Activated Carbon, Biochar and Charcoal: Linkages and Synergies across Pyrogenic Carbon’s ABCs

Abstract

Biochar and activated carbon, both carbonaceous pyrogenic materials, are important products for environmental technology and intensively studied for a multitude of purposes. A strict distinction between these materials is not always possible, and also a generally accepted terminology is lacking. However, research on both materials is increasingly overlapping: sorption and remediation are the domain of activated carbon, which nowadays is also addressed by studies on biochar. Thus, awareness of both fields of research and knowledge about the distinction of biochar and activated carbon is necessary for designing novel research on pyrogenic carbonaceous materials. Here, we describe the dividing ranges and common grounds of biochar, activated carbon and other pyrogenic carbonaceous materials such as charcoal based on their history, definition and production technologies. This review also summarizes thermochemical conversions and non-thermal pre- and post-treatments that are used to produce biochar and activated carbon. Our overview shows that biochar research should take advantage of the numerous techniques of activation and modification to tailor biochars for their intended applications.

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Review
Activated Carbon, Biochar and Charcoal: Linkages
and Synergies across Pyrogenic Carbon’s ABCs
Nikolas Hagemann 1, *ID , Kurt Spokas 2ID , Hans-Peter Schmidt 3ID , Ralf Kägi 4,
Marc Anton Böhler 5and Thomas D. Bucheli 1
1Agroscope, Environmental Analytics, Reckenholzstrasse 191, CH-8046 Zurich, Switzerland;
thomas.bucheli@agroscope.admin.ch
2United States Department of Agriculture, Agricultural Research Service, Soil and Water Management Unit,
St. Paul, MN 55108, USA; kurt.spokas@ars.usda.gov
3Ithaka Institute, Ancienne Eglise 9, CH-1974 Arbaz, Switzerland; schmidt@ithaka-institut.org
4Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department Process Engineering,
Überlandstrasse 133, CH-8600 Dübendorf, Switzerland; ralf.kaegi@eawag.ch
5Eawag, Swiss Federal Institute of Aquatic Science and Technology, Application and Development,
Department Process Engineering, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland;
marc.boehler@eawag.ch
*Correspondence: nikolas.hagemann@agroscope.admin.ch; Tel.: +41-58-462-1074
Received: 11 January 2018; Accepted: 1 February 2018; Published: 9 February 2018
Abstract:
Biochar and activated carbon, both carbonaceous pyrogenic materials, are important
products for environmental technology and intensively studied for a multitude of purposes. A strict
distinction between these materials is not always possible, and also a generally accepted terminology
is lacking. However, research on both materials is increasingly overlapping: sorption and remediation
are the domain of activated carbon, which nowadays is also addressed by studies on biochar.
Thus, awareness of both fields of research and knowledge about the distinction of biochar and
activated carbon is necessary for designing novel research on pyrogenic carbonaceous materials.
Here, we describe the dividing ranges and common grounds of biochar, activated carbon and other
pyrogenic carbonaceous materials such as charcoal based on their history, definition and production
technologies. This review also summarizes thermochemical conversions and non-thermal pre- and
post-treatments that are used to produce biochar and activated carbon. Our overview shows that
biochar research should take advantage of the numerous techniques of activation and modification to
tailor biochars for their intended applications.
Keywords:
activated charcoal; pyrolysis; activation; modification; pyrogenic organic matter;
adsorbent; negative-emission technology
1. Introduction
Both biochar and activated carbon are pyrogenic carbonaceous materials (PCM). They are
produced by thermochemical conversion of carbonaceous feedstock (pyrolysis or/and activation).
Biochar is produced from sustainably sourced biomass and is used for non-oxidative applications in
agriculture (e.g., in soil) and is also discussed as a raw material for industrial processes. By definition,
it is used for carbon sequestration [
1
,
2
]. Hence, if “biochar” is used as a fuel, i.e., it is burned and the
carbon is transformed (oxidized) into CO
2
, it is actually classified as charcoal. Activated carbon is
produced from any carbon source (fossil, waste or renewable) and engineered to be used as sorbent to
remove contaminants from both gases and liquids [
3
,
4
]. Thus, it is defined as a material for contaminant
sorption without exigencies in regard to the sustainability of its production nor to the fate of the carbon
after its use. Both materials have their distinct history, widely separated scientific communities and
Water 2018,10, 182; doi:10.3390/w10020182 www.mdpi.com/journal/water

Water 2018,10, 182 2 of 19
separated bodies of literature. Unfortunately, a generally accepted terminology and definition is
lacking [1,4].
