Fruit waste extracts metal contamination from water

Technological advances have often come at a cost to the environment. While the industrial revolution gave us mechanisation, it also heralded the start of steadily increasing levels of CO2 in the atmosphere. Synthetic dyes might have given us colours of every hue, but they also polluted many waterways.

Nuclear power provides cheap electricity, but also gave us Chernobyl and Fukushima. In these hopefully more enlightened days, emphasis is being placed on using technology to clean up, rather than pollute, the environment.

Unfortunately, while it is simple to introduce a pollutant into a natural system, it takes rather more effort to remove it, and many laboratories around the world are therefore spending significant amounts of time and money developing new techniques to clean up contaminated ecosystems.

One area that has received much attention is the removal of toxic metal ions from river water and wastewater (a metal ion is a metal atom that bears a charge, usually positive - metals exist as ions when they are dissolved in water).

This can be achieved, with varying degrees of efficiency, by passing the water through finely divided solid materials such as silica and charcoal, which bind the metal ions to their surface.

Specially designed organic molecules which have high affinities for metal ions can also be attached to these solid supports, to further improve the effectiveness of this process.

However, the preparation of these materials and their regeneration following use is often too expensive to allow their application on a large scale, and alternative solids which can remove metal ions are actively being sought.

This brings me to a recent paper with the rather surprising title: Banana Peel Applied to the Solid Phase Extraction of Copper and Lead from River Water: Preconcentration of Metal Ions with a Fruit Waste, by a team of chemists from Brazil.

Building on work which showed solid so-called "waste" natural products such as sugar-cane bagasse (fibrous material left after sugar-cane crushing), peanut shells and apple waste could be used to extract metal ions from water, they studied the ability of minced banana peel to remove both lead and copper ions from water.

The minced banana peel was prepared from dried banana skins in a ball mill to give particles having diameters in the range 35-45 micrometres (about the width of a human hair). This powder was tested two ways; added directly to a water sample containing lead and copper ions and stirred, or a water sample containing lead and copper ions was passed through the powder.

In both cases, the powder showed an excellent affinity to lead and copper ions, retaining over 95% of both, in various conditions. It outperformed other natural products (clay, volcanic rock, peanut husks, sawdust), and could be reused 11 times with no capacity loss.

Given New Zealand is 31st in the world with respect to per-capita consumption of bananas (17.81 kg per person per year), there must be a lot of potentially valuable banana peel going to waste in this country. Any budding entrepreneurs out there?

• Dr Blackman is an associate professor in the chemistry department at the University of Otago.