Noncompetitive and Competitive Adsorption of Heavy Metals in Sulfur-Functionalized Ordered Mesoporous Carbon

Abstract

In this work, sulfur-functionalized ordered mesoporous carbons were synthesized by activating the soft-templated mesoporous carbons with sulfur bearing salts that simultaneously enhanced the surface area and introduced sulfur functionalities onto the parent carbon surface. XPS analysis showed that sulfur content within the mesoporous carbons were between 8.2% and 12.9%. The sulfur functionalities include C-S, C═S, -COS, and SO_x. SEM images confirmed the ordered mesoporosity within the material. The BET surface areas of the sulfur-functionalized ordered mesoporous carbons range from 837 to 2865 m²/g with total pore volume of 0.71-2.3 cm³/g. The carbon with highest sulfur functionality was examined for aqueous phase adsorption of mercury (as HgCl₂), lead (as Pb(NO₃)₂), cadmium (as CdCl₂), and nickel (as NiCl₂) ions in both noncompetitive and competitive mode. Under noncompetitive mode and at a pH greater than 7.0 the affinity of sulfur-functionalized carbons toward heavy metals were in the order of Hg > Pb > Cd > Ni. At lower pH, the adsorbent switched its affinity between Pb and Cd. In the noncompetitive mode, Hg and Pb adsorption showed a strong pH dependency whereas Cd and Ni adsorption did not demonstrate a significant influence of pH. The distribution coefficient for noncompetitive adsorption was in the range of 2448-4000 mL/g for Hg, 290-1990 mL/g for Pb, 550-560 mL/g for Cd, and 115-147 for Ni. The kinetics of adsorption suggested a pseudo-second-order model fits better than other models for all the metals. XPS analysis of metal-adsorption carbons suggested that 7-8% of the adsorbed Hg was converted to HgSO₄, 14% and 2% of Pb was converted to PbSO₄ and PbS/PbO, respectively, and 5% Cd was converted to CdSO₄. Ni was below the detection limit for XPS. Overall results suggested these carbon materials might be useful for the separation of heavy metals.