δ13C values of coal-derived PAHs from different processes and their application to source apportionment

The 13C/12C isotopic ratios for coal-derived polycyclic aromatic hydrocarbons (PAHs) from a number of processes encompassing low and high temperature carbonisation, gasification and combustion have been determined using gas chromatography–isotope ratio mass spectrometry (δ13C GC–IRMS). The results, in conjunction with those for PAHs released under controlled laboratory pyrolysis conditions, indicate that the primary control on the isotopic values of coal-derived PAHs is likely to be the extent of ring growth required to form PAHs during processing. Thus, for relatively mild conversion processes such as low temperature carbonisation where the major aromatics are alkyl substituted 2–3 ring PAHs, the isotopic signatures are similar to those of the parent coals (−24 to −25‰ for UK bituminous coals). However, the δ13C values for the PAHs become lighter in going to high temperature carbonisation (−25 to −27‰), gasification (−27 to −29‰ for old Town gas plants in the UK) and combustion (−29 to −31‰) as the extent of ring condensation increases and confirming that the PAHs are not released as primary volatiles. To demonstrate the potential of applying these differences to source apportion environmental PAHs where major inputs from coals can be expected, soil and vegetation samples taken close to a low temperature carbonisation plant (Bolsover, North Derbyshire) have been analysed. In addition to low temperature coal tar, significant inputs of PAHs from transport fuels, high temperature carbonisation and possibly combustion (coal/biomass) have been inferred from the isotopic ratios, taken in conjunction with the differences in alkyl substitution patterns.