Versatile Surface Modification of Ceramsite Via Honeycomb Calcium-aluminum-silicate-hydrate and Its Functionalization by 3-thiocyanatopropyltriethoxysilane for Enhanced Cadmium(II) Removal

A low-cost and efficient filter medium for Cd(II) removal was prepared by anchoring -SCN functional groups (by 3-thiocyanatopropyltriethoxysilane, TCPS) on ceramsite via the approach of synthesizing a honeycomb calciumaluminum-silicate-hydrate (C-A-S-H) layer as intermediate. The specific surface area of ceramsite was increased enormously by more than 50 times because of the modification of honeycomb layer. Moreover, the abundant Si-OH bonds existing in the structure of CAS-H can serve as active sites for TCPS. The combined effects ensure that the hybrid filter medium (named ceramsite/C-A-S-H/TCPS) demonstrated a high Cd(II) adsorption capacity of 18.27 mg·g 1 for particle size of 0.1-0.6 mm, 12.63 mg·g −1 for 0.6-1.25 mm and 8.64 mg g −1 for 1.25-2.35 mm. The Cd(II) adsorption capacity per unit area of ceramsite/C-A-S-H/TCPS (0.1-0.6 mm) is up to 4.07 mg·m −2 , which is much higher than that of many nano-adsorbents. In addition, ceramsite/C-AS- H/TCPS could maintain a high removal efficiency (> 85%) in a wide range of pH 3-11 and showed excellent selectivity in the presence of competing ions. Furthermore, Cd(II) could be desorbed from ceramsite/C-A-S-H/TCPS composites with nearly 100%, suggesting the potential application in recycling of heavy metal ions.