Oxidation of sulfide ion in synthetic geothermal brines at carbon‐based anodes

Abstract

AbstractGeothermal brines associated with natural gas extraction present an environmental problem because of the need to avoid the escape of toxic and odorous hydrogen sulfide. In this work, carbon‐based anodes (including graphite, granulated activated carbon (GAC), and industrial coke) were used in the electrochemical oxidation of sulfide ion in synthetic geothermal brines in both batch and flow cells, with a view to remediating this troublesome contaminant. Experiments were carried out in alkaline solution (to prevent volatilisation of H2S), in the absence or presence of added chloride ion and naphthenic acids (NAs). The product spread was variable due to the large number of competing reactions, including formation of polysulfide, oxidation to sulfate (either directly or via hypochlorination), sacrificial anode oxidation combined with sulfide trapping, and Kolbe oxidation of NAs combined with sulfide trapping. From the technological perspective, the product distribution is immaterial, because sour brines contain such high concentrations of inorganic and naphthenate salts that reinjection of the treated brine will always be required. The most efficient systems, on the basis of charge per mol sulfide remediated, employed packed bed reactors with ground coke or GAC anodes. In these reactors, the disappearance of sulfide ion was promoted, as expected, at low flow rate and high current. However, the packed beds were limited to low applied current, in order to avoid compromising the electrical conductivity of the anode bed by the production of gas (O2).