Natural emulsion gels and lecithin-based sorbents: A potential treatment method for organic spills on surface waters

Natural sorbents for the removal of a free toluene phase from water were made using lecithin, food-grade oils (canola, sunflower, safflower and corn oil) and water, or lecithin, hydroxystearic acid (HSA) and canola oil, or lecithin, HSA and soy wax. Lecithin (5 g), food-grade oils (5 mL), and water (5 mL) formed emulsions, in which clusters of water droplets were dispersed in oil, as probed using confocal microscopy and cryo-scanning electron microscopy (cryo-SEM). These emulsions were gel-like, with shear elastic moduli (G’) greater than the shear viscous moduli (G”). Emulsion gels obtained with lecithin, canola oil and water could absorb up to 47% (volume based) of toluene freely floating on deionised water in 20 h. G’ increased from 1621 ± 203 Pa to 6372 ± 168 Pa upon mixing with up to 20% of toluene (volume based), and decreased to 2130 ± 376 Pa and to 846 ± 60 Pa with 33% and 47% toluene (volume based), respectively. However, the gels remained cohesive enough to be recovered from water even with 47% toluene, facilitating it removal. The gels lost instead cohesiveness with 67% of toluene. Similar trends were observed with all other food-grade oils used and in the presence of CaCl₂ salt. With 35 g/L NaCl and 33% toluene gels less cohesive than with DI or CaCl₂ salt. Optical microscopy showed that lecithin formed thick, heterogeneous films at the oil-water interface in the absence of toluene. Toluene addition to lecithin in canola decreased the interfacial tension (as probed with the pendant drop method), rendered the lecithin interfacial films homogeneous and improved the miscibility between the oil and the water phase, as observed through confocal microscopy. Toluene addition also affected the lamellar swelling of lecithin bilayers, which were studied using Wide and Small Angle X-ray Scattering (WAXS and SAXS). Addition of HSA or HSA and soy wax to the emulsion gels increased their shear viscoelastic moduli before and after mixing with toluene. Gels comprised of HSA (0.5 g), lecithin (4.5 g), canola oil (2.5 mL) and water (5 mL) or of HSA (0.5 g), lecithin (3 g), soy wax (1.5 g) and water (5 mL) could absorb approximately 10 mL of toluene (i.e. 67% of the lecithin gel volume) in 20 h. Addition of HSA also increased the cohesiveness of gels with toluene and 35 g/L NaCl.