Sorption of Borate and Fluoride on Bimetallic Mg-Al and Mg-Fe Oxides in Aqueous Solutions

Abstract

Two bimetallic oxides were formed at 873 K by calcination of hydrotalcite ([(M₁^II_1-xM₂^III_x (OH)₂] (A^n—) _x/n·mH₂O, where M₁ is Mg, M₂ is Al or Fe, A is anionic species, and x is 0<x<1) , for borate and fluoride sorption experiments in aqueous solutions. While the XRD patterns for both bimetallic oxides were assigned to MgO without secondary metallic compounds, SEM-EDX results showed that the secondary metallic elements were evenly distributed on the surfaces of both oxides. When borate and fluoride were sorbed on bimetallic oxides in the Mg—Al system, Al(OH) ₄^— and Mg²⁺ ions were initially released followed by sorption onto the solid phase, but Fe was not released in the bimetallic Mg—Fe oxide system. Regeneration of hydrotalcite was successfully achieved in the Mg—Al system upon borate and fluoride sorption, resulting in greater sorption efficiency with the Mg—Al system as compared to the Mg—Fe system. XPS results for the bimetallic oxides showed that the surface composition was identical to the molar ratio of Al/Mg in the bulk phase of the bimetallic oxide in the Mg—Al system, thus enhancing the regeneration of hydrotalcite.