86 Figure 5.4 Experimental results and model predictions of DCMD membrane wetting experiments with different critical concentrations. 93 Figure 6.1 SEM images of local surface morphology for PVDF membrane, composite membrane and omniphobic membrane.
Introduction
- The Emerging Need for Hypersaline Industrial Wastewater Desalination
- Membrane Distillation as a Promising Solution
- Challenges with Conventional Hydrophobic MD Membranes
- Membrane Fouling
- Membrane Wetting
- The Applications of Membranes with Special Wettability
- Objectives and Hypothesis
- Structure of the Dissertation
As shown in Figure 1.2, in the presence of oil droplets in the feed solution of MD operation, the membrane pore would be easily clogged, leading to a significant decrease in vapor flux[32]. As is well known, the wetting of membrane pores can be induced by various wetting agents, such as low surface tension and water-miscible liquids (e.g., alcohols) and amphiphilic molecules (e.g., surfactants).
Impact of Surface Wettability on Fouling Resistance of MD Membranes
- Introduction
- Materials and Methods
- Materials and Chemicals
- Composite Membrane Fabrication
- Membrane Characterizations
- Preparation and Characterizations of Oil Feed Solution
- Membrane Fouling Test
- Force Measurement of Underwater Oil-Membrane Interaction
- Results and Discussions
- Morphology and Wetting Properties of the Pristine and Modified Membranes
- Properties of Oily Feed Solution
- Membrane Performance in Direct Contact Membrane Distillation
- Underwater Oil-Membrane Interaction
- Implications
In-air water CA of (D) the modified surface of the composite membrane, (E) the unmodified surface of the composite membrane, and (F) the pristine PVDF membrane. Underwater oil CA of (G) the modified surface of the composite membrane, (H) the unmodified surface of the composite membrane, and (I) the pristine PVDF membrane.
Influence of Surface Charge on Fouling Resistance of MD Membranes
Introduction
An effective approach to mitigating oil fouling in MDs is to develop membranes with an underwater oleophobic surface that resists oil adhesion when submerged in water. Following this principle, composite MD membranes with a hydrophobic substrate and an underwater oleophobic top surface have been developed using hydrogel-nanoparticle composites to mitigate oil fouling [100,141].
Materials and Methods
- Materials and Chemicals
- Fabrication of the Underwater Oleophobic Membranes
- Composite membrane coated with polydopamine (PDA) and SiNPs
- Composite membrane coated with PDDA and SiNPs
- Characterizations of Morphology and Wetting Property
- Preparation and Characterization of Oily Feed Solution
- Streaming Potential Measurement of the Membranes
- Membrane Fouling Tests
- Underwater Oil-Adhesion Force Measurements
The morphology of the oil droplets in the prepared solution was observed using an optical microscope (Nikon LV100, Japan). DCMD experiments were performed with the saline crude oil emulsion to compare the fouling behaviors of the pristine PVDF membrane and the fabricated composite membranes.
Results and discussions
- Membrane Surface Properties
- Membrane Surface Morphology
- Membrane Surface Potential
- Membrane Surface Wetting Property
- Properties of Saline Crude Oil Emulsion
- Membrane Performances in MD Experiments
- Force Spectroscopy with Oil Droplet Probe
- Implications of Electrostatic Interaction
- The Relative Irrelevance of the DLVO Approach
Cross-sectional SEM images of (D) the commercial PVDF membrane, (E) the PDA/SiNPs-PVDF composite membrane, and (F) the PDDA/SiNPs-PVDF composite membrane. Surface AFM images of (G) the commercial PVDF membrane, (H) the PDA/SiNPs-PVDF composite membrane, and (I) the PDDA/SiNPs-PVDF composite membrane.
Conclusion
However, since adhesion free energy is a thermodynamic property, it still cannot explain the dynamic behavior of the PDA/SiNPs-PVDF membrane in CA measurement and force spectroscopy. We speculate that such dynamic wetting behavior involves redistribution of natural anionic surfactants at the crude oil–water interface. These anionic surfactants have lateral mobility along the interface and can therefore migrate under the influence of an electric field.
When the oil droplet was in contact with a positively charged surface, these anionic surfactants gradually concentrated to the contact area and thereby increased the local electronegativity, leading to a higher electron donor parameter of the oil surface and thus a stronger attractive AB interaction between the oil droplet and the membrane.
Mechanism of Membrane Pore Wetting in Membrane Distillation
Introduction
Previous studies have shown that the presence of low-surface-tension (LST) or amphiphilic agents in the feed solution induces pore wetting in MD with hydrophobic membranes. This theoretical framework can explain pore wetting induced by lowering the surface tension of the feed solution via the addition of LST and water-miscible liquid, such as alcohol, into the aqueous feed solution. In this study, we investigate the use of an impedance-based technique to monitor pore wetting in MD.
