25th International Meeting on Organic Geochemistry IMOG 2011

P-014
Evaluation of hydropyrolysis as a method for the quantification
of black carbon via the testing of standard reference materials
Will Meredith 1 , Emma Tilston 2 , Philippa Ascough 2 , David Large 1 , Colin Snape 1 , Michael
Bird 3
1 Universtiy of Nottingham, Nottingham, United Kingdom, 2 SUERC, East Kilbride, United Kingdom, 3 James
Cook University, Cairns, Australia (corresponding author:william.meredith@nottingham.ac.uk)
Black carbon (BC) is the aromatic, recalcitrant product
of incomplete combustion of biomass and fossil fuels.
An accurate and reproducible method for BC
quantification is desirable if we are to assess its
occurrence and stability in a range of environments.
Currently a variety of thermal, chemical and optical
methods are used which inevitably give a wide range
of results, as demonstrated by Hammes et al (2007)
in the BC inter-comparison study [1].
A new approach is hydropyrolysis (hypy), in which
pyrolysis assisted by high hydrogen pressure (15
MPa) facilitates reductive removal of labile organic
matter. The high hydrogen pressure and slow heating
rate employed, together with the presence of a Mo
based catalyst prevent generation of secondary char.
Here, we present results evaluating the potential of
hypy for the isolation and quantification of BC from
the 12 reference materials used in the BC intercomparison
study, comprising 3 laboratory-produced
BC-rich materials, 5 BC-containing environmental
matrices and 4 non-BC containing potentially
interfering materials [1].
By increasing the maximum hypy temperature in 25°C
increments from 450°C to 595°C (Fig. 1), it is possible
to identify conditions under which lignocellulosic and
other labile organic carbon material (e.g. humic acids,
lipids, proteins etc) are fully removed (by 550°C), but
at which degradation via hydrogasification of the BC
component has not yet commenced (over 575°C).
This plateau in the carbon content of the hypy
residues (as measured by elemental analysis)
between 550 and 575°C is apparent in the soil and
sediment samples, suggesting that hypy reproducibly
isolates a carbonaceous fraction of consistent relative
stability in samples from different environments.
The resulting BC contents (as determined at 550°C)
for all 12 samples fall within the range reported in the
BC inter-comparison study [1], with BC also
distinguished successfully from the potentially
interfering materials. Hypy is also able to differentiate
―low BC‖ biochars from ―high BC‖ n-hexane soot.
Additionally, the results are highly reproducible, with
BC determinations from triplicate analyses typically
within ±1%.
Fig. 1. BC (%) as proportion of organic carbon (OC)
as measured for the environmental matrices.
Within BC-containing environmental matrices hypy is
able to discriminate between those with high fossil
fuel derived BC contents, dominated by soot (e.g. the
marine sediment and urban aerosol from
industrialised environments), and those with lower BC
contents derived from vegetation burning (e.g.
agricultural soils).
It appears that hypy can reduce labile organic matter
to volatile products in a controlled manner, and so
isolate rapidly and reproducibly the most resistant BC
fraction from carbonaceous samples, independent of
the environmental matrices in which it is found. This
makes it an attractive new approach for quantification
of BC across a wide range of sample types.
[1] Hammes, K., et al. (2007) Global Biogeochemical
Cycles 21, GB3016, doi: 10.1029/2006GB002914.
162