[Preparation of Mg-Al-Me(Me=La, Ce, Zr) Composite Oxides for Efficient Fluoride Uptake]

The Mg-Al-Me(Me=La, Ce, Zr) composites were prepared by co-precipitation method of Mg, Al, Me(Me=La, Ce, Zr) salt solutions with a molar ratio of 20:1:4. The sample were calcined at 500℃ for 5h as the adsorbents for removal of fluoride. The adsorbents were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The patterns of SEM indicated that three calcined sorbents were somewhat agglomerated particles to form a sheet structure after adsorption. The patterns of XRD showed that the three adsorbents fored a metal composite oxide. The effects of adsorption time, initial concentration of fluorine and coexisting ions (Cl^-, NO₃^-, SO₄^2-, CO₃^2-, HCO₃^-, PO₄^3-) fluorine adsorption properties on three adsorbents were tested by batch adsorption experiments. The results showed that the kinetics of fluorine on three adsorbents were well fitted by pseudo-second-order model. And the adsorption rate of fluoride on three adsorbents decreased in the order of Mg-Al-Zr > Mg-Al-La > Mg-Al-Ce. Regression results of adsorption rate controlling step test by moving boundary model indicated that intra-particle diffusion rate was not the only rate-limiting step in the adsorption of fluoride on the three materials. The equilibrium isotherm showed that the adsorption capacity of fluoride uptake by three adsorbents decreased in the order of Mg-Al-La > Mg-Al-Ce > Mg-Al-Zr. The adsorption isotherm of three adsorbents was very well described by the Langmuir models, and their linear correlations were 0.9958, 0.9790, 0.9975, respectively. The maximum adsorption capacities of fluoride on Mg-Al-La, Mg-Al-Ce, Mg-Al-Zr calculated by Langmuir model were as high as 54.22, 51.65, 50.89 mg·g^-1, respectively. The results showed that the co-existing anions such as CO₃^2-, HCO₃^-, PO₄^3- had an inhibitory effect on the Mg-Al-La, Mg-Al-Ce, Mg-Al-Zr adsorption of fluoride. The effect of coexisting anions on fluoride adsorption increased in the order of Cl^- < NO₃^- < SO₄^2- < CO₃^2-≈HCO₃^-4^3-.