Biochar and its Application

Abhishek Kumar1, Tanushree Bhattacharya2,

1
Research Scholar, Department of Civil & Environmental Engineering, Birla Institute
of Technology, Mesra, Ranchi
2
Assistant Professor, Department of Civil & Environmental Engineering, Birla Institute
of Technology, Mesra, Ranchi.
Email- tbhattacharya@bitmesra.ac.in

Abstract

The boom in the field of science and technology was a blessing in disguise but lately it has
been found to be highly detrimental. Apart from the contamination of environment, issues of
climate change, fossil fuel depletion and excessive land degradation have come to the fore.
Pollutants, especially heavy metals, threaten our planet enormously with their everlasting
persistence in soil and the inevitable toxicity on organisms. Tremendous efforts have been
made to remediate contaminated soils. However, there is an imminent need of a potential
solution which can simultaneously deal with the various surmounting issues. Biochar
possesses properties of large surface area, high carbon sequestration, and surplus nutrient
content and even there is a potential to produce bio-oil and syngas. Therefore, biochar can be
used to reduce the bioavailability of pollutants, especially heavy metals, in soil, tackle the
issues of climate change, by sequestering carbon, and depleting fossil fuels via bio-oil and
syngas production. Further, biochar enhances soil productivity which can aid in dealing with
food security issues. It was therefore considered essential to acknowledge ourselves about
biochar, the properties its possesses, the mechanism used for capturing pollutants, its
advantages, disadvantages, the research done in relation with biochar and the scope for
research.

Introduction

The later part of the eighteenth century saw an unprecedented and drastic transformation in
the socio-economic lives of people living in the European continent. This complete process
later came to be known as Industrial Revolution. With further growth of science and
technology, it later spread to various parts of the world. Not only the pace of life increased,
but the comfort level enhanced significantly. Ideas metamorphosed into reality. Technology
became an essence of anything and everything what one could think of. It became an
inevitable part of life. But this inevitability had major fallouts. The negative consequences of
this growth and development begun coming to the fore. Major fallouts include the monster of
climate change and the pollution caused because of heavy metals. Climate change has already
affected the mother Earth drastically. On the other hand, none of the spheres of our
environment has remained immune from heavy metal contamination. Once the heavy metal
enters into the environment, it keeps getting transported from one sphere to the other and is

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bound to stay in the environment for a very long period of time. These toxic metals have
affected the plants and the animals thoroughly and have polluted our surroundings
excessively. Therefore, it becomes necessary to look for remediation techniques to not only
minimise the damage caused because of heavy metals but also tackle the issue of climate
change. In the recent years, biochar has remarkably been shown to contain the damage caused
because of heavy metal pollution and simultaneously deal with climate change.

What is biochar?

It is a carbonaceous product (carbon-neutral or even carbon-negative) obtained from the

thermochemical conversion of biomass in an oxygen-limited condition (International Biochar
Initiative, 2012). The biochar produced differs in terms of physical and chemical
characteristics based on the temperature that is used for pyrolysis and the type of feedstock
used. It can potentially be used to reduce the bioavailability and leachability of heavy metals
in contaminated soils. It is also able to improve the quality of the contaminated soil and has a
remarkable reduction in heavy metal uptake by various crops. Further it can be produced
from anything containing biomass thereby reducing waste. Recently it has been shown to
tackle climate change, produce bio-fuel and enhance soil productivity. Therefore, application
of biochar can potentially be a remarkable solution not just for the remediation of
contaminated soils, but even for tackling the major issues of climate change, fossil fuel
depletion and food shortage.

Properties of Biochar

Various studies have been conducted which show that the source of feedstock and heat
treatment temperatures are the two major factors that determine the physiochemical
properties of biochar.

Chemical properties

Atomic ratios- The process of biochar formation involves modifications in the chemical
structure, which mostly includes detachment of various functional groups. The hydrogen and
oxygen containing groups are released which decrease the respective ratios with carbon (the
O/C-ratio decreases at a higher rate as compared to the H/C ratio).

Elemental composition- Rise in reaction temperature causes an increase in carbon content

whereas hydrogen and oxygen contents decrease. Biochars produced at high temperatures
may have carbon contents higher than 95% and oxygen contents lesser than 5%. The
hydrogen content of wood varies between 5-7% which is reduced during pyrolysis to even
less than 2%. Correlation between the treatment temperature and the nitrogen content is not
clear enough in overall comparison.

