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dition of distilled water. Crystal samples for the scanning electron microscope were carefully rinsed with distilled water, allowed to dry at room temperature, and glued to specimen stubs with conductive epoxy resin. Samples were coated with gold-palladium and 310

Ladung der Cut -1onen wird dadurch soweit reduziert, daL3 der Elektronentransfer vom Perhydroxyl-Radikal energetisch unmoglich wird. Ein autokatalytischer Reaktionsmechanismus, der zwei benachbarte Reaktanten umfaot, ist dann die einzige verbleibende katalytische Reaktionsmoglichkeit. Die Rolle von Cu(1)-Oxyd, welches fur die ausgepragte Lichtabhangigkeit von Oszillationsfrequenz und -amplitude verantwortlich ist, wird weiter studiert. Die Untersuchungen lassen schlieBen, daR Kupferionen in Sulfidoberflachen bei der katalytischen Zersetzung von Wasserstoffperoxyd eine analoge Rolle als Redox-Partner spielen wie Kupferionen in Losungen. Das Elektrodenpotential erlaubt es, ihre katalytische Reaktivitat zu kontrollieren und energetisch so anzupassen, daL3 periodisches katalytisches Verhalten eintritt.

Sixteen metal sulphides with widely differing solid state, chemical and electrochemical properties have been investigated with respect to their suitabilityasanenergy source for Thiobucillus ferrooxiduns. The most critical rate-determining parameter for bacterial oxidation (in the absence of any electron acceptors, e.g. Fe3+) was the solubility product of the sulphide. Deviations from the systematic dependence are, however, observed when a large concentration of the holes are present in the semiconducting sulphide (p-type conduction), or when holes are generated through electron extraction from the sulphide surface (e.g. by Fe3+). The experimental results are consistent with a mechanism in which Th. ferrooxiduns utilises H+ and Fe3+ as catalytic agents which break surface bonds by chemical and electrochemical mechanisms, respectively. They are subsequently recycled while the bacterium oxidises the generated surface products, -SHS-and SO, etc., to sulphate. On the basis of the derived mechanism it should be possible to estimate and predict the suitability of sulphides for bacterial oxidation and to analyse the influence of crystalline quality and impurities, as well as the composition of the solution, on the rate of oxidation.

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