Ozone-treated activated alumina as an alternative adsorbent to remove fluoride from water: Conventional and Bayesian approaches to evaluate the isotherms, kinetics, and thermodynamics

Abstract

Activated alumina (AA) is a widespread adsorbent for removing fluoride from water. Despite its high affinity and selectivity for fluoride, its use has some limitations. In addition to the low adsorption capacity of this adsorbent and the long time required to reach equilibrium, the pH of the aqueous matrix must be less than 6.0 for AA to achieve high levels of fluoride removal. This study investigated the potential application of ozone-treated activated alumina (OAA), a new adsorbent material obtained by the ozonation of AA, to remove fluoride from water in a wide pH range. The performance of OAA on fluoride adsorption under different conditions of pH and dosage (g/L) was evaluated using a central composite rotational design (CCRD). As a result, using an adsorbent dosage of 4.35 g/L, OAA achieved high fluoride rates (>94%) over a wide range of pH values (4−8). The Minimum Square Method (MSM) and Monte Carlo Markov Chain (MCMC) methodologies obtained kinetic and equilibrium parameters. The Liu model was the most suitable isotherm model, which obtained a qm of 15.80 mg/g. Among the kinetic models, the general order model was the one that best fitted experimental data. Through thermodynamic analysis, the nature of fluoride adsorption on OAA was found to be endothermic and spontaneous. The fluoride removal efficiency of OAA (98.7%) was higher than AA (46.7%) at the same experimental conditions, showing the modification of AA by using the ozonation process was effective. Finally, the influence of co-existing ions (Cl-, SO42-, NO3-) and the regeneration and reuse of OAA were also evaluated.