Probing Silica-Kaolinite Interactions with Sum Frequency Generation Spectroscopy

Treating the oil sands tailings ponds is a major challenge because of the vast amounts of tailings and the need for a reliable treatment technique for releasing water and generating the highly consolidated material required for land reclamation. Treatment with chemicals such as lime (calcium (hydr)oxide) is a promising technology for tailings dewatering and consolidation, particularly at higher pH. Given that kaolinite and silica minerals are the main constituents of many oil sands, we have investigated the influence of lime and NaOH addition on the silica/aqueous kaolinite interface over the pH range 7.4-12.4 using vibrational sum frequency generation spectroscopy (SFG). With lime addition, at pH 12.0 and above we observe a complete disappearance of the vibrational features of the interfacial water molecules for planar silica in contact with an aqueous dispersion of kaolinite particles. A concurrent increase in the amount of adsorbed kaolinite on the silica surface at pH 12.0 and above is observed, shown in the increased intensity of the kaolinite SFG peak at 3694 cm^-1. This suggests that the absence of water features in the SFG spectra is associated with conditions that facilitate dewatering. With NaOH addition, however, the interfacial water SF intensity is still significant even under highly alkaline conditions despite the increase in adsorbed kaolinite at high pH. To better understand the SFG observations and get a deeper insight into the chemistry of the silica/aqueous kaolinite interface, we measure the ζ-potential on the planar silica/aqueous interface and kaolinite aqueous dispersions under the same pH conditions with NaOH and lime addition.