Immobilization of Pb2+ and Cd2+ on the novel calcium/magnesium silicate and their transformation in the presence of phosphate

Adsorbent development is a key step toward the removal of heavy metals (HMs) from water. The transformation of fixed HMs in the environment is of the utmost importance for successful remediation treatments. Herein, novel calcium silicate (CS) and magnesium silicate (MS) adsorbents were prepared by a composite-hydroxide-mediated method, which were used to remove Pb2+ and Cd2+ from water, and the synthesis mechanism was studied in detail. Our results showed that CS/MS exhibited a high uptake of Pb2+ and Cd2+, which were 970.87/1594.81 and 334.45/448.43 mg g−1, respectively. Various characterization techniques confirmed the high adsorption capacities of the adsorbents for heavy metal ions, and the identified adsorption mechanisms were ion exchange, chemical precipitation, and surface-complexation. Afterward, the aging test of the heavy metal-adsorbent complex was conducted in a solution containing phosphate (P) to investigate the effect of P on the speciation of HMs fixed by CS and MS. The results revealed that the immobilized HMs transformed into a more stable phosphates phase to different degrees; in addition, the content of P significantly decreased in the aging process, which was beneficial for controlling the eutrophication of water bodies. In summary, CS and MS effectively immobilized HMs in the investigated aqueous system.