Potentiometric and calorimetric study of fluoride adsorption onto mesoporous alumina nanoparticles

Fluoride contamination in groundwater has been recognized as one of the serious problems for water quality worldwide, and it is classified as one of the contaminants of water for human consumption by the World Health Organization (WHO). Besides the natural geological sources of fluoride enrichment in groundwater, fluoride-containing minerals are used in several industries which contribute to fluoride pollution. The effluents of these industries have fluoride concentrations extending from ten to thousands of mg·L-1. The WHO has established that acceptable F- concentration in water should be < 1.5 mg·L-1. Depending on the concentration and the consumption lasted of water with fluoride excess the effects can be very harmful for health. Nowadays many methods exist to remove fluoride from wastewater, such as adsorption, precipitation, electrocuagulation or ion-exchange. Adsorption shows considerable potential for fluoride removal from contaminated water, offering strategic advantages when compared to other techniques. Improvement of well-designed adsorbents is therefore of a great importance to the effective application of adsorption in water remediation. A wide variety of materials has been developed and used for fluoride removal from wastewater. However, the lowest limit achieved for fluoride removal with most adsorbents is greater than 2 mg·L-1. Nanomaterials can offer several advantages in waste water treatments over traditional methods, such as high surface area and a short diffusion route. The high surface in relation to their volume offers the opportunity to treat contaminated water using nanoparticles instead of using large scale solid: therefore nanomaterials are expected to give better results than traditional sorbents (higher loading, lower residual concentration) with the advantage of cost and materials savings. Preliminary studies have shown that alumina has high affinity for fluoride and that mesoporous alumina has high adsorption efficiency. The scope of this work is to perform a thermodynamic study to characterize fluoride adsorption process on nano-alumina and screen the synthesized materials to obtain an optimal candidate for applications. We synthesized and characterized mesoporous alumina nanoparticles by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and then carried out adsorption studies. Fluoride adsorption has been investigated using a combined experimental approach which includes potentiometry (using a F- selective electrode) and, for the first time in this field, isothermal titration calorimetry. In our method, potentiometric data have been fitted with a Langmuir isotherm to obtain the value of Kads. Then, the Kads has been used to calculate the free F- concentration for each titrant addition in the calorimetric titrations in order to fit the experimental heat and ultimately obtain the Hads value for the fluoride adsorption.