resources

Review
Synthesis Methods, Properties, and Modifications of
Biochar-Based Materials for Wastewater Treatment: A Review
Bryan Díaz 1,2 , Alicia Sommer-Márquez 2 , Paola E. Ordoñez 1 , Ernesto Bastardo-González 2 , Marvin Ricaurte 1, *
and Carlos Navas-Cárdenas 2,3, *

1 Grupo de Investigación Aplicada en Materiales y Procesos (GIAMP), School of Chemical Sciences and
Engineering, Yachay Tech University, Hacienda San José s/n y Proyecto Yachay, Urcuquí 100119, Ecuador
2 Catalysis Theory and Spectroscopy Research Group (CATS), School of Chemical Sciences and Engineering,
Yachay Tech University, Hda. San José s/n y Proyecto Yachay, Urcuquí 100119, Ecuador;
ebastardo@yachaytech.edu.ec (E.B.-G.)
3 Departamento de Ciencias de la Energía y Mecánica, Universidad de las Fuerzas Armadas ESPE,
Av. Gral. Rumiñahui s/n, Sangolquí 171103, Ecuador
* Correspondence: mricaurte@yachaytech.edu.ec (M.R.); canavas3@espe.edu.ec (C.N.-C.)

Abstract: The global impact of water and soil contamination has become a serious issue that affects
the world and all living beings. In this sense, multiple treatment alternatives have been developed
at different scales to improve quality. Among them, biochar has become a suitable alternative for
environmental remediation due to its high efficiency and low cost, and the raw material used for
its production comes from residual biomass. A biochar is a carbonaceous material with interesting
physicochemical properties (e.g., high surface area, porosity, and functional surface groups), which
can be prepared by different synthesis methods using agricultural wastes (branches of banana rachis,
cocoa shells, cane bagasse, among others) as feedstock. This state-of-the-art review is based on
a general description of biochar for environmental remediation. Biochar’s production, synthesis,
and multiple uses have also been analyzed. In addition, this work shows some alternatives used
to improve the biochar properties and thus its efficiency for several applications, like removing
Citation: Díaz, B.; Sommer-Márquez, heavy metals, oil, dyes, and other toxic pollutants. Physical and chemical modifications, precursors,
A.; Ordoñez, P.E.; Bastardo-González, dopants, and promoting agents (e.g., Fe and N species) have been discussed. Finally, the primary
E.; Ricaurte, M.; Navas-Cárdenas, C.
uses of biochar and the corresponding mechanism to improve water and soil quality (via adsorption,
Synthesis Methods, Properties, and
heterogeneous photocatalysis, and advanced oxidation processes) have been described, both at
Modifications of Biochar-Based
laboratory and medium and large scales. Considering all the advantages, synthesis methods, and
Materials for Wastewater Treatment:
A Review. Resources 2024, 13, 8.
applications, biochar is a promising alternative with a high potential to mitigate environmental
https://doi.org/10.3390/ problems by improving water and soil quality, reducing greenhouse gas emissions, and promoting
resources13010008 the circular economy through residual biomass, generating value-added products for several uses.

Academic Editor: Katarzyna

Keywords: biochar-based materials; wastewater treatment; soil amendment; biochar synthesis
Pietrucha-Urbanik
methods; biochar modification; biochar activation
Received: 14 November 2023
Revised: 13 December 2023
Accepted: 28 December 2023
Published: 5 January 2024 1. Introduction
Biochar is an important, interesting, low-cost material with various agricultural, in-
dustrial, and scientific applications. Biochar is a name given to vegetable-derived charcoal,
which can be used as an agent to improve soil and water quality [1–3]. This carbon-rich
Copyright: © 2024 by the authors.
substance can be produced by the carbonization of biomass residues (e.g., wood, dung,
Licensee MDPI, Basel, Switzerland.
This article is an open access article
manure, or leaves) in thermal conversion processes, such as pyrolysis, torrefaction, and
distributed under the terms and
hydrothermal carbonization (HTC) [4–6]. Among them, pyrolysis is the most common pro-
conditions of the Creative Commons cess to obtain biochar under anaerobic conditions and high temperatures [7–9]. In addition,
Attribution (CC BY) license (https:// heat, syngas, liquid fuels, and pyroligneous acid (wood vinegar) are also generated during
creativecommons.org/licenses/by/ this process [10–12].
4.0/).

Resources 2024, 13, 8. https://doi.org/10.3390/resources13010008 https://www.mdpi.com/journal/resources

Resources 2024, 13, 8 2 of 33

The HTC is a novel technology that produces carbonaceous materials, e.g., biochar.
This process has received much attention due to its eco-friendly, cost-effective, and straight-
forward approach [8,13,14]. During the HTC, the raw material is treated at high pressures
and temperatures to produce various biochar-based materials with a high calorific value,
low humidity, and high combustion performance [15,16], known as hydrochar [17]. In turn,
the hydrochar can be used to generate energy in fuel cells [15], for gas storage [18], as a soil
amendment [19], as a catalyst [20], and as an adsorbent to retain pollutants in water, such
as heavy metals [14,21] and dyes [14,22].
The term biochar has been used in recent times; however, the origins of its concept are
ancient. The Amazon basin has zones up to 2 m deep of terra Preta, a mixture of very fertile
dark-colored soil with high carbon content, ceramic fragments, and organic debris, that
has supported the agricultural needs of the Amazonian people for centuries [23–25]. The
name biochar is related to a carbonaceous material used for environmental rehabilitation,
especially for soil improvement and water treatment. In addition, biochar has potentially
significant implications for climate change mitigation, for example, in capturing CO2 from
air and industrial sources [26,27]. Likewise, using the gases generated during the pyrolysis
process, biochar production may be integrated with other processes, such as bioenergy
generation [6,28].
The use of biochar for agricultural and environmental purposes has been thoroughly
researched and evaluated. This substance benefits agriculture and the environment in
various ways, and its soil persistence and nutrient retention capabilities make it an ex-
cellent soil additive for enhancing crop yields. Biochar can be applied to mitigate soil
contamination by immobilizing heavy metals and organic pollutants [29–32]. Heavy met-
als in soils are extremely damaging contaminants that hinder soil qualities necessary for
successful crop performance [25,33,34]. Heavy metals are not biodegradable and remain
in polluted soil and water for long periods [35,36]. Soil contamination by heavy metals
(e.g., Cd, Cr, Hg, Pb, Cu, Zn, As, Co, Ni, and Se) and persistent organic pollutants (POPs,
such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), poly-
chlorinated dibenzo-dioxin (PCDD), and polychlorinated dibenzofurans) is a global issue
that threatens human life and health [37]. Removing heavy metals from polluted soils can
be expensive and time-consuming for agriculture [33,38]. However, biochar can stabilize
Cd, Cu, Ni, Pb, and Zn in soil and reduce their bioavailability through enhanced sorption
(based on electrostatic attraction, ion exchange, and surface complexation) and chemical
precipitation (incurred by raising the soil pH and adding of carbonate and phosphate
ash) [29,39–41]. Thus, the physicochemical properties of biochar can be considered to aid in
the adsorption of heavy metals and organic contaminants in soils, which is highly beneficial
for environmental mitigation [34,42–44].
On the other hand, biochar can be used as a cleaning agent for polluted water, whether
in any industrial or agricultural field [41,45–47]. Due to the importance of hydrogeological
resources, it is critical to care for and protect them, along with technical and scientific efforts
to mitigate the negative environmental impact of the world. The number of contaminating
compounds introduced into aquatic ecosystems is rising because of the varied uses of
water to fulfill various human activities [48,49]. Various components, such as pesticides,
pathogens, heavy metals, detergents, dyes, medications, and personal care and hygiene
products, alter the water quality [33,48,50,51]. One of the major water pollutants is oil
activity, which, due to the release of oil into the environment, involves operations related
to the exploitation and transport of hydrocarbons, resulting in the gradual deterioration of
the environment [52–55], directly affecting the water and, as a result, the affectation of the
ground. When hydrocarbons combine with water, they form an impenetrable barrier that
limits the growth of biological activity, causing direct harm to fauna and soils that rely on
this already polluted water [56,57].
Biomass conversion favors the production of a carbonaceous material (i.e., biochar)
with interesting physicochemical properties, which allow it to be used as an adsorbent for
both organic and inorganic compounds present in water [49,58,59]. Among these properties
Resources 2024, 13, 8 3 of 33

