Bioresource Technology 337 (2021) 125451

Contents lists available at ScienceDirect

Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech

Review

Production and beneficial impact of biochar for environmental application:

A comprehensive review
Yuwen Zhou a, Shiyi Qin a, Shivpal Verma a, Taner Sar b, Surendra Sarsaiya c,
Balasubramani Ravindran d, Tao Liu a, Raveendran Sindhu e, Anil Kumar Patel f,
Parameswaran Binod e, Sunita Varjani g, Reeta Rani Singhnia f, Zengqiang Zhang a,
Mukesh Kumar Awasthi a, b, *
a
College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi Province 712100, PR China
b
Swedish Centre for Resource Recovery, University of Borås, Borås 50190, Sweden
c
Key Laboratory of Basic Pharmacology and Joint International Research Laboratory of Ethnomedicine of Ministry of Education, Zunyi Medical University, Zunyi,
Guizhou, China
d
Department of Environmental Energy and Engineering, Kyonggi University, Youngtong – Gu, Suwon 16227, South Korea
e
Microbial Processes and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram, Kerala,
695019, India
f
Department of Marine Environmental Engineering, National Kaohsiung University of Science and Technology, Kaohsiung City, Taiwan
g
Gujarat Pollution Control Board, Gandhinagar, Gujarat 382010, India

H I G H L I G H T S G R A P H I C A L A B S T R A C T

• Review the existing knowledge on the

biochar properties and its future
perspectives;
• Biochar derived from physic-chemical
and bio-engineering techniques from
feedstock’s;
• Different modification technologies
were mentioned in this review;
• Modified biochar had better function in
application compared with original
biochar;
• Providing cumulative information for
environmental application of biochar.

A R T I C L E I N F O A B S T R A C T

Keywords: This review focuses on a holistic view of biochar, production from feedstock’s, engineering production strategies,
Biochar its applications and future prospects. This article reveals a systematic emphasis on the continuation and
Engineering techniques development of biochar and its production methods such as Physical engineering, chemical and bio-engineering
Applications, biomass and feedstock’s
techniques. In addition, biochar alternatives such as nutrient formations and surface area made it a promising
cheap source of carbon-based products such as anaerobic digestion, gasification, and pyrolysis, commercially
available wastewater treatment, carbons, energy storage, microbial fuel cell electrodes, and super-capacitors
repair have been reviewed. This paper also covers the knowledge blanks of strategies and ideas for the future

* Corresponding author at: College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi Province 712100, PR China.
E-mail address: mukeshawasthi85@nwafu.edu.cn (M.K. Awasthi).

https://doi.org/10.1016/j.biortech.2021.125451
Received 12 May 2021; Received in revised form 18 June 2021; Accepted 19 June 2021
Available online 24 June 2021
0960-8524/© 2021 Elsevier Ltd. All rights reserved.
Y. Zhou et al. Bioresource Technology 337 (2021) 125451

in the field of engineering biochar production techniques and application as well as expand the technology used
in the circular bio-economy.

1. Introduction branch, which is the solid production from biomass pyrolysis (Yang
et al., 2020). Biochar is a carbon-rich material produced from organic
With the development of economy and the improvement of living ­ raw materials by pyrolysis under conditions of limited oxygen, which
standards, more and more bio-waste resources (organic, agricultural feedstock is available from a wide range of sources as mentioned above
and woody wastes) are generated in daily life (AEBIOM Statistical (Wang and Wang, 2019). Biochar is a promising alternative to activated
Report, 2019). It is estimated that around 822 million tons of rice husk carbon owing to its high specific surface area, non-carbonized compo­
waste are generated annually worldwide, and the number of crops was nent content, porous structure, and high variability of surface functional
wasted by the resident in Europe has reached 700 million tons every ­ groups (Rajec et al., 2016). Modification of biochar, including surface
year (Donner et al., 2021; Dunnigan et al., 2018). In 2017, the European oxidation, metal oxide impregnation, and functionalization, can be
Union (EU) generated 144 kilotons of oil equivalent biological waste performed in order to modify the physical and chemical properties of
from different sources (wood waste, agricultural, and organic). 70% of biochar to better suit environmental needs (Ahmed et al., 2016).
them burnt directly or after compression. Nearly 12% were applied for Biochar has the great potential for application and has been used as
biogas production. 12% of the waste biomass were used to produce an alternative to activated carbon. The low cost of biochar production,
biofuels, including 76% biodiesel, 17% bioethanol and 7% others. The its wide availability, and its role in soil remediation are attracting
remaining 6% of the waste biomass was either landfilled or composted increasing attention (Tang et al., 2019). Biochar, because of its unique
(AEBIOM Statistical Report, 2019).Table 1. properties, can not only have significant implications on the remediation
The common treatment technologies to dispose these agricultural of targeted contamination in the soil, but can also improve soil prop­
wastes are composting or open burning, which have the disadvantages erties (Kumar et al., 2021). Biochar enhanced the physical properties of
of producing greenhouse (CO2, CH4, and N2O) and pollution gases (H2S, soil by improving its water holding capacity, moisture levels, and oxy­
SO2, and NH3), and reducing the utilization of value of agriculture waste gen content. Biochar chemical properties, such as contaminant fixation
(Dunnigan et al., 2018; Yang et al., 2021). However, the technology of and carbon sequestration, are also improved. At the same time, biochar
producing biochar through the pyrolysis of the agricultural waste has leads to changes in soil microbial abundance and diversity. Biochar
many superiorities and opportunities, including large quantity, wide helps soil C sequestration and reduces greenhouse gas (e.g., CO2)
sources, large application fields (Xiao et al., 2018). The prepared bio­ emissions (Frišták et al., 2018). Biochar can also improve agricultural
char can be used in wastewater treatment, soil remediation, filter filler land and its impact on the uptake and accumulation of potentially toxic
and so on. When biochar is used in agricultural land, it can hold the metals in cultivated plants (Zhang et al., 2020a,b). In this article, we
nutrients in soil and improve agricultural sustainability and yield (Hu reviewed the understanding and frontier applications of biochar in
et al., 2020a,b). In addition, it can also sequester carbon and reduce recent years, especially in recent five years. The production technology
carbon emission. For example, Lima et al. found that the use of 1 m3 of of biochar was discussed, and the modification of biochar was deeply
Dillenia excelsa wood wastes is equivalent to reducing CO2 emissions by summarized. The application of biochar in different environmental
1687 kg (Lima et al., 2020). The use of organic wastes for agricultural media was introduced. In addition, we forecast and suggest the devel­
production can return the nutrients it contains to the soil and develop a opment prospect of biochar. This paper is instrumental in providing
circular economy (Kwoczynski and Čmelík, 2020). The production of theoretical basis for the promotion of biochar.
biochar from biomass is an efficient and environment friendly method of
biomass treatment (Fig. 1). The main raw materials for biochar pro­ 2. Production from various feedstock’s
duction are animal manure, agriculture and forest residues, industrial
bio-wastes, and marine & aquatic organism etc. Pyrolysis of biomass The utilization of biomass rich in cellulose, hemicellulose, lignin and
produces a range of solids, liquids, and gases. Biochar is an important other organic matters to produce biochar has become a popular topic in

Table 1
Removal mechanism and capacity for various pollutants by biochar.
Contaminants Pyrolysis Feedstock Initial Removal capacity Removal mechanisms Reference
temperature (℃) concentration
(mg/L)

