Banana peels help detect toxins in water
Slippery helper Brazilian researchers have found an unexpected helper in the struggle against contaminated drinking water: Bananas.
In a new study, published in the journal Industrial & Engineering Chemistry Research, minced banana peels were able to bind and accumulate trace amounts of lead and copper in river water, making the toxic metals 20 times easier to detect with crude equipment.
The findings offer a new source of hope to people in developing countries, where water quality can be poor and the latest water-screening technologies hard to come by.
But no one should rush out and put mushed bananas into dirty water to make it potable, the researchers say. Instead, the technique might someday find its way into industrial settings as a cheap and non-toxic helper in the effort to ensure clean drinking supplies.
"The surprise came when I found its extraction capacity, which is higher than other similar materials constructed under chemical reactions, such as modified silica, alumina and cellulose," says Gustavo Castro, an analytical chemist at the Biosciences Institute at Botucatu, Brazil.
"All these materials are produced in the laboratory with the same objective - to remove metals from water," he says. "However, they present high costs, and in their preparation, some toxic residues are produced."
Heavy metals like copper and lead are common contaminants in industrial and agricultural run-off. Even at extremely low concentrations in drinking water, the metals can be toxic to human health, with effects ranging from nausea to liver and brain damage. But they can be hard to detect at such low doses.
In the search for greener ways to both find and remove metals from water, research groups have been working with sugar cane, coconut fibres, apple peels and more.
Castro and colleagues were the first to test banana peels, which contain proteins that are known binders of metal.
The researchers started with flasks of water that contained pre-determined levels of positively charged copper and lead ions. They added dried and ground banana peels. Then, they stirred. After a few minutes, Castro says, there was less metal in the water than there was at the beginning of the experiment. That showed that the peels had bound the metals.
The technique worked even at high levels of pH, which would be useful in waste flows from industrial sources. And the banana peels retained their metal-binding powers for more than 10 cycles of testing.
Clumping toxins together
Banana peels can't actually be used to remove metals from water or to clean up contamination, says Ashok Gadgil, an environmental engineer at the University of California, Berkeley. Instead, their value lies in their ability to gather together trace amounts of copper and lead and make the metals easier to detect.
The maximum allowable level of lead in drinking water, according to the US Environmental Protection Agency, is just 15 parts per billion. Levels this low can easily escape many kinds of equipment. In the new study, banana peels increased the concentration of both metals by a factor of 20, making them far easier to sense, even with basic tools.
"Anybody who has a relatively insensitive instrument will be happy to find a 20-fold improvement in the concentration of something they want to look for," says Gadgil.
"This is something that is interesting for people who have limited access to highly sophisticated instrumentation. They could use this as a pre-concentrator so that they could then detect minute quantities of metal, even with equipment that has high detection limits."
Before applying banana peels to the task of water monitoring in the real world, however, Gadgil urged more tests on a wider range of banana types at various levels of ripeness.
"I would want to know if a banana in Bangladesh works the same way as a banana in Brazil," he says. "Chemistry is tricky. I would want to be damn confident of a method of analysis before jumping on an action plan."