Clinoptilolite, a natural zeolite, was used as an adsorbent for fluoride adsorption.  In order to enhance the clinoptilolite sorption capacity for fluoride, it chemically modified using phosphoric acid and investigated the sorption of fluoride to acid-modified clinoptilolite by a series of batch experiments using fluoride concentrations in solution ranged from 2 to 10 mg/l. The influence of acid concentration, contact time, solution pH, initial concentration and adsorbent mass has been studied. It was found that the acid modified clinoptilolite exhibits higher adsorption to fluoride than unmodified clinoptilolite and adsorption of the fluoride is influenced by several parameters such as initial concentration, adsorbent dosage and solution pH. The results showed that pH has a moderate effect on fluoride adsorption in the pH range of 4 to 9. Meanwhile maximum absorption efficiency of acid modified clinoptilolite in fluoride removal shows improvement due to 0.7 molar phosphoric acid. The equilibrium adsorbed amount also increased with the initial concentration of the metal ions, as expected. Among the models tested, the adsorption equilibrium for fluoride was best described by the Freundlich model. Finally, results of this study demonstrated the effectiveness and feasibility of acid modified clinoptilolite for removal of fluoride ions from aqueous systems.

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