Mg-Fe composite oxides with various mixing ratios were synthesized at different calcination temperature, to use them as anion removal agents. Crystal structure and specific surface area of the Mg-Fe composite oxides were evaluated. The B and As(III) removal tests from dilute aqueous solution (initial concentration: 20 mg/dm3) were conducted by using the Mg-Fe composite oxides. The predominant factors for removing them efficiently were considered.
The Mg-Fe composite oxides having various specific surface area and different crystal structure are obtained, depending on the chemical composition and the calcination temperature. When the mixing ratios are set to Mg:Fe = 1:1, 2:1 and 3:1, respectively, Mg-Fe type LDH is mainly formed as a precursor before calcination. Amorphous composite oxide is obtained by the calcination of Mg-Fe type LDH at 400°C. Approximately, the specific surface area of Mg-Fe composite oxides is increasing with an increase in the mixing ratio of Fe. On the other hand, the specific surface area of them also tends to decrease as the calcination temperature increases over 600°C.
As the B removal mechanism from dilute aqueous solution, it is considered that (1) the formation of Mg(OH)2 on the particle surface by the hydration of Mg-Fe composite oxides during removal operation and (2) the reconstruction of LDH structure by the partial hydration of Mg-Fe composite oxides are predominant. It is also found that (1) the hydration on the surface of MgO particles, (2) the reconstruction of LDH structure by the partial hydration, (3) the affinity with As(III) due to Fe and (4) the high specific surface area are effective for the As(III) removal.