Energy content- Because of the higher carbon content present in biochar, the energy content
increases with increase in temperature. The most significant increase is seen at temperatures
between 250-350°C. Temperatures of 700 °C lead to an increase in the energy content from

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15-20 MJ/kg for raw biomass to 30–35 MJ/kg for biochar (this energy content can be
compared to that of anthracite).

Fixed carbon and volatile matter- After removing the volatile components, the carbon
content that remains in the solid structure is called fixed carbon. The fixed carbon content of
raw biomass is in the range of 10–30%. It increases from 10 to 90% (or even more) at
temperatures of 700 °C, while the volatile matter decreases from 90 to 10% (or even less) for
the same temperatures.

Structural composition- It is composed of cellulose, hemicellulose, lignin, etc. depending

upon the biomass used for biochar preparation and pyrolysis temperatures involved. There is
near total decomposition of hemicellulose and the partial decomposition of cellulose and
lignin. Treatment beyond 650 °C decomposes almost all of the holocellulose (i.e. cellulose
and hemicellulose). On the contrary, the temperatures required for decomposing lignin are
several hundred degrees higher than that for holocellulose.

Functionality- There is detachment of various functional groups and there is simultaneous

release of oxygen and hydrogen. This leads to presence of lesser number of functional groups
and more of aromatic structures in biochars with low H/C-ratios (corresponding to a higher
degree of carbonization) than compared to low-temperature chars (Conti et al., 2014). This is
so because aromatic structures are highly thermodynamically stable. The maximum
aromaticity is achieved between 500-800 °C. Even the entire carbon of some biochars may be
bound in aromatic structures at this temperature range.

The functional groups, their type and amount, influence biochar’s alkalinity (and therefore
their ability to neutralize acids in soils). The partial detachment of functional groups (such as
carboxyl or hydroxyl groups) causes formation of unpaired negative charges which enables
them to accept protons. An increase in treatment temperature contributes to an increase in
alkalinity (Fidel et al., 2017).

pH value- A significant outcome of the increase in alkalinity is an increase in the pH-value of

biochar. Typically, raw biomass is slightly acidic or mildly basic with pH-values of 5-7.5.
The functional groups that get detached during pyrolysis are predominantly acidic in nature.
Their release renders the remaining solid more basic. Further, the relative content of the ash
(co-incidentally basic in nature) is increased during the process. Maximum achievable pH-
values could be in the range of 10-12 for temperatures above 500 °C.

Cation exchange capacity- The cation exchange capacity (CEC) is defined as the amount of
exchangeable cations (e.g. Ca2+, Mg2+, K+, Na+, NH4+) that a material can hold. It is an
outcome of negative surface charges which attract cations. The cation exchange capacity
therefore directly depends on the surface structure, where the functional groups provide
surface charges, and the surface area, that makes the surface charges accessible. The highest
cation exchange capacities pertain to biochars produced at relatively low temperatures since
sufficient amount of functional groups remain to provide negative charges.

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Ash content and composition- The ash content of biochar hugely depends upon the ash
content in the parent biomass. This varies greatly and depends not only on the type of
biomass, but also on the harvesting techniques considered. Water and volatile matter are
released but a large part of the ash remains in the solid product. The ash content increases
with increase in temperature. A comprehensive review on ash composition presented by
Vassilev et al. shows that the main components of biomass ash are SiO 2, CaO, K2O, P2O5,
Al2O3 and MgO.

Self-heating and degradation during storage- The process of carbonization results in change
in some of the properties of the biomass, which can influence the risk of spontaneous
combustion. The highly volatile content of untreated biomass, which causes self-heating, get
significantly diminished during carbonization, rendering the biochar thermal stability
(Dzonzi-undi et al., 2014). Also, it would be more resilient to spontaneous combustion than
untreated biomass. A moderate water content of biomass enables microbial activity, which is
also responsible for self-heating. Initially, the water content of biochar is very low. However,
the porous structure of biochar readily absorbs moisture from the surroundings, leading to an
increase in water content and this enables microbial activity (which also may contribute to
self-heating).

