IISc researchers produce a superior water filter

A membrane capable of producing safe drinking water by filtering out objects at the nanoscale level and killing the commonly found pathogenic bacteria E. coli has been developed by a team led by Dr. Suryasarathi Bose, Assistant Professor, Department of Materials Engineering, IISc, Bengaluru.

The membrane is produced by mixing two polymers — poly vinylidene fluoride (PVDF) and poly methyl methacrylate (PMMA) — that become miscible at about 220 degree C. While PVDF crystallises during cooling, PMMA does not and separates out; the PMMA is removed using a solvent. This property of selective crystallisation and separation is taken advantage of to produce a nano-porous membrane.

As the membrane is about 1 mm thick, a combination of fine pores and channels are produced when the PMMA is removed. The average pore size is 50 nm. “The blend of two polymers is known, but we have been able to use that to produce a nano-porous structure by removing the PMMA,” said Prof. Bose. The results of the work were published last week in the Journal of Materials Chemistry A.

Last year, Dr. Bose had produced a novel membrane that had micron-sized pores (0.57-0.68 microns). It was produced by mixing two polymers polyethylene (PE) and polyethylene oxide (PEO) at 180 degree C. In this case, the micron-sized pores were produced by removing the water-soluble PEO.

Unlike the micro-filtration achieved using the PE polymer, the nano-pore structure produced now has greater advantages. “It can support a reverse osmosis membrane,” he said. “It can enhance the efficiency of a RO membrane if placed before it.” The nano-porous membrane can filter the water and send semi-pure water to the RO membrane. As a result, the RO membrane will require lesser pressure to produce pure water.

The nano-sized porous structure can prevent bacteria from passing through the pores as bacteria are typically micron-sized. However, the bacteria can form a biofilm on the structure. As a result, the filter’s efficiency will be reduced within a short period.

To prevent this and to kill the bacteria, they mixed silver, titanium dioxide and carbon nanotubes to the PVDF-PMMA mixture. Due to polarity and specific interaction with PVDF, all the three added materials got embedded only on the PVDF.

The three nanoparticles serve two important purposes. First, the nanoparticles promote PVDF crystallisation at a much faster rate. As a result of faster crystallisation, defective crystals are developed. “We get nanopores of uneven sizes (50-100 nm) and these increase the flow rate of water and hasten the filtration process,” Prof. Basu said. “Under 25 psi water pressure, the flow rate is more than 2 litres per meter square second.”

The second advantage of silver, titanium dioxide and carbon nanotubes that are embedded on membrane is their ability to kill E. coli bacteria. Silver leaches in water and when the ions so released kill the bacteria by destroying the integrity of the cell and by damaging the cell proteins and terminating the DNA replication.

Titanium dioxide also kills the bacteria. Though its antibacterial property is best in the presence of UV light, the present study did not use UV light.

Carbon nanotubes kill the bacteria through direct physical contact — the roughness of the nanotubes kills the bacteria.

A combination of all the three nanoparticles was superior in killing the pathogenic bacteria E. coli.

Published in The Hindu on February 25, 2015

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