Influence of Surfactant Headgroup Charge Density on Removal of Aromatic Contaminants from Water by Surfactant-Modified Montmorillonite

Recent reports estimate that petroleum production is causing freshwater resources to decrease at an alarming rate. The development of organically modified clay minerals (organoclays) has been an area of interest for use in... [ view full abstract ]

Recent reports estimate that petroleum production is causing freshwater resources to decrease at an alarming rate. The development of organically modified clay minerals (organoclays) has been an area of interest for use in adsorbing these pollutants out of water. Primarily, research has been focused on organoclays that have been modified with hexadecyl trimethylammonium bromide (HDTMA), a surfactant that presents environmental risks (cytotoxicity) in part due to the high charge density of its head-group. In this work, we compared the conventional HDTMA organoclays with clays made using two lower charge-density cationic head groups, HDTM-phosphonium and HD-pyridinium. The clays were characterized (surfactant loading, clay mineral interlayer spacing magnitude, and interlayer surfactant crystallinity), and the mechanism and magnitude of uptake of ethylbenzene, a model aromatic petroleum hydrocarbon, examined. Ethylbenzene uptake effectiveness (K_D) was highest on average for HDTMA modified clays. However, the clay with the highest uptake efficiency (K_OC) was a HDPy-modified clay, suggesting there are interactions between pi-systems of the surfactant and the organic contaminant. All HDTMP organoclays showed lower K_D and K_OC than HDTMA organoclays, suggesting cation-pi surfactant-contaminant interactions, which would be weaker for HDTMP, are involved. Understanding the mechanism of contaminant uptake is crucial to design of improved organoclays.