are a high porosity, surface area, and various surface functional groups, such as carboxyls
(–COOH), phenols, and hydroxyls (–OH) [21,36,59,60]. All the properties have allowed the
use of biochar for wastewater treatment to be increased, considering that the adsorption
process with biochar has significant advantages compared to other treatment methods, like
the low cost, easy operation, and easy maintenance [61–64]. Thus, biochar has become a
substance that meets the requirement of improving the quality of wastewater generated
in industries or any other field, taking advantage of its obtaining from agricultural waste
resources.
There is a high amount of organic and inorganic waste in the world. The most common
organic wastes are categorized as plastic wastes (e.g., bottles, plastics) or residual biomass
according to their origin. In many countries, especially in South America, a considerable
amount of agricultural residues (e.g., banana husk, cocoa, coffee, rice husk, corn, or oil
palm) is generated daily, which have not been used for any application. Furthermore, most
of them, when discarded, generate environmental damage, such as the emission of toxic
gases into the atmosphere and water pollution [65,66]. In this sense, using a substance
produced by residual biomass (i.e., biochar) that helps reduce environmental pollution is
highly beneficial. Considering this fact, agricultural waste might be used as raw materials
to mitigate the damage caused by contaminants worldwide at different scales. Thus, using
carbonaceous materials produced by agricultural wastes would be an economical and
low-cost alternative, which can also generate an added value to the residual biomass that is
rejected or wasted.
Unlike other works, this review article focuses on several lines of interest, starting with
an overview of biochar synthesis methods, e.g., pyrolysis and hydrothermal carbonization.
Likewise, several types of biomass used as raw material to obtain this carbonaceous
material have been listed and analyzed. In addition to these topics, an exhaustive review
has been conducted on some methods to modify the properties of biochar, considering both
the physical and chemical methods. Moreover, several applications of the use of biochar
have been widely studied, and how their efficiency can be correlated to physicochemical
properties, along with different contaminant removal mechanisms (e.g., adsorption and
advanced oxidation processes). Finally, a summary of the pros and cons of the use and
applications of biochar for environmental remediation has been made, identifying that this
carbonaceous material can still be improved, but the results are promising so that it can
continue to be used and thus encourage the circular economy in the world.

2. Biochar: Feedstock, Synthesis Methods, and Properties

Biochar mainly comprises carbon (~60–90%), although it may also contain oxygen,
hydrogen, and inorganic ash depending on the source biomass [67]. Biochar conversion
is considered more environmentally benign than coal combustion, as biomass is carbon
neutral [68]. Generally, biochar has a high surface area (above 100 m2 /g), which depends
on the raw material and the synthesis conditions [69]. As a result, biochar can be used in a
variety of nonfuel applications, such as chemical adsorption (e.g., water treatment [21,70])
and carbon storage [68]. In addition, this carbonaceous material has also been used as a
soil fertilizer [71,72].

