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TECHNICAL REPORTS
1001
Terra Preta research gave evidence for the positive in uence of
charred organic material (biochar) on infertile tropical soils.
Facing global challenges such as land degradation, fossil energy
decline, water shortage, and climate change, the use of biochar
as a soil amendment embedded into regional matter cycles seems
to provide an all-round solution. However, little is known about
biochar e ects on individual ecosystem processes. Besides, the term
biochar is used for a variety of charred products. erefore, the
aim of this study was to investigate principal material properties
of di erent chars to establish a minimum set of analytical
properties and thresholds for biochar identi cation. For this
purpose, chars from di erent production processes (traditional
charcoal stack, rotary kiln, Pyreg reactor, wood gasi er, and
hydrothermal carbonization) were analyzed for physical and
chemical properties such as surface area, black carbon, polycyclic
aromatic hydrocarbons, and elemental composition. Our results
showed a signi cant in uence of production processes on biochar
properties. Based on our results, to identify biochar suitable for
soil amendment and carbon sequestration, we recommend using
variables with the following thresholds: O/C ratio <0.4, H/C ratio
<0.6, black carbon >15% C, polyaromatic hydrocarbons lower
than soil background values, and a surface area >100 m2 g−1.
One Step Forward toward Characterization: Some Important Material
Properties to Distinguish Biochars
Sonja Schimmelpfennig* and Bruno Glaser
B from the discovery of
fertile black soils in Amazonia (Terra Preta do Indio).
ese soils raised global interest because of their high
humus contents and enhanced soil fertility compared with the
surrounding infertile tropical soils (Sombroek, 1966; Zech et
al., 1990). More detailed research identi ed charred organic
material as a key component of Terra Preta, partly explaining
its unique properties (Glaser et al., 2001). Moreover, it has
been recently shown that organic waste in the form of excre-
ment and bones was added to soils and mixed with charred
material (Glaser and Birk, 2011). Charred organic material, as
known from black carbon (BC)/pyrogenic carbon research, is
relatively recalcitrant against degradation due to high amounts
of aromatic carbon (C) structures (Goldberg, 1985). Over
time, these aromatic structures are partially oxidized, generat-
ing edges of the aromatic backbone exhibiting a high nutrient
bonding capacity (Glaser et al., 2001; Glaser et al., 2002b).
Subsequently, nutrients from organic waste bound reversibly
and in a plant-available way to the edges of charred material,
and sustainable and fertile soils with horizons up to 2 m were
created (Glaser, 2007; Glaser and Birk, 2011).
Apart from the Terra Preta phenomenon, the addition of
charred organic material to soil was practiced in ancient agri-
cultural systems all over the world (Kleber et al., 2003; Ogawa,
1994). Furthermore, a considerable amount of pyrogenic C
is accumulated in the environment consistently by forest and
vegetation res (Glaser and Amelung, 2003). e occurrence
of combustion-derived organic material in the environment
can best be described by the combustion continuum (Fig. 1).
e combustion continuum is used to describe the di er-
ent forms of pyrogenic C in the environment (Hedges et al.,
2000). It includes slightly charred biomass, char, and charcoal
as well as soot and graphitized C (Fig. 1), varying consider-
ably in size, composition, and environmental performance
(Masiello, 2004). Certain ranges in the combustion contin-
uum are commonly de ned as BC (Forbes et al., 2006) or bio-
char (Karaosmanoglu et al., 2000; Lehmann, 2007). For this
Abbreviations: BC, black carbon; BET, Brunauer–Emmett–Teller; HTC, hydrothermal
carbonization; HTT, highest treatment temperature; PAC, polyaromatic
hydrocarbon; PAH, polycyclic aromatic hydrocarbon.
S. Schimmelpfennig and B. Glaser, Soil Physics Section, Univ. of Bayreuth,
Universitätsstr. 30, 95440 Bayreuth, Germany; Bruno Glaser, Soil Biogeochemistry,
Martin-Luther-Univ. Halle-Wittenberg, von-Seckendor -Platz 3, 06120 Halle,
Germany; S. Schimmelpfennig, current address: Dep. of Plant Ecology, Justus-
Liebig-Univ., Heinrich-Bu -Ring 26-32 (IFZ) 35392 Gießen, Germany. Assigned to
Associate Editor Warren Busscher.
Copyright © 2012 by the American Society of Agronomy, Crop Science Society
of America, and Soil Science Society of America. All rights reserved. No part of
this periodical may be reproduced or transmitted in any form or by any means,
electronic or mechanical, including photocopying, recording, or any information
storage and retrieval system, without permission in writing from the publisher.
J. Environ. Qual. 41
doi:10.2134/jeq2011.0146
Supplemental data le is available online for this article.
Received 16 Apr. 2011.
*Corresponding author (Sonja.schimmelpfennig@bot2.bio.uni-giessen.de).
© ASA, CSSA, SSSA
5585 Guilford Rd., Madison, WI 53711 USA
Journal of Environmental Quality
ENVIRONMENTAL BENEFITS OF BIOCHAR
SPECIAL SECTION
Published July, 2012
1002 Journal of Environmental Quality
reason, in some studies BC is used synonymously with biochar
(Warnock et al., 2007). More recently, Lehmann and Joseph
(2009) proposed the use of the term biochar for describing
charred organic matter that is applied to soil to improve soil
properties. However, when looking at the original Terra Preta
concept, in addition to improving soil fertility, long-term sta-
bility (at least for 2000 yr) should be a key feature of biochar
for C management.
Biochar research has revealed positive in uences of biochar
on nutrient retention; water-holding capacity; and microbial,
fungal, and mycorrhizal growth, resulting in increased yields
and seedling growth (Gaskin et al., 2008; Kim et al., 2007;
Kwapinski et al., 2010). e possibilities for using biochar for
mitigating climate change by C sequestration or waste man-
agement are also worth mentioning (Kwapinski et al., 2010;
Macías and Camps Arbestain, 2010). Studies have shown that
biochar’s properties and its e ects on plants and ecosystems vary
widely due to di erent production processes and raw materials.
For instance, studies on the e ects of forest- re–derived and
vegetation- re–derived pyrogenic C on soil and plant growth
revealed an improved growth of larch seedlings (Makoto et al.,
2010). ese results could be repeated and con rmed for larch
seedlings grown in soil amended with larch biochar produced
at temperatures of 400°C, whereas biochar produced at 800°C
had no e ect on larch seedling growth (Makoto et al., 2011).
