Abstract
Internal concentration polarization (ICP) is a major issue in forward osmosis (FO) as it can significantly reduce the water flux in FO operations. It is known that a hydrophilic substrate and a smaller membrane structure parameter (S) are effective against ICP. This paper reports the development of a thin film composite (TFC) FO membrane with a hydrophilic mineral (CaCO3)-coated polyethersulfone (PES)-based substrate. The CaCO3 coating was applied continuously and uniformly on the membrane pore surfaces throughout the TFC substrate. Due to the intrinsic hydrophilicity of the CaCO3 coating, the substrate hydrophilicity was significantly increased and the membrane S parameter was reduced to as low as the current best of cellulose-based membranes but without the mechanical fragility of the latter. As a result, the ICP of the TFC-FO membrane could be significantly reduced to yield a remarkable increase in water flux without the loss of membrane selectivity.
Highlights
Due to the different water transport mechanisms in RO and forward osmosis (FO), the conventional thin film composite (TFC)-RO membranes show rather poor water permeability in FO processes[9,10,11]
In comparison with other hydrophilic polymers such as polyvinyl alcohol (PVA), the negative charge developed after polyacrylic acid (PAA) dissociation in water could chemically bond Ca+ through electrostatic forces to initiate the CaCO3 coating[32,33,34,35]
Since a smaller S value is indicative of a smaller Internal concentration polarization (ICP) effect, a higher water flux is expected of these CaCO3-coated membranes in the FO operation
Summary
Due to the different water transport mechanisms in RO and FO, the conventional TFC-RO membranes show rather poor water permeability in FO processes[9,10,11]. Through the electrostatic interaction between the carboxylate groups of PAA and Ca2+, the coverage and continuity of the CaCO3 coating in PES could be progressively built up to improve the hydrophilicity of the PES substrate without the trade off in its mechanical strength. We apply a previously reported alternative soaking process (ASP)[30,31] to produce TFC-FO membranes with a CaCO3-coated PES substrate.
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