2.1. Differences between Biochar, Activated Carbon and Charcoal

The carbon family involves interesting materials, such as biochar, charcoal, and acti-
vated carbon. These carbonaceous materials share the essence and origin, which is carbon.
The most significant distinction is their synthesis methods, conditions, and applications.
On the one hand, coal results from coalification, i.e., a geological process involving biomass
conversion with water and sediments. Peat and lignite are intermediate stages of this
process [73].
On the other hand, charcoal, biochar, and activated carbon are products of thermo-
chemical processes, and they are defined as pyrogenic carbonaceous materials (PCM) [24].
Biochar and activated carbon are frequently used in agriculture for environmental reme-
 massmass
mass conversion
conversion
conversion with with
with water
water
water and and
and sediments.
sediments.
sediments. Peat Peat
Peat
and and
and lignite
lignite
lignite are
areare intermediate
intermediate
intermediate stages
stages
stages of ofof
thisthis
this process
process
process [73].
[73].
[73].
OnOn On the
thethe
otherother
otherhand,hand,
hand, charcoal,
charcoal,
charcoal, biochar,
biochar,
biochar, and and
and activated
activated
activated carbon
carbon
carbon are
areare products
products
products of ofofthermo-
thermo-
thermo-
chemical
chemical
chemical processes,
processes,
processes, and and
andtheythey
they are
areare defined
defined
defined as asaspyrogenic
pyrogenic
pyrogenic carbonaceous
carbonaceous
carbonaceous materials
materials
materials (PCM)
(PCM)
(PCM) [24].
[24].
[24].
Resources 2024, 13, 8 4 ofreme-
33
Biochar
Biochar
Biochar and and
and activated
activated
activated carbon
carbon
carbon are
areare frequently
frequently
frequently used
used
used inagriculture
in in agriculture
agriculture for
forfor environmental
environmental
environmental reme-
reme-
diation,
diation,
diation, such such
such asfiltering
as as filtering
filtering and and
and purification.
purification.
purification. Meanwhile,
Meanwhile,
Meanwhile, charcoal
charcoal
charcoal isused
is is
used used for
forfor heating
heating
heating and and
and
cooking
cooking
cooking [74,75].
[74,75].
[74,75]. These
These
These materials’
materials’
materials’ physical
physical
physical and and
and chemical
chemical
chemical properties
properties
properties areareare similar
similar
similar since
since
since they
they
they
diation,
share
share
share thesuch
the
the ascarbonaceous
same
same
same filtering and purification.
carbonaceous
carbonaceous origin.
origin.
origin. Meanwhile,
However,
However,
However, they
they
they charcoal
have
have
have is properties
singular
singular
singular used for heating
properties
properties that
that
that and
distin-
distin-
distin-
cooking
guish
guish
guish [74,75].
them.
them.
them. These
Table
Table
Table materials’
11describes
describes
1 describes physical
the
thethe marked
marked
marked and chemical
difference
difference
difference properties
between
between
between these
these are
these similar since
carbonaceous
carbonaceous
carbonaceous they
materi-
materi-
materi-
share
als the
and same
the carbonaceous
main origin.
characteristics However,
that define they
them have
and singular
differ from properties
each other. that distin-
alsals
andand thethe main
main characteristics
characteristics thatthat define
define them
them and
and differ
differ fromfrom each
each other.
other.
guish them. BiocharTableis1adescribes
member the
ofthemarked
the carbon difference
family between
that, when these
mixed carbonaceous
with other materialspro-
species,
Biochar
Biochar is is a member
a member of of
the carbon
carbon family
family that,
that, when
when mixed
mixed with
with other
other species,
species, pro-
pro-
andduces
the main new characteristics
hybridized that define
nanomaterial them and
biochar-baseddiffer from
materials each other.
[76]. They have novel phys-
duces
duces new new hybridized
hybridized nanomaterial
nanomaterial biochar-based
biochar-based materials
materials [76].
[76]. They
They have
have novel
novel phys-
phys-
Biochar is a member of the carbon family that, when mixed with other species, pro-
icochemical
icochemical
icochemical properties
properties
properties and and
and are
areare highly
highly
highly effective
effective
effective for
forfor degrading
degrading
degrading water
water
water pollutants
pollutants
pollutants through
through
through
duces new hybridized nanomaterial biochar-based materials [76]. They have novel physic-
adsorption,
adsorption,
adsorption, heterogeneous
heterogeneous
heterogeneous photocatalysis,
photocatalysis,
photocatalysis, and and
and advanced
advanced
advanced oxidation
oxidation
oxidation processes.
processes.
processes. Braghiroli
Braghiroli
Braghiroli
ochemical properties and are highly effective for degrading water pollutants through
etal.
et et
al. al.[77]
[77] [77]
havehave
have reported
reported
reported aahigh
a high high sorption
sorption
sorption capacity
capacity
capacity ofphenols
of of phenols
phenols and and
and chemical
chemical
chemical intermediates
intermediates
intermediates
adsorption, heterogeneous photocatalysis, and advanced oxidation processes. Braghiroli
withwith
with the
thethe use use
use
of ofofactivated
activated
activated biochar
biochar
biochar and and
and other
other
other biochar-based
biochar-based
biochar-based materials
materials
materials for
forfor treating
treating
treating phenolic
phenolic
phenolic
et al. [77] have reported a high sorption capacity of phenols and chemical intermediates
compounds,
compounds,
compounds, such such
such as asasphenol,
phenol,
phenol, bisphenol
bisphenol
bisphenol A,A,A,p-nitrophenol,
p-nitrophenol,
p-nitrophenol, and and
and pentachlorophenol,
pentachlorophenol,
pentachlorophenol, are
areare toxic
toxic
toxic
with the use of activated biochar and other biochar-based materials for treating phenolic
tohealth
to to health
health and and
and the
thethe environment
environment
environment [78].
[78].
[78].
compounds, such as phenol, bisphenol A, p-nitrophenol, and pentachlorophenol, are toxic
to health and the environment [78].
Table
Table
Table 1.1.Description
Description
1. Description ofthe
of of
the the most
most
most influential
influential
influential carbonaceous
carbonaceous
carbonaceous materials.
materials.
materials.
Table 1. Description of the most
Biochar
Biochar
Biochar influential carbonaceous Activated
Charcoal
Charcoal
Charcoal materials.
Activated
Activated Carbon
Carbon
Carbon Ref.
Ref.
Ref.
Carbonaceous
Carbonaceous
Carbonaceous material
material
material with
with
with a aa
Carbonaceous
Carbonaceous
Carbonaceous material
material
material
Biochar pro-pro-
pro- Charcoal Activated Carbon Ref.
highhigh
high surface
surface
surface area area
area produced
produced
produced bybybythe
the the
ducedduced
duced from from
from organic
organic
organic matter,
matter,
matter, ItItaisisporous
It is aaporous
porous black
black
black solid
solid
solid Carbonaceous material with ofa of
Carbonaceous material thermochemical
thermochemical
thermochemical conversion
conversion
conversion ofor-
or- or-
Definitionsuch
Definition
Definition such
such as asasresidual
residual
residual biomass, material
biomass,
biomass, material
material made
made
made upupup ofcarbon
of of carbon high surface area produced by
carbon [73] [73]
[73]
produced from organic matter, It is a porous black solid ganic ganic
ganic matter,
matter,
matter, followed
followed
followed
the thermochemical conversionby by
by
an an
an acti-
acti-
acti-
Definition and and
and it it
it
has has
has environmental
environmental
environmental
such as residual biomass, and in inin
its its
its amorphous
amorphous
amorphous state.
state.
state.
material made up of carbonvation [73]
vation
vation
of process
process
process
organic matter, to
to to
boostboost
boost its
itsits
followed adsorp-
byadsorp-
adsorp-
an
and itand
and energetic
hasenergetic
energetic applications.
applications.
applications.
environmental and in its amorphous state.
tiontion
tion capacity.
capacity.
capacity.
activation process to boost its
energetic applications.
Agricultural
Agricultural
Agricultural residues:
residues:
residues: rice
rice
rice adsorption capacity.
Agricultural
hulls,
hulls,
hulls, manure.
manure.
manure. residues: rice Petroleum
Petroleum
Petroleum residues,
residues,
residues, agricultural
agricultural
agricultural
Feedstock hulls, manure.
Feedstock
Feedstock Hardwood
Hardwood
Hardwood Petroleum residues, agricultural [6]
[6][6]
Feedstock Trees,
Trees,
Trees, shrubs,
shrubs,
shrubs, grasses,
grasses,
grasses, and and
and Hardwood residues,
residues,
residues, and and
and biomass
biomass
biomass in ingeneral.
in general. [6]
general.
Trees, shrubs, grasses, and residues, and biomass in general.
wood.wood.
wood.
wood.
High High
High adsorption
adsorption
adsorption and and
and poros-
poros-
poros-
CharacteristicsHigh adsorption and porosityHigh
Characteristics
Characteristics
Characteristics High
High
High burnability
burnability
burnability
burnability HighHigh
High
High adsorption
adsorption
adsorption
adsorption [73]
[73] [73]
[73]
ity
ityity
There are two main processes:
There
There
There are
areare
two two
two main
main
main processes:
processes:
processes: car- car-
car-
carbonization (pyrolysis,
Pyrolysis,
Pyrolysis,
Pyrolysis,
Pyrolysis, gasification,
gasification,
gasification,
gasification, torre-
torre-
torre- Kiln-calcined
Kiln-calcined
Kiln-calcined
Kiln-calcined bonization
bonization
bonization (pyrolysis,
(pyrolysis,
(pyrolysis, gasification,
gasification,
gasification,
Production
Production torrefaction, HTC.
Production
Production gasification, torrefaction, and [6,73][6,73]
[6,73]
[6,73]
Slow pyrolysis
faction,
faction,
faction, HTC.HTC.
HTC. Slow Slow
Slow pyrolysis
pyrolysis
pyrolysis torrefaction,
torrefaction,
torrefaction,
HTC), and
followed and
and HTC),
byHTC),
HTC),
an followed
followed
followed
activation
bybyprocess.
by
anananactivation
activation
activation process.
process.
process.
Expensive:
Expensive:
Expensive:
Expensive: high-temperature
high-temperature
high-temperature
high-temperature
Cost
Cost Low
Cost
Cost Low
Low
Low cost
cost
cost
cost LowLowLow
Low cost
cost
cost
cost [6] [6]
[6][6]
costs.
costs.
costs.
costs.

