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| Titel |
Interactions between cations and peat organic matter monitored with NMR wideline, static and FFC NMR relaxometry |
| VerfasserIn |
Gabriele E. Schaumann, Pellegrino Conte, Alexander Jäger, Giuseppe Alonzo, Marko Bertmer |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250044843
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| Zusammenfassung |
| The molecular size of humic substances is still under debate and is believed to range up to
several hundred thousands Dalton, although a number of recent studies suggest much lower
molecular weights. Nowadays an increasing number of authors suggest a model of molecular
aggregates. One explanation why results on the molecular mass of humic materials are
contradictory, may be that individual OM molecules are linked via intermolecular
interactions, by bridges of water molecules or by cations bridging cation exchange sites
(Schaumann, 2006a, b). Properties of such cross-linked systems can be similar to
macromolecular systems revealing covalent cross-links. In this context, multivalent cations
play an important ecological role, serving as reversible cross-linking agent. Formation and
disruption of such cation bridges may close or open sorption sites in soil organic
matter. Although cross-linking by multivalent cations has been proposed in many
studies, the cross-linking effect has not yet been demonstrated on the molecular
scale.
The objective of this study was to investigate the interactions between cations and peat
organic matter using NMR wideline techniques as well as static and fast field cycling
(FFC) NMR relaxometry. Peat treated with solutions containing either Na+, Ca2+ or
Al3+ was investigated in air-dried state for longitudinal relaxation times (T1) and
NMR wideline characteristics. T1 distributions were separated into two Gaussian
functions which were interpreted to represent two proton populations belonging to
two environments of differing mobility. The relaxation rates (R1 = T1-1) in the
cation treated samples spread over a range of 87-123 s-1 (R1a: fast component) and
32-42 s-1 (R1b: slow component). The rates in all treatments are significantly
different from each other. and decrease in the order conditioned
sample > desalinated sample > Na-treated sample. The treatment with multivalent
cations affects R1a and R1b in different ways and needs more detailed explanation.
Wideline proton NMR spectra can be used to quantify proton containing material,
mainly water, based on their mobility. Spectra were decomposed into a Gaussian and
Lorentzian line and changes to mobility after heat treatment indicate the water binding
strength. In this study, differences in the various NMR parameters on the cation
treatments will be presented and discussed with respect to the crosslinking hypothesis. |
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