One-pot synthesis of amorphous calcium phosphate/Fe3O4 composites and the application in the removal of cadmium

Abstract Amorphous calcium phosphate (ACP) is a promising material for cadmium removal due to its irregular surface structure. Nevertheless, the preparation of ACP has not been trivial owing to the facile transformation of the meta-stable state of ACP into the crystalline phase of hydroxyapatite (Hap). Herein we report a new preparation, by which ACP can be synthesized at ease through a one-pot synthesis. The utility of citric acid, together with, preformed Fe3O4 renders the formation of ACP/Fe3O4 magnetic composites, hampering the aggregation of calcium phosphates and hence hindering the transformation of ACP into undesired Hap. The well-controlled syntheses are illustrated and proved to be pH-dependent. The cadmium adsorption of the ACP/Fe3O4 composite is well-fitted with Langmuir isotherm giving a maximum capacity of 454.5 mg g―1 and shows a pseudo-second order kinetic behavior. The superior cadmium adsorption capacity of the composite is top ranked among previously reported adsorbents. The thorough investigation underpins the adsorption mechanism, whereby the specific replacement of Ca2+ ions in ACP network by Cd2+ is highly selective over other M(II) ions such as Co2+ and Ni2+. Furthermore, the superparamagnetic nature of the composites allows a convenient separation of the materials after use.