Removal of Cd(II) by micellar enhanced ultrafiltration: Role of SDS behaviors on membrane with low concentration

Abstract Surfactant monomers tend to adsorb and aggregate on membrane, which plays a crucial role in removal of metal ions by micellar enhanced ultrafiltration (MEUF) at low concentration level. In this paper, MEUF of cadmium(Ⅱ) (Cd(Ⅱ)) with low sodium dodecyl sulfate (SDS) concentration was studied. Ultrafiltration performance was evaluated in terms of rejection ratios of Cd(Ⅱ) and SDS and relative flux of membranes. The results showed that almost 90% of Cd(Ⅱ) were removed by MEUF when SDS concentration was merely 4 mmol/L. This indicates that reducing the dosage of anionic surfactant in removal of metal ions by MEUF is possible. The retention rates of SDS ranged from 20% to 60%. Relative flux dramatically declined when SDS concentration increased from 0.4 mmol/L to 4 mmol/L. In addition, SDS behaviors on membrane during ultrafiltration were analyzed based on Zhu and Gu's adsorption model and Hermia's blocking models. When large amounts of monomeric SDS existed in the bulk solution, SDS monomers adsorbed on membrane and aggregated into hemimicelles. With SDS concentration gradually increasing, standard pore blocking and cake filtration occurred because SDS hemimicelles deposited on membrane. Cd(Ⅱ) rejection was related to the fouling layer formed by deposition of SDS on membrane.