Permeability and selectivity of reverse osmosis membranes: correlation to swelling revisited.
Membrane swelling governs both rejection of solutes and permeability of polymeric membranes, however very few data have been available on swelling in water of salt-rejecting reverse osmosis (RO) membranes. This study assesses swelling, thickness and their relation to water permeability for four commercial polyamide (PA) RO membranes (SWC4+, ESPA1, XLE and BW30) using atomic force microscopy (AFM) and attenuated total reflection Fourier transform IR spectroscopy (ATR-FTIR). ATR-FTIR offered a significantly improved estimate of the actual barrier thickness of PA, given AFM is biased by porosity ("fluffy parts") or wiggling of the active layer or presence of a coating layer. Thus obtained intrinsic permeability (permeability times thickness) and selectivity of aromatic polyamides plotted versus swelling falls well on a general trend, along with previously reported data on several common materials showing RO and NF selectivity. The observed general trend may be rationalized by viewing the polymers as a random composite medium containing molecularly small pores. The results suggest that the combination of a rigid low dielectric matrix, limiting the pore size, with multiple hydrophilic H-bonding sites may be a common feature of RO/NF membranes, allowing both high permeability and selectivity.
University of Zagreb, Faculty of Chemical Engineering and Technology, Marulićev trg 19, 10000 Zagreb, Croatia. Electronic address: edraz@fkit.hr.
Emil
E
Dražević
University of Zagreb, Faculty of Chemical Engineering and Technology, Marulićev trg 19, 10000 Zagreb, Croatia.
Krešimir
K
Košutić
Technion - Israel Institute of Technology, Wolfson Department of Chemical Engineering, Technion City, 32000 Haifa, Israel. Electronic address: vfreger@tx.technion.ac.il.
Viatcheslav
V
Freger
D004560:Electricity; D008567:Membranes, Artificial; D018625:Microscopy, Atomic Force; D009757:Nylons / Q000737:chemistry; D009995:Osmosis; D010539:Permeability; D017550:Spectroscopy, Fourier Transform Infrared; D014867:Water
D008567:Membranes, Artificial; D009757:Nylons; D014867:Water
Dielectric exclusion; Nanofiltration; Permeability; Reverse osmosis; Salt rejection; Swelling