A mercury spill is probably one of the peskiest chemical spills for those attempting the cleanup. Mercury is one of the few elements on the Periodic Table which is a liquid in its natural form at room temperature. Most elements are either solids or gases. Because the mercury is a liquid, and because it has an unusually high surface tension, mercury which is spilled stays in droplet form but it splits into hundreds of tiny mercury spheres when it hits a hard surface. The tiny droplets don’t stick to the surface but scatter everywhere, in all directions. Trying to sweep the mercury into a managable pile just results in the droplets splitting into even smaller drops and skimming across the surface. When you also consider that mercury is highly toxic, you begin to see the frustration that can arise from even a small mercury spill.
Most mercury “spill kits” designed to clean up after a mercury spill rely on high surface area adsorbants such as activated charcoal. Mercury doesn’t react with the charcoal, and doesn’t really “wet” the surface of the carbon, but it does give you a sweepable mass which you can (with some effort) collect into a dustbin. This approach is messy and it doesn’t work very well, but it’s better than nothing. Other mercury remediation techniques include a vacuum flask attached to a long glass pipette (essentially a large eyedropper). The mercury droplets are sucked into the flask where they reform a pool of mercury which can be disposed of in a separate container. This method is very labor-intensive and doesn’t work so well for very fine droplets, as each individual particle of mercury has to be (literally) chased after with the pipette.
Neither of these approaches does anything to address the problem of mercury-polluted water. Communities which rely on rivers, lakes, and wells for their water supply run the danger of mercury poisoning. The mercury (a runoff from industrial pollution) concentrates itself in the riverbeds and can be stirred up / disturbed as the result of fishing and other human activities. Thankfully, recent results from Northwestern University (published in the journal Science) outlines a new type of approach that can clean up mercury metal spills and also remove dissolved mercury ions from water supplies. Their approach is based on a dehydrated three-dimensional gel. The chemists take powdered silicon salts along with either sulfur or selenium in combination with a small amount of platinum, and formed a slurry in water. These slurrys began to “gel” as a result of a polymerization reaction that takes place between the different salts present. Drying these gels using carbon dioxide resulted in a “puffed out”, extremely light, dry foam that is almost entirely empty space (filled with air) with extremely thin walls of sulfur / selenium / silicon.
These dried gels resemble aerogels (a well-known silicon dioxide foam) but these new foams have sulfur and selenium atoms in place of the oxygen atoms. The finalized structure is extremely porous, with a molecular structure that resembles a twisted maze of caverns and passages. The walls of the passages are lined with sulfur atoms, which form a particularly strong interaction with mercury. The mercury-sulfur bond is very strong, and so the formation of the bonds is energetically favored.
The gels were tested against both mercury metal spills as well as contaminated drinking water that contained dissolved mercury. In all cases, the gels were found to removing 99+% of the mercury contamination, and they did so selectively. Other important elements of the water – dissolved salts, or the water molecules themselves – were unaffected. This is because the sulfur linings of the molecular passages are selective for mercury; other elements do not react. As the mercury metal / mercury-containing water flowed through the pores of the dried gel, the atoms bonded to the sulfur constituents and remained stuck in place.
These gels are currently being considered for large-scale decontamination of drinking water. Researchers are also investigating the possibility of switching other elements for the sulfur / selenium components, in hopes of producing a high-surface area catalyst for useful chemical transformations.
The source of this article can be found at: http://www.sciencemag.org/cgi/reprint/317/5837/490.pdf