Dynamic molecular structure of plant biomass-derived black carbon (biochar)

TLDR: A molecular-level assessment of the physical organization and chemical complexity of biomass-derived chars and, specifically, that of aromatic carbon in char structures suggests the existence of four distinct categories of char consisting of a unique mixture of chemical phases and physical states.

Abstract: Char black carbon (BC), the solid residue of incomplete combustion, is continuously being added to soils and sediments due to natural vegetation fires, anthropogenic pollution, and new strategies for carbon sequestration (“biochar”). Here we present a molecular-level assessment of the physical organization and chemical complexity of biomass-derived chars and, specifically, that of aromatic carbon in char structures. Brunauer−Emmett−Teller (BET)−N2 surface area (SA), X-ray diffraction (XRD), synchrotron-based near-edge X-ray absorption fine structure (NEXAFS), and Fourier transform infrared (FT-IR) spectroscopy are used to show how two plant materials (wood and grass) undergo analogous but quantitatively different physical−chemical transitions as charring temperature increases from 100 to 700 °C. These changes suggest the existence of four distinct categories of char consisting of a unique mixture of chemical phases and physical states: (i) in transition chars, the crystalline character of the precursor ma...

Figures

Figure 4: Dynamic molecular structure of plant biomass-derived black carbon (biochar) across a charring gradient and schematic representation of the four proposed char categories and their individual phases. (A) Physical and chemical characteristics of organic phases. Exact temperature ranges for each category are controlled by both charring conditions (i.e., temperature, duration, and atmosphere) and relative contents of plant biomass components (i.e., hemicellulose, cellulose, and lignin). (B) Char composition as inferred from gravimetric analysis. Yields, VM, fixed-C, and ash contents are averaged across wood and grass chars. Relative contributions above 700˚C are estimates.

Fig. 3. The large number of peaks observed at low temperatures (100-300˚C) demonstrates how the greater variety of C forms present in fresh plant material remains preserved. The 1s-π* C=C transition at 285.3 eV corresponding to H-, CH3- or C-bonded aromatic C (24) is more prominent for wood than for grass, which can be ascribed to the higher lignin content of woody material. Further, lignin in coniferous wood (here Ponderosa Pine) is comprised of phenolic monomers derived from coniferyl alcohol, forming a peak at 287 eV which is prominent in the wood spectra (25). However, we also expect a contribution of the mixed 1s-3p/σ* resonance at this energy that correlates with aliphatic C-H. A second common feature associated with aromatic C is a broader band between 292 and 295 eV (1s-σ* resonance) (26). Spectra of grass chars show additional features in the intermediate region (286-287.5 eV). Here resonances of carbonyl groups (286.4 eV), aliphatic C-H and phenolic C-OH (287.1-287.3 eV) are located. At higher energies, carboxyl C-OOH (288.6 eV) and C-O (289.3 eV) of (hemi)cellulose and lignin are found (see peak assignments in Table S-2).

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Lawrence Berkeley National Laboratory
Lawrence Berkeley National Laboratory
Title
Dynamic molecular structure of plant biomass-derived black carbon (biochar)
Permalink
https://escholarship.org/uc/item/177491q3
Author
Keiluweit, M.
Publication Date
2010-06-04
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California

Dynamic Molecular Structure of Plant Biomass-derived Black Carbon (Biochar)
Marco Keiluweit
†
, Peter S. Nico
‡
, Mark G. Johnson
§
, and Markus Kleber
†,
*
Department of Crop and Soil Science, Oregon State University
University of California - Berkeley, Lawrence Berkeley National Laboratory, Earth Sciences
Division
U.S. Environmental Protection Agency, National Health and Environmental Effects Research
Laboratory, Corvallis

†
Oregon State University
‡
Lawrence Berkeley National Laboratory
§
U.S.
Environmental Protection Agency