Zn(II), Pb(II) and Cd 350–750 Phytoremediation 36.2, 41.0, and — Adsorption (Huang et al., 2018)
(II) residue 33.8
As(V), Pb(II) and 550 Sugarcane leafy — 157 mg/g, 103 mg/ Carb π-π* transition and carbon (Li et al., 2018)
methylene blue trash g and 297 mg/g π-electron
Pb (II) 350 Coconut shell 1.4 3.94 mg/g Electrostatic attraction, ion (Hao et al., 2020)
exchange, precipitation, and surface
complexation
Cd(II), Cu(II), and Pb 600 Cow bone meal 50, 50, and 120 75.15, 163.80, and Cation exchange, π-electrons, (Xiao et al., 2020)
(II) 389.51 mg/g electrostatic interactions
Phenol 800 Bagasse 20 Completely remove Chemically bonded and (Zhang et al., 2020)
electrostatically attracted on surface
oxygen sites
Hazardous landfill 700 waste palm shell 660 595 mg/g H-bond; Electrostatic attraction (Lam et al., 2020)
leachate
Cr(VI) 800 Marine macroalgae 30 95.23 Adsorption; magnetism (Wang et al., 2020)
Cr (VI) and 800 Corn straw 200 and 40 116,97 and 450.43 Pore filling and π-π stacking (Qu et al., 2021)
naphthalene interactions
Sulfamethoxazole 700 Sugarcane bagasse 100 400 Oxygenated functional groups; (Prasannamedha
electron donor;strong H bond et al., 2021)

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Y. Zhou et al. Bioresource Technology 337 (2021) 125451

the researcher around the world, as biochar plays an increasingly

important role in reducing greenhouse gas (GHG) emission, carbon
sequestration, absorbing contaminants, soil remediation and compost­
ing (Yaashik et al., 2020). The physical (pH, surface area, electrical
conductivity, and pore size), chemical (Functional groups, cation ex­
change capacity, and nutrition) and biological properties of biochar are
affected by the feedstock, preparation conditions and preparation pro­
cess. Biochar produced under low temperature pyrolysis is rich in
bioavailable nutrients, which can improve soil physico-chemical prop­
erties, microbial abundance and promote plants growth after adding it
into soil (Hu et al., 2020a; El-Bassi et al., 2021). At present, a variety of
methods for producing biochar have been studied according to the re­
quirements of biochar characteristics. Thermochemical conversion
technologies contained pyrolysis, gasification, torrefaction and hydro­
thermal carbonization (Fig. 2) (Yaashikaa et al., 2019; Chen et al.,
2020a,b).
The difference between these methods is biomass type and the pro­ Fig. 2. Process of biomass to biochar conversion.
cess conditions like temperature, pressure, heating rate and residence
time, etc. Pyrolysis under low-oxygen or non-oxygen conditions is one of physical or chemical modification improve its characteristics for using it
the most common and oldest technologies for the biochar production. in special application fields (Zhuang et al., 2020). For instance, modi­
During the pyrolysis process, biochar not only will be produced, but also fication for biochar could be achieved through acid treatment, alkaline
generates liquid (bio-oil) and gas (syngas) that could be used as energy treatment, oxidant oxidation, compositing, sulfuration, nitrogenation
substances to meet energy crisis (Chi et al., 2020). Pyrolysis is classified and physical activation, etc. Biochar with nitro-generation, physical
into three types of technologies according to heating rate, temperature, activation modification, and acid treatment can enhance alkalinity/
pressure and residence time etc., which contains slow pyrolysis, inter­ polarity, increase surface area/porosity and remove impurities respec­
mediate pyrolysis and fast pyrolysis (Ahmad et al., 2020). In contrast to tively. Modified biochar with more excellent properties will have more
fast pyrolysis, the heating time of slow pyrolysis that is up to several advantages when we use it in soil amendment, electrochemical material,
hours or days is longer than it and with a slow heating rate (0.1-1℃/ catalyst and adsorbent, etc. (Huang et al., 2021). The benefits of uti­
min). The one of reason that slow pyrolysis is widely utilization to lizing modified biochar in applications will give us more unexpected
produce biochar for its high solid yield. With the increase of tempera­ effects in the future.
ture, the mass fraction range of pyrolysis solid increases. However, fast
pyrolysis at a faster heating rate and high temperature are produce more
liquid and gas like bio-oil, CO, H2 and syngas (Yaashik et al., 2020; 2.1. Physical properties
Ahmad et al., 2020).
Intermediate pyrolysis while its conditions are between slow pyrol­ 2.1.1. Density and porosity
ysis, and fast pyrolysis had a better balance between the yields of solid The density and porosity of the biochar will affect its mobility in
(biochar) and liquid (bio-oil), but it is rarely adopted to realize the nature, its interaction with the hydrologic cycle of the soil, and its
production of biochar. Although, the conventional biochar is with many suitability as a natural niche for small soil microbes. However, the wide
excellent properties (Fig. 3) in application, it is also essential to make range of biochar porosity characterization, making it difficult to find
suitable ways to link body structures of quantity and porosity to

Fig. 1. Biochar production and its applications.

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Y. Zhou et al. Bioresource Technology 337 (2021) 125451

Fig. 3. General properties of biochar.

environmental effects. The characteristics of biochar (e.g., density, and Therefore, it is necessary to reduce the energy consumption to improve
porosity), however, largely depend on the origin of the biomass, pre- the product quality, i.e., porosity; ii) The activation temperature is a
treatment, and production conditions of biochar. Therefore, greater dominant factor to add specific area; the average surface area of biochar
effort should be placed to control the combined effects of key parameters which is made from both wood residues increases to about 12.0 m2g− 1 as
such as biomass type, production conditions (e.g., temperature, pres­ the temperature increases from 700 to 900◦ C; iii) pilot scale technology
sure, resident time, and renewable gas), and post-treatment conditions has produced powerful functional biochars similar to laboratory-scale
(e.g., physical, chemical, or joint) to increase and maximize the biochar technologies that can increase the benefits of using thermochemical
properties of the target response (Lee et al., 2017a,b). The mass of biomass conversion, and increase profits with the various products in
biochar particles is measured using a pycnometer method. Quantitative the biorefinery industry (Lee et al., 2017c). Pore space was defined by
calculations were calculated using biochar concentrations that could be imaging analysis of X-ray tomography images and, in addition, nano-
placed in a 20 mL stainless steel cylinder with minimal pressure scale porosity was evaluated by helium ion microscopy (Hyväluoma
(Askeland et al., 2019). However, biochar made of eucalypt wood low et al., 2018). Hydrothermal treatment or oxygenation is another com­
density had a potency of up to 35% of water and up to 45% of the mon way to promote biochar porosity. Ongoing research, including the
available water than biochar made of high-quality eucalyptus wood identification of chemical conditions and fractions with plasma biochars
(Werdin et al., 2020). In physical use, the biochar porosity and surface containing radicals that are still lacking (Hu et al., 2020b). Future
area increased exposure to the flow of CO2 gas agents, vapor or mixture research should focus on developing or building technologies to achieve
above 700◦ C (Anto et al., 2021). In addition, biochar increased carbon improved performance and a biochar density and porosity structure
content, cation exchange power, water-holding capacity and reduced simultaneously.
bulk density in mineral soil (Jeyasubramanian et al., 2021).
Activation is the most widely used and effective way to promote 2.1.2. Specific surface area
biochar density and porosity, especially chemical synthesis (Leng et al., The carbonization process escapes gas, which affects the surface area
2021). Nitrogen/phosphorus (N/P) improved biochar with strengthened of the biomass by affecting changes in porosity. Biochar has a large
porosity were produced by guanidine phosphate (GP) by step-one py­ specific surface area, and it has a high cation exchange capacity. We
rolysis of bamboo sawdust. Biochar-produced biochar pyrolysis at 450◦ C often use Brunauer–Emmett–Teller (BET) analysis to determine the
produced richer groups (4.1 at% for P, 11.3 at% for N), resulting in specific surface area, i.e., the amount of gas absorbed in a defined sce­
higher aquatic potency of Cd(II) (60.3 mg/g), as well as Cu(II) (81.7 mg/ nario using nitrogen at 77 k, nitrogen as adsorbate but sometimes carbon
g), and Pb(II) (166.2 mg/g). Pb (II) was rapidly regenerated with min­ dioxide is used instead. In general, along with increasing temperature,
eral formulations, including Pb5(PO4)3Cl and Pb5(PO4)3OH (Meng and the specific surface area increases, but the use of carbon dioxide pro­
Hu, 2021). The biochar adsorption capacity relies largely on the number duces a larger specific surface area. Biochar has a large surface area and
of positive/negative charged groups on the surface, and the porosity is porosity, which increases the soil’s water-holding capacity and nutrient
distributed by volume size. Compared to inactive biochar, H3PO4- uptake (Shareef and Zhao, 2017).
treated pine tree sawdust biochar introduced an advanced microporosity
and the P–O–P bonds incorporation into the biochar matrix, expressing 2.1.3. Pore size and pore volume distribution
over 20% increase in Pb(II) adsorbed potency (Hu et al., 2020). In Although biochar has a large surface area, its ability to utilize water
addition, active temperature of pyro-gasification, wood residue type, and adsorb gases is limited by the size of the pores. The biochar pores
and CO2 gas blow rate influenced the activated biochar porosity. The stride across several orders of magnitude and are itemized as micropore
major conclusion are as following: i) The pyrolysis or torrefaction in the (0.0001–0.05 µm), medium (0.002–0.05 µm) and large (pore size
pretreatment stage has little effect on the porosity of activated carbon, 0.05–1000 µm). The methods of determining pore volume is usually

