Effect of humic acids on bitumen films at the oil-water interface and on emulsion stability: Potential implications for groundwater remediation

This study investigates the effect of humic acids (HA) on the stability of toluene/water emulsions in the presence of bitumen. Bottle tests showed that emulsions were unstable with bitumen or HA added separately, but were stable when HA and bitumen were simultaneously present in the system. The stability of the mixed HA/bitumen system was explained on the basis of the synergistic interactions between HA and bitumen, which produced interfacial films with high compressional and shear visco-elastic moduli, as probed with the oscillating barrier method using a Langmuir trough and with an interfacial shear rheometer, respectively. The shear rigidity of HA and bitumen films was measured at pH 3, 5.5 and 10 and was found to exhibit a marked pH-dependence. The shear visco-elastic moduli were greatest at pH = 3, at which the electrostatic repulsion between HA and bitumen was likely lower than at pH 5.5 and 10. In addition to changing the bitumen's films rigidity, HA affected the morphology of interfacial bitumen films, as probed in situ using Brewster Angle Microscopy at the toluene-water interface.