Adsorptive oxidation of sulfides catalysed by δ-MnO₂ decorated porous graphitic carbon composite

Removal of dissolved sulfide contaminants from aqueous model solution using bio-derived porous graphitic carbon (PGC) impregnated with δ-MnO₂ was investigated. The composite adsorbent was synthesized using the chemical wet deposition method wherein MnO₂ was deposited on carbon walls through an in-situ reaction between permanganate and ethanol. Formation of transition metal oxide of manganese in the form of birnessite nanoparticles on interconnected PGC cell structure was confirmed by transmission electron microscopy, scanning electron microscopy, elemental analysis, and X-Ray diffraction characterization studies. The composite nanomaterial was tested for sulfide removal from aqueous solution at various conditions, including the pH, adsorbent dosage, initial solution concentration, and contact time. Adsorption results demonstrated an excellent adsorption capacity of ca. 90% within 20 min of contact time at 298 K. Equilibrium data collected from batch adsorption experiments fitted well with the Langmuir isotherm model (K_L = 190 L/mg; R² = 0.99). The maximum adsorption capacity of the composite was estimated as 526.3 mg S^2-/g at highly alkaline conditions compared to ca. 340 mg/g for a δ-MnO₂ adsorbent. Adsorptive oxidation of sulfides on composite MnO₂-PGC adsorbent was found to be controlled by the chemisorption process in accordance with the pseudo-second-order reaction model. Characterization of spent adsorbents revealed that sulfide was removed through adsorptive oxidation resulting in the formation of agglomerated particles of metal sulfate complexes and elemental sulfur. Analysis of reaction mechanism revealed that both MnO₂ and PGC played a role in the adsorptive oxidation of sulfides to CaSO₄ and elemental sulfur.