Synthesis of calcined La-doped layered double hydroxides and application on simultaneously removal of arsenate and fluoride

A co-precipitation method has been developed to prepare the Mg/Al layered double hydroxides (Mg/Al LDHs) and La-doped Mg/Al LDHs (Mg/Al/La LDHs). The products of calcined Mg/Al LDHs (Mg/Al LDOs) and Mg/Al/La LDOs are obtained by further calcination at 450 degrees C for 24 h. Characteristics of the materials are evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). Adsorption behaviors of Mg/Al LDOs and Mg/Al/La LDOs have been contrastively investigated for arsenic and fluoride. Results show that doping La3+ into Mg/Al LDOs can modify its surface and structure, resulting in enhanced adsorption capacity of anions. A pseudo-second order kinetic model describes the adsorption kinetics of As(V) and F- on the Mg/Al/La LDOs, implying the mechanism of chemisorption. Langmuir and Freundlich isotherm models are used to fit adsorption data and the maximum uptake amount of As(V) and F- are calculated as 87.30 and 62.33 mg/g, respectively. Adsorption of As(V) and F- are also investigated as a function of initial concentrations, contact time, and media pH. The influence of co-existing anions indicate that the adsorption percentage of As(V) decreased in the order Cl- approximate to SO42- > NO3- > CO32- > HPO42-, and that of F- decreased in the order NO3- approximate to Cl- > SO42- > HPO42- > CO32-. It is found that Mg/Al/La LDOs keep the sorption efficiencies over 5 cycles. These results demonstrate that the Mg/Al/La LDOs can be employed as a low-cost and efficient adsorbent for treatment of undesirable anions.