Electrochemical oxidation of sulfide ion in synthetic sour brines using periodic polarity reversal at Ebonex® electrodes

The Magneli phase Ti4O7 (Ebonex®) was used as both anode and cathode in the electrochemical oxidation of sulfide ion in alkaline solution in the absence and presence of chloride and naphthenate ions. Ebonex anodes gradually lost their activity through the formation of an over-oxidized surface layer, but their activity could be maintained by periodic polarity reversal. In the context of the current paradigm for the mechanistic behaviour of oxide-based anodes, Ti4O7 has properties that combine those of “inactive” anodes (formation of hydroxyl radicals) and “active” anodes (formation of a higher oxide at the surface), with the exception that the higher oxide in the case of Ti4O7 is TiO2, which is incapable of substrate oxidation. Sulfate is the major oxidation product, especially in the presence of chloride, an ubiquitous component of sour brines, via mediated electro-oxidation to hypochlorite. Unlike at boron-doped diamond anodes, at which sulfide is oxidized with near-quantitative current efficiency, significant parasitic oxidation of water to O2 occurs at Ebonex, and oxidation of sulfide requires ~16 F mol–1, corresponding to a 50% current efficiency.