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.
Subject(s)
Bioreactors , Conservation of Natural Resources , Desulfovibrio , Palladium/isolation & purification , Platinum/isolation & purification , Rhodium/isolation & purification , Catalysis , Electrochemistry , Motor Vehicles , Palladium/metabolism , Platinum/metabolism , Rhodium/metabolismABSTRACT
Knowledge of instrumental complications is vital both in interpreting experimental observations and in achieving true results. In a study of palladium electrodeposition on highly ordered pyrolytic graphite by in situ electrochemical scanning tunnelling microscopy, some unexpected experimental artefacts caused by instrumental design and tip-surface interactions have been recognised. The electrodeposition system employed in our in situ studies has been found to be very sensitive to an initial cathodic current surge when the potentiostat control was applied to the electrodes at the measured open circuit potential. As a result, palladium and/or hydrogen were immediately deposited on the graphite surface within a small radius under the imaging tip. The cause of this current surge was investigated and found to be related to the performance of the potentiostat. The deposit was removed either by anodic potential sweeps or under positive local electric fields of the imaging tip. Further cathodic deposition was possible on the surge-induced deposit. Characterisation, by ex situ x-ray diffraction, atomic force microscopy, scanning transmission electron microscopy and high resolution electron microscopy, showed that the deposits were Pd. Use of the various techniques demonstrated their complementarity in studies of the structure and dimensions of surface deposits.
Subject(s)
Electrochemistry , Graphite , Microscopy/methods , PalladiumABSTRACT
The solubility product of cadmium sulphide has been measured in three acidic media by precipitation and dissolution techniques. The values of log K(s0) after correction for complex formation are -27.3 +/- 0.6 in all the media examined. An X-ray and microscopic examination of the precipitates shows an increase in crystallinity on aging, although Ostwald ripening was not observed.