Illustrative
Illustrative
Illustrative im-
im-
im-
Illustrative
age
image
age
age

Filtration,
Filtration,
Filtration, water
water
water
Filtration, treatment,
treatment,
treatment,
water treatment, Energetic
Energetic
Energetic use
Energetic use
use
use asfuel
as as
fuel
as fuel (cook-
(cook-
(cook-
fuel Water
Water
Water filtration,
filtration,
filtration,
Water filtration, aesthetic
aesthetic
aesthetic
aesthetic uses,
uses,
uses,
uses,
Uses
Uses
Uses
Uses [73,79–81]
[73,79–81]
[73,79–81]
soilsoil
soil remediation.
remediation.
remediation.
soil remediation. ing).
ing).
ing).
(cooking). medical
medical
medical uses,
medical uses,
uses,
uses, water
water
water
water treatment. [73,79–81]
treatment.
treatment.
treatment.

Compared to pure nano-photocatalysts, biochar-supported catalysts have larger sur-

face areas, are more porous, have higher catalytic capacities, and are more stable [82].
Biochar may support hosting different catalytic nanoparticles because of its unique surface
features, readily modifiable functional groups, chemical stability, and electrical conductiv-
ity [83,84].

2.2. Feedstock for Biochar Production

Biomass is living or once-living organic matter that can serve as a versatile renewable
source for environmental and energy applications (e.g., electricity generation, heat provi-
Resources 2024, 13, 8 5 of 33

sion) and for the production of many types of biofuels, compost, pharmaceutical products,
other chemicals, and biomaterials, like biochar. Almost all organic materials (such as tree
bark, nut shells, crop residues, and manure) can be used as feedstock for biochar using
appropriate equipment [85–87]. Biomass as an initial resource can come from animal, veg-
etable, or human-generated waste, such as industrial or municipal waste (sewage) [66,88].
Biochar-based materials can have different characteristics and properties depending on
the biomass used as feedstock, which, in turn, will allow the carbonaceous material to be
used in specific applications. Table 2 shows the most common feedstock used to produce
biochar, which can be any organic matter, from plant materials to industrial waste.

Table 2. Commonly used feedstock for biochar production [89–92].

Biochar Feedstock Examples

Leftovers from a meal (eggshells), expired, stale, and blemished fruits and
Food wastes vegetables (banana peels, pineapple peels, cauliflower leaves, peanut
shells, avocado shells, etc.).
Sewage sludge Sewage sludge from the municipal wastewater treatment process.
Animal waste Manures from cows, pigs, and chickens.
Industrial waste Plastics as binders, Bioenergy residues
Forest chips produced from logs, whole trees, logging residues, stumps,
pinecones, hardwood, etc.
Wood chips
Wood residue chips produced from untreated wood residues, recycled
wood, and offcuts (rose stems, bambu, guadua, etc.).
Manure and other wastes from farms, poultry houses, and slaughterhouses.
Agricultural waste
Harvest waste (herbs, grass, etc.); fertilizer run-off from fields.
Forestry waste Bark, sawdust, timber slash, and mill scrap.

Biochar is commonly produced from vegetal residues called cellulosic biomass, such
as firewood or rice residues. In recent years, other raw materials have been studied to
produce biochar, such as algae, food waste, manure, and animal tissue [63,90,91], obtaining
interesting results regarding its physical-chemical properties and applications. On the other
hand, raw materials with a high biomass content, such as sewage and municipal solid
waste (MSW), cannot be considered a suitable feedstock for biochar production since they
may include contaminating components that can affect the biochar performance for soil or
water treatments [93].

2.3. Synthesis Methods Used to Prepare Biochar

Biomass can be transformed using thermochemical conversion processes, like pyrolysis
or HTC treatment, to produce biogas, liquid fuels (e.g., bio-oil), and solid materials, such
as biochar [8,63]. Biomass valorization can be conducted through basic processes (e.g.,
pyrolysis, gasification, torrefaction, anaerobic digestion, or combustion), in which the
organic matter can be transformed into heat, electricity, or by-products, like biochar-based
materials [94]. Thermochemical conversion encompasses the degradation of biomass
structure in either an oxygenic or anoxygenic atmosphere at high temperatures [95]. Biochar
production begins from the initial conversion of biomass through thermochemical processes
until a carbonaceous material with desired physicochemical properties is obtained. The
operating principles, synthesis conditions, and their effect on biochar production for the
most common thermochemical conversion processes are detailed below:

2.3.1. Pyrolysis
In the pyrolysis process, the biomass source, which was previously mentioned, is
subjected to a thermal treatment to produce biochar and other by-products. Depending on
the operating conditions, biogas, liquid bio-oil, and biochar can be generated during this
process [96]. It is essential to note that biomass must be previously dried and ground to
Resources 2024, 13, 8 6 of 33

obtain a carbonaceous material of high quality and yield. The heating process is conducted
at high temperatures (400–800 ◦ C) without oxygen and allows biomass conversion into
by-products for several applications, such as energy and environmental remediation. The
by-products can be used as energy or residual heat to contribute to the pyrolysis or thermal
treatment of the raw material. This thermal process releases the lowest percentage of
carbon back into the atmosphere [58,71,97]. According to the literature, there are two types
of pyrolysis: slow and fast pyrolysis, which depend on the temperature conditions and
the heating rate [98]. The slow pyrolysis is conducted at temperatures ranging from 250
to 600 ◦ C using heating rates of 1–10 ◦ C/min [90,99]. While fast pyrolysis is based on
the thermal conversion of biomass at temperatures above 600 ◦ C, using heating rates
higher than 50 ◦ C/min [100]. The concentration and the physicochemical properties of
the products formed (e.g., biogas, bio-oil, and biochar) can vary depending on the type of
pyrolysis. Thus, during the slow pyrolysis of biomass, a large amount of biochar can be
produced, generating low concentrations of gases and liquids with a high content of highly
contaminant volatile organic compounds (VOCs) [101].
On the other hand, fast pyrolysis is mainly used to produce a high concentration of
liquids (e.g., biofuel) with better physicochemical quality than those produced by slow
pyrolysis, achieving a lower VOC content and a higher concentration of log-chain hydrocar-
bons [102]. Both types of pyrolysis can be used to produce biochar. However, the properties
(e.g., carbon content, density, water retention capacity, functional groups, surface area) and
applications of the carbonaceous materials will be different. Slow pyrolysis can be the best
way to obtain water and soil remediation biochar. Meanwhile, fast pyrolysis can be the best
route to produce biochar as fuel or precursor to other materials [101,103].
The pyrolysis of biomass modifies the size and arrangement of the carbonaceous
structures, enhancing the physicochemical properties of the products obtained during the
process [104]. In general, this impact becomes more robust at higher treatment temperatures.
To obtain a higher biochar production yield, the temperature interval for pyrolysis should
be around 400–800 ◦ C [105,106]. Lua et al. [81] reported that by raising the pyrolysis
temperature from 250 to 500 ◦ C, the specific surface area can increase from 170 to 480 m2 /g,
which has been related to the increased evolution of volatile matter in pistachio nut shells,
resulting in an improved pore growth at the biochar surface, reaching total pore volume
values of 0.47 cm3 /g (at 500 ◦ C), which were much higher than those obtained at a pyrolysis
temperature of 250 ◦ C (0.193 cm3 /g). This has been related to elevated temperatures
supplying activation energy, which can favor conversion reactions, resulting in higher
degrees of order in the carbonaceous structures [107].