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Y. Zhou et al. Bioresource Technology 337 (2021) 125451

performed applying nitrogen adsorption, similar to standard Bru­ increase in biochar electrical conductivity (EC) with increasing tem­
nauer–Emmett–Teller (BET) analysis, so the same analysis limitations is perature may be related to the loss of biomass volatiles during carbon­
also applicable. The biochar pore structure is decisively consisted of ization, the increase in CEC (due to the presence of exchangeable ions on
micro-pores, along with increasing temperature, the total pore volume the biochar surface), and the increase in effective nutrients (Chandra
increased. Micro-pores could occupy more than 80% of the total pore and Bhattachary, 2019).
volume (charcoal from safflower cakes, and temperatures 400–600◦ C)
or occupy more than 80% of the surface area (charcoal, temperatures 2.2.3. Cation exchange capacity (CEC)
450–550◦ C), respectively, the quantity of micro-pores in untreated The functional groups on the biochar surface are mainly negatively
agricultural waste (stover and straw) was found to be less than 10% as a charged, which makes it less able to adsorb anions and more able to
comparison (Weber and Quicker, 2018). Particle size is inversely related exchange cations. The presence of a large number of abundant minerals
to reaction temperature. That is, high heating temperatures produce (e.g., K, Ca, Na, and Mg) in biochar facilitated the formation of surface
smaller biomass particles, whereas long residence times and low heating functional groups, resulting in a high CEC value for biochar (Li et al.,
rates produce larger biomass particles. 2019). Over time, lower CEC leads to a more rapid decline in soil pH
(Teutscherova et al., 2017). Along with the pyrolysis temperature
2.1.4. Mechanical stability increased, the disappearance of some acidic functional groups causes the
The mechanical strength of biochar is largely determined by its CEC of biochar to decrease (Li et al., 2018a).
density. Therefore, the release of substances or the evaporation of water
during the heating process affects the mechanical stability of the biochar 3. Engineering biochar production techniques
by affecting its density. Biochar with high compressive strength could
only be made from high lignin and highly density content raw feed­ Biochar is produced through the pyrolysis of biomass. The products
stocks. The biochar mechanical strength can also be determined by of pyrolysis mainly included synthetic gas, bio-oil and biochar (Fig. 2).
biomass nano-composite properties, orientation, and moisture content, Below 300◦ C, biomass dehydrates and dries and releases the gas in the
as these properties are virtually unaffected by the pyrolysis process pores, mainly some inorganic gas, such as N2 and CO2. As the temper­
(Weber and Quicker, 2018). The anisotropy of the charring process ature increases, the long carbon chain breaks and splits to form small
makes the biochar less mechanically stable than the parent. The loss of organic molecules. A large amount of bio-oil is precipitated from
hydrophilic oxygen-containing functional groups on the biochar surface biomass, including organic acids, aromatic hydrocarbons, tar, methanol,
and the decrease in oxygen-carbon (O/C) and hydrogen-carbon (H/C) acetone, acetic acid, and etc., In the meantime, also releases some un­
ratios greatly affect the stability of biochar when the temperature saturated gaseous hydrocarbons (e.g., CH4). The solid component at this
increases. time is coke. The coke yield might be an indirect indicator of the amount
of the heavy compounds in the bio-oil. When the temperature reaches
2.2. Chemical properties about 500–700◦ C, biomass will be further decomposed, the amount of
bio-oil is very small, a lot of syngas is released, volatile organic com­
2.2.1. Elemental composition pounds (VOCs) leave the biomass, and the final non-volatile components
Biochar has lower volatile matter content than bio-wastes. Biochar is are coke and biochar. The proportions of these products depend on the
formed from non-volatile compounds (e.g., aromatic substances) and temperature range, pressure, and residence time, etc. (Tomczyk et al.,
volatile compounds (e.g., aliphatic substances), which have a high C and 2020).
low O content and aliphatic substances are more affected by increasing Based on the residence time, temperature, and heating rate, pyrolysis
temperature (Ortiz et al., 2020). During the reaction process, the carbon can be divided into slow, fast and flash pyrolysis (Chi et al., 2020)
content of the biochar increases considerably, while the hydroxide (Fig. 2). Biomass pyrolysis for a long time (a few hours to a few days) at a
content is lower. Untreated wood usually has a carbon content of slightly temperature lower than 400◦ C with a very low heating rate under 10◦ C
more than 50% and an oxygen content of slightly more than 40% (by min− 1 can yield 30% biochar. The process is called as slow pyrolysis.
weight, no dry ash). During biochar production, the most significant Fast pyrolysis temperature is high, generally between 500 and 700◦ C.
changes occurred at the temperature range of 200–400◦ C. Both values Compared with other pyrolysis processes, the heating rate is faster,
gradually approached the limit values of 100% (carbon) and 0% (oxy­ which is between 10 and 100℃/s. The resident time less than 2 s. The
gen) at higher temperatures. The carbon contents of high-temperature biochar obtained by this method has higher oxygen content and lower
biochar can reach more than 95%, and the oxygen contents are less calorific value. Biomass is burnt at high temperatures in flash pyrolysis,
than 5%. The hydrogen content of wood varies from 5% to 7%, and mostly higher than 1000◦ C. The heating rate sometimes is very fast.
decreases to less than 2% during pyrolysis above 700◦ C, or even less Temperature reach 1000℃ within a short time, mostly less than dozens
than 1% in some case (Weber and Quicker, 2018). Higher pyrolysis of seconds. Such conditions cause a high production of bio-oil and low
temperatures result in lower hydrogen-carbon and Oxygen-carbon ratios biochar yield in the process (Gabhane et al., 2020). However, slow py­
for biochar, and lower temperatures can be used to produce biochar rolysis tends to yield higher proportions of biochar because of its longer
when we use biomass to improve soil quality (Sato et al., 2019). residence time and slower heating rates. In absence of O2, slow pyrolysis
performed at moderate temperature (350–550◦ C) and long residence
2.2.2. pH and electrical conductivity (EC) time caused higher yield of biochar (30%) than the fast pyrolysis, flash
Primary biomass is usually slightly acidic or slightly alkaline, with a pyrolysis or gasification. Engineering biochar production techniques can
pH between 5 and 7.5 (Weber and Quicker, 2018). Pyrolysis causes some be roughly divided into physical-engineering techniques, chemical-
of the acidic functional groups of the biochar (e.g., carboxyl, hydroxyl, engineering techniques, and biological- engineering techniques (Fig. 4).
or carbonyl groups) to separate early in the carbonization process, ul­
timately resulting in an increase in the pH of the biochar. Temperature is 3.1. Physical-engineering techniques
one of the most critical variables that affects the biochar pH. The main
reason for the increase in pH is the breakage and fracture of weak bonds Physical modification is a way to enhance or enlarge some charac­
(e.g., hydroxyl bonds) in the biochar structure at high temperatures (Li teristics of biochar through some physical methods. The biochar pro­
et al., 2018a,b). The liming effect of biochar has important applications duction strongly depends upon several variables like biomass
in regulating greenhouse gas emissions from acid soils, reducing CO2 characteristic (e.g., biomass type, moisture content, bulk density, and
and N2O emissions from soils due to an increase in soil pH, and reducing particle size), reaction conditions (e.g., resident time, temperature, and
nitrogen fertilizer use due to the use of biochar (Wu et al., 2018). The heating rate), environment conditions (e.g., pressure, type of carrier gas,

