Resumen de Synthesis, characterization and theoretical calculations of ZnTiO3 for the adsorption and photocatalytic removal of Methylene Blue dye
- español
Actualmente, los nanomateriales han despertado gran interés para aplicaciones ambientales, como la remoción de colorantes en aguas residuales. La presente tesis tuvo por finalidad, la preparación de materiales estructurados con propiedades adsorbentes y fotocatalíticas para la remoción del colorante catiónico azul de metileno (MB) en soluciones acuosos. Para ello, inicialmente se recolectaron y caracterizaron varias arcillas ecuatorianas que fueron utilizadas como soportes de fotocatalizadores preparados a base de zinc y titanio. Los fotocatalizadores ZnTiO3/TiO2, ZnTiO3/TiO2/La y TiO2 (fase anatasa) fueron sintetizado mediante el método sol-gel. Las arcillas, G-Clay y R-Clay fueron utilizadas como fuentes de materia prima para sintetizar zeolitas FAU y LTA, respectivamente, mediante fusión alcalina y método hidrotermal. Las zeolitas se combinaron con sus arcillas precursoras y con los fotocatalizadores para preparar pellets que fueron utilizados exitosamente en ensayos de remoción de MB en soluciones acuosas. Las isotermas experimentales se ajustaron al modelo de Langmuir y las cinéticas experimentales al modelo de pseudo-segundo orden, lo cual evidenció una adsorción de monocapa en una superficie que contenía un número infinito de sitios idénticos, manteniendo una tasa de adsorción constante durante el proceso
- català
Actualment, els nanomaterials han despertat gran interès per a aplicacions ambientals, com la remoció de colorants en aigües residuals. La present tesi va tenir per finalitat, la preparació de materials estructurats amb propietats adsorbents i fotocatalítiques per a la remoció del colorant catiònic blau de metilè (MB) en solucions aquoses. Per a això, inicialment es van recol·lectar i van caracteritzar diverses argiles equatorianes que van ser utilitzades com a suports de fotocatalitzadors preparats a base de zinc i titani. Els fotocatalitzadors ZnTiO3/TiO2, ZnTiO3/TiO2/La i TiO2 (anatasa) van ser sintetitzat mitjançant el mètode sol-gel. Les argiles G-Clay i R-Clay van ser utilitzades com a fonts de matèria primera per a sintetitzar zeolites FAU i LTA, respectivamente, mitjançant fusió alcalina i mètode hidrotermal. Les zeolites es van combinar amb les seves argiles precursores i amb els fotocatalitzadors per preparar pèl·lets que van ser utilitzats amb èxit en proves de remoció de MB en solucions aquoses. Les isotermes experimentals es van ajustar al model de Langmuir i les cinètiques experimentals al model de pseudo-segon ordre, la qual cosa va evidenciar una adsorció de monocapa en una superfície que contenia un nombre infinit de llocs idèntics, mantenint una taxa d'adsorció constant durant el procés
- English
Nanoscience, as well as nanotechnology, have merged as a dynamic research field in recent years. The exponential growth in this interdisciplinary research field is due to the improved characteristics of nanomaterials and their various applications.
Currently, nanomaterials have aroused great interest for environmental applications, such as the removal of colorants in wastewater. To this end, various nanomaterial architectures have been designed to allow their practical use in adsorption and photocatalysis processes. Zinc (Zn) and titanium (Ti) oxides are widely used photocatalysts for contaminant removal due to their unique properties. On the other hand, clay materials have proven to be excellent sources of raw material for the synthesis of zeolites, as well as effective supports for photocatalysts. Both clays and zeolites have important adsorbent properties and, together with photocatalytic materials, allow the preparation of materials with improved properties for the practical and effective treatment of wastewater.
The purpose of this thesis was the preparation of structured materials with adsorbent and photocatalytic properties for the removal of the cationic methylene blue (MB) dye in aqueous solutions. To do this, initially, 12 Ecuadorian clays were collected and characterized, which were used as supports for photocatalysts prepared based on zinc and titanium. The photocatalysts ZnTiO3/TiO2, ZnTiO3/TiO2/La and TiO2 (anatase phase) were synthesized by the sol-gel method setting a calcination temperature of 500 °C. ZnTiO3/TiO2 and ZnTiO3/TiO2/La were obtained with a particle size less than 100 nm, setting a ZnO:TiO2 molar ratio of 1:3. Photocatalysts were evaluated in methylene blue photocatalytic degradation tests, and as a result, it was obtained that ZnTiO3/TiO2 and ZnTiO3/TiO2/La had a better degradation capacity than TiO2 (anatase phase).
The photocatalysts were impregnated in the 12 Ecuadorian clays and then structured in the form of pellets of 0.2 cm (diameter) and 1.0 cm (length). The materials thus prepared were again evaluated regarding their photocatalytic activity and their adsorption capacity. The tests were carried out at room temperature, using a Bach-type reactor, both in the presence of solar light and UV light. The removal of methylene blue by adsorption and photocatalysis was verified from the discoloration of the solution and quantified by UV-Vis spectrophotometry at 623 nm. Clay12 (or R-Clay, due to its red color) impregnated with TiO2 showed the best photocatalytic activity. The X-ray Fluorescence (XRF) results showed that this clay had TiO2 and Fe2O3 in its composition, which could improve its photocatalytic activity. Clay6 (or G-Clay, due to its gray color) had the best methylene blue adsorption capacity. The results of the X-ray Diffraction (XRD) allowed concluding that the presence of the metahalloysite phase improved the adsorption capacity of the clay. In addition, this clay contained several oxides with exchange cations that could also contribute to its high adsorption capacity.
Both clays, G-Clay and R-Clay, were used as raw material sources to synthesize FAU and LTA zeolites, respectively. The zeolites were synthesized by alkaline fusion and hydrothermal method, presenting good textural and morphological characteristics, suitable for adsorption processes. The zeolites were combined with their precursor clays and with ZnTiO3/TiO2 to prepare pellets that were successfully used as adsorbents in MB removal tests in aqueous solutions. The experimental isotherms were adjusted to the Langmuir model and the experimental kinetics to the pseudo-second-order model, which showed monolayer adsorption on a surface that contained an infinite number of identical sites, maintaining a constant adsorption rate during the process.
Finally, a comparative study was carried out using computational simulation to determine the MB adsorption mechanism on the surfaces (101) of ZnTiO3 and TiO2 (anatase phase). The DFT (Density Functional Theory) calculations were developed using the VASP code (Vienna Ab initio Simulation Package) with the Perdew-Burke-Ernzerhof (PBE) functional in the generalized gradient approximation (GGA). The results showed that the adsorption of MB on the surface (101) of ZnTiO3 is stronger than on the surface (101) of TiO2. Furthermore, the semi-perpendicular orientation was the most likely molecular approach for the surfaces of both oxides. Our theoretical study verified that ZnTiO3 has better MB adsorption energy than TiO2 in the anatase phase, which is important to enhance a subsequent degradation process. The large bandgap obtained by DFT calculations also showed that ZnTiO3 can potentially be used as a photocatalyst, which would allow complete degradation of MB after being adsorbed.
Therefore, considering only the structure of the band, ZnTiO3 fully meets the necessary requirements to be a photocatalyst. But, as already mentioned, in addition to the band structure, the adsorption capacity is also very important for photocatalytic materials.
In this way, the feasibility of using ZnTiO3 as an adsorbent and photocatalytic material for the removal of methylene blue in aqueous systems was experimentally and computationally corroborated. In this way, ZnTiO3 constitutes an efficient alternative material for various technological and environmental applications.
