A novel electrobiotechnology for the recovery of precious metals from spent automotive catalysts

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Abstract
Platinum group metals are routinely used in automotive catalysts but recycle technology lags behind demand. There is no available 'dean technology' and leach solutions (e.g. aqua regia) to solubilise the metals from scrap are highly aggressive. A microwave-assisted leaching method was developed which gave 80% metals recovery, with the leach time reduced from 2 h to 15 min using 50% (aq.) diluted aqua regia to give potentially a more biocompatible leachate. Desulfovibrio desulfuricans reduces soluble platinum group metals to cell-bound insoluble base metals (e.g. Pd(II) --> Pd(0)). For use, biofilm was immobilised on a Pd-23% Ag solid alloy membrane which delivered H. to the cells via an electrochemical chamber at the back-side. The biomass-coated Pd-Ag alloy electrode was used in a flow-through reactor for recovery of Pd, Pt and Rh from aqua regia leachates (pH 2.5) of spent automotive catalysts with up to 90% efficiency at a flow residence time of 15 minutes. Free cells did not reduce platinum group metals from the leachates but the electrobioreactor did so using biofilm-cells pre-loaded with Pd(0). Reactors lacking biomass or reactors with heat-killed biofilm removed less platinum group metals, via electrochemically-synthesised H. reductant alone. The use of an active biofilm layer in a flow-through electrobioreactor provides a simple, dean and rapid potential recycle technology.