Efficient phosphate removal by Mg-La binary layered double hydroxides: synthesis optimization, adsorption performance, and inner mechanism.

Abstract

Layered double hydroxides (LDH) hold great promise as phosphate adsorbents; however, the conventional binary LDH exhibits low adsorption rate and adsorption capacity. In this study, Mg and La were chosen as binary metals in the synthesis of Mg-La LDH to enhance phosphate efficient adsorption. Different molar ratios of Mg to La (2:1, 3:1, and 4:1) were investigated to further enhance P adsorption. The best performing Mg-La LDH, with Mg to La ratio is 4:1 (LDH-4), presented a larger adsorption capacity and faster adsorption rate than other Mg-La LDH. The maximum adsorption capacity (87.23 mg/g) and the rapid adsorption rate in the initial 25 min of LDH-4 (70 mg/(g·h)) were at least 1.6 times and 1.8 times higher than the others. The kinetics, isotherms, the effect of initial pH and co-existing anions, and the adsorption-desorption cycle experiment were studied. The batch experiment results proved that the chemisorption progress occurred on the single-layered LDH surface and the optimized LDH exhibited strong anti-interference capability. Furthermore, the structural characteristics and adsorption mechanism were further investigated by SEM, BET, FTIR, XRD, and XPS. The characterization results showed that the different metal ratios could lead to changes in the metal hydroxide layer and the main ions inside. At lower Mg/La ratios, distortion occurred in the hydroxide layer, resulting in lower crystallinity and lower performance. The characterization results also proved that the main mechanisms of phosphate adsorption are electrostatic adsorption, ion exchange, and inner-sphere complexation. The results emphasized that the Mg-La LDH was efficient in phosphate removal and could be successfully used for this purpose.