In this thesis the degradation process of some organic pollutants was studied in a new prototype of atmospheric plasma reactor which uses a radial electrical discharge for the plasma generation (Big RAdial Plasma reactor, BigRAP). The discharge is applied on the surface of the solution, using an electrodes configuration recently patented by the research group, consisting of a pointed electrode, to which a high voltage is applied, and a grounded ring counter electrode partially immersed into the solution. This discharge is particularly efficient in the degradation of surfactant substances. This specific reactor allows, moreover, the treatment of different volumes of solution and thus makes possible to study the effect of the volume of the solution on the degradation efficiency. In the thesis work, solutions containing metolachlor, triton X-100 and phenol were considered and for each contaminant two series of treatments were carried out, repeating every experiment twice: in the first series different volumes of solutions containing the same initial concentration of pollutant were treated, while in the second series different volumes of solutions containing the same number of moles of pollutant were considered. For almost all treatments a stream of argon was bubbled into the solution, allowing mixing and assuring the plasma production in argon atmosphere. Metolachlor, which is an herbicide belonging to the class of persistent organic pollutants (POPs), was the first pollutant treated. After thirty minutes of treatment, the contaminant was almost completely degraded by plasma. Comparing the different degradation curves, it is observed that the degradation kinetics is faster for the treatment of the smallest volume, and gets slower as larger volumes are treated: this trend was also observed for all the other pollutants taken into consideration in this thesis. The two series of treatments with metolachlor were also performed without argon bubbling, maintaining, however, a flow in the upper part of the reactor to allow the production of plasma in argon atmosphere: these treatments induced a limited degradation of the pollutant, thus highlighting the importance of bubbling in mixing the solution and in the degradation process. The second pollutant treated was triton X-100, which is a commonly used substance in the laboratory for cell lysis, to extract proteins and permeabilize the membranes of living cells. From a chemical point of view, triton X-100 is a non-ionic surfactant molecule characterized by a hydrophilic and a hydrophobic portion. In the various treatments of triton X-100, the degradation was complete, confirming the high effectiveness of the reactor in treating surfactant chemical species. The third pollutant treated was phenol, which is used in laboratory and in industrial processes as an intermediate for organic synthesis; nowadays it is very widespread and easily found in waters and sediments. Phenol is not a surfactant compound, and so the treatments led to the degradation of maximum 80% of the pollutant in solution. Finally, some experiments were performed in which solutions containing metolachlor or phenol with an excess of tert-butanol, a known scavenger of •OH radicals, were treated to evaluate whether this radical was involved in the degradation process of these pollutants. In both cases, a little difference wase observed in the degradation with and without t-butanol, so additional experiments were performed to quantitatively evaluate the degradation of t-butanol itself: these experiments highlighted that t-butanol degrades significantly less in solutions that also contain the pollutants. It was therefore concluded that, although the relative reactivity of phenol and metolachlor in the plasma reactor does not reflect their relative reactivity with •OH, this radical is involved in the process leading to their degradation.
In questo lavoro di tesi è stato studiato il processo di degradazione di alcuni inquinanti organici in un nuovo prototipo di reattore al plasma atmosferico a scarica radiale (Big RAdial Plasma reactor - BigRAP). Il plasma viene prodotto nel reattore tramite l’applicazione di scariche elettriche sulla superficie della soluzione. Il reattore è costituito da un elettrodo a punta, cui è applicato un alto voltaggio, e da un elettrodo ad anello posizionato a livello della superficie della soluzione e posto a potenziale di terra. Il reattore permette il trattamento di volumi diversi di soluzione, consentendo di studiare l’effetto del volume della soluzione sull’efficienza della degradazione, ed è particolarmente efficiente nella degradazione di sostanze surfattanti. Nel lavoro di tesi sono state trattate soluzioni contenenti metolachlor, triton X-100 e fenolo, e per ogni inquinante sono state eseguite due serie di trattamenti, ripetendo ogni esperimento due volte: nella prima serie sono stati trattati volumi diversi di soluzione con la stessa concentrazione inziale di inquinante, mentre nella seconda serie sono stati trattati volumi diversi di soluzioni contenenti lo stesso numero di moli di inquinante. Per quasi tutti i trattamenti un flusso di argon è stato fatto gorgogliare nella soluzione, permettendone il rimescolamento e garantendo la produzione del plasma in argon. Il primo inquinante trattato è stato il metolachlor, un erbicida appartenente alla classe degli Inquinanti organici persistenti (POPs). Dopo trenta minuti di trattamento, il contaminante viene degradato quasi completamente dal plasma. Confrontando le diverse curve di degradazione si è osservato che la cinetica di degradazione è più veloce per il trattamento del volume più piccolo e rallenta man mano che si trattano volumi maggiori: questo andamento è stato osservato anche per gli altri inquinanti. Solo per il metolachlor le due serie di trattamenti sono state eseguite anche senza gorgogliamento di argon, mantenendo però un flusso nella parte superiore del reattore per