Physical properties

Density and porosity- While the weight-based energy density of biochar increases with the
treatment temperature, the bulk density (the volume specific weight of a bulk material in a
heap or pile) shows the opposite trend. During pyrolysis, porous biochar is left behind. The
higher the porosity, the lighter would be biochar per unit volume. Upon increasing the
pyrolysis temperatures, higher porosities in the final product is obtained.

Surface area- The total surface area of the biomass changes just like the porosity changes as
an outcome of the escaping volatile gases during the carbonization process. A large surface
area is connected to a number of other biochar properties (e.g. cation exchange capacity or
water holding capacity). There is a general increase with increase in temperatures. However,
after an initial increase, the surface area of biochars may decrease again at high temperatures
(800-1000 °C).

Pore volume and pore size distribution- Pores in biochar span several orders of magnitude
and can be classified into macropores (with a pore diameter of 1000–0.05 μm), mesopores
(0.05–0.002 μm), and micropores (0.05–0.0001 μm; Brewer et al., 2014). The total pore
volume increases with increase in temperature (Fu et al., 2012). The pore structure of
biochars contains numerous micropores (more than 80% of the total pore volume).

Hydrophobicity and water holding capacity- There are two important processes which occur
during pyrolysis and influence the hydro-properties of biochars: a decrease in functional
groups altering the material’s affinity to water and an increase in porosity that changes the
amount of water which can be adsorbed. Despite the fact that biochars have been studied
extensively, the interaction with water is not well understood and contradicting findings exist
in the literature studied (Chun et al., 2004 and Zornoza et al., 2016). Hydrophobicity is

4
caused because of surface functional groups, whereas water holding capacity depends on the
porosity of the biochar’s bulk volume. It is believed that increase in pyrolysis temperature
would result in even higher hydrophobic character of the biochar (since more number of polar
surface functional groups will be getting removed and the aromaticity would be increasing).
On the contrary, Biochars produced at high temp are expected to hold more water in their
porous structure as confirmed by experiments in Gray et al. (2014) and Zhang and You
(2013).

Mechanical stability- The mechanical stability of biochar (in general) is lower than that of the
parent biomass since structural complexity is lost during carbonization (Byrne et al., 1991).
However, it does not continuously decrease during pyrolysis. The mechanical stability
generally correlates inversely with the porosity (and therefore directly to the density) of the
biochar. Biochar with a high mechanical stability can be produced from feedstock with high
density and high lignin content.

Grindability- The change in mechanical stability during carbonization leads to a brittle

biochar with improved grindability compared to the parent material. Torrefaction of biomass
with a larger amount of hemicellulose (for e.g. agricultural residues) results in a better
grindability as compared to woody biomass.

Thermal conductivity and heat capacity- A higher density in biochar is generally associated
with a higher thermal conductivity. So the development of a porous structure causes a
decrease in thermal conductivity of biochars (when compared to their parent biomass). The
value of heat capacity depends on the temperature at which the measurement is carried out.
This was well demonstrated by Dupont et al., who had determined the heat capacity of woody
biomass at room temperature to be about 1300 J/(kgK) and that of the biochar produced at
500 °C to be around 1000 J/(kgK).

Electromagnetic properties- The electric properties significantly change throughout the

process of carbonization. The reduction in functional groups and the appearance of
conjugated double bonds cause an increase in conductivity for increasing pyrolysis
temperatures. Due to the electromagnetic shielding efficiency, biochar can be used as an
additive in various composite materials (e.g. building materials such as cement). Further,
their effectiveness in shielding against electromagnetic interferences is enhanced.

Production of Biochar

The production of biochar is mainly dependent upon two important factors- biomass used and
pyrolysis.

Biomass

In theory, all types of organic material can be used for production of biochar (Kuppusamy et
al., 2016). But it is different in reality. When we consider the costs involved in biochar
production (and various regulations pertaining to it), the feedstock abundance reduce
(Tripathi et al., 2016).

