Direct evidence from hydropyrolysis for the retention of long alkyl moieties in black carbon fractions isolated by acidified dichromate oxidation

Abstract

Chemical oxidation with acidified potassium dichromate is one of the more commonly used of a range of available methods for the quantification of black carbon (BC) in soils and sediments. There are potential uncertainties with this method however, with indications that not all non-BC material is susceptible to oxidation. An emerging approach to BC quantification is hydropyrolysis (hypy), in which pyrolysis assisted by high hydrogen pressure facilitates the reductive removal of labile organic matter, so isolating a highly stable portion of the BC continuum that is predominantly composed of >7 ring aromatic domains.Here, results from the hypy of the BC fraction isolated by dichromate oxidation (BCdox) from a BC-rich soil are presented, which demonstrated that 88% of the total carbon initially defined as BC was stable under hypy conditions (defined as BChypy). More notably, hypy allowed the non-BChypy fraction to be characterised. In addition to a number of PAHs, the non-BChypy fraction was also found to contain a significant abundance of n-alkanes, with a marked predominance of even-numbered homologues. These compounds are probably derived from lipids, hydrogenated during hypy, which survived dichromate oxidation due to their hydrophobic nature. Hypy of the dichromate oxidation residue from a sample of Green River shale, known to contain no BC of pyrogenic origin revealed that the significant apparent BCdox content (BC/OC=5.7%) was also largely due to the presence of n-alkanes within the oxidation residues. The distribution of these compounds, biased towards longer chain homologues with no significant even/odd preference, indicated that they were largely derived from long n-alkyl chains within this highly aliphatic matrix.Hypy therefore provides compelling direct evidence for the incomplete removal of non-BC material by dichromate oxidation from both a BC-rich soil and a BC-free oil shale, with the molecular characterisation of the non-BChypy fraction allowing the potential sources of this material to be deduced.