Forward osmosis (FO) has received considerable interest for water- and energy-related applications in recent years. FO does not require an applied pressure and is believed to have a low fouling tendency. However, a major challenge in FO is the lack of high performance FO membranes. In the current work, novel nanofiltration (NF)-like FO membranes with good magnesium chloride retention were synthesized using layer-by-layer (LbL) assembly. The membrane substrate was tailored (high porosity, finger-like pores, thin cross-section, and high hydrophilicity) to achieve a small structural parameter of 0.5 mm. Increasing the number of polyelectrolyte layers improved the selectivity of the LbL membranes while reducing their water permeability. The more selective membrane 6#LbL (with 6 polyelectrolyte layers) had much lower reverse solute transport compared to 3#LbL and 1#LbL. Meanwhile, the FO water flux was found to be strongly affected by both membrane water permeability and solute reverse transport. Severe solute reverse transport was observed for the active-layer-facing-draw-solution membrane orientation, likely due to the suppression of Donnan exclusion as a result of the high ionic strength of the draw solution. In contrast, the active-layer-facing-feed-solution orientation showed remarkable FO performance (15, 20, and 28 L/m².h at 0.1, 0.5, and 1.0 M MgCl₂, respectively, for membrane 3#LbL using distilled water as feed solution), superior to other NF-like FO membranes reported in the literature. To the best of the knowledge of the authors, this is the first work on the synthesis and characterization of LbL based FO membranes.
Transport of water, solutes, and
contaminants through a thin film
composite (TFC) membrane is governed by the intrinsic structure of
its polyamide separation layer. In this work, we systematically characterized
the nanoscale polyamide structure of four commercial TFC membranes
to reveal the underlying structure–property relationship. For
all the membranes, their polyamide layers have an intrinsic thickness
in the range of 10–20 nm, which is an order of magnitude smaller
than the more frequently reported apparent thickness of the roughness
protuberances due to the ubiquitous presence of nanovoids within the
rejection layers. Tracer filtration tests confirmed that these nanovoids
are well connected to the pores in the substrates via the honeycomb-like
opening of the backside of the polyamide layers such that the actual
separation takes place at the frontside of the polyamide layer. Compared
to SW30HR and BW30, loose membranes XLE and NF90 have thinner intrinsic
thickness and greater effective filtration area (e.g., by the creation
of secondary roughness features) for their polyamide layers, which
correlates well to their significantly higher water permeability and
lower salt rejection. With the aid of scanning electron microscopy,
transmission electron microscopy, and tracer tests, the current study
reveals the presence of nanosized defects in a polyamide film, which
is possibly promoted by excessive interfacial degassing. The presence
of such defects not only impairs the salt rejection but also has major
implications for the removal of pathogens and micropollutants.
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