Removal of Divalent Lead cation from Aqueous Streams using Micellar-Enhanced Ultrafiltration
The increasing contamination of urban and industrial waste water by toxic metal ions is a worrying environmental problem. These inorganic micro-pollutants are of considerable concern because they are non-biodegradable, highly toxic and in some cases have a probable carcinogenic effect. If directly discharged into the sewage system they may seriously affect the operation of biological treatment systems and render the activated sludge unsuitable for application to agricultural land. The traditional techniques for the removal of metal ions from aqueous effluents are incapable of reducing concentrations to the levels required by law (reduction or lime precipitation) or prohibitively expensive (ion exchange, activated carbon adsorption or electrolytic removal). The use of membrane separation processes to treat waste water containing toxic metal ions is today an attractive and suitable technique, and it can easily be included in the process, which is the reason why membrane separations are being used more and more frequently. Separations can be carried at room temperature, the modular membrane surface can be easily adjusted to the wastewater flows, and various industrial membranes are now available. In order to retain metallic ions, reverse osmosis (or at least nanofiltration) can be used, due to the size of the ions in aqueous solutions. However, the usual permeate fluxes of reverse osmosis membranes are limited and require high transmembrane pressure, which makes the process expensive. During the last decade, there has been a constantly increasing level of interest and research efforts in order to improve the performances of surfactant-based separation processes. In the present study an attempt is made to remove lead(II) ions from synthetic aqueous solutions by micellar-enhanced ultrafiltration (MEUF) using the anionic surfactant sodium dodecylsulfate (SDS). The study has been carried out at a temperature of 318 oK and on a laboratory scale. Ultrafiltration experiments were carried out with a tangential cell system. The inlet flux was held constant (up to 0.5 m.s-1) and the drop in pressure was varied from 1 to 3 bars by restricting the outlet tube. Polysulfone membranes with a molecular weight cut-off (MWCO) of 10,000 Da and an effective filtration area of 30 cm2 were used. The influence of the operating parameters on permeate flux and lead rejection was studied. Rejection coefficients of 99% are reached under optimal conditions of pressure, feed concentration in SDS, tangential velocity of the feed, and percent filtered volume. An ionic exchange model has been used to study the interaction between the lead cation (Pb2+) and sodium dodecylsulfate micelles. The model used to fit the experimental data is an ionic micelle in which the electric double layer is divided into a diffuse outside layer and a Stern layer inside the shear surface. Assuming that divalent cations are strongly attached to the micellar surface and located specifically in the Stern layer of the micelle, it has been found that adsorption in the Stern layer is well described by a Langmuir isotherm. From this isotherm, ion exchange constants for Pb2+ with the Na+ counter-ion have been determined Ke (Na+ /Cd2+) -=1.39, and have been compared with those obtained for other cations (Cd2+, Mg2+) in the same media. In order to determine the performance of the MEUF process in acidic streams, studies were performed at constant cation concentrations and various pH values. The pH variation was obtained by the addition of HClO4 or H3PO4. Rejection remains higher than 95% provided the pH is maintained higher than 1.8.