Novel Application of Laboratory Instrumentation Characterizing Mass Settling Dynamics due to Oil-Mineral-Microbial Interactions

Dr. Tian-jian Hsu (CACR professor), Dr. Leiping Ye (CACR postdoc), Dr. Andrew J. Manning (HR Wallingford, UK, professor) and James G. Holyoke (Civil Engineering Undergraduate student)
Sponsor: GoMRI (CSOMIO consortium)

Since the 2010 Deepwater Horizon oil spill disaster, the fate of leaked oil became one of the most concerning issues in the coastal ocean. So far, there around one-fifth of the oil fate is remains unaccounted for. A majority of the unaccounted oil is believed to sink onto the seabed through the formation of Marine Oil Snow or Oil-Mineral Aggregates. Hence, how the oil droplets interact with the Suspended Particle Materials (SPM) in the water column, the resulting formation of oil-SPM aggregates, and their settling velocity is an important science issue for a more comprehensive prediction and mitigation of oil spill in the coastal ocean.

As part of the Consortium for Simulation of Oil-Microbial Interactions in the Ocean (CSOMIO), the primary work we mainly focus on here at Center for Applied Coastal Research is the laboratory experimental studies of flocculation processes for oil-SPM aggregates in order to improved parameterizations in the regional-scale coastal models for oil-sediment-biogeochemical system. Oil-mineral flocs have been successfully created from a series of laboratory flocculation experiments with sea-water, crude oil and clay minerals (Bentonite and Kaolin clay) using a Magnetic Stirrer laboratory desk setup (Figure 1). In order to obtain high quality floc population data, a novel floc video instrument LabSFLOC-2 (Laboratory Spectral Flocculation Characteristics) (Figure 2) has been adopted for the first time to study oil mineral aggregates (OMAs). Meanwhile, a Vectrino-II profiling velocimeter (Figure 3) is adopted to measure flow turbulence statistics. The experimental results reveal that the OMAs can easily form in any oil, cohesive sediment and seawater mixtures. However, in bentonite clay cases, the oil flocs tend to be much larger those in kaolin clay cases, which could result in the significant variability of the floc sizes vs. flocs settling velocities (Figure 4). An integrated analysis of floc settling statistics (Figure 4 left) using LabSFLOC-2 and the structure of oil-flocs using digital microscope images allows a full understanding on the physics and its modeling.

Figure 1. Top: The overview of the desk experimental setup of the stir-jar tests in CACR. Bottom: The LabSFLOC-2 system for real-time observation of settling statistics of oil-flocs.

Figure 2. The Vectrino II setup with stir-jar system to monitor the flow velocities and turbulence.

Figure 3. Sample results from oil-bentonite mixture. The left panel shows the floc size and settling velocity for each individual floc with a total of 1592 flocs analyzed in this sample. The three diagonal lines present contours of constant Stokes equivalent effective density: pink = 1,600 kg·m-3, green = 160 kg·m-3 and red = 16 kg·m-3. The right graph shows a sample of OMAs image from the digital microscope camera (~ ×40).