Fluoride and metal ions removal from water by adsorption on nanostructured materials

Nowadays the environmental pollution is a great global enemy, being one of the problems that most affect the whole world. This PhD thesis focuses on the elimination of certain aqueous contaminants, such as fluoride or different metal ions. Nanostructured materials have been applied as innovative adsorption method to remove the aforementioned pollutants. These materials present a high surface area in a very small volume, being ideal materials for the treatment of wastewater. The first chapter of the thesis focuses on the removal of fluoride from contaminated water. Hierarchical alumina microspheres (HAM) have been selected as the optimum material, due to their high surface area and porosity, as well as the stability of the material. HAM have been synthesized using the published methodology with significant modifications, and have subsequently been fully characterized with techniques such as SEM, TEM, XRD, DLS or BET. Potentiometric studies have been performed to determine the fluoride remaining in solution. The obtained results have been adjusted with the Langmuir and Freundlich model to describe the adsorption mechanism. Accurate data on the enthalpy associated to the adsorption process allow the design of the best conditions both for the uptake and for the eventual successive release of a given chemical species. In previous works, the enthalpy associated to fluoride adsorption (∆Hads) has been calculated by the van’t Hoff equation. However, many studies considered the discrepancies between enthalpy obtained directly (ITC) and from van’t Hoff equation and evidenced the large uncertainties associated to the latter method. In this work, ITC is applied for the first time to obtain direct determination of ∆Hads for fluoride ion adsorption by HAM to provide independent and more robust thermodynamic parameters. The second part of the thesis focuses on the removal of heavy and precious metals from contaminated water. In this case, magnetic nanoparticles (SPION, Super Paramagentic Iron Oxide Nanoparticles) have been chosen as adsorbent. Magnetic materials may represent an interesting tool for the removal/recovery of metal ions from aqueous media, as they can be dispersed in the sample and easily recovered by using a magnetic field. However, for the adsorption of metal ions, the unmodified SPION has been demonstrated to have a small adsorption capacity. One of the advantages of this material is that its surface is easily modifiable by adding an organic ligand. Therefore, following the HSAB theory, ligands with functional groups such as -SH or -RSR- have been selected for SPION modification. The aim of this study is to synthesize and functionalize SPION with sulphur containing groups for the selective removal of heavy metals and for the recovery of precious metals from water, characterizing the adsorption processes in terms of loading capacity and thermodynamic parameters. SPION have been synthesized and functionalized with 3-mercaptopropionic acid (3-MPA) following the procedure published in the literature and then characterized by SEM, TEM, BET, FT-IR, XRD and TGA, while the metal adsorption process has been studied using a new methodology, which combines ICP and ITC. While in previous works ΔHads (adsorption enthalpy) related to metal adsorption have been calculated by the van’t Hoff equation, ITC is applied for the first time for the direct determination of ΔHads. Data obtained by ICP have been fitted with a Langmuir isotherm to obtain the value of the adsorption constant (Kads). Then, the Kads has been used to calculate the free metal concentration for each titrant addition in the calorimetric titrations in order to fit the experimental heat and ultimately obtain the ΔHads value for the metal adsorption. Moreover, ITC is also applied as a screening of the adsorbent material, in order to discriminate the optimal candidate for metal removal/recovery applications.