2.3.2. Torrefaction
Similarly to pyrolysis, torrefaction is a thermochemical process based on biomass con-
version into value-added products, e.g., biochar, biogas, and bio-oil [108]. However, this
process differs from pyrolysis in operating conditions and formed product types. Torrefac-
tion is a thermal process based on biomass dehydration, carbonization, and caramelization
at relatively low temperatures (i.e., 200 to 300 ◦ C) without oxygen [58]. Biochar is the only
product generated during this process. However, the physicochemical properties (e.g.,
structural characteristics) of the carbonaceous material produced are inferior to those of
pyrolysis [62,91].

2.3.3. Hydrothermal Carbonization

Hydrothermal carbonization (HTC) is a thermochemical technology for processing
biomass with high moisture content in a hot compressed water system [14]. The main
product of HTC is hydrochar, a type of biochar produced in this way. Apart from this car-
bonaceous material, aqueous (nutrient-rich) and gas phases (mainly CO2 ) can be produced
depending on the operating conditions [109]. The carbon-rich hydrochar can be employed
as fuel, coal substitute, gasification feedstock, soil additive for nutrient enrichment, or as
an adsorbent or precursor of activated carbon [110]. The advantage of the HTC process is
Resources 2024, 13, 8 7 of 33

that biomass may be transformed into carbonaceous solids without an energy-intensive

drying procedure or an anoxygenic atmosphere. Likewise, toxic chemical compounds and
residual micropollutants are also avoided during HTC [111].
As mentioned below, HTC is a thermochemical process that uses heat to transform
wet biomass feedstock into hydrochar. HTC is performed in a reactor at temperatures
ranging from 120 to 300 ◦ C under autogenous (self-generated) pressure or under pressure
(2–6 MPa) with feedstock residence periods ranging from 0.5 to 8 h [112–114]. HTC offers
a key advantage over other high-temperature thermochemical conversion processes (e.g.,
pyrolysis) because it is possible to use wet waste without a pre-drying process [19,115].
HTC may use a variety of feedstock, including aquatic biomass, agricultural waste, and
industrial and animal waste [11]. Water is a favorable medium for heat transfer in HTC.
However, there may be some mass transfer restrictions if the particle size variability in the
feedstock is too large (above 2 cm) and the reaction time is too short (less than 30 min) [19].
As a result, particle size should be constant to provide uniform heat and mass transfer.
On the other hand, the aqueous slurry needs to be centrifuged or filtered to separate the
process water and particulates (wet cake). Biomass conversion processes mainly depend
on the feedstock, the desired final product, and its corresponding use. Table 3 shows a
summary of the typical thermochemical conversion processes, temperature conditions, and
the products obtained in each of them.

Table 3. Thermochemical conversion processes to obtain biochar used in different fields [9,116,117].

Temperature
Process Feedstock Final Product Uses
Interval (◦ C)
Rice husk, cocoa
Torrefaction 200–300 Biochar Soil conditioner
husk
Fuel (cooking,
Wood, agricultural Syngas,
Pyrolysis 300–800 heat),
waste biochar
soil amendment
Water filtration and
Compost (green
adsorption of
Slow waste) woody Activated
350–700 contaminants
pyrolysis prunings, grass biochar
(gasses, solids,
clippings
liquids)
Fast Agricultural waste Soil conditioner,
450–550 Biochar
pyrolysis and crops plant growth
Rice husk, manure,
Solid fuel, soil
150–400 algae, corn stover,
HTC Hydrochar amendment,
(High pressure) biosolids, food
adsorbent
waste
Agricultural waste,
Combustible, Biochemicals, fuel
manure, food
Gasification >800 ethane, (low yield, high
residues, sewage
methane reactivity)
sludge

2.4. Methods of Biochar Activation and Modification

Physical and chemical activation methods can enhance biochar properties, such as
impregnation or adding dopants or additives in the carbonaceous structure. Physical
activation is accomplished by processing biochar with oxidizing agents, mostly steam or
carbon dioxide, at temperatures ranging from 500 to 1000 ◦ C. Water is a smaller molecule
than carbon dioxide, which favors its penetration into the biochar pores [19,118], enhancing
its morphological properties, like surface area and porosity [119]. Figure 1 shows some
routes of physical and chemical activation of biochar, as well as the chemical compounds
and thermal conditions used to enhance its physicochemical properties. Notably, the
 carbon dioxide, at temperatures ranging from 500 to 1000 °C. Water is a smaller molecu
than carbon dioxide, which favors its penetration into the biochar pores [19,118], enhan
ing its morphological properties, like surface area and porosity [119]. Figure 1 shows som
Resources 2024, 13, 8
routes of physical and chemical activation of biochar, as well as the chemical 8 ofcompoun
33

and thermal conditions used to enhance its physicochemical properties. Notably, the p
rosity, surface chemistry, and yields of carbon-based adsorbents produced significan
porosity, surface chemistry, and yields of carbon-based adsorbents produced significantly
depend on the
depend biomass
on the biomasscomposition
composition ofof feedstock
feedstock and and the synthesis
the synthesis conditions
conditions [120]. [120].

FigureFigure 1. Activation
1. Activation routesofofbiochar
routes biochar and
andactivated carbon.
activated (Reprinted
carbon. from [120]).
(Reprinted from©[120]).
2018 by©
the2018 by
authors.
authors.
Many modification methods (e.g., chemical, physical, and biological routes) have
Many modification
been studied to improvemethods (e.g.,
the properties chemical,
of biochar usedphysical, and biological
for environmental routes) ha
purposes [63].
been The
studied
widelytoused
improve
method the
hasproperties of biochar
been the chemical usedAcid
alteration. for environmental purposes [6
modification, alkalinity
modification, oxidizing agent modification, metal salts, or oxidizing
The widely used method has been the chemical alteration. Acid modification, agent modification are alkalin
the most common. In contrast, steam and gas purging have been the most common types of
modification, oxidizing agent modification, metal salts, or oxidizing agent modificati
physical modification [19]. Figure 2 describes a categorization scheme of the modification
are the most most
methods common. In contrast,
often reported in thesteam and
literature gasThe
[121]. purging have been
modification method the most
that has comm
typesattracted
of physical modification
scientific attention for [19]. Figure
producing 2 describes
sorbents for wateratreatment
categorization
involves scheme
embedding of the mo
different elements into the biochar framework before, during, or after the
ification methods most often reported in the literature [121]. The modification method th thermochemical
conversion.
has attracted Physical activation
scientific attentionofforbiochar using steam
producing and chemical
sorbents activation
for water with acidic
treatment involves e
and alkaline solutions is usually performed after pyrolysis. However, remarkable results
have been seen when chemical activation is performed before pyrolysis [122].
 bedding different elements into the biochar framework before, during, or after the ther-
mochemical conversion. Physical activation of biochar using steam and chemical activa-
tion with acidic and alkaline solutions is usually performed after pyrolysis. However, re-
Resources 2024, 13, 8
markable results have been seen when chemical activation is performed before9 pyrolysis
of 33

[122].