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Y. Zhou et al. Bioresource Technology 337 (2021) 125451

Fig. 4. Postulated mechanisms of biochar interactions with inorganic contaminants.

and blow rate of carrier gas), reaction device and catalyst etc. (Tripathi other organic compounds with stronger chemical bonds were degraded
et al., 2016). Physical techniques in biochar modification are more in the last stage (Cárdenas-Aguiar et al., 2017). A higher biochar yield
convenient in the aspects of cost and time while chemical activation by was appeared than with others with a process of slow pyrolysis. At lower
acids or alkali requires more time to complete the reaction in pre and pyrolysis temperatures, the yield of biochar was increased due to the
post treatment of the samples (Banerjee et al., 2016). Physical modifi­ partial pyrolysis of the cake, meanwhile, dehydration and carbonization
cation can be carried out from temperature, pressure, initial pH, parti­ of cellulose and lignin. Generally, in the pyrolysis process, with the in­
cles sizes, gas/steam activation, microwave modification, magnetic crease of temperature, the yield of biochar decreased and the yield of
method and other aspects. syngas increased. Low pyrolysis temperature is beneficial to the for­
mation of biochar yield.
3.1.1. Temperature
The changes of physicochemical properties and biochar structure are 3.1.2. Pressure
closely related to pyrolysis temperature. Chemical bonds are rearranged The state that the pressure factor has the greatest influence on the
and new surface functional groups are generated (phenol, pyridine, modification result is vacuum pyrolysis. The most important effect of
carboxyl, lactone and etc.) at higher temperature. These surfaces func­ pressure means that the modification process is vacuum pyrolysis.
tional groups can play a significantly role as electron donor and During the vacuum pyrolysis, pressure range was controlled between
acceptor. It improves the activity of biochar and changes the perfor­ 0.05 and 0.20 MPa and temperature was from 450◦ C to 600◦ C. Under
mance of biochar macroscopically. On the contrary, in case of low vacuum conditions, only using vacuum or low pressure to remove the
temperature during pyrolysis, the biochar produced the smoother steam generated during pyrolysis process can effectively prevent the
exterior structure and exposed fewer active functional groups on its volatilization of inorganic compounds, which has a significantly positive
surface. impact on the product yield and quality. Vacuum pyrolysis is an
Biochar production can be divided into three stages: pre-pyrolysis; important method to produce high quality biochar.
main-pyrolysis and formation of carbonaceous products (Lee et al., Pyrolysis under high pressure requires more strict reaction condi­
2017c). In the first stage, ambient temperature shall not exceed 200◦ C. tions and higher economic cost. High pressure puts forward high re­
Evaporation of moisture and light volatiles occurs at this time. The quirements for pyrolysis device. The operating pressure has a certain
moisture content evaporation leads to breaking of chemical bonds and effect on the yield of biochar. The influences of peak temperature
forms –CO, –COOH groups and hydroperoxide (Cárdenas-Aguiar et al., (400–550◦ C) and absolute pressure (0.1–1.5 MPa) on pyrolysis process
2017). The second stage temperature from 200 to 500◦ C. It is a period of of two-phase olive mill wastes were investigated (Panwar et al., 2019).
rapid devolatilization and decomposition of hemicellulose and cellulose. With the increase of peak temperature and absolute pressure, the yield
When the temperature is above 500◦ C, it enters the last stage. Lignin and of biochar decreased, while the yield of fixed carbon increased.

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Y. Zhou et al. Bioresource Technology 337 (2021) 125451