However, as the proposed applications of biochar and activated carbon increasingly overlap,
awareness of the “other” domain in each case can be beneficial. As an example, nowadays both biochar
and activated carbon are used for soil remediation, which before has been solely an application of
activated carbon [
5
,
6
]. When the activated carbon is not removed after the application and if this
activated carbon was produced from renewable feedstock and is complying to further
specifications [2,7]
,
it can be considered as biochar. Moreover, there is an increasing need for specialized sorbents
in environmental technology including waste-water treatment and soil remediation, resulting in
numerous studies on biochar-based materials that may replace conventional activated carbon [
8
,
9
].
Thus, the aim of this review is to provide an overview on definitions, uses and production of biochar
and activated carbon and related pyrogenic carbonaceous materials to highlight their similarities
and differences.
2. Pyrogenic Carbonaceous Materials (PCM): Brief History and Definitions
Pyrogenic carbonaceous material is defined by Lehmann and Joseph (2015) [
1
] to describe
“all materials that were produced by thermochemical conversion and contain some organic C”. For this
chapter, we follow broadly both their definition of PCM as the umbrella term as well as their definitions
of the different classifications of PCMs.
All carbonaceous materials ultimately originate from biomass. Coal results from coalification, i.e.,
a geological process after biomass is covered with water and sediment. Peat and lignite are intermediate
stages of this process. Charcoal, biochar and activated carbon are products of thermochemical processes
and defined thus as PCM. Thus, the latter three materials are similar with respect to elemental
composition and prevailing chemical bonds, but each has its own history, distinct properties and,
especially, specific applications (Figure 1). In addition to the above materials, natural pyrogenic matter
and other solid products obtained by pyrolysis classify as PCM. This review, however, does not cover
carbonaceous matter produced in liquid phase, such as the products of hydrothermal carbonization
(hydrochar) or liquid crystals; or in gas phase, such as soot, carbon black and carbon nanotubes [
3
].
Still, charcoal, biochar and activated carbon contain particles (e.g., condensates) that can be classified
as soot [
1
]. There is increasing research interest in hydrochar that is suggested as a biofuel, as an
adsorbent in soil and water, as well as a catalyst or for energy storage [
10
]. Thus, there is a considerable
overlap of potential applications of hydrochar with those of biochar, activated carbon and charcoal.
However, due to fundamental differences in history, production process and technology as well as
both physical and chemical structure, hydrochar is not discussed in this review [10,11].
2.1. Natural Pyrogenic Organic Matter, Black Carbon and Char
Natural pyrogenic organic matter (PyOM) [
12
] also termed “pyrogenic carbon in soils” [
13
] is
a ubiquitous constituent of soil organic matter and originates from vegetation fires (e.g., forest or
prairie fires). PyOM represents a relevant pool for the global carbon cycle and has similar effects as
anthropogenic addition of PCM to soil, i.e., biochar [12–15].
Char is a material that results from incomplete combustion of natural or anthropogenic origin
and is sometimes used synonymously for PyOM [1,15].
Black carbon is probably the most common term for PCM dispersed in the environment across
the atmosphere, water bodies, soils and sediments. It may be of anthropogenic (combustion of fossil
fuels—soot) or natural (vegetation fires) origin [1,16].

Water 2018,10, 182 3 of 19
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Figure 1. Overview on feedstock (orange), treatments for production (blue) and applications (grey) of
pyrogenic carbonaceous materials (PCM). This figure intends (1) to highlight the numerous
possibilities for combining thermochemical conversions both with each other as well as with non-
thermal pre- and post-treatments; (2) to provide an overview on feedstock and applications; and (3)
to answer the question if a PCM can be classified as a biochar [1,2], which depends on the intended
application (non-oxidative for carbon sequestration), sustainable production (sustainably sourced
biomass, no fossil feedstock) and chemical properties (carbon content e.g., >50% [2] or >10% [7], and
a low content of pollutants as detailed in the guidelines of the European Biochar Certificate [2] or the
International Biochar Initiative [7]). This figure does not aim to provide a full list of potential feedstock
or applications.
2.2. Charcoal
Charcoal is defined as carbonized wood used mainly as fuel or as reductant in industry, e.g., to
reduce oxidized iron ores in iron and steel production. The production of gunpowder containing 15%
charcoal was the principal driver for research on charcoal [17] This included trials on different types
of wood as feedstock and on production technologies overall aiming at low ash contents to enable a
clean combustion. Charcoal was and is mainly used as a smokeless fuel for cooking, heating and steel
production [15]. It can be considered as the oldest chemical product, as already e.g., the Tyrolian ice
man was tattooed, potentially for medical purposes, by using charcoal as a pigment more than 5000
years ago [18,19]. However, medical use of charcoal is nowadays included in discussions about
biochar.
According to the United Nations Food and Agriculture Organization (FAO) [20], 50 million tons
of wood charcoal were produced worldwide in 2015, which is three times more than in 1961.
