Protonated covalent organic frameworks for green and effective recovery of Au(I) from thiosulfate solutions: Performance, DFT calculations, and mechanism insights

The recovery of Au(I) from thiosulfate leaching systems is important to ensure the sustainable use of strategic metal resources. In this study, two ionic covalent-organic frameworks (iCOFs), Tmp-Tpa COF-Cl and Tmp-Tpa COF-SO4, were synthesized through a post-modification strategy guided by theoretical calculations. Acid modification redistributed the surface charge of Tmp-Tpa COF due to the protonation of nitrogen atoms, which enabled the efficient and selective recovery of Au(I) from a thiosulfate solution. The Au(I) adsorption capacities of Tmp-Tpa COF-Cl and Tmp-Tpa COF-SO4 were 69.8 mg/g and 71.3 mg/g, respectively, which were significantly higher than those of previously reported materials. Adsorption equilibrium isotherms and kinetic studies revealed that the adsorption of Au(I) onto iCOFs conformed to the Langmuir and quasi-second-order kinetics model, showing that Au(I) underwent uniform monolayer chemisorption on the surface of the iCOFs. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculations indicated that the adsorption of Au(I) onto Tmp-Tpa COF-Cl and Tmp-Tpa COF-SO4 was mainly because acid modification converted –C = N to –C = NH+-, which enhanced the strength of electrostatic attraction and coordination with Au(S2O3)23− complexes. Acidic thiourea rapidly eluted Au(I) adsorbed onto the iCOFs and simultaneously regenerated Tmp-Tpa COF-Cl. Even during the fifth adsorption/desorption cycle, the adsorption and desorption efficiencies of the iCOF still reached 86.6 % and 97.7 %, respectively. In addition, Tmp-Tpa COF prepared by Knoevenagel condensation was inexpensive (less than $50/kg) and did not require the use of organic solvents, enabling large-scale green production. This work provides an innovative method for the efficient, selective, and high-capacity recovery of gold from thiosulfate leachate.