Síntesis, caracterización y aplicación de nanomateriales de carbono para el tratamiento de aguas residuales

Abstract

ABSTRACT Nowadays, the international concern about the pollution of the aquatic environment has significantly increased. One of the most important aspects is its contamination by organic substances, which have been not eliminated by conventional biological treatments, due to their chemical nature or being in a high concentration. These types of pollutants are classified as Persistent Organic Pollutants (POPs). In reference to this situation, it has to be emphasized that this type of substances, affect to numerous functions of the organism, like the hormonal or endocrinal system. Hormonal alterations have been highlighted in the different species of nature, including humans. The entry of these substances into the aquatic environment can carry serious risks. Therefore, it is very important to look for new technologies to eliminate this type of contaminants, being used as tertiary treatments. In this sense, the adsorption process is a very viable option, among others for its fast efficiency and low cost of process. The use of carbonaceous nanomaterials as adsorbents and supports of catalysts, has demonstrated its good efficiency in the different processes of water decontamination. These materials have more favorable characteristics, such as: high specific surface area values, their ability to operate in acidic and basic media, their structure can be modified, being presented with different sizes and shapes, and their surface chemistry, among others. Considering all these premises, different graphenic materials have been designed with different textural, structural, morphological and surface chemistry characteristics. Among the great variety of persistent organic compounds, in this doctoral thesis, four of them with different nature have been selected: a surfactant type, "Triton X-100", two chlorinated aromatic compounds, 2,4- dichlorophenoxyacetic acid and 2, 4-dichlorophenol, and a gasoline additive, methyl tert-butyl ether. For the different adsorption processes, graphenic materials have been used: graphene oxides with different grain size, 10 and 325 mesh, their respective reduced graphene oxides and the latter with groups of nitrogen anchored on their surface. Also, in order to compare their possible application as adsorbents in real processes, their results have been compared with those obtained using commercial materials: activated carbon, and different high-surface area mechanically modified graphites. For all of these carbonaceous materials, an exhaustive textural, structural, superficial and morphological characterization has been carried out. Using the Adsorption- Desorption technique of liquid N2 at 77 K, the specific surface area of each adsorbent was calculated using the Brunauer Emmet and Teller equation. On the other hand, thanks to the technique of X-ray diffraction, the number of sheets for the graphenic materials has been deduced, as well as the distance between them. Besides, it has made it possible to check whether the oxidation process of the starting graphite has been effective. In addition, using the Raman technique, the degree of structural defects possessed by the different graphite materials has been shown. These last results have been related to those obtained by thermogravimetry analyses under air atmosphere, verifying the temperature at which the material begins to get oxidized. Using this technique, under helium atmosphere, has quantified the weight loss during the deflagration process of graphene oxides in the conditions of production. Thanks to the analysis of the micrographs obtained by Field Scanning Electron Microscopy, the morphology of the different graphenic and graphitic materials has been verified. It has been possible to relate these results to those obtained by the different textural, structural and gravimetric techniques. The use of Xray Photoelectronic Spectrometry techniques and Fourier Transform Infrared Spectroscopy and Attenuated Total Reflection have allowed us to know the surface atomic percentage of C, O and N of the materials, as well as each of their contributions. In addition, the different characterization techniques have also justified the different types of interactions produced between the pollutant and the adsorbent. Considering the kinetic results obtained from the adsorption process for the Triton X-100 with graphenic materials, the real adjustment of the different equations and empirical models to the experimental data has been verified. The reduced materials have a higher initial velocity. The fact that they have much larger exposed surface and presents little residual oxygen, causes their adsorption kinetics of the contaminant to be unaffected by diffusion problems, thus, their initial velocities are instantaneous. Also, their adsorption isotherms have been developed. The materials with greater amount of surface oxygen, graphene oxides, have the lower capacity of adsorption compared to their reduced ones, due to the production of micelles, after the critical micellar concentration. They occupy more surface, and therefore, minimize the adsorption capacity of the materials. Therefore, the subsequent adsorption study has been focused on materials with less oxygen. It has been verified that reduced graphene oxides without nitrogen, have displayed the maximum adsorption capacity, being the one with smaller granulometry the best adsorbent. In addition, the production of physisorbed contaminant layers has been demonstrated by thermogravimetric analysis after adsorption. The different adsorption capacities have been related to the physicochemical properties of the adsorbents, where structural defects play a very important role. On the other hand, for the commercial materials, it has been demonstrated that the active carbon has a lower initial adsorption value than the high surface graphites and, in addition, a lower adsorption capacity. For the former, their behaviors have been justified in terms of their graphitic character, as measured by Raman. In this case, the material that contains less residual oxygen, and more graphitic domain, displays greater adsorption capacity. Comparing these results with those obtained by the graphenic materials, it has been concluded that the latter ones, suppose an improvement in the adsorption process of Triton X-100. On the other hand, the adsorption process for chlorinated aromatic contaminants, 2,4-dichlorophenoxyacetic acid and 2,4-dichlorophenol has been studied. Their adsorption kinetics using graphenic materials has been influenced by the different diffusion problems, where it has been verified that for graphene oxides, there is a slower limiting stage, the internal diffusion. Thanks to the study of X-ray diffraction before and after adsorption, it has been demonstrated that the interlaminar distance of the