Kinetics of the electrochemical deposition of sulfur from sulfide polluted brines

Abstract

The kinetics of the electrochemical oxidation of sulfide ions in salt water were studied using rotating graphite disc electrodes, polarization techniques, Electrochemical Impedance Spectroscopy (EIS), X-ray Photoelectron Spectroscopy (XPS) and Electron Dispersion Spectroscopy (EDS). Elemental sulfur was shown to be the final product under various temperatures, potentials and times of electrolysis, in amounts that increased with increase in the above variables. The rate of the process is controlled by electron transfer across the interface, while diffusion in the electrolyte has only a modest effect. The apparent reaction orders with respect to the sulfide concentration and pH are 0.60 and 0, respectively. The proposed overall reaction is: $${\hbox{HS}^{-}_{\rm (aq)} \to \hbox{S} + \hbox{H}^{+} + 2\hbox{e}},$$ while the rate determining step is: $${\hbox{HS}^{-}_{\rm (aq)}\to \hbox{HS}_{\rm ads} + \hbox{e}}.$$ The charge transfer coefficient is αa = 0.23 and the standard rate constant at the equilibrium potential is $${k^{\circ}=2.2 \times 10^{-7}}$$ cm s−1. The degree of coverage of the electrode with sulfur and the polarization resistance of the interface increase, while the current decreases, with the time of electrolysis as more sulfur is deposited on the electrode surface.