Approximately 25% of globally harvested wood is used for charcoal production; however, the FAO
admits that there is large uncertainty in this data [21]. Two thirds of the production is located in
Africa [20,22], where it is an important commodity that may even be subject to trade bans if controlled
by large criminal organizations to finance their illegal activities as e.g., in Somalia [23]. Across the
African continent, charcoal is still an important domestic fuel, but it is also exported e.g., to Europe,
Figure 1.
Overview on feedstock (orange), treatments for production (blue) and applications (grey) of
pyrogenic carbonaceous materials (PCM). This figure intends (1) to highlight the numerous possibilities
for combining thermochemical conversions both with each other as well as with non-thermal
pre- and post-treatments; (2) to provide an overview on feedstock and applications; and (3) to
answer the question if a PCM can be classified as a biochar [
1
,
2
], which depends on the intended
application (non-oxidative for carbon sequestration), sustainable production (sustainably sourced
biomass, no fossil feedstock) and chemical properties (carbon content e.g., >50% [
2
] or >10% [
7
], and a
low content of pollutants as detailed in the guidelines of the European Biochar Certificate [
2
] or the
International Biochar Initiative [
7
]). This figure does not aim to provide a full list of potential feedstock
or applications.
2.2. Charcoal
Charcoal is defined as carbonized wood used mainly as fuel or as reductant in industry, e.g.,
to reduce oxidized iron ores in iron and steel production. The production of gunpowder containing
15% charcoal was the principal driver for research on charcoal [
17
] This included trials on different
types of wood as feedstock and on production technologies overall aiming at low ash contents to enable
a clean combustion. Charcoal was and is mainly used as a smokeless fuel for cooking, heating and steel
production [
15
]. It can be considered as the oldest chemical product, as already e.g., the Tyrolian ice man
was tattooed, potentially for medical purposes, by using charcoal as a pigment more than 5000 years
ago [18,19]. However, medical use of charcoal is nowadays included in discussions about biochar.
According to the United Nations Food and Agriculture Organization (FAO) [
20
], 50 million
tons of wood charcoal were produced worldwide in 2015, which is three times more than in 1961.
Approximately 25% of globally harvested wood is used for charcoal production; however, the FAO
admits that there is large uncertainty in this data [
21
]. Two thirds of the production is located in
Africa [
20
,
22
], where it is an important commodity that may even be subject to trade bans if controlled

Water 2018,10, 182 4 of 19
by large criminal organizations to finance their illegal activities as e.g., in Somalia [
23
]. Across the
African continent, charcoal is still an important domestic fuel, but it is also exported e.g., to Europe,
where it is almost exclusively used for barbeques, because in industrial processes it was mostly replaced
by fossil coal, which is cheaper [
24
]. In Brazil, however, charcoal is still used in metallurgy due to its
low ash content [
25
] and lower local price compared to coal. Other uses include the production of
sodium cyanide, carbon disulfide or silicon, as a filling compound for bottled gas, as lining of molds
in metal foundries and for steel hardening, as a cementation granulate, and for pyrotechnics and
explosives as well as electrodes and batteries [26].
Charcoal can be produced in industrial retorts that allow the use of side products (wood vinegar,
acetic acid, smoke flavors) and state-of-the-art treatment of exhausts [
26
]. However, often it is still
produced using very basic technologies like earth mounds that do not treat exhaust and thus are a
concern to health and the environment due to emissions of carbon monoxide, methane, fine particulate
matter and volatile organic compounds [15,21,25].
2.3. Activated Carbon
Activated charcoal was initially defined as “any form of carbon capable of adsorption” [
27
].
Historically, it began with the use of charcoal as a sorbent being traced back to both the Roman
and Chinese Empire, and potentially even further. The Romans realized that charcoal can purify
water; a property we still utilize. However, despite this long history of charcoal’s use for purification,
it took humans over 3000 years to optimize charcoal for the removal of specific contaminants. In 1863,
Smit [
28
] observed that charcoal removes oxygen from air for an extended period of over a month.
However, not all charcoals had the same capacity, with animal bone-based charcoal having a higher
capacity than wood-based charcoals. This was the starting point for a quickly growing field of research
about differences and characteristics of charcoals that result in different sorption properties.
It was noticed early in the 19th century that charcoal also contained a variety of sorbed organic
compounds [29]. Thereby, the first “activation” strategies were aimed at reducing absorbed chemical
species after production. Chaney (1919) [
30
] considered thermal activation to be the only technique
capable of removing the adsorbed organic compounds from the surface of the charcoal. The charcoal
skeleton remains and new surface area (pores) are exposed. Thus, chemical groups become “active”,
i.e., available for sorption. With this, the science of charcoal activation and activated carbon began.
