Synthesis of Mg-Al LDH and its calcined form with natural materials for efficient Cr(VI) removal

Layered double hydroxide (LDH) has been extensively studied due to its excellent capacity of heavy metal adsorption. However, the synthesis of LDH is always complex and requiring pure chemical reagents. In this study, two solid minerals, MgO and metakaolin (MK), derived from abundant natural resources, were used as raw materials to synthesize Mg-Al LDH via a facile one-pot method. The compositions and morphologies of synthetic products and its calcined form(C-LDH) were characterized by X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Cr(VI) adsorption by the LDH and C-LDH was systematically studied including the effect of contact time, initial pH value, temperature, liquid/solid ratio, and initial Cr(VI) concentration. The adsorption kinetics and isotherms were modelled, and thermodynamic parameters were determined. Results showed that the LDH and C-LDH exhibited high adsorption capacities (23.5-33.2 mg/g and 33.2-38.9 mg/g, respectively) within a wide pH range of 1.75-6.0, and maintained a higher adsorption capacity in all cases afterwards. The pseudo-second-order kinetic model fitted the kinetic data well in which the Langmuir model described the adsorption isotherms the best. The adsorption process was endothermic and spontaneous. The adsorption mechanisms may include anion exchange, redox reaction, and electrostatic adsorption. Moreover, the effect of the synthesis conditions (i.e., alkali concentration, temperature and liquid/solid ratio) on the adsorption characteristics were discussed. And XPS, UV-Vis Spectrophotometer (UV-Vis) and FTIR analysis confirmed Cr(VI) adsorption on the LDH and C-LDH surface, followed by its subsequent reduction to Cr(III). It has been revealed that the LDH/C-LDH synthesized in this work exhibited higher Cr(VI) removal capacities compared to the ones reported in the literature, demonstrating the significant potential of synthesizing high performance absorbents by low-cost natural materials.