The demographic and industrial development that has characterized the last decades has increased the consumption of water resources to the point of making the study and development of industrial techniques suitable for the purification of huge volumes of wastewater essential. Among the most used technologies, those that are most convenient are part of the advanced oxidation processes, in which, due to the action of hydroxide radicals, the organic compounds are completely reduced to CO2 and H2O: this type of radicals, in fact, are highly reactive and therefore able to degrade any organic component in a non-selective manner. The heterogeneous photocatalysis, belonging to the advanced oxidation processes, is based on the excitation of a solid semiconductor: TiO2, being highly stable, non-toxic, easy and relatively inexpensive to produce and despite having some limitations, including the poor absorption of photons in the Solar spectrum, is one of the most studied materials for this type of application. In order to improve some properties, a composite catalytic material was synthesized through the addition of 0.1% of reduced graphene. In order to understand the photocatalytic mechanism in detail, the physical-chemical properties of the catalyst are studied through advanced laboratory techniques: X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), UV-Vis spectroscopy, nitrogen adsorption isotherms, mercury intrusion porosimetry, laser diffraction and zeta potential. Despite the presence of reduced graphene decreases the probability of recombination of the electron-hole pairs, it results that the main properties of the composite remain unchanged with respect to the starting TiO2. The experimental measurements, carried out in a Pyrex reactor with a volume of 1 liter irradiated with 6 Daylight and 4 Blacklight Blue lamps and subsequently analyzed with TOC, ionic chromatography and liquid chromatography, made it possible to optimize the degradation process by determining the optimal catalyst concentration (50, 100 or 200 ppm) to remove a certain amount of alacloro (5, 10 or 20 ppm), highlighting the formation of large quantities of reaction intermediates which make the TOC degradation decidedly slower than the degradation of alachlor. Subsequently, in order to derive a universal kinetic model applicable to all types of reactors, a kinetic expression was derived with two explicit dependencies: the concentration of contaminant and the absorption rate of photons, which represents the main difference between catalysis and photocatalysis. It has been considered that radical reactions occur between adsorbed species and that the overall reaction rate can be identified with the reaction rate between hydroxide radicals and adsorbed alacloro molecules (RDS). To study the photon absorption profile of the catalyst were carried out spectrophotometric measurements, which reported the relative values of transmittance, diffuse transmittance and diffuse reflectance, useful for obtaining the values of the main optical coefficients such as the scattering and absorption coefficients. In order to obtain the photon absorption profile along the main direction of the reactor, it is necessary to solve the radiative transfer equation (RTE) and integrate it along the entire reactor volume. In particular, the resolution method adopted is the Monte-Carlo technique, an iterative procedure by which the real values of the optical coefficients and of the spectrum of specific spectral intensity are obtained, then correlated to the profile of the local photon absorption speed (LVRPA). The derived model, in relation to the experimental results, provides values with a maximum total error of less than 4% and can therefore be considered reliable. Once all the intrinsic properties of the catalyst were studied, the influence of an extrinsic parameter, such as the effect of the oxidizing agent, was studied. The use of hydrogen peroxide (H2O2), although possessing a higher oxidizing power than oxygen due to a greater production of hydroxide radicals, did not however give satisfactory results, also taking into consideration the higher costs of the operation. Finally, the synthesized catalyst showed excellent stability properties while maintaining the degradation properties of the organic contaminant for at least 30 hours of irradiation.
