Enhanced separation performance of carbon nanotube–polyvinyl alcohol composite membranes for emulsified oily wastewater treatment under electrical assistance

Membranes provide an effective solution to treat emulsified oily wastewater, but usually have inherent drawbacks: permeability and selectivity trade-off as well as serious membrane fouling. In this study, carbon nanotube-polyvinyl alcohol (CNT-PVA) composite membranes with high water flux and good electrical conductivity were fabricated and then served as cathode during the filtration of n-hexadecane-in-water emulsion (n-emulsion) and cutting fluid emulsion (c-emulsion) to improve the oil removal efficiency and alleviate membrane fouling. Results show that oil removal rates increase from 85.2% to 97.6% for n-emulsion and from 56.7% to 83.0% for c-emulsion after the application of −1.5 V. The permeation flux after an operation of 60 min with −1.5 V are 2.3 times and 2.2 times of those without potential for n-emulsion and c-emulsion, respectively. Additionally, less oil fouling can be observed from the SEM images of the membrane surface and fouling can be mitigated according to the analysis of fouling model. Cyclic voltammetry and zeta potential analysis demonstrate that electrostatic repulsion rather than electrochemical reactions is responsible for the enhanced oil removal rate and antifouling ability in the filtration process. Furthermore, the results of long term filtration with five runs indicate that the application of potential onto the membranes maintains remarkable stability in high oil removal rate and high flux recovery rate.