Tuning Separation Processes Using Electroactive Membranes for Oil Fouling Prevention and Salt Rejection Optimization
Membrane technology is an advanced water treatment process that well addresses water scarcity. It provides versatile treatment processes to expand freshwater resources, including seawater and brackish water desalination, wastewater and contaminated groundwater recycling. However, membranes are prone to fouling. In addition, they suffer from low water recovery, as well as a trade-off between water permeability and salt selectivity in desalination processes. Here, we incorporated carbon nanotubes (CNT) based electro-conducting thin film into commercial membranes and evaluated their viability as an effective approach to minimize membrane fouling along with adjusting the membrane’s selectivity toward different salt ions.
We first studied the fouling of model oil (hexadecane) emulsions stabilized by anionic, cationic and nonionic surfactants in a crossflow filtration system using ultrafiltration (UF) and nanofiltration (NF) membranes. In the UF filtration experiments, emulsions stabilized with the cationic surfactant quickly fouled the negatively charged UF membranes. Anionic and non-ionic surfactants stabilized emulsions, on the other hand, experienced less fouling. NF membranes exhibited exponential fouling with all types of surfactant stabilized emulsions. When 10 mM NaCl was used as the electrolyte, the differences between the surfactants were quenched. We demonstrated that the electrostatic interaction between the membrane surface and emulsion droplets was of key importance in membrane fouling. Therefore, we utilized electro-conducting CNT membranes to treat emulsion droplets at ionic strengths as high as 100 mM. Membrane fouling was reduced dramatically when electrical potentials were applied to the membrane surface. We concluded that the reduced fouling was due to less oil coalescence, which was caused by a re-distribution of charged surfactant molecules at the oil/water interface in response to the electric field. Finally, we fabricated the electro-conducting NF membrane by incorporating multi-walled CNT (MWCNT) and single/double-walled CNT (S/DWCNT) network into the polyamide (PA) film. By tuning the electrical potentials applied to the membrane surfaces, the membrane’s ion selectivity was dramatically changed. In particular, the application of negative potentials assisted MWCNT-PA membrane to better reject NaCl but allowed more NaCl to pass through S/DWCNT-PA membranes. We concluded that the phenomena resulted from different membrane film structures.