Impact of mixing and resting times on the droplet size distribution and the petroleum hydrocarbons’ concentration in diluted bitumen-based water-accommodated fractions (WAFs)

Abstract

The preparation of Water-accommodated Fractions (WAFs) and chemically enhanced WAFs (CEWAFs) are essential for evaluating oil toxicity. The Chemical Response to Oil Spills: Ecological Research Forum (CROSERF) method was widely adopted, with variables (e.g., mixing time, oil loading, etc.) being continuously changed among research groups, which limits the cooperation in this area. Herein, we conducted WAF and CEWAF experiments using two loadings of diluted bitumen (Dilbit): 1 g/L and 10 g/L. For the CEWAF, the dispersant to oil ratio was 1:20. We investigated the impact of three mixing durations (18 h, 42 h, and 66 h) and two resting times (6 h and 24 h) on the droplet size distribution (DSD) and accommodated oil concentration. This would be highly beneficial for analyzing toxicity from oil spills, especially when considering the toxic effect of both suspended oil droplets and dissolved hydrocarbons. The DSD results and oil chemistry analysis showed that at a low oil loading concentration (1 g/L), both WAFs and CEWAFs had the same DSD, with an average d50 (volume median diameter) of 3.38 ± 0.70 μm and 3.85 ± 0.63 μm, respectively. At a high oil loading concentration (10 g/L), the WAFs had an average d50 of 3.69 ± 0.52 μm, showing no correlation with mixing and resting time. The DSD of CEWAFs increased significantly at 42 h mixing and 24 h resting time, with oil concentration reaching equilibrium after 42 h mixing. Therefore, WAFs appears to require only 18 h mixing and 6 h resting, while it is recommended to have 42 h mixing and 24 h resting for CEWAFs at high dilbit oil loading concentrations.