An efficient nanosheet-laminated photocatalytic membrane with high water permeance and photocatalytic activity has been successfully created by an international team of researchers under the direction of Kobe University academics. Since the membrane’s photocatalytic characteristics successfully reduce fouling when exposed to light, cleaning is made simpler.
(a) Electron microscope image showing a cross-section of the nanosheet-laminated photocatalytic membrane developed in this study. (b) Comparison of how different combinations of nanosheets affect water permeation speed. (c) Changes in the rate constant of the rhodamine B photodegradation reaction depending on the combination of nanosheets (inset: photos showing the dye solution before and after photoirradition). Image Credit: Kobe University.
By adhering 2D nanomaterials (nanosheets) to porous support, researchers created this membrane.
This cutting-edge membrane technology may be used to purify water, which has the potential to assist both the global environmental and energy concerns by ensuring clean energy and safe drinking water supplies. It is envisaged that this would accelerate the transition towards carbon-neutral, sustainable communities.
A research team from Kobe University’s Graduate School of Science, Technology, and Innovation/Research Center for Membrane and Film Technology (Associate Professor Keizo Nakagawa, Professor Tomohisa Yoshioka, and Professor Hideto Matsuyama) made this advancement in conjunction with Professor Takashi Tachikawa of the university’s Molecular Photo Science Research Center.
The research group also includes Associate Professor Chechia Hu of the National Taiwan University of Science & Technology and Professor Shik Chi Edman Tsang from Oxford University
On April 7th, 2022, the “Chemical Engineering Journal” published the findings for the first time.
Main Points
A new nanosheet-laminated photocatalytic membrane that exhibits high permeance and photocatalytic activity was successfully created by the researchers.
Both water permeability and photocatalytic activity were significantly increased by the combined effects of the nanosheet materials. Membrane clogging was also decreased by the photocatalytic property (fouling). The researchers suggest employing this membrane in a renewable energy water purification process (light).
Research Background
The lack of adequate access to water in many parts of the world is becoming a bigger issue as a result of climate change, population growth, and economic expansion in emerging nations. According to reports, water shortages will affect two-thirds of the world’s population by 2025.
The widespread adoption of water recycling and cleaning technologies as well as the effective use of water production technologies (such as saltwater desalination) are essential to preventing these acute water shortages.
900 water purification facilities now employ the membrane filtering process because it consistently and reliably produces water of high quality. Membrane fouling, on the other hand, is an issue when the membrane that separates and eliminates impurities from the water gets blocked. When membrane fouling takes place, it becomes impossible to get the necessary volume of treated water.
As a result, the membrane has to be cleaned or changed. Many studies into different fouling prevention techniques have been conducted in the past to address this problem, but an adequate solution is yet to be identified.
There is a technique that uses less energy and is less harmful to the environment. This entails adding a photocatalytic material (like titania) to the membrane and using photocatalysis to remove contaminants. Such a membrane must, however, have strong photocatalytic activity and visible light responsiveness in addition to the ability to treat water. This necessitates that the designer takes the structure and material of the membrane into account while designing the membrane.
The nanofiltration membrane created by this research team functions by using 2D channels among its layers of nanosheets. Scientists generated the 2D channels between the nanosheets by laminating niobate nanosheets, a type of metallic oxide nanosheet with each sheet being about a nanometer thick and a few hundred nanometers broad, onto a porous support membrane.
In this work, it was shown that the water permeance of the niobate nanosheet-layered membrane was significantly improved while the photocatalytic activity was significantly increased by the addition of carbon nitride nanosheets, which are sensitive to visible light. Additionally, the membrane’s photocatalytic characteristics totally resolved the problem of the membrane’s permeance being lowered as a result of fouling.
Research Methodology
Simple vacuum filtration of nanosheet materials (colloidal solutions) onto polymer supporting membranes can result in nanosheet-laminated membranes. In this work, the research team created an ultra-thin membrane made of laminated nanosheets with a thickness of around 100 nm.
Molecular weight fractionation and X-Ray diffraction measurements showed that introducing carbon nitride (*5) nanosheets into a niobate nanosheet-laminated membrane would enable the researchers to control the diameter of nanochannels between the layers.
In terms of membrane performance, the laminated nanofiltration membrane with a 74:25 ratio of niobate (HNB3O8) to carbon nitride (g-C3N4) nanosheet preserved its separation performance while exhibiting an eightfold increase in water permeance.
Visible light could be absorbed, thanks to the incorporation of carbon nitride nanosheets, which improved photocatalytic activity. Additionally, the photodegradation of cationic dyes (rhodamine B) by the membrane was significantly enhanced by this combination of nanosheets.
When the developed composite membrane is used as a separation membrane, the niobate nanosheets give the laminated membrane its structure, while the carbon nitride is introduced between these layers and acts as a spacer. As a result, the laminated membrane’s channels widen, dramatically accelerating the rate of water penetration.
By manipulating the channel structure, it is possible to separate 90% of a dye from the water, which has a molecular weight of about 1000. The membrane’s photocatalytic functionality is as follows: the niobate nanosheets serve as catalytic promoters and the carbon nitride nanosheets serve as photocatalysts that absorb visible light.
The team also found that by properly managing the band structure, the electrons could move around more effectively, which led to a sharp rise in photocatalytic activity. These findings served as the foundation for the researchers’ use of the membrane for water purification and their experiment on membrane fouling utilizing Bovine Serum Albumin (BSA) as the foulant.
The membrane’s water penetration speed dropped to 1/5 of normal due to BSA fouling. However, by irradiating the composite nanosheet membrane, the researchers were able to totally restore its permeance.
Further Research
The researchers successfully created a membrane that exhibits good water permeance and photocatalytic activity by combining several types of nanosheets to create 2D nanochannels. By altering the kind of nanosheet, it is anticipated that additional advancements in membrane performance and photocatalytic activity may be accomplished. This will allow for more precise control over the development of 2D nanochannels and the band structure.
In order to refine the photocatalytic process and expand the membrane area, the researchers will next work toward practical and industrial applications.
A JSPS KAKENHI Grant-in-Aid for Scientific Research (c) (Grant Number: JP19K05121) and the Ministry of Science and Technology, Taiwan, both provided funding for this work (Grant Number: 110-2221-E-011-012-MY3).
Journal Reference:
Imoto, S., et al. (2022) HNb3O8/g-C3N4 nanosheet composite membranes with two-dimensional heterostructured nanochannels achieve enhanced water permeance and photocatalytic activity. Chemical Engineering Journal. doi.org/10.1016/j.cej.2022.136254.