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

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.

Linking Increased Isotope Fractionation at Low Concentrations to Enzyme Activity Regulation: 4-Cl Phenol Degradation by Arthrobacter chlorophenolicus A6

“…aurescens TC1 or of 2,6-dichlorobenzamide degradation by Aminobacter sp. MSH1, the present study reveals a different pattern of microbial adaptation to low concentrations: bacteria downregulated their enzymatic activity instead of running into limited substrate supply. Hence, the pattern observed here reveals another “end-member behavior” on the scale of physiological adaptation of degrading bacteria that can be expected in low-energy environments.…”

Methodological Advances to Study Contaminant Biotransformation: New Prospects for Understanding and Reducing Environmental Persistence?

“…Isotope fractionation is fully expressed if enzymatic turnover is limiting but becomes masked if mass transfer into bacterial cells becomes rate-determining. With bacteria cultivated on atrazine in a chemostat, a drastic decrease in the level of isotope fractionation was observed at a threshold concentration of ∼50 μg/L atrazine, , and a similar decrease in isotope fractionation at lower concentrations was observed for the herbicide metabolite 2,6-dichlorobenzamide (BAM) in an inoculated sediment tank . First, this direct observation of mass transfer limitation demonstrated that diffusion into the cells through the cell membrane can become rate-limiting when concentrations decrease to levels close to the Monod/Michaelis–Menten constant of enzymatic breakdown − (Figure , right side).…”

“…With bacteria cultivated on atrazine in a chemostat, a drastic decrease in the level of isotope fractionation was observed at a threshold concentration of ∼50 μg/L atrazine, 73,74 and a similar decrease in isotope fractionation at lower concentrations was observed for the herbicide metabolite 2,6-dichlorobenzamide (BAM) in an inoculated sediment tank. 92 First, this direct observation of mass transfer limitation demonstrated that diffusion into the cells through the cell membrane can become rate-limiting when concentrations decrease to levels close to the Monod/ Michaelis−Menten constant of enzymatic breakdown 93−95 (Figure 1, right side). Second, it revealed that the onset of mass transfer limitation occurs at residual concentrations that are orders of magnitude higher than the nanogram per liter concentrations at which products of biotransformation are observed by LC-HRMS in the environment.…”

Accurate Removal of Toxic Organic Pollutants from Complex Water Matrices

“…The conditions during actual water and wastewater treatments are distinguished from lab conditions that are usually deionized water, distilled water and simple-component electrolyte solutions. The organic interferents of natural organic matter (NOM), effluent organic matter ( E f OM) and other representative organics of higher concentration are considered as the main interferents in surface water and effluent from wastewater plants. ,− ,− They are composed of thousands of different organic molecules, and their molecular weight distribution ranges from 10 2 to 10 5 Da. In addition, they contain both hydrophilic and hydrophobic components and have manifold functional groups of −OH, −NH 2 , −COOH, and complex rings.…”