Modelling the dynamics of pentachlorophenol bioavailability in column experiments

The aqueous-phase concentration of an organic pollutant found in a subsurface environment is often assumed to be its bioavailable concentration. However, the aqueous-phase concentration does not adequately reflect the dynamics of contaminant availability to microbes in flow-through systems. This paper assesses the effects of interacting processes such as sorption, biodegradation, and transport on contaminant bioavailability, and the fraction of the bioavailable contaminant that is taken up by microbes. The evolution of the bioavailable and uptake fractions is studied in two ways. Firstly, column experiments are conducted in which the introduced contaminant (pentachlorophenol, PCP) can flow through the columns, be consumed by microorganisms, or be sorbed by a solid matrix. Secondly, a phenomenological model (Flow/Sink/Reservoir model) that illustrates the dynamic nature of bioavailability and quantifies the uptake fraction is developed, based on a flow balance. Results show that after 60 h of sorption-limited bioavailability, the microorganisms induce desorption, so that the sorbed pool becomes bioavailable and bioavailability is limited by the PCP injection rate. A conclusion is drawn that the aqueous-phase concentration is a poor indicator of contaminant bioavailability to microbes.