Abstract
In the aftermath of oil spills in the sea, clouds of droplets drift into the seawater column and are carried away by sea currents. The fate of the drifting droplets is determined by natural attenuation processes, mainly dissolution into the seawater and biodegradation by oil-degrading microbial communities. Specifically, microbes have developed three fundamental strategies for accessing and assimilating oily substrates. Depending on their affinity for the oily phase and ability to proliferate in multicellular structures, microbes might either attach to the oil surface and directly uptake compounds from the oily phase, or grow suspended in the aqueous phase consuming solubilized oil, or form three-dimensional biofilms over the oil–water interface. In this work, a compound particle model that accounts for all three microbial strategies is developed for the biodegradation of solitary oil microdroplets moving through a water column. Under a set of educated hypotheses, the hydrodynamics and solute transport problems are amenable to analytical solutions and a closed-form correlation is established for the overall dissolution rate as a function of the Thiele modulus, the Biot number and other key parameters. Moreover, two coupled ordinary differential equations are formulated for the evolution of the particle size and used to investigate the impact of the dissolution and biodegradation processes on the droplet shrinking rate.
Highlights
After a natural or accidental release of crude oil in the sea, part of the oil ends up in the form of droplets moving through the seawater column
A few remarks are in order with regard to the theoretical modeling of each one of the three basic modes of biodegradation; that is, direct interfacial uptake, bioreaction in the bulk aqueous phase, and bioreaction in a biofilm formed around the droplet
A compound particle model of the core-shell type is developed for the microbial degradation of solitary oil microdroplets and takes into account three fundamental biodegradation modes, namely the direct interfacial uptake at the oil surface, the bioreaction in the bulk aqueous phase, and the bioreaction in a biofilm formed around the droplet
Summary
After a natural or accidental release of crude oil in the sea, part of the oil ends up in the form of droplets moving through the seawater column. It is anticipated that in the long run, most of the released oil in the sea is consumed by autochthonous oil-degrading microorganisms (bacteria, fungi, yeasts) that have developed appropriate machinery for accessing and assimilating oily substrates [8,9,10] In this way, crude oil enters as a nutrient into the marine food chain. Microbes grow suspended in the bulk aqueous phase and uptake-dissolved and micellar oil compounds This strategy has been observed, for example, in pure cultures of Gram-negative microbes, mainly of Pseudomonas species, that have a hydrophilic cell surface and produce biosurfactants of low molecular weight (e.g., rhamnolipids). Before proceeding with the mathematical analysis, certain key considerations on modeling the different biodegradation modes (Section 2.1) and a set of basic hypotheses (Section 2.2) are set forth
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