Furthermore, the effect of different pressure, peak temperature and gathering pollutants that causes more serious regional or point source
particle size on the stability of the vine shoot-derived biochar was pollution. Smaller biochar particles either move downward in the soil
studied (Manya et al., 2014). The results showed that applying pyrolysis layer or are easily eroded by wind or water. Ball milled biochar and
under both high temperature and high pressure can maximize the yields immobilization technology combining used in pollution treatment is
of pyrolysis gas, but reduce the production of char. Just increasing both a future consideration and a new trend of development.
pressure makes a different manifestation in pyrolysis component. The
increase of pressure from atmospheric pressure has a significant effect 3.1.5. Gas/steam activation
on the pyrolysis behavior. The production of both biochar and fixed Steam activation is a commonly used physical activation method
carbon has increased under high pressure. This discovery maybe due to which can promote the formation of macropores, meso and micro-pores
the secondary reaction enhancement. Therefore, increasing pressure can during the decomposition of volatile components. The activated carbon
also be regarded as a promising way that improve the potential stability has larger total pore volume, larger inner surface, wider pore width
of biochar effectively. distribution, and higher proportion of meso-pores with the increasing
resident time of activation (Muthmann et al., 2020). It also helps to
3.1.3. Influence of initial pH expose the metal content of biochar to increase the amount of cation
This pH does not refer to the pH of biochar, but the pH of a medium. exchange capacity (CEC). This improvement may be due to the devel­
The biochar acts on wastewater, the initial pH of wastewater has an opment of activated carbon leading to the formation of pores in biochar,
impact on the function of biochar. PH value is one of the parameters that the increase of specific surface area and its surface activity degree. This
have to be mentioned in the process of biochar adsorption, because it has activation method can also remove incomplete combustion parts and
an important influence on the metal morphology and surface charge of other impurities from biochar. In addition, the utilization of functional
adsorbent at the same time. The study of pH value is also helpful to groups on the surface of biochar was reduced by steam activation
understand the binding mechanism of combining various metal ions (Panahi et al., 2020). The surface area and pore volume of biochar might
with activated biochar (Tang et al., 2018). Most of the heavy metals be enhanced by CO2 modification and the activated centers were also
(HMs) show free-state in acidic condition and are easy to migrate, on the built up on the surface of biochar. Shao et al. (2018) observed that the
contrary, most of them exist in the combined state under the alkaline micro-pore capacity, micro-pore area and specific surface area of engi­
condition and are not easy to migrate. The initial pH value of the solu­ neered biochar prepared from corncob feedstock increased dramatically
tion has a significant effect on the molecular structure of heavy sub­ in CO2 atmosphere. The specific surface area increased from 56.91 m2/g
stances, the surface charge of biochar, the ionic state of surface to 755.34 m2/g after CO2 modification.
functional groups and the active center of modified activated biochar,
thus affecting the adsorption performance. The modified pine biochar 3.1.6. Microwave modification
was impregnated with magnetic ferromanganese oxide to obtain the Microwave is a kind of electromagnetic basic irradiation which is
maximum specific Brunauer–Emmett–Teller surface area, pore volume efficiently easy to control in application at the 0.03–300 GHz fre­
and average adsorption pore size. In addition, further, the best adsorp­ quencies and wavelengths of 0.01–1 m. The microwave pyrolysis can be
tion performance was observed at solution pH 4 (Liang et al., 2019). The carried out at low temperature of 200-300℃ in reaction, and the yield of
adsorption of inorganic nonmetallic ions by biochar is also affected by biochar can reach more than 60 wt%. It shows strong energy-saving
pH value. The results were observed that the adsorption of phosphorus ability and high efficiency. Microwave pyrolysis using electromagnetic
on Mg/Al modified biochar is highly pH dependent. With the increase of radiation as heat source is a kind of thermal degradation in inert envi­
pH value from 3 to 11, the adsorption capacity decreases successively. ronment. The use of microwave radiation causes the dipole rotation of
Therefore, pH 3 was an ideal condition for this study, which was useful particles and generates heat from the interior of the material. Compared
to adsorb pollutants (Li et al., 2016). with the traditional heat transfer from surface to interior part by con­
duction mechanism, microwave pyrolysis allows both interior and sur­
3.1.4. Particles sizes face area to be heated simultaneously. Microwave modification has
Particles sizes mainly affect the specific surface area of biochar. In economically lower production cost and potentially shorter processing
general, the smaller biochar particles sizes are, the larger the specific time to produce desired products than traditional pyrolysis. This pro­
surface area is. With large specific surface area of biochar has strong vides strongly power for the conversion of biomass into produce biochar
adsorption properties. The dominant reason for the adsorption capacity by microwave pyrolysis technology.
of activated biochar is higher specific surface area. Compared with
activated carbon, however, raw biochar is easy to produce, but its 3.1.7. Magnetic biochar
adsorption capacity is far limited. The method of changing the particle Biochar used for composting or adsorption of HMs will produce
sizes of biochar to enhance adsorption capacity is ball milling. In the secondary pollution after separation, because its HMs content is too
process of ball milling, the functional groups in biochar can also be high, which hinders the regeneration and recycling of biochar. An
modified in the presence of appropriate chemical reagents in addition to effective solution is to combine suitable magnetic media e.g., zero valent
specific surface area and micropore. After 2 h of milling, the yielding iron (Fe0), gamma-Fe2O3, CoFe2O4, and Fe3O4 with biochar. Engineered
adsorbents are approximately found as 200 nm in mean diameter. After biochar can be collected easily from aqueous solution by applying
ball milling for 3 h, the degradation rate of tetracycline adsorbed on magnetic force through this strategy. Nevertheless, Bare Fe3O4 nano-
biochar/Fe3O4 highly reached 99% (Shan et al., 2016). Through suffi­ particles tend to be instable because of their easy detachment from
cient ball milling, nano sized biochar can be prepared, and its perfor­ biochar matrix. Hence, it is a challenge to immobilize Fe3O4 nano-
mance in removing organic and inorganic pollutants is equivalent to that particles on biochar matrix. Amino-terminated 3-Triethoxysilylpropyl­
of activated carbon and carbon nanotubes (Shan et al., 2016). Ball amine (TSA) with like-hand structure was used as coupling agent to
milling is not only an economical and simple method to prepare high- connect biochar and Fe3O4 particles, which ensured that the nano-
efficiency adsorbent for the removal of pollutants from environment, particles could be encapsulated on the surface of biochar (Zhou et al.,
but also an anticipated technology to degrade pollutants adsorbed on 2018a,b). After magnetic modification, the removal rate of As and Cr for
adsorbent and reduce its environmental risk. However, the application sludge biochar increased extremely from less than 5% and 3% to more
of the ball milled biochar in the open environment is difficult to recover, than 80% and 25% respectively (Kasiuliene et al., 2018).
and it is easy to cause secondary pollution if it is not treated well. If used
biochar can’t be separated from the environment in time, it will not play
the role that it should have at first of controlling pollutants, just