Figure 2. 2.
Figure Biochar
Biocharmodification methods
modification methods to to enhance
enhance properties.
properties. (Reprinted
(Reprinted fromwith
from [121], [121], with permis-
permission
sion from Elsevier). © 2017 Elsevier
from Elsevier). © 2017 Elsevier Ltd. Ltd.

Interestingand
Interesting andnovel
novel physicochemical
physicochemical activation
activationmethods of biochar
methods seek seek
of biochar to improve
to improve
functional
functional stability,and
stability, andthese
thesecan
canbe
be based
based on
on its
its modification
modification with
withother
otherspecies.
species.InIn this
this biochar-based
sense, sense, biochar-based composites
composites can
can be be prepared
prepared by impregnating
by impregnating or coating
or coating their
their surface
surface with metal oxides, clays, carbonaceous structures (e.g., graphene oxide or carbon
with metal oxides, clays, carbonaceous structures (e.g., graphene oxide or carbon nano-
nanotubes), complex organic compounds, such as chitosan, among others [32,123,124].
tubes), complex organic compounds, such as chitosan, among others [32,123,124].
2.4.1. Physical Activation
2.4.1. Physical
PhysicalActivation
activation enhances the surface pores of biochar and can also modify its
chemical
Physicalproperties (e.g.,enhances
activation surface functional
the surface groups,
poreshydrophobicity,
of biochar and andcan
polarity) [125]. its
also modify
Steam activation enhances the surface area and porosity of biochar [126].
chemical properties (e.g., surface functional groups, hydrophobicity, and polarity) Zhang et al. [127][125].
have reported sludge-based pyrolysis to produce biochar,
Steam activation enhances the surface area and porosity of biochar which was activated using
[126]. Zhang et al. a [127]
physical activator (CO2 ) to enhance its adsorption capacity of Pb2+ from an aqueous so-
have reported sludge-based pyrolysis to produce biochar, which was activated using a
lution. The results revealed that the physical activation with CO2 enhanced the specific
physical activator (CO 2) to enhance its adsorption 2+ capacity of Pb 2+ from an aqueous solu-
surface area by more than ten times, and its Pb adsorption capacity increased from
tion.
7.6The
mg/gresults revealed
to 22.4 that the
mg/g [127]. Thephysical
biochar activation
activation with CO22 enhanced
with CO aided in the the specific sur-
introduc-
face area by more than ten times, and its Pb 2+ adsorption capacity increased from 7.6 mg/g
tion of oxygen-containing functional groups. On the other hand, biochar activation with
to 22.4 mg/g [127].
CH3 COOK The biochar
also enhanced activation
the pore structure with CO2 aidedbiochar,
of sludge-based in the increasing
introduction of oxygen-
its surface
area morefunctional
than ten times, fromOn81 m 2 /g to 908 m2 /g, reaching a Pb2+ adsorption capacity
containing groups. the other hand, biochar activation with CH3COOK also
of 47.6 mg/g
enhanced [127].
the pore structure of sludge-based biochar, increasing its surface area more than
ten times, from 81 m2/gactivation,
During physical to 908 m2/g, biochar
reachingis exposed to a required
a Pb2+ adsorption amountofof47.6
capacity oxidizing
mg/g [127].
agents, such as steam, ozone, carbon dioxide, or air, at temperatures typically above
During physical activation, biochar is exposed to a required amount of oxidizing
500 ◦ C [128]. These oxidizing chemicals enter the biochar structure and gasify the carbon
agents, such as steam, ozone, carbon dioxide, or air, at temperatures typically above 500
atoms, opening and expanding previously inaccessible pores [129]. This type of activation
°C can
[128]. Thesea oxidizing
produce biochar with chemicals enter areas
larger surface the biochar structure
and generate andamount
a large gasifyof the carbon at-
surface
oms, opening
oxygen and expanding
functional groups, which previously
frequently inaccessible pores
serve as active [129]. This
adsorption sitestype of activation
for pollutant
canremoval
produce a biochar with larger surface areas and generate a large amount of surface
[129].
 oxygen functional groups, which frequently serve as active adsorption sites for pollutant
removal [129].
Another physical process, like steam activation, is gas purging, in which gases (such
as carbon dioxide) are mixed with the accessible amorphous carbon at the biochar surface
Resources 2024, 13, 8
in a restricted oxygen environment to produce carbon monoxide [130]. Moreover, carbon10 of 33
monoxide formation can increase the biochar surface area, improving its microporous
structure and pore volume [27].physical process, like steam activation, is gas purging, in which gases (such
Another
as carbon dioxide) are mixed with the accessible amorphous carbon at the biochar surface
in a restricted oxygen environment to produce carbon monoxide [130]. Moreover, carbon
2.4.2. Chemical Activation
monoxide formation can increase the biochar surface area, improving its microporous
The most typical routeand
structure forpore
modifying
volume [27].the type and number of functional groups at
the biochar surface is chemical activation, which involves doping a chemical agent into its
2.4.2. Chemical
structure. In this process, the raw Activation
material (i.e., biomass) is impregnated with a chemical
The most typical
agent, and the combination is subsequently route forthermally
modifying treated
the type to
andobtain
numbera of functional groups at
biochar-doped
the biochar surface is chemical activation, which involves doping a chemical agent into its
material [129]. During the process, the chemical agent can act as an activator, which favors
structure. In this process, the raw material (i.e., biomass) is impregnated with a chemical
sample dehydration and
agent, andprevents the generation
the combination of tarthermally
is subsequently and volatile
treated chemicals, thus in-
to obtain a biochar-doped
creasing the yield of the carbonization
material [129]. During process [131].
the process, the In addition,
chemical agentthese activators
can act can bewhich
as an activator,
used to increase the biochar-specific surface and pore volume and generate functional thus
favors sample dehydration and prevents the generation of tar and volatile chemicals,
increasing the yield of the carbonization process [131]. In addition, these activators can
groups in its structure.
be used to increase the biochar-specific surface and pore volume and generate functional
Depending ongroups
the final
in itspurpose
structure. of the carbonaceous material, acid or alkali activa-
tion can be employed. When Dependingsoil on
amendment or water
the final purpose of the purification (heavy acid
carbonaceous material, metal or color-
or alkali activation
ant adsorption) is performed,
can be employed. acidic
Whenactivation is preferred
soil amendment or waterover alkali (heavy
purification activation
metal[132].
or colorant
Alkali activation isadsorption)
more related is performed, acidic activation
to producing materials is preferred over storage
for energy alkali activation [132].orAlkali
[133,134]
activation is more related to producing materials for energy storage [133,134] or electro-
electrochemical processes because of their high capacitances [132,133]. The impregnation
chemical processes because of their high capacitances [132,133]. The impregnation of
of specific elementsspecific
or promoters
elementsto or increase
promotersbiochar adsorption
to increase capacitycapacity
biochar adsorption has been haswidely
been widely
reported for water purification.
reported for water Figure 3 describes
purification. Figuresome biochar
3 describes somemodification routes.routes.
biochar modification

Figure 3. Classification of biochar

Figure medication
3. Classification routes.
of biochar (Reprinted
medication routes.from [19], from
(Reprinted with[19],
permission from from
with permission
Elsevier). © 2019 Elsevier Ltd.© 2019 Elsevier Ltd.
Elsevier).

