There is an age-old practice of storing drinking water in copper vessels to rid the water of bacteria. Scientific evidence of copper vessel’s antibacterial property became available in March 2012 when a paper in the Journal of Health, Population and Nutrition found that water stored in copper vessels for 16 hours killed pathogenic bacteria such as E. coli and cholera causing Vibrio cholerae O1, and Salmonella species. Now, researchers at the Indian Institute of Science (IISc) Bengaluru have developed a water-filter membrane with copper ions to make drinking water safe. The results were published in the journal Nanoscale.
A team of researchers led by Prof. Suryasarathi Bose from the Department of Materials Engineering at IISc made the commonly used polyvinylidene fluoride (PVDF) water-filter membrane to prevent biofouling and kill bacteria. To do this they first made the inert PVDF membrane functional by blending it with a polymer (styrene maleic anhydride or SMA).
Though copper oxide is an excellent antibacterial agent, it can be toxic if the concentration of copper in the water exceeds 1.3 ppm (WHO standard). So the researchers coated copper oxide with a biocompatible polymer (polyphosphoester or PPE) for controlled release of copper ions. A porous gel-like structure of copper oxide coated with the polymer was used for coating the membrane. The polymer used for coating copper has anti-fouling property.
The SMA polymer coated on the membrane, which gets partially hydrolysed when in contact with water, interacts with the outer membrane of the bacteria to produce disc shaped structures. “It solubilises the membrane protein but does not kill bacteria. But the interaction with the bacterial cell membrane leads to release of a particular enzyme (phosphatase and/or phospholipase),” says Prof. Bose.
This enzyme released from the bacteria cleaves the polymer coating found on copper oxide resulting in controlled release of copper ions from the membrane into water. “In the absence of bacteria, the amount of copper ions released is 0.035 ppm at the end of three days and 0.13 ppm at the end of 30 days,” says Nagarajan Padmavathy from the Department of Materials Engineering at IISc and the first author of the paper.
But when water with very high concentration of bacteria (10,000 colony-forming units of bacteria per ml of water) was used, the amount of copper ions in water at the end of four hours was 1.6 ppm, which is more than the WHO limit. “At 1.6 ppm, the copper ions in water can be toxic,” says Prof. Bose.
But the ability of copper ions to kill bacteria found in high concentration was four orders of magnitude higher at the end of four hours. “The ability to kill bacteria is more than 98%,” says Padmavathy.
According to Prof. Bose, the cleaving of the polymer coating is slow and takes about a year for it to be completely degraded. So the amount of copper ions released might not be high.
These membranes with controlled release of copper ion may turn out to be a potential candidate for water purification applications with enhanced antibacterial and antifouling performances.
In the pipeline
“We have to carry out more studies to understand the amount of copper a person would be exposed to over time by drinking the water filtered by copper-containing membrane. And also the amount of copper ions released into water from the membrane over a period of time when bacteria-containing water is continuously filtered,” says Prof. Bose.
Explaining why they chose to work with copper oxide than silver, Prof. Bose says: “Unlike silver, copper is inexpensive. But more importantly, copper oxide provides ample opportunities to harness different surface functional groups and polymers unlike silver.”