Electrochemical Oxidation of Hydrogen Sulfide in Polluted Brines Using Porous Graphite Electrodes under Geothermal Conditions

Abstract

Hydrogen sulfide contaminates the geothermal brines that are encountered in the drilling of wells. It promotes the corrosion of the drilling equipment, pipelines, vessels, forms sulfide scales, which plug tubulars, and degrades the quality of the produced oil. This work aims to study the feasibility of electrochemical removal of H2S from such brines by anodic oxidation. Porous graphite electrode was used to achieve electrochemical oxidation of sulfide ions in chloride brines in an autoclave under high temperature and pressure. The X-ray photoelectron spectroscopy (XPS) and Energy Dispersive Spectroscopy (EDS) proved that sulfur was identified as the final reaction product under various potentials and temperatures. In low temperature, the sulfur was detected on the electrode surface, while in high temperature the resulting sulfur flows out in the electrolyte so that it is not detected on the electrode surface. The rate of sulfide oxidation depends on the potential, temperature and the sulfide concentration. The obtained limiting currents are lower than those predicted from mass transfer correlations, which result from the effects of the deposited sulfur on the internal surface of the porous graphite electrode. The large internal surface area of the used electrodes improved the removal efficiency of sulfide ions and this is very clearly noticed from the current transient's results. In high temperature, the limiting current is re-increased due to the liberation of sulfur from the electrode surface, which reactivates electrode surface and the rate of oxidation is increased again.