Study On Influencing Factors,Mechanism And Kinetic Model Of Aggregation Between Coastal Spilled Oil And Suspended Sediment

With the increase of global demand for petroleum products and the scale of marine oil transportation,offshore oil spill accidents occur frequently.In the offshore area with high load of suspended matter,the spilled oil will break up and disperse into small oil droplets under the action of surface waves,then interact with the suspended sans to form the oil-suspended sediment aggregate（OSA）.OSA is easy to settle down due to the high density,which poses a major damage to the living environment of marine benthos.Therefore,it is of great significance to explore the formation and influencing factors of OSA for further assessment of the settlement and effect of coastal oil spill.In this study,mesoscale simulation experiments were carried out in a wave tank,and the aggregation between coastal spilled oil and suspended sediment was systematically studied according to the technical route:screening influencing factors-exploring action mechanism-establishing mathematical models.The effects of wave energy and water temperature on the dispersion of five oils were firstly investigated.After that,oils with moderate dispersion effect were selected,and the effects of oil properties,sediment concentration and size and environmental factors on the formation of OSA were evaluated.Combined with experimental data,prediction model of OSA density was established and key properties of sediment which have relatively significant effect on OSA formation was determined using the partial least squares regression analysis.Based on these,the possible interaction forces between oil droplets and suspended particles were analyzed using analysis technologies such as confocal microscopy,and the contribution degrees of different forces were qualitatively analyzed.Finally,kinetic model of OSA formation reflecting the mechanism of oil-sediment aggregation and properties of suspended sediment was established.The main results are as follows:（1）The light and medium crude oil could easily break up into the water column,and rapidly migrate around under the action of waves.The ODE continuously increased over time and tended to be stable after reaching the maximum.The dispersion of heavy crude oil and fuel oil were both poor.The corresponding droplet size was large and the oil refloating was obvious.The ODE rapidly increased with time until reaching the maximum and then gradually decreased.There was a negative exponential function between the kinematic viscosity(υ_oil)and equilibrium ODE（ODE_e）,while the oil-water interfacial tension(IFT_ow)showed a negative linear relationship with ODE_e.Compared with water temperature,wave energy has a more significant effect on the oil dispersion.With the attenuation of wave energy,the submergence of floating oil became shallower and the size of dispersed oil increased,leading to the obvious buoyancy of oil droplets.Regardless of the wave energy and oil type,the volume mean diameter（VMD）of oil droplets all sharply dropped within 30 min and continued to decrease afterwards,tending to be stable after reaching the minimum.（2）The higher IFT_owof oil,the lower proportion of effective aggregation between oil droplets and natural sediment.When wave energy was large enough and water temperature was higher than 25℃,the higher specific gravity of oil droplets andυ_oil,the higher proportion of oil settlement after aggregating with suspended sediment,the maximum could reach 23%.When water temperature was between 15℃and 25℃,the equilibrium oil sinking efficiency（OSE_e）first rose and then declined asυ_oilincreased.When water temperature was below 10℃,OSE_ewas always low no matter what kind of oil,less than 10%overall.When water temperature was constant,the maximum oil sinking efficiency(E_max)showed allometry with the increase of energy dissipation rate（ε）.There was a critical ε for different oils.Whenεwas below this threshold,E_maxwas less affected byε.Beyond this threshold,E_maxsharply increased with the increasingε.When wave energy was higher,more multi-droplet OSAs containing large size of trapped oil tended to form.Meanwhile,the particle content and density of OSA also significantly increased.（3）For the natural sediment with different concentration and size,OSE of Roncador all rapidly increased within 60 min.Since then,the growth rate slowed down and tended to be stable,the maximum could reach 15%.Under higher initial concentration of particle(C_par0),the equilibrium time of OSA formation significantly reduced,and the density,oil content and size of trapped oil of OSA increased.Therefore,the threat to the benthos was magnified.When C_par0exceeded 100 mg/L,the particles acted as chemical dispersant which could appropriately reduce IFT_ow.This effect became more obvious with the continuous increase of C_par0,which also promoted the fragmentation and dispersion of oil into the water column.Small particles were easier to catch more dispersed oil and combine with oil droplets faster.However,the flaky OSA tended to form by the large particles,and the corresponding density and size was also high.（4）Combined with the above test data,prediction models of OSA density,E_maxand M_{par E}were established through the partial least square analysis.Results demonstrated that wave energy,particle concentration and size were the three significant factors determining the OSA density.C_par0was also the most important factor influencing the oil and particle content of OSA.Cation exchange capacity（CEC）of particle also had a large impact on the capture of oil and self-agglomeration of sediments by affecting the electromotive properties.Particle size played a fourth leading role in governing the oil content of OSA but had little effect on the particle content.However,the hydrophobicity has nonsignificant effect on the two processes.（5）Based on the above experiments,the process and mechanism of the aggregation between oil droplets and suspended particles were discussed and analyzed by using modern analytical techniques such as three-dimensional electron microscopy and electrochemistry.Results showed that the OSA formation mainly depends on the van der Waals force,electrostatic double-layer force and projectile force under the action of wave.Van der Waals force drives the physical adsorption between oil and particles,and the adsorption effect mainly depends on the specific surface area.The electrostatic double layer force indirectly promotes the combination of oil and particles through the directional migration of cations caused by the interaction of some groups on the particle surface or diffusion double layer,which largely depends on the cation exchange capacity.The particle size and hydrophobicity determine the collision strength between oil and particle,and then affect the projectile force.The higher the wave energy,the greater contribution of projectile force to the formation of OSA.With the increase of seawater salinity,electrostatic double layer force and van der Waals force gradually dominated the OSA formation.（6）The classical attenuation kinetics of dispersed oil was improved and applied for the nonlinear fitting analysis of OSE,solving the particle characteristic coefficient（α）under different conditions.Multiple linear regression equation betweenαand sediment properties affecting the micro force of OSA formation such as cation exchange capacity,specific surface area and size was established.On this basis,the obtained oil dispersion model and OSA density prediction model were combined,and the final kinetic model of OSA formation was established,which can accurately predict the variation OSE over time under high C_par0.The above results can provide technical support for maritime and marine environmental protection departments to evaluate the sedimentation and hazard scope of coastal oil spill.