Synthesis and characterization of nanostructured adsorbents for the removal of inorganic and organic pollutants from water

In recent years water contamination is reaching alarming levels, since the concentration of pollutants present in seas, lakes and streams has far exceeded water self-purifying capacity. The most widespread sources responsible for water pollution are urban and industrial dumps containing a vast range of dangerous substances. Despite wastewaters are always subjected to purification treatments, a complete remediation is not always possible. The most common pollutants are heavy metal ions and organic compounds such as phenolic compounds, polybrominated diphenyl ethers (PBDEs), polycyclic aromatic hydrocarbons (PAHs), pesticides and synthetic dyes. All these substances are highly stable and can easily bioaccumulate in living organisms as xenobiotic molecules, causing chronic and acute toxicity or carcinogenic and mutagenic effects. Furthermore, substances as dyes can cause changes to the aquatic ecosystem as they absorb the sunlight limiting its penetration into deep waters, thus inhibiting the photosynthesis of aquatic plants and thus limiting the water re-oxygenation capacity. In such a scenario, in the last few decades the scientific community has put a great deal of effort into the improvement of wastewater remediation processes. Among the various treatments, adsorption is one of the most useful thanks to its simplicity and low cost. The great advancement in nanotechnology has paved the way for new highly effective nanostructured adsorbent materials. These generally porous nanoadsorbents are characterized by a high surface area and high surface/volume ratio, which greatly influence their adsorption capacity. The purpose of this thesis work was the development of new mesoporous adsorbents, namely functionalized ordered mesoporous silica (OMS) and metal organic frameworks (MOFs) for the removal of heavy metal ions and organic dyes two types of pollutants commonly present in wastewaters.. Both OMS and MOFs are characterized by high surface area, high surface/volume ratio, geometrical order, and easy synthesis or functionalization. SBA-15 OMS and Fe-BTC type MOF have been successfully synthesized and characterized by means of different techniques, such as small angle X rays scattering (SAXS), powder X-rays diffraction (XRD), N2 physisorption, transmission and scanning electron microscopy (TEM and SEM), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). SBA-15 was functionalized with two different organic ligands, namely triethylenetetramine (TETA) and 2,8-dithia-5-aza-2,6-pyridinophane macrocycle (PyNS2), to obtain two different adsorbents, named SBA-TETA and SBA-PyNS2 which have been tested against some heavy metal ions (Cu2+, Zn2+ and Cd2+). The Fe-BTC, without further modifications, was instead tested as an adsorbent of two highly toxic organic dyes, such as Alizarin red S (ARS) and Malachite Green (MG). All adsorption experiments were monitored using Inductive Coupled Plasma Optical Emission Spectroscopy (ICP-OES) or UV-Vis Spectroscopy. This allowed the experimental determination of the adsorption capacity q of the three adsorbents, their thermodynamic and kinetic parameters. In the case of SBA-15-based adsorbents, further investigations on their properties were carried out by means of potentiometric titrations.