Synergistic immobilization of UO₂²⁺ by novel graphitic carbon nitride @ layered double hydroxide nanocomposites from wastewater

Cotton-like graphitic carbon nitride @ layered double hydroxides nanocomposites (g-C₃N₄@Ni-Mg-Al-LDH) was fabricated using lamellar g-C₃N₄ and rod-like Ni-Mg-Al-LDH as monomer molecules by one-step hydrothermal synthesis strategy, and it exhibited superior adsorption performance to U(VI) from wastewater based on strong synergistic effects among metal–oxygen functional groups (Mg[sbnd]O, Al[sbnd]O and Ni[sbnd]O) and free-metal functional groups (O[sbnd]C[dbnd]O, C[sbnd]O[sbnd]C, C[sbnd]O, C[sbnd]C and [sbnd]NH₂/[sbnd]NH[sbnd]/=[dbnd]N[sbnd]). The adsorption of U(VI) on g-C₃N₄@Ni-Mg-Al-LDH was controlled by outer-sphere surface complexation or ion exchange, which was evidenced with batch adsorption experiments and SEM, EDS, elemental mapping, FT-IR, XRD and XPS characterizations. Moreover, the maximum adsorption capacity of U(VI) on g-C₃N₄@Ni-Mg-Al-LDH (99.7 mg·g⁻¹) was approximately 1.5 times higher than that of U(VI) on Ni-Mg-Al-LDH (59.8 mg·g⁻¹) and approximately 3.0 times higher than that of U(VI) on g-C₃N₄ (31.1 mg·g⁻¹) at 298.15 K, and a multilayer adsorption was existed in a low concentration of UO₂²⁺, while a monolayer adsorption was appeared in a high concentration of UO₂²⁺. The results also showed that the adsorption reaction of U(VI) was a typical endothermic and spontaneous process because of ΔH^θ > 0, ΔS^θ > 0 and ΔG^θ < 0. Thus, it can provide new highlights to improve the physicochemical properties of low-efficiency adsorbents based on synergistic effect through polymerization reaction, and it can be selected as superior synthetic route for the synthesis of excellent adsorbents in environmental remediation.