Morphological and chemical properties of black carbon in physical soil fractions as revealed by scanning electron microscopy and energy-dispersive X-ray spectroscopy

Soils contain significant amounts of black carbon (BC) from biomass and fossil fuel combustion. However, its origin, morphology, and chemistry have remained obscure. Here, we examined BC in particle-size and density fractions of the surface soil of a Haplic Chernozem using a scanning electron microscope (SEM) coupled to an energy-dispersive X-ray spectrometer (EDX) in order to investigate the morphological and chemical properties of BC as a function of its origin and fate in soils. The results showed that BC did not only occur as well-defined but also as SEM-amorphous particles. The BC particles exhibited different morphologies ranging from spherical to irregular shapes and from smooth to rough surfaces. Particles with similar morphologies were found in different soil fractions, indicating that BC from different sources is present in the soil, dominated by soot-BC from coal (and oil) combustion and char-BC from coal combustion and biomass burning. The identity of BC was ascertained by atomic O/C ratios ≤ 0.33. Within a BC particle, the O/C ratio increased from interior to exterior surfaces. The mean degree of oxidation increased significantly with an increase in the size of the particle fraction and an increase in the density of the fraction. The presence of inherently light BC in heavy mineral fractions as well as SEM-visible mineral associations with BC particles provided evidence that the partially oxidized BC chemically interacted with the mineral phase, presumably resulting in a protection of the enclosed BC against further decomposition in soil.