Researchers from the IISc have developed a new way to design thin, porous membranes that can be used for water filtration. By carefully mixing two polymers, and adding some nanostructures, they obtained membranes with ultrasmall holes in it. They have also shown that these membranes are more efficient in killing the bacteria commonly found in drinking water.
Membrane-based water purification methods have many advantages over the conventional techniques. As Dr Suryasarathi Bose, who led the study, says, “the main advantage of membrane based filtration system is the fact that it works without the addition of chemicals, with a relatively low energy and easy and well-arranged processing”.
Polymers, like plastic, have an interesting character: a new class of materials of desired properties can be obtained by carefully blending two known polymers. IISc researchers chose a well studied combination to mix: PVDF/PMMA (poly(vinylidene fluoride)/poly(methyl methacrylate)). They also added different nanostructures to the resulting membrane, and studied how they influenced the killing of Escherichia coli, the most common microorganism in drinking water. Their findings are published in Journal of Materials Chemistry A.
The researchers first mixed PVDF and PMMA polymers and allowed the blends to crystallize, and removed the PMMA phase before obtaining the final membrane. They developed different configurations of the membrane, each with a different nanostructure embedded in it. Some of the configurations had just nanoparticles, whereas a few others had carbon nanotubes decorated with nanoparticles. In general, the addition of nanostructures allowed water seep through the membrane quickly, thus speeding up the filtration process. Also, membranes with embedded nanostructures were found to be more efficient in killing the Escherichia coli, the microorganism targeted in the study. Overall, carbon nanotubes decorated with both silver and titanium dioxide nanoparticles were found to be the quickest and most efficient in inactivating the bacteria. According to the researchers, this is because of the cumulative effect of the antibacterial properties of the nanoparticles and the carbon nanotubes.
Antibacterial properties of silver and titanium dioxide nanoparticles are well known. Silver nanoparticles damage cell proteins, and trigger a process that will ultimately abort DNA replication. Titanium dioxide has a different modus operandi: it forms some hydroxyl free radicals and peroxides, which kill bacteria. Even carbon nanotubes, in addition to it’s impressive mechanical, physical and electrical properties, can also kill bacteria. Though the antibacterial properties of these individual nanostructures are quite well studied, scientists are not sure how effective they would be, when they attack in a group.
According to Dr Bose, “The major challenge in front of membrane technology is to prevent the growth of unwanted bacteria and other micro organisms to increase its shelf life”. This research shows that, when membranes, nanoparticles, and carbon nanotubes hunt in a group, getting rid of such bacteria may not remain a big challenge for a long time.
About the authors:
Suryasarathi Bose is an Assistant Professor at the Department of Materials Engineering. Giridhar Madras is a Professor at the Department of Chemical Engineering. Maya Sharma is a PhD student at the Center for Nano Science and Engineering. All are at the Indian Institute of Science, Bangalore.
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