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The properties of biomass feedstock looked at are moisture content, ash content, calorific
value, percentage of lignin, cellulose, and hemicellulose, fractions of fixed carbon and
volatile components. A wide variety of feedstock biomasses have been used for the
production of biochar such as kitchen waste, agricultural waste, forest residues, bioenergy
crops etc. Organic biomass is used as feedstock not only for biochar production but also for
compost preparation; therefore it becomes necessary to ensure that there is no competition for
feedstock in either of the two applications. A high yield of biochar is derived from biomass
which has more lignin and less cellulose. The higher lignin content increases further with the
porosity of biochar. Additionally, the volatile component, water content, particle size and
shape of biomass also affect the property of biochar obtained.

Pyrolysis

Biochar is produced from the thermal conservation of biomass feedstock. Various techniques
have been used such as torrefaction (a pyrolytic process primarily at low temperature), slow
pyrolysis, intermediate pyrolysis, fast pyrolysis, gasification, hydrothermal carbonization and
flash carbonization (Zhang et al., 2013). Broadly, biochar can be produced in traditional
earthen charcoal kilns and modern biochar retorts (Asensio et al., 2013).

In pyrolysis, the biomass is heated at relatively low temperatures (300–900 ∘C) in the absence
of oxygen. This technology can be distinguished by the residence time, the pyrolytic
temperature of the material (e.g., slow and fast pyrolysis process), pressure, size of the
adsorbent, the heating rate and lastly the method used (e.g., pyrolysis started by the burning
of fuels, by electrical heating, or by microwaves). Pyrolysis has been considered as the most
cost-effective and highly efficient method for biochar production (Cha et al., 2016). The
biochar properties are heavily affected by the extent of pyrolysis (pyrolytic temperature and
pressure) and entirely by the size of biomass and kiln (or furnace) residence time (Asensio et
al., 2013). Vapour residence time determines the rate at which volatile and gases are removed
in a kiln (Meyer et al., 2011). Secondary reactions occur as a result of prolonged residence
time (notably the reactions of tar on biochar surfaces and charring of the tar rather than
additional combustion or processing outside the kiln) (Oliver et al., 2013). For gasification in
pyrolysis, the biomass feedstock is oxidized in the gasification chamber at a temperature of
about 800∘C (Oliver et al., 2013).

Presently, slow pyrolysis is widely used for biochar production because of the higher biochar
yield, although there does exist some fallouts such as energy inefficiency and the demand for
long production duration (Tripathi et al., 2016).

A table was prepared for various biomasses used for biochar production and the properties
associated with such biochar by Ahmad et al., 2014.

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Table 1: Properties of biochar produced from various feedstock.

