Synthesis and characterization of heavy metal-based hydroxyapatite for batch adsorption of turquoise blue dye: equilibrium, kinetic, and thermodynamic studies

In this study, different metal hydroxyapatite (HAP) nanoparticles were synthesized at a laboratory scale. All the methods used were very economical because all reagents used in this work were cheap and easily available in laboratories. The synthesized products were characterized by X-ray diffractometer (XRD), Brunauer-Emmett-Teller and Barrett-Joyner-Halenda, scanning electron microscopy, and Fourier transform infrared spectroscopy analysis to confirm the synthesis of respective products by ensuring the presence of phosphate and hydroxyl functional groups. These metal HAP nanoparticles were applied in water treatment applications as adsorbents for the elimination of turquoise blue dye from the aqueous solution. Batch experiments were performed, and all effective parameters were optimized. Their optimized values were as follows: pH = 8, dosage of adsorbent = 0.05 g, contact time = 75 min, temperature = 30 C-degrees, and dye concentration = 75 ppm. The order of adsorption capacity of four different metal HAP products was determined as calcium-HAP (Ca-HAP) > lead-HAP (Pb-HAP) > strontium-HAP (Sr-HAP) > barium-HAP (Ba-HAP). Ca-HAP, Ba-HAP, Sr-HAP, and Pb-HAP have the highest concentration of 0.5 N, giving 54-69.29% desorption. Moreover, the effect of surfactants and electrolytes was also studied. Langmuir isotherm and pseudo-second-order kinetic model were best fitted for turquoise blue dye adsorption. The reaction was exothermic and spontaneous in nature. HIGHLIGHTS Synthesis of nano metal hydroxyapatite (HAP) by the wet precipitation method. Applications of HAP as a useful material in turquoise blue dye adsorption. The order of adsorption capacity of four different metal HAP products was determined as Ca-HAP > Pb-HAP > Sr-HAP > Ba-HAP. The Langmuir model and pseudo-second-order kinetic model were best fitted.