Numerical Investigation of Hypoxic Effects on the Biodegradation of Oil Microdroplets

We present an extended compound particle model for the biodegradation of solitary oil microdroplets moving through a seawater column. The compound particle is of the core-shell type and consists of an oily core that is successively surrounded by an ultrathin bioreactive skin of oleophilic microbes and another bioreactive biofilm shell. The extended model accounts for the counter transport of dissolved oxygen and multiple oil components of varying bioavailability and toxicity within the biofilm that covers and degrades the oily droplet. The governing advection-diffusion-bioreaction PDEs are solved numerically to calculate the droplet shrinking rate as a function of the drifting speed, the non-linear microbial kinetics, the biofilm thickness, and other key parameters. A system of coupled ODEs is also formulated for the evolution of the compound particle dimensions. The impact of hypoxic and inhibitory conditions on the droplet biodegradation rate is quantified and critically discussed in connection with the potential for the - very slow, but feasible- anaerobic hydrocarbon degradation in oxygen-depleted marine waters and sediments.