Pyrolysis Surfac
Feedstock temp. Yield Ash pH C H O N e area References
(m2 g
(∘C) (%) (%) (%) (%) (%) (%) 1
)
Canola straw 400 27.4 – – 45.7 – – 0.19 – Tong et al. (2011)
Corn cobs 500 18.9 13.3 7.8 77.6 3.05 5.11 0.85 0.0 Mullen et al. (2010)
Corn stover 450 15.0 58.0 – 33.2 1.40 8.60 0.81 12.0 Lee et al. (2010)
Corn stover 500 17.0 32.8 7.2 57.3 2.86 5.45 1.47 3.1 Mullen et al. (2010)
Cottonseed hull 200 83.4 3.1 – 51.9 6.00 40.5 0.60 – Uchimiya et al. (2011)
Cottonseed hull 800 24.2 9.2 – 90.0 0.60 7.00 1.90 322.0 Uchimiya et al. (2011)
Fescue straw 100 99.9 6.9 – 48.6 7.25 44.1 0.64 1.8 Keiluweit et al. (2010)
Fescue straw 700 28.8 19.3 – 94.2 1.53 3.60 0.70 139.0 Keiluweit et al. (2010)
Oak bark 450 – 11.1 – 71.2 2.63 12.9 0.46 1.9 Mohan et al. (2011)
Oak wood 400–450 – 2.9 – 82.8 2.70 8.05 0.31 2.7 Mohan et al. (2011)
Orange peel 150 82.4 0.5 – 50.6 6.20 41.0 1.75 22.8 Chen and Chen (2009)
Orange peel 700 22.2 2.8 – 71.6 1.76 22.2 1.72 201.0 Chen and Chen (2009)
Peanut shell 300 36.9 1.2 7.8 68.27 3.85 25.89 1.91 3.1 Ahmad et al.(2012a)
Peanut shell 700 21.9 8.9 10.6 83.76 1.75 13.34 1.14 448.2 Ahmad et al.(2012a)
Peanut straw 400 28.2 – – 42.90 – – 1.50 – Tong et al. (2011)
Pine needles 100 91.2 1.1 – 50.87 6.15 42.27 0.71 0.7 Chen et al. (2008)
Pine needles 700 14.0 2.2 – 86.51 1.28 11.08 1.13 490.8 Chen et al. (2008)
Pine shaving 100 99.8 1.2 – 50.60 6.68 42.70 0.05 1.6 Keiluweit et al. (2010)
Pine shaving 700 22.0 1.7 – 92.30 1.62 6.00 0.08 347.0 Keiluweit et al. (2010)
Pinewood 700 – 38.8 6.6 95.30 0.82 3.76 0.12 29.0 Liu et al. (2010)
Poplar wood 400 32.0 3.5 9.0 67.30 4.42 – 0.78 3.0 Kloss et al. (2012)
Rice husk 500 – 42.2 – 42.10 2.20 12.10 0.50 34.4 Liu et al. (2012)
Saw dust 450 – 1.1 5.9 72.00 3.50 24.41 0.08 – Lin et al. (2012)
Saw dust 550 – 2.8 12.1 85.00 1.00 13.68 0.30 – Lin et al. (2012)
Soybean stover 300 37.0 10.4 7.3 68.81 4.29 24.99 1.88 5.6 Ahmad et al.(2012a)
Soybean stover 700 21.6 17.2 11.3 81.98 1.27 15.45 1.30 420.3 Ahmad et al.(2012a)
Soybean straw 400 24.7 – – 44.10 – – 2.38 – Tong et al. (2011)
Spruce wood 400 36.0 1.9 6.9 63.50 5.48 – 1.02 1.8 Kloss et al. (2012)
Spruce wood 525 – 4.7 8.6 78.30 3.04 – 1.17 40.4 Kloss et al. (2012)
Wheat straw 400 34.0 9.7 9.1 65.70 4.05 – 1.05 4.8 Kloss et al. (2012)
Wheat straw 525 – 12.7 9.2 74.40 2.83 – 1.04 14.2 Kloss et al. (2012)
Chicken litter 620 43-49 53.2 - 41.50 1.20 0.70 2.77 - Ro et al. (2010)
Poultry litter 350 54.3 30.7 8.7 51.07 3.79 15.63 4.45 3.9 Cantrell et al. (2012)
Poultry litter 700 36.7 46.2 10.3 45.91 1.98 10.53 2.07 50.9 Cantrell et al. (2012)
Tire rubber 200 93.5 15.0 - 74.70 6.38 3.92 - - Lian et al. (2011)
Tire rubber 800 43.0 10.5 - 86.0 0.87 2.16 0.47 50.0 Lian et al. (2011)

Mechanism used for containment of pollutant by biochar

Carbonaceous materials have been used for a long time as sorbents for contaminants in soil
and water. The mechanism used for capturing the contaminants could be described as
follows:

Sorption: The process of sorption of organic and inorganic contaminants from water/soil onto
biochar is one of the mechanisms used. Factors such as high surface area and microporosity
of biochar, pH and ionic strength affect the sorption of organics/inorganics onto biochar. It is

7
interesting to note that biochars produced at higher temperatures exhibit higher sorption
efficiency for remediation of organic contaminants in soil and water. The process of
adsorption is remarkable in case of metallic contaminants.

Hydrogen bond formation: Polar compounds are adsorbed by the hydrogen bonds formed
with the oxygen containing molecules (carboxyl, hydroxyl, and phenolic surface functional
groups) of the biochar (Sun et al., 2011), while non-polar compounds access hydrophobic
sites on biochar surfaces in the absence of hydrogen bonding between water and oxygen
containing functional groups (Ahmad et al., 2012a).

Electrostatic attraction/repulsion: The interaction between positively charged cationic

organic contaminants and negatively charged biochar surfaces is another possible mechanism
for containing pollutants (Xu et al., 2011). Electrostatic outer-sphere complexation due to
metallic exchange with K+ and Na+ available in the biochar.

Diffusion: Non-ionic compounds diffuse into the non-carbonized and carbonized fractions of
biochar (an effective sorption mechanism).

