Fouling of nanofiltration membranes by organics, colloids and their combinations in cross-flow filtration: Effects of the presence of various ions on membrane-foulant non-electrostatic interactions

The presence of macromolecules such as organic and colloidal foulants in the feed has been reported to result in membrane fouling. Fouling is aggravated when the organics and colloids co-exist in the feed due to synergistic effects. In addition, the presence of ions has been observed to promote organic fouling as the ions (especially Ca2+) act as a bridge binding foulants to the membrane (membrane-Ca2+-foulant complexation). Different cations have been reported to have different effects on membrane fouling. However, there are no clear explanations for the fluctuations in the fouling trends. Mostly, the deviations have been linked to the affinity of the cations to bind foulants to the membrane (membrane-foulant interactions) and also complex organic foulants. Membrane fouling in the presence of Na+, Ca2+ and La3+ as model cations is systematically investigated in more detail in this study. Sodium alginate, latex and silica are used as model organic and colloidal foulants. Specifically, more attention is paid on the effects of the presence of various cations on membrane-foulant as well as foulant-foulant non-electrostatic interactions, which determine initial and later membrane fouling, respectively. These interactions were computed from contact angles of membranes and foulants based on the Lifshitz-van der Waal and acid-base interaction energies. Fouling (especially organic fouling) was exacerbated in the presence of cations due to organic complexation and and the reduction in foulant–membrane repulsive interactions. Initial and later membrane fouling related well with membrane-foulant as well as foulant-foulant interactions. Membrane-foulant interactions were more attractive for fouling in the presence of La3+, intermediate for Ca2+ and least for Na+ for all fouling types. This correlated well with alginate and latex fouling trends where fouling was severe in the presence of La3+ and least in the presence of Na+. However, there was insignificant effect for the addition of cations on silica fouling showing that not only membrane-foulant non-electrostatic interactions control flux decline. For combined fouling, different observations were made. However, fouling was highest for fouling in the presence of Ca2+. Fouling was not only influenced by membrane-foulant interactions but also competition for cations between organic and colloidal foulants. It was concluded from these experiments that the extent of membrane fouling is controlled by the affinity of foulants for the membrane surface. These affinity interactions are dependent on the presence of various cations in the feed water

Effect of multivalent cations on membrane-foulant and foulant-foulant interactions controlling fouling of nanofiltration membranes

Insight was gained into the various factors influencing membrane fouling in the presence of different cations. Membrane-foulant interaction energies were computed from contact angle measurements following the Lifshitz-van der Waals/acid-base approach. A unique approach was used to investigate cake-enhanced concentration polarization (CECP) effects. Calcium worsened organic fouling through organic-calcium complexation and increasing foulant affinity for the membrane surface, whereas magnesium and lanthanum had less influence on membrane fouling as they moderately decreased the foulants' affinity for the membrane surface. Fouling was facilitated by permeation drag and reduction in membrane-foulant electrostatic repulsive interactions due to slight neutralization of foulant and membrane zeta potential. Although cake layers formed in the presence of magnesium and lanthanum had low resistance to water flow, the cake layers resulted in severe CECP effects. Initial membrane fouling rates related well to energies of adhesion, whereas later fouling rates did not show clear correlation to energies of cohesion

In Situ Generation of Fouling Resistant Ag/Pd Modified PES Membranes for Treatment of Pharmaceutical Wastewater

In this study, Ag and Pd bimetallic nanoparticles were generated in situ in polyethersulfone (PES) dope solutions, and membranes were fabricated through a phase inversion method. The membranes were characterized for various physical and chemical properties using techniques such as FTIR, SEM, AFM, TEM, EDS, and contact angle measurements. The membranes were then evaluated for their efficiency in rejecting EOCs and resistance to protein fouling. TEM micrographs showed uniform distribution of Ag/Pd nanoparticles within the PES matrix, while SEM images showed uniform, fingerlike structures that were not affected by the presence of embedded nanoparticles. The presence of Ag/Pd nanoparticles resulted in rougher membranes. There was an increase in membrane hydrophilicity with increasing nanoparticles loading, which resulted in improved pure water permeability (37–135 Lm2h−1bar−1). The membranes exhibited poor salt rejection (<15%), making them less susceptible to flux decline due to concentration polarization. With a mean pore radius of 2.39–4.70 nm, the membranes effectively removed carbamazepine, caffeine, sulfamethoxazole, ibuprofen, and naproxen (up to 40%), with size exclusion being the major removal mechanism. Modifying the membranes with Ag/Pd nanoparticles improved their antifouling properties, making them a promising innovation for the treatment of pharmaceutical wastewater