As previously mentioned, the most common activators are alkalis (KOH, NaOH, and
As previously ZnCl
mentioned, the most common activators are alkalis (KOH, NaOH, and
2 ) [130] and acids (citric, nitric, sulfuric, and phosphoric) [119,135,136]. H3 PO4 is
ZnCl2) [130] and acids (citric, nitric,
commonly used as an sulfuric,
activatorand phosphoric)
because it can promote[119,135,136]. H3POprocesses
the bond breakage 4 is com-while

monly used as an activator

maintainingbecause it can
the internal porepromote the bond
structure [137]. breakageofprocesses
The distribution whilein the
chemical agents
precursor before carbonization plays a vital role in the final
maintaining the internal pore structure [137]. The distribution of chemical agents in the product’s porosity improvement
and functionality [138]. According to Fierro et al. [139], the effect of the added quantity of
precursor before carbonization plays a vital role in the final product’s porosity improve-
phosphoric acid for the activation of carbon derived from rice straw is essential to increase
ment and functionality [138].
its yield untilAccording to Fierro
a certain quantity. Whenetthey
al. used
[139],aH the effect of the added quan-
3 PO4 : biomass ratio equal to 1 (ranging
tity of phosphoric acid
from for
0 to the
1.6),activation of carbon
the carbon yield increasedderived
by up tofrom
10%. rice straw is essential to
Moreover, when the ratio was more
increase its yield until a certain quantity. When they used a H3PO significant than 1, an increase
4: biomass in the
ratio equalpercentage
to
of the carbonization yield was not observed. However, the specific surface area of the
1 (ranging from 0 to 1.6), the carbon yield increased by up to 10%.
Moreover, when the ratio was more significant than 1, an increase in the percentage
of the carbonization yield was not observed. However, the specific surface area of the
Resources 2024, 13, 8 11 of 33

carbonaceous material increased from 520 to 786 m2 /g. In this work, the volume of the
pores was highly variable, and no tendency to deformation was observed. Likewise,
Zakaria et al. [140] have reported that the effect of phosphoric acid to obtain mangrove-
based activated carbon (with the H3 PO4 : precursor ratios of 3, 4, and 5) on its production
yield and surface characteristics are also notable. They observed a gradual decrease in the
yield of activated carbon (45–41%) as the ratio increased from 3 to 5. Other authors have
also reported this fact [141–144]. Thus, it is noticeable that this trend is independent of the
raw material. However, the carbon production yield depends on the raw material, as seen
in Table 4.

Table 4. Enhanced carbon production yield by activation with H3 PO4 .

H3 PO4 : Biomass Yield

Raw Material Ref.
Impregnation Ratio (%)
Rice straw 1.0 51.9 [139]
Mangrove pile 3.0 44.7 [140]
Paulownia wood 1.0 42.0 [143]
Olive stone 1.5 36.8 [141]
Apricot shell 1.0 26.2 [144]
Jackfruit peel waste 1.0 56.3 [142]
Rubber wood sawdust 1.5 63.0 [145]

According to the activated biochar definition [1,5–7,10], only rice straw, jackfruit
peel waste, and rubber wood sawdust are considered activated biochar. The activation
mechanism is related to the H3 PO4 :biomass ratio, temperature, and time [140,143,145,146].
Textural and morphology features are affected depending on time contact and temperature.
Low activation time and temperature result in incomplete carbonization and a higher
yield [146,147]. An appropriate H3 PO4 :biomass ratio, temperature, and activation time
lead to improvement of the surface area and pore volume. However, beyond that, those
properties can decrease, and it is because the increase in pore size leads to the collapse of
the tiny pores [146].
H2 SO4 and HNO3 have also been employed as activating agents of biochar. In general,
the presence of H2 SO4 during the biochar synthesis does not alter its structural properties
(e.g., specific surface area and pore volume). However, this acid promotes the sulfonation
reaction, generating polar functional groups (e.g., sulphonic groups –SO3 H) at its sur-
face [119,148,149], which, in turn, enhances its performance for several applications like ion
and pollutant adsorption [148,150,151], biodiesel production [152,153] and other catalytic
processes [119,154]. Likewise, HNO3 -based species can modify the physicochemical prop-
erties of biochar-based materials and, thus, their performance in a specific application. Its
presence promotes the generation of many types of surface functional groups through the
oxidation and nitration of aromatic rings on the surface of biochar-based materials [132].
Moreover, HNO3 can remove partially combusted volatiles and impurities from the
surface of biochar, enhancing its surface area and pore volume [155]. Güzel et al. [156] and
Hadjittofi et al. [157] have demonstrated that nitric acid-activated biochar-adsorbents can
effectively remove methylene blue and Cu2+ from aqueous solutions, respectively. In both
cases, the activated carbonaceous materials exhibited higher adsorption capacities than
non-activated biochar, attributed to the larger surface area, the lower point of zero charges,
and more oxygen functional groups, like carboxylic, phenolic, and lactonic moieties.
On the other hand, using bases during the biochar activation can generate positive
electrostatic charges on their surface, which generates a solid affinity for adsorbed neg-
atively charged pollutants [119]. Among the bases used as biochar activators, KOH has
been widely used because of the special features that it gives to biochar. Biochar properties
(e.g., textural and morphological) can be improved using this chemical. The activation
Resources 2024, 13, 8 12 of 33

properties depend on the KOH: biochar ratio, temperature, and time [158–160]. Porosity de-
velopment is associated with gasification (CO2 production) [161]. Different authors report
different values of reached specific surface areas: 621 m2 /g [161], 912.73 m2 /g [159], and
2201 m2 /g [160]. Their results differ due to the previously mentioned parameters and the
synthesis process. Higher surfaces are obtained when the first raw material is converted to
biochar followed by a post-chemical activation (KOH) [158,160] rather than direct one-pot
pyrolysis and chemical activation [159,161]. Likewise, Trakal et al. [162] studied the effect
of chemical activation on the removal efficiency of Cu from an aqueous solution using
pure amorphous biochar and activated biochar (BCact ). In this work, chemical activation
with 2 M KOH substantially raised the total pore volume of biochar, obtaining values of
0.01 and 8.74 mL/g for amorphous biochar (surface area = 9.80 m2 /g) and BCact (surface
area = 11.6 m2 /g), respectively. These results correlated with the Cu adsorption capacity,
which was more significant for BCact (10.3 mg/g) than that obtained with amorphous
biochar (8.77 mg/g).

