Post-Formation of Oil Particle Aggregates: Breakup and Biodegradation

Biological/physical particles interact to degrade marine oil spills

After marine oil spills, suspended physical particles and extracellular polymeric substances (EPS) secreted by bacteria can aggregate with oil to form marine oil snow (MOS), which determines the vertical migration and biodegradation processes of the submerged oil. Here, we investigated the biodegradation of oil spills during the formation of MOS under different average energy dissipation rates (ε) and different ratios of particles. Furthermore, we elucidated the biodegradation mechanism of oil spills from a spatiotemporal perspective. The ε plays a major role (either promoting or inhibiting) in the biodegradation effect of oil spills, and there is a proportional threshold for biological/physical particles, which can regulate the ε's effect on degradation. The oil-water interfacial tension, the encapsulation of oil droplets by particles, hydrogen bonds, and the vertical distribution of oil droplets (suspended or deposited) will also jointly affect the particles threshold on this basis, thereby influencing the biodegradation of oil spills. When the proportion of XG exceeds the threshold (kaolinite: XG = 1:3 at 150 rpm and 1:1 at 200 rpm), the originally promotive role of ε on n-alkane degradation shifts to inhibition, while its inhibition impact on PAHs biodegradation shifts to enhancement, respectively. Notably, in nearshore and extreme environments (storm or strong wave conditions), particles are more conducive to the degradation of n-alkanes and PAHs, respectively. This study will further broaden the research perspective on the environmental behavior of marine oil spills in the presence of MOS and providing a theoretical basis for predicting the fate of oil spills in nearshore environments.

The vertical transport and fate of MPs-oil composite pollutants in nearshore environment

MPs-oil composite pollutants interact with particles to form MPs-oil-particles aggregates (MOPAs) in nearshore environment. In this study, we investigated vertical transport and fate of MPs-oil composite pollutants mediated by particles under various time scales, proposed and elucidated associated mechanisms. Majority of MPs with -CH2 suspended in water columns and particles with Si-O and O-H adsorbed MPs-oil composite pollutants in sediment phase, which caused differences in morphology structure and composition. The MOPAs with spherical or irregular three-dimensional in water columns can transport to sediment phase, resulting in more than 79 % lamellar MOPAs and more than 63 % oil in sediment phase. Besides, we demonstrated that degraded small-sized MPs-oil composite pollutants can resuspend into water columns. The mass of n-alkanes in sediment phase (< 45 μg) was lower than in water columns (< 120 μg) during degradation process. More importantly, during the intermediate stage of degradation, the size of oil droplets on surface of MPs decreased and particles trapped them to sediment phase, resulting in a V-shaped curve of mass changes of C14-C35 in water columns. Our research fills the gap in the field of MPs-oil composite pollutants in water columns and sediment phase, which can provide theoretical support for their disposal.

Oil spills in coastal regions of the Arctic and Subarctic: Environmental impacts, response tactics, and preparedness

Coastal areas of the Arctic and Subarctic are vulnerable to accidental oil spills, impacting the ecosystem, society, and economy. This article provides a comprehensive overview of oil spill pollution issues in cold regions, focusing on environmental impacts, oil transport and fate, coastal/shoreline response measures, and the state of current international policies and regulations. Numerous studies have described the potential effects of oil pollution (crude oil and refined products) on wildlife (invertebrates, fish, birds, and marine mammals) and coastal communities within the Arctic and Subarctic regions. The observed detrimental effects are influenced by the oil fate and transport processes, including physiochemical attenuation and biodegradation, natural dissolution/dispersion following point-source release (surface and subsurface), entrainment by sea ice, and stranding onto shorelines (in which the residual oil may be translocated). Measures such as natural attenuation, bioremediation, manual removal, in situ burning, and washing/flooding are available for spill response in coastal regions. Case studies in cold regions are illustrated for a better analysis of practical response methods, implying that shoreline cleanup operations in the Arctic and Subarctic are more challenging than those in more temperate and populated regions because of environmental and logistical challenges. Regarding preparedness, a number of national and international policies, regulations, and guidelines have been established to advance oil spill prevention and response measures within the Arctic and Subarctic regions. Based on the state of knowledge presented in this review, recommendations are made for future research on oil spill pollution in coastal regions of the Arctic and Subarctic.

Mechanism of nearshore sediment-facilitated oil transport: New insights from causal inference analysis

A quantitative understanding of spilled oil transport in a nearshore environment is challenging due to the complex physicochemical processes in aqueous conditions. The physicochemical processes involved in oil sinking mainly include oil dispersion, sediment settling, and oil-sediment interaction. For the first time, this work attempts to address the sinking mechanism in petroleum contaminant transport using structural causal models based on observed data. The effects of nearshore salinity distribution from the estuary to the ocean on those three processes are examined. The causal inference reveals sediment settling is the crucial process for oil sinking. Salinity indirectly affects oil sinking by promoting sediment settling rather than directly affecting oil-sediment interaction. The increase of salinity from 0‰ to 35‰ provides a natural enhancement for sediment settling. Notably, unbiased causal effect estimates demonstrate the strongest positive causal effect on the settling efficiency of sediments is posed by increasing oil dispersion effectiveness, with a normalized value of 1.023. The highest strength of the causal relationship between oil dispersion and sediment settling highlights the importance of the dispersing characteristics of spilled oil to sediment-facilitated oil transport. The employed logic, a data-driven method, will shed light on adopting advanced causal inference tools to unravel the complicated contaminants' transport.

