Selective Magnetic Filtration of Noble Metals From Dilute, Acidic Streams Using Carbon-Coated Nanomagnets

Magnetic filtration provides an efficient way to control large liquid volumes with a small amount of a specific reagent. At present, the lack of sufficiently stable nanomagnets has limited the magnetic filtration to routine processing in biochemical separations, particularly in diagnostics. Unfortunately these high-price applications are limited to water and neutral pH as the present oxide-based nanomagnets have a poor stability and low binding capacities. Recently we prepared metallic nanoparticles with a graphene-like carbon coating of 1-2 nm by reducing flame spray synthesis [1,2]. This coating protects the metal nanoparticles from oxidation up to temperatures of 190°C and against dissolution in acids. On the other hand it allows the introduction of covalently bound functional groups via diazonium chemistry [3] or physical adsorption, giving access to organic chemistry as a method to design the particle surface. The functionalized magnetic particles represent a promising modular platform for removing a wide range of contaminants. Attaching a thiourea-like chelating agent allows the efficient and selective removal of precious metal ions in highly acidic solutions. The low costs and high particle stability favor this processing method and associated material for large-scale separation application where metal ions are present at ultra low concentrations. This contribution will show how noble metal can be rapidly collected from acidic mining streams down to the microgram per liter level [4]. [1] R. N. Grass, W. J. Stark, J. Mater. Chem. 2006, 16, 1825. [2] R. N. Grass, M. Dietiker, R. Spolenak, W. J. Stark, Nanotechnology 2007, 18, 035703. [3] R. N. Grass, E. K. Athanassiou, W. J. Stark, Angewandte Chemie 2007, 26, 4909 [4] M. Rossier, F. M. Koehler, E. K. Athanassiou, R. N. Grass, B. Aschlimann, D. Guenther, W. J. Stark , manuscript in preparation (2009) Figure 1: Selective gold and platinum extraction in a solution composed of 1g/l Fe, Cu and Zn and 1mg/l Au and Pt.