Overcoming the trade off between the permeation and rejection of TFN nanofiltration membranes through embedding magnetic inner surface functionalized nanotubes

Abstract

Today, one of the most imortant environmental problems is the treatment of industrial wastewaters containing heavy metal ions and salts. In this research, thin-film nanocomposite (TFN) membranes were developed through incorporation of modified titanate nanotube (TNT) and halloysite nanotube (HNT) into the polyamide (PA) active layer to improve the performance of the NF membranes. At first, the internal surface of the nanotubes was coated with different polymers to lessen the inner diameter of the nanotubes. Then, the inner surface coated nanotubes were magnetized through placing Fe3O4 on their outer surface. In case of magnetized nanotubes, the fabrication of the membranes was done in the absence and in the presence of the external magnetic field, which made the nanotubes to align in a regular pattern across the PA layer, smoothing the membranes' surface and lowering the contact angle. It is expected that the magnetized nanotubes can make suitable arrangement in the structure of thin layer in the presence of the magnetic field and show better performance. The nanotubes act as the channels for water transport and in case of inner coated nanotubes, reject more ions through the steric hindrance. The pure water permeation of the membranes, modified with polystyrene inner coated TNT and HNT increased by 71.23% and 80.27%, respectively compared to the pristine TFC membrane without any significant changes in the rejection of Na+ and Cu2+; a suitable trade-off between the permeation and the ion rejection. In addition, the membranes modified by magnetized polyaniline inner coated TNT and HNT showed 36.24% and 75.62% more water permeation compared to the pristine TFC membranes while the rejections of Na+ and Cu2+ have not been changed remarkably. The results of this research showed that inner coating and magnetization of the nanotubes can be considered as a novel method to enhance the efficiency of TFN membranes for wastewaters treatment.