Characterisation of aged black carbon using pyrolysis-GC/MS, thermally assisted hydrolysis and methylation (THM), direct and cross-polarisation 13C nuclear magnetic resonance (DP/CP NMR) and the benzenepolycarboxylic acid (BPCA) method

• Published in: Organic geochemistry Vol. 39; no. 10; pp. 1415 - 1426
• Main Authors: Kaal, Joeri, Brodowski, Sonja, Baldock, Jeff A., Nierop, Klaas G.J., Cortizas, Antonio Martínez
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.10.2008; Elsevier Science
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; Marine and continental quaternary; Soil and rock geochemistry; Soils; Surficial geology; Iberian Peninsula; Southern Europe; Europe; Spain; carbohydrates; models; biomass; lipids; Curie point; gas chromatograms; oxidation; climate; polycyclic aromatic hydrocarbons; networks; nuclear magnetic resonance; mass spectroscopy; benzene; acids; pyrolysis; carbon; proteins; terrestrial environment; organic materials; charcoal; NMR spectra; soils; toluene; hydrolysis
• ISSN: 0146-6380; 1873-5290
• DOI: 10.1016/j.orggeochem.2008.06.011

Aged black carbon (BC) from biomass burning is difficult to identify chemically when it is mixed with other forms of soil organic matter (SOM). As a consequence, the natural abundance of aged and degraded BC is unknown. We carried out a molecular characterisation of up to ∼7000 yr old charcoal and NaOH-extractable SOM obtained from a colluvial soil in NW Spain using Curie point pyrolysis-GC/MS and THM. Black C was tentatively quantified using solid state 13C CP and DP NMR in conjunction with a molecular mixing model (MMM), and the use of BPCAs. Not surprisingly, the charcoal consisted for the most part of chemically distinctive BC moieties, as concluded from CP and DP NMR–MMM (>72% BC-inherent C) and the BPCA method (30–40%). Charcoal produced mainly benzene, toluene and polycyclic aromatic hydrocarbons (PAHs) upon pyrolysis and THM. The SOM was a mixture of BC-derived (producing benzene, toluene, PAHs and benzonitrile on pyrolysis) and non-BC lipid, carbohydrate and protein-derived OM. Benzenes, PAHs and benzonitrile accounted for 41–54% (pyrolysis-GC/MS) and 34–58% (THM) of total identified peak area in extractable SOM, comparable with the 32–41% BC obtained using DP NMR–MMM. Combined results suggested that the SOM BC was a partially oxidised, N-containing, weakly condensed aromatic network. Apart from increased oxidation with age, ∼700 and 7000 yr old BC gave similar molecular fingerprints. The results contribute to our knowledge of the chemistry of BC, a potential climate mediator, and its fate in the terrestrial environment.