Lo sviluppo demografico ed industriale che ha caratterizzato gli ultimi decenni ha aumentato il consumo di risorse idriche al punto da rendere fondamentale lo studio e lo sviluppo di tecniche industriali atte alla purificazione di enormi volumi di acque reflue. Tra le tecnologie più utilizzate quelle che risultano più convenienti fanno parte dei processi di ossidazione avanzata, nei quali, per effetto dell’azione di radicali idrossido, i composti organici vengono completamente ridotti a CO2 e H2O: questo tipo di radicali, infatti, sono altamente reattivi e quindi in grado di degradare qualunque componente organico in maniera non selettiva. La fotocatalisi eterogenea, appartenente ai processi di ossidazione avanzata, si basa sull’eccitazione di un semiconduttore solido: TiO2, essendo altamente stabile, non tossico, facile e relativamente economico da produrre e pur presentando alcune limitazioni, tra cui lo scarso assorbimento di fotoni nello spettro solare, è uno dei materiali più studiati per questo tipo di applicazioni. Al fine di migliorarne alcune proprietà è stato sintetizzato un materiale catalitico composito tramite l’aggiunta di 0.1% di grafene ridotto. Al fine di comprendere dettagliatamente il meccanismo fotocatalitico sono studiate le proprietà fisico-chimiche del catalizzatore tramite avanzate tecniche di laboratorio: diffrazione a raggi X (XRD), microscopio elettronico a trasmissione (TEM), microscopio elettronico a scansione (SEM), spettroscopia UV-Vis, isoterme di adsorbimento con azoto, porosimetria ad intrusione di mercurio, diffrazione laser e il potenziale zeta. Nonostante la presenza di grafene ridotto diminuisca la probabilità di ricombinazione delle coppie elettrone-lacuna, risulta che le principali proprietà del composito rimangano invariate rispetto al TiO2 di partenza. Le misure sperimentali, condotte in un reattore Pyrex dal volume di 1 litro irradiato con 6 lampade Daylight e 4 Blacklight Blue e successivamente analizzate con TOC, cromatografia ionica e cromatografia liquida, hanno permesso di ottimizzare il processo di degradazione determinando la concentrazione ottimale di catalizzatore (50, 100 o 200 ppm) per rimuovere una determinata quantità di alacloro (5, 10 o 20 ppm), evidenziando la formazione di grandi quantità di intermedi di reazione che rendono la degradazione di TOC decisamente più lenta della degradazione dell’alacloro. Successivamente, al fine di derivare un modello cinetico universale ed applicabile a tutti i tipi di reattori, è stata derivata un’espressione cinetica con due dipendenze esplicite: la concentrazione di contaminante e la velocità di assorbimento dei fotoni, che rappresenta la principale differenza tra catalisi e fotocatalisi. Si è considerato che le reazioni radicaliche avvengano tra specie adsorbite e che la velocità globale di reazione possa essere identificata con la velocità di reazione tra i radicali idrossido e le molecole di alacloro adsorbite (RDS). Per studiare il profilo di assorbimento di fotoni da parte del catalizzatore, sono state condotte delle misure spettrofotometriche che hanno riportato i relativi valori di trasmittanza, trasmittanza diffusa e riflettanza diffusa, utili per poi ottenere i valori dei principali coefficienti ottici quali il coefficiente di scattering e di assorbimento. Al fine di ottenere il profilo di assorbimento di fotoni lungo la principale direzione del reattore è necessario risolvere l’equazione di trasferimento radiativo (RTE) e integrarla lungo tutto il volume del reattore. In particolare, il metodo di risoluzione adottato è la tecnica di Monte- Carlo, un procedimento iterativo mediante il quale si ottengono i valori reali dei coefficienti ottici e dello spettro di intensità spettrale specifica, poi correlati al profilo della velocità locale di assorbimento di fotoni (LVRPA). Il modello derivato, in relazione ai risultati sperimentali, fornisce valori con un errore totale massimo inferiore al 4% e può quindi considerarsi affidabile. Una volta studiate tutte le proprietà intrinseche al catalizzatore, si è studiata l’influenza di un parametro estrinseco, quale l’effetto dell’agente ossidante. L’utilizzo di acqua ossigenata (H2O2), pur possedendo un potere ossidante più elevato dell’ossigeno per effetto di una maggiore produzione di radicali idrossido, non ha però dato dei risultati soddisfacenti, tenendo anche in considerazione i maggiori costi dell’operazione. Il catalizzatore sintetizzato, infine, ha presentato ottime proprietà di stabilità mantenendo inalterate le proprietà di degradazione del contaminante organico per almeno 30 ore di irradiazione.
