A new model for the biodegradation kinetics of oil droplets: application to the Deepwater Horizon oil spill in the Gulf of Mexico

Abstract

Oil biodegradation by native bacteria is one of the most important natural processes that can attenuate the environmental impacts of marine oil spills. Existing models for oil biodegradation kinetics are mostly for dissolved oil. This work developed a new mathematical model for the biodegradation of oil droplets and applied the model to estimate the time scale for oil biodegradation under conditions relevant to the Deepwater Horizon oil spill in the Gulf of Mexico. In the model, oil is composed of droplets of various sizes following the gamma function distribution. Each oil droplet shrinks during the microbe-mediated degradation at the oil-water interface. Using our developed model, we find that the degradation of oil droplets typically goes through two stages. The first stage is characterized by microbial activity unlimited by oil-water interface with higher biodegradation rates than that of the dissolved oil. The second stage is governed by the availability of the oil-water interface, which results in much slower rates than that of soluble oil. As a result, compared to that of the dissolved oil, the degradation of oil droplets typically starts faster and then quickly slows down, ultimately reaching a smaller percentage of degraded oil in longer time. The availability of the water-oil interface plays a key role in determining the rates and extent of degradation. We find that several parameters control biodegradation rates, including size distribution of oil droplets, initial microbial concentrations, initial oil concentration and composition. Under conditions relevant to the Deepwater Horizon spill, we find that the size distribution of oil droplets (mean and coefficient of variance) is the most important parameter because it determines the availability of the oil-water interface. Smaller oil droplets with larger variance leads to faster and larger extent of degradation. The developed model will be useful for evaluating transport and fate of spilled oil, different remediation strategies, and risk assessment.

Biodegradation of dispersed Macondo crude oil by indigenous Gulf of Mexico microbial communities

“…The introduction of chemical dispersants reduced the surface tension of the oil and created droplets smaller than those formed by natural dispersions or releases from natural seeps. The small size of the oil droplets formed also increased the surface area available for microbial colonization, and significantly enhanced the biodegradation; one of the most important process for the removal of the oil released from the DWH spill and retained in the water column (Atlas and Hazen, 2011;Meckenstock et al, 2014;Vilcaez et al, 2013).…”

The influence of droplet size and biodegradation on the transport of subsurface oil droplets during the Deepwater Horizon spill: a model sensitivity study

“…With the advent of technologies that allow oil recovery from hundreds of meters below the sea surface, a better understanding of oil droplet formation, evolution, transport, and degradation in deep waters is necessary to predict the fate and transport of subsurface oil spills (Yapa et al 2012). In addition, this information is necessary to fully understand the impact of dispersants applied at the wellhead which greatly affects droplet sizes , Vilcáez et al 2013, Nagamine 2014 and to better constrain the estimate of the mass of initially suspended droplets in deep hydrocarbon plumes, which is one of the largest uncertainties in the hydrocarbon budget of the Deepwater Horizon (DH) spill (Ryerson et al 2012).…”

Dynamic behavior of a floating water droplet with hydrocarbon-degrading bacteria

“…On water surface, the spilled oil is commonly dispersed into small droplets, which are then decomposed naturally by microorganisms. However, model prediction [2] and lab-scale reactors [3] have indicated that this process can take up to few months, during which the oil droplets can have adverse impacts on marine habitats.…”

“…Furthermore, the oil droplets can concentrate in certain area of the marine system and signify the environmental impacts locally. The effective bio-degradation process also requires nutrients [2] which are limited in water bodies. Most critically, dispersing process relies on dispersants, which can be more toxic than the oils themselves [6].…”