As important a technology as water filters are, they get less effective as bacteria builds up on their surface. Now, researchers at Washington University in St. Louis have developed a new type of membrane that reduces such biofouling by actively killing bacteria with graphene – and in an interesting twist, it's built by bacteria.
To make the new membrane, the researchers first feed a sugary substance to Gluconacetobacter hansenii. The bacteria then use that substance to produce cellulose nanofibers in water. While those fibers are growing, the team adds graphene oxide flakes to add stability and durability to the membrane.
"This is like 3D printing with microorganisms," says Young-Shin Jun, co-lead author of the study. "We can add whatever we like to the bacteria nanocellulose during its growth. We looked at it under different pH conditions similar to what we encounter in the environment, and these membranes are much more stable compared to membranes prepared by vacuum filtration or spin-coating of graphene oxide."
With their role as traitors to the bacteria world done, the next step is to kill off the Gluconacetobacter by treating the membrane with a base solution. At the same time, the graphene oxide is "reduced" by removing the oxygen groups in it. That makes it reactive to light, which can be used to kill other bacteria that get trapped on the membrane.
"If you want to purify water with microorganisms in it, the reduced graphene oxide in the membrane can absorb the sunlight, heat the membrane and kill the bacteria," says Srikanth Singamaneni, co-lead author of the study.
To test the membrane's bacteria-killing prowess, the researchers exposed it to E. coli. Three minutes of exposure to light was enough to heat the membrane to over 70° C (158° F), hot enough to kill the bugs. The sterilized membrane was also apparently able to filter water twice as fast as existing membranes, under high pressure.
Tests of membranes made without the reduced graphene oxide found that, predictably, the E. coli survived.
The researchers say that the new design should be relatively inexpensive to produce, which should help the millions of people in developing countries that don't have easy access to clean drinking water. That said, it may be tricky to implement in current water filtration systems, but the team suggests that future designs could be made to use LEDs or tiny generators that produce light and heat from the flow of the water itself.
The research was published in the journal Environmental Science & Technology.