Formation of surface complexes: Surface complexes are formed between cations and active
functional groups (–COOH and –OH) on the biochars (Tong et al., 2011). They are
dependent upon pH and the ionic radius. The smaller the ionic radius of metals will be, the
greater would be the adsorption capacity (Ngah and Hanafiah, 2008).

Precipitation: For example, lead forms precipitates of lead-phosphate-silicate in biochar. Co-

precipitation and inner-sphere complexation formed between metals and organic
matter/mineral oxides of biochar.

It is worth mentioning that sorption of organic contaminants by biochars is more favoured

than that of inorganic contaminants.

Figure 1: Postulated mechanisms of the interactions of biochar with organic contaminants.

Circles on biochar particle show partition or adsorption (Source: Ahmad et al., 2014)

8
Figure 2: Postulated mechanisms of biochar interactions with inorganic contaminants. Circles
on biochar particle show physical adsorption (Source: Ahmad et al., 2014)

Applications of biochar

The various properties of biochar including the high carbon content, larger surface area, well-
developed porous structures, sufficiently enriched surface functional groups, etc. make the
biochar potentially a huge prospect for a variety of applications in the twenty-first century.
Some of the applications could be listed out as follows:

Climate change mitigation

Biochar production has been proposed as one of the best ways to mitigate climate change.
This is done so by sequestering carbon in soil. Biochar shows long-term stability in soil, and
this acts as a key factor in decreasing the carbon dioxide emissions into the atmosphere
(Singh et al., 2012). A recent experiment had estimated that the mean residence time of
carbon in biochars could vary from 90 to even 1600 years, depending upon the labile and
intermediate stable carbon components (Singh et al., 2012). Further, studies have even shown
that biochar can reduce nitrous oxide and methane emissions from soil by biotic and abiotic
mechanisms (Zweiten et al., 2009). Woolf et al. (2010) has proposed a sustainable biochar
concept by which emissions of greenhouse gases (including CH 4 and N2O) can be avoided by
pyrolysis of the waste biomass. Simultaneously, the bioenergy that is produced during the
process of pyrolysis offsets fossil fuel consumption. Further, half of the carbon fixed in
biomass during photosynthesis is retained. Interestingly, biochar has been estimated to be
capable of tackling 12% of current anthropogenic carbon emissions.

Soil improvement

Biochar has high organic carbon content making it potentially a soil conditioner. It does so by
improving the physicochemical and biological properties of soils. Further, soil water
retention capacity elevates with increase in organic carbon (~18%). Sohi et al. (2009) has
reported a reduction in nutrient leaching due to biochar application. Generally, biochar has a

9
neutral to alkaline pH which induces a liming effect on acidic soils, thereby possibly
enhancing plant productivity. Remarkable increase in seed germination, plant growth, and
crop yields have also been reported in soils amended with biochars. Application of biochar
together with organic or inorganic fertilizers can even multiply crop yields. Further, increase
in microbial population coupled with microbial activity in soils amended with biochar has
also been seen (Lehmann et al., 2011). These influence the biogeochemical processes in soils.
On the contrary, Weyers and Spokas (2011) have reported short-term negative effects (or the
long-term null effects) on earthworm population in soils amended with biochar. However,
wood-based biochars showed null to even positive impacts on earthworm population. Li et al.
(2011) has recommended application of wet biochar to soil, as it could help mitigate negative
impact on earthworms by preventing desiccation.

Waste management

Waste biomass is used for the production of biochar. This is not only economical but
simultaneously beneficial. Thus, biochar has great potential for waste management
originating from animals or plants as it decreases the pollution associated with it. In the
recent past, pollution has emerged to be the greatest hazard and multiple options have been
sought for to tackle this menace. Biochar production gives us an excellent alternative to
overcome this issue. Waste biomass is vast and highly varied in nature. It includes crop
residues, forestry waste, animal manure, food processing waste, paper mill waste, municipal
solid waste, and sewage sludge (Cantrell et al., 2012). It is interesting to note that pyrolyzing
the waste biomass, particularly animal manure and sewage sludge, even kills any member of
the microbial population present. Hence, it also reduces the associated environmental health
effects (Lehmann and Joseph, 2009). However, the persistence of toxic heavy metals in
biochar developed from sewage sludge and municipal solid waste need to be carefully taken
care of and handled properly before its long-term application to soils.

