Exploring and Evaluation of CaAl Hydrotalcite-like Adsorbents on Phosphate Recycling

We synthesized CaAl-Cl layered double hydroxide (CaAl-LDH) by co-precipitation and evaluated its performance in phosphate adsorption. Calcium fluoride (0.25 mol) and sodium meta-aluminate (0.1 mol) were poured into a beaker that contained 0.2 mol sodium hydroxide. The suspension was aged at 25 degrees C for 4 h under vigorous stirring. The acquired sludge was filtered and washed using distilled water, dried at 60 degrees C, and then ground to powder for further study. Phosphate stock solution was prepared using sodium di-hydrogen phosphate, and used for adsorption kinetic and isotherm research. The impact factors (e.g., competitive anions, adsorbent dosage, and adsorption time) of phosphate removal by CaAl-LDH were researched using industrial effluents. Adsorption processes were studied in 50 mL centrifuge tubes, with shaking by a thermostatic oscillator for planned time intervals. Mixtures were filtered through a 0.22 mu m membrane before analysis. Samples before and after phosphate adsorption were characterized by scanning electron microscopy to observe morphological characteristics. Thermo-gravimetry/differential scanning calorimetry was used to study the sample thermo-stability. Metal ions (e.g. calcium, aluminum) and phosphate concentration were determined by inductively coupled plasma atomic emission spectrometry. We observed a typical hydrotalcite structure in the CaAl-LDH by scanning electron microscopy, which indicates that the obtained materials are well-defined hexagonal platelet-like particles. The synthesized CaAl-LDH had good thermo-stability based on thermo-gravimetry/differential scanning calorimetry, and pyrolysis showed four mass loss stages. From batch adsorption experiments, we found that the adsorption fitted a Langmuir model (R-2=0.997) very well; the maximum adsorption capacity (q(max)) of 162.3 mg/g corresponded to the experimental data (166.9 mg/g), and is significantly higher than previous reports. The adsorption kinetics followed a pseudo-second-order model (R-2=0.998). Additionally, co-existing competitive anions influenced phosphate adsorption in the order of CO32- > SO42- > NO3-. Adsorbent dosage and contact time could also impact CaAl-LDH phosphate adsorption capacity. CaAl-LDH in this study could be applied in the recovery and reuse of phosphate from industrial wastewater because of its outstanding performance and stability.