Conductive CNT-PVDF membrane for capacitive organic fouling reduction

Organic fouling of ultrafiltration (UF) membranes results in decreased water flux and increased energy requirements. Modification of UF membrane surfaces is one possible method to mitigate natural organic matter (NOM) fouling, yet to date; most modifications have been passive. In this study, we investigate the use of a carbon nanotube-polyvinylidene fluoride (CNT-PVDF) porous non-Faradaic cathode on top of a UF membrane to actively produce negative surface charges via capacitive charging. The study is divided into three elements: (1) modification of the UF system with the capacitive CNT-PVDF electrodes and determination of the optimal electrode-membrane configuration, (2) analysis of the fouling mitigation mechanism, and (3) evaluation of the practical potential of capacitive fouling reduction. All experiments were completed in the cross-flow configuration. The optimal electrode-membrane configuration for organic fouling reduction was when the permeate first flowed through the porous anode, then the CNT-PVDF cathode, and finally the polyethersulfone (PES) UF membrane. The extent of capacitive fouling reduction was determined to be a function of anode material, ionic strength, and cathode potential. The primary fouling reduction mechanism is the potential-induced cathodic negative surface charges that increase the Derjaguin–Landau–Verwey–Overbeek (DLVO) energy barrier and decrease the collision efficiency of negatively-charged organic matter with the membrane surface. The capacitive system has potential to reduce energy requirements by up to 2-fold as compared to the unmodified UF system when challenged with 10 ppm NOM solutions at low ionic strength.