Highly effective removal of lead and cadmium ions from wastewater by bifunctional magnetic mesoporous silica

Abstract A bifunctionalized magnetic mesoporous silica (NZVI-SH-HMS) material with thiol and nanometer zero-valent iron immobilized on the surface was synthesized at ambient temperature by gel-sol and wet impregnation methods, which was successfully used to remove lead and cadmium ions from aqueous solution. SEM, TEM images and FTIR spectra showed that thiol and NZVI were stable and evenly dispersed onto the surface of HMS. N2 adsorption–desorption isotherms indicated NZVI-SH-HMS owned good porous properties with a large specific surface area (312.84 m2/g) and proper pore size (2.56 nm). XRD and XPS results confirmed the material was superparamagnetic and the main process for removing heavy metals was the formation of Pb3(CO3)2(OH)2, PbS, CdxFe(1-x)(OH)2 and Cd(OH)2. The theoretical maximum adsorption capacity of NZVI-SH-HMS was 487.8 mg Pb(II)/g and 330.0 mg Cd(II)/g, respectively. Adsorption isotherm and kinetics data fitted perfectly to Langmuir isotherm model and followed the pseudo-second-order kinetics. Batch experiments indicated that the main adsorption mechanisms could be precipitation/co-precipitation and surface complexation between the heavy metal ions and the adsorbents. Additionally, NZVI-SH-HMS exhibited excellent adsorption capacities and sorption recyclability in various simulated wastewater (the surface water, tap water, sewage plant wastewater and domestic wastewater), which showed the promising capabilities for the removal of heavy metal ions in actual water environment. These results suggested that the synergistic effects of NZVI and thiol greatly improved the adsorption performance of the bifunctionalized magnetic mesoporous silica materials.