Pregnant women rejoice; Australian scientists Max Worthington and Justin Chalker, along with a team of researchers, have found a method to remove mercury from water using industrial waste and orange peels. High concentrations of mercury in food and water can have toxic effects on the nervous, digestive and immune systems, and on lungs, kidneys, skin and eyes, and the answer, at least a large part of the answer, grows on trees.
The synthetic material in question is sulfur-limonene polysulfide, which combines sulphur, a petroleum byproduct, and limonene, the main component of orange oil. Both of these substances are produced in the order of tens of thousands of tons each year and then discarded. When reacted together and combined, these compounds create a soft red rubber that essentially pulls mercury out of water.
Ocean mercury levels have tripled since the beginning of the industrial revolution. Mercury has permeated sea-life, posing a threat to fish-consuming humans. This is why pregnant women are advised to avoid seafood; fetuses are especially susceptible to the negative health outcomes of mercury exposure. Children exposed to high levels exhibit clear signs like mental retardation, seizures, and cerebral palsy. Mercury, however, can negatively affect children in more subtle ways at lower doses – causing behavior problems, memory deficits, and shorter attention spans. This new compound discovered by Worthington and Chalker, therefore, has huge implications. The sulfur-limonene polysulfide is cheap to produce and can be diffused through large bodies of water with relative ease.
As with all things, however, there may be a catch. Limonene is considered highly toxic to aquatic life. This new substance will require extensive toxicity research to ensure that it is safe for our oceans. Preliminary tests have shown promising results; liver cells exposed to water containing the substance showed no change in viability. If the new substance can be effective while the concentration of limonene remains below the threshold for adverse effects, there are huge implications to this discovery.
The team set out to create a polymer or plastic out of something in large supply. When they assert that the material is easy to make, they are not over-exaggerating. To get the materials to react, sulfur was melted and then heated to 170 degrees Celsius (338 degrees Fahrenheit). An equal amount of limonene was added to the molten sulfur, and a red wax-like substance resulted once cooled to room temperature. The first shape the polysulfide was molded into was a lego brick, as researcher Michael Crockett explains, “just because we found a high temperature food mold that we were able to pour the material into.” This original manifestation may have been unintentionally yet perfectly appropriate; the pliability of the material implies that it has the potential to be used in coating pipes or molded into any number of shapes, providing a truly ubiquitous material.
In their testing of the substance, the team explored its abilities to sequester metals from water. They began with palladium, which is another metal byproduct which pollutes water. When tested, the new material removed 42% of palladium from water within the first hour, then reached an equilibrium in which no more could be sequestered. But, this finding showed that sulfure-limonene polysulfide had promising abilities to remove soft metals from water.
This led the researchers to test its affinity for mercury. “When an aqueous solution of HgCl2 (10 mM) was added to the surface of the polysulfide, the result was a surprise: a bright yellow deposit formed that remained immobilized on the polysulfide. The deposit typically appeared within 30 min and remained on the polysulfide, even after washing with water.” Sulfur is already used in the disposal of mercury, but because of its structure is difficult to form into anything else useful in regards to mercury. Sulfur-limonene polysulfide, however, traps the mercury and keeps it there.
Worthington and Chalker then went on to challenge the substance’s mercury-removal power in a manner more applicable to the natural environment. They spiked river water and soil deposits with mercury and then added the polysulfide. Again, insoluble mercury deposits formed on its surface. The telltale yellow deposits remained adhered in the soil suspension as well, indicated its ability to remove mercury even in these more complex conditions.
Crockett, an American researcher who did the initial synthesis, says the team intentionally made the material, based on “theories about the kinds of properties that it would have just based on the nature of sulfur.” What they didn’t expect, however, that it would “react specifically with mercury to give a color change.” Most of the research on materials to sequester mercury concerned granular or powdered substances, but this gel-like substance can be put to a much wider variety of uses. The study was just published Tuesday October 20, so it hasn’t been widely read, but according to Crockett, they have already seen some interest in the material from governmental and/or private entities. Furthermore, the team has seen “some luck with lead binding to the material as well, but those results are qualitative. We have reason to believe it could remove most heavy metals.”
The uncovered sulfide is highly advantageous for three distinct reasons: it is cheap to produce, it is easy to produce on a mass scale, and the yellow deposits formed by the mercury serve as an easily identifiable indication of its efficacy. The coolest part, as Chalker puts it, is that “it literally grows on trees.”
— Patricia Kane