The first patents for charcoal activation were granted in the 1920s [
31
–
33
]. However, there was
little theoretical grounding explaining the observations of increased sorption capacities. Several of
the pioneering works were largely empirical and focused on different activation temperatures as
well as different chemicals, but were lacking a focused hypothesis or a mechanism. It was generally
assumed that the increased sorption was due to the increased surface area (and maybe even new
pores) [
34
]. Activation in the 1950s was further defined as “any process which selectively removes
the hydrogen or hydrogen-rich fractions from a carbonaceous raw material in such a manner as to
produce an open, porous residue” [
35
]. Chemical and structural alterations leading to increased surface
areas and tailored modifications of chemical moieties are still the key elements of activation today.
However, also important are the secondary traits of mechanical strength, total porosity, surface charge,
and diffusivity [36].
Theoretical studies were initiated by Wright [
37
] studying bromine sorption on charcoal and
activated charcoal. A comparable bromine sorption capacity was observed before and after activation.
The mechanism of bromine sorption had changed; from anion substitution prior to activation to
chemical addition reactions to carbon after thermal activation. It was hypothesized that the activation
increased the presence of unsaturated C-bonds. Focusing on this mechanism, Lowry [
38
] observed
in 1930 that the sorption capacity of activated charcoal was correlated to the total hydrogen content
(presence of unsaturated/non-aromatic C-bonds). With the advent of new instrumentation to
investigate crystalline structures, Ruff and colleagues [
39
] provided evidence that, in addition to
changes of the surface chemistry, distinct alterations in the physical structure occur. They observed

Water 2018,10, 182 5 of 19
small, isolated fragments of graphite and the activated charcoal lacked the semi-continuous graphite
sheet character that was present prior to activation.
There are several complicating factors that hamper the identification of the sorption mechanisms.
A first factor is that charcoal is being altered, even at ambient conditions. Shelton [
40
] noted in 1920 a
three-fold change in the sorption capacity for N
2
, following storage at laboratory conditions. A second
factor is that even the inorganic precipitated salts on the charcoal (ash content) contributed to sorption
processes [41].
Aside from sorption, activated carbon also can act as a catalyst, e.g., in anaerobic reduction of
azo dyes, and supports the formation of radicals to oxidize dyes or pollutants in an (electrochemical)
oxidative treatment of industrial effluents [
42
,
43
] and is further used for electrodes in microbial fuel
cells [43,44].
Today, there are several markets that are increasing the global demand for activated carbon.
These include drinking-water conditioning, energy-storage technology as well as environmental
technology driven by new legislation (e.g., imposing clean-up requirements for mercury emissions
from power plants) [
45
]. The annual net value of the activated carbon market is estimated at $3.0
billion (USD) [
45
]. Powdered activated carbon accounts for approximately 50% of the total activated
charcoal market in 2015 [
45
]. Granular activated carbon (GAC) was approximately 30% of the total
market (removal of volatile organic compounds (VOCs) and chlorine), with polymer-coated, extruded,
impregnated-activated carbons accounting for the remaining 20% [45].
2.4. Biochar
The discussion on biochar originates mainly in the research about the highly fertile anthropogenic
soils rich in pyrogenic organic matter such as the Amazonian dark earth (also called “Terra Preta
do Indio”). Here, charred biomass is considered as a key agent transforming poor soil into highly
productive agroecosystems [
46
,
47
]. Thus, biochar was originally defined as carbonized biomass
(in early publications, also still called “charcoal” [
48
]) used for agricultural purposes. While the initial
debate focused on the direct application of biochar to soil, nowadays the so-called cascade use of
biochar describes it as a multifunctional material that can be used to address several challenges within
a life cycle (Figure 1) [
49
,
50
]. For example, biochar can be used as a feed supplement to improve the
health and productivity of e.g., ruminants or poultry [
51
]. When the fed biochar is excreted again with
the animal feces, it reduces odors and nutrient losses from the manure, both in fresh form, as well
as during its composting. Eventually, when the biochar containing manure or compost is applied to
soil, biochar serves as a slow-release fertilizer [
52
]. Also, biochar is used in biodegradable packaging
materials to extend the shelf-life of fruits and vegetables, eventually improving soil quality after
co-composting the packaging boxes [50].
Recently, the application of biochar as an amendment in concrete has been suggested to reduce
the amount of cement [
53
] or sand [
54
] needed. Furthermore, recent studies investigate the use of
biochar as a raw material for electrodes in microbial fuel cells [
55
] or for new types of supercapacitors,
which requires post-pyrolysis treatment (modification) of the initial biochar [56].
Carbon sequestration is either a direct goal or indirect positive outcome of all biochar applications.
Applied to soil or to concrete, biochar is stable for decades to centuries [
57
,
58
] and thus stores
carbon that was previously removed from the atmospheric carbon pool by photosynthesis. Biochar
amendment to soil is, therefore, discussed as one of the most promising negative emission
technologies [59,60].
