*2.4. Analysis Method*

The particle specific surface area of the leach residue was determined by laser diffraction (Mastersizer 2000, Malvern Panalytical Ltd., Malvern, UK). After the leaching experiment, the leaching solution was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES; Optima Model 5300 DV, Perkin Elmer, Norwalk, CT, USA), to determine the amount of metal leaching. The chemical characterization of the surface species was performed by XPS (K-Alpha+, Thermo Fisher Scientific, Waltham, MA, USA) to determine As, Fe, and S elements in the measured sample. The refractory gold concentrate and leach residue were subjected to XRD (X'Pert Pro MRD, PANalytical, Almelo, Netherlands) analysis. Cu-Kα X-rays were used and 2θ of 10◦–70◦ was analyzed with an acceleration voltage of 40 kV, a current of 30 mA, and a scanning speed of 2◦/min.
