Simultaneous oxidation of cyanide and thiocyanate at high pressure and temperature

Graphical abstract: - Highlights: • The oxidation rate of SCN⁻ was greatly enhanced by the presence of CN⁻. • The degradation of mixtures was significantly affected by temperature and pressure. • A free-radical pathway was proposed, CN⁻ and CNO⁻ being the reaction intermediates. • The principal reaction products were found to be HCOO⁻, NH₃ and SO₄²⁻. • One of the parallel routes gives the found products and the other N₂, CO₂ and H₂. - Abstract: Thiocyanate and cyanide are important contaminants that frequently appear mixed in industrial effluents. In this work the wet oxidation of mixtures of both compounds, simulating real compositions, was carried out in a semi-batch reactor at temperature between 393 K and 483 K and pressure in the range of 2.0–8.0 MPa. The presence of cyanide (3.85 mM) increased the kinetic constant of thiocyanate degradation by a factor of 1.6, in comparison to the value obtained for the individual degradation of thiocyanate, (5.95 ± 0.05) × 10⁻⁵ s⁻¹. On the other hand, the addition of thiocyanate (0.98 mM) decreased the degradation rate of cyanide by 16%. This revealed the existence of synergistic and inhibitory phenomena between these two species. Additionally, cyanide was identified as an intermediate in the oxidation of thiocyanate, and formate, ammonia and sulfate were found to be the main reaction products. Taking into account the experimental data, a reaction pathway for the simultaneous wet oxidation of both pollutants was proposed. Two parallel reactions beginning from cyanate as intermediate were considered, one yielding ammonia and formate and the other giving carbon dioxide and nitrogen as final products