Adsorption equilibrium and kinetics of fluoride on sol–gel-derived activated alumina adsorbents

Abstract

Adsorption equilibrium and kinetics of fluoride on a sol–gel-derived activated alumina and its modifications with calcium oxide or manganese oxide were studied to explore the feasibility of applying these adsorbents for fluoride removal from drinking water. The activated alumina adsorbents were characterized with SEM/EDS and N 2-adsorption for their chemical and pore textural properties. The adsorption isotherms were correlated with the Langmuir and Freundlich adsorption equations. The fluoride adsorption isotherms on the sol–gel-derived activated alumina followed the Freundlich model while the fluoride adsorption isotherms on the calcium oxide- or manganese oxide-modified activated alumina adsorbents followed the Langmuir model. The calcium oxide-modified alumina adsorbent showed the highest fluoride adsorption capacities of 0.99 and 96.23 mg/g at fluoride concentrations of 0.99 and 432 mg/L, respectively. A pseudo-second-order model and an intraparticle kinetic model fitted well the adsorption kinetic data. It was found that both external and intraparticle diffusions contribute to the rate of removal of fluoride from the activated alumina-based adsorbents produced in our laboratory. The adsorption kinetic models evaluated in this work fitted well the adsorption uptake of fluoride from a Mexican groundwater on both calcium oxide- and manganese oxide-modified alumina adsorbents.