Separation of Excess Fluoride from Water Using Amorphous and Crystalline AlOOH Adsorbents

Aluminum hydroxide is an effective defluoridation adsorbent; however, the poor defluoridation performance limits its wide application. In this work, amorphous and crystalline AlOOH adsorbents are synthesized through hydrolysis of Al salts, and their defluoridation performances are evaluated in terms of adsorption capacity and rate, sensitivity to pH value, and water quality after defluoridation. The defluoridation performance of AlOOH is closely related to the hydrolysis pH value, but hardly to the type of Al salts. The adsorbent can remove >95% fluoride in the first 2 min and reach adsorption equilibrium within 2 h, and the maximum defluoridation capacity is 41.9 mg/g. Furthermore, the adsorbent exhibits an excellent defluoridation efficiency at a wide pH range of 4.5-10.5. After fluoride removal, the adsorbents prepared at pH values of 6 and 7 exhibit low residual Al concentration. The Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) results confirm that the fluoride removal mechanism is the ligand exchange between fluoride and hydroxyl groups. The excellent defluoridation capacity and low residual Al demonstrate that AlOOH is a potential adsorbent for fluoride separation from water.