New strumming sleight for old tunes: Exploration on the sorption mechanisms of Cd(II) by hydroxyapatite synthesized with different molar Ca/P ratios
Hydroxyapatite (HAP) has been widely used for the preconcentration and stabilization of various heavy metal pollutants in both the water and soil environment. Nonetheless, there still exists a gap between its synthesis conditions, physicochemical properties and removal performances. Under such a research background, three HAP materials were synthesized via a modified co-precipitation approach by varying initial Ca/P molar ratios (i.e., 4/3, 5/3 and 6/3) and used for capturing toxic cadmium (Cd(II)) from an aqueous solution. According to the experimental results, the three synthesized HAP samples were all the Ca-deficient type and the actual Ca/P molar ratio was in the increasing order of HAP4/3 < HAP5/3 < HAP6/3. In particular, the HAP4/3 sample exhibited higher Cd(II) removal performance due to its higher specific surface area (SSA) value and abundant Ca-deficient sites. More than 93% of Cd(II) was removed within an extremely short period of 5 min and a complete removal was achieved at a prolonged contact time of 24 h. The sorption behavior of Cd(II) was independent of ionic strength variation and scarcely influenced by various inorganic electrolyte ions, low molecular weight organic acids (LMWOAs) and competing heavy metal ions . The desorption data of Cd(II)-containing HAP phases, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses pointed to a predominant removal mechanism of lattice incorporation with a minor contribution of inner-sphere surface complexation. The experimental findings demonstrated the significant application potential of Ca-deficient HAP material for the cost-effective treatment of Cd(II)-polluted water systems.
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