Specifically, we perform DCMD experiments with a commercial hydrophobic polyvinylidene fluoride (PVDF) membrane and induce pore wetting by adding surfactants to the feed solution. Utilizing this impedance-based technique, we comparatively investigate pore wetting behaviors in direct contact membrane distillation (DCMD) with feed solutions containing two different types of wetting agents.
Theory and Model Development
- General Criterion of Pore Wetting
- Membrane Wetting by Alcohol Solution
- Membrane Wetting by Surfactants
For pore wetting with Triton X-100 solution, the liquid–air interface (ie, the wetting boundary) propagates forward toward the distillate when the transmembrane pressure exceeds the LEP corresponding to the Triton X-100 concentration at the liquid–air interface. The relevant LEP for the propagation of the wetting boundary (i.e., the liquid–air interface) depends on the interfacial concentration of surfactants, which in turn is controlled by the transport of surfactants to the wetting boundary. An important boundary condition is that the concentration of surfactant at the wetting boundary is such that the corresponding LEP is equal to ∆𝑃.
In other words, the wetting boundary is depleted of surfactants due to the continuous and rapid removal of surfactants from the solution via adsorption on the pore surface near the boundary. As a result, the 𝛾𝐿 of the wetting boundary is close to the 𝛾𝐿 of wetting-free water due to the very low interfacial wetting concentration.
Materials and Methods
- Materials
- Surface Tension Measuremnts
- Principle and Experimental Setup of impedance-based Wetting Monitoring
- MD Wetting Experiments
A potentiostat (SP-150, BioLogic Science) was coupled to the DCMD system to perform in situ measurements of the impedance across the membrane during MD. Two titanium electrodes (1 cm × 0.5 cm) were placed in direct contact with the two sides of the MD membrane. For each membrane wetting experiment, both the time-resolved impedance and the salt rejection rate of the membrane were monitored.
Real-time salt rejection of the membrane was obtained from real-time water flux and distillate conductivity. Where 𝑍0 is the initial transmembrane impedance, 𝑍(𝑡) is the transmembrane impedance at time 𝑡, 𝑑0 is the thickness of the membrane, which was 180 µm for the membrane used in this study.
Results and Discussion
- Ethanol-Induced Wetting: a Simple Application of the General Wetting Criterion
- Surfactant-Induced Wetting: a Dynamic Process Limited by Surfactant Transport
The experiment shown in Figure 4.3b is an example of inducing wetting by lowering the LEP by reducing the surface tension of the feed solution via the addition of ethanol. A counterintuitive prediction from Equation 4.9 is that the rate of surfactant-induced pore wetting is independent of ∆𝑃. In other words, a lower critical surfactant concentration at the wetting boundary does not lead to a lower equilibrium surface concentration of the adsorbed surfactants on the pore surface near the wetting boundary.
In any case, the aqueous surfactant concentration at the wetting boundary cannot increase to the critical concentration required for forward propagation of the boundary until the surrounding pore surface is almost completely saturated with the adsorbed surfactants. In this case, ∆𝑃 is always higher than the LEP of the feed solution, regardless of the surfactant concentration at the wetting limit.
Conclusion
Kinetic Model for Surfactant-Induced Pore Wetting in Membrane Distillation
Introduction
Interestingly, despite the technological successes in the development of wetting prevention measures, the exact mechanism of pore wetting in MD is not fully understood. In a recent study, we showed the fundamental difference between pore wetting induced by alcohol, with which the impact of surface adsorption is negligible, and by surfactants, with which the impact of surface adsorption is significant[219]. Our study also suggests that, while the presence of wetting agents in feed solution promotes wetting, the adsorption of wetting agents on pore surface does not promote wetting by making the pores hydrophilic, but rather deters pore wetting through the surface tension of the feed solution at the liquid-air interface[219].
In this study, we investigate the kinetics of pore wetting in MD with feed solutions containing surfactants. We use sodium dodecyl sulfate (SDS) as a model surfactant to validate several key features predicted by the model, and then study the membrane pore wetting induced by different surfactant species with the model.
Theory and Model Development
The adsorption causes a decrease in the SDS concentration at the wetting boundary and consequently leads to an increased LEP (calculated from the reduced boundary SDS concentration), which hinders the further progression of the wetting boundary in the membrane pore. On the other hand, convective transport of SDS tends to increase the SDS concentration at the wetting boundary. In other words, diffusion tends to decrease the SDS concentration at the wetting limit when the bulk SDS concentration is low.
Regardless of the SDS concentration in the feed solution, the transport of SDS within an ideal cylindrical pore can be described by Eq. Given the position of the wetting boundary as quantified by l(t), the SDS concentration profile can be obtained using Eqs.
