Evaluation of Acid Activation under the Adsorption Capacity of Double Layered Hydroxides of Mg–Al–CO3 Type for Fluoride Removal from Aqueous Medium

The main objective of this work was to investigate the effect of acid activation on the adsorption ability of synthesized double layered hydroxides (DLH) for fluoride ions present in aqueous solution. Samples of DLH synthesized presented an enhancement of adsorption capacity for fluorides related to their precursors, a consequence of substitution of CO 3 2– anions (after acid degradation) present on the DLH structure by monovalent anions (chloride and formate) that are more easily interchangeable compared to fluoride ions. Both variables evaluated in the activation step (acid concentration and contact time) presented a positive effect on the adsorption capacity for fluoride ions. Results demonstrated the necessity of a rigorous control of these variables to avoid complete degradation of the DLH structure. The adsorption assays permitted the conclusion that an enhancement on adsorption capacity was verified at pH 3.5 (compared to pH 5.0) and temperature of 50 °C (compared to 10 °C), indicating the endothermic nature of the process (Δ S = +28.35 J·K –1 ·mol –1 ). The high adsorption capacity of fluoride ions observed in this work (303.54 mg·g –1 ) demonstrates the potential of these adsorbents toward treatment of water contaminated with this element.