Enhancing boron rejection in low-pressure reverse osmosis systems using a cellulose fiber–carbon nanotube nanocomposite polyamide membrane: A study on chemical structure and surface morphology

Abstract

Nanocomposite membranes based on carbon nanotubes (CNTs) or cellulose nanofibers (CNFs) are promising solutions for water filtration technologies such as seawater desalination, brackish water purification, and wastewater treatment. In this study, we investigated the effect of integrating CNTs and CNFs into a polyamide membrane (CNT–CNF–PA) on boron rejection in low−pressure reverse osmosis (RO) systems (0.75 MPa). A NaCl (500 ppm) and boron (5 ppm) based solution at pH = 7 was tested in an RO filtration system. The CNT–CNF–PA membrane exhibited a high boron rejection rate (up to 74%) and high permeation (greater than 1.0 m3/m2∙day). The chemical structure played a significant role in boron rejection performance. We conducted principal component analysis (PCA) to statistically study the changes in bond vibrations obtained by Fourier transform infrared (FTIR) spectroscopy, comparing the oxygenated and hydrogenated changes to sodium chloride rejection, boron rejection, and water permeation of the membranes. Additionally, the surface morphology, i.e., roughness, exhibits a direct positive effect on boron rejection, whereas the pore structure is linked to both sodium chloride and boron rejections. Molecular dynamic simulations revealed that the CNF and CNT structures suppress the hydrogen bonds between the PA matrix and boric acid, negatively affecting its diffusion and rejection. These findings help clarify the boron rejection mechanism, enabling further development of PA membranes.