Energy production

Remarkably, another potential use, which could be associated with conversion of waste
biomass to biochar, is the production of bioenergy. This bioenergy will serve as an alternative
to fossil fuel and will likely be associated with lower carbon emissions (Bolan et al., 2013a).
However, bioenergy production is hugely dependent on the conditions involved in the
process of pyrolysis. Slow pyrolysis has been shown to produce a lower yield of liquid fuel
and more biochar, while fast pyrolysis generates more of liquid fuel (bio-oil) and less of
biochar (Mohan et al., 2006). However, the production of bioenergy from biomass is still
controversial and there is need for further research.

Capturing the contaminants

Environmental remediation has recently been recognized as a promising area where biochar
can be successfully applied (Ahmad et al., 2014). Organic contaminants of the greatest
concern have been pesticides, herbicides, polycyclic aromatic hydrocarbons, dyes, and
antibiotics. Simultaneously, inorganic contaminants are of a wide variety too. But, metals
have been of the gravest concern. Unlike organic contaminants, metals are non-biodegradable

10
and their bioavailability makes them highly toxic to living organisms (Zhang et al., 2013).
Biochar has been emerging as an excellent option to reduce the bioavailability of
contaminants in the environment (Sohi, 2012). It is a very effective environmental sorbent for
organic and inorganic contaminants both in soil and water. Recently, biochar has been
applied as a novel carbonaceous material to adsorb metals in soil and water.

Composting

Recent studies show that biochar has a great potential for enhancing the process of
composting. It has become evident that biochar addition in composting can improve its
physicochemical properties. enhance the microbial activities and promote organic matter
decomposition. However, there is need to explore the mechanism of biochar addition on
composting and evaluate the agricultural/environmental performances of biochar compost.

Advantages

We have already seen the multiple uses that biochar can potentially offer. Biochar can
potentially help in climate change mitigation which can aid in averting major modifications
on our mother Earth, soil productivity enhancement (coupled with its additional role in
composting) which can simultaneously aid in tackling the issue of food security, solve the
menace of pollution by contaminant capture and solid waste management by its conversion
into biochar and lastly biofuel production, which will minimise the depleting trend of fossil
fuels.

Apart from these applications, biochar has several other advantages. It is less expensive than
activated carbon. The cost of biochar production is approximately one-sixth of commercially
available activated carbon (Ahmad et al., 2012a). It is generally obtained at low temperature
and does not require further processing to be activated (Ahmad et al., 2012a). It shows
significant adsorption for both organic as well as inorganic contaminants. Biochar contains a
non-carbonized fraction (such as oxygen containing carboxyl, hydroxyl, and phenolic surface
functional groups) that interacts with soil contaminants (Uchimiya et al., 2011b). It shows a
significant amount of ion exchange capacity (‘cation exchange capacity’), a property that is
found minimal or nearly absent in activated carbons. This property is due to residual
carboxylic acid functionalities on the biochar, which get removed (along with other residual
side chain aliphatic groups) in the process of activation in case of activated carbons. It has the
ability to promote living microbiology in the soil. Therefore, it enhances the food web in soil.
It has greater water holding capacity compared with activated carbons. It has low density
therefore it provides additional voids and aeration in the soil. It reduces loss of nutrients
through leaching (Ventura et al., 2013). Biochar prepared from wastes such as sewage sludge
and manure have a high nutrient content. Therefore, it enriches soil quality.

Disadvantages/ Drawbacks/ Limitations

Apart from the advantages of using biochar, there are likely fallouts that need to be dealt
with. We need to consider health aspects related with biochar production. Issues such as
gaseous aerosol emissions during improper pyrolysis may arise and therefore need to be

