The Electro‐oxidation of Some Insoluble Inorganic Sulfides, Selenides, and Tellurides in Cationic Surfactant‐Aqueous Sodium Hydroxide Systems

Abstract

A study was made of the electro-oxidation of several sulfides, selenides, and tellurides in cationic surfactant-aqueous sodium hydroxide suspensions in a slurry cell and a sandwich cell. Because the cationic surfactant increased the oxidation potential of water, it was possible to observe and study separate voltammetric waves for the oxidation of the sulfides to sulfur, sulfite, and sulfate; the selenides to selenium, selenites, and selenates; and the tellurides to tellurium, tellurite, and.tellurate. Superimposed, in some cases, on these oxidation waves were waves for the oxidation of the metal ions to higher oxidation states. Controlled potential coulometric studies of these oxidations showed that it was possible to perform each of these oxidations with essentially 100% current efficiency. The oxidation potentials of the sulfides to sulfur reflected differences which could be attributed to differences in crystallization energies. In cases where the oxidation of the metal ion did not interfere, the potentials for oxidation of sulfur to sulfite and sulfite to sulfate were independent of the metal ion because these reactions were the same. In a similar manner, the oxidation of the selenium to selenite, selenite to selenate, tellurium to tellurite, and tellurite to tellurate all showed potentials that were independent of the metal ion that was present. The oxidation of the selenides to selenium and tellurides to tellurium all occurred at the same potential and this potential was, within experimental error, equal to the first oxidation peak potential for the surfactant. This led to the conclusion that these oxidations all proceeded by a mechanism that involved electrochemical oxidation of the surfactant followed by chemical oxidation of the solid.Recently, as part of a series of studies on the uses of cationic surfactants in the electro-oxidation of insoluble, difficulty oxidizable compounds (1-8), it was reported that it was possible to electrolytically oxidize iron pyrite (9, 10) and galena (11) powders in a cationic surfactant-aqueous sodium hydroxide system. Previously, electrolytic oxidation studies of these sulfide minerals to obtain answers to questions in the field of mineral processing have primarily utilized crystalline electrodes or powders mounted in graphite paste or epoxy cements (12-26).This paper is a report on an extension of the previous studies (9-11) to electro-oxidation of several other sulfides and some selenides and tellurides suspended in surfactant systems. The purpose of these studies was to investigate the possible use of these surfactant systems in electroanalysis of insoluble powders and in areas of massive electrolysis such as electrolytic roasting of minerals.The previous studies (1-11) have shown that cationic surfactants affect these electro-oxidations in two ways. They solubilize the solids and form a hydrophobic film on the electrode, increasing the oxidation potential of water by 0.9V. Anionic and neutral surfactants solubilize insoluble solid powders but do not appreciably increa...

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