Among the consequences of the smelting accident of a source of Cesium-137 that occurred in 1989 at the Metalli Capra S.p.a. metal refinery (BS), the release prohibition of leachate waters from the landfill where the contaminated waste is nowadays stored, represents an inconvenient environmental and logistical problem. The specific activity from Cesium-137 of these waters, although attributable to a very low concentration of the contaminant (picograms of Cesium-137 per gram of leachate water), is in fact slightly higher than the limit for the release of 0.1 Bq/g imposed by the Legislative Decree 101/20. The objective of this thesis work is therefore to find an extraction system which is highly effective in removing the little amount of Cesium-137 contained in these fluids, in order to lower their specific activity below the limit and thus allow their treatment at purifiers. Nanomagnetic particles (MNPs), surface-functionalized with chemical groups or chelating agents specific to Cesium-137, represent a practical method of decontamination because, once the pollutant has been captured, the extractant is easily removed from the water by means of an external magnet. Among the various nanomagnetic particle-based extraction systems tested in this thesis work, surface functionalization with the potassium-copper hexacyanoferrate complex proved to be the most performing. The magnetite, produced by the co-precipitation method, is firstly coated with polyethylenimine, which provides amine groups needed to coordinate copper ions. The latters are in turn responsible for hexacyanoferrate ions coordination, leading to the formation of the KCu HCF complex, which is specific and highly effective in removing Cesium. Batch experiments conducted on aqueous solutions artificially contaminated with stable Cesium have concluded that the adsorption equilibrium is reached rapidly within 5 minutes of contact time and that the extraction efficiency is not affected by the co-presence in solution of Cesium-competitive cations such as Sodium and Potassium, present in high concentration in the saline waters of the Metalli Capra landfill. Different combinations in terms of extractant dosage and contact time have been tested on leachate waters from three different tanks. After treatment spectrometric analyses show that in general the Cesium-137 residual specific activity decreases as the concentration of extractant and the contact time increase; the chemical composition of the fluid influences the extraction, in fact, the presence of a massive amount of organic requests the use of a greater amount of extractant. Already starting with 300 or 450 ppm of extractant (depending on the lower or higher presence of organic material in the fluid) and 30 minutes of contact, residual specific activities well below the 0.1 Bq/g limit are obtained. If low dosages of extractant are preferred, contact time can be increased. In this regard, starting from the experimental data, a simulation of the contact times required in order to obtain a certain decontamination factor, for a certain dosage, has been conducted for each fluid: contact times increase in accordance with the required decontamination factor. In conclusion, a comparison between the costs of the treatment (per liter of leachate water) with KCu HCF MNPs and a recent proposal aimed at recycling weakly contaminated waters to produce cement, has revealed that the two strategies are economically comparable. Therefore, the affordability of KCu HCF MNPs, combined with the advantage of having low volume solid radioactive waste and of course a very high extraction efficiency toward Cesium-137, make the system a viable solution to the decades-long problem of the Metalli Capra landfill.
Tra le conseguenze dell’incidente fusorio di una sorgente di Cesio-137 verificatosi nel 1989 presso la raffineria Metalli Capra S.p.a. (BS), il divieto di rilascio delle acque di percolato della discarica in cui i rifiuti contaminati sono stati stipati, rappresenta uno scomodo problema ambientale e logistico. L’attività specifica da Cesio-137 di queste acque, seppur riconducibile ad una ridottissima concentrazione del contaminante (picogrammi di Cesio-137 per grammo di acqua di percolato) è infatti leggermente superiore al limite per il rilascio di 0,1 Bq/g imposto dal Decreto Legislativo 101/20. L’obiettivo del presente lavoro di tesi è dunque trovare un estraente che sia estremamente efficace nella rimozione del Cesio-137 contenuto in questi fluidi, al fine di abbassarne l’attività specifica sotto il limite e consentirne il trattamento presso impianti di depurazione. Le particelle nanomagnetiche (MNPs), funzionalizzate superficialmente con gruppi chimici o chelanti specifici per il Cesio, costituiscono un pratico metodo di decontaminazione perchè, a valle della cattura del contaminante, l’estraente viene facilmente rimosso dalle acque per mezzo di un magnete esterno. Tra i diversi sistemi di estrazione a base di particelle nanomagnetiche sperimentati in questo lavoro di tesi, la funzionalizzazione superficiale con il complesso potassio-rame esacianoferrato (KCu HCF MNPs) si è dimostrata essere la più prestante. La magnetite, prodotta tramite il metodo della coprecipitazione, viene dapprima ricoperta con la polietilenimina, che fornisce i gruppi amminici necessari per coordinare gli ioni rame; questi ultimi sono a loro volta responsabili della coordinazione degli ioni esacianoferrato, che porta alla formazione del complesso KCu HCF, specifico ed altamente efficace nella rimozione del Cesio. Esperimenti in batch condotti su soluzioni acquose artificialmente contaminate con Cesio stabile hanno concluso che l’equilibrio di assorbimento è raggiunto rapidamente a partire da 5 minuti di tempo di contatto e che l’efficacia di estrazione non è inficiata dalla compresenza in soluzione di cationi competitivi al Cesio come Sodio e Potassio, presenti in elevata concentrazione nelle acque saline della discarica Metalli Capra. Diverse combinazioni in termini di dosaggio dell’estraente e tempo di contatto sono state testate sulle acque di percolato provenienti da tre cisterne differenti. L’analisi spettrometrica a valle del trattamento ha in linea generale evidenziato che