Chemical regeneration of magnesium oxide used as a sorbent for fluoride

Abstract

► Use of MgO in repeated calcination regarding its sorption of F − and chemical stability was tested. ► The principal mechanism to immobilize F − is based on formation of Mg(OH)2. ► The basicity of MgO was considered to be responsible for the removal of F − through hydration. ► NaMgF 3 and Mg were evaporated by the calcination at 1273 K of the solid residues after sorption of F − . Use of MgO for repeated calcination with regard to its sorption density for F − and chemical stability was examined. Magnesium oxide was produced by the calcination of MgCO 3 at 1273 K for 1 h. The sorption of 9.82 mM F − on the calcined product and the calcinations of solid residues were carried out five times to evaluate trends in sorption density for F − and the stability of the chemically regenerated sorbents. The order of sorption density of F − ( Q /mol g −1 ) apparently seems to depend on the specific surface area. However, Q′ values after normalization of the sorption density for the specific surface area ( Q′ /mmol m −2 ) were found to be correlated with the solid basicities of the calcined products which were derived from CO 2 -TPD curves for the calcined products. The number of weak base sites, calculated from the peak intensity at 373 K in CO 2 -TPD, was considered to be responsible for the removal of F − through hydration. Large quantities of NaMgF 3 and elemental Mg were evaporated by the calcination at 1273 K of the solid residues formed after sorption of F − .