Probing the antifouling mechanism of the oil-water separation membranes with ZnO nanostructures

Nanostructures have been commonly incorporated into oil-water separation membranes to improve their antifouling properties. However, the antifouling mechanism of the membrane with different nanostructures is still unclear. In this work, ZnO nanostructures, i.e., nanorods, hollow nanorods and randomly assemble nanosheets, were constructed on polyvinylidene fluoride (PVDF) membrane surfaces. The resulted membranes exhibited improved antifouling properties in comparison with the pristine PVDF membrane, and the hollow nanorods showed the best antifouling property. The antifouling mechanism was investigated through direct force measurements between oil drops and membrane surfaces using atomic force microscopy. The theoretical analysis of approach force curves indicated that the repulsive electrical double layer force was enhanced at higher pH and in the presence of ZnO nanostructures, which prevented the oil attachment on the membrane surface. The retraction force curves suggested that the adhesion between oil and membrane was significantly affected by the ZnO nanostructures, and the adhesion followed the trend of hollow nanorods < randomly assemble nanosheets < nanorods. Consequently, the oil drops on the membrane with hollow nanorods could be easily removed by water rinsing, which agreed with the flux recovery results. This work provided a fundamental understanding of the antifouling mechanism of the oil-water separation membranes with nanostructures, and a useful guidance for constructing suitable surface structures for efficient oil-water separation membranes.