Abstract
Asphaltenes generally aggregate, then precipitate and deposit on the surfaces of environmental media (soil, sediment, aquifer, and aquitard). Previous studies have recognized the importance of asphaltene aggregates on the wettability of aquifer systems, which has long been regarded as a limiting factor that determines the feasibility and remediation efficiency of sites contaminated by heavy oils. However, the mechanisms/factors associated with precipitant effects on asphaltene aggregates structure, and how the precipitant effects influence the wettability of surfaces remain largely unknown. Here, we observe the particle-by-particle growth of asphaltene aggregates formed at different precipitant concentrations. Our results show that aggregates for all precipitant concentrations are highly polydisperse with self-similar structures. A higher precipitant concentration leads to a more compacted aggregates structure, while precipitant concentration near to onset point results in a less compact structure. The well-known Smoluchowski model is inadequate to describe the structural evolutions of asphaltene aggregates, even for aggregation scenarios induced by a precipitant concentration at the onset point where the Smoluchowski model is expected to explain the aggregate size distribution. It is suggested that aggregates with relative high fractal dimensions observed at high precipitant concentrations can be used to explain the relatively low Stokes settling velocities observed for large asphaltene aggregates. In addition, asphaltene aggregates with high fractal dimensions are likely to have high density of nanoscale roughness which could enhance the hydrophobicity of interfaces when they deposit on the sand surface. Findings obtained from this study advance our current understandings on the fate and transport of heavy oil contaminants in the subsurface environment, which will have important implications for designing and implementing more effective and efficient remediation technologies for contaminated sites.
Highlights
Crude oil is a common soil contaminant usually originating from a variety of sources, including petroleum spills and fuel storage tank leakages
Prior to the aggregation study, preliminary experiments were conducted to determine the instantaneous onset point of precipitation of asphaltene which is defined as the concentration of n-heptane φ that has to be added to the model oil to observe insoluble asphaltenes via optical microscopy in less than 10 min of mixing with the precipitant
We observed asphaltene aggregates formation at different precipitant concentrations using in situ observations combined with advanced statistical analyses
Summary
Crude oil is a common soil contaminant usually originating from a variety of sources, including petroleum spills and fuel storage tank leakages. Upon its release to the ground surface, crude oil has the propensity to migrate into the subsurface where it can persist as hydrophobic contaminants and occasionally leach to nearby ground and surface waters. Driven by significant adverse environmental impacts and risks, the detection and remediation of crude oil contaminated soils and aquifers have yielded a large amount of research interest in the past decades [1,2,3,4,5]. Solvent extraction is a well-accepted method for the removal of hydrophobic organic contaminants from soil. Water 2020, 12, 2116 describes the process and solvents used for crude oil contaminated soil remediation in detail [6]. Solvent extraction can trigger changes in the solvation characteristics of crude oil potentially undermining the efficiency of the remediation process
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