11
taken care of. Environmental pollution from dust, erosion and leaching of biochar particles
could be a possibility. The fine ash associated with biochar is a perfect source for dust, posing
a risk for respiratory diseases. In some cases, yields may decline because of the sorption of
water and nutrients by the biochar, which reduces their availability for the crops. There could
be expected sorption of residual herbicides and pesticides which could subsequently have
implications on the efficacy of these products. Long-term removal of crop residues, like
stems, leaves and seed pods, to be used for production of biochar can reduce overall soil
health by diminishing the number of soil microorganisms and disrupting internal nutrient
cycling. There is a possibility of negative impact on soil biota such as short-term negative
impacts on earthworm population density. Though, biochar did not appear to cause
significant long-term impacts (Weyers and Spokas, 2011). The reduction in nitrous oxide
emissions is not universal and emissions even increase sometimes. Contrasting results were
seen in field trials by Angst et al. (2014) and Verhoeven and Six (2014). Having discussed
the possible disadvantages of using biochar, it needs to be clearly mentioned that they can be
properly dealt with by appropriate care taken during its production stage. Further there is a
dire need of extensive research so that any issue arising because of its usage is aptly resolved.

Conclusion

Remarkable work has been done in the field of biochar nationally as well as internationally.
Biochar has produced from substances (such as plant residue, sewage sludge, animal litter,
water-hyacinth and even algae) in deficient or no oxygen pyrolysis conditions (usually below
700 °C). Biochar properties are affected by pyrolysis temperature and the type of biomass
used for biochar production. Characterization of biomass can be used for specific purposes
such as cost effective hydrogen sulphide removal from biogas. The specificity of biochar may
be enhanced by certain modifications. For example, spherical biochar has been prepared by
employing a two-step thermal technology to potato peel waste. Further, magnetic biochar
have been produced which can be easily recovered using external magnetic field and show
greater ability to remove pollutants. Biochar production minimizes waste and makes waste
profitable. Further, it reduces organic pollutants and heavy metal concentrations (even under
semi-arid conditions). Interestingly, soil contaminated with petroleum can be efficiently
remediated by adding biochar. There is even possibility of using biochar for efficient dye
removal and degradation. Biochars selectively absorb crude oil from oil/water biphasic
mixtures. Biochar could even be used as an amendment in metal contaminated soil for
improving growth and yield in crops. Use of biochar as an additive for the composting
provides favourable conditions (such as high porosity, large surface area, etc.) because of
which biochar can reduce the length of the composting process and further enhance the value
of compost. Biochar can have positive or negative effect on rhizosphere organisms depending
on biomass origin, pyrolysis temperature, application method, dose, etc. Additionally, long
terms effects are positive and increase soil biota activity. There could be short-term negative
to long-term null impacts on earthworms. Biochar significantly decreases total nitrogen losses
and greenhouse gas emission and therefore aid in climate change mitigation.

Biochar is a very promising and environment-friendly material in a broad range of

disciplines. It has already been established that biochar can be made from anything and

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everything containing biomass. This causes a gap of global standardisation as regards
biochar. Various countries have formed voluntary biochar quality standards. There even
exists a regulatory gap. Further, application rate of biochar need to be determined for each
soil type so that possible detrimental effects due to over-application are avoided. Most of the
experiments have been carried out in laboratory conditions. There arises a need of large scale
field studies and trials so that real time application of biochar is understood more
appropriately. Biochar has been used to capture and store a wide variety of contaminants.
Therefore, stability of biochar needs to be adequately acknowledged for safety purposes.
Further, there is a need of an environmental risk assessment that includes the impacts of these
on terrestrial or aquatic ecosystems. Cleaner technology has to be developed so that there is
no scope for release of any kind of gaseous release during the production of biochar. A
comparative study is needed for biochar produced both from different materials and at
different temperatures and its effect on the process of composting. Further, the suitability of
biochar and compost for specific soil (and plants) need to be found out. Dosage of biochar for
composting and the effect of aging on biochar’s composting also need additional attention.
Although, the total environmental life-cycle assessment has been conducted for biochar in
some of the cases, there lies a knowledge gap for a wide variety of feedstock. Biochars could
be developed which bind to soil in a better fashion. The contaminants captured by the biochar
need to be safely removed (and stored) so that the biochar becomes available for reuse.
Biochar activation could be another significant area to make the application of biochar more
specific to remove particular contaminants. Application of biochar on soil in extreme weather
conditions such as heavy rainfall, drought, etc. need to researched upon. This becomes
excessively important with changing climate scenarios.

It seems that biochar can just be the perfect solution for the various issues threatening our
planet. Adequate research in this field can turn out to be the most beneficial step humankind
can take.

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