Enhanced fluoride adsorption and regeneration efficiency of cross-linker-free mesoporous hydroxyapatite/chitosan nanocomposites

Abstract

The presence of F− ions in water is either a helpful or harmful outcome, contingent upon the concentration level of the pollutant. In this work, hydroxyapatite/chitosan (HAp/CS) nanocomposites were prepared incorporating varying weight percentages of chitosan with HAp, using co-precipitation followed by lyophilization without employing any cross-linking agents for the adsorption of F− ions. An increase in the concentration of chitosan in the HAp matrix significantly enhanced the colloidal stability, F− ion adsorption capacity, and reduced the particle size (length 38% and width 86%) when compared to HAp. The respective maximum F− adsorption capacity of HAp/CS1, HAp/CS2, HAp/CS3, and HAp/CS4 nanocomposites were 56 mg/g, 80 mg/g, 82 mg/g, and 100 mg/g at pH 7, which was considerably higher (10 times) than HAp when compared to other existing reports. The adsorption isotherm and kinetics of HAp and HAp/CS nanocomposite adsorbent confirmed the monolayer and strong chemisorption mechanisms. The surface morphology of the adsorbent after adsorption of F− ions changed from rod to sphere along with an increase in particle size, as confirmed by HR-TEM. Moreover, FTIR and XPS analysis revealed the surface interaction between the adsorbent and F− ions through ion exchange and electrostatic interaction, which lead to the formation of fluorapatite. HAp/CS4 showed 2.5 times higher sustainable regeneration efficiency (90% in 30 min) up to 7th cycle than HAp. Furthermore, the solution pH (5–7) of treated water was maintained at neutral pH after fluoride adsorption, rendering the water potable. Hence, these results demonstrate that chitosan plays a significant role in the removal of F− ions and this composite is ideal for F− ion adsorption applications.Graphical abstract