Removal Of Cadmium In The Aqueous Solution Via Micellar Enhanced Ultrafiltration

Micellar-enhanced ultrafiltration, a surfactant-based separation process, is promising in removing multivalent metal ions from aqueous solutions. The micellar-enhanced ultrafiltration (MEUF) of cadmium from aqueous solution using anionic surfactants sodium dodecylsulfate (SDS) was investigated systematically in this paper. The micelle size and zeta potential of SDS micelles were determined by dynamic light scattering measurements (DLS). The effects of feed concentration, pH, transmembrane pressure, nonionic surfactants, membranes and salts were studied. The preliminary investigation on the mechanism of micellar-enhanced ultrafiltration was also studied by nuclear magnetic resonance mensuration.According to the results of dynamic light scattering measurements, large premicelle was found in solution with SDS concentration lower than cmc. The formation process of micelle is thought to be a stepwise process. The micelle size decreased with the increasing surfactant concentration. The monomer molecules in the small micelle may be arranged more orderly than those in the large micelle. The zeta potential increased with cadmium concentration, more Cd2+ ions were bound with micelle. The micelle size also decreased with the increasing nonionic surfactant concentration, the surface charge density of micelles was reduced rapidly. The high Cd2+ rejection were obtained when the SDS concentration lower than the CMC of SDS (8 mmol/L). This may attribute to the concentration polarization and the decrease of cmc because of the addition of Cd2+. When the feed Cd2+ concentration were fixed, the Cd2+ retention increased with feed SDS concentration first, then attained a plateau at higher SDS concentration. The Cd2+ retention increased slightly with feed Cd2+ concentration first then decreased rapidly with the fixed feed SDS concentration. Large numbers of H+ compete with Cd2+ to bind with micelles. The Cd2+ retention would decline if the solution pH lowers than 3. The permeate flux increased with the transmembrane pressure then keep constant. The Cd2+ retention increased with the nonionic surfactants concentration then decreased. The addition of nonionic surfactants increased the number of micelles and also decreased the counter-ion binding degree of mixed micelle.The specific flux values of various membranes decreased in the order as follows: P30 > P10 > C30 > P8 > C10. The specific flux gradually decreased with the increase of transmembrane pressure. When the ratio of surfactant to metal concentration was fixed at 10, the Cd2+ retention of P8 membrane decreased with the increase of transmembrane pressure and feed concentration. However, the Cd2+ retentions of P10 and P30 membranes increased with transmembrane pressure firstly then decreased. The higher the feed solution, the more the Cd2+ retention decreased. The reason for this is the different effects of concentration polarization which enhanced by the increase of TMP and feed concentration. No significant influence of pressure on the rejection of Cd2+ was observed during the experiments with two cellulose membranes. The rejections of Cd2+ of cellulose membranes were much lower than those of polyethersulfone membranes. This can be attributed to the lower concentration polarization because of the much smaller water permeate flux of cellulose membranes.The rejections of metal ions strongly depend on the valence of metal ion. The electrostatic attraction of micelle and metal ions with the same valence were much closed. The rejection of Cd2+,Cu2+,Ni2+ were almost the same. The rejections of Cd2+ decreased rapidly with the NaCl concentration. This can be attributed to the competition of Na+ and Cd2+ for the binding with micelles. The stronger the complexation of Cd2+ and the anion in solution, the more the rejection of Cd2+ decreased. The complexation ability of anions decreased in the order as follows: Cl- > SO42- > NO3-≈CO32-.23Na+ magnetic resonance mensuration chemical shift increased with the Cd2+ concentration. It means that more Na+ change from the binding state to free state. The electrostatic attraction of Cd2+ and micelle were stronger.