Removal of metal ions from aqueous solutions using lecithin enhanced ultrafiltration

This work is concerned with an alternative method for metal ion removal from aqueous solutions - surfactant enhanced ultrafiltration. Surfactant monomers aggregate above a certain concentration, specific to the surfactant, to form micelles. Anionic surfactant micelles will attract and bind metal cations. Free metal ions and surfactant monomers pass freely through an ultrafiltration membrane, but if the micelle-metal ion complex is sufficiently large it is rejected. Research reported in this thesis has been carried out on well defined aqueous solutions containing only one type of metal ion together with the natural surfactant lecithin. Lecithin is a food grade by-product of the soybean processing industry and it was chosen because it is non-toxic, biodegradable, abundant and inexpensive. It has a high molecular weight of about 750 Daltons and forms large size micelles. The main aim was to identify the basic mechanisms which influence the permeate flux and rejection levels of the process. The project was carried out in three stages. Stage one was the characterisation of the feed solution which included the determination of the critical micelle concentration using surface tension measurements, measurement of micelle size and zeta potential using a Malvern zeta sizer and visualisation of the micelle shape using scanning electron microscopy of freeze fractured lecithin solution droplet. In the second stage filtration experiments were carried out at a wide range of lecithin concentrations, metal ion concentrations and operating conditions. The experiments were run for 5 hours, by which stage a steady state condition was reached in all cases. Permeate samples were taken after I, 3 and 5 hours. Permeate flux was monitored throughout the experiment. The following properties were monitored for the feed solution at the beginning and the end of each experiment and for all permeate samples: lecithin concentration, copper concentration, pH, conductivity. In the 3 stage Electron Dispersive Analysis by X-ray (EDAX), Scanning Electron Microscopy (SEM) and a X-ray Photoelectron Spectrum Technique (XPS) were employed to investigate any membrane feed solution interactions. The results of the 3 stages were used to identify the basic mechanisms which control the permeate flux levels and the extent of component rejection in lecithin enhanced ultrafiltration.