permettere comunque la produzione del plasma in argon: questi trattamenti hanno indotto una degradazione limitata dell’inquinante, evidenziando quindi l’importanza del gorgogliamento nel rimescolare la soluzione e nel processo di degradazione. Il secondo inquinante trattato è stato il triton X-100, comunemente usato in laboratorio per lisi cellulare e altre applicazioni biologiche. Il triton X-100 è una molecola surfattante non ionica e mostra proprietà tensioattive, avendo una porzione idrofilica e una idrofobica. Nei vari trattamenti la degradazione è stata completa, confermando l’elevata efficacia del reattore nel trattare specie chimiche surfattanti. Il terzo inquinante trattato è stato il fenolo, usato in laboratorio e nei processi industriali come intermedio per sintesi organica. Il fenolo non presenta carattere surfattante, e infatti i trattamenti hanno portato alla degradazione di massimo l’80% dell’inquinante in soluzione. Infine, sono stati eseguiti alcuni esperimenti in cui sono state trattate soluzioni contenenti metolachlor o fenolo in eccesso di tert-butanolo, noto scavenger di radicali •OH, per valutare se fosse coinvolto questo radicale nel processo degradativo degli inquinanti. In entrambi i casi si sono osservate differenze poco apprezzabili nelle degradazioni con e senza t-butanolo, per cui sono stati eseguiti degli esperimenti aggiuntivi per valutare quantitativamente la degradazione del t-butanolo stesso: questi esperimenti hanno evidenziato che il t-butanolo si degrada significativamente di meno in soluzioni che contengono anche gli inquinanti. Si è dunque concluso che, nonostante la reattività relativa di fenolo e metolachlor nel reattore al plasma non rispecchi la loro reattività relativa con •OH, questo radicale è coinvolto nel processo che porta alla loro degradazione.
Studio dell’effetto del volume nel trattamento di soluzioni contenenti contaminanti organici in un reattore al plasma a scarica radiale
MIOTTO, ALBERTO
2023/2024
Abstract
In this thesis the degradation process of some organic pollutants was studied in a new prototype of atmospheric plasma reactor which uses a radial electrical discharge for the plasma generation (Big RAdial Plasma reactor, BigRAP). The discharge is applied on the surface of the solution, using an electrodes configuration recently patented by the research group, consisting of a pointed electrode, to which a high voltage is applied, and a grounded ring counter electrode partially immersed into the solution. This discharge is particularly efficient in the degradation of surfactant substances. This specific reactor allows, moreover, the treatment of different volumes of solution and thus makes possible to study the effect of the volume of the solution on the degradation efficiency. In the thesis work, solutions containing metolachlor, triton X-100 and phenol were considered and for each contaminant two series of treatments were carried out, repeating every experiment twice: in the first series different volumes of solutions containing the same initial concentration of pollutant were treated, while in the second series different volumes of solutions containing the same number of moles of pollutant were considered. For almost all treatments a stream of argon was bubbled into the solution, allowing mixing and assuring the plasma production in argon atmosphere. Metolachlor, which is an herbicide belonging to the class of persistent organic pollutants (POPs), was the first pollutant treated. After thirty minutes of treatment, the contaminant was almost completely degraded by plasma. Comparing the different degradation curves, it is observed that the degradation kinetics is faster for the treatment of the smallest volume, and gets slower as larger volumes are treated: this trend was also observed for all the other pollutants taken into consideration in this thesis. The two series of treatments with metolachlor were also performed without argon bubbling, maintaining, however, a flow in the upper part of the reactor to allow the production of plasma in argon atmosphere: these treatments induced a limited degradation of the pollutant, thus highlighting the importance of bubbling in mixing the solution and in the degradation process. The second pollutant treated was triton X-100, which is a commonly used substance in the laboratory for cell lysis, to extract proteins and permeabilize the membranes of living cells. From a chemical point of view, triton X-100 is a non-ionic surfactant molecule characterized by a hydrophilic and a hydrophobic portion. In the various treatments of triton X-100, the degradation was complete, confirming the high effectiveness of the reactor in treating surfactant chemical species. The third pollutant treated was phenol, which is used in laboratory and in industrial processes as an intermediate for organic synthesis; nowadays it is very widespread and easily found in waters and sediments. Phenol is not a surfactant compound, and so the treatments led to the degradation of maximum 80% of the pollutant in solution. Finally, some experiments were performed in which solutions containing metolachlor or phenol with an excess of tert-butanol, a known scavenger of •OH radicals, were treated to evaluate whether this radical was involved in the degradation process of these pollutants. In both cases, a little difference wase observed in the degradation with and without t-butanol, so additional experiments were performed to quantitatively evaluate the degradation of t-butanol itself: these experiments highlighted that t-butanol degrades significantly less in solutions that also contain the pollutants. It was therefore concluded that, although the relative reactivity of phenol and metolachlor in the plasma reactor does not reflect their relative reactivity with •OH, this radical is involved in the process leading to their degradation.| File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.12608/61954