Microwave-assisted rapid synthesis of Al-doped Fe3O4/hydroxyapatite magnetic nanocomposites in pectin matrix for removal of fluoride from aqueous solution.

The Al-doped Fe₃O₄/hydroxyapatite (HAP) nanocomposites prepared in pectin matrix was employed as adsorbent that offers high adsorption capacity for fluoride, magnetic separability and good stability. Powder X-ray diffraction and transmission electron microscopy characterizations were used to determine the structure and morphology of as-prepared samples. The needle-like Al-dopedFe₃O₄/HAP nanocrystals were coated with pectin molecules, which not only induced the nucleation, but inhibited the further growth of nanocrystals. The results of thermogravimetric analysis and Fourier transform infrared spectra showed that the obtained products are organic--inorganic nanocomposites; and the Al doping in composites increased the amount of hydroxyl groups, which could supply more adsorption sites for fluoride. In bath experiments, the adsorption capacity of Al-doped samples was significantly higher than that of pure HAP due to the high affinity of aluminium toward fluoride. The effect of reaction parameters such as pH, calcination temperature, co-existing anions and Al/Ca molar ratio on the adsorption capacity was also investigated to determine optimum adsorption condition. The Al-doped Fe₃O₄/HAP nanocrystals exhibited high adsorption rate for fluoride during the first 30 min and reached equilibrium within 80 min. The adsorption kinetics and isotherms were well-fitted by the pseudo-second-order kinetic model and the Freundlich isotherm model, respectively. In addition, the adsorption mechanism of fluoride on Al-doped Fe₃O₄/HAP nanocomposites was investigated, which can be mainly attributed to electrostatic attraction and ion-exchange. [ABSTRACT FROM AUTHOR]