Influence of nickel doping on the moisture adsorption properties of magnesium aluminate spinel: thermodynamic and kinetic analysis

Abstract

Magnesium aluminate spinel (MAS) has been widely investigated due to its exceptional properties, which include a high melting point, thermal stability, chemical inertness, abundant vacant sites, substantial porosity, and strong mechanical resilience. The hygroscopic nature of MAS continues to be a major drawback that limits the range of possible uses. In this study, the hygroscopic nature of MAS is investigated for its potential in atmospheric moisture capture. Preliminary thermodynamic and kinetic modeling was performed on experimental data to gain a deeper understanding into the material's surface properties and the fundamental adsorption mechanisms. MAS and Ni doped derivatives were synthesized by a cost-effective coprecipitation method. X-ray diffraction studies confirmed a cubic spinel phase without any impurity phases. Different samples with the general composition of Mg_1−xNi_xAl₂O₄; x = 0.2, 0.6, 0.8 were tested on an indigenously designed moisture harvester, and it was found that the adsorption process depends critically on both available humidity and adsorption duration. As the relative humidity varies from 45–85%, Mg_1−xNi_xAl₂O₄ with x = 0.2 and x = 0.6 exhibited a water adsorption capacity of 1.6 mg g⁻¹ to 4.3 mg g⁻¹ and 2.7 mg g⁻¹ to 12 mg g⁻¹, respectively, while at the 45–75% RH, Mg_1−xNi_xAl₂O₄ with x = 0.8 exhibited a water adsorption capacity of 8 mg g⁻¹ to 13 mg g⁻¹. The adsorption capacity was determined by gravimetric analysis using weight measurements of the sample both before and after adsorption. Langmuir, Freundlich, and Temkin adsorption isotherm models were employed to analyze adsorption data. Thermodynamic analysis revealed that adsorption is exothermic and spontaneous, while kinetic studies showed it follows first-order behavior.