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

The Mg-Al layered double hydroxides are intercalated with inorganic sulfides of sodium trithiocarbonate (Na₂CS₃), sodium sulfide (Na₂S), and organic sulfides of sodium dimethyldithiocarbamate (SDTC), trithiocyanuric acid (TMT) via the co-precipitation method for the purpose of removing Cu²⁺ and Pb²⁺ 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 Cu²⁺ align more closely with the Langmuir model, suggesting a preference for monolayer adsorption, whereas Pb²⁺ 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 Cu²⁺ and Pb²⁺, 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 Cu²⁺/Pb²⁺. The sorption mechanisms for Cu²⁺ and Pb²⁺ are explored: Cu²⁺ preferentially reacts with interlayer sulfides in LDHs, subsequently displacing hydroxyl groups from the LDHs layers to form precipitates, whereas Pb²⁺ 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 Cu²⁺ and Pb²⁺, with minimal risk of secondary pollution. Metal speciation analysis reveals that organic sulfur-intercalated LDHs provide more stable sorption for Cu²⁺ and Pb²⁺.