(777c) Polydopamine Modified Commercial Thin Film Composite Membranes for Forward Osmosis: Flux and Desalination Performance

Forward osmosis
is a water treatment technology seen a tool to aid in the treatment,
desalination, and purification of feed waters often unsuitable for other
pressure driven membrane separation processes[1-3]. Until recently there was a single commercially available membrane for use in forward osmosis processes; the cellulose triacetate manufactured and sold by Hydration Technology Innovations (HTI^TM). Being made from cellulose acetate this membrane is vulnerable to hydrolysis under alkaline conditions, reducing membrane selectivity. In recent years commercial and academic interest has focused on developing more chemically resilient forward osmosis membranes. Thin film composite (TFC) membranes have garnered great interest as an alternative platform for forward osmosis. 

TFC comprise the
vast majority of commercial reverse osmosis and nanofiltration membranes being
used for such purposes over the last 30 years. TFC membranes are built upon a
three tiered structure comprising of an
ultra-thin aromatic polyamide layer supported by a polysulfone (PSu) or
polyethersulfone (PES) layer that is cast onto a polyester (PET) nonwoven.
Early work using TFC membranes in FO by McCutcheon found the performance of TFC
RO membranes to be inferior to that of CA FO membranes[4]. Later
work found, lack of support layer wetting, leading to severe internal
concentration polarization (ICP) and hindered osmotic flux due to a reduced
effective porosity[5]. A possible solution to this problem is to make a membrane with
an intrinsically hydrophilic support; however, this would require a retuning of
the delicate interfacial polymerization process, which can be impacted by the
support layer properties. An alternative is to take an existing membrane that
has been optimized for superior permselectivity and modify the support layer to
increase its hydrophilicity.

Later work by Arena utilized polydopamine (PDA) for
TFC membrane support modification for improved hydrophilicity observing
a significant improvement in the water flux of PDA modified TFC RO membranes in
the pressure retarded osmosis (PRO) orientation with no transmembrane pressure[6]. The
work presented here is a follow up on that work finding similar performance
improvements with these same PDA modified membranes in the forward osmosis (FO)
orientation. Additionally these membranes were tested under desalination
conditions using the ammonia-carbon dioxide (NH₃-CO₂).
Water and ion fluxes were measured from these tests and found unequal rates of
cation and anion transport through these membranes.

Additionally water and ion fluxes for NH₃-CO₂
draw solution with varying ammonia to carbon dioxide ratios were studied using
commercial membranes, seeking to gain better understanding on the effects of
varying ammonia to carbon dioxide ratios on membrane water and ion fluxes.

Figure 1: Solute fluxes for osmotically driven sodium
chloride rejection. The lined bar represents ammonia species reverse solute
fluxes, the solid bar represents sodium ion forward solute fluxes, and the
cross-hatched bard represents chloride ion forward solute fluxes at 23±1 °C,
0.25 m/s draw and feed cross-flow velocity.

Works Cited

[1]  McGinnis RL, Hancock NT,
Nowosielski-Slepowron MS & McGurgan GD. Pilot demonstration of the NH₃/CO₂
forward osmosis desalination process on high salinity brines. Desalination
(2013) 312: pp. 67-74.

[2]  Cath TY, Childress AE &
Elimelech M. Forward osmosis: principles, applications, and recent developments.
Journal of Membrane Science (2006) 281: pp. 70-87.

[3]  Hancock NT, Black ND &
Cath TY. A comparative life cycle assessment of hybrid osmotic dilution
desalination and established seawater desalination and wastewater reclamation
processes. Water research (2012) 46: pp. 1145-1154.

[4]  McCutcheon JR, McGinnis RL
& Elimelech M. A novel ammonia-carbon dioxide forward (direct) osmosis
desalination process. Desalination (2005) 174: pp. 1-11.

[5]  McCutcheon JR &
Elimelech M. Influence of membrane support layer hydrophobicity on water flux
in osmotically driven membrane processes. Journal of Membrane Science (2008)
318: pp. 458-466.

[6]  Arena JT, McCloskey BD,
Freeman BD & McCutcheon JR. Surface modification of thin film composite
membrane support layers with polydopamine: enabling use of reverse osmosis
membranes in pressure retarded osmosis. Journal of Membrane Science (2011)
375: pp. 55-62.