Adsorption mechanism and synthesis of adjustable hollow hydroxyapatite spheres for efficient wastewater cationic dyes adsorption

The efficient adsorption of toxic dyes has been an important issue in water pollution control. Nowadays, researchers are experimenting with various methodologies to determine the optimal conditions for synthesizing hydroxyapatite (HAP) particles with a fine, hollow morphology, which has been a major difficulty in the field of study. In this research work, controlled hollow mesoporous HAP particles were successfully prepared using a CaCO3 core as a template at various hydrothermal reaction times. It was noted that by adjusting the hydrothermal process, the hollow morphology of the HAP particles could be regulated, with the best hollow mesoporous hydroxyapatite (HM-HAP) particles formed at the 6 h reaction period. Adsorption characteristics were determined via the adsorption of cationic dyes (methylene blue and methyl violet) onto synthesized HM-HAP particles. Langmuir’s adsorption isotherm was the best fit for describing the adsorption of methylene blue (MB) and methyl violet (MV) (R2 = 0.99 and 0.98), indicating monolayer adsorption. For kinetic studies, the adsorption of MB and MV fits well with the pseudo-second-order model (R2 = 0.99), demonstrating the chemisorption process. The maximal uptake capacities determined using the Langmuir model were 143 mg/g and 453 mg/g for MB and MV, respectively. According to the results of the sorption-desorption experiment, it was possible to renew and reuse synthesized HM-HAP for a maximum of four cycles. Therefore, the HM-HAP particles synthesized in this study were demonstrated to be effective adsorbents for the elimination of MB and MV in aqueous solutions.