Dynamic of the structural alteration of biochar in ancient Anthrosol over a long timescale by Raman spectroscopy.

Daniel Vieira De Sousa,Carlos Ernesto R G Schaefer,Maria Jacqueline Rodet,João Carlos Ker,Luciano Moura Guimarães,Jorlandio Francisco Félix,Jorge Paz-Ferreiro

doi:10.1371/journal.pone.0229447

Daniel Vieira De Sousa, Carlos Ernesto R G Schaefer + Show 5 more

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https://doi.org/10.1371/journal.pone.0229447

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Abstract

The presence of biochar with high carbon accumulation capacity and nutrient adsorption is causally associated with archeological soils. Although this type of soil organic matter has been known for a long time, the knowledge of its structure and environmental behavior is still limited. This work used Raman spectroscopy to obtain structural information and identify alterations in biochar particles. To this end, we studied biochar particles found in an archaeological site with a temporal window lasting 12451 to 11080 yr cal BP. The molecular, structural and sp2/sp3 characteristics of the charcoal particles were determined at the time of burning and associated with the temperature, time and characteristics of the burnt material. We propose that the process of oxidation of the biochar occurs during the first 2000 years after its genesis. The oxidation process is a reflection of decreases in the number of defects related to sp2 bonds on amorphous carbons and increases in the number of defects associated with ionic impurities, which clearly indicate the interaction between biochar particles and the soil matrix. The data confirm the hypothesis that the persistence of biochar in the environment is due to its graphite structure and suggest that over a 12000 year timeframe, biochar particles undergo several changes that occur in the disordered phase and are rapidly oxidized.

Highlights

Biochar can be conceptualized by two distinct perspectives: the “chemical” perspective and the "utility" perspective
Structural characteristics of standard charcoal pyrolyzed in muffle ovens The X-ray diffraction (XRD) analyses (Fig 3) revealed the changes of the biomass during the pyrolysis reaction and showed that the full width half maximum (FWHM) for the lattice planes (hkl planes (002), (004), (100)) decreased with increasing temperature
This work supports previous findings that show that charcoal will suffer alteration processes over time, and our results support the following conclusions. i) The most significant changes occur in the crystalline lattice during the first 2000 years following the contact of the charcoal particle with the soil matrix. ii) The amorphous fraction is more altered, which is reflected in the decreased number of defects associated with the D1 band in relation to the D4 band

Summary

Introduction

Biochar can be conceptualized by two distinct perspectives: the “chemical” perspective and the "utility" perspective. In the “chemical” perspective, biochar can be understood as the solid product of biomass pyrolysis that formed by the incomplete combustion of organic compounds [1,2,3] and is composed of a graphite microstructure with ordered and disordered phases [3]. The ordered phase is attributed to polyaromatic nanographite domains, which confer structural stability and long-term permanence in the environment. The disordered/ amorphous phase is the most susceptible to changes. This phase is composed of carboxylic and aliphatic groups [4, 5], which are responsible for generating electrical charges and adsorbing.

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