Kwapinski et al. (2010) reported enhanced maize seedling
growth for soil amended with miscanthus biochar (produced
at temperatures of 600°C) as well; however, the same feed-
stock pyrolyzed with lower temperatures (400°C) had nega-
tive impacts on seedling growth. Furthermore, Gundale and
DeLuca (2006) reported an increased sorption of phytotoxic
compounds for biochar produced at temperatures of 800°C
compared with low-temperature biochar (350°C) from the
same feedstock (wood from ponderosa pine and douglas r),
resulting in more favorable soil condi-
tions for plant roots sensitive to phyto-
toxic compounds.
ese ndings imply that the e ect
of biochar and pyrogenic C on soil
cannot be generalized and that a closer
examination should be taken of biochar
production processes. Biochar produc-
tion processes vary widely in terms of
temperature, pressure, oxygen levels,
duration, and e ciency. ese pro-
cess parameters in uence physical and
chemical properties as well as the struc-
ture of obtained chars.
e most frequent chemical reactions
associated with the carbonization pro-
cess are dehydration, decarboxylation,
dehydrogenation, and demethylation
(Amonette and Joseph, 2009; Richter
and Howard, 2000). Dehydration can
be described as loss of hydroxyl groups,
decarboxylation is the loss of carboxy
groups and emission of noncondensable
gases such as CO and CO2, dehydroge-
nation is the oxidation of saturated to
unsaturated hydrocarbons generating
aromatic structures, and demethylation is the elimination of
methyl groups (Fox and Whitesell, 2004).
By means of the chemical reactions during the carboniza-
tion process, thermochemical alteration of biomass leads to
increasing amounts of aromatic C structures, depending on
process conditions (Baldock and Smernik, 2002). Due to
di erences in technical process parameters and the di erent
intensities of chemical reactions during carbonization, it was
our objective to de ne material properties that would allow
the identi cation of a biochar’s potential performance, even if
a detailed description of the production process was not avail-
able. We focused on analyzing the elemental composition and
BC contents as indicators for the degree of carbonization and
their stability against degradation. Furthermore, we measured
the surface area because it in uences soil ecology and water
balance in biochar-amended soils. As a toxicity indicator, we
determined the amount of polyaromatic hydrocarbons (PAHs)
in the biochar samples.
Elemental compositions, plotted as hydrogen (H)/C vs.
oxygen (O)/C ratios (Van Krevelen diagrams), have been used
in coal research to describe maturity, decomposition rate, and
thereby combustion behavior of fossil chars and coal (Hammes
et al., 2006; van Krevelen, 1957). e H/C and O/C ratios can
therefore serve as indicators for the degree of carbonization of
biochars, with high ratios pointing to considerable amounts of
primary plant macromolecules, such as carbohydrates and cel-
lulose (Chun et al., 2004).
Applied to BC and biochar, the atomic ratios of C, H, and
O can also serve as indicators for chemical composition and car-
bonization grade. Kuhlbusch and Crutzen (1995) used element
ratios to characterize BC from vegetation res and suggested a
H/C ratio of ≤0.2 to be suitable for de ning black C.
Fig. 1. The combustion continuum, used to describe the multiple forms of pyrogenic carbon to be
found in the environment. Modi ed after Masiello (2004) and Hedges et al. (2000) and supple-
mented with own data.
www.agronomy.org • www.crops.org • www.soils.org 1003
Black carbon is commonly de ned as a continuum of prod-
ucts from combustion processes (Forbes et al., 2006) that are
chemically characterized by stable, aromatic C structures. Its
structure renders BC very stable and recalcitrant to degrada-
tion in soils (Schmidt and Noack, 2000). ese ndings cor-
respond with the results of Glaser et al. (2001), who identi ed
pyrogenic C with condensed aromatic structures (i.e., BC) as
the main contributor to the fertility of Terra Preta. Black C is
generated during incomplete combustion of biomass and fossil
fuels, with temperature being a crucial in uence on its struc-
ture (Masiello, 2004). e sources of BC range from forest and
grassland res to biomass and fossil fuel burning from house-
holds, tra c, and industries. Small BC particles such as soot
are easily taken up by wind, are transported as aerosols in the
atmosphere, and accumulate with precipitation and uvial as
well as eolian deposition in sediments and soils. Due to its
condensed aromatic structures, BC is chemically recalcitrant
against degradation (Forbes et al., 2006). Schmidt and Noack
(2000) described BC as one of the most stable forms of C with
respect to degradation from chemical oxidation, depending on
its size. Preston and Schmidt (2006) point out that the stabil-
ity of BC from oxidation increases in particular with increasing
portions of soot.
Subsequently, long-term C sequestration in soils in form
of BC represents a potential tool for counteracting increasing
atmospheric CO2 levels (Forbes et al., 2006). Additionally,
e ects such as increased cation exchange capacity, an improved
water balance, and enhanced microbial activity strongly pro-
mote the application of BC as a soil amendment (Liang et al.,
2006; Pietikäinen et al., 2000). Furthermore, BC exhibits high
sorptive capacity for hydrophobic organic pollutants such as
PAHs, depending on its degree of aromatization (Smernik,
2009). Adsorption of PAH onto BC particles reduces their
bioavailability, slows their biodegradation rates, and preserves
them in sediments (McElroy et al., 1989).
e surface area of biochar can be an indicator of its poros-
ity, mainly for pores of ≤2 nm size. e rationale behind this
is that micropores ≤2 nm contribute mostly to the surface
area due to the coverage of nitrogen (N) with a molecular size
of 0.35 nm (Downie et al., 2009). Such micropores on the
biochar surface can be created by the loss of volatile elements
such as CO2, H2O, and CO and by the loss of volatizing tar
during the production process (Zabaniotou et al., 2008). As
a consequence, the Brunauer–Emmett–Teller (BET) surface
area of biochar may allow prediction of the e ect of biochar
amendment on the moisture levels in soil. Because water can
be held within the pores of the char by capillary forces, it can
function like a sponge by taking up water under wet conditions
and releasing it when soil conditions become dry (Shinogi and
Kanri, 2003). Glaser et al. (2002a) reported an 18% higher
eld capacity of charcoal-amended anthrosols compared with
the surrounding soils without charcoal. However, accord-
ing to Tryon (1948), the improvement of soil water retention
depends on soil texture. Besides the potential positive e ects
on water storage, a higher surface area indicates a larger area
of retreat for microbes and micro ora, which build niches for
microscopic competitive soil biota. Biochar amendments to
soil can therefore change the microbial composition consid-
erably ( ies and Rillig, 2009). Moreover, BET surface area
has provided evidence of adsorption behavior of biochar for
organic contaminants in soil. Because the BET surface area, as
well as aromaticity, increase with the degree of carbonization,
a high BET surface area may indicate higher adsorptive capac-
ity for pesticides and aromatic and polycyclic aromatic hydro-
carbons (PAHs) in soils (Chen et al., 2008; Yu et al., 2010).
is can be explained by the hydrophobicity of the carbonized
material and the organic contaminants that strongly bind via
Van der Waals forces (Verheijen et al., 2010). Hence, the BET
surface area can be used for evaluating the e ect of biochar on
the aforementioned soil properties.
