C 1s K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy for characterizing functional group chemistry of black carbon

Abstract

Black carbon (BC) is considered ubiquitous in soil organic matter (OM) and therefore plays an important role in soil biogeochemistry. Its complexity, particularly within environmental matrices, presents a challenge for research, primarily as a result of techniques which may favor detection of certain functional group types rather than capturing total sample C. The objective of this study was to utilize carbon (C) 1s near edge X-ray absorption fine edge structure (NEXAFS) spectroscopy to characterize the C chemistry of a broad range of BC materials. Characteristic resonances in the NEXAFS spectra allowed direct molecular speciation of the total C chemistry of the reference materials, environmental matrices and potentially interfering materials, obtained from an earlier BC ring trial. Spectral deconvolution was used to further identify the functional group distribution of the materials. BC reference materials and soils were characterized by a large aromatic C region comprising around 40% of total absorption intensity. We were able to distinguish shale and melanoidin from BC reference materials on the basis of their unique spectral characteristics. However, bituminous coal shared chemical characteristics with BC reference materials, namely high aromaticity of more than 40% identified by way of a broad peak. Lignite also shared similar spectra and functional group distributions to BC reference materials and bituminous coal. We compared the results of spectral deconvolution with the functional group distributions obtained by way of direct polarization magic angle spinning (DPMAS) 13C nuclear magnetic resonance (NMR) spectroscopy. Correlations between aromatic type C values for DPMAS 13C NMR and NEXAFS gave r 2 = 0.633 ( p < 0.05) and the values for NEXAFS were around 30–40% lower than for 13C NMR. Correlations were also drawn between the aromatic C/O-alkyl C ratio values for the two methods ( r 2 = 0.49, p < 0.05). Overall, NEXAFS was applicable for a wide range of environmental materials, such as those measured, although some limitations for the technique were addressed.