l’attività residua specifica da Cesio-137 diminuisce all’aumentare della concentrazione di estraente utilizzata e del tempo di contatto; si è inoltre riscontrato che la presenza di una massiccia quantità di organico nel fluido ostacola l’estrazione, rendendo necessario l’utilizzo di una maggiore quantità di estraente. Già partendo da 300 o 450 ppm di legante (a seconda della minore o maggiore presenza di materiale organico nel fluido) e 30 minuti di contatto si ottengono attività specifiche residue ben inferiori al limite di 0,1 Bq/g. Laddove fosse preferibile prediligere bassi dosaggi di estraente, si può agire sull’aumento dei tempi di contatto. A questo proposito, a partire dai dati sperimentali, per ciascun fluido è stata condotta una simulazione dei tempi di contatto necessari al fine di ottenere un certo fattore di decontaminazione per uno specifico dosaggio. Ne è emerso che i tempi di contatto aumentano concordemente con il fattore di decontaminazione richiesto. Per concludere è stata effettuata un’analisi economica mirata a comparare i costi complessivi di trattamento (per litro di acqua di percolato) con KCu HCF MNPs rispetto ad una recente proposta che riciclerebbe queste acque debolmente contaminate per produrre del cemento. I costi risultano del tutto comparabili, per cui, l’accessibilità economica delle KCu HCF MNPs, unita al vantaggio di avere rifiuto radioattivo solido di volume ridotto ed una elevata efficacia di estrazione del Cesio-137, rendono il sistema una valida soluzione per il problema decennale della discarica Metalli Capra.
Selective separation of Cesium-137 in leachate waters by chemically functionalized magnetic nanoparticles : the case of metalli Capra landfill
Vanellone, Silvia
2021/2022
Abstract
Among the consequences of the smelting accident of a source of Cesium-137 that occurred in 1989 at the Metalli Capra S.p.a. metal refinery (BS), the release prohibition of leachate waters from the landfill where the contaminated waste is nowadays stored, represents an inconvenient environmental and logistical problem. The specific activity from Cesium-137 of these waters, although attributable to a very low concentration of the contaminant (picograms of Cesium-137 per gram of leachate water), is in fact slightly higher than the limit for the release of 0.1 Bq/g imposed by the Legislative Decree 101/20. The objective of this thesis work is therefore to find an extraction system which is highly effective in removing the little amount of Cesium-137 contained in these fluids, in order to lower their specific activity below the limit and thus allow their treatment at purifiers. Nanomagnetic particles (MNPs), surface-functionalized with chemical groups or chelating agents specific to Cesium-137, represent a practical method of decontamination because, once the pollutant has been captured, the extractant is easily removed from the water by means of an external magnet. Among the various nanomagnetic particle-based extraction systems tested in this thesis work, surface functionalization with the potassium-copper hexacyanoferrate complex proved to be the most performing. The magnetite, produced by the co-precipitation method, is firstly coated with polyethylenimine, which provides amine groups needed to coordinate copper ions. The latters are in turn responsible for hexacyanoferrate ions coordination, leading to the formation of the KCu HCF complex, which is specific and highly effective in removing Cesium. Batch experiments conducted on aqueous solutions artificially contaminated with stable Cesium have concluded that the adsorption equilibrium is reached rapidly within 5 minutes of contact time and that the extraction efficiency is not affected by the co-presence in solution of Cesium-competitive cations such as Sodium and Potassium, present in high concentration in the saline waters of the Metalli Capra landfill. Different combinations in terms of extractant dosage and contact time have been tested on leachate waters from three different tanks. After treatment spectrometric analyses show that in general the Cesium-137 residual specific activity decreases as the concentration of extractant and the contact time increase; the chemical composition of the fluid influences the extraction, in fact, the presence of a massive amount of organic requests the use of a greater amount of extractant. Already starting with 300 or 450 ppm of extractant (depending on the lower or higher presence of organic material in the fluid) and 30 minutes of contact, residual specific activities well below the 0.1 Bq/g limit are obtained. If low dosages of extractant are preferred, contact time can be increased. In this regard, starting from the experimental data, a simulation of the contact times required in order to obtain a certain decontamination factor, for a certain dosage, has been conducted for each fluid: contact times increase in accordance with the required decontamination factor. In conclusion, a comparison between the costs of the treatment (per liter of leachate water) with KCu HCF MNPs and a recent proposal aimed at recycling weakly contaminated waters to produce cement, has revealed that the two strategies are economically comparable. Therefore, the affordability of KCu HCF MNPs, combined with the advantage of having low volume solid radioactive waste and of course a very high extraction efficiency toward Cesium-137, make the system a viable solution to the decades-long problem of the Metalli Capra landfill.| File | Dimensione | Formato | |
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2022_10_Vanellone_01.pdf
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Descrizione: Testo della tesi
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12.61 MB
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12.61 MB | Adobe PDF | Visualizza/Apri |
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2022_10_Vanellone_02.pdf
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Descrizione: Sommario esteso
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1.14 MB
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1.14 MB | Adobe PDF | Visualizza/Apri |
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https://hdl.handle.net/10589/192129