Before establishing biochar as a soil amendment, due to
its predicted and expected positive e ects, it is also important
to consider possible negative in uences on the environment.
ese might arise from the presence of toxic substances such
as PAH in biochar. As byproducts from incomplete com-
bustion processes, PAHs can be found ubiquitously in the
environment. eir chemical structure makes them barely bio-
degradable and recalcitrant to oxidation (Preston and Schmidt,
2006). Most research on PAHs focused on its sources, environ-
mental fate, and pathways in hydrological, atmospherical, and
geological systems (Chiou et al., 1998; Richter and Howard,
2000). Wilcke et al. (2000) reported increased amounts of
PAHs in organic soils compared with mineral soils depending
on location and composition of the study site, with the high-
est accumulation rates found in forest and urban soils. ese
ndings can be explained by the precipitation of products
from incomplete combustion processes from industry, tra c,
and private households as well as from forest and wild res,
which have been identi ed as major sources of PAHs in the
environment (Glaser et al., 2005; Lima et al., 2005). During
combustion, high temperature breaks down complex chemical
structures of the raw material, of which some can be resynthe-
sized to PAHs. Because biochar is a product of pyrolysis and
incomplete combustion, contamination with PAHs seems pos-
sible. Hence, content and composition of PAHs in biochar are
important parameters for evaluating the suitability of biochar
for soil amendment and to derive threshold values for biochar
application to soil. Based on the analytical properties described
and the desired features of stability, a large surface area, and low
toxicity levels, we di erentiate the biochar materials according
to their production processes. With the characterization of bio-
char material we strive for standard values, rendering biochar a
reliable tool for environmental management.
Materials and Methods
Description of Investigated Biochars
and Corresponding Raw Material
We compared biochars produced in professionally engineered
reactors, such as the Pyreg reactor, with biochar produced in
traditional char burners and wood gasi ers and with the hydro-
thermal carbonization (HTC) technique. In total, 66 biochar
samples were investigated. One third of the samples were made
from wood, the most traditional feedstock for common char-
coal, usually charred to increase its e ciency for energy-generat-
ing purposes. All the other raw materials can be characterized as
organic leftovers from di erent cropping systems in various parts
of the world. An overview of the studied feedstocks, combined
1004 Journal of Environmental Quality
with the respective processing technique, is given in Table 1. We
categorized all biochar samples in six groups according to the
applied production process as outlined in the following section.
Charcoal Stack Biochar
Six biochars from di erent feedstocks were produced in tra-
ditional charcoal burners. Charcoal stacks are the traditional
way of converting mainly wood to charcoal with the purpose
of increasing its heating value. e temperature settings inside
the kilns cannot easily be managed and may vary according to
construction and size. On average, charcoal production tem-
peratures for our samples were about 340°C. Duration of the
charring process also varied between the individual chars pro-
duced in the kilns.
Hydrothermal Carbonization Material
Twenty-one samples produced using HTC were studied. e
HTC technique can be described as combined dehydration
and decarboxylation of a fuel to raise the C content with the
aim of achieving a higher calori c value. Practically, organic
material is pyrolyzed for 4 to 24 h up to 200°C under pres-
sure of around 20 bars in a moist atmosphere. Due to the
absence of oxygen, the HTC process has a C e ciency of close
to one (Titirici et al., 2007). e resulting end product can be
described as brownish, oil-like slurry with C present in solid
particles or dissolved in water. By drying the slurry, material
with char-like properties can be obtained, sometimes de ned
as HTC coal or HTC material (Funke and Ziegler, 2010).
Nineteen of our samples were produced by the Suncoal
company (Königs Wusterhausen, Germany) from sugarcane
residues. Two other samples made from bark and sugar beet
were supplied by Hydrocarb (Kirtorf-Arnsheim, Germany).
An overview of di erent feedstocks and catalysts of the HTC
material is given in Supplemental Table S1.
Rotary Kiln Biochar
Sixteen biochars were produced in rotary kilns with outside heat-
ing. Ten samples were made from bamboo in China with a verti-
cally constructed rotary kiln at temperatures varying from 400
to 600°C in 50°C steps. Six samples were taken from a home-
engineered, horizontally constructed kiln from Switzerland
produced at temperatures of 650°C. Generally, rotary kilns are
externally heated, cylindrical-shaped pyrolyzers. In most cases,
the initial biomass is continually moved by rotating spirals inside
the kiln. Pyrolysis gas is discharged and can be used for heating
purposes or for driving the process. Rotary kilns are suitable for
carbonization of organic material because the process conditions
can be managed accurately and biochar properties can be easily
related to adjusted parameters (Brown, 2009).
Pyreg Biochar
Six biochars were produced with the Pyreg reactor, with two
made out of wheat, one out of sewage sludge, one out of lop,
one out of maize silage, and one out of bark and needles. e
average pyrolysis temperature was around 500°C. e Pyreg
process is a patented pyrolysis process. It is characterized by
allothermic gasi cation of biomass, a low oxygen burner, and
the uncoupling of biochar. Part of the heat generated by burn-
ing the pyrolysis gases in a ameless oxidation burner is reused
for maintaining the pyrolysis process. Because ameless oxida-
tion prevents the formation of NOx and dust, pyrolysis gases
from Pyreg reactors do not need further cleaning, making them
ready to use for further energy generation. Moreover, by dis-
charging the pyrolysis gases from the solid residues, condensa-
tion of the gases onto the solid charred residues (biochar) with
possible negative e ects can be prevented.
Wood Gasi er Biochar
Eight biochars were produced from wood feedstock in di er-
ent wood gasi ers following the Spanner RE2 technology with
Table 1. Biochar samples, arranged after feedstock and production processes. The rst number is the number of samples;the second number refers
to highest production temperature (°C).
Material Production process
Charcoal stack HTC† Others Pyreg Rotary kiln Wood gasi er Total
Animal meal 1 1
Bamboo 10/350–550 10
Bark 1/200 1
Bark/needles 1/550 1
Coconut shells 2/350&650 2
Lop 1/550 1
Maize 1/350 1/550 2
Peanut shells 1/500 1
Rice hulls 1/350 1
Sewage sludge 1/550 1
Sugar beet 1/200 1
Sugar cane 19/200 19
Girasol 1/800 1
Walnut shells 1/800 1
Wheat 2/550 2
Wood 4/350 2/n.d.‡ 7/750 8/800 21
Total 6 21 8 6 17 8 66
† Hydrothermal carbonization.
