Removal of Pb(II) from aqueous solution using hydroxyapatite/calcium silicate hydrate (HAP/C-S-H) composite adsorbent prepared by a phosphate recovery process

In this study, phosphate was recovered from wastewater by C-S-H, and its phosphate-rich product HAP/C-S-H was reused as a composite adsorbent to remove Pb(II) from aqueous solution. The samples were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), zeta potential and specific surface areas analysis. The adsorption of Pb(II) on C-S-H and HAP/C-S-H were investigated at different pH, ionic strength, contact time, initial concentration and temperature. The results showed that the adsorption capacities of Pb(II) were heavily dependent on pH and slightly dependent on ionic strength. The adsorption kinetics and adsorption isotherms were well described by pseudo-second-order model and Langmuir model, respectively, and the thermodynamic parameters indicated that the adsorption process was endothermic and spontaneous. The adsorption capacities of Pb(II) on C-S-H and HAP/C-S-H could reach 543.5 mg/g and 946.7 mg/g, respectively, when the pH was 5.0, the temperature was 298 K and the ionic strength was 0.01 M. All the results indicate that HAP/C-S-H, recovered from phosphate wastewater, is an effective material for Pb(II) removal from aqueous solution, providing double environmental benefits for resource recovery and pollution control.