Werner et al. (2017) [
61
] suggested pyrogenic carbon capture and storage (PyCCS) as a more
sustainable and carbon-efficient method than conventional biomass-energy carbon capture and storage
(BECCS—capturing and geological storage of CO
2
derived from burning of biomass). They calculated
that the 1.5
◦
C goal as specified in the Paris Agreement [
62
] can be achieved through plantation-based
PyCCS, but would require the conversion of natural vegetation to biomass plantations in the order of
146–3328 Mha globally.

Water 2018,10, 182 6 of 19
2.5. Other Solid Products Obtained from Pyrolysis
Pyrogenic carbonaceous materials can be produced from all kind of feedstock beyond biomass or
fossil carbon sources. Here, we briefly introduce products obtained from thermal treatment of bones,
sewage sludge and scrap tires. Due to low carbon content, products obtained from bones and sewage
sludge cannot be classified as activated carbon or biochar by strict definition [
2
,
4
], but these materials
may be used as sorbents and/or soil amendments. However, for sewage sludge and especially for
scrap tires there are also considerable sustainability concerns, especially the high content of heavy
metals that would not allow them to be classified as biochars [2].
Bone char, also called spodium [
4
] or bone black or animal char [
63
], is a blackish material obtained
from the thermal treatment of bones, that only contains approximately 11% carbon, but up to 78%
calcium phosphate [
4
]. It was first mentioned in the United Kingdom in 1815 [
63
] and its primary
use was in the sugar industry for refining the sugar during its production process. For this purpose
oxidative activation was not necessary, as the calcium phosphate of the bone turns into a highly porous
structure already during carbonization [
4
]. Still, bone char can also be activated or modified by various
techniques [
64
]. Today, lab-based studies investigate amongst other things the suitability of bone char
for the removal of arsenic [
65
] and cadmium [
66
] from water. Other studies suggest its use as a soil
amendment, especially as a source of phosphorous [67].
Sewage sludge is a relevant resource. In Germany, for example, 2 million metric tons of dry
matter sewage sludge are produced annually [
68
], which equals approximately 20–25 kg per capita and
contains 3.5% nitrogen and 5.5% phosphorous (P
2
O
5
). Due to hygienic concerns and its considerable
content of both organic and inorganic contaminants, its direct application in agriculture is increasingly
restricted by legislation. Pyrolysis of sewage sludge and biochar-like application e.g., to paddy soil
is suggested as a promising management practice, which eliminates the organic contaminants in the
sewage sludge by thermochemical conversion, reduces the availability of selected heavy metals and
promotes plant growth. However, these finding are based on a greenhouse study, and long-term effects
need further investigation [
69
]. Also, sewage sludge can be used as a feedstock for the production of
activated carbon [70–72].
Scrap tires are a considerable environmental issue, with the European Union (EU), United States
and Japan together producing about 6 Gt of this complex, non-recyclable waste [
73
]. There is intensive
research on scrap-tire pyrolysis that mainly aims at high yield of liquids and gases for fuel and energy
generation [
74
,
75
]. The solid residue can be used as a precursor for activated carbon. Despite its high
carbon content, it cannot be defined as a biochar, as it is not produced from biomass and contains e.g.,
up to 2% of Zn.
3. Producing Pyrogenic Carbonaceous Materials: Thermal Conversions and Additional
Treatments
Pyrogenic carbonaceous materials are produced by thermo-chemical conversion of
carbon-containing precursors. Thermo-chemical conversions are defined based on temperature,
duration and the presence or absence of oxidants and classified as either pyrolysis, gasification
or activation. These classifications may be further distinguished in sub-classifications [25].
In a nutshell, pyrolysis and gasification result in a solid residue as the desired main product
(pyrolysis) or as a desired or often undesired side product (gasification) that is enriched in carbon by
removing hydrogen, oxygen and other elements. Activation is the optimization of sorption capacity by
increasing the specific surface area, which can be achieved either by direct activation (one step) or by
an activation after pyrolysis (two steps) [
3
,
4
]. Modification is the introduction of non-carbon moieties
to the surface of carbonaceous materials to improve their sorption capacity for specific sorbates [
76
].
Modification can be done after or instead of activation.

Water 2018,10, 182 7 of 19
3.1. Pyrolysis and Gasification
In the literature, the term pyrolysis is used to describe the production of charcoal, biochar and
precursors for activated carbon. Carbonization is used alternatively and exchangeable.
By definition, pyrolysis is the thermo-chemical treatment of a feedstock in the strict absence of
oxygen or any other additional oxidant [
25
]. Still, partial oxidation might happen e.g., due to the
inherent content of oxygen in biomass or lack of air-tightness e.g., in traditional kilns of mounds.