‡ Not determined.
www.agronomy.org • www.crops.org • www.soils.org 1005
process temperatures of up to 1200°C. e solid remains of the
process are a char-like material. Wood gasi ers ,such as the one
developed by the company Spanner Re2, are in use in regional
bioenergy systems in villages or on farms. If the char-like solid
remains of the process turn out to be suitable as a soil amend-
ment, its use would pose an additional bene t for farmers.
Methods
Elemental Composition
Total C and N was determined using an NA 1108 elemental ana-
lyzer (CE Instruments, Mailand, Italy) coupled to a delta S isotope
ratio mass spectrometer (Finnigan MAT, Bremen, Germany) via a
Con ow III interface (Finnigan MAT). Total amounts of O and
H were quanti ed using an HT analyzer (HEKA-Tech, Wegberg,
Germany) coupled to a Delta V plus isotope ratio mass spectrom-
eter ( ermo-Fisher, Bremen, Germany).
Black Carbon (Pyrogenic Carbon) Content
Due to various origins, BC cannot be described as one single
chemical substance but rather as a continuum of carbonaceous
material. e chemical structure of BC is characterized by a high
amount of aromatic and graphitic structures, being chemically
relatively inert to degradation (Freier et al., 2010; Glaser et al.,
2005; Schmidt and Noack, 2000). To account for this feature
but not to quantify the entire continuum of BC, we applied the
molecular marker method described by Freier et al. (2010) with
slight modi cations. In detail, for the elimination of polyvalent
cations and hydrolyzable organic material, 50 mg charred mate-
rial was subjected to digestion with 10 mL of 4 mol L−1 TFA for
4 h at 105°C. e residue of this chemical process was collected
by ltration through a glass ber lter (GF 6; Whatman, Fisher
Scienti c, Germany), rinsed several times with deionized water,
and dried at 40°C for 4 h. For oxidation, the residue was trans-
ferred into a quartz digestion tube, and 2 mL of 65% HNO3
were added. e mixture was heated to 170°C and held at this
temperature in a high-pressure digestion apparatus for 8 h, and
BC was oxidized to benzenepolycarboxylic acids (Glaser et al.,
1998). e solution was poured through an ashless cellulose lter
(Schleicher & Schuell 5893; Whatman) into 25-mL glass cups.
To ensure that enough material for representative sampling was
available, around 0.1 g of ground and glown quartz was added
to the ltrate. e cups were kept at 80°C under a fume hood to
remove acid and water and cooled in a desiccator. e residues
were then homogenized with a tipped glass stick. Concentration
of BC C was determined in the same way as described for total C.
Brunauer–Emmett–Teller Surface Area
e BET analysis was performed to determine the speci c sur-
face area of the biochars. e analysis starts with cooling the
solid matter (in our case biochar) to the temperature of liquid
N (77K). en, in eight steps, a distinct amount of gaseous N
is added to the sample. e BET surface describes the area cov-
ered by a dinitrogen (N2) monolayer sorbed to the surface of a
sample. Because a N2 molecule that sorbs to solid matter takes
up a certain area (0.35 nm2), with the assumption of complete
monolayer occupancy the surface area can be calculated with
the BET equation (Eq. [1])
AS = Vm*NA*am [1]
where AS is the surface area of the sample, Vm the capacity of
the monolayer to hold a de ned amount of gas in mol, NA
Avogadro’s number, and am is the space required by one con-
densed adsorbed N2 molecule. For analysis, approximately
1 g of biochar was poured into a glass tube and dried under
vacuum in a VacPrep 061 (Micromeritics, Aachen, Germany)
until a pressure of 55 μHg was reached. e BET surface of the
dried sample was determined by using a Gemini 2360 Surface
Area Analyzer (Micromeritics).
Polycyclic Aromatic Hydrocarbons
e USEPA de ned 16 PAHs as priority pollutants being
hazardous to the environment. ese 16 USEPA-PAHs were
analyzed for all biochars. Determination and quanti cation of
the 16 USEPA-PAHs was performed by the TÜV (Technischer
Überwachungsverein) Rheinland, section Nürnberg. e PAHs
were extracted from the samples with n-hexane in soxhlets for
8 h. e extract obtained was quanti ed using gas chromatog-
raphy–mass spectrometry. Deuterated naphthaline-d8, pyrene-
d10, and benzo(a)pyrene-d12 were used as internal standards.
Statistics
All statistical analyses were conducted using the Statistica 6
software package (Statsoft, Inc., Tulsa, OK). Data were tested
for normal distribution with the Kolmogorov-Smirnov test.
In the case of normal distribution of the data, we applied the
Tukey HSD post hoc test for comparison of means. If normal
distribution of the data could not be achieved via logarithmic
transformation, the nonparametric Mann-Whitney U test was
applied. e discriminant analysis was performed with log-
transformed data (common logarithm) or nontransformed
data, depending on optimum satisfaction of the normality dis-
tribution. Because the discriminant analysis is relatively robust
against violation of normality assumption, parameters that
only approximately met normality distribution were included.
Levels of signi cance were de ned as highly signi cant (p ≤
0.01), signi cant (p > 0.01 and p < 0.05), and weakly signi -
cant (p > 0.05 and < 0.1).
Results and Discussion
Van Krevelen Diagram
e elemental composition of the di erent biochar samples was
visualized by a Van Krevelen diagram plotting H/C vs. O/C ratios
(van Krevelen, 1957). Figure 2 shows that the sample set could
be divided into two groups, one with a high H/C ratio (ranging
from 0.6 to 1.3, mainly represented by HTC material), and the
other group with an H/C ratio ≤0.6, including all other biochar
samples. As illustrated, the H/C ratio of most samples resembles
that of stone coal, indicating high aromaticity and molecular
homogeneity (Mackenzie, 2005). Hammes et al. (2006) found
that a H/C ratio of around 0.3 points to substances with a very
highly condensed aromatic ring system, whereas an H/C ratio of
≥0.7 indicates noncondensed aromatic structures such as lignin
(Fig. 2). Material with an H/C ratio ≤0.2 can be de ned as BC
following Kuhlbusch and Crutzen (1995). Accordingly, following
the Van Krevelen diagram, all Pyreg materials but the one from
sewage sludge, some wood gasi er and rotary kiln material, and
one other sample (active coal) with a C content of nearly 100%
1006 Journal of Environmental Quality
can be classi ed as BC, which is characterized as being hardly
degradable in the environment (Glaser et al., 2005; Hedges et al.,
2000; Schmidt and Noack, 2000).
e O/C ratio of the samples showed smaller variation,
with the ratio of most chars ranging from close to 0 to 0.2.
