## 3 Results and Discussion
### XPS Analysis  
XPS was an effective method for investigating the chemical states and elemental makeup of the molybdenum surface by measuring the binding energies of emitted electrons [36; 37; 38; 39]. To elucidate the mechanism by which the STG system with Cu\({}^{2+}\) affected the floatability of molybdenum, XPS analyses were performed on molybden samples subjected to various treatment conditions. Figure 9 presents the XPS survey spectra of molybden samples under different treatment conditions. The spectrum of untreated molybdenite (Figure 9) was relatively clean, showing only minimal oxygen signals, likely due to air contamination or slight surface oxidation. Similarly, the spectrum of molybdenite treated with STG alone closely matched that of untreated molybdenite. After Cu\({}^{2+}\) treatment, a distinct Cu 2p peak appeared, confirming the chemical binding of Cu\({}^{2+}\) to the molybdenite surface. In the STG system, Cu\({}^{2+}\) led to a significant increase in Cu 2p peak intensity, suggesting enhanced formation of Cu complexes on the molybdenite surface. To assess changes in surface composition quantitatively, the atomic concentrations of key elements in each sample were determined (Table 1). The data showed that the Cu content on the untreated and STG treated molybdenite surfaces were, respectively, only 0.19% and 0.22%, which were possibly due to the experimental error because molybdenite surface and STG molecule alone do not contain Cu-related substances and should not cause significant changes in Cu content by themselves. After Cu\({}^{2+}\) treatment, this value increased to 0.91%, confirming the effective adsorption of Cu\({}^{2+}\). Following the addition of STG, the surface Cu content increased to 3.65%, consistent with the enhancement of the Cu 2p peak intensity. This increase was likely due to the preferential binding of -COO- in STG molecules to Cu\({}^{2+}\), which captured additional Cu\({}^{2+}\) from the pulp, thereby raising the surface Cu content. Concurrently, untreated molybdenite exhibited a baseline C content of 12.90%, primarily attributed to atmospheric CO\({}_{2}\) physisorption. For Cu\({}^{2+}\)-treated samples, C content markedly increased to 18.48%, likely due to Cu\({}^{2+}\)-catalyzed oxidation of adsorbed CO\({}_{2}\) into carbonate species. STG-treated  
Figure 8: Residual STG ratio under different reagent conditions. samples showed a slightly reduced C content (17.21%), potentially reflecting residual physically adsorbed STG from incomplete cleaning. Importantly, sequential Cu\({}^{2+}\)/STG treatment yielded the highest C content (18.50%), consistent with intensified C 1s spectral signals. This enhancement was mechanistically explained by Cu\({}^{2+}\) providing active surface sites that promoted STG chemisorption, with carbon-containing moieties in STG acting as supplementary carbon sources. Similarly, untreated molybden exhibited a characteristic S content of 50.88%, predominantly attributed to lattice S\({}^{2}\) species. Following Cu\({}^{2+}\) treatment, the S content decreased to 45.19%, which could be ascribed to Cu\({}^{2+}\)-induced sulfur vacancies and partial conversion to Cu-S species. STG-treated samples exhibited an intermediate S content of 47.63%, likely due to residual STG molecules partially masking sulfur signals. Notably, the sequential Cu\({}^{2+}\)/STG treatment resulted in the lowest S content (44.85%), likely due to two synergistic mechanisms: (1) Cu\({}^{2+}\)-mediated bridging of STG molecules, forming a dense organic film that enhanced surface shielding, and (2) the synergistic effect between Cu\({}^{2+}\) and STG, exacerbating sulfur etching during interfacial reactions. These combined processes effectively suppressed sulfur detection while promoting the accumulation of copper and carbon on the molybden surface. Figure 9: XPS survey spectra of molybdenite: (**a**) untreated, (**b**) exposed to Cu\({}^{2+}\), (**c**) exposed to STG, and (**d**) exposed to both Cu\({}^{2+}\) and STG.