Electrochemical oxidation of sulfide ion at a Ti/IrO2–Ta2O5 anode in the presence and absence of naphthenic acids

Abstract

Electrochemical oxidation of sulfide ion at a Ti/ IrO 2 -Ta 2 O 5 anode followed partial order kinetics (between current and mass transport control) in the absence and presence of chloride ion and of naphthenic acids, at sulfide concentrations typical of sour brines. The desired outcome was to promote the 2-electron oxidation of sulfide to elemental sulfide rather than the 8-electron oxidation to sulfate. Although elemental sulfur accumulated to some extent at low conversion of sulfide, sulfate ion became the principal product as the reaction progressed. At high conversion, the overall current efficiencies were typically higher than 50%, with material balance about 90%. However, this anode material was gradually poisoned by sulfide in long term use.

Anodic Oxidation of Sulfide to Sulfate: Effect of the Current Density on the Process Kinetics

“…1,2 The presence of S 2-in sewage systems is a common problem due to the stimulation of the metabolic activity of sulfate-reducing bacteria, given the high organic load and low dissolved oxygen. 3 Sulfide is one of the contaminants in effluents generated in the extraction of oil and gas and in certain types of wastes from oil refinery, 4 as well as in wastewaters from the leather industry that present high concentration in sulfide ion. 5 Concrete corrosion problems have also been reported for S 2-concentration between 0.1 and 0.5 mg L -1 .…”

“…The biological oxidation to remove S 2-can also be performed, but it is slow and applies only to low S 2-concentrations. 4 The electrochemical processes are a promising technique for the resolution of pollution problems, since they present (i) versatility: direct or indirect oxidation, phase separation, biocides functions, treatment of many different contaminants; and (ii) energetic efficiency: electrochemical processes generally have lower temperature requirements, the potential can be controlled and electrodes and cells can be designed to minimize energy losses due to voltage drops, poor current distribution and secondary reactions. 9 In the recent years, there has been a growing interest in these processes for the treatment of wastewaters containing recalcitrant and biotoxic compounds.…”

“…8 These results are significantly different from those obtained in the oxidation of S 2-using a Ti/IrO 2 -Ta 2 O 5 anode, with kinetic orders ranging from 0.5 and 0.9. 4 The feasibility of S 2-electro-oxidation was also observed in the electrolysis of a tannery wastewater with Ti/MO (MO = metal oxide) anodes, without, however, being identified the formed intermediates. 25 Despite the studies found in the literature describing the S 2-anodic electro-oxidation, only a few are focused on the electro-oxidation of sulfide to sulfate.…”

“…Thus, the formation and deposition of sulfur seems to be influenced by the S 2-concentration. The electrochemical oxidation of S 2-was also performed by Waterston et al, 8 using a boron-doped diamond (BDD) anode, and it was found that its conversion to SO 4 2-occurred almost quantitatively with a current efficiency of 90%. This situation seems to show the best performance of this electrode material, BDD, when compared to others.…”

“…7 Thus, it can be removed from the aqueous solution without the production of sludge, by oxidation to elemental sulfur or oxyanions, like SO 4 2-, which are ecologically benign. 8 According to the potential diagrams , using NaCl as supporting electrolyte, and observed sulfur deposition on the surface of the graphite anode, a fact that was also observed later.…”