Systematic screening of layered double hydroxides for phosphate removal and mechanism insight

To optimize the chemical composition of layered double hydroxides (LDH) as phosphate adsorbents and further discuss the adsorption mechanism, 14 LDH were synthesized from theoretically feasible, inexpensive and innoxious (or low noxious) precursors through the coprecipitation method. The morphology, chemical composition and nanostructure of LDH were confirmed by scanning electron microscopy equipped with energy dispersive spectrometer, X-ray diffraction and Fourier transform infrared spectroscopy. Then their adsorption performance were evaluated and compared. Phosphate adsorption by the selected LDH as a function of pH and competing anions were subsequently studied. In a 20 mg-P/L phosphate solution at pH 5.5, LDH with trivalent cation Al3+ had a better performance than Fe3+-based LDH. Phosphate adsorption kinetics of the as-synthesize LDH fitted well with the pseudo-second-order model. Mg2-Al1-Cl-LDH (corresponding to LDH with cations of Mg2+ and Al3+ in Mg2+/Al3+ molar ratio of 2.0 and interlayer anion of Cl−) had a higher phosphate removal and faster kinetics than others did. The isotherm data of Mg2-Al1-Cl-LDH showed a good compliance with Langmiur model, and the maximum adsorption capacity reached 76.1 mg/g under 25 °C. The phosphate adsorption mechanisms were deduced as ion exchange, electrostatic attraction, physical adsorption and ligand exchange. Specifically, ion exchange was the main mechanism.