A "sponge" that can get gold out of waste water

Gold has been the basis of currency for many civilizations throughout history. Despite this, only ~190,000 tons of gold have been mined to-date, an amount that readily fits into a box ~20 meters on each side.

Nowadays gold is primarily used in electronics, where it is irreplaceable due to its unique properties. However, diminishing gold supplies and a continuous rise in the production of electronics have led the European Union to label this precious metal as a critical resource. Considering it can take as much as a ton of ore to yield enough gold for 40 cell phones, it is no surprise that extracting this commodity from sources other than virgin mines is becoming increasingly important.

While gold can be found in a number of different sources, such as electronic waste, sea water, fresh water, waste water, and sewage sludge, there are currently no materials reported that can selectively extract gold from such complex media.

Recently the laboratory of Professor Wendy L. Queen at EPFL developed a "sponge" that can mine gold from a variety of complex liquids. The porous material, referred to as Fe-BTC/PpPDA, is constructed by a metal-organic framework (MOF) and polymer building blocks, and has a very large internal surface area, which allows it to adsorb up to 1 gram of gold per gram of material.

In this work, published in the Journal of the American Chemical Society, the new material was tested in highly complex real-world samples. The PhD student responsible for the work, Daniel T. Sun, in collaboration with Dr Natalia Gasilova, has shown that these materials can remove gold in as little as two minutes from river water, sea water, and solutions obtained from electronic waste. Further, the sponge can be destroyed after metal extraction, leaving behind 23.9-karat gold, the highest purity reported to date for such an extraction method.

Last, due to recent reports that suggest that ~1.5 million Euros worth of gold goes into the Swiss sewage system annually, the group has also obtained waste water and sewage sludge ash from a local waste water treatment plant. The group has demonstrated that the sponge can effectively remove the gold leaving all other metals behind. The group is now exploring other MOF-Polymer composites for the extraction of a variety of contaminates and other high value commodities from water.