“A ton of silt contains 0.5 to 1 gram of gold and 7 to 10 gram of silver. In a city like Brussels, with 1 million inhabitants, you could easily gain 5 to 10 kilograms of gold, 70 to 100 kilograms of silver and certainly a few hundred grams of platinum in a year. ”
These are the words of Natacha Brion, a researcher affiliated with the Analytical, Environmental and Geochemistry group of the Vrije Universiteit Brussel (VUB). She coordinates a project on urban mining, to recover the high concentrations of valuable precious metals from the sludge during water treatment. “This is done with special bacteria that can convert sulfur in the sludge into sulfuric acid. Subsequently, almost all heavy metals dissolve in this sulfuric acid, so that they do not complicate purification. ” For the second phase, the VUB collaborates with its French-speaking and Brussels counterpart, the Université Libre de Bruxelles (ULB). Although nothing has been tested in practice, Brion is hopeful about the laboratory experiments with the nanoparticles that should ensure that the precious metals can be “fished” with a magnet. "UCL is going to develop magnetic nanoparticles with a molecule on them that specifically binds one element, such as gold."
More than one technique
People in the Netherlands are also particularly interested in the possibilities and benefits of urban mining. Scientists from the Dutch water research institute KWR also start from the sludge, but instead of adding magnetic nanoparticles to it, they burn the sludge. “After combustion, the concentration of the metals is even higher; this makes it easier to recover the metals, ”says senior researcher Kees Roest. “Or you can use electrochemistry: using electrical current to cause certain metals to precipitate. Or, for example, you can use resin to "fish" specific metals from the solution. So we rather look at a train of techniques. ”