Please cite this article in press as: W. Meredith, et al., Direct evidence from hydropyrolysis for the retention of long alkyl moieties in black carbon fractions isolated by acidified dichromate oxidation, J. Anal. Appl. Pyrol. (2012), http://dx.doi.org/10.1016/j.jaap.2012.11.001 ARTICLE IN PRESS G Model JAAP-2855; No. of Pages 8 Journal of Analytical and Applied Pyrolysis xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Journal of Analytical and Applied Pyrolysis journa l h o me page: www.elsevier.com/locate/jaap Direct evidence from hydropyrolysis for the retention of long alkyl moieties in black carbon fractions isolated by acidified dichromate oxidation W. Meredith a,∗ , P.L. Ascough b , M.I. Bird c , D.J. Large a , C.E. Snape a , J. Song d , Y. Sun d,e , E.L. Tilston b,1 a Department of Chemical and Environmental Engineering, Faculty of Engineering, University of Nottingham, NG7 2RD, UK b Scottish Universities Environmental Research Centre (SUERC), Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride G75 0QF, UK c School of Earth and Environmental Sciences and Centre for Tropical Environmental and Sustainability Science, James Cook University, Cairns, Queensland 4870, Australia d State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, PR China e Department of Earth Science, Zhejiang University, No. 38 Zheda Road, Hangzhou 310027, PR China a r t i c l e i n f o Article history: Received 1 July 2012 Accepted 6 November 2012 Available online xxx Keywords: Black carbon Pyrogenic carbon Hydropyrolysis Dichromate oxidation a b s t r a c t 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 (BC dox ) 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 BC hypy ). More notably, hypy allowed the non-BC hypy fraction to be characterised. In addition to a number of PAHs, the non-BC hypy 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 BC dox 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-BC hypy fraction allowing the potential sources of this material to be deduced. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Black carbon (BC), also known as pyrogenic carbon is the carbon- rich, recalcitrant product of the incomplete combustion of biomass and fossil fuels [1], that has both high aromaticity and high resis- tance to oxidative degradation [2,3]. An accurate and reproducible method for the quantification of BC is desirable if we are to assess its occurrence and stability in a range of environments. Currently a variety of thermal, chemical, spectroscopic, molecular marker and optical methods are used, which inevitably give a wide range of ∗ Corresponding author. Tel.: +44 0115 951 4198; fax: +44 0115 951 3898. E-mail address: william.meredith@nottingham.ac.uk (W. Meredith). 1 Present address: Crop and Soil Systems, Scotland’s Rural College (SRUC), West Mains Road, Edinburgh EH9 3JG, UK. results, as previously described in a number of BC inter-comparison studies [4–6]. As BC is typically isolated via operational, rather than chemical parameters, individual methodologies can only identify BC from a specific portion of the BC continuum, with no one method able to isolate or quantify BC across the whole range [5–7]. Chemical oxidation by acidified potassium dichromate is cur- rently one of the most widespread methods employed for BC determination [2,8], as it requires little specialist equipment, with the BC content of the sample isolated due to its chemical recalci- trance relative to the more labile non-BC material [4]. However, the strong chemical oxidation procedures used to remove the non-BC material have been reported to attack BC, with the concomitant potential to underestimate BC [9]. While in contrast, there are also reports of oxidation residues containing non-BC, paraffinic struc- tures, which would imply an overestimation of the true BC content [9]. 0165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2012.11.001