Materials and Methods
- Materials and Chemicals
- Surface Tension Measurements
- Intrinsic Contact Angle Measurements
- Determination of LEP
- Surface Excess Concentration
- DCMD Wetting Experiments
The shape of the air bubble was analyzed using the instrument's built-in software to obtain the surface tension of the solutions. The roughness of the reconstructed PVDF film was measured by atomic force microscopy (AFM, Dimension ICON. For each SDS concentration, CA measurements were performed at 5 different positions on the reconstructed PVDF film, and the mean and standard deviation of the measurements are reported.
With the surface tension of the feed solution and intrinsic CA of the feed solution on the reconstructed PVDF film, we use Eq. The real-time salt rejection of the membrane was calculated using the monitored vapor flux and conductance data.
Results and Discussion
- The Impact of SDS Concentration
- The Impact of Vapor Flux
- The Impact of Transmembrane Hydraulic Pressure
- Membrane Wetting Kinetics for Different Surfactant Species
For model simulation, c0 and Jw are based on the experimental conditions, and the critical SDS concentration c∗ in the wetting limit is assumed to be 0.42 mM, which corresponds to a LEP of 6 kPa. Wetting can occur in the absence of vapor flux due to transport of SDS to the wetting boundary via diffusion if the concentration of bulk surfactant is sufficiently high. Regardless of c0, a higher c∗ always contributes negatively to the transport of SDS to the wetting boundary.
The pseudo force equilibrium assumption requires that the actual surfactant concentration at the wetting boundary be kept as 𝐶′. According to the kinetic model, the critical concentration (𝐶′) at the wetting boundary affects the diffusive transport of surfactants because it affects the concentration gradient.
Conclusion
The theoretical predictions are simulated using the full kinetic model of surfactant-induced pore wetting as reported in our previous publication [238] and briefly summarized in Appendix D. The random choice of a 𝐶′ for SDS will lead to a large range of predicted 𝑡𝑤𝑡s from 𝑡𝑤𝑡s to infinity (i.e., the membrane will never become wet), highlighting the importance of diffusion in the axial transport of SDS in the pore wetting process. We believe the developed model is universally applicable for predicting kinetics of pore wetting induced by other surfactants or any highly surfactants that (1) can significantly change the surface tension of an aqueous solution even at low concentration, and (2) strongly adsorb on the pore surface.
On the contrary, it discourages pore wetting by continuously removing surfactant at the wetting boundary, thereby increasing its LEP. For different types of surfactants, surface excess concentration has a strong influence on surfactant-induced pore wetting kinetics.
Stategies for Fouling and Wetting Mitigation in Membrane Distillation Opertation
- Introduction
- Materials and Methods
- Materials and Chemicals
- Fabrication of Composite Membrane with Superhydrophilic Skin Layer
- Membrane Characterization
- Crude Oil-in-water Emulsions
- Membrane Fouling Test
- Oil-probe Force Spectroscopy
- Results and Discussion
- Membrane Morphology
- Membrane Wetting Properties
- Characterizations of Crude Oil-in-Water Emulsions
- Membrane Performance with Oil-in-Water Emulsion without Synthetic
- Membrane Performance with Oil-in-Water Emulsion Stabilized by Synthetic
- Force Spectroscopy to Elucidate Oil-Membrane Interaction
- Conclusion
The wetting properties of the omniphobic membrane were significantly different from those of the PVDF and CTS/PFO membranes. The underwater adhesion force between the oil droplet and the PVDF membranes (~200 μN) was significantly stronger than that of the composite membrane and the omniphobic membrane. First of all, no strong adhesive force was detected in the contact between the oil droplet and the superhydrophilic surface of the composite membrane.
The force curve for the interaction of the oil droplet with the omniphobic membrane in the presence of surfactants was similar to that with the composite membrane in DI water. However, the omniphobic membrane differed from the hydrophobic membrane in that the oil droplets could not penetrate the pores of the omniphobic membrane due to the reentrant structure.
Conclusions and Future Work
In this chapter, we systematically investigated the impact of surface wetting properties on membrane wetting and/or fouling behavior in MD. The pore size distribution of the membrane was examined using a Capillary Flow Porometer, CFP (Porolux 1000, IB-FT GmbH, Germany). As 𝑃𝑓 increases further, 𝜃𝑓 will reach a maximum, 𝜃𝑓,𝑚𝑎𝑥, which is determined by the surface tensions of the feed solution and the membrane material.
The second part of the second integral of the RHS is the amount of surfactant in the solution within the pore. Each curve represents the change in salt rejection rate in a wetting experiment, and the color of the curve specifies the type of surfactant used in the experiment. The purple dashed line indicates a salt rejection rate of 99%, which is defined as the criterion for membrane pore breakthrough.
Rubner, et al., Patterned superhydrophobic surfaces: towards a synthetic mimic of the namib desert beetle, Nano Lett.
