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Adsorption and organisation of molecules at the solid/liquid interfaces on nanostructure materials

  • Assaf, Marwa
Publication Date
Dec 10, 2021
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Water pollution is one of our most strategic recurring problems that are usually a result of human activities. The source of these contaminants is mostly the organic pollutants (dyes, pesticides, phenols, drugs…) that are considered complex and poorly degradable. The decontamination processes are often based on the use of adsorbents with specific properties such as high adsorption capacity and accessibility, which is the case of nanostructure materials such as zeolites, clays, porous silica….Our objectives were to study in details the fundamental physicochemical and adsorption mechanisms at the solid liquid interface. Particularly, we focus on the impact of porosity on the interactions with materials having high sorption capacity, the different levels of porosity (micro and meso), the effect of the material Si/Al ratio and the solvent on the adsorption process. A well-known faujasite type zeolite (FAU) has been selected due to its possibility to obtain hierarchical faujasite. Thus, modified faujasite was post synthesized using an alkaline treatment in the presence of surfactant (structuring agent). Several characterization methods have been carried out such as N2-Sorption-Isotherm, 29Si-NMR, X-ray fluorescence, and X-ray diffraction to evaluate structural and textural properties.Several organic molecules were chosen for their properties of size, charge, functional group to determine their sorption efficiency. Cationic molecules showed the highest adsorption capacity compared to the neutral and anionic molecules.The experimental conditions tested (isotherm protocols with the suspension preparation, equilibrium times, solvents, and Solid/liquid ratio) are essential steps to select the conditions to carry out the adsorption isotherm. These experiments have been performed for linear dye adsorption onto commercial Faujasite.The displacement enthalpy associated with the adsorption of the dye molecules was estimated using Isothermal Titration Calorimetry (ITC). This approach offers the possibility to evaluate the effect of porosity, and the surface chemistry (hydrophilicity) using direct measurements as a function of the loading. In addition, Second Harmonic Scattering (SHS) has been used as a local technique to evaluate the interaction and organization of the dyes at the interface.A high sorption capacity is obtained for the linear dye (sDiA hemicyanine) in contrast to the bulky dyes (Basic Fushin) towards the commercial FAU. The sorption capacity of sDiA remains constant except for the highly modified FAU which becomes lower. However, for BF, this capacity increases progressively with increasing the porosity. Other studies were performed for sDiA onto FAU in the presence of various solvents. The adsorption capacity in the presence of solvents is lower than in water. This decrease is probably due to the difference in the activity of the bronsted acid which leads to less organized molecules at FAU surfaces.The SHS measurements analysis allows to quantify the correlations in the molecular system. The analysis puts in evidence the indicator called i4 highly related to molecular interactions and organization. This i4 parameter is high for the linear dye for conditions: water solvent and microporous solid, which indicates a high correlation between the dye’s molecules. In addition, the high absolute value of the displacement enthalpy reveals a strong interaction between this dye and FAU material. These values become less significant in mesoporous FAU. However, in the case of bulky molecules, the i4 is not significant for both FAUs, and the displacement enthalpy was not determined for FAU and its similar to sDiA in the case of mesoporous FAU.Finally, the confinement effect on the adsorption of sDiA and Basic Fushin has been highlighted when the enthalpy and the i4 indicator become more negative. Thus, we succeed to combine such macroscopic and local approaches to evidence the effect of confinement on the sorption behavior.

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