7
Y. Zhou et al. Bioresource Technology 337 (2021) 125451

3.2. Chemical-engineering techniques of acid modified biochar has increased.

Chemical activation is the most extensively used activation method. 3.2.3. Alkali-treatment
Chemical activation is also known as wet oxidation. Chemical modifi­ The alkali-treatment of biochar improves the oxygen-containing
cation is more widely utilized than physical modification, because it has functional groups, pore volume, and surface area on the ultimate
some remarkable characteristics such as the following: (1) demands modified biochar. KOH and NaOH are the most commonly used alkali-
short disposal time and low activation temperature; (2) activated bio­ activating reagents. The methods of alkaline activation are soaking in
char is provided with larger surface area; (3) more yield production of alkaline agents at room temperature, then soaking and stirring continue
activated biochar; and (4) micro-porous structure is well developed and taking as long as 6–24 h that depends on the type of raw materials of
evenly distributed (Panwar and Pawar, 2020). Chemically activated biochar. After washing the biochar, further pyrolysis was carried out in a
biochar has large specific surface area and developed microporous reactor at 300 to 700◦ C for 1–2 h in nitrogen environment. The surface
structure, which has been widely and vitally used in the environmental of biochar generated positive surface charge in alkaline modification,
applications. The adsorption capacity of biochar is conducive to remove which was advantageous to absorb pollutants with negative charge.
pollutants from air, water, and soil effectively. Oxidizing agent modifi­ KOH-modified biochar is considered as an economically cost-effective
cation, acid modification, alkaline modification, and metal salts modi­ and environment-friendly adsorption material for the removal of HMs
fication are mainly included in Chemical modification. Acids like H2SO4, in wastewater. The previous publication shown that after NaOH treat­
HNO3, HCl, H3PO4, and H2O2, alkali (such as NaOH and KOH) and ment on biochar, the adsorption capacity of walnut biochar HMs
oxidizing reagents have been already used in chemical-engineering increased exponentially by 3–5 times (Liang et al., 2019).
techniques as some of the chemicals to acquire target properties for
engineering biochar. 3.2.4. Metal salts impregnation
Impregnated biochar is instrumental in improving the physico­
3.2.1. Oxidizing modification chemical properties of biochar by forming composite materials, so as to
Acid or alkali can be utilized for biochar oxidation, and the cation improve the mass yield and adsorption performance of modified bio­
exchange capacity, functional group utilization, micropore and specific char. The catalytic, adsorption, and magnetic properties of biochar can
surface area of biochar could be improved. The appropriate oxidant be improved by impregnated biochar with metal oxides or metal salts. At
effectively improves the adsorption capacity and uptake capacity of present, there are some biomass-derived modified biochar that
biochar to HMs. The reason caused these improvements might be impregnated with metal oxides, including CaO, ZnO, FeO, MnO etc. and
attributed to the induced affinity binding between metals with positively metal salts including LaCl3, MgCl2, AlCl3, FeCl3 etc. Biochar have been
charged and carbonyl with negatively charged groups in biochar. Oxides activated to achieve the aim which could improve the adsorption ca­
improve the biochar physico-chemical properties by forming new pacity for adsorbing anions. The biochar impregnated with oxide ex­
composites sites, which normally enhances the adsorption capacity of hibits new hydroxyl groups, larger pore size and better surface area. The
biochar. The oxide-modified biochar indicated higher thermal stability studies indicated that the biochar treated with zinc had an excellent
than the original biochar, which may be owing to the existence of removal effect on Pb(II) and p-nitrophenol in waste water (Wang et al.,
thermally inactive minerals as diluents. Compared with other activation 2017). The removal efficiency of contaminant by adding zinc to func­
processes, oxide-derived biochar generally reveals stronger capacity of tional materials was investigated. The results showed that during the
metal binding. Oxide biochar has more abundant surface functions than preparation process zinc was helpful to form hydroxyl groups on the
other activated biochar. Such engineered biochar could normally be ZnO particles surface, which is conducive to the removal of organic
promoted as a dependable method for adsorption of organic and inor­ pollutants and HMs.
ganic pollutants from drinking water and waste water. Sulfamethoxa­
zole was almost completely removed (>97%) within 30 min by adding 3.3. Biological- engineering techniques
potassium permanganate and biochar powder into sulfamethoxazole
solution at the same time. The degradation of sulfamethoxazole medi­ 3.3.1. Microorganism immobilization technology
ated via highly oxidative intermediate manganese species could be Cell immobilization can be used in microbial treatment to limit mi­
boosted by biochar treated with KMnO4 (Tian et al., 2019). croorganisms in a certain spatial area, so as to maintain biological ac­
tivity and high proliferation rate. In addition, immobilization provides
3.2.2. Acid-treatment stronger resistance to toxic compounds, higher cell density and better
Acid activation is a certainly essential means for subsequent appli­ reusability. Meanwhile, biochar has been also regarded as a promising
cation to regulate the physicochemical properties of biochar by material to immobilized microorganisms due to its rich porosity, virous
importing acidic functional groups on the surface of biochar. Removal of surface functional groups, large specific surface area and cost-
metal impurities is the central reason for the acidic modification of effectiveness. Recent biochar studies have explored its strength as a
biochar, in which the acidic functional groups alter the physical and carrier of microbial inoculation. Biofilm theory is proposed to explain
chemical properties of biochar to a great extent. Strong acid could microorganism immobilization technology. In order to adhere and
strengthen the adsorption capacity of modified biochar via increasing attach to the surface of biochar, living cells will secrete multiple poly­
the size of surface area and quantity of functional groups connected on mers and fix themselves on the substrate, forming an extracellular ma­
biochar surface. The H3PO4–derived modified biochar has larger surface trix -enclosed microbial biofilm (Frankel et al., 2016; Lin et al., 2021b).
area and more oxygen-containing functional groups, which leads to The close proximity biofilm-embedded cells might permit metabolite or
higher adsorption of some heavy metal ions than raw biochar (Panwar genetic exchange, thus promoting the degradation and adsorption of
and Pawar, 2020). On the contrary, weak acids such as tartaric, acetic, pollutants.
and citric acids treated on biochar could also form carboxyl functional To immobilize the microorganisms on biochar, biochar is firstly,
groups on the surface of biochar, however, the specific surface area of sterilized at 121℃ for 20 min then add cell suspension. The mixture was
biochar declined slightly after modification (Sun et al., 2015). Acid cultured on a rotary shaker at 150 rpm and 30◦ C for 24 h until the cells
activation could decrease the pH value of biochar, thus the biochar at were adsorbed on the biochar pores and surface (Lou et al., 2019). The
lower pH has a broad application prospect in alkaline calcareous soil. application of biochar immobilized bacteria in soil can improve the
Compared with other activation methods, acid modified biochar has a removal rate of Cd indirectly by promoting plant growth, so as to
smaller surface area, which may be the result of pore structure failure. improve the phytoremediation effect (Chuaphasuk and Prapagdee,
However, compared to other modification processes, the oxygen content 2019). Phosphate-solubilizing bacteria (PSB) with biochar as carrier can

