Synthesis and physicochemical characterization of new calcined layered double hydroxide Mg Zn Co Al-CO3; classical modeling and statistical physics of nitrate adsorption

Abstract

This research focuses on the study of a new material, a layered double hydroxide, which was synthesized using a co-precipitation method at pH constant and, then, tested for its retention of nitrate anions. The LDH was prepared from four metal cations Mg-Zn-Co-Al to produce the quadratic material mentioned, Mg Zn Co Al-CO3, which was calcined to obtain metal oxides. Different techniques of characterization were considered, namely FTIR, X-ray diffraction, BET, SEM and EDX. Adsorption experiments for nitrate were carried out to test the effect of contact time, adsorbent mass, pH values, mass effect, and initial surfactant concentrations. When equilibrium was reached after 300 min (R2 = 0.998), the Pseudo-second order model was found to fit accurately the kinetic data. The intraparticle diffusion model recorded a rate constant of between 0.277 and 0.757 (g/mg. min−0.5) and the Sips adsorption isotherm best fitted the experimental data obtained. Maximum adsorption capacity for nitrate was found to be 55.97 mg g−1 which was in accordance with the experimental results. Thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic. The values of ΔH˚ and ΔS˚ of the adsorption process were 14.81 and 59.04 kJ mol−1, respectively. The low value of ΔH˚ (<40 kJ mol−1) indicated that the adsorption process was associated with physical forces. Finally, for a deeper insight into the nitrate adsorption mechanism, the statistical physics model was considered to quantify the number of adsorbed nitrate molecules per site, the anchorage number, the receptor sites density, the adsorbed quantity at saturation, the concentration at half saturation and the molar adsorption energy. A statistical physics model, with two binding sites, proved to be the best option to characterize and interpret the removal of nitrate on the tested LDH.