Rhamnolipid based micellar-enhanced ultrafiltration for simultaneous removal of Cd(II) and phenolic compound from wastewater

Treatment of wastewater containing heavy metal ions and phenolic compound is a growing challenge due to their coexistence in many industrial effluents. The present study was aimed at the investigation of simultaneous removal of Cd+2 and p-cresol from aqueous solution by micellar-enhanced ultrafiltration (MEUF) with rhamnolipid (RHL) as a biosurfactant. MEUF experiments were performed according to statistical design considering the factors of solution pH (6.5-8.5), concentration of RHL (50-650 mg/L), Cd+2 (30-70 mg/L) and p-cresol (50-150 mg/L) in a stirred cell using 10 kDa flat sheet polyethersulfone membrane. The process was optimized to obtain the best possible feed conditions by the simultaneous maximization of the rejection of both Cd+2 and p-cresol, using the response surface methodology. The maximum rejection coefficient of 98.8% and 25% for Cd+2 and p-cresol, respectively were obtained for following optimal conditions: RHL: 370 mg/L, Cd+2:60 mg/L, p-cresol: 75 mg/L and pH 7.8. Dynamic light scattering measurement showed that small-sized rhamnolipid vesicles (10-50 nm) were formed more than 93% in presence of Cd+2 and p-cresol, in the experimental range of RHL concentration greater than its critical micelle concentration (CMC), which was found to be 50 mg/L determined by surface tension measurements. Under the optimal feed conditions, the effects of various hydrodynamic conditions such as transmembrane pressure, stirrer speed and feed temperature on permeate flux and rejection of both pollutants were studied. Also, the performance of MEUF process was assessed through estimation of the distribution coefficient, vesicle loading and vesicle binding constants for both Cd+2 and p-cresol.