The Role of Bio-physical Cohesion on Oil-Mineral Flocculation and Settling in Seawater

Bio-physical cohesive particles, such as Extracellular polymeric substances (EPS) and mineral clay in aquatic systems, play an important role in determining the transport of spilled oil and its eventual fate in aquatic environments where suspended sediment and microbial activities are significant and diverse. A series of stirring jar tests have been conducted to understand the multiple-structures characteristics of the oil-mineral aggregates (OMAs) as well as EPS-oil-mineral aggregates (EPS-OMAs), and their difference with two types of natural common mineral (Kaolinite and Bentonite) aggregates. OMAs and EPS-OMAs have been successfully generated in the laboratory with artificial seawater, Texas crude oil (Dynamic viscosity: 7.27×10^-3 Pa·s at 20 °), natural clay minerals (Bentonite and Kaolin clay) and Xanthan gum powder (a proxy of natural EPS) using a magnetic stirrer, which provides a high level of homogenous flow turbulence (Turbulence dissipation rate ε estimated to be about 0.02 m²·s^-3). The high-resolution microscopy results show that EPS, kaolinite and bentonite lead to distinguished oil floc structures respectively because of the different stickiness character of EPS and mineral clay particles. With relatively low cohesion, kaolinite particles (Flocculation rate (R_f): 0.2131) tend to attach oil droplets surface and become dominant in microflocs (<160 μm) in the mixture sample. Bentonite particles (R_f: 0.3487) with much higher cohesion can absorb with oil droplets and dominant in Macroflocs (>160 μm). EPS with the highest biological cohesion can bond multiple small oil droplets and form web structure trapping oil and minerals. With the addition of EPS into the mineral mixture, flocculation rate increased evidently both in kaolinite and bentonite samples. In the EPS, minerals and oil mixture, oil prefers more to aggregate with bentonite than kaolinite, and EPS contributes to form more extra-large size flocs with low density. The settling velocity of the OMAs and EPS-OMAs are highly related the mineral types and EPS component. A complete discussion and synthesis will be presented in the conference.