Compound Particle Model for the Microbial Degradation of Solitary Oil Microdroplets Traveling through a Water Column*

A compound particle model is developed for the biodegradation of oil microdroplets moving through a water column. The compound particle consists of an oily core that is successively surrounded by a bioreactive skin of negligible thickness and another bioreactive shell of finite thickness. The bioreactive skin represents a layer of superhydrophobic microbes that uptake oil directly from the oily phase, whereas the bioreactive shell represents a distinct biofilm phase. The new model accounts for all three modes of biodegradation: interfacial uptake, bioreaction in the bulk aqueous phase, and bioreaction in a biofilm formed around the droplet. Equations have been established for the determination of the droplet shrinking rate and the evolution of the compound particle dimensions as functions of the drifting speed, the microbial kinetics, the biofilm thickness, the diffusivity and solubility ratios. Numerical analysis is used to extend the domain of validity of the model by taking into account the effects of multiple oil components, oxygen limitation and biofilm erosion. Computational findings will be discussed in relation to observations from the biodegradation of hexadecane droplets by Marinobacter sp. in a microfluidic setting.