A DFT study on the speciation of aqueous gold and copper cyanide complexes

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The speciation of copper and gold cyanide complexes was investigated using Density Functional Theory (DFT) in order to understand the behaviour of these complexes in aqueous solutions. To represent the local environment, two methods of implicit (COSMO) and explicit water molecules were utilized and the results were compared to evaluate their capability. The hydration state of the species, stability constants, cluster formation, interaction of different species and finally the gold leaching character of copper cyanide complexes were investigated in details.The solvation energy of Au(CN)2−, Cu(CN)2−, Cu(CN)3−2 and Cu(CN)4−3 were calculated as −41.1, −42.5, −98.1 and −168.7 kcal·mol−1 with 4, 4, 6 and 8 water molecules in the first shell of hydration, respectively. A high level of cluster formation was observed for both dicyano copper and gold complexes. Cu(CN)2− displayed the highest level of interaction with Au(CN)2− and mixed cluster formation of dicyano complexes was proposed as a mechanism of copper cyanide adsorption on the activated carbon. Other species including tri and tetracyano complexes were found to be well dispersed with minimal interaction with other species. The leaching potential of copper cyanide complexes was proved by computational modelling for the first time, here. Cyanide ions are released from all three different order copper cyanide complexes, while only released CN− from Cu(CN)3−2 and Cu(CN)4−3 dissolves gold and Cu(CN)2− passivates the gold surface by formation of Au(CN)-Cu(CN) layer. The results of this computational modelling study can be used in providing a more in depth understanding of the chemistry of gold hydrometallurgy in the presence of other metal ions.