The adsorption equilibrium of a fluoride solution on 1-2 mm granular activated alumina modified by Fe-2(SO4)3 solution was investigated. The experiments were conducted using a wide range of initial fluoride concentrations (0.5 to 180 mg.L-1 at pH similar to 7.0) and an adsorbent dose of 1.0 g.L-1. The application of Langmuir and Freundlich adsorption isotherm models (linear and nonlinear forms) generally showed that a single Langmuir or Freundlich equation cannot fit the entire concentration gap. Experimental data on low equilibrium concentrations (0.1 to 5.0 mg.L-1) was in line with both Langmuir and Freundlich isotherm models, whereas that of high equilibrium concentrations (5.0 to 150 mg.L-1) was more in line with the Freundlich isotherm model. A new Langmuir-Freundlich function was used for the entire concentration gap, as well as for low and high concentrations.