Optimizing dual-layer composite membranes with functionalized CNT loadings using response surface methodology

ABSTRACT This study employed central composite design (CCD) within the framework of response surface methodology (RSM) to investigate the direct and interactive influences of critical synthesis parameters on the performance of polyethersulfone (PES)/polyamide (PA) double layer composite membranes for nanofiltration (NF) application. Independent variables included PES, piperazine (PIP), and trimesoyl chloride (TMC) concentrations, along with interfacial polymerization (IP) reaction time. The objective was to simultaneously optimize water permeability (WP) and salt rejection to achieve the highest NF membrane performance. Optimal parameters for maximum WP and salt rejection were determined: PES concentration at 14.0 wt%, PIP concentration at 1.5 w/v%, TMC concentration at 0.2 w/v%, and reaction time at 45 s. In addition, membrane performance was improved by incorporating functionalized multi-walled carbon nanotubes (f-MWCNTs) into the PES ultrafiltration substrate, enhancing hydrophilicity and porosity. The resulting PA NF membrane developed on a substrate containing 0.15 wt% f-MWCNT (TFNC) exhibited WP of 11.84 LMH/bar, surpassing the WP of the same NF membrane on an unloaded substrate (TFC) by approximately 20%, while maintaining nearly identical salt rejection levels. Salt rejection followed the order: Na2SO4 > MgSO4 > NaCl, suggesting Donnan-exclusion mechanisms primarily governed the separation process due to repulsion forces between the negatively charged membrane surface and anions.