Differentiation of charcoal, soot and diagenetic carbon in soil: Method comparison and perspectives

The various sources of pyrogenic and coalified carbon (black carbon, BC) in soil have considerable structural heterogeneity, making the quantification of BC a challenge. This study was aimed at evaluating the capability of different detection procedures to recover different types of BC from soil. We added defined quantities of urban dust (UD, NIST SRM1649a), diesel particulate matter (DPM, NIST SRM2975), charcoal, lignite, bituminous coal and wood to four topsoil samples. Mixtures were analyzed by way of chemothermal oxidation (CTO), thermal gradient oxidation (ThG), the benzene polycarboxylic acid method (BPCA) and mid-infrared spectroscopy (MIRS). CTO returned good quantification of soot BC in the pure DPM, yet the recovery of soot BC from soil was unsatisfactory (18-270%). ThG gave good precision but lower values for pure soot BC. It severely overestimated the BC content for all soil-standard mixtures. The BPCA method gave a low return for soot BC, but for the spiked soil it reliably detected charcoal and coalified C (69-107% avg. recovery) but underestimated soot BC (52-90% recovery of DPM). Linear coherence in specific MIR vibrations was found in one component soil-BC mixtures for each BC type. Applying these standard calibrations to multi-component mixtures allowed detecting charcoal and a quantification of soot BC (88% avg. recovery) via MIRS, but ignored the presence of diagenetic C. We see the greatest potential in differentiating soot from charcoal in soil by employing a combination of chemical and thermal oxidation and MIRS, while the differentiation from diagenetic C is not possible yet. (C) 2012 Elsevier Ltd. All rights reserved.