Polyelectrolyte multilayer films as backflushable nanofiltration membranes with tunable hydrophilicity and surface charge

A diverse set of supported polyelectrolyte multilayer (PEM) membranes with controllable surface charge, hydrophilicity, and permeability to water and salt was designed by choosing constituent polyelectrolytes and by adjusting conditions of their deposition. The membranes were characterized in terms of their water and MgSO4 permeabilities and resistance to colloidal fouling. The commercial nanofiltration membrane (NF270) was used as a comparative basis. Highly hydrophilic and charged PEMs could be designed. For all membranes, MgSO4 permeability coefficients of NF270 and all PEM membranes exhibited a power law dependence on concentration: Ps [is proportional to] C-[tau], 0.19 < [tau] < 0.83. PEM membranes were highly selective and capable of nearly complete intrinsic rejection of MgSO4 at sufficiently high fluxes. With the deposition of colloids onto the PEM surface, the separation properties of one type of polyelectrolyte membrane showed similar rejection and superior flux properties compared to NF270 membranes. We hypothesize that a PEM-colloid nanocomposite was formed as a result of colloidal fouling of these PEM films. The feasibility of regenerating the PEM membranes fouled by colloids was also demonstrated. In summary, the PEM-based approach to membrane preparation was shown to enable the design of membranes with the unique combination of desirable ion separation characteristics and regenerability of the separation layer

Adsorption of Anionic or Cationic Surfactants in Polyanionic Brushes and Its Effect on Brush Swelling and Fouling Resistance during Emulsion Filtration

Atom transfer radical polymerization of ionic monomers from membrane surfaces yields polyelectrolyte brushes that swell in water and repel oil droplets to resist fouling during filtration of oil-in-water emulsions. However, surfactant adsorption to polyelectrolyte brushes may overcome this fouling resistance. This work examines adsorption of cationic and anionic surfactants in polyanionic brushes and the effect of these surfactants on emulsion filtration. <i>In situ</i> ellipsometry with films on flat surfaces shows that brushes composed of poly­(3-sulfopropyl methacrylate salts) (pSPMK) swell 280% in water and do not adsorb sodium dodecyl sulfate (SDS). pSPMK-modified microfiltration membranes reject >99.9% of the oil from SDS-stabilized submicron emulsions, and the specific flux through these modified membranes is comparable to that through NF270 nanofiltration membranes. Moreover, the brush-modified membranes show no decline in flux over a 12 h filtration, whereas the flux through NF270 membranes decreases by 98.7%. In contrast, pSPMK brushes adsorb large quantities of cetyl­trimethyl­ammonium bromide (CTAB), and at low chain densities the brushes collapse in the presence of this cationic surfactant. Filtration of CTAB-stabilized emulsions through pSPMK-modified membranes gives minimal oil rejection, presumably due to the brush collapse. Thus, the fouling resistance of polyelectrolyte brush-modified membranes clearly depends on the surfactant composition in a particular emulsion

Behavior of oil droplets at the membrane surface during crossflow microfiltration of oil–water emulsions

A fundamental study of microfiltration membrane fouling by emulsified oil was conducted using a combination of real-time visualization, force balance on a droplet, and permeate flux analysis. The model 0.1% v/v hexadecane-in-water emulsions contained sodium dodecyl sulfate (0.1 mM, 0.4 mM, or 0.8 mM) to regulate interfacial tension. Direct Observation Through the Membrane tests with AnoporeMOE (Min. of Education, S’pore

Effect of cross-flow velocity, oil concentration and salinity on the critical flux of an oil-in-water emulsion in microfiltration

Microfiltration is an attractive means for treating oily wastewater, especially when the size of the oil droplets are micrometer-sized since the conventional techniques become deficient. A systematic study on the critical flux of oil-in-water emulsion, which behaves differently from other colloidal foulants with regards to deformation, coalescence and splitting, has not been carried out to date. This was the goal of the current study, which employed the Direct Observation Through the Membrane (DOTM) technique to characterize the critical flux of oil-in-water emulsions of various concentrations, and at various cross-flow velocities (CFV) and salt concentrations. Five observations can be highlighted here. Firstly, the oil droplets with a mean droplet diameter of approximately 5 µm exhibited critical fluxes equal to or greater than latex particles of 10 µm. This is likely due to the twin effects of membrane oleophobicity promoting back-transport of the oil foulant from the membrane and the presence of a droplet size distribution with larger drops that can enhance the shear-induced diffusion of the average drops. Secondly, the critical flux values did not agree with the model that is valid for the size range the mean droplet diameter falls in, but instead agreed with the model adapted for smaller particulate foulants. Thirdly, the increase in the critical flux with CFV was more significant for the lower oil concentration. Fourthly, a striping phenomenon was observed at higher oil concentrations and lower CFV values. Striping was not observed for latex particles. Fifthly, the critical flux decreased with salt concentration. These findings highlight the unique fouling behavior of oil-in-water emulsions in microfiltration.EDB (Economic Devt. Board, S’pore)MOE (Min. of Education, S’pore

Differentiating the inhibitory effects of natural organic matter constituents on photocatalytic treatment processes

International audienc

Analysis of microstructural properties of ultrafiltration cake layer during its early stage formation and growth

