Sulfur impurity removal in magnesium aluminate spinel by acid treatment

Abstract

MgAl2O4 is a versatile and stable oxide that is useful in various applications. However, the presence of impurities originating as a part of the powder synthesis process can potentially impair desired properties. In this work, we performed acidic treatments with mixtures of HNO3 and H2SO4 on commercial MgAl2O4 nanopowders. The treated powder structure and morphology was found to be largely retained, while changes in specific surface area, adsorbed CO2, and sulfur content were detected. Two sulfur species populations were identified (i.e., S2+ and S6+) and their concentration throughout the surface and bulk of the particles was analyzed. Notably, S2+ was beforehand unknown as a contaminant in MgAl2O4. The S2+ concentration could be diminished below detection limit of XPS by increasing the percentage of H2SO4. The insights gained on sulfur impurities present in commercial nanopowders and their removal possibilities or limitations can help utilize such oxides in applications sensitive to chemical purity.