graphene oxides are active centers of adsorption, producing intercalated compounds. However, in the reduced graphenic materials, adsorption has only been detected due to external diffusion. As for the Triton X-100, a study of their adsorption isotherms has been carried out, where different relationships have been found depending on the studied pollutant. For 2,4-dichlorophenoxyaceticacid, the adsorptive driving force has been found to be the residual surface oxygen of the materials, developing hydrogen-bonding interactions between pollutant-adsorbent. Therefore, the material with highest adsorption is the one with the maximum amount of surface oxygen, the graphene oxide with the lowest particle size. On the other hand, using 2,4-dichlorophenol as a pollutant type, it has been shown that the higher the specific surface area of the adsorbent is, as measured by Adsorption-Desorption N2 at 77 K, the more adsorption capacity is achieved. Therefore, reduced graphene oxide with lower particle size has the highest adsorption capacity. In this case, the driving force of adsorption, are the π-π bonds. It has been possible to verify the different interactions between adsorbent-contaminant by an exhaustive study of infrared spectroscopy of the samples after adsorption. On the other hand, when high surface area graphites are used, it is found that there is a greater capacity of adsorption, in both contaminants, for the material that more surface area possesses. So, higher quantity per m2 of material is adsorbed from the 2,4-DCP. Due to the co-existence of these two contaminants at the same time in the wastewater, an adsorption study has been carried out in mixtures at different concentrations (50 and 150 mg / L). The battery of graphenic materials with smaller particle size was chosen. It has been shown that using graphene oxide as adsorbent, the best experimental results have been obtained, since there are cooperative adsorptions between both pollutants. In addition, thanks to a detailed study of X-ray and Infrared Diffraction, the importance of the interlaminar spacing as an active adsorption center has been demonstrated. On the other hand, for the two reduced materials, worse results of adsorption have been obtained, thus competitive effects between both contaminants have been produced. The fact of anchoring nitrogenous functional groups has broken down the aromatically degree at the surface of the material, and consequently, lower adsorption capacities have been obtained for this material. Comparing different materials, it has been possible to conclude that, as in the adsorption process of Triton X-100, the graphenic materials represent an improvement in the adsorption process of chlorinated aromatic compounds. Finally, a study of the accelerated Fenton catalytic process was carried out at the research center of Helmholtz Center for Environmental Research in Leipzig, Germany. Two materials of carbonaceous nature, reduced graphene oxide and high surface area graphite, with an active phase of Ru and Fe, for the monometallic and Ru-Fe catalysts for the bimetallic ones have been selected as materials. An exhaustive characterization of the different materials has been carried out, where X-ray diffraction has only shown the detection of the iron species for the monometallic catalysts. The surface study by X-ray Photoelectron Spectroscopy has demonstrated the importance of the surface groups in the synthesis of the iron catalysts. Also, it has been possible to define their different oxidation states, as well as each of its contributions. The catalyst with high surface area graphite as support, presents smaller amounts of Fe2+. In addition, the production of ruthenium metal after reduction of the catalysts at 400 °C under hydrogen atmosphere, has been justified by the existing displacements of Ru 4d. It has been shown that using a reduced graphene oxide support; higher interaction strength has been obtained with Ru for the bimetallic catalyst, compared to the high surface area graphite. Thanks to Transmission Electron Microscopy technique, the morphology of the Ru particles and Fe species, as well as their distribution on the surface of the supports, and their average diameter have been established. In addition, the different results have been corroborated with those obtained by X-ray Photoelectron Spectroscopy, since it has been justified that the catalysts that have presented stronger interactions, have a smaller particle diameter and consequently a better distribution. MTBE has been used as a pollutant since it is widely used as a gasoline additive. It also has a very high solubility, conferring it a great mobility towards the aqueous medium. Early catalytic results have shown the effectiveness of the accelerated Fenton process using carbonaceous catalysts. However, a very surprising experimental fact has been detected, since oxidation products have been found in a unique hydrogenation atmosphere. Due to the complexity and scientific interest, it has been decided to follow this line of research. Therefore, certain aspects of the process has to be taken into account. It has been found that the optimum pH of the reaction has a value of 3. However, not all acidifying agents are valid, since the only one that operates in favor of the degradation of MTBE is HNO3. By comparing the pH's throughout the reaction, it has been found that using a monometallic catalyst of Ru there has been a sudden change in its initial value towards more basic pH's. This fact has been related to the production of NH4 + radicals. On the other hand, if a monometallic Fe catalyst is used, its pH value has not been changed since its oxidized form is not capable of promoting the H2 molecule. In contrast, the bimetallic catalyst has caused a very slow and more controlled variation of pH throughout the reaction, always maintaining the acidic nature of the medium. On the other hand, the production of H2O2 as an “in-situ” oxidizing agent has been justified. Subsequently, OH generation has been detected as radical species, since the reaction does not take place if it is carried out in the presence of an acceptor or "scavenger", such as tert-butyl alcohol. In addition, it has been important to check out the need for a Ru metal phase for the smooth operation of the reaction. Ru has to be in the metallic form for the promotion of the H2 molecule and its subsequent migration by "spillover" effect. Finally, the reaction has been carried out with the bimetallic graphite catalyst in order to justify the reproducibility of the reaction for other carbonaceous catalysts. Its good application has been approved. Comparing these two catalysts, it has been shown that the worst activity of this catalyst has been found for the latter, due to its larger particle size, weaker interactions between the support and the Ru particle, and the lower ratio between iron species, Fe 2+ / Fe 3+.