Precipitation of cadmium in water by the addition of phosphate solutions prepared from digested samples of waste animal bones

Chemical neutralization of aqueous cadmium by adding soluble phosphorus compounds has been successfully applied to treat contaminated waters. However, the reagent costs are relatively high. The present work proposes a novel alternative approach for removing aqueous cadmium from wastewaters through an in-situ precipitation reaction promoted by adding phosphate solutions prepared by acid digestion of waste animal bones. As the major components of the biologic phosphates, calcium hydroxyapatite Ca10(PO4)6(OH)2, and tricalcium phosphate Ca3(PO4)2 powders were firstly synthesized and used as model sources of PO4–3 ions to investigate the optimal neutralization conditions. The extraction at pH 7 revealed that equilibrium was rapidly established, and the kinetics data could well be described by the pseudo-second-order model. This model is related to chemical bonding reactions between Cd2+ from synthetic wastewater with a Cd2+ concentration of 50 mg L–1 and the added bone-derived ions (Ca2+, PO4–3), forming Ca(10–x)Cdx-phosphate apatites with Cd molar fractions x up to ~8.7. The precipitates were characterized by different spectroscopic techniques (Fourier transform infrared, X-ray diffraction, and scanning electron microscopy- energy dispersive X-ray spectroscopy). The experiments revealed that this novel approach enables removing high degrees (up to ~99%) of aqueous cadmium from contaminated waters with Cd2+ concentrations within the range of 50-500 mg L–1.