"Insights from isotope fractionation on limitations of micropollutant biodegradation—evaluating mass-transfer limitation and biodegradation in a bench-scale aquifer"
Anthropogenic organic micropollutants threaten groundwater quality worldwide. Identifying the bottlenecks of biodegradation and finding solutions to eliminate organic micropollutants in groundwater is a daunting task. Two possible limiting factors have been hypothesized as the bottlenecks for biodegradations under oligotrophic conditions: (1) Mass-transfer limitation from the bulk phase to the bioavailable phase, particularly the mass-transfer process through the bacterial cell membrane, and (2) physiological limitation (e.g., downregulation of functional genes). Compound-specific isotope analysis (CSIA) creates new windows to glimpse mass-transfer limitation and physiological limitation during biodegradation. Decreased isotope fractionation due to mass-transfer limitation through the bacterial cell membrane during organic contaminant biodegradation by suspended bacteria has been observed in chemostats at low concentrations (< Monod constant). However, there is a lack of studies mimicking a realistic aquifer setting where bacteria attached to sediments are exposed to oligotrophic environments. This thesis evaluates the limitations of micropollutant biodegradation by applying CSIA in a bench-scale aquifer in three sequential experiments.
Firstly, isotope effects during aqueous diffusion/dispersion were investigated to address a potentially confounding factor in data interpretations (section 2.1). Secondly, in order to identify mass-transfer limitations at low concentrations, distributions of isotope fractionations, concentrations, and biomass along a concentration cross gradient were verified by reactive transport simulations (section 2.2). Finally, to explore the remediation strategies for biodegradation enhancement and pinpoint the potential limiting factors in the degradation-stimulated system, the response of the contaminant-degradation activity to a priming strategy (a transient inlet supply of elevated contaminant concentration) and temporal flow fluctuation were characterized by applying CSIA and concentration analysis.