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.
Hong
Seong Uk
SU
Department of Chemical Engineering, Hanbat National University, Daejeon 305-719, South Korea.
Malaisamy
Ramamoorthy
R
Bruening
Merlin L
ML
D000838:Anions / Q000737:chemistry*; D004573:Electrolytes / Q000737:chemistry*; D005374:Filtration; D005459:Fluorides / Q000737:chemistry* / Q000302:isolation & purification*; D049329:Nanostructures / Q000737:chemistry*; D011312:Pressure
D000838:Anions; D004573:Electrolytes; D005459:Fluorides