*correspondingauthorphone:(01)‐541‐737‐5718;email:markus.kleber@oregonstate.edu


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2
Abstract
Char black carbon (BC), the solid residue of incomplete combustion, is continuously being
added to soils and sediments due to natural vegetation fires, anthropogenic pollution, and new
strategies for carbon sequestration (‘biochar’). Here we present a molecular-level assessment of
the physical organization and chemical complexity of biomass-derived chars and, specifically,
that of aromatic carbon in char structures. BET-N
2
surface area, X-ray diffraction (XRD),
synchrotron-based Near-edge X-ray Absorption Fine Structure (NEXAFS), and Fourier
transform infrared (FT-IR) spectroscopy are used to show how two plant materials (wood and
grass) undergo analogous, but quantitatively different physical-chemical transitions as charring
temperature increases from 100 to 700˚C. These changes suggest the existence of four distinct
categories of char consisting of a unique mixture of chemical phases and physical states: (i) in
transition chars the crystalline character of the precursor materials is preserved, (ii) in amorphous
chars the heat-altered molecules and incipient aromatic polycondensates are randomly mixed,
(iii) composite chars consist of poorly ordered graphene stacks embedded in amorphous phases,
and (iv) turbostratic chars are dominated by disordered graphitic crystallites. The molecular
variations among the different char categories translate into differences in their ability to persist
in the environment and function as environmental sorbents.

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3
Introduction
Black carbon (BC) is an important constituent of soils and sediments (1-4). BC has
received much attention for three reasons. First, there is a general lack of knowledge of the
processes that lead to the loss of BC from soils and sediments which prevents a clear
understanding of fluxes into and out of the Earth’s slow cycling C pools (3). Second, the addition
of synthetic BC (“biochar”) in soils combined with bioenergy production has been suggested as a
means to mitigate climate change (5, 6). Finally, BC in soils and sediments is recognized as an
effective sorbent for potentially hazardous organic compounds (2, 7). Evidently, there is
gathering interest in understanding the behavior of BC; precise information regarding the
structure and properties of BC is needed. However, BC is not a well-defined chemical substance
and encompasses C forms with varying degrees of aromaticity such as partly charred plant
matter, char, soot, and graphite (8). This fact creates difficulties in quantifying BC
concentrations in natural environments (9), it complicates the identification of biochars with
properties beneficial to soils (5), and results in large variations in the sorptive potential of BC
(10). Masiello (3) summarized the nature of the problem when she stated: “discrepancies
between BC studies occur at least in part because of a lack of a common model of BC.”
Biomass-derived char BC is defined as the solid residue of incomplete combustion. A
widely accepted conceptual approach to represent the transient chemical properties of char BC is
based on the gradual increase in aromaticity observed for the heat-induced transformation of
plant biomass into char (3, 11). This concept is commonly referred to as the “combustion
continuum” and assumes that, with increasing charring temperature, plant biomass undergoes
chemical transformations leading to the formation of aromatic ring structures, followed by a
progressive condensation of smaller aromatic units into larger conjugated sheets (4). Recently,

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Knicker and co-workers (12, 13) conceptualized char BC created at a temperature of 350˚C and
in the presence of O
2
as a heterogeneous mixture of thermally altered biomacromolecules with
substantial substitution with O, H, and S and average cluster sizes of aromatic units smaller than
six rings.
While Knicker et al. did not attempt to relate physical properties of their char BC to its
molecular structure, the recent past has revealed indications of the occurrence of nonlinearities
and phase transitions during the thermal decomposition of biomass, which can be explained only
by relatively abrupt changes in physical properties of chars, namely crystallinity and porosity.
For instance, N
2
-accessible surface area (SA) of char BC exhibits a rapid increase at intermediate
charring temperatures (5, 10). This is approximately the same temperature region where X-ray
diffraction data (14-16) show a transition from low-density disordered C to the formation of
turbostratic crystallites.
It thus appears critical for the prediction of fate and reactivity of char BC to relate the
evolution of the chemical structure of chars created across a relevant temperature range to their
physical properties. Consequently, the primary objective of this study is to integrate physical and
chemical information into a comprehensive model for the physical nature of plant biomass-
derived char. We test the hypothesis that physical transitions expressed by changes in SA along a
representative charring temperature range are reflected in corresponding changes in crystal
structure as determined by X-ray diffraction. We further explore the extent to which physical
phase transitions as observed by Paris et al. (16) are reflected in chemical changes, and test
whether this information can be combined with chemical data to recognize specific categories of
chars.

Frequently asked questions

Q1. What are the contributions in "Dynamic molecular structure of plant biomass-derived black carbon (biochar) " ?

In this paper, the authors used X-ray diffraction data to predict the fate and reactivity of black carbon ( BC ) in a plant biomass-derived char.