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(1) Membranes for addressing the challenge of emerging pollutants
(1) Membranes for addressing the challenge of emerging pollutants
Emerging pollutants (EPs) such as antibiotics, steroids, or fluorinated compounds are now commonly found in environmental waters. Current wastewater treatment technologies cannot address the issue, suggesting the need for improved materials and systems.
A current research direction in our lab is the development of photocatalytic membrane reactors (PMRs) for the removal of EPs from water. By combining the catalytic properties of certain nanomaterials with the separation capabilities of membranes, we aim to enhance the overall EP removal rate. We either incorporate nanomaterials directly into the membrane (as shown in the SEM image located at the bottom right) or apply them through a spray-coating approach.
For instance, in our recent studies (article here), we have managed to achieve unprecedented removal of tetracyline, an antibiotic, and are now shifting focus toward the removal of steroid hormones and microplastics.
Some of these studies are performed in collaboration with Prof. Chechia Hu at National Taiwan University of Science & Technology (more about him here).
(2) Vapor-Induced Phase Separation (VIPS) Membranes and Their Applications in Direct Contact Membrane Distillation for Desalination and Wastewater Treatment
(2) Vapor-Induced Phase Separation (VIPS) Membranes and Their Applications in Direct Contact Membrane Distillation for Desalination and Wastewater Treatment
Our team focuses on a process that aims to form membranes from a gaseous atmosphere of non-solvent, the so-called vapor-induced phase separation process or VIPS. Like the more common liquid-induced phase separation (LIPS) process, it is regulated by the interplay between thermodynamic, mass transport, viscoelastic properties, and sometimes chemical reactions. But a different boundary condition, gas/liquid as opposed to liquid/liquid, is often associated with slower mass transfers. In other words, membranologists can gain better control over membrane formation using VIPS than LIPS. We are interested in gaining knowledge on the important features essential to the fine-tuning of VIPS membranes, and we spend effort on correlating aspects of thermodynamic and mass transfers to the structures obtained. We investigate the role of process and formulation parameters on the final morphology of the films, and look for potential applications of these matrices.
We have also recently been exploring the potential of VIPS membranes for efficient membrane desalination (article here) or pollutant removal by direct contact membrane distillation (DCMD). DCMD is a promising separation process that utilizes a semipermeable membrane to separate two liquid phases based on their vapor pressure differences. One of the critical components in DCMD is the membrane itself, which should possess several key properties, such as high hydrophobicity, selective permeability to water vapor, and mechanical stability, to ensure efficient and sustainable performance. The unique features of VIPS membranes make them promising candidates for numerous applications of DCMD (desalination of seawater, removal of metal ions from wastewater, etc.).
Seawater is collected at the beach nearby our University (left photograph), and then used in our DCMD process (photograph in the middle). The temperature gradient between the feed side and the permeate side (schemed on the right) enables the transfer of water. We keep the feed temperature at a relatively low level (60 Celcius degrees or below) to minimize the energy requirements.
(3) Membrane formation
(3) Membrane formation
The performance of a membrane depends on its structure, which is influenced by a variety of formulation and process parameters. These parameters include polymer concentration, nature of the solvent/non-solvent system, presence of additives, dissolution temperature, non-solvent bath/atmosphere temperature, and non-solvent activity, among others. They affect the composition path of the polymeric system during phase inversion and the kinetics of phase separation. In the example below, three types of structures are observed, which were obtained from a similar polymer but through different membrane formation mechanisms. Each structure has its advantages, drawbacks, and potential applications. A long-term goal of our group is to predict the structure and resulting properties (such as porosity and pore size) of membranes based on the knowledge of formulation and process parameters, without the need for membrane preparation.
(4) Green membranes and antifouling green membranes
(4) Green membranes and antifouling green membranes
Membrane fouling arises from the interactions between solutes and the membrane material. It is inevitable since membranes aim to separate solutes. Rejected solutes may establish low-energy interactions with the membrane material on its surface, and so, can be easily removed by a simple cleaning procedure. This is referred to as reversible fouling. On the contrary, irreversible fouling may occur if strong hydrophobic or electrostatic interactions are established. Fouling causes flux decline. Consequently, the separation process needs to be temporarily stopped to wash the membrane. If irreversible fouling dominates, the membrane has to be replaced, ultimately leading to production delay and process costs increase.
There are strategies to limit fouling. In our lab, we focus primarily on biofouling. It concerns interactions of the membrane with proteins, bacteria and other kinds of cells (eg.: blood cells if the membrane is used in blood-contacting devices). It can be mitigated by incorporating so-called anti-biofouling materials in the dope solution (in-situ modification) or by coating/grafting these materials on the surface of membranes during a post-treatment procedure (surface modification). An example of grafting process is seen in the video, where a hydrophobic membrane is being modified by plasma-enhanced chemical vapor deposition (PECVD). We work primarily with derivatives of poly(ethylene glycol) (PEGylated copolymers) as well as with zwitterionic materials.
Green membranes are membranes formed from as many environment-friendly materials as possible. Very common polymers used to fabricate porous membranes are polysulfone (PSf), polyvinylidene difluoride (PVDF), polyethersulfone (PES), etc. Their solvents (N-methylpyrrolidone (NMP), dimethylacetamide (DMAC), dimethylformamide (DMF), etc.), employed in large quantities for membrane preparation in industry, are toxic. So, their use is getting restricted and even banned in some cases. As such, membranologists need to find alternatives greener or green solvents that could still lead to the desired membrane properties in terms of pore size, porosity, etc.
In our lab, we have been fabricating membranes for many years using NMP or DMAC but are now switching to greener solvents including dimethyl sulfoxide, triethyl phosphate, cyrene, gamma-valerolactone, etc. The tricky part is to maintain the properties of the membranes, that is, to be able to prepare membranes with similar separation performances. Switching solvents means redefining the thermodynamic stability of the polymer/solvent/non-solvent system, as well as the kinetic of phase inversion. It results in a change of membrane structure, and so, of membrane properties. Our team also makes efforts to develop not only green membranes with a controlled structure, but also green antifouling membranes.
(5) Membranes for blood filtration
(5) Membranes for blood filtration
We have designed membranes able to selectively catch blood cells in collaboration with Prof. Yung Chang (more about him here). For example, leukodepletion filters can be designed by incorporating functional materials able to selectively interact with leukocytes. We often use the electrospinning process to prepare such membranes. We also work with non-woven commercial materials which we modify by coating or grafting some functional groups with the view to imparting functional properties (hemocompatibility, cell catching) to the membrane.
Graphical abstract from one of our articles in the Journal of Membrane Science
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