Hydrothermal carbonization samples plus two samples from
charcoal stacks and one from a wood gasi er stood out with
ratios from 0.2 to 0.4. Following the combustion continuum as
shown in Fig. 1, the samples have properties similar to charcoal
(O/C ratio, 0.2–0.4) or soot (O/C ratio, <0.2). e majority of
the samples from Pyreg, rotary kiln, wood gasi ers, and char-
coal stacks exhibited O/C ratios similar to soot. Because some
of the samples showed an H/C ratio ≤0.2, as is typical for BC, a
nearly complete carbonization of biochar seems possible for all
production processes. However, exceptionally high O/C ratios
have been found for two wood gasi er biochars and one Pyreg
sample (sewage sludge), pointing to low carbonization grades.
As shown by the three dotted lines in Fig. 2, indicating
the reaction pathways of dehydration, decarboxylation, and
demethylation, the elemental ratio allows interpretation of
chemical composition and state of carbonization of the mate-
rial. e HTC material was not as carbonized as the other
samples. e H/C ratio of the HTC material points to a low
grade of demethylation and decarboxylation. is is con rmed
by the chemical similarity of HTC material with brown coal,
indicating an early stage of carbonization, characterized by
chemical structures (e.g., methoxyl, hydroxyl, and other func-
tional groups) and incomplete demethylation, re ected by low
H/C and O/C ratios (Mackenzie, 2005). In turn, according
to Yonebayashi (1988), lower H/C and O/C ratios of char-
coal stack, Pyreg, rotary kiln, and wood gasi er
biochars mean fewer functional groups on the
char’s surface and higher amounts of stable C
compounds (Yonebayashi, 1988). Also, Chun
et al. (2004) found that high H/C and O/C
ratios of char can be explained by uncharred
leftovers of primary plant macromolecules such
as carbohydrates and cellulose.
e Van Krevelen diagram of the stud-
ied samples indicates an in uence of process
parameters on the chemical reactions during
the production course and consequently on
the chemical composition of the nal prod-
ucts. Potential key factors are temperature, pro-
cess duration, and the presence of oxygen. For
example, Baldock and Smernik (2002) found
that H/C and O/C ratios of laboratory-pro-
duced wood biochars decreased signi cantly
with increasing production temperatures and
duration of the combustion process. Krull et
al. (2009) reported that a conversion of organic
material to aromatic C structures is highest at
process temperatures >350°C, resulting in low
H/C and O/C ratios. According to Hammes
et al. (2008), temperatures higher than 500°C
lead to a H/C ratio <0.5, although this inter-
relation depends on oxygen amount and tem-
perature ramping during the heating process.
is means that the number of functional
groups of organic matter decreases with increasing thermal
treatment. Up to process temperatures of 350°C, functional
groups, such as carboxyl and hydroxyl groups, are maintained,
but they are eliminated at temperatures >500°C. Based on
these observations, the position of HTC material in the dia-
gram, indicating high H/C and O/C ratios, becomes clear. e
HTC process is mainly driven at temperatures around 200°C,
leaving many plant-derived functional groups undisturbed.
Besides the parameters already discussed, the type of feedstock
was shown to have e ects on elemental composition of charred
material (Demirbas, 2004).
e O/C ratio, being an indicator for the presence of polar
functional groups, can give information on surface hydrophi-
licity and hydrophoby of the charred material. Polar functional
groups on the surface of biochar can bind water by electrostatic
interactions, and hydrogen bondings can thereby act as water
adsorption centers (Gustkiewicz and Orengo, 1998). is cor-
responds to the ndings of Chun et al. (2004), who found
that biochar from wheat residues with an O/C ratio of 0.29
showed higher water uptake by o set with water vapor than
biochar of the same material with O/C ratios of 0.05 and 0.09
and activated coal with an O/C ratio of 0.06. Hence, the HTC
samples and some wood gasi er biochars appear to exhibit
higher hydrophilicity than the other materials.
Functional groups could have an in uence on the stabil-
ity of biochar by preventing a dense, graphite-like structure of
the material (Laine and Yunes, 1992). All examined materials,
except HTC material, exhibit low H/C and O/C ratios similar
to stone coal, indicating chemical stability against microbial
degradation compared with other organic materials, such as
Fig. 2. van Krevelen diagram illustrating atomic ratios of materials under study in compari-
son to assorted organic material such as cellulose, lignin, and brown-, bituminous-, and
stone coal (data adapted from Kim et al. [2007] and Funckelman and Orem [2005]). The
square marks the biochars with desirable ratios for soil amendment, concerning stabil-
ity against decomposition; encircled values mark outliers according to the Grubbs test
(B arnett, 1994). The lines for decarboxylation, dehydration, and demethylation are drawn
according to van Krevelen (1957). HTC, hydrothermal carbonization.
www.agronomy.org • www.crops.org • www.soils.org 1007
cellulose, lignin, or brown coal, which is more easily degraded
(Preston and Schmidt, 2006). erefore, the O/C and H/C
ratios of the HTC material, similar to those of brown coal and
lignin, point to higher chemical reactivity and coherently lower
stability in soils. ese results are in general accordance with
earlier studies: Research on the chemical stability of pyrolysis-
derived biochar predicts residence times in soils of about 2000
yr (Kuzyakov et al., 2009), whereas HTC material is assumed
to be degraded after decades (Steinbeiss et al., 2009).
e elemental ratios of H, O, and C can be used to di erenti-
ate material obtained by di erent charring processes. Desirable
ratios of biochar for soil application with the e ect of sequester-
ing C are H/C ≤0.6 and O/C ≤0.4, specifying material with con-
siderable aromatic C compounds and low amounts of functional
groups. In that sense, material from charcoal stacks, rotary kilns,
wood gasi ers, and the Pyreg reactor are, with few exceptions
(two wood gasi er chars and one sewage sludge biochar; see
encircled values in Fig. 2), suitable for long-term C sequestration
in soils. Hydrothermal carbonization materials can be degraded
more easily in soils due to higher amounts of functional groups
resulting from lower carbonization intensity. erefore, at the
current process conditions, HTC materials cannot be recom-
mended as a soil amendment for long-term C sequestration.
Black Carbon
High amounts of BC are desirable for biochar to be used as a
soil amendment because it represents a stable form of organic
matter (Schmidt and Noack, 2000) with positive in uences
on soil fertility (Glaser, 2007; Glaser et al., 2001; Liang et al.,
2006; Pietikäinen et al., 2000). e mean BC content of our
samples varied from 5% (HTC) to 30% BC C (charcoal stack)
(Fig. 3). ese results show that HTC material has signi cantly
lower BC contents than all other biochars (Tukey HSD test; p
< 0.05). e low BC contents of HTC material point to low
stability in the soil, which corresponds to much of the cur-
rent literature (Steinbeiss et al., 2009). Moreover, these results
con rm the ndings of the elemental composition with higher
H/C and O/C ratios of HTC material compared with pyroly-
sis-derived biochars, pointing to lower stability against degra-
dation. erefore, the BC content of HTC material underlines
that this material is not as suitable for long-term C sequestra-
tion as biochar from other production processes.
ese results demonstrate a predominant in uence of pro-
duction processes on the formation of aromatic C structures,
corresponding to results discussed in the Van Krevelen diagram
section and to previous ndings reported by Masiello (2004).
