Micellar-enhanced ultrafiltration for the removal of cadmium and zinc: Use of response surface methodology to improve understanding of process performance and optimisation

In this study, removal of cadmium and zinc from their respective water samples was conducted by micellar-enhanced ultrafiltration (MEUF), using sodium dodecyl sulfate (SDS) as the surfactant. Response surface methodology (RSM) was used for modelling and optimising the process, and to gain a better understanding of the process performance. Face Centred Composite (CCF) Design was used as the experimental design. The factors studied were pressure (P), nominal molecular weight limit (NMWL), heavy metal feed concentration (CZn, CCd) and SDS feed concentration (CSDS). Using RSM the retention of heavy metals was maximized while optimising the surfactant to metal ratio (S/M). Response surface plots improved the understanding the effect of the factors on permeate flux. Concentration polarisation was negligible and therefore, high NMWL membranes with high pressure provided high flux with negligible effect on the retention of heavy metals. The optimal conditions of zinc removal were CSDS = 13.9 mM, CZn = 0.5 mM, NMWL = 10 kDa and P = 3.0 bar, and for cadmium removal CSDS = 14.2 mM, CCd = 0.5 mM, NMWL = 10 kDa and P = 3.0 bar. The retentions achieved were 98.0 ± 0.4% for zinc and 99.0 ± 0.4% for cadmium. To improve resource efficiency, the surfactant was reclaimed after use; 84% of the initial SDS was recovered by precipitation.