8
Y. Zhou et al. Bioresource Technology 337 (2021) 125451

directly immobilize Pb by increasing the release of phosphorus, thus extremely activates HMs resistant bacteria to enhance the immobiliza­
forming stable pyromorphite precipitation (Chen et al., 2019). The tion of HMs in sheep manure compost (Liu et al., 2021). They found that
removal of phosphorus (P), nitrogen (N) and bioavailable carbon as the most effective biochar addition to bacterial diversity was 7.5%. The
measured by chemical oxygen demand (COD) by microorganisms using surface functional groups of biochar could be related to the immobili­
biochar as carrier has also been widely concerned. zation activities of metals (Yang et al., 2018). Aromatic carbon groups
could improve the Pb immobilization. Inorganic mineral (Ca2+, K+,
3.3.2. Anaerobic digested (AD) Mg2+, and Na+) richness also helps to increase the activity in trapping
The pH value and redox potential of biomass anaerobic digestion metals (Yang et al., 2018). Biochar applications not only reduce HMs but
(AD) residues are more suitable, which can be effectively used in the also promote dissolved organic matter and microbial growth (El-Naggar
production of engineered biochar. The biochar prepared from AD et al., 2020).
treated biomass has high quality in anion exchange capacity, cation In recent studies, different types of applications with biochar treat­
exchange capacity, specific surface area, and surface negatively charge ments have been started to increase metal bioremediation. Bioremedi­
for contaminant treatment, compared to pristine biochar. Biochar which ation uses biological metabolic activities to reduce the content of HMs or
prepared from AD treated biomass and then pyrolysis has revealed change their states. Phytoremediation, animal remediation and micro­
excellent removal efficiency for cationic dyes and HMs. The preparation bial remediation are three methods of bioremediation technology. And
of biochar from anaerobic digestion residues can not only reduce the microbial remediation is the most popular and efficacious direction in
cost of waste treatment and disposal, but also a persistent way to pro­ HMs pollution treatment. Since microorganisms can synthesize extra­
duce bioenergy economically and environmentally. Biochar application cellular enzymes, which can be played an important role in metal
in anaerobic digestion can enhance the efficiency of AD. The most likely removal by attaching to the surface of biochar (Sanchez-Hernandez
explanation for these results is that biochar acts as an electron mediator et al., 2019; Hu et al., 2020b). Qi et al. (2021) reported that the com­
or shuttle rather than an electron catheter to promote interspecific direct bination of biochar and mixed bacteria MB9 (Bacillus subtilis, Bacillus
electron transfer is the key mechanism of biochar promoting interspe­ cereus, and Citrobacter sp.) significantly improved the remediation effi­
cific electron transfer (Wang et al., 2020a,b). The redox active organic ciencies of uranium and cadmium. In addition, MB9 bacteria were well
functional groups on the surface of biochar are more conducive to colonized after bioremediation treatments, and this application in
electron exchange between electron acceptor methanogens and electron farmland can be both environmentally friendly and cost-effective
donating bacteria, compared to directly attach on biochar. remediation (Qi et al., 2021). Earthworms like microorganisms may
act as a biological vector that facilitates the adsorption of enzymes on
4. Environmental application of biochar biochar (Sanchez-Hernandez et al., 2019). Xiao et al. (2021) reported
that earthworm (Eisenia fetida), alone or combined with biochar or Ba­
Biochar is the carbon-rich product which generated from feedstock’s cillus megatherium could be an alternative method for cadmium reme­
such as wood, crop residues, organic wastes, dairy manure, sewage diation. Interestingly, Wang et al. (2020c) reported that the cadmium
sludge, industrial wastes (olive oil mill water) (Sohi et al., 2009; Zhang concentration was decreased while the available arsenic concentration
et al., 2020b). When biochar is used in large areas such as agricultural with biochar applications was increased. It was also reported that the
lands, it can remove harmful materials such as pollutants and HMs, and accumulation of arsenic and cadmium in the plant decreases, and cad­
is used as a natural absorbent material for waste management. Similarly, mium accumulates to a large extent in the worms in this environment
it is used as a natural filter for removing antibiotics, pesticides and (Wang et al., 2020a). Huang et al. (2020) reported that the application
pharmaceuticals in wastewater treatments. In addition to its usage ad­ of earthworms with biochar had a negative effect on worms, and also
vantages, waste minimization is also successfully achieved due to its suggests considering the potential risks of biochar on organisms.
production from various wastes. The activity of biochar applications can be increased not only by
using micro or macro-organisms together but also with different com­
4.1. Toxic metals remediation binations. Cadmium removal was successfully done by using combina­
tion of maize straw biochar, wheat straw biochar and ferrous sulfate or
Compost could be contaminated with HMs (Cd, Pb, Cr, Cu, and Zn) combined with ferrous sulfate and pig manure (Chen et al., 2021).
and organic compounds such as polycyclic aromatic hydrocarbons Biochar and compost mixtures can also affect the mobility of HMs by
(PAHs), fertilizers, pesticides, antibiotics, and polychlorinated biphenyls changing the physicochemical properties of the medium in which HMs
(PCBs) due to its feedstock come from different materials. Heavy metal exist (Liang et al., 2017). The combination of different biochar types
accumulation in composting can affect compost fertility, quality, mi­ (wood biochar and sewage sludge derived biochar) provide both waste
crobial diversity and activities (Lin et al., 2021a), and contaminated treatment and toxic metals remediation (Penido et al., 2019). Corn
agricultural products grown can pose a risk to human health. Heavy stalks, mushroom residues, vegetable straw and poultry manure were
metals spread through the food chain, hydraulic power, human migra­ used as raw materials for united composting. And it was found that the
tion and other means to various places. Similarly, organic compounds bioavailability of Cd was significantly reduced after treatment with
are also potential pollutants which adverse effect on plant growth and wheat stack biochar (WB) and rice husk biochar (RB) especially RB, and
microbial communities (Dutta et al., 2017). As was controlled by biochar adsorption, but there was little effect on Cu
Ahmad et al. examined the properties of variable biochar on metal and Zn (Zhang et al., 2021b). With the widespread use of pesticides in
removal (Ahmad et al., 2017). For this, the HMs removal rates were agricultural lands, it has become an important source of environmental
investigated via biochar obtained at 300◦ C and 700◦ C from different pollutants such as heavy metal pollution. The methods applied in heavy
type of agricultural wastes (pine needles, peanut shells and soybean metal bioremediations are similarly used in the improvement of organic
stover). In alkaline environment, Pb and Cu mobility was more effective materials such as pesticides (Cao et al., 2009; Zhang et al., 2013). In
by biochar produced at 300◦ C. On the contrary, activities of biochar some cases, the application of biochar may reduce the degradation and
produced at 700◦ C showed better performance in decreasing mobility of mineralization of micro-pollutants such as 2, 4-dichlorophenol and
Zn and Pb in the acidic environment (100%) (Ahmad et al., 2017; Zhang phenanthrene and cause their accumulation (Gu et al., 2016). Biochar
et al., 2020a). In addition, the pH might be changed after addition of can be modified and used actively to remove pollutants. For example,
biochar. Changing the pH can provide advantages for heavy metal biochar enzymatic activation can be achieved in the existence of
immobilization (Jiang et al., 2012). In acidic environment, heavy metals earthworms. Biochar-bound enzymes (alkaline phosphatase, β-glucosi­
are easy to exist in free form, while in alkaline environment, most of dase, arylsulfatase, and carboxylesterase) could supply adsorption of
them are in combined form with strong stability. Cow manure biochar organophosphorus (23–100% of control activity) and methyl carbamate

9
Y. Zhou et al. Bioresource Technology 337 (2021) 125451

pesticides (37–57%) (Sanchez-Hernandez et al., 2019). Table 2

Current studies on the main effects of biochar for environmental management.
4.2. Carbon sequestration Biochar types Compost Influence References
substrates
Carbon sequestration is the capture and ensuing storage of carbon in Biochar mixed Straw and Reduce the duration of (Qu et al., 2020)
order to prevention it from being released into the atmosphere (Duku with gypsum chicken composting decreased
et al., 2011; Hu et al., 2020a). It results from the double benefits of long- manure nitrogen and carbon
term carbon sequestration and potentially positive soil change with the mixture losses and latent
ecological danger
application of biochar to agriculture/soil (Ennis et al., 2012; Ten­ composting quality
enbaum, 2009). Characteristic properties of biochar (fixed carbon, sur­ and nutrient retention
face area, ash content, pH, and etc.) could be depend on feedstocks and are improved by
biochar process conditions (Manyà, 2012). The carbon content of bio­ biochar mixed with
gypsum
char is generally inversely related to the feedstock’s ash content
Biochar obtained Urban organic Organic matter loss (Tessfaw et al.,
(Windeatt et al., 2014). Natural minerals present in feedstocks that used from Khat solid waste was reduced the 2020)
for biochar formation can interact with organic materials in pyrolysis, straw increasing of
and might affect the properties of the pyrolysis product (Nan et al., concentration occurred
2019). Nan et al. (2019) investigated the effects of natural minerals on in P, K, Ca, Mg and Zn
The contents of few
carbon conversion by pyrolysis of peanut hull, barley grass, sewage heavy metals such as
sludge, and cow manure. Removing natural minerals could decrease low Mn, Cu, Fe were lower
molecular weight organic compound emissions and retain more carbon than the control
(3.5–30.1%) in biochar during pyrolysis (Nan et al., 2019). Biochar can Grape husks Municipal Level of potassium (Vamvuka et al.,
biochar solid wastes were increased by 76% 2020)
be classified into two groups which are labile and recalcitrant carbon
Heavy metals
depending on carbon content (Zheng et al., 2019). Labile carbon could concentrations were
easily be applied by soil microorganisms in biochar application, and reduced by 40%-95%
carbon mineralization can be restored. On the other hand, recalcitrant Sewage Sludge- Zn-mining area Cd, Pb, and Zn (Penido et al.,
carbon is more difficult to decompose and can be available in the soil for derived bioavailability were 2019)
biochar (SSB) reduced
more time (Chen and Frank, 2020; Gascó et al., 2019). Soil organic Waste disposal
carbon (SOC) can affect the microbial activity of the soil and then lead to problems could be
increased organic matter decomposition and CO2 production (Jílková solved
et al., 2020; Liu et al., 2020). Both increasing and decreasing carbon Municipal solid Benzene/m- Biochar can be an (Jayawardhana
waste-derived xylene alternative and an et al., 2019a,
emissions were observed in biochar applications. Especially, biochar
biochar economical adsorbent 2019b)
used in acidic environments caused more CO2 emissions due to probably Volatile organic
more biochar degradation, while total CO2 emissions were decreased in compounds could be
alkaline environments (Sheng et al., 2016; Sheng and Zhu, 2018). On the removal via biochar
other hand, biochar applied to the soil helps to significantly reduce applications
Biochar Agricultural Biochar can be (Meng et al.,
emissions of CO2, N2O, and CH4 (Renner, 2007; Yang et al., 2020). Along waste effective inoculum 2020)
with these, biochar application has been proposed as a promising addition in solid-state
treatment for carbon sequestration and global warming mitigation anaerobic digestion
(Sheng et al., 2016; Sheng and Zhu, 2018; Lin et al., 2021b). Methane generation
and volatile fatty acids
degradation were
4.3. Solid waste management promoted
Carbon dioxide
Biochar as a carbonaceous material has been widely used for both utilization were
remediation of HMs and organic compounds, and solid waste manage­ improved
Biochar from Switch grass Toxicity of (Sun et al., 2020)
ment can also be achieved by generating biochar from various waste lignocellulose hydrolysate lignocellulose derived
(Table 2) (Jayawardhana et al., 2019a, 2019b; Lee et al., 2017; Penido feedstocks microbial inhibitory
et al., 2019; Shaaban et al., 2018). In studies conducted with biochar on compounds were
organic solid waste composting, the biochar addition ratio has been removed.
Microbial growth and
evaluated and generally ranges between 2 and 20% (Xiao et al., 2017;
bio-productivity were
Tessfaw et al., 2020). Depending on the type of biochar, the removal enhanced
levels of nitrogenous compounds were varied in waste composting (Qu
et al., 2020). In some cases, it is suggested that composting enriched
with some metals such as phosphorus, calcium, magnesium can be used (Margot et al., 2015; Rogowska et al., 2020). Biochar has porous
as biofertilizer (Tessfaw et al., 2020; Vamvuka et al., 2020). Biochar can structure and surface functional groups, and with these properties it can
be evaluated as an alternatively pH buffer and source of mineral sup­ be evaluated as a potential adsorbent (Ahmad et al., 2014; Peiris et al.,
plementation and thus can play a protective role in waste recovery 2017). Biochar adsorbents have been used to remove various contami­
(Meng et al., 2020; Sun et al., 2020, 2018). Addition of biochar into the nants such as antibiotics, pesticides, pharmaceuticals and personal care
solid-state anaerobic digestion medium can be improved the production products from aquatic environments (Ahmed et al., 2017; Thompson
of volatile fatty acids and methane generation from agricultural wastes et al., 2016; Tran et al., 2020; Zheng et al., 2010). Huggins et al. (2016)
(Meng et al., 2020) Table 3. observed that macro-porous biochar has high adsorption capacity at
high COD, PO4 and NH4 concentrations, and that it also high perfor­
4.4. Water and wastewater mance during column treatment of high COD containing wastewater.
Particle size of biochar used in wastewater treatment is an important
Wastewater treatment plants (WWTPs) has been aimed to decline the parameter since it affects the required contact time for wastewater
negative effects of organic micro-pollutants (pharmaceuticals, antibi­ treatment. Zheng et al. (2010) studied the removal of pesticides (atra­
otics, etc.) that affect to aquatic environment and drinking water quality zine and simazine) with biochar obtained from waste biomass with