Photocatalytic removal of organic compounds from wastewater : synthesis, physico-chemical characterization and kinetic modelling of TiO2 nanocomposite
FARINA, ANDREA
2018/2019
Abstract
The demographic and industrial development that has characterized the last decades has increased the consumption of water resources to the point of making the study and development of industrial techniques suitable for the purification of huge volumes of wastewater essential. Among the most used technologies, those that are most convenient are part of the advanced oxidation processes, in which, due to the action of hydroxide radicals, the organic compounds are completely reduced to CO2 and H2O: this type of radicals, in fact, are highly reactive and therefore able to degrade any organic component in a non-selective manner. The heterogeneous photocatalysis, belonging to the advanced oxidation processes, is based on the excitation of a solid semiconductor: TiO2, being highly stable, non-toxic, easy and relatively inexpensive to produce and despite having some limitations, including the poor absorption of photons in the Solar spectrum, is one of the most studied materials for this type of application. In order to improve some properties, a composite catalytic material was synthesized through the addition of 0.1% of reduced graphene. In order to understand the photocatalytic mechanism in detail, the physical-chemical properties of the catalyst are studied through advanced laboratory techniques: X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), UV-Vis spectroscopy, nitrogen adsorption isotherms, mercury intrusion porosimetry, laser diffraction and zeta potential. Despite the presence of reduced graphene decreases the probability of recombination of the electron-hole pairs, it results that the main properties of the composite remain unchanged with respect to the starting TiO2. The experimental measurements, carried out in a Pyrex reactor with a volume of 1 liter irradiated with 6 Daylight and 4 Blacklight Blue lamps and subsequently analyzed with TOC, ionic chromatography and liquid chromatography, made it possible to optimize the degradation process by determining the optimal catalyst concentration (50, 100 or 200 ppm) to remove a certain amount of alacloro (5, 10 or 20 ppm), highlighting the formation of large quantities of reaction intermediates which make the TOC degradation decidedly slower than the degradation of alachlor. Subsequently, in order to derive a universal kinetic model applicable to all types of reactors, a kinetic expression was derived with two explicit dependencies: the concentration of contaminant and the absorption rate of photons, which represents the main difference between catalysis and photocatalysis. It has been considered that radical reactions occur between adsorbed species and that the overall reaction rate can be identified with the reaction rate between hydroxide radicals and adsorbed alacloro molecules (RDS). To study the photon absorption profile of the catalyst were carried out spectrophotometric measurements, which reported the relative values of transmittance, diffuse transmittance and diffuse reflectance, useful for obtaining the values of the main optical coefficients such as the scattering and absorption coefficients. In order to obtain the photon absorption profile along the main direction of the reactor, it is necessary to solve the radiative transfer equation (RTE) and integrate it along the entire reactor volume. In particular, the resolution method adopted is the Monte-Carlo technique, an iterative procedure by which the real values of the optical coefficients and of the spectrum of specific spectral intensity are obtained, then correlated to the profile of the local photon absorption speed (LVRPA). The derived model, in relation to the experimental results, provides values with a maximum total error of less than 4% and can therefore be considered reliable. Once all the intrinsic properties of the catalyst were studied, the influence of an extrinsic parameter, such as the effect of the oxidizing agent, was studied. The use of hydrogen peroxide (H2O2), although possessing a higher oxidizing power than oxygen due to a greater production of hydroxide radicals, did not however give satisfactory results, also taking into consideration the higher costs of the operation. Finally, the synthesized catalyst showed excellent stability properties while maintaining the degradation properties of the organic contaminant for at least 30 hours of irradiation.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/150166