Chemical regeneration of magnesium oxide used as a sorbent for fluoride

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 ^-.