It is highly probable that di erences in feedstock within a
single production process have a considerable e ect on the BC
content of the biochar. Nevertheless, these di erences could not
be analyzed statistically due to an insu cient number of replica-
tions for the individual production processes in this study.
Brunauer–Emmett–Teller Surface Area
Results of the BET analysis show that the production processes
of biochar in uence BET surface area. Especially the surface
area of HTC material, with a mean value of 8 m2 g−1, is sig-
ni cantly lower than that of biochar from the other processes
(Tukey HSD test; p < 0.05) (Fig. 4). Furthermore, the surface
area of traditional charcoal is lower than Pyreg, rotary kiln,
and wood gasi er biochars. e BET surface area of rotary
kiln and other biochars is highly variable. is can be due to
the large variety of feedstock and process parameters of these
samples. Our data indicate that high mineral ash containing
biochar from feedstock such as animal meal, sewage sludge, or
rice hulls tends to exhibit low BET surface areas. Further stud-
ies are necessary to test the importance of raw materials for the
characters of biochar.
In addition to the feedstocks, highest treatment temperature
(HTT) also in uenced BET surface area. Variability of process
temperature was particularly noticeable in rotary kiln, charcoal
stack, and other biochars. It is known also from the literature
that BET surface area is in uenced by parameters of the pro-
duction process, such as heating rate, HTT, and reaction time
(Downie et al., 2009; Glaser et al., 2002a; ies and Rillig,
2009; Tryon, 1948). However, current information on heating
rate and reaction time was not widely available. Nevertheless,
samples were checked for correlations between HTT and BET
surface area. For this, all samples were sorted after HTT, inde-
pendent from their production processes. We found a highly
signi cant correlation (p < 0.01) between HTT and BET sur-
face area for process temperatures up to 750°C (Fig. 5). Also,
Keiluweit et al. (2010) found that surface areas from grass- and
pine-originated biochar produced in a mu e furnace at tem-
peratures ranging from 100 to 700°C with 100°C temperature
steps increased continually along the temperature gradient.
e positive correlation between HTT and BET surface area
can be explained by changes in the chemical structure of the
feedstock during the production process. With higher tem-
peratures, fused-ring aromatic C structures are generated. ey
provide a matrix in which micropores, accountable for higher
surface areas, can develop (Brown, 2009).
Fig. 3. Comparison of means of the black carbon contents of the
biochar samples sorted by production process. Letters indicate
signi cant di erences (Tukey HSD test; p < 0.05). Error bars show SE.
HTC, hydrothermal carbonization.
1008 Journal of Environmental Quality
However, according to our data, the BET surface area seems
to peak at HTT of 750°C. Such a peak in HTT vs. surface
area correlation corresponds with the observation of Chun et
al. (2004). ey report signi cantly lower surface areas for low-
temperature (300 and 400°C) wood biochar compared with
biochar from the same feedstock produced at temperatures of
500 and 600°C. In their study, a peak of the surface areas is
described for temperatures of 600°C, followed by a decrease
of BET values of biochar produced at 700°C. e subsequent
decline can be explained by changes in the chemical structure
and pore deformation, especially the destruction of ne pore
structures at 700°C (Chun et al., 2004). Additionally, at around
700 to 800°C, though variable with feedstock, formation of tur-
bostratical C structures can be observed (Brown, 2009; Downie
et al., 2009). A turbostratic, instead of a graphitic, structure
is favorable because of its high porosity and surface area.
Turbostratic structures have more spacing between the planes
due to disordered arrangements of the C molecules (Fig. 6).
Regarding the ecological relevance of BET surface area, our
data show that Pyreg, rotary kiln, and wood gasi er biochars
exhibit a large surface area with a positive in uence on soil
moisture and soil biota. A large surface area indicates a high
amount of micropores (Atkinson et al., 2010), and as a result
biochar provides habitats for microorganisms (Joseph et al.,
2010) and fungi (Saito, 1990). Furthermore, due to the pre-
dominantly aromatic structure of these materials, organic pol-
lutants can bind to a large hydrophobic surface, preventing
them from leaching to the groundwater. Hydrothermal carbon-
ization material exhibits a signi cantly smaller surface area with
less possibility to bind organic pollutants. Additionally, it has
fewer micropores to bind water or serve as habitat for microor-
ganisms. Moreover, the generally acidic pH (4.1–4.7) of HTC
material (Fuertes et al., 2010; Rillig et al., 2010) could enforce
its rather unfavorable e ect on microorganisms because most
soil biota prefer circumneutral pH values (Chan and Xu, 2009;
Tolksdorf-Lienemann and Rebling, 2009). Materials from char-
coal stacks and other biochars were not signi cantly di erent
from the rest of the sample set. All other investigated biochars
were heterogeneous with respect to BET surface area, compris-
ing samples with extremely small BET surface area (1.74 m2 g−1)
produced from animal meal but also samples with extremely
large BET surface area (1700 m2 g−1), such as active coal.
Polycyclic Aromatic Hydrocarbons
Figure 7 shows the total EPA-PAH amounts of the di erent
biochars in the sample set. Material from rotary kilns, Pyreg,
others, and HTC and has the lowest amount of PAHs, whereas
biochar from charcoal stacks tends to have higher mean values.
e highest PAH content was found in wood gasi er biochar,
di ering signi cantly (Mann-Whitney U test; p < 0.05) from
all other groups except charcoal stacks. ese results lead to the
assumption that PAH formation is in uenced considerably by
process parameters of biochar production.
e technological setup may signi cantly in uence the
amount of PAH in the biochars. Concerning wood gasi -
ers, temperature during gasi cation and the cooling of the
gas afterward can likely explain the high PAH contents in
these biochars. Because the highest treatment temperatures of
wood gasi ers are ≥800°C (McKendry, 2002), evaporation of
biomass-enclosed oil (generally at about 500°C; Pakdel and
Roy, 1991) and subsequent tar generation seems possible.
Temperatures of ≥700°C can lead to chemical alteration of the
vapors toward highly aromatic, deoxygenated tar with a high
fraction of condensed PAHs (Ledesma et al., 2002; Pakdel and
Roy, 1991). Due to the construction of wood gasi ers, gas and
solid residues are not separated in the cooling phase of the pro-
cess, allowing condensation of volatile tar components into the
Fig. 4. Comparison of means of the Brunauer–Emmett–Teller (BET)
surface area of the biochar samples sorted by production process.
