Unveiling the mechanisms behind contradictory fouling and rejection behaviors in dual-layer pH-responsive CuO/SiO2 membranes via film theory and DSPM modeling

Abstract

This study examines the effects of membrane top layer and sublayer modifications on filtration performance, focusing on drug doxorubicin. The membranes were modified with CuO (top layer), SiO2 (sublayer), and simultaneous presence of CuO and SiO2 nanoparticles, alongside pH adjustment, with their effects analyzed using mathematical modeling. CuO nanoparticles elevated flux by 18 %. SiO2 nanoparticles further boosted it by 127 % and reduced reversible fouling by 27 %. The simultaneous presence of nanoparticles improved flux by 82 %, with irreversible fouling limited to only 2.5 %. Additionally, pH alteration from 8 to 2, augmented flux by 471 % and reduced reversible fouling by 71 %, showing strong pH responsiveness. These performance enhancements were consistent with membrane's characteristics. In contrast to these anticipated trends, simultaneous addition of nanoparticles led to a 69 % rise in reversible fouling, which did not correlate with membrane's characteristics and could not be explained experimentally. Mathematical modeling through film theory found a 13 % rise in polarization modulus as the cause of this behavior. Moreover, CuO incorporation resulted in a 12 % reduction in rejection, which was not associated with membrane's characteristics and could not be explained experimentally. Mathematical modeling through DSPM model, by considering the difference between surface charge density and effective charge density, identified the reduction of 32 % in membrane effective charge density as the cause of this unexpected behavior. Thus, efforts to enhance flux or reduce pore size through membrane modifications were found ineffective, as modeling revealed that increased polarization and the dominant role of Donnan exclusion limited performance improvements. © 2025 Elsevier B.V.