So-called fast pyrolysis (sometimes called “flash pyrolysis”) and intermediate pyrolysis aim at
the production of a mainly liquid product within less than 30 s. This is also often carried at lab scale
for mechanistic studies and in combination with analytical chemistry (e.g., mass spectrometry) to
investigate the pyrolytic behavior of different feedstock during thermochemical conversion [77].
Slow pyrolysis takes minutes to days and aims mainly at a solid product, e.g., biochar or charcoal.
If slow pyrolysis is performed at low temperatures (<300
◦
C), the process can also be called torrefaction,
which is so far mainly used to increase the caloric value before combustion for energy production or as a
preparatory step for downstream chemical treatment of biomass. At higher temperature (300–900
◦
C),
slow pyrolysis can also be called carbonization [25].
Gasification is defined as thermo-chemical treatment optimized for the yield of gaseous fuels
that are mainly used for the generation of heat and electricity [
25
]. This is achieved by the presence
of oxygen during the high-temperature (>800
◦
C) process at stoichiometric rates of 0.15 to 0.28 of
supplied oxygen to carbon present in the feedstock [78].
In practice, it is often difficult to categorize actual technologies according to these definitions
(Table 1). The conditions of pyrolysis of e.g., biochar widely define their resulting properties and allow
the design of biochars to address specific challenges such as soil quality [79].
Table 1.
Parameters of thermal conversion processes of biomass. Not all parameters are strictly in line
with the definitions stated in the text. Note that these are only examples that do not aim to provide an
extensive overview.
Process Temperature Range
[◦C]
Heating Rate [◦C
s−1]
Vapor Residence
Time Primary Product
Slow pyrolysis
550–950 [80]
600 [81]
500 [82]
“low-moderate” [83]
400 [84]
350–700 [85]
0.1–1 [80]
“low” [81]
1–100 [85]
5–30 min [81,82]
“long” [83]
Hours–days [84]
Hours [85]
35% Char [82–84]
15–40% Solid,
20–55% Liquid,
20–60% Gas [85]
Fast and
intermediate
pyrolysis
850–1250 [80]
650 [81]
500 [82–84]
450–550 [85]
10–200 [80]
“very high” [81]
>1000 [85]
0.5–5 s [81]
1s[82]
<2 s [83]
<1 min [85]
bio-oil [81]
75% liquid [82–84]
50–70% liquid [85]
Gasification
>750 [82]
>800 [83]
750–900 [84]
>800 [85]
Variable [85]
10–20 s [82]
“long” [83]
Seconds–minutes [85]
85% Gas [82–84]
90–100% Gas [85]
3.2. Chemical Activation
To yield a chemically activated PCM, feedstock material (coal, lignite, peat or biomass) is mixed
(impregnated) with concentrated aqueous solutions of an activation agent (Table 2), of which ZnCl
2
,
KOH and H
3
PO
4
are the most common ones on industrial scale [
3
]. Feedstock-to-agent ratios (degree
or coefficient of impregnation) are typically in the range of 1:0.5 to 1:3 based on dry matter. The effect
of the activation agent increases with increasing dose. Thus, the degree of impregnation can be seen as
an analog to the magnitude of burn-off in physical activation as discussed below [
4
,
76
,
86
]. However,
the extent of activation is also defined by the selection of the chemical itself, the intensity of the

Water 2018,10, 182 8 of 19
mixing, and the temperature and the duration of the subsequent activation [
3
]. At high chemical
concentration or excess reaction time, the pore volume decreases due to the physical collapse of
the carbon structure [
86
]. Mixtures often are kneaded thoroughly to initiate the degradation of the
feedstock. ZnCl
2
applied to wood sawdust results e.g., in a brown plastic paste [
4
]. This mass is
then activated, i.e., it is heated in the absence of oxygen (pyrolysis). Temperatures can be lower than
for physical activation (600–700
◦
C for ZnCl
2
[
4
]) but KOH for example still requires temperatures
above 850
◦
C [
3
]. Finally, the product must be washed to remove the activation agent, which often
can be reused or recycled, although the activation agent can partially volatilize during the activation
process [
4
]. Thus, chemical activation at industrial level demands considerable effort in the treatment
of exhaust and washing effluent to minimize its environmental impact, which in turn increases the
costs of this process.
Although the mechanisms of activation vary between activation agents, there are common
principles [
3
]: already during the intensive mixing, the original structure of the feedstock starts to
degrade. In plant biomass, bonds between cellulose molecules loosen and ions of the activation agent
occupy the resulting voids and thus define the microporosity created during the following activation
that becomes available after washing of the activated carbon. Furthermore, the activation agents avoid
the formation of tar and thus prevent the clogging of pores.
Table 2. Typical agents used for chemical activation of biomass or carbonaceous materials.
