In this work, a mesoporous nanocast carbon prepared using SBA-15 as a template was modified by surface oxidation to produce unique catalysts for sulfide oxidation/elimination from an aqueous medium. Different characterization techniques (BET, SEM, Raman, FTIR, potentiometric titration, elemental analyses, cyclic voltammetry) showed that treatment with concentrated HNO₃ at 80 °C for 5, 15 and 30 min attacks the carbon structure creating different oxygen surface functionalities in concentrations varying from 0.4–1.1 mmol g⁻¹ with a small effect on the surface area (1080–1148 m² g⁻¹) and pore volumes (micropore 0.17–0.20 cm³ g⁻¹ and mesopores 0.36–0.50 cm³ g⁻¹). These materials showed high activities for the oxidation of sulfide in an aqueous medium forming polysulfides, e.g., S₂²⁻, S₃²⁻ and S₄²⁻, which are rapidly eliminated from the aqueous medium. These results are discussed regarding efficient sulfide oxidation at redox surface oxygen sites leading to the formation of higher polysulfides followed by adsorption of these relatively large and more hydrophobic molecules into the mesopores.