Once the juice is removed, a full quarter of an orange’s mass comes from its peel. The orange tree is putting a great deal of energy into constructing this material, that more often than not simply becomes landfill.

In an effort to make better use of this resource, it has been tested as a material for water purification. Cellulose and lignin, the main components of orange peel, are quite polar – a property which enables powdered orange peel to absorb various pollutants. Unfortunately, though, after the powder has removed the pollutants, its extraction from the newly clean solution can be difficult.

To address this problem, a research team in Italy has developed a technique for making orange peel easier to remove after it has successfully done its job. Powdered orange peel is added to a gel which also contains silk fibroin and then this gel is dried with supercritical carbon dioxide.  The resulting crystalline foam allows water to readily penetrate it and can easily be removed from solution.

Magnified images of the foam.

In order to examine the utility of this new technology the team used methylene blue as a test pollutant.  This vivid dye allows for easy spectroscopic measurements of the pollution remaining in solution. It was found that the incorporation of the powdered orange peel in to the crystalline foam did not have a large impact on its ability to absorb pollutants.

The team then assessed the impact of a number of variables on the technology. pH changes between two and eight did not affect the capacity of the foam.  This compares favorably with the simple powder, which does not absorb pollutants effectively below pH five.  In contrast, the foam absorbs pollutants much more slowly than the powder, which reaches equilibrium with the pollutants in 15 minutes, in contrast to the foam, which requires a full 24 hours.

This novel crystalline foam paves the way for waste orange peel to be effectively used in water remediation.  Although the foam acts more slowly than the powder, it removes the need for a costly and often time consuming removal step. So next time you finish an orange, hold onto that peel!

Effect of pH and time on dye removal.