios of the contaminant plume using a reactive transport model that accounted for a mass-transfer limitation into bacterial cells, where apparent isotope enrichment factors *ϵ decreased strongly below concentrations around 600 μg/L BAM. For the biodegradation of organic micropollutants, mass transfer into the cell emerges as a bottleneck, specifically at low (μg L-1) concentrations. Neglecting this effect when interpreting isotope ratios at field sites may lead to a significant underestimation of biodegradation.

Original languageEnglish
Pages (from-to)7386-7397
Number of pages12
JournalEnvironmental Science and Technology
Volume55
Issue number11
DOIs
StatePublished - 1 Jun 2021

Keywords

  • 2,6-dichlorobenzamide
  • CSIA
  • GC-IRMS
  • bioavailability
  • flow-through system
  • reactive-transport model

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