Polyelectrolyte and silver nanoparticle modification of microfiltration membranes to mitigate organic and bacterial fouling.

Membrane fouling remains one of the most problematic issues surrounding membrane use in water and wastewater treatment applications. Organic and biological fouling contribute to irreversible fouling and flux decline in these processes. The aim of this study was to reduce both organic and biological fouling by modifying the surface of commercially available poly(ether sulfone) (PES) membranes using the polyelectrolyte multilayer modification method with poly(styrenesulfonate) (PSS), poly(diallyldimethylammonium chloride) (PDADMAC), and silver nanoparticles (nanoAg) integrated onto the surface as stable, thin (15 nm) films. PSS increases the hydrophilicity of the membrane and increases the negative surface charge, while integration of nanoAg into the top PSS layer imparts biocidal characteristics to the modified surface. Fouling was simulated by filtering aqueous solutions of humic acid (5 and 20 mg L(-1)), a suspension of Escherichia coli (10(6) colony-forming units (CFU) mL(-1)), and a mixture of both foulants through unmodified and modified PES membranes under batch conditions. Filtration and cleaning studies confirmed that the modification significantly reduced organic and biological fouling.

Separation of fluoride from other monovalent anions using multilayer polyelectrolyte nanofiltration membranes.

Nanofiltration (NF) is an attractive technique for reducing F- concentrations to acceptable levels in drinking water, but commercial NF membranes such as NF 270 and NF 90 show minimal Cl-/F- selectivity. In contrast, simple layer-by-layer deposition of 4.5-bilayer poly(styrene sulfonate) (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) films on porous alumina supports yields NF membranes that exhibit Cl-/F- and Br-/F- selectivities>3 along with solution fluxes that are >3-fold higher than those of the commercial membranes. Fluoride rejection by (PSS/PDADMAC)4PSS membranes, which is >70%, is independent of pressure over a range of 3.6 to 6.0 bar, suggesting that the primary transport mechanism in these films is convection. Moreover, the fact that Br-/F- selectivity is 12% higher than Cl-/F- selectivity suggests that discrimination among the monovalent ions is based on size (Stokes radius). Chloride/fluoride selectivities are essentially constant over Cl-/F- feed ratios from 1 to 60, so these separations will be viable over a range of conditions. Interestingly, PSS/protonated poly(allylamine) films show little Cl-/F- selectivity, and the selectivity of PSS/PDADMAC membranes is a strong function of the number of deposited layers, indicating that NF properties are very sensitive to film structure.

High-flux nanofiltration membranes prepared by adsorption of multilayer polyelectrolyte membranes on polymeric supports.

Layer-by-layer deposition of anionic and cationic polyelectrolytes readily converts polymeric ultrafiltration membranes into materials capable of nanofiltration. ATR-FTIR spectra confirm that layer-by-layer deposition occurs on the ultrafiltration substrates, and adsorption of as few as 2.5 bilayers of poly(styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) or 3.5 bilayers of PSS/poly(diallyldimethylammonium chloride) (PDADMAC) reduces the molecular weight cutoff of polyethersulfone ultrafiltration supports from 50 kDa to <500 Da. Deposition of multilayer polyelectrolyte films on 300 and 500 kDa membranes also decreases molecular weight cutoffs, but solute rejections are significantly lower when using these supports, suggesting that the polyelectrolyte films do not completely cover large (0.2-0.4 microm in diameter) pores. On the 50 kDa substrates, PSS/PDADMAC films containing 3.5 bilayers exhibit a 95% rejection of SO(4)(2-) and a chloride/sulfate selectivity of 27, whereas 4.5-bilayer PSS/PAH coatings show a glucose/raffinose selectivity of 100. Pure water flux for [PSS/PAH](3)PSS-coated membranes at 4.8 bar is 1.6 m(3)/(m(2)day), which is more than 2-fold higher than that through a commercial 500 Da membrane.

Polyurethane and sulfonated polysulfone blend ultrafiltration membranes. I. Preparation and characterization studies.

Polyurethane (ether type) and sulfonated polysulfone (sodium salt form) in the presence of polyethylene glycol 600 were blended in various compositions using N,N'-dimethylformamide as solvent and used for preparing ultrafiltration membranes by the phase inversion technique. Polymer blend composition, additive concentration, and casting and gelation conditions were optimized. Blend membranes were subjected to ultrafiltration characterizations such as compaction, pure water flux, water content, and membrane resistance. The membranes were also subjected to the determination of pore statistics and molecular weight cutoff determination studies using dextran of different molecular weights. Surface morphology of the membranes was analyzed using scanning electron microscopy at different magnifications. The effects of polymer composition and additive concentration on the above parameters were analyzed and the results are compared and discussed with those of pure sulfonated polysulfone membranes. The derived pore size, porosity, and number of pores have a remarkable interrelationship and also have a definite role and relationship with the molecular weight cutoff, morphology, and flux performance of the membranes.