Letters indicate signi cant di erences (Tukey HSD test; p < 0.05).
Error bars show SE. HTC, hydrothermal carbonization.
Fig. 5. Brunauer–Emmett–Teller (BET ) surface area means and stan-
dard error as a function of highest treatment temperature. Data for
hydrothermal carbonization samples taken from Ge ers (2010).
www.agronomy.org • www.crops.org • www.soils.org 1009
solid residues. erefore, high PAH contamination of wood
gasi er biochars seems plausible (Pakdel and Roy, 1991) and
could be veri ed by our data. Small amounts of PAH forma-
tion are also seen at pyrolysis process temperatures from 350
to 600°C, explaining the presence of PAH in biochar from the
other production processes. However, PAHs formed at these
temperatures are mostly branched and thus are less toxic to the
environment (Garcia-Perez, 2008).
To determine di erences in the abundance of single PAHs
in the biochar groups, we performed a discriminant analysis.
Other biochars were excluded from this analysis because pro-
duction techniques and process parameters are highly variable.
Separation of the remaining groups was possible by means of
the discriminant functions Root 1 and Root 2 (Fig. 8). Both
roots are highly signi cant at p < 0.01 according to the chi
square test of successive roots.
From the factor structure matrix (Table 2), it can be seen
that naphthalene is the most in uential PAH, loading posi-
tively on Root 1, followed by acenaphthene and acenaph-
thylen. Phenanthrene loads with
the highest negative value on Root
1. is suggests that the simple
naphthalene/phenanthrene ratio
might be useful to di erentiate the
biochar groups from one another.
In Fig. 9, naphthalene/phenan-
threne ratios are visualized for the
biochar groups.
e lowest naphthalene/phenan-
threne ratio was obtained for HTC
material. is is in accordance with
the result of the discriminant analy-
sis in Fig. 8, showing a good separa-
tion of most HTC samples from all
other biochar groups. Hydrothermal
carbonization samples are located
on the negative (phenanthrene) side
of Root 1, indicating low naphtha-
lene (possibly low acenaphthene
and acenaphthylene) to phenan-
threne ratios. Material from charcoal
stacks, Pyreg, and wood gasi ers
exhibits higher naphthalene/phen-
anthrene ratios (Fig. 9), situated
neither clearly on the negative nor
on the positive (naphthalene) side
of Root1. Biochar from rotary kilns have the highest naphtha-
lene/phenanthrene ratios, as re ected by positive values on Root
1. Biochars from other processes show the largest variability due to
the heterogeneity of their production processes.
No such predominant in uence of single PAHs can be seen
on Root 2. According to the factor structure matrix (Table
Fig. 6. Schematic illustration of turbostratic and graphitic carbon
structures. Adapted from Dasgupta and Sathiyamoorthy (2003).
Fig. 7. Comparison of means of the summed 16 priority pollutant
polycyclic aromatic hydrocarbons de ned by the USEPA (EPA-PAHs)
in the biochar samples, sorted by production process. Letters
show signi cant di erences between the samples, with p < 0.05 as
determined with the Mann-Whitney U test. Error bars show SE. HTC,
hydrothermal carbonization.
Fig. 8. Scatterplot of canonical scores as functions of the phenanthrene/naphthalene ratio and the total
priority pollutant polycyclic aromatic hydrocarbons (PAHs) de ned by the USEPA contents. HTC, hydro-
thermal carbonization.
1010 Journal of Environmental Quality
2), all 16 EPA-PAHs are negatively correlated with Root 2.
e factor structure coe cients, as well as the total PAH con-
tent, are similar for a variety of single PAHs. erefore, we
interpreted Root 2 as the visualization of the sum of PAHs.
In accordance with the comparison of means (Fig. 7), wood
gasi er biochars show the highest amounts of PAHs, plotting
on the negative side (high ΣPAH) of Root 2. e groups HTC,
Pyreg, and rotary kiln are located in a similar range from zero
to positive values (low ΣPAH) of Root 2. Biochar from char-
coal stacks showed high variability.
To summarize, with the help of discriminant analysis, indi-
vidual biochar groups can be separated by their PAH composi-
tion as follows. Biochar from wood gasi ers and charcoal stacks
typically exhibit high overall PAH contents. Biochar from rotary
kilns is characterized by low overall PAH content and high naph-
thalene/phenanthrene ratios. Pyreg biochar and HTC material
have rather low overall PAH content as well but have intermedi-
ate and low naphthalene/phenanthrene ratios, respectively.
To bring our results into a practical application context for
biochar, PAH contents have to be set in relation with given
threshold values. e legal framework for biochar as a soil amend-
ment is not speci ed and should be subject to future research.
Binding threshold values for PAHs in soil exist in the form of
precautionary values (“Vorsorgewert”) and remediation values
(“Prüfwert”) according to the “Bundesbodenschutzverordnung”
(BBodSchV, German federal soil protection ordinance).
Precautionary values describe thresholds above which harm-
ful changes of the soil have to be assumed. In the BBodSchV,
threshold values are given for the sum of the 16 EPA-PAHs but
not for individual PAHs (except for benz(a)pyrene). According
to the BBodSchV§4.2, the precautionary values are 3 mg PAH
kg−1 for soils with humus content <8% and 10 mg kg−1 for soils
with higher humus content. e remediation value, indepen-
dent from humus content, is given for 20 mg PAH kg−1 soil
(Σnaphthalene not included). e precautionary values for
benz(a)pyrene are 0.3 and 1 mg kg−1 soil, respectively.
In the following discussion, we estimate the maximal bio-
char amount that should be added to soils with respect to the
given precautionary values. It is assumed that biochar is applied
to soil depths of up to 30 cm, which is an adopted working
depth in agriculture. We calculated with a bulk density of the
surface soil of 1.3 g cm−3.
e results presented in Table 3 show that, on average, only
0.39 or 1.38 kg m−2 wood gasi er biochar can be applied to soils
before exceeding precautionary values for ΣPAHs (depend-
ing on soil humus content and soil-inherent PAH concentra-
tions). In contrast, 300 to 1000 kg m−2 of Pyreg biochar can
be added to soil before the precautionary values are reached.
Recommendable application rates of HTC, rotary kiln, char-
coal stack, and other biochars lie within this range.
Due to low benz(a)pyrene contents of the biochars studied,
only ΣPAH contents are relevant if threshold values for appli-
cation of biochar to soil are considered (Tables 3 and 4).
To conclude, biochars from di erent technological processes
can be distinguished by their PAH composition. Especially the
naphthalene/phenanthrene ratio and the total PAH content
appear to be useful parameters to distinguish biochars from
di erent production processes. Due to its high PAH content,
biochar from wood gasi ers seems to be disadvantageous for
soil amendment. Hydrothermal carbonization material and
Pyreg and rotary kiln biochar show signi cantly lower PAH
content. Soil amendment with these biochars does not exceed
critical values and is safe for the environment. All other mate-
rials show high variation within the groups, possibly due to
di erences in feedstock.
