Chapter 2 Impact of Surface Wettability on Fouling Resistance of MD Membranes
2.3. Results and Discussions
2.3.3. Membrane Performance in Direct Contact Membrane Distillation
DCMD experiments with feed solution containing crude oil, using the fabricated composite membrane and pristine PVDF membrane, demonstrated drastically different fouling behaviors between the two membranes (Figure 2.4). Significant fouling was observed in DCMD experiments with the pristine PVDF membrane, as indicated by an appreciable decline of water vapor flux in a short period of time (red circles in Figure 2.4). Since the pristine PVDF membrane was underwater hydrophobic and oleophilic (Figure 2.2I), the oil droplets would tend to attach onto the surface of the pristine PVDF membrane via the long-range hydrophobic-hydrophobic interaction.
The attachment and accumulation of the oil droplets on the membrane surface resulted in blockage of membrane pores that served as the pathways for water vapor, and consequently led to the decline of water vapor flux. In addition, because the oil droplet sizes were significantly larger than the membrane pore size, the fouling occurred almost immediately when the MD operation started.
20000 4000 6000 8000 10
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Oil Droplet Size (d.nm)
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Figure 0.4 Normalized water (vapor) flux for the fabricated composite membrane (blue squares) and pristine PVDF membrane (red circles) with feed solution containing 1,000 ppm crude oil in the DCMD experiments. The time-dependent water fluxes were normalized by the initial water fluxes. The cross flow velocities of the streams in the membrane cell of all DCMD experiments were 9.6 cm/s and 4.3 cm/s in the feed stream and distillate stream, respectively. The feed and distillate temperatures were 60°C and 20°C, respectively. The initial water fluxes for PVDF membrane and composite membrane were 30.96 L m-2 h-1, and 26.15 L m-2 h-1, respectively.
In comparison, the DCMD performance of the composite membrane with asymmetric wettability was significantly more stable. When challenged by the oily feed solution that markedly fouled the pristine PVDF membrane, the fabricated composite membrane maintained a stable water vapor flux without significant flux decline in the 36-hour operation (blue squares). In addition, the salt rejection rate of the composite membrane always remained ~100%.
The anti-oil-fouling property of the composite membrane is attributable to the in-air hydrophilic and underwater oelophobic CTS-PFO/SiNPs nanocomposite coating on the membrane surface. The hydrophilic moieties on the chitosan strongly interact with water and keep the membrane surface hydrated, providing a hydration layer that deters the oil droplet from attaching[128,129]. Such anti-fouling mechanism for the composite membrane is similar to the mechanism for its underwater oleophobicity suggested by the underwater oil CA measurements.
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Composite PVDF
N o rm a liz e d W a te r F L u x ( J /J
0)
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The low surface tension fluoro- functional groups on PFO, on the other hand, may further contribute to fouling mitigation due to its weak interaction with the oil foulants. The comparative advantage of the membranes with both hydrophilic and oleophobic functional groups, over those with only hydrophilic functional groups, in the effectiveness of fouling control, has been reported in literature for microfiltration membranes[86].
The fact that the composite membrane maintained a perfect salt rejection throughout the experiment suggests that the PVDF substrate of the composite membrane remained hydrophobic and functioned as a barrier for liquid transport as expected. The prevention of undesired wetting of the hydrophobic substrate was achieved by carefully coating only the membrane surface with the CTS-PFO/SiNPs nanocomposite by maintaining a low spraying rate, a small spraying angle, and a high degree of nebulization. Other common surface modification approaches, such as dip- coating or spin-coating[106,108,110,111], may result in membranes that are partially or entirely wetted when immersed in water. Furthermore, even in the presence of naturally occurring surfactants that wetted the pristine PVDF membrane, the highly hydrophilic coating on the composite membrane helped alleviate surfactant-induced wetting and maintained an uncompromised salt rejection. A similar phenomenon has been observed in other studies in which composite membranes were challenged with alternative wetting agents[52]. However, the exact mechanism behind this anti-wetting property of the highly hydrophilic coating remains unclear.
It is worth noting that the initial water vapor flux with the fabricated composite membrane (26.15 L m-2 h-1) was lower than that with the pristine PVDF membrane (30.96 L m-2 h-1). The reduction of the initial water flux might be attributable to the blocking of the membrane pores by the CTS-PFO/SiNPs nanocomposite as suggested by the SEM image presented in Figure 1B.
However, the detrimental impact of the nanocomposite coating on the vapor permeability of the membrane might not be as significant as the portion of the pore blockage suggests, because CTS- PFO/SiNPs coating was a porous and superhydrophilic hydrogel network that liquid water might freely penetrate.
On the other hand, the hydrodynamic condition in such a hydrogel network was stagnant compared to that in the cross-flow feed stream as a result of both the absence of hydrodynamic mixing and strong interaction between water and hydrophilic functional groups in such a network.
The relatively ineffective mass and heat transfer of liquid water in the coating layer enhanced temperature polarization[16,136,137], curtailing the driving force for vapor transfer and
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consequently reducing the water vapor flux. This effect of enhanced temperature polarization is a very important, if not the dominant, contribution to the reduction of initial vapor flux.