A comparative study of the adsorption kinetics of AuCN2− and AuS2O323− by prepared activated carbon

The adsorption behaviour of two gold complexes, ${\left(\mathrm{Au}{\left(\mathrm{CN}\right)}_2\right)}^{-}$ and ${\left(\mathrm{Au}{\left(S_2{O}_3\right)}_2\right)}^{3-}$, was examined using activated carbon (AC) derived from Parinari macrophylla walnut shells. The AC samples were prepared by physical activation under N₂ or CO₂ atmospheres at a final pyrolysis temperature of 900 °C. Structural, chemical, and adsorption properties were characterized through N₂ adsorption isotherms, Boehm titration, infrared spectroscopy, and iodine and methylene blue indices. Adsorption experiments were conducted with 2 g·L⁻¹ of AC in 10 mg·L⁻¹ Au solutions, over an 8 h contact period, and kinetic modelling was performed using pseudo-first order and pseudo-second order equations. Activated carbons obtained under N₂ and CO₂ atmospheres exhibited specific surface areas of 596 and 1167 m²·g⁻¹, respectively, with total pore volumes of 0.261 and 0.582 cm³·g⁻¹, and microporosity exceeding 55 %. Surface chemistry analysis revealed a predominantly basic character, with aromatic CC and carbonyl (C=O) functionalities as the main groups. Gold adsorption efficiencies ranged from 33.87 % to 99.49 %, confirming high aqueous-phase adsorption performance. For both complexes, adsorption kinetics were best described by the pseudo-second-order model, and particle size had a marked effect on adsorption rates.