Solubilization mechanism of diesel in saponin micelle for contaminated montmorillonite washing

Abstract

The biosurfactant was used to remove diesel from contaminated montmorillonite, which was enhanced by humic acid sodium salt (HASS) addition or microbubbles resulted from sonication. It was found that montmorillonite exhibited strong adsorption for saponin, causing the effective critical micelle concentration shift of saponin from 0.2 g L-1 to 0.6 g L-1. The washing efficacy (diesel removal) with 0.6 g L-1 saponin and 2% HASS (HASS/saponin ratio = 2%) increased 14.9 % compared to the control condition (without HASS), whereas the diesel removal with microbubble enhancement (25 min sonication) increased 15.2 % compared to the control condition. Bright-field microscopy, microscope, DLS, PSS, and TEM were used to measure the size of micelles and count micelle numbers. In the addition of 2% HASS, the mean diameter of saponin micelles decreased from 824.9 ± 12.3 nm to 91.3 ± 7.6 nm, and the number of micelles increased from 42,000 to 110,000 at saponin solution of 0.3 g L-1. The hydrophobic core volume increased accordingly. HASS molecule might cause the extension of hydrophobic interior space in micelles to solubilize the hydrocarbon contaminant. In addition to micelles solubilization, the microbubbles enhancement washing process increased the diesel removal by floatation, cavitation in sonication.