The use of Raman spectroscopy to characterize the carbon materials found in Amazonian anthrosoils

Abstract

This manuscript presents the Raman characterization of the stable carbon materials found in a special type of soil, named Terra Preta de Índio (Indian Dark Earths). The Terras Pretas de Índios have been studied as a potential model for sustainable agriculture in the humid tropics. The stability of organic matter and the long‐term high‐level of ion exchange capacity in these soils are due to unusually high amount of black carbon. Here, we show how Raman spectroscopy can be used to characterize the stable carbon content in the Terras Pretas de Índios (TPI‐carbons). The tangential stretching mode (G band) and the disorder‐induced mode (D band) are analyzed in comparison to laboratory‐produced amorphous carbons at different degree of disorder used here as reference materials. Statistical analysis show predominance of sp2 phase and crystallite sizes within the limit range between nanographite and amorphous carbons, while Raman mapping of a TPI‐carbon grain shows that the surface is more disordered than the grain core. The analysis used here can also differentiate the TPI‐carbon structures from different types of charcoal.