Surface evolution of synthetic bilgewater emulsion

Bilgewater is a regulated shipboard produced waste stream that often contains oil-in-water emulsion. Fundamental knowledge of emulsion surface changes is required for improved wastewater treatment; however, limited information is currently available. We have reported the first surface characterization of synthetic bilgewater emulsions using time-of-flight secondary ion mass spectrometry (ToF-SIMS) coupled with optical microscopy. A Navy standard bilgewater solution consisting of a hydrocarbon and detergent mixture is used as the synthetic bilgewater emulsion model. Both fresh and aged emulsion samples are analyzed to determine their droplet size distributions (DSDs) and surface chemical composition. Our results show that fresh emulsions are largely mono-modal with hydrocarbon fragments as the main surface composition. Aged emulsions are also mono-modal with slightly larger size. Both SIMS spectral comparison and Principal Component Analysis (PCA) show that some surfactant components appear on the fresh emulsion surface while larger molecular weight components appear at the aged bilge droplet surface. Our results indicate that the oil-water interface evolves after emulsion droplet formation. More importantly, surface evolution not only changes the bilgewater DSD, but also alters the surface chemical composition and reactivity.