Understanding the development of microstructure of cake layer during filtration can help establish effective operating conditions and membrane cleaning cycles. In this work, dynamic behavior of cake layers formed during early stage of ultrafiltration (UF) of colloids was analyzed in terms of cake porosity, fractal structure and evolution of fouling mechanisms. Experimental data on membrane fouling by SiO2 colloids under different conditions were recorded to study the effect of pH (1.0–8.9), salinity (0–3.5 %wt. NaCl) and nanoparticle concentration (30–1000 ppm). The results show time dependent behavior of cake porosity with a steady state value lower at lower pH and at higher salinity of the continuous phase. Multiple fouling mechanisms were identified to occur sequentially with cake filtration as the dominant mechanism. High fractal dimension (2.45–2.94) was obtained in all conditions tested. Fractal dimension of a growing cake is shown to undergo a sharp transition to a higher value, pointing to cake restructuring during filtration

Microfiltration of saline crude oil emulsions: Effects of dispersant and salinity

Dispersants reduce oil-water interfacial tension making the separation of oil-water emulsions challenging. In this study, crude oil stabilized by the dispersant, Corexit EC9500A, was emulsified in synthetic sea water using a range of Corexit/crude oil concentration ratios (up to 10% by volume). With an interfacial tension of only 8.0 mJ/m2 at 0.5 mL(Corexit)/L, approximately 50% of the crude was dispersed into droplets <10 µm. Near complete rejection of oil in crossflow separation tests was accompanied by a precipitous flux decline attributable in part to dispersant- and salinity-induced decrease in membrane's oleophobicity (4.2 mJ/m2 decrease in surface energy). Screening of electrostatic interactions prompted oil coalescence that occurred at the membrane surface but not in the bulk of the emulsion. Real-time in situ visualization by Direct Observation Through Membrane gave direct evidence of surface coalescence pointing to both its detrimental effects (spread of contiguous films) and possible advantages (removal of large droplets by crossflow shear).This material is based upon work supported in part by the National Science Foundation Partnerships for International Research and Education Program under Grant IIA-1243433. SK was supported by a scholarship from the Turkish Petroleum Corporation. CAH also acknowledges MSU Environmental Science and Policy program for the support via the doctoral recruiting fellowship. ISK’s research stay at MSU was funded by Kyiv Mohyla Foundation of America

Aqueous Swelling of Zwitterionic Poly(sulfobetaine methacrylate) Brushes in the Presence of Ionic Surfactants

Superhydrophilic polyzwitterionic brushes resist fouling, but free ions may screen zwitterion charges and alter brush hydration. This work examines the effect of ionic surfactants on polyzwitterionic brush swelling. In situ ellipsometry shows that the swelling of poly­[2-(methacryloyloxy)­ethyl­dimethyl-(3-sulfopropyl)­ammonium hydroxide] (PMEDSAH) brushes depends on surfactant charge and concentration as well as film thickness. Solutions containing ≥6 mM sodium dodecyl sulfate (SDS) increase the swollen thicknesses of PMEDSAH brushes 2- to 9- fold with respect to thicknesses in water, and increases in swelling are especially high (6- to 9- fold) for thin films. Surfactant adsorption likely breaks ionic cross-links in brushes to enhance swelling, and immersion of brushes in 500 mM NaCl also leads to extensive swelling. Fitting of in situ ellipsometry data suggests that highly swollen films consist of a relatively dense, collapsed base layer covered by dilute brushes that contain about 98% water. At 10 or 20 mM surfactant concentrations, dodecyltrimethyl­ammonium bromide (DTAB) yields much smaller increases in swelling than SDS, presumably because of the hydrophobicity of DTAB. Fluxes through PMEDSAH-modified microfiltration membranes are higher with 2 mM DTAB than with 6 mM SDS, consistent with the higher swelling of thin PMEDSAH brushes in the SDS solution

Aqueous Swelling of Zwitterionic Poly(sulfobetaine methacrylate) Brushes in the Presence of Ionic Surfactants

Superhydrophilic polyzwitterionic brushes resist fouling, but free ions may screen zwitterion charges and alter brush hydration. This work examines the effect of ionic surfactants on polyzwitterionic brush swelling. In situ ellipsometry shows that the swelling of poly­[2-(methacryloyloxy)­ethyl­dimethyl-(3-sulfopropyl)­ammonium hydroxide] (PMEDSAH) brushes depends on surfactant charge and concentration as well as film thickness. Solutions containing ≥6 mM sodium dodecyl sulfate (SDS) increase the swollen thicknesses of PMEDSAH brushes 2- to 9- fold with respect to thicknesses in water, and increases in swelling are especially high (6- to 9- fold) for thin films. Surfactant adsorption likely breaks ionic cross-links in brushes to enhance swelling, and immersion of brushes in 500 mM NaCl also leads to extensive swelling. Fitting of in situ ellipsometry data suggests that highly swollen films consist of a relatively dense, collapsed base layer covered by dilute brushes that contain about 98% water. At 10 or 20 mM surfactant concentrations, dodecyltrimethyl­ammonium bromide (DTAB) yields much smaller increases in swelling than SDS, presumably because of the hydrophobicity of DTAB. Fluxes through PMEDSAH-modified microfiltration membranes are higher with 2 mM DTAB than with 6 mM SDS, consistent with the higher swelling of thin PMEDSAH brushes in the SDS solution