Removal of Pb(II), Cd(II) and Hg(II) from aqueous solution by mercapto-modified coal gangue

A low-cost mercapto-modified coal gangue (CG-SH) was fabricated by modification of coal gangue (CG) with (3-mercaptopropyl) trimethoxysilane. The structure and composition for as-prepared CG-SH were characterized by using X-ray diffraction (XRD), Fourier transfer infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray fluorescence (XRF). Results indicated that larger amounts of mercapto-groups (-SH) was successfully introduced onto CG, which followed by acted as active sites for the removal of heavy metal cations, such as Pb(II), Cd(II) and Hg(II). The factors that affected the adsorption equilibrium as well as the removal efficiency, i.e., contact time, initial concentration, pH and temperature, were investigated in detail. The adsorption isotherms for Pb(II), Cd(II) and Hg(II) were well fitted with Langmuir model. The maximum adsorption capacity of CG-SH for Pb(II), Cd(II) and Hg(II) were calculated to be 332.8, 110.4 and 179.2 mg g^-1, respectively. The adsorption for Pb(II), Cd(II) and Hg(II) on CG-SH could be well described by pseudo-second-order kinetic model. And thermodynamic analysis suggests that the adsorption process for Pb(II) is exothermal, while that for Cd(II) and Hg(II) are endothermal. The results suggest CG-SH have great potential to be used as efficient absorbent for the removal of heavy metal cations from water.