IISc: novel membrane filters water, kills bacteria

A low-cost water purification membrane capable of filtering out objects greater than one micron size and also killing E. coli bacteria has been developed by the Bangalore-based Indian Institute of Science (IISc) researchers and Steer Engineering Pvt. Ltd, Bangalore. The results of the study were published recently in the Journal of Materials Chemistry A.

The novel membrane with pores as small as 0.57-0.68 microns was developed by mixing two polymers — polyethylene (PE) and polyethylene oxide (PEO) — at 180 degree C. Unlike PE, PEO is water-soluble and the two polymers are immiscible.

“We want them to be immiscible as we want to remove one of them, in this case the PEO which is water soluble,” said Dr. Suryasarathi Bose, Assistant Professor, Department of Materials Engineering, IISc. He is the corresponding author of the paper. Tiny holes came into being once the water-soluble polymer was removed.

In order to create pores that are sub-micron in size, the amount of PEO polymer distributed in the PE matrix should also be sub-micron in size. To achieve this, only a tiny amount of PEO was taken compared with PE. Also, the polymers mixed at 180 degree C were sheared at high speed to produce tiny droplets of PEO. “The higher the shear rate, the smaller the droplets,” Prof. Bose said.

The PEO droplets on the matrix were then removed to create tiny holes. “We take the mixture and dip it in water. As soon as we dip the mixture, the PEO gets dissolved in water leaving behind tiny holes in the PE matrix,” he explained.

Going further, the researchers rendered the membrane antibacterial against E.coli. For this, grapheme oxide (GO) was mixed with PE and PEO and the graphene oxide was made functional with amine groups. The tablet-shaped samples with tiny holes are hydrophobic (water repellent) in nature.

Since the holes were sub-micron in size, pressure was required to force water through the holes. “The pressure used varied from one to seven psi. Flux [rate of water flow] increased as the pressure applied increased,” said Prof. Bose.

“Since pressure was used to force water through the tiny holes, the hydrophobic nature of the tablet would not have mattered,” said Prof. Bose.

The membrane filtered all solid substances that were more than one micron in size. “By itself, the membrane cannot remove salinity. For that, one has to use reverse osmosis,” Prof. Bose said.

When put into a colony of E. coli bacteria, the tablets reduced the E. coli colony at 37 degree C; measurements were made 24 hours after the tablets were left in the bacterial colony.

There are two ways in which the bacteria may be getting killed. The roughness of the grapheme oxide surface is one factor while the interaction of the amine group of the tablet with the phosphate group of the lipids present in the cell could be another. “The amine group destroys the integrity of the cell membrane,” he said.

Since the tablet is hydrophobic in nature, the killed E. coli to a large extent do not stick to the membrane.

Though it cannot remove salinity, its ability to filter particles larger than one micron makes the membrane attractive.

“This membrane can support reverse osmosis but can’t replace it. If we use this membrane before RO then the efficiency of RO membrane will increase,” Prof. Bose stressed. “When we supply pure water [to RO unit], the pressure on RO motor will come down. So the efficiency of RO will increase.”