Evaluating crude oil distribution tendencies in a multi-phase aquatic system: Effects of oil type, water chemistry, and mineral sediment

Planning for effective response to crude oil spills into water depends on evidence of oil behavior, including its tendency to become distributed throughout an aquatic system. An improved laboratory method is employed to quantitatively assess crude oil distribution among different layers that form after mixing within a multi-phase system of water and sediment. Mixtures of conventional crude oil or diluted bitumen with different water types in the presence or absence of mineral sediment are first mixed by a standard end-over-end rotary agitation protocol. After a settling period, each mixture's visibly distinct floating, surface oil (e.g., slick or emulsion), subsurface bulk water, and bottom layers are then separated. Finally, the masses of oil, water, and sediment constituting each layer are isolated, quantified, and compared. The novel results reveal how component properties affect oil distribution among layers to inform spill behavior models, risk assessments, and response plans, including applications of spill-treating agents.

Microbial remediation of oil-contaminated shorelines: a review

Frequent marine oil spills have led to increasingly serious oil pollution along shorelines. Microbial remediation has become a research hotspot of intertidal oil pollution remediation because of its high efficiency, low cost, environmental friendliness, and simple operation. Many microorganisms are able to convert oil pollutants into non-toxic substances through their growth and metabolism. Microorganisms use enzymes' catalytic activities to degrade oil pollutants. However, microbial remediation efficiency is affected by the properties of the oil pollutants, microbial community, and environmental conditions. Feasible field microbial remediation technologies for oil spill pollution in the shorelines mainly include the addition of high-efficiency oil degrading bacteria (immobilized bacteria), nutrients, biosurfactants, and enzymes. Limitations to the field application of microbial remediation technology mainly include slow start-up, rapid failure, long remediation time, and uncontrolled environmental impact. Improving the environmental adaptability of microbial remediation technology and developing sustainable microbial remediation technology will be the focus of future research. The feasibility of microbial remediation techniques should also be evaluated comprehensively.

Field fluorometers for assessing oil dispersion at sea

Oil dispersion by the application of chemical dispersants is an important tool in oil spill response, but it is difficult to quantify in the field in a timely fashion that is useful for coordinators and decision-makers. One option is the use of rugged portable field fluorometers that can deliver essentially instantaneous results if access is attainable. The United States Coast Guard has suggested, in their Special Monitoring of Applied Response Technologies (SMART) protocols, that successful oil dispersion can be identified by a five-fold increase in oil fluorescence. Here we test three commercial fluorometers with different excitation/emission windows (SeaOWL, Cyclops 7FO, and Cyclops 7F-G) that might prove useful for such applications. Results show that they have significantly different dynamic ranges for detecting oil and that using them (or similar instruments) in combination is probably the best option for successfully assessing the effectiveness of oil dispersion operations. Nevertheless, the rapid dilution of dispersed oil means that measurements must be made within an hour or two of dispersion, suggesting that one feasible scenario would be monitoring ship-applied dispersants by vessels following close behind the dispersant application vessel. Alternatively, autonomous submersibles might be pre-deployed to monitor aerial dispersant application, although the logistical challenges in a real spill would be substantial.

The Role of Bacteria in the Formation and Migration of Oil-Particle Aggregates (OPAs) after Marine Oil Spills and the Associated Mechanism

Oil spills interact with mineral particles to form oil-particle aggregates (OPAs), which promotes the oil's natural diffusion and biodegradation. We investigated the effect of bacteria on the formation and vertical migration of OPAs under different concentrations and types of particles and proposed and elucidated an oil-particle-bacteria coupling mechanism. The depth of particle penetration into oil droplets (13-17 μm) was more than twice that of the nonbacterial group. Oil that remained in the water column and deposited to the bottom decreased from 87% to 49% and increased from 14% to 15% at high/low concentration, respectively. Interestingly, the median droplet diameter showed a negative correlation (R² = 0.83) and positive correlation (R² = 0.60) at high/low concentration, respectively, with the relative penetration depth first proposed. We further demonstrated that bacteria increased the penetrating depth by a combination of reducing/increasing the interfacial tension, reducing the oil amount (C₁₇-C₃₈) in the OPAs, and increasing the particle width. These effects reduced the droplet size and ultimately changed the vertical migration of OPAs. Finally, we provided a simple assessment of the vertical distribution of OPAs in nearshore environments based on experimental data and suggested that the role of bacteria in increasing the depth of particles penetrating into the oil droplets should not be ignored. These findings will broaden the research perspective of marine oil spill migration.