Hydrogen sulfide (H₂S) is a toxic and corrosive gas. It occurs naturally in aquatic environments and is a common contaminant of sewage, industrial wastewater and gas streams. Various methods have been applied for its removal but generally issues exist such as high cost, chemicals consumption and sludge production. Electrochemical treatment, whereby sulfide is oxidized and NaOH can be recovered via cathodic water splitting, provides an interesting alternative, as it is chemical-free and may allow for the recovery of elemental sulfur (S^o) and caustic (NaOH) as by-products (Vaiopoulou et al., 2016). Whereas the operational costs, mainly power consumption, could be limited, particularly anodes come at high cost and limited lifetime due to passivation or poisoning of the active sites.

Mixed metal oxide (MMO) coated titanium electrodes are widely used in electrochemical processes such as chlorine and oxygen generation, due to their long life and reduced energy consumption, and have been recently investigated for electrochemical treatment of sulfide streams. In the particular case of spent caustic streams (SCS) where a combination of high alkalinity, sulfide concentration and current densities takes place, MMO coated titanium anodes have to our knowledge not been evaluated. Therefore, we tested 6 electrode materials (Ir MMO, Ru MMO, Pt/IrOx, Pt, PbOx and TiO₂/IrTaO₂) to determine how the different catalyst coatings affected the sulfide oxidation efficiency and the stability and lifetime of the electrode when applied for the treatment of SCS.

A first series of tests was conducted in a multielectrode reactor, to assess the activity and stability of the different electrode materials. Activity tests were performed in a batch mode and controlled potentiostatically at 50 A m^-2 current density, while stability tests were performed in a continuous mode for 305 h and controlled at a current density range from 50 to 200 A m^-2. The counter electrode used was stainless steel mesh and the anolyte consisted of 50 mM Na₂S and 50 mM NaOH. At regular intervals cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed. The change in the electrode surface texture and composition upon sulfide oxidation was examined with scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX).

Sulfide removal in an alkaline electrochemical cell is proceeding through a number of different oxidation reactions, that happen in a wide range of anode potentials and result in the production of S^o or different sulfur oxyanions (Mao et al., 1991). This distribution was also observed in this study, with the sulfur species produced from sulfide oxidation differing with the anode materials, yet with S^o as the first product of sulfide oxidation, followed by S₂O₃^2-, SO₃^2- and SO₄^2-.This work demonstrated that Ru MMO was the most active material towards S^o recovery with a Coulombic efficiency (CE%) of 65 %. Moreover, it performed sulfide oxidation at an onset anode potential of 0.78 V vs. SHE and completed the batch cycle with an average of 1.1 ± 0.3 V vs. SHE.

The Ru MMO was also found to be the most stable electrode with only a decrease of 17% of the initial Ru/Ti ratio after 305 hours. This reduced catalyst over base metal ratio can be explained by the sulfur deposition observed on the surface of the electrode. More specifically, the Ru MMO electrode preserved a stable potential of 1.57 V vs. SHE at a high current density of 200 A m^-2, which is in good agreement with the SEM/EDX results. In contrary, the harsh conditions of high alkalinity and sulfide concentration affected the stability of the Pt/IrOx electrode which showed an increase of the anode potential from 2.79 to 6.61 V vs. SHE at 200 A m^-2. The decline of stability was confirmed with SEM/EDX, where no significant amount of the Pt and Ir catalysts were found on the surface of the electrode at the end of the test.

This study confirms the importance of the anode material selection as a crucial aspect of the electrochemical cells for sulfide oxidation. The choice of the anode materials can directly affect the capital costs of the application and it also has a direct impact on the energetic efficiency and the operational maintenance of the system. The electrochemical sulfide oxidation on Ru MMO demonstrated that a combination of activity and stability is a desired property for the anode material, although it still needs to be determined whether these results will compensate for the high costs of most of the precious metals used in MMO coated electrodes.