Oil-Mineral Flocculation and Settling Dynamics in Saline Water

Cohesive particles in aquatic systems play an important role in determining the eventual fate of spilled oil via the generation of Oil-Mineral Aggregates (OMAs). Series of laboratory experiments have been conducted aiming at filling the knowledge gap regarding how cohesive clay particles influence the accumulation of petroleum in the coastal and marine environments through sedimentation. OMAs have been successfully created in a stirring jar with saline water (salinity≈35 PPT), crude oil and two types of the most common natural cohesive minerals, Bentonite and Kaolinite clay. The magnetic stirrer maintains a constant level of high flow turbulence with turbulence dissipation ε estimated to be about 0.02 m²·s^-3. We observe distinctly different OMAs due to these two mineral types in digital microscopy with oil-Kaolinite forming droplets OMAs and oil-Bentonite forming flakes OMAs (see Figure 1). A high-resolution video instrument LabSFLOC-2 (Laboratory Spectral Flocculation Characteristics, 2.0 version) has been used to obtain OMAs characteristics such as floc numbers(N), floc size(D), settling velocity(W_s), effective density(ρ_e) and fractal dimension(f_n). The results show that adding oil into Kaolinite flocs dramatically decreases the W_s and ρ_e especially for the flocs > 160 μm. However, for the pure Bentonite or mixed Kaolinite-Bentonite flocs, adding oil contrarily increases the settling velocity and effective density also largely in the flocs > 160 μm. The f_n shows all around 2.4 ~ 2.6 except for the large oil-Kaolinite showing a value around 2.0 due to the incorporation of oil droplets.

Different OMA structures, resulting density and settling velocity can be explained by the stickiness of the mineral particles. Generally, it has been found that the stickiness (directly links to the mineral flocculation rate) of Bentonite clay is much higher than Kaolinite clay particles. Low cohesion Kaolinite particles generally attach to oil droplets without significantly changing the droplets. High cohesion bentonite clay can directly absorb oil into the aggregate structure and the droplets can no longer exist, which leads to a denser floc, especially in microflocs. This study suggests that Bentonite participation can be more complicated for numerical modeling of OMAs and more future investigation is warranted.