Decomposition of Cyanide Solution Containing Ag ( I ) by Photo-Catalytic and/or Ozone Oxidation Route

Abstract

To contribute to the treatment of spent solution discharged from industry, decomposition reaction of cyanide solution containing Ag(I) by the photocatalytic and/or ozone oxidation route was investigated. The main findings obtained were as follows:
1) Ag(CN)₂^- ion in cyanide solution is photocatalytically reduced to elemental Ag by TiO₂ or ZnO catalyst powder, but exhibits an induction period at the initial stage under the air bubbling condition. By contrast, there was no induction period under N₂ gas bubbling condition. Precipitation of Ag powder was far slower than that under air bubbling. Simultaneously CN^- ion is oxidized to CNO^-, and subsequently to NO₃^- and CO₃^2-. Oxidation of CNO^- ion was significantly slower under N₂ gas bubbling than under air bubbling.
2) ZnO is a more efficient photocatalyst for Ag( I ) cyanato complex, however limited consumption of the ZnO catalyst is unavoidable.
3) Using ozone as a decomposing agent for cyanide solution containing Ag(CN)₂^-, AgCN is precipitated. However, complete removal of Ag( I ) is not expected by this method. In contrast, the addition of KCl to the solution leads to complete Ag( I ) removal, producing AgCl instead of AgCN.
4) Using ozone and photocatalytic reactions at the same time, production of CNO^- ion by oxidation of the CN^- ion can proceed faster than that using each reaction independently, precipitating Ag₂O₂ powder. However, complete removal of Ag( I ) is not expected. AgCl instead of Ag₂O₂ is recovered by the addition of KCl to the solution.