Synthesis of magnetic calcium phosphate composites using citric acid as additive for removal of cadmium from aqueous solution

Abstract

Magnetic calcium phosphate composites were successfully synthesized in the presence of citric acid via one-pot synthesis. The phase of calcium phosphates was normally found as hydroxyapatite phase (Hap). However, the metastable phase of amorphous calcium phosphate (ACP) could occur in the presence of preformed Fe3O4 particles and citric acid. Fe3O4 might prevent the aggregation of calcium phosphate nuclei, and citric acid was found to inhibit the transformation of ACP into Hap. The inhibiting ability of citric acid depended strongly on the species of citric acid in the solution. The fully-deprotonate form promoted Hap, while the less deprotonated species yielded ACP. The materials were characterized by XRD, FTIR, elemental analysis, DSC, surface area analyzer, TEM, SEM, and VSM. Under external magnetic field, the used adsorbents were removed easily from aqueous solution showing high potential for convenient separation.

Among the prepared magnetic calcium phosphate composites, the ACP/Fe3O4/citric sample having Ca/P ratio of 1.5 shown the highest cadmium capacity. The adsorption isotherm and kinetics of this adsorbent were fitted with Langmuir model and pseudo-second-order model, respectively. The maximum adsorption capacity achieved from the Langmuir model was 270.3 mg g-1. The sorption mechanism was found to be cation exchange between Ca2+ in calcium phosphate materials and Cd2+ in the solution. Higher cadmium capacity was achieved from the materials having lower crystallinities. The cadmium adsorption capacity increased when the initial pH of the Cd2+ solution increased from 2 to 3, and remained constant between pH of 3 to 7.