Investigation Of The Interaction Mechanisms Between Surfactant Micellar Solutions And Metal Ions

In their using or disposal processes, surfactant solutions always exhibit different characteristics or treatment effects because of the different species or concentrations of the metal ions contained in natural waters or in the flocculants used. So it's necessary to investigate thoroughly the interactions between surfactant micellar solutions and different metal ions.This study used anionic surfactant sodium dodecyl sulphate (SDS) as model, under two constant temperatures: 30℃ and 10℃, investigated the interaction mechanisms between this kind of surfactant micellar solutions and several commonly encountered metal ion solutions (Al2(SO4)3, FeCl3, PAC, CaCl2 and MgCl2). According to the different phenomenon observed in each system with the increasing of the metal ion concentration, and taking into consideration the dominant species form of each metal ion at the pH value of the very system, the process probably occurred in each system was discussed, and the main interaction mechanism between SDS micelles and metal ions was speculated, too. The SEM photographs of the precipitates verified the previous conclusion from another side.Results suggested, in SDS-Al2(SO4)3 system, Al3+ was the dominant species of Al, andadsorption-charge neutralization was the main flocculation mechanism. When the added Al concentration was 1/3 of the initial SDS concentration, every three surfactant head groups on micellar surface were bound together by one Al3+ ion, charge neutralization condition was achieved and the optimum flocculation region began. When the added Al concentration was excessive, SDS and Al formed aqueous complex, which resulted in the redissolution of the precipitates and finally the solution became clear again. In SDS-FeCl3 and SDS-PAC systems, Fe(OH)2+ and Alb were the dominant species of Fe and Al, respectively. Adsorption-bridging effect was the main flocculation mechanism. No redissolution phenomenon was observed in the whole experimental range. In SDS-CaCl2 system, the precipitation phenomenon was owing to the crystallization induced by the solubility product of Ca(DS)2. But in SDS-MgCl2 system, no turbidity or precipitation phenomenon was observed in the whole experimental range. For all systems, in the optimum flocculation region, micellar forms of SDS were eliminated completely and part of the free ones were also eliminated by the sweeping effect of the precipitates.This paper can provide important references for studying the various applications of surfactant solutions and for developing new-style hardwater-resistent surfactants. On the other hand, it's also helpful in developing high-efficiency compound flocculants that suitable for the treatment of this kind of wastewater.