With the increase of global demand for petroleum products and maritime transportation, many petroleum hydrocarbon contaminants entered the marine environment through various oil spill accidents (He et al., 2018; Qi et al., 2018). Because of the toxic effect of oil on the aquatic ecology, it is necessary to clean it up in time once the oil spill accident occurs (French et al., 2004). The application of oil dispersant is a traditional means of oil spill removal (Prince and Butler, 2016). During the Deepwater Horizon spill in 2010, more than 2 million gallons of oil dispersant were sprayed onto the oil using aircraft and ships (McNutt et al., 2012; Hansel et al., 2015). The dispersant contain amphiphilic molecules (“surfactants”) in a solvent base (Fernandes et al., 2019), and the surfactants break up the oil slick into micron-sized droplets, which are further carried below the water surface by waves. The oil is degraded by various micro-organisms present in the water column (Clayton et al., 1993; Li et al., 2008).
However, in estuaries and coastal waters, due to the high concentration of particulate matter in the water column and the strong shear effect of waves, dispersed oil droplets are easy to aggregate with suspended particulate matter to form the oil-mineral aggregate (OMA), thus leading to the sinking of spilled oil (Fitzpatrick et al., 2015; Silva et al., 2019). Under appropriate conditions, up to 20% of the spilled oil can interact with particulate matter, and the oil content of the formed OMA after spraying oil dispersant accounts for 65% of the total spilled oil (Bandara et al., 2011). As polycyclic aromatic hydrocarbons (PAHs) persist much longer in the sediment phase than at the sea surface (Harayama et al., 1999), these oil-bearing aggregates will exist on the seabed for a long time, resulting in excessive petroleum hydrocarbon contaminant, especially toxic PAHs, posing a serious threat to both the benthic communities and marine environment (Gao et al., 2019; Qi et al., 2021; Suneel et al., 2021). Common oil spill responses are mostly applicable to the floating oil but ineffective for the sunken oil (IMO, 2012; Usher, 2009). Remarkably, the sunken oil cannot be easily monitored by typical visual observation or remote sensing techniques (Yu et al., 2021; Hammoud et al., 2019), posing challenges to the cleaning operations (Jacqueline, 2008).
Except for the physical adsorption force, there may be a more potent chemical force between oil droplets and particles (Lambert and Variano, 2016), thus the morphological structure of OMA is usually stable. Zhao et al. (2017) analyzed the 3D structure of OMA using a confocal microscope imaging technology and found that particles could be embedded into the oil droplets at depth of 2–10 µm. The factors that affect the OMA formation are very complex, such as oil characteristics (concentration, droplets size, viscosity, density and polar component), particulate matter characteristics (concentration, particle size, density, hydrophobicity, composition and zeta potential) and environmental conditions (temperature, salinity and mixing energy) (Loh et al., 2014; Gao et al., 2018). Many scholars have studied the above influencing factors on OMA formation and summarized some empirical results. Hua et al. (2018) proved that oil viscosity is a key factor affecting the aggregation process of oil and suspended particulate matter, lower oil viscosity was conducive to the stable formation of OMA. Moreira et al. (2015) found that polar components (asphaltenes and resins) in oil can improve the adhesion between oil droplets and suspended particles, thus contributing to the formation of OMA. Yu et al. (2019a) found that under the same mass concentration, more oil droplets can be trapped by smaller particles, and larger particles tend to interact more quickly with oil droplets. Strong hydrophobicity of solid particles can promote their affinity with the oil droplets, making them easier to form OMA (Loh and Yim, 2016). Unfortunately, it remains unknown which of the above factors has a more significant effect on the oil-mineral aggregation.
Besides, few studies have focused on the formation of OMA in the presence of chemical dispersant and related conclusions have been still a matter of controversy. Khelifa et al. (2008) proposed that in the marine environment where the mixing energy and suspended particulate concentration are usually low, the use of oil dispersant would be beneficial to the formation of OMA. The surfactants in the dispersant can significantly reduce the oil-water interfacial tension and viscosity, making the spilled oil disperse into a large number of emulsified droplets and maintain stable (Cai et al., 2017). Li et al. (2007) found that the synergistic effect of dispersant and mineral fines enhances the transfer of oil from the surface downward into the water column. However, some studies have found that excessive dispersant can inhibit the formation of OMA (Yu et al., 2019a; Sørensen et al., 2014; Page et al., 2002), but the specific cause has not been confirmed. Furthermore, previous studies often ignored the effect of dispersant on the physicochemical properties of oil and minerals, so this study will start from these aspects to fill the knowledge gap.
The overall goal of this study is to investigate the effect of chemical dispersant on the surface properties of minerals and spilled oil, and determine the mechanism of OMA formation in the presence of chemical dispersant. The specific objectives are to : 1) examine the variation of zeta potential, cation exchange capacity (CEC) and contact angle (CA) of minerals, viscosity of oil and oil-water interfacial tensions (IFTow) with dispersant-to-oil ratios (DOR), 2) evaluate the dispersion and sedimentation of oil with medium viscosity in a wave tank at different DOR, and 3) analyze the influencing factors of the OMA formation.