10
Y. Zhou et al. Bioresource Technology 337 (2021) 125451

Table 3
Elemental analysis and proximate analysis of various biomass.
Biomass Elemental analysis (dry basis, wt%) Proximate analysis (wt%) HHV (MJ/kg) Reference

C H O N S Volatile matter Moisture Fixed carbon Ash

Goat manure 42.08 5.62 39.85 1.45 <1.00 70.13 8.92 4.18 16.77 – (Erdogdu et al., 2019)
Corn cob 44.11 7.98 46.75 0.70 0.46 78.32 3.04 14.39 4.25 17.219 (He et al., 2021)
Cattle manure 30.96 2.34 63.62 2.67 0.41 52.96 6.75 5.41 34.88 12.008 (He et al., 2021)
Spirulina 44.16 6.97 29.86 10.69 1.16 80.67 4.62 7.55 7.16 19.40 (Niu et al., 2021)
Sargassum 28.50 2.78 21.97 2.13 1.19 50.04 4.01 2.52 43.43 10.54 (Niu et al., 2021)
Food waste 47.5 3.9 6.6 31.3 0.4 71.3 3.3 15.1 10.3 22.2–23.7 (Lee et al., 2021)
Rice husk 37.8 5.5 39.30 0.3 – 73.9 Dry basis 9 17.1 15.3 (Fleig et al., 2021)
Cattle manure 30.96 2.34 63.62 2.67 0.41 52.96 6.75 5.41 34.88 12.01 (He et al., 2020)
Swine manure 51.18 6.82 16.82 3.01 0.84 66.72 Dry basis 11.95 21.33 – (Zhou et al., 2018a)
Beef manure 41.85 4.90 32.36 32.36 0.49 68.87 Dry basis 13.08 18.05 – (Zhou et al., 2018b)
Chicken manure 30.89 4.39 24.77 2.28 0.54 61.42 Dry basis 1.44 37.14 – (Zhou et al., 2018a)
Cow manure 67.28 6.22 23.14 3.36 0 59.24 Dry basis 17.32 23.44 – (Lin et al., 2021b)
Dairy cattle manure 38.5 5.0 42.4 3.4 0.5 55.6 3.8 16.0 24.6 – (Jung et al., 2021)

different particle sizes. The treatment time increased up to 2–5 days Declaration of Competing Interest
from 1 day when the increased particle size (<0.125 and <0.250 mm) of
the biochar is applied instead of smaller size (<0.075 mm) (Zheng et al., The authors declare that they have no known competing financial
2010). interests or personal relationships that could have appeared to influence
Biochar can also be used in various applications by replacing filter the work reported in this paper.
materials. Granular biochar can be considered as an alternative product
at low cost and when compared to activated carbon used in water Acknowledgement
treatment (Huggins et al., 2016; Lin et al., 2017; Thompson et al., 2016).
Dalahmeh et al. (2018) also tried to remove pharmaceutically active The authors are grateful to the Shaanxi Introduced Talent Research
compounds (PhACs) in sewage plants by using biochar filters instead of Funding (No. A279021901), China and The Introduction of Talent
sand filters. It has been reported that biochar type filters were more Research Start-up fund (No. Z101022001), College of Natural Resources
effective to remove of carbamazepine and metoprolol, and it had similar and Environment, Northwest A&F University, Yangling, Shaanxi Prov­
efficient with sand type filters for removing of ranitidine and caffeine. ince 712100, China. The support from all the colleagues and research
Also, biochar filters are more effective in removing organic matter and staff members is greatly acknowledged for their constructive advice and
nitrogen than sand filters, but both filters have been reported to be poor help.
in phosphorus removal (Dalahmeh et al., 2018). In addition, biochars
can be chemically modified to change their polarity, surface area and References
pore volume (Jung et al., 2013). Due to the structural properties of
chemically activated biochars, their adsorption capacities may be AEBIOM Statistical Report, 2019.
Ahmad, J., Patuzzi, F., Rashid, U., Shahabz, M., Ngamcharussrivichai, C., Baratieri, M.,
changeable depending on their target chemical hydrophobicity and 2020. Exploring untapped effect of process conditions on biochar characteristics and
molecular/chemical structures (Jung et al., 2013, Lin et al., 2021b). By applications. Environ. Technol. Innov. 21, 101310.
modifying the surface structure of biochar with magnesium ions, ion Ahmad, M., Lee, S.S., Lee, S.E., Al-Wabel, M.I., Tsang, D.C., Sik Ok, Y., 2017. Biochar-
induced changes in soil properties affected immobilization/mobilization of metals/
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