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Assessment of a molecular marker method to determine the pyrogenic carbon component in charcoals and soils


Schneider, M P W. Assessment of a molecular marker method to determine the pyrogenic carbon component in charcoals and soils. 2011, University of Zurich, Faculty of Science.

Abstract

Fire residues such as charcoals or soot stem from incomplete combustion during wildfires or anthropogenic burning of fossil fuels and are found ubiquitously in the environment. Pyrogenic carbon (PyC) contained in these fire residues forms a small but significant part of the global carbon cycle. PyC can be described as a continuum of molecules of different degree of aromatic condensation formed when organic matter or biomass is exposed to heating. Generally, the degree of conden-
sation and the stability of PyC are assumed to increase with increasing heat treatment. Because PyC has not a defined chemical structure and comprises a heterogeneous mixture of all kinds of fire-
altered materials, qualitative besides quantitative information is needed in order to characterize PyC adequately. Precise estimation of sources, fluxes, and sinks of PyC are needed to come to a global PyC budget. This requires a method which gives reliable results for PyC concentrations in all environmental compartments. Benzene polycarboxylic acids (BPCA), a group of seven molecular markers specific for PyC, are a promising tool for PyC characterization in environmental matrices. In the scope of this thesis, the BPCA method was systematically tested and two analytical methods for BPCA detection were compared. Charcoals derived from wood (Castanea sativa Mill.) and grass (Oryza sativa L.) were produced under laboratory conditions at maximum temperatures of 200 °C to 1000 °C. These charcoals served as model compounds to calibrate qualitative and quantitative information provided by the BPCA method. Currently, two methods are used for BPCA isolation and quantification, a traditional method based on gas chromatography (GC-BPCA), and a modified method using liquid chromatography (LC-BPCA). Applying both methods to the model charcoals, LC-
BPCA improved reproducibility and increased yields of BPCA molecular markers, compared to GC-
BPCA. For both methods, charcoals produced at intermediate temperatures (400-700 °C) gave highest
yields of molecular markers. In addition to quantitative information, the BPCA method provides information about the degree of condensation in fire residues. The yields of one of the marker molecules, B6CA (mellitic acid), proved to increase systematically with increasing formation temperature of charcoals, thus reflecting the increasing degree of condensation. Furthermore, in this study the BPCA method was applied to environmental samples to trace changes of charcoal upon ageing in soils. For solubility of charcoals the results showed that only a small fraction of charcoal is soluble in water, but that the soluble fraction increases strongly with increasing functionalization upon ageing. The similarity in the distribution of the seven BPCA molecular markers suggested a similar degree of condensation of charcoal-derived soluble PyC and molecular structures found in rivers and coastal waters. In a chronosequence of soils which have been converted to agricultural land use by slash-and-burn up to 100 years ago, changes in PyC quantity and quality were investigated. With time the charcoal chemical quality, as measured by BPCA molecular markers for PyC, did not change and charcoal stocks did not show a clear decrease, which is opposite to previous results. The results indicate that charcoal may resist (bio-)chemical degradation even when exposed to intense weathering in a tropical climate. In this work it could be shown that BPCA are a valuable tool to determine PyC in the environment, as it detects PyC over a broad range of the combustion continuum and simultaneously allows to estimate the degree of aromatic condensation in PyC. Future tasks include comparison of BPCA results obtained for laboratory charcoals with those obtained for charcoals formed during wildfires, analysis of changes in BPCA pattern upon ageing under laboratory conditions and the further development of compound specific radiocarbon dating for BPCA molecular markers.

Abstract

Fire residues such as charcoals or soot stem from incomplete combustion during wildfires or anthropogenic burning of fossil fuels and are found ubiquitously in the environment. Pyrogenic carbon (PyC) contained in these fire residues forms a small but significant part of the global carbon cycle. PyC can be described as a continuum of molecules of different degree of aromatic condensation formed when organic matter or biomass is exposed to heating. Generally, the degree of conden-
sation and the stability of PyC are assumed to increase with increasing heat treatment. Because PyC has not a defined chemical structure and comprises a heterogeneous mixture of all kinds of fire-
altered materials, qualitative besides quantitative information is needed in order to characterize PyC adequately. Precise estimation of sources, fluxes, and sinks of PyC are needed to come to a global PyC budget. This requires a method which gives reliable results for PyC concentrations in all environmental compartments. Benzene polycarboxylic acids (BPCA), a group of seven molecular markers specific for PyC, are a promising tool for PyC characterization in environmental matrices. In the scope of this thesis, the BPCA method was systematically tested and two analytical methods for BPCA detection were compared. Charcoals derived from wood (Castanea sativa Mill.) and grass (Oryza sativa L.) were produced under laboratory conditions at maximum temperatures of 200 °C to 1000 °C. These charcoals served as model compounds to calibrate qualitative and quantitative information provided by the BPCA method. Currently, two methods are used for BPCA isolation and quantification, a traditional method based on gas chromatography (GC-BPCA), and a modified method using liquid chromatography (LC-BPCA). Applying both methods to the model charcoals, LC-
BPCA improved reproducibility and increased yields of BPCA molecular markers, compared to GC-
BPCA. For both methods, charcoals produced at intermediate temperatures (400-700 °C) gave highest
yields of molecular markers. In addition to quantitative information, the BPCA method provides information about the degree of condensation in fire residues. The yields of one of the marker molecules, B6CA (mellitic acid), proved to increase systematically with increasing formation temperature of charcoals, thus reflecting the increasing degree of condensation. Furthermore, in this study the BPCA method was applied to environmental samples to trace changes of charcoal upon ageing in soils. For solubility of charcoals the results showed that only a small fraction of charcoal is soluble in water, but that the soluble fraction increases strongly with increasing functionalization upon ageing. The similarity in the distribution of the seven BPCA molecular markers suggested a similar degree of condensation of charcoal-derived soluble PyC and molecular structures found in rivers and coastal waters. In a chronosequence of soils which have been converted to agricultural land use by slash-and-burn up to 100 years ago, changes in PyC quantity and quality were investigated. With time the charcoal chemical quality, as measured by BPCA molecular markers for PyC, did not change and charcoal stocks did not show a clear decrease, which is opposite to previous results. The results indicate that charcoal may resist (bio-)chemical degradation even when exposed to intense weathering in a tropical climate. In this work it could be shown that BPCA are a valuable tool to determine PyC in the environment, as it detects PyC over a broad range of the combustion continuum and simultaneously allows to estimate the degree of aromatic condensation in PyC. Future tasks include comparison of BPCA results obtained for laboratory charcoals with those obtained for charcoals formed during wildfires, analysis of changes in BPCA pattern upon ageing under laboratory conditions and the further development of compound specific radiocarbon dating for BPCA molecular markers.

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Additional indexing

Item Type:Dissertation
Referees:Schmidt M W I, Dittmar T
Communities & Collections:07 Faculty of Science > Institute of Geography
Dewey Decimal Classification:910 Geography & travel
Language:English
Date:2011
Deposited On:19 Mar 2012 15:18
Last Modified:05 Apr 2016 15:39
Number of Pages:69
Free access at:Related URL. An embargo period may apply.
Related URLs:http://opac.nebis.ch/F/?local_base=NEBIS&CON_LNG=GER&func=find-b&find_code=SYS&request=006955292

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