Abstract
Thiol-functionalized mesoporous calcium silicate (MCS-SH) was synthesized using post-grafting with calcium nitrate tetrahydrate and sodium metasilicate nonahydrate as raw materials, cetyl trimethyl ammonium bromide as template, and (3-mercaptopropyl)trimethoxysilane as modifying agent. The structure and composition were characterized using X-ray diffraction, Fourier-transform infrared spectroscopy, Brunauer–Emmett–Teller surface analysis, thermogravimetry–differential thermal analysis, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectrometry. The adsorption performance and thermodynamic and kinetic characteristics of MCS-SH for Cd2+, Cu2+, Pb2+, and Cr3+ were investigated. Results showed that MCS-SH possessed a slit-pore structure with a specific surface area of 129.32m2g−1 and pore size that was mainly distributed in 5–49nm, which reduced by 28.81m2g−1 and 2nm compared with those of unmodified mesoporous calcium silicate (MCS), respectively. The amount of SH grafted to MCS-SH was 0.4594mmolg−1, according to the determined sulfur content. The maximum adsorption capacities for Cd2+, Cu2+, Pb2+, and Cr3+ were 601.51, 509.56, 618.09, and 334.81mgg−1 at 293K, respectively, which were much higher than those reported in the literature, and follow the order of Pb2+>Cd2+>Cu2+>Cr3+. The equilibrium data of the four heavy metal ions adsorbed by MCS-SH fitted the Langmuir model and, especially, the Redlich–Peterson model well. The adsorption processes were all endothermic, entropy increasing, and spontaneous. The adsorption of MCS-SH for Cd2+, Cu2+, Pb2+, and Cr3+ was rapid and attained equilibrium within 60min. The adsorption kinetics can be well fitted by the pseudo-second-order model, and the adsorption activation energy followed the order of Cr3+ (29.7526kJmol−1)>Pb2+ (21.5840kJmol−1)>Cu2+ (19.6988kJmol−1)>Cd2+ (18.5377kJmol−1). The adsorption mechanisms include physical adsorption, chemical adsorption (especially surface complexing adsorption), and ion exchange, with chemical adsorption being the dominant mechanism. MCS-SH exhibited an excellent performance as adsorption material for Cd2+, Cu2+, Pb2+, and Cr3+.
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