Activation Agent References
ZnCl [87–90]
FeCl3[91,92]
H2SO4[71,87,93–95]
H3PO4[96–106]
HCl [107]
HNO3[108]
NaOH/KOH [107,109–124]
Na2CO3/K2CO3[102,119,125,126]
Urea [111]
3.3. Physical Activation
Physical or thermal activation is the partial oxidation of a carbonaceous precursor or intermediate
material to increase its porosity. Physical activation is the oldest method of preparing activated
carbon, where the term “physical” stems from the early misconception that steam would only
physically volatilize and remove condensates from charcoal. However, at high temperatures
(>750 ◦C)
,
H
2
O becomes an oxidant for carbonaceous materials (Table 3). Thus, “physical” activation is also
based on oxidative reactions i.e., chemical processes.
Looking at physical activation performed as a second step after carbonization, some authors
distinguish two stages of activation: during the first stage of activation, disorganized (amorphous)
carbon-like tar is burnt (oxidized) with the effect that clogged pores open. During the second stage,
parts of the carbon crystallites are also oxidized [
4
]. The first stage primarily increases the specific
surface area; the second stage both further increases the specific surface area by creating new (micro) pore
space or creating new interconnections between pores, but also changes the surface chemistry [
127
,
128
].
Oxidized functional groups including phenolic, ketonic and carboxylic groups are created on the
aromatic surfaces of the carbonaceous material [3,4].

Water 2018,10, 182 9 of 19
Table 3.
Activation of carbonaceous materials by oxidative gases (“physical activation” or “thermal
activation”).
Oxidant Idealized Reaction [3,4] Energy Balance [3] References
Steam/H2OC+H2O→CO + H2
2C + H2→2C(H) 1endothermic [72,95–97,112,118,120,
124,125,129–158]
CO2
C + CO2→C(O) +CO 1
C(O) →CO
sum: C + CO2→2CO
endothermic [106,129,133,139,140,145,
153,159]
Air/O2C+O2→CO2exothermic [104,105,114,119,139,158,
160–165]
Note: 1C(H) and C(O) refer to surface hydrogen and oxygen complexes (chemisorbed hydrogen/oxygen) [3].
The degree of physical activation can be characterized by the burn-off, i.e., the mass loss during
activation [
4
,
76
,
86
]. In general, higher burn-off correlates with higher porosity of the product. However,
burn-off above 40–50% may result in a net destruction of porosity as the external burning of the
carbonaceous materials may outweigh the formation of porosity [128].
Oxidation by limited amounts of air was used, for example, to modify the properties of
biochar by Suliman and colleagues [
164
]. They used low-temperature physical activation by oxygen
(“post-pyrolysis oxidation” at 250
◦
C) to optimize biochars for their application to increase soil water
retention. Xiao and Pignatello used a similar technique at 400
◦
C [
165
]. However, the use of oxygen
or air results in rapid reactions, which may hinder a controlled production process. Thus, for the
production of activated carbon with a higher burn-off, often steam (H
2
O) or CO
2
are preferred. Here,
chemisorbed oxygen and hydrogen (C(O) and C(H); chemical equations in Table 3) on the carbonaceous
surface inhibit the oxidation and thus slow down the activation process [
3
]. C(O) is an intermediate
of the activation using CO
2
, and C(H) is a byproduct due to the formation of hydrogen during
steam activation.
The porosity of activated carbon depends on a multitude of factors including feedstock,
temperature, duration of the activation process, and the choice of the oxidant. Oxidation with CO
2
first
results in the opening of new pores. Steam activation, however, widens existing microporosity quite
quickly. Thus, CO
2
predominantly supports the creation of microporous materials and steam-derived
activate carbons rather show a wider pore-size distribution [128].
Aside from H
2
O, CO
2
and O
2
, also chlorine, ammonia, sulphur and sulphur dioxide can be used
as agents for physical activation, although they are rarely used [
4
]. The use of ammonia or sulphur
dioxide can be also considered as a modification, as discussed below.
3.4. Modification
Aside from the porous morphology, the chemistry of the surface defines the sorption
characteristics of an activated carbon [
127
]. Modifications aims at the optimization of the surface
chemistry toward the sorption of specific sorbates. Rivera-Utrilla [
76
] defines the categories of
oxidation, sulfuration, nitrogenation and coordinated ligand functionalization treatments.
Oxidative modification of carbonaceous materials is not the same as activation, as the oxidation is
carried out by hydrogen peroxide (H
2
O
2
) [
108
,
160
] or nitric acid (HNO
3
) at low temperatures (ambient
to 100
◦
C) and mainly aims at the creation of oxidized functional groups and not at an increase in
surface area. In fact, HNO
3
is especially known to decrease the specific surface area and the pore
volume due to the destruction of the porous structure [
76
]. Oxidation of activated carbon was reviewed
in depth by Daud and Houshamnd [
127
]. Oxidative modification with ozone can also be considered as
physical/thermal activation.
