Isolation and characterisation of organic components in sediments from an estuarine environment

Humic substances (humic acids and fulvic acids) and humin were isolated from sediments from an estuarine environment. The clay-size fractions from the sediment cores were also isolated to access the organic matter in its natural state (without being subjected to chemical pre-treatments or to extraction procedures). Organic carbon compositions of the sediments were low, resulting in low yields of humic substances and of humin. High concentrations of urea added to aqueous alkaline media improved the extraction yields of humic substances, and these isolates had enhanced compositions from lignin and from peptide structures. Humin was solubilised using acidified dimethyl sulfoxide, and insoluble humin was concentrated using hydrofluoric acid. These humin isolates are very similar but the insoluble humins have larger compositions of highly ordered aliphatic hydrophobic species that are responsible for the insoluble nature to these fractions. Nuclear magnetic resonance (NMR) spectroscopy of the organic matter associated with the clay-sized fraction of the sediments illustrated that the extraction solvent sequence does not alter the organic fractions. The low concentrations of organic carbon in the clay-sized fraction hinder in-depth characterisation studies, but such studies are enhanced when the clay-sized fraction has minimal pre-treatment (10 % hydrofluoric acid). High ash compositions and analyses of the elemental compositions show that silicates are selectively removed, but the remaining inorganic species concentrated in the demineralised clay-sized fraction have a negative impact on characterisation studies. Analyses show that marine organic matter is incorporated in the humic substances whereas the humins are dominated by terrestrial organic matter. Peptides are shown to make major contributions to the humic substances and humin, and there is evidence to indicate that a large proportion of these structures are from microbial inputs. Evidence for lignin is very strong in the base of the sediment cores suggesting that Galway Bay was a terrestrial environment prior to a sea level rise. Carbohydrates are shown to persist at depth due to the presence of highly ordered cellulose. Labile carbohydrates and peptides are preserved as a result of their associations with hydrophobic domains provided by the aliphatic hydrocarbons, and inorganic colloids such as iron hydroxides. Humic substances will eventually become re-mineralised with time whereas the humin components persist in the environment. These therefore represent a more stable sink for organic carbon. Swollen-state 13C NMR show that there are regions of the organic compositions inaccessible to the solvent and components in these species are responsible for the refractory natures of the organic materials.