Smart ultrafiltration: pH-modulated polyacrylic acid-grafted polyethersulfone (PAA-g-PES) membranes for efficient natural organic matter (NOM) foul mitigation and cleaning process
Funeka Matebese, Meladi L. Motloutsi, Mabore J. Raseala, Richard M. Moutloali
Abstract
• Increased hydrophilic PAA grafts on PES enhanced morphology and roughness. • The membrane’s PAA grafts collapsed with pH increased flux while reducing solute rejection. • Increased repulsion forces between -COOH groups improved rejection and antifouling as a function of pH. • Backwashing with alkaline solutions led to high foulant removal from the membrane surfaces. • PAA-g-PES membranes exhibited improved flux, antifouling profile, and enhanced reusability. The presence of natural organic matter (NOMs) in surface water complicates drinking water treatment by forming toxic byproducts, posing significant health-related problems. The purpose of this research is to evaluate the efficiency of pH-responsive membranes in mitigating fouling and facilitating backwashing of NOM deposits on and within the membrane surface. NOM is predominantly hydrophobic, necessitating the use of hydrophilic membranes to prevent fouling and improve rejection and backwashing processes. Polymeric membranes were fabricated using polyacrylic acid-grafted polyethersulfone (PAA- g- PES). PAA is known for its hydrophilic and pH-sensitive properties. The characteristics of the PAA-g-PES polymer and its ultrafiltration (UF) membranes were confirmed through TGA, FTIR, zeta potential, SEM, and AFM, respectively. Their pH sensitivity was confirmed by degree of swelling (DOS), water contact angle (WCA), and pure water flux studies. To assess the impact of the pH-responsiveness of PAA brushes, NOM-impacted surface water was tested at different pH levels (3, 7, and 9). Fluorescence excitation-emission matrix (FEEM) analysis revealed that the presence of PAA improved NOM removal, achieving an average removal rate of 65.6% across all components. This enhanced NOM removal can be attributed to swollen grafts, increased surface hydrophilicity, and electrostatic repulsion interactions. At low pH levels, dominant hydrophobic interactions caused graft shrinkage, resulting in high-water permeation. The best-performing membrane exhibited a flux recovery ratio ( FRR ) of 84.4% after more than 9 hours of fouling and cleaning, demonstrating the efficacy of pH-sensitive membranes. High pH solutions showed improved fouling resistance and backwashing efficiency, proving to be an effective approach for widespread use.