Comparative study of different organic and inorganic sulfide intercalated layered double hydroxides to remove Cu(Ⅱ) and Pb(Ⅱ) from aqueous solutions.

The Mg-Al layered double hydroxides are intercalated with inorganic sulfides of sodium trithiocarbonate (NaCS), sodium sulfide (NaS), and organic sulfides of sodium dimethyldithiocarbamate (SDTC), trithiocyanuric acid (TMT) via the co-precipitation method for the purpose of removing Cu and Pb in aqueous solutions. The sorbents are characterized by XRD, FT-IR, BET, SEM, XPS and zeta potential. The sorption kinetic data fits well with the pseudo-second-order model and the intra-particle diffusion model. The sorption isotherm data for Cu align more closely with the Langmuir model, suggesting a preference for monolayer adsorption, whereas Pb sorption better follows the Freundlich model, indicating heterogeneous multilayer adsorption, thus revealing distinct sorption processes. Among the sulfur-intercalated LDHs, SDTC@LDH exhibits the highest sorption capacity for Cu and Pb, reaching 357.14 mg/L and 274.76 mg/L, respectively. Molecular simulations further elucidate the superior performance of organic sulfur-intercalated LDHs, demonstrating higher binding energies and stronger affinity between organic sulfides and Cu/Pb. The sorption mechanisms for Cu and Pb are explored: Cu preferentially reacts with interlayer sulfides in LDHs, subsequently displacing hydroxyl groups from the LDHs layers to form precipitates, whereas Pb first bonds with hydroxyl groups on the LDHs layers, causing structural collapse, followed by capture by exposed interlayer sulfides. Dynamic sorption experiments demonstrate that SDTC@LDH can treat larger volumes of aqueous solutions compared to other adsorbents, indicating that SDTC@LDH possesses superior sustained sorption capacity in water treatment applications. Coexisting ion experiments, pH tests, reusability studies, and sulfide/heavy metal leaching assessments confirm the high selectivity and stability of the adsorbents for Cu and Pb, with minimal risk of secondary pollution. Metal speciation analysis reveals that organic sulfur-intercalated LDHs provide more stable sorption for Cu and Pb.