The electro-oxidation of sulfite and its radicals has a wide range of applications from gold electrowinning to wastewater treatment, hydrogen production and in the food industry (1-4). While efforts have been made to study the electro-oxidation of sulfite ions on noble metals (e.g. Au) as well as carbon surfaces, metal oxides in general have not been thoroughly investigated for this electrochemical reaction (1, 5). During gold recovery from thiosulfate solutions, Au⁺ ions are reduced to Au from the thiosulphate gold complex on the cathode while oxygen evolution occurs on the anode via the following reactions (6):

Au(S₂O₃)₂^3- + e^- = Au + 2S₂O₃^-2, E⁰= 0.15 V (1)

4OH^- = 2H₂O + O₂ + 4e^-, E⁰= 0.4 V (2)

To drive the cathodic reaction, high current densities are used, which result in high anodic potentials on the stainless-steel anodes. Such high anodic potentials may cause transpassive corrosion of the anode as well as significant, and undesirable, changes in solution chemistry (7).

This study systematically investigated the effect of operating factors on the corrosion behavior of stainless steel anodes in thiosulfate solutions during gold electrowinning. Further, novel non-precious group materials such as manganese oxides and perovskites were used as electrocatalysts for sulfite electro-oxidation to ultimately enhance the activity and long-term durability of the anodes in the gold electrowinning process (8). A 2ⁿ half-fraction factorial design study was performed to find the effect of five important parameters involved in the gold electrowinning process (i.e. thiosulfate concentration, sulfite concentration, sulfate concentration, temperature and applied anodic current density), on the anodic potential of the stainless steel anode. As shown in the Fig. 1A, increasing the sulfite concentration and temperature of the gold electrowinning solution lead to a low anodic potential of 550 mV which is about 250 mV lower than the transpassive corrosion potential for the 316 stainless steel anodes. To further reduce the anodic potential and employ the high sulfite concentration (i.e. 3000-32000 ppm) in the gold electrowinning solution, non-precious metal oxides were tested to facilitate the electrowinning process by catalyzing the following sulfite electro-oxidation reaction instead of the oxygen evolution reaction (eq. 2):

SO₃^2- + 2OH^- = SO₄^2- + H₂O + 2e^-, E⁰= -0.93 V (3)

Fig. 1B shows that a combination of manganese oxide and perovskite electrocatalysts provides promising electrocatalytic activity and durability for sulfite electro-oxidation (i.e. up to 500 mV lower onset-potential over 7 hours of testing at 3.5 mAcm^-2) compared to the Ni, commercial Ag/AgO, Pt, carbon, commercial Pb-2 wt% Ag and stainless steel electrodes tested here.

Fig. 1 A) Surface plot produced from the 2ⁿ half-fraction factorial design study on the effect of five electrowinning parameters on the performance of 316 stainless steel gold electrowinning anode. B) A comparison between the electrocatalytic activity of various electrocatalyst materials for sulfite electro-oxidation in a solution of 100 ppm NaCl + 3000 ppm Na₂SO₄ + 32000 ppm Na₂SO₃ at 3.5 mA cm^-2 and 45 °C. All MnO₂-based catalysts are under the license with Catalyst Square Materials Ltd (8).

References:

1. P. Westbroek, J. De Strycker, K. Van Uytfanghe and E. Temmerman, Journal of Electroanalytical Chemistry, 516, 83 (2001).
2. J. A. Allen, G. Rowe, J. T. Hinkley and S. W. Donne, International Journal of Hydrogen Energy, 39, 11376 (2014).
3. R. Ojani, J. B. Raoof and A. Alinezhad, Electroanalysis, 14, 1197 (2002).
4. E. M. Contreras, Industrial & Engineering Chemistry Research, 47, 9709 (2008).
5. A. G. Zelinsky and B. Y. Pirogov, Electrochimica Acta, 231, 371 (2017).
6. C. Abbruzzese, P. Fornari, R. Massidda, F. Vegliò and S. Ubaldini, Hydrometallurgy, 39, 265 (1995).
7. N. J. Laycock, R. C. Newman and J. Stewart, Corrosion Science, 37, 1637 (1995).
8. E. Gyenge and P. Hosseini-Benhangi, An oxygen electrode and a method of manufacturing the same, in, U.S. (15/251,267) and Canadian (2,940,921) patent applications (Filed on August 30, 2016).