The PhD research project was aimed to improve the scientific knowledge about the composition and behaviour of volatile organic compounds (VOCs) in volcanic and hydrothermal systems, focusing on (i) primary processes occurring at high temperatures in deep fluid reservoirs and (ii) secondary processes occurring during the uprising of hydrothermal fluids toward the surface. The first goal was achieved following both experimental and empirical approaches. Laboratory experiments were performed in order to investigate the reaction pathways for benzene production under hydrothermal conditions, confirming that the aromatic compounds can be efficiently produced through dehydrocyclization and aromatization of normal-alkanes with cyclics as by-products. Moreover, the pivotal role of minerals in controlling organic reactivity and organic reaction pathways was demonstrated. The analysis of VOCs in fumarolic and venting gases from four volcanic-hydrothermal systems in the Mediterranean area characterized by different temperature and redox conditions at depth (Solfatara Crater, Nisyros Island, Poggio dell'Olivo and Cava dei Selci), supported by thermodynamic and experimental data, evidenced a strict control of physicochemical conditions of deep hydrothermal reservoirs on the composition of VOCs emitted at the surface. Alkanes were the most abundant VOCs, with decreasing abundances at increasing carbon chain length, in agreement with thermodynamic data. At relatively high temperatures, saturated hydrocarbons may undergo dehydrogenation to alkenes and/or dehydrocyclization and subsequent aromatization, as experimentally demonstrated. Accordingly, aromatics were enriched in fumarolic fluids from high temperature active volcanic systems, whilst cyclics were more abundant in hydrothermal systems characterized by lower temperatures, likely due to incomplete aromatization. The occurrence and abundances of S-bearing compounds were related to sulphur fugacity, whilst O-bearing compounds were mainly related to shallow processes. Interstitial soil gases were characterized by remarkably different compositions of VOCs when compared to those recorded in the fumarolic and venting gases, suggesting the relevant importance of secondary processes occurring at depth (dehydrocyclization of alkanes producing an enrichment in cyclics relative to fumarolic fluids) and at shallow depths (DMS oxidation, microbial production of O-bearing compounds). In particular, microbially-driven processes likely play a major role in modifying the composition of VOCs prior to their emission from soils in volcanic and hydrothermal systems.

Volatile Organic Compounds (VOCs) from Volcanic and Hydrothermal Systems: Evidences from Field and Experimental Data / Stefania Venturi. - (2017).

Volatile Organic Compounds (VOCs) from Volcanic and Hydrothermal Systems: Evidences from Field and Experimental Data

VENTURI, STEFANIA
2017

Abstract

The PhD research project was aimed to improve the scientific knowledge about the composition and behaviour of volatile organic compounds (VOCs) in volcanic and hydrothermal systems, focusing on (i) primary processes occurring at high temperatures in deep fluid reservoirs and (ii) secondary processes occurring during the uprising of hydrothermal fluids toward the surface. The first goal was achieved following both experimental and empirical approaches. Laboratory experiments were performed in order to investigate the reaction pathways for benzene production under hydrothermal conditions, confirming that the aromatic compounds can be efficiently produced through dehydrocyclization and aromatization of normal-alkanes with cyclics as by-products. Moreover, the pivotal role of minerals in controlling organic reactivity and organic reaction pathways was demonstrated. The analysis of VOCs in fumarolic and venting gases from four volcanic-hydrothermal systems in the Mediterranean area characterized by different temperature and redox conditions at depth (Solfatara Crater, Nisyros Island, Poggio dell'Olivo and Cava dei Selci), supported by thermodynamic and experimental data, evidenced a strict control of physicochemical conditions of deep hydrothermal reservoirs on the composition of VOCs emitted at the surface. Alkanes were the most abundant VOCs, with decreasing abundances at increasing carbon chain length, in agreement with thermodynamic data. At relatively high temperatures, saturated hydrocarbons may undergo dehydrogenation to alkenes and/or dehydrocyclization and subsequent aromatization, as experimentally demonstrated. Accordingly, aromatics were enriched in fumarolic fluids from high temperature active volcanic systems, whilst cyclics were more abundant in hydrothermal systems characterized by lower temperatures, likely due to incomplete aromatization. The occurrence and abundances of S-bearing compounds were related to sulphur fugacity, whilst O-bearing compounds were mainly related to shallow processes. Interstitial soil gases were characterized by remarkably different compositions of VOCs when compared to those recorded in the fumarolic and venting gases, suggesting the relevant importance of secondary processes occurring at depth (dehydrocyclization of alkanes producing an enrichment in cyclics relative to fumarolic fluids) and at shallow depths (DMS oxidation, microbial production of O-bearing compounds). In particular, microbially-driven processes likely play a major role in modifying the composition of VOCs prior to their emission from soils in volcanic and hydrothermal systems.
2017
Franco Tassi, Everett Shock, Orlando Vaselli
ITALIA
Stefania Venturi
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Utilizza questo identificatore per citare o creare un link a questa risorsa: https://hdl.handle.net/2158/1077611
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