Combination of Physical-Chemical Parameters
to Distinguish Biochar
To determine if the combination of analyzed parameters is
suitable to distinguish the biochars from one another, we per-
formed a discriminant analysis, including the parameters total
Table 2. Factor structure coe cients for the discriminant functions
Root 1 and Root 2 (polycyclic aromatic hydrocarbons/biochars).
Variable Root 1 Root 2
Naphthalene 0.184154 −0.474015
Acenaphthylene 0.085529 −0.311497
Acenaphtene 0.120407 −0.561022
Fluorine 0.036946 −0.526867
Phenanthrene −0.088090 −0.566059
Anthracene 0.075041 −0.510700
Fluoranthene −0.032718 −0.574521
Pyrene −0.023699 −0.581886
Chrysene −0.024334 −0.495021
Benzo(a)anthracene 0.028439 −0.444469
Benzo(b+k) uoranthene −0.036900 −0.387283
Benzo(a)pyrene 0.005800 −0.394795
Dibenzo(a,h)anthracene 0.040669 −0.269691
Indeno(c,d)pyrene −0.031109 −0.264652
Benzo(g,h,i)perylene −0.030744 −0.226279
USEPA-PAH† 0.063328 −0.495910
† Polycyclic aromatic hydrocarbons de ned by the USEPA.
Fig. 9. Naphthalene/phenanthrene ratios for group means with SE.
Letters show signi cant di erences (Tukey HSD test, p < 0.05). HTC,
hydrothermal carbonization.
www.agronomy.org • www.crops.org • www.soils.org 1011
EPA-PAHs, BC contents, and the BET surface area of the
samples. e results are shown in Fig. 10.
e factor structure matrix (Table 5) shows that BC is the
most prominent parameter on Root 1 with a negative factor
structure coe cient. e parameter BET is dominant in the
same direction, indicating a positive correlation between BET
and BC contents. On Root 2, all three parameters correlate
positively with the discriminant function, PAHs having the
most prevalent in uence, followed by BET values. Root 1
therefore mainly re ects BC, whereas Root 2 is dominated by
PAHs; BET surface is the second-most important factor on
both roots.
Concerning our data set, HTC material plots explicitly on
the positive side of Root 1, indicating that it di ers consider-
ably from the other groups by low BC contents. In contrast,
high BC contents specify the charcoal stack, Pyreg, and rotary
kiln groups. High variation in BC contents characterizes wood
gasi er biochars.
Looking at Root 2, the most prominent group plotting
almost exclusively on the positive side is the wood
gasi er biochar group, pointing to comparably
high PAH contents. Data points di er in the BET
plot from the right bottom (high values of Root 1,
low values of Root 2) to the left top (low values of
Root 1, high values of Root 2). Accordingly, the
scatter plot of canonical scores shows that biochar
from the charcoal stack and rotary kiln groups can
be divided into two subgroups according to their
BET surface area values. In the case of rotary kiln
biochars, this e ect can be traced back to di erent
reactor designs and feedstocks. Reactor type 1 is
arranged horizontally, and biochar feedstock was
exclusively wood. Reactor type 2 is arranged verti-
cally, and bamboo was used as a raw material. e
variability in Pyreg biochar can also be explained
by the di erences in process engineering (some
Pyreg samples are made with prototypes) and feed-
stock. A clear example of the e ect of a di erent
feedstock is the Pyreg sewage sludge sample, plot-
ting far from the other Pyreg samples positively on
Root 1, indicating low BC contents.
In summary, for the sample set and the parameters analyzed,
the discriminant analysis was a suitable tool to determine di er-
ences and similarities among the biochar groups. Biochar from
di erent production processes could be divided roughly by the
parameters ∑PAH, BET surface, and BC, although a detailed
speci cation and description of the production parameters was
not available.
Conclusions
Our results suggest that the di erent production processes
can be listed according to their suitability to produce biochar
with desirable properties for soil amendment in the following
way: pyreg > rotary kiln > charcoal stack > others > wood
gasi er > HTC.
To ensure the quality of biochar for environmental man-
agement, we highlight the need for analytical guideline values.
We strongly propose the use the elemental ratios of H/C and
O/C and the BC concentration as tools to de ne desirable
properties of biochar for soil amendment, mainly concerning
Table 3. Maximum biochar amounts for biochar addition to soil, considering the precautionary values for the sum of the 16 polycyclic aromatic
hydrocarbons of the German national soil conservation act.
Biochar type HTC Pyreg Others Wood gasi er Rotary kiln Charcoal stack
Mean values (ΣEPA-PAH), mg kg−16 3.9 14 2945 606.1 663.63
Possible biochar addition (kg m−2 soil)
for soil with humus contents <8% 195 300 83.6 0.39 1.93 1.76
Possible biochar addition (kg m−2 soil)
for soil with humus contents > 8% 650 1000 278.6 1.3 6.43 5.88
Table 4. Maximum biochar amounts for biochar addition to soil, considering the precautionary values for benz(a)pyrene of the German national soil
conservation act.
Biochar type HTC† Pyreg Others Wood gasi er Rotary kiln Charcoal stack
Mean values benz(a)pyrene, mg kg−10.16 0.05 0.43 9.45 1.34 0.73
Possible biochar addition (kg m−2 soil)
for soil with humus contents <8%
365.63 1170 273 12.38 117 161
Possible biochar addition (kg m−2 soil)
for soil with humus contents >8%
1218.75 3100 910 41.27 390 537.93
† Hydrothermal carbonization.
Fig. 10. Scatterplot of canonical scores as functions of black carbon content showing
Brunauer–Emmett–Teller (BET) surface area and polycyclic aromatic hydrocarbons
(PAHs) de ned by the USEPA contents. HTC, hydrothermal carbonization.
1012 Journal of Environmental Quality
the stability of biochar against degradation in soil. As guide-
line values, we recommend an H/C ratio <0.6 and an O/C
ratio <0.4 as well as BC concentrations >15% based on C. For
a good biochar performance in soil, the surface area should
be >100 m2 g−1, and the PAH contents should lie within the
thresholds of national or international regulations. Standards
concerning biochar production processes would be help-
ful for designing the appropriate product for application to
soil. Furthermore, our sample set consisted of biochars from
di erent production processes as well as feedstocks. Further
research is necessary to better separate biochar characteristics
due to feedstocks and production technique.
Acknowledgments
We thank Priyanka Pandey for help with the laboratory work. e
German ministry for education and research (BMBF Pyreg project
01LY0809F) is acknowledged for nancial support of our biochar studies.
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