Unraveling pH Effects on Ultrafiltration Membrane Fouling by Extracellular Polymeric Substances: Adsorption and Conformation Analyzed with Localized Surface Plasmon Resonance

Abstract

Extracellular polymeric substances (EPSs) can conform and orient on the surface according to the applied aquatic conditions. While pH elevation usually removes EPSs from membranes, small changes in pH can change the adsorbed EPS conformation and orientation, resulting in a decrease in membrane permeability. Accordingly, EPS layers were tested with localized surface plasmon resonance (LSPR) sensing and quartz crystal microbalance with dissipation monitoring (QCM-D) using a hybrid sensor. A novel membrane-mimetic hybrid QCM-D–LSPR sensor was designed to indicate both “dry” mass and mechanical load (“wet” mass) of the adsorbed EPS. The effect of pH on the EPS layer’s viscoelastic properties and hydrated thickness analyzed by QCM-D corroborates with the shift in EPS areal concentration, ΓS, and the associated EPS conformation, analyzed by LSPR. As pH elevates, the processes of (i) elevation in EPS layer’s thickness (QCM-D) and (ii) decrease in the EPS areal density, ΓS (LSPR), provide a clear indication for changes in EPS conformation, which decrease the effective ultrafiltration (UF) membrane pore diameter. This decrease in the pore diameter together with the increase in surface hydrophobicity elevates UF membrane hydraulic resistance.

Fouling of Reverse Osmosis Membrane with Effluent Organic Matter: Componential Role of Hydrophobicity

“…To assess the dry adsorbed molecular mass on the polyamide surface, an LSPR membrane-mimetic sensor was fabricated (Supporting Information), and the affinities of the different fractions for polyamide were examined (Figure d). The dry-molecular-mass surface concentration, Γ S , of the adsorbed organic fractions on the polyamide surface was indicated by the shift in light-extinction maxima, Δλ, and the dry mass was calculated using eqs and .…”

“…Although the HPOA layer had a relatively high hydraulic resistance, it did not reduce the permeate flux more than the other polar fractions or the EfOM mix, likely because of its relatively low adsorption of dry molecular mass. To measure the amount of the “dry” as well as the hydrated molecular mass of these fractions that was adsorbed to the polyamide surface under parallel flow conditions, the fractions’ affinity was tested in both QCM-D and LSPR membrane mimetic sensors, similar to the recent study of Ran et al…”

“…Complementary analyses of the fractions by EEM and DLS and the fractions’ intrinsic hydraulic resistance improve our understanding of their fouling behavior. Data acquired using the LSPR membrane-mimetic sensor (developed according to our previous study) indicate that the reduction in RO permeate flux of all adsorbed fractions is correlated with their dry molecular mass.…”

“…Comparisons of fraction adsorption on RO membrane active layers on QCM-D sensors and the resulting flux reduction have revealed differences that are likely due to the hydration of fouling layers affecting the QCM-D analysis. 46 Complementary analyses of the fractions by EEM and DLS and the fractions' intrinsic hydraulic resistance improve our understanding of their fouling behavior. Data acquired using the LSPR membrane-mimetic sensor (developed according to our previous study 47 ) indicate that the reduction in RO permeate flux of all adsorbed fractions is correlated with their dry molecular mass.…”

Indirect Evidence that Colloidal Deposition Inside a Hollow Fiber Ultrafiltration Membrane Exacerbated Fouling during Municipal Wastewater Reclamation

“…An important and continued gap in the literature is our understanding of the role of nanocolloid size relative to the UF pore size during combined organocolloid fouling while reclaiming secondary effluents. It is emphasized that deconstructing “colloidal” and “organic” matter is futile and unnecessary because they commonly exist as organocolloids in real-world wastewaters. ,,− Thus, efforts to parse their roles and individually investigate colloid and organic fouling are not expected to be beneficial in practice because they act in unison.…”