New material created from orange peel cleans up mercury pollution

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Sulfur-limonene polymer is a plastic made from citrus peel (here shown molded into the shape of a plastic building block) that can both detect and absorb mercury pollution(Credit: Flinders University)

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Since the beginning of the industrial age, mercury pollution has increased steadily in our environment, particularly in rivers and oceans. As a result, high-level predators in our waterways often contain very high levels of mercury, and eating fish containing this neurotoxin can lead to serious health issues. Now Australian scientists working at Flinders University have discovered a simple and efficient way to remove mercury from the environment by using a material made from recycled waste citrus peel.

Attempts in the past at removing mercury from ore processing, such as the EPA's MCS system, often involve large machinery and complex processes that are well less than 100 percent efficient, and not really suited to marine environments. And, mercury-adsorption materials made from aluminum oxide hold some promise, an efficient, easy, cheap method to remove mercury levels to safe drinking levels with a few treatments has been a long-held goal.

The new substance from Flinders University, however, is capable of removing mercury from both soil and water, is made from renewable, recycled, organic waste, and provides an efficient and sustainable way for continuous mercury pollution removal.

The key ingredient in the new material is sulfur-limonene polysulphide, which is a polymer created from sulfur and limonene – a substance found in the oily skin of citrus fruits. With both of the major ingredients being leftover waste from the petroleum and citrus industries, respectively, there are many millions of tons of both discarded around the world on a yearly basis.

In attempting to find a use for these by-products, scientists have sought to create polymers such as paints, plastics, and coatings from them and reduce the use of petroleum. Incorporating what is known as waste-valorization (basically, any industrial processing activity directed at reusing, recycling, or composting wastes into useful products or sources of energy), Flinders University researchers took the two industrial by-products of sulfur and limonene to create a new polymer.

"More than 70 million tonnes of sulphur is produced each year by the petroleum industry, so there are literally mountains of it lying, unused, around the globe, while more than 70 thousand tons of limonene is produced each year by the citrus industry (limonene is found mainly in orange peels)." said Doctor Chalker of Flinders University. "So not only is this new polymer good for solving the problem of mercury pollution, but it also has the added environmental bonus of putting this waste material to good use while converting them into a form that is much easier to store so that once the material is 'full' it can easily be removed and replaced."

Initially interested in simply producing a new polymer in this way, the researchers were surprised to discover that the material they had created also seemed to bind very well to heavy metals. This, the researchers surmised, was due to metal's affinity to bond to metal.

Using this knowledge, the researchers sought to bond mercury to their new substance, and found that after just one treatment almost 50 percent of mercury in solution was removed. According to the researchers, subsequent treatments are able to reduce the level of mercury almost to the point that it is drinkable.

Another surprising advantage to using sulfur-limonene polysulphide to absorb mercury is that the material changes color in response to its exposure to mercury. That is, it has a chromogenic (color producing) effect, and so can also be used as a detector for mercury pollution.

Currently exploring commercialization of this technology, the Flinders University researchers hope to use this material to leach mercury from soil and groundwater, and also study its use in water filters for potable water.

With hopes to inspire scientists and engineers to think of new and interesting methods to incorporate these materials to solve urgent environmental challenges, the researchers also want to partner and collaborate with industries and environmental agencies to create the new material in bulk for use in large-scale mercury pollution clean-up projects. They are also considering looking for investment opportunities for a startup company.

The results of this research will soon be published in the journal Angewandte Chemie International Edition


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