Adsorption of perchlorate from water using calcined iron-based layered double hydroxides

In this study, a new calcined iron-based layered double hydroxide material was synthesized to adsorb perchlorate from water. The MgFe-CO ₃ layered double hydroxides (MgFe-LDH) were prepared at pH 9-10 and with different molar Mg/Fe ratios (2-5) using a co-precipitation method, and then calcined at a temperature range of 300-900°C. Results showed that the best synthesis conditions to maximize the perchlorate adsorption capacity of the calcined MgFe-CO ₃ layered double hydroxide (MgFe-CLDH) were the calcination temperature of 550°C and [Mg]/[Fe]=3. Furthermore, the adsorbents and their adsorption products were characterized by X-ray, FT-IR and thermogravimetric analyses. The layered double hydroxide structures in the adsorbent were lost during calcination at 550°C, but were reconstructed subsequent to adsorption of perchlorate, indicating that the 'memory effect' appeared to play an important role in perchlorate adsorption. The perchlorate adsorption pattern was well described by the pseudo-second-order kinetic model, while the Freundlich isotherm provided the best fitting of adsorption isotherms with the experimental data at chemical equilibrium at 25°C. Indeed, an initial solution pH of 4-10, a higher adsorbent dose, and a lower initial perchlorate concentration typically favored the removal of perchlorate from water. Furthermore, co-existing anions, including PO ₄ ^3-, SO ₄ ^2-, Cl ^- and NO ₃ ^-, inhibited the perchlorate adsorption, to different degrees. At 25°C, MgFe-3 CLDH=1.33g/L (calcination temperature=550°C and [Mg]/[Fe]=3), and the initial solution pH of 4-10, 2000μg/L of perchlorate was almost all adsorbed within 720min. This study demonstrated that the new calcined iron-based layered double hydroxide was a promising adsorbent for control of the perchlorate pollution in water.