Sulfurization by treatment with SO
2
or H
2
S also may have a destructive effect on porosity but
may, for example, considerably increase the sorption capacity for mercury [76,166].

Water 2018,10, 182 10 of 19
Nitrogenation or nitrogen modification of activated carbon is mainly achieved by treatment with
ammonia (NH
3
) in a gaseous or dissolved state to increase polarity and basicity in order to optimize
sorption, but also the catalytic properties of the activated carbon [
76
,
167
]. For biochars, for example,
Yang and Jiang used nitration with subsequent reduction to improve the sorption of copper [168].
Functionalization with coordinated ligands mainly aims at the optimization of metal sorption on
activated carbon, although it seems that it could easily be adapted to optimize, for example, nutrient
carrier behavior of biochar for soil amendment. For this purpose, activated carbons are treated with
complex organic liquids. These treatments can be performed with a multitude of agents and conditions
as reviewed by Rivera-Utrilla [76].
3.5. Non-Thermal Pre- and Post-Treatments
Aside from thermal treatments, there are numerous other steps that may be part of the production
process of biochar or activated carbon (Figure 1). Prior to thermal treatments, feedstock can be blended
to optimize the conditions for downstream pyrolysis and/or to optimize the resulting pyrogenic
carbonaceous material. Moist and dry feedstock can be blended to optimize moisture content and
energy density to avoid the necessity for a drying process, for example by blending manures with
straw [
169
]. Also, biomass can be treated with metal salts to create biochars optimized, for example
for the sorption of arsenic [
170
]. In contrast to chemical activation, the dosage of salts to the biomass
feedstock are considerably lower and the metal salt is applied not as a catalyst, but to provide specific
sorption sites in the final product. However, potential interactions of the intended functions should
still be considered when planning such experiments.
After thermal treatments, the resulting pyrogenic carbonaceous material can be impregnated, for
example with metals, to create additional sorption sites, e.g., by precipitating iron(oxy)hydroxides on a
biochar (inorganic impregnation) to optimize its specific sorption e.g., of arsenic [
171
]. The results may
be similar to metal-blended biochars, although the production process differs considerably (addition of
metal before or after thermal treatment). Recently, the formation of an organic coating was described
on biochar as the result of the interaction of natural organic matter with biochar, which allows the
optimization of nutrient retention by biochar after production and prior to their use in soil [
172
–
174
].
The oldest post-thermal treatment is washing, for example of activated carbon, to remove residual
condensates and ashes in order to further optimize sorption [
142
] or to simply remove the agents of
chemical activation [4].
In essence, there are numerous potential combinations of pre- and post-thermal treatments with
pyrolysis, activation and/or modification. However, they should be selected with care: a lower
number of different production steps ease the repetition of a specific protocol, both for further scientific
purposes or potential large-scale application.
4. Conclusions and Future Research
This review provides an overview of definitions of biochar and activated carbon. We highlighted
that the strict distinction of biochar and activated carbon is mainly based on the final use of these
materials. We revealed that research on activated carbon and charcoal is much older than biochar
science and provides an immense source of information beneficial for the biochar community.
For example, partial activation with adapted protocols might help to “tailor” biochars for specific
applications. This may enable preparation of an organic coating on biochar, which was recently
identified as the key to explaining the capture and slow release of nutrients by co-composted biochar.
Sorption of organic molecules to form the organic coating is likely to be facilitated after a surface
oxidation during the early stages of composting [
172
], which could potentially also be achieved by
physical activation.
Looking at steam activation, we showed that the moisture of the feedstock is an important
parameter when producing biochar via slow pyrolysis. Feedstock moisture is not just a question of the
energy balance of biochar production, but has also the potential to define biochar properties. Although

Water 2018,10, 182 11 of 19
moisture can increase biochar yield at low temperatures [
175
], the formation of steam and subsequent
steam activation might result in lower yield and partially activated biochar in high temperature
production scenarios (>800 ◦C).
For comparative studies of biochar and activated carbon, both materials could be produced from
the same feedstock using different impregnation ratios and burn-off rates, respectively, producing
comparative series from non-activated to fully activated biochar/activated carbon. Such studies will
allow an overall economic evaluation, as there will be a trade-off of, for example, contaminant sorption
capacity (increases with increasing degree of activation) and yield (decreases with increasing degree of
activation).
Acknowledgments:
N.H., H.-P.S., R.K., M.B. and T.D.B. thank the Federal Office for the Environment, Switzerland
(FOEN/BAFU) for funding of the project “EMPYRION—Sustainable removal of organic micro-pollutants from
sewage using pyrolysis products”. USDA is an equal opportunity provider and employer.
Author Contributions:
The review was jointly designed and written by all authors. N.H. coordinated this process.
Conflicts of Interest: The authors declare no conflict of interest.
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