Transport of micelles of cationic surfactants through clinoptilolite zeolite

Natural clinoptilolite is capable of removing metal cations from water due to its high cation exchange capacity. The affinity of the zeolite for cations makes it a good candidate to remove cationic surfactants from water. The transport of hexadecyltrimethylammonium (HDTMA), at a solution concentration greater than its critical micelle concentration (CMC), through zeolite columns of different particle sizes resulted in a retardation factor of 50–60. The capacities of HDTMA sorption at the breakthrough point CB are 82–83 mmol kg−1 at 20–25 PVs and 140–160 mmol kg−1 at 40–45 PVs for the 1.4–2.4 mm and 0.4–1.4 mm zeolite, respectively. While, the capacities of HDTMA sorption at the exhaustion point CE are 185–187 mmol kg−1 at 110–120 PVs and 195–210 mmol kg−1 at 90–100 PVs, for the 1.4–2.4 mm and 0.4–1.4 mm zeolite, respectively. In the early stage of the transport experiment, the HDTMA effluent concentration was close to zero while the counterion Br− concentration was about 50% of the input concentration, indicating a complete removal of HDTMA but only a partial removal of bromide. At any given time, the amount of surfactant removed is about twice that of counterion Br− removed and twice that of metal cations released. The concurrent removal of counterion accompanying surfactant sorption was attributed to HDTMA admicellar sorption by the zeolite, indicating that both cation exchange and hydrophobic bonding are dominant mechanisms for HDTMA removal when its input concentration is greater than the CMC. Therefore, when designing an adsorption bed to remove surfactant from aqueous solutions with concentrations greater than the CMC, partially release of counterion needs to be taken into consideration.