Unveiling the synergistic mechanism of magnesium hydroxide and silica in modulating polyethersulfone membrane structure and function

Abstract

In this study, an environmentally benign dual-additive modification strategy was investigated by incorporating magnesium hydroxide and silica into PES membranes to simultaneously tailor surface chemistry and pore morphology during non-solvent-induced phase separation. Membranes were fabricated using a fixed Mg(OH)₂ content with varying silica concentrations and characterized by FESEM, ATR-FTIR, gravimetric porosity analysis, water contact angle measurements, and filtration performance testing, including pure-water flux, humic acid rejection, and flux recovery ratio. All modified membranes retained the asymmetric structure of pristine PES but developed thinner selective layers and more elongated finger-like macrovoids as silica content increased up to the optimum composition (M-MS2.5). At this condition, porosity increased to 67.73 %, the O–H absorption band became more pronounced, and the water contact angle decreased from ∼80° to 53.4° (≈33 % reduction), indicating enhanced interfacial hydrophilicity. These structural and chemical enhancements led to improved transport and fouling behavior, with water flux approaching ∼80 L m⁻² h⁻¹ and the flux recovery ratio increasing to 90.91 %. Although humic acid rejection decreased from 79.8 % (pristine) to 69.19 % at the optimum silica loading (2.5 %), it remained within a reasonable ultrafiltration performance range. At higher silica concentrations, however, particle agglomeration and localized structural densification were observed, resulting in increased contact angle, reduced flux, and partial recovery of rejection, indicating a limited compositional window for effective synergy between silica and Mg(OH)₂. Overall, the intermediate silica loading (2.5 wt%) provided the most favorable balance between permeability, antifouling stability, and rejection, suggesting strong potential for application in high-throughput treatment of NOM-rich waters.