Characterization of black carbon in geosorbents at the nanometer scale by STEM–EDX elemental mapping

The lack of a universal method for black carbon (BC) quantification and the need to describe the intra-particle/aggregate distribution of BC in natural matrices/geosorbents prompted the search for new methods. This study explored the feasibility of using analytical electron microscopy to characterize black carbon residing in natural matrices. Soot, char, graphite and other carbonaceous samples were examined using energy dispersive X-ray (EDX) analysis in transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). TEM–EDX analysis (point analysis) showed that char, soot and graphite all have a characteristic O/C atomic ratio of ⩽ 0.1 when interrogated at the submicrometer scale, implying this BC elemental signature may be useful in identifying BC at this scale. The presence of a universal elemental signature suggested that it may be more proper to describe different condensed carbonaceous materials as having different BC contents, rather than emphasizing that there are different types of BC, each with a different bulk characteristic O/C. Subsequently, the mapping of BC in STEM–EDX (areal analysis) was demonstrated on sediment samples with varying soot content at a resolution of ∼10 nm. Several standard carbonaceous materials were also examined with quantitative STEM–EDX, yielding BC content comparable with other conventional BC quantification methods. The power of STEM–EDX in revealing morphological features and distribution of various chemical components at nanometer scale was also highlighted. The analytical approach presented here will be valuable for the characterization of BC and the study of nanometer scale organic-mineral association in sediments and soils.