Effect of metal ions on gold adsorption onto activated carbon in the acidic thiourea solution: Experiment study and DFT calculation

Thiourea leaching has been regarded as a promising method to substitute cyanidation for gold recovery from ores or concentrates. Activated carbon used in traditional cyanide leaching solution can also be used for gold recovery from thiourea leaching solution. In this paper, the effect of metal ions including Cu2+, Ag+, Pb2+ and Zn2+ on gold adsorption onto activated carbon from acidic thiourea solution was studied for the first time. Results showed that the adsorption process of activated carbon for gold can be well described by the Freundlich isotherm model, and followed the Pseudo-second kinetic model. The adsorption of gold onto activated carbon in thiourea solution may be a ligand exchange reaction. The S of the activated carbon is the electron-accepting atoms, which can form the bond of Au-S with Au in Au[CS(NH2)2]2+. The presence of Pb2+ and Zn2+ did not interfere gold adsorption at gold concentration of 5 mg/L, but hindered gold adsorption at gold concentration of 100 mg/L. When Cu2+ and Ag+ were added in the whole studied concentration range, the gold adsorption could be significantly declined. The adsorption capacity of gold onto activated carbon in the presence of metal ions followed the sequence of Cu2+ < Ag+ < Pb2+ < Zn2+ < no addition of metal ions. Meanwhile, metal ions also adsorbed onto activated carbon, and their adsorption capacity on activated carbon obeyed the descending order of Cu2+ > Ag+ > Pb2+ > Zn2+, consistent with the complexing ability of metal ions with thiourea. The decrease of active site of activated carbon occupied by metal ions was responsible for the decrease of gold adsorption. Based on the DFT analysis, it can be inferred that the ability of metal ions to lose electrons caused the different influence of metal ions on gold adsorption.

Automated summary

Thiourea leaching is a promising alternative to cyanidation for gold recovery. This study investigated the effect of metal ions on gold adsorption onto activated carbon from acidic thiourea solution. Metal ions such as Cu2+, Ag+, Pb2+, and Zn2+ interfered with gold adsorption, with Cu2+ and Ag+ having the greatest impact. Metal ions also adsorbed onto activated carbon, with their adsorption capacity following the order of Cu2+ > Ag+ > Pb2+ > Zn2+. The decrease in active sites on activated carbon occupied by metal ions resulted in a decrease in gold adsorption.