2.5. Properties That Biochar Modification Processes Can Improve

Biochar modifications can enhance its structure and physicochemical properties (e.g.,
an increase in the surface area, the generation of oxygen-containing functional groups, and
the increase in aromaticity, among others [163]), favoring its ability to adsorb contaminants,
such as heavy metals [164]. It is due to generating active sites for specific uses, like in
catalysis, water treatment, anaerobic digestion, soil remediation [93], supercapacitors, and
fuel cell applications [118].
The pore size and surface functional groups of biochar are significant features that
influence its efficiency as a pollutant adsorbent [165,166]. The surface functional groups in
biochar are responsible for their strong metal adsorption ability [164]. Metal adsorption by
biosorbents can occur via complexation between metals and different functional groups
on the biosorbent surface or through electrostatic attractions between metal cations with
negative charges and the functional groups at its surface [163]. According to Choudhary
et al. [167], functional groups can act as adsorption sites for metal attraction and are located
throughout the biochar matrix. In this sense, it is necessary to smash the biochar structure
to expose a higher amount of functional groups and, therefore, to promote its efficiency for
pollutant removal [167]. Considering this fact, heavy metal adsorption by biochar can occur
at its surface (outer pores) as well as within the pore structure of the carbonaceous mate-
rial (inner pores), depending on the type and amount of surface functional groups [164].
Likewise, the removal of other types of pollutants, like dyes [140,168,169], oil [170], pes-
ticides [171], and pharmaceuticals [93,172], using biochar can occur through monolayer
adsorption. During these treatment processes, the chemisorption predominates through
the complexation, coordination, ion exchange, and chelation between pollutants and the
carbonaceous materials surface, depending on the functional groups and the structural and
other physicochemical properties of the biosorbents. These biochar properties depend on
the raw material, synthesis method, activation routes, and the use of dopants, composites,
and additives described below.

2.6. Dopants, Composites, and Additives Used to Improve Biochar Properties

Many attempts have been made to activate biochar without external doping agents,
such as gas, steam, microwaves, acids, alkalis, and oxidants [118,129]. On the other hand,
adding other materials to the biochar structure has been a novel strategy to produce
composites with interesting properties, which can be used in several applications. Some of
these strategies and applications of the carbonaceous materials are described in Table 5:
Resources 2024, 13, 8 13 of 33

Table 5. Some chemicals used to modify the properties and applications of biochar samples prepared
by pyrolysis.

Improvement Properties
Raw Materials Applications Ref.
Method
Physical activation with CO2 Adsorption of Pb2+
Sludge-based Chemical activation with from an aqueous [127]
CH3 COOK solution
Corn cobs, stalks, and reeds Acidic activation (H2 SO4 ) Sodium ions removal [148]
H2 SO4
Giant reed stalks Removal of ammonium [150]
(post-combustion)
H2 SO4
Peanut shells Toxic organic pollutants [151]
(post-combustion)
HNO3 Methylene blue
Weeds [156]
(post-combustion) adsorption
HNO3
Cactus fibers Cu2+ adsorption [157]
(post-combustion)
KOH (post-combustion);
Pomegranate residue;
KOH + Toluene Battery performance [149]
grapefruit peel
(post-combustion)

2.6.1. Dopants for Biochar

Adding a precursor or dopants can improve the physicochemical properties of biochar-
based materials. Dopants promote the carbonaceous material’s reactivity, making it an
interesting material for catalytic applications. Metallic and non-metallic dopants have been
widely used. For example, the modification of biochar with transition metals, like iron,
can enhance its specific surface area and the adsorption affinity. In contrast, modifications
with non-metals and alkali/alkaline earth metals can decrease the property above [173].
Mašek et al. [44] have reported that potassium doping can increase the carbon-sequestration
potential of biochar by 45%, making it an important strategy to prevent global warming.
When dopants modify biochar, its functionality can be altered and could improve its
performance for several environmental and energetic applications. Minerals, inorganic
species, metals, and metal oxides have been the most common dopants to functionalize the
biochar structure. Their presence in the carbonaceous matrix displays a significant improve-
ment in adsorption performance, as well as in the selectivity of certain pollutants [174].
Jha [175] studied the effect of three chemical dopants on pollutant absorption using biochar-
based adsorbents. These dopants were zinc oxide (ZnO), thiol (–SH), and manganese oxide
(MnO2 ), which exhibited the highest pollutant removal. Other types of dopants have been
used to promote the physicochemical properties of biochar and, therefore, its performance
in a specific application. Figure 4 shows some dopants and precursors used to enhance the
biochar surface.
Doping techniques and procedures, such as impregnation, are the most common
methods used for generating changes in the structure of biochar. Di Stasi et al. [176]
produced activated biochar by wet impregnation using cerium nitrate hexahydrate or urea
as dopant agents. The aqueous solutions were stirred at 80 ◦ C until the water evaporated
entirely. Subsequently, the samples were dried at 110 ◦ C and then calcined in a reactor at
550 ◦ C for 3 h in an inert environment (N2 atmosphere).
OR PEER REVIEW 14 of 33
Resources 2024, 13, 8 14 of 33

Figure 4. Elements and compounds

Figure 4. Elementsused to improveused
and compounds thetobiochar
improvestructure.
the biochar(Reprinted from [120]).
structure. (Reprinted from [120]).
© 2018 by the authors.
© 2018 by the authors.
For water purification, well-developed porosity and hydrophobic surfaces are required
Doping techniques and procedures,
to effectively such ascapacity
enhance the adsorption impregnation,
of organic or are the most
inorganic common
pollutants on biochar-
methods used for generating changes
based sorbents. in the structure
The adsorption of inorganicof biochar. Di Stasi
or polar organic et al. [176]requires
contaminants pro- the
duced activated biochar by wet impregnation using cerium nitrate hexahydrate or urea as [60].
presence of surface oxygen functional groups to improve the electrostatic attraction
dopant agents. TheUnfortunately, sometimes biochar has a moderate to low surface area and a limited number
aqueous solutions were stirred at 80 °C until the water evaporated
of surface functional groups, which limits their performance [129]. For this reason, it
entirely. Subsequently, the samples
is necessary were dried
to functionalize at 110 °C
the biochar andtothen
surface calcined
improve in a reactor
its properties at its
and, thus,
550 °C for 3 h in an performance
inert environment
in a specific(Napplication.
2 atmosphere).The surface chemistry of biochar can also be altered
For water purification, well-developed
by doping heteroatoms such as porosity
N, P, S, andandmetalhydrophobic
oxide from various surfaces
sources are re-Some
[120].
modifications have been proposed to improve the adsorption
quired to effectively enhance the adsorption capacity of organic or inorganic pollutants capability of biochar-based
materials, which are described below.
on biochar-based sorbents. The adsorption of inorganic or polar organic contaminants re-
quires the presence2.6.2.
of surface
Iron-Dopedoxygen functional groups to improve the electrostatic at-
Biochar
traction [60]. Unfortunately,
Among sometimes
the dopantsbiochar has abiochar
used during moderate to low
synthesis, onesurface areacommon
of the most and a and
limited number of effective
surface has been iron and
functional its species,
groups, which likelimits
iron oxide
their(Feperformance
2 O3 ) [177–179]. [129]. For
The presence of iron species in biochar can promote several properties and enhance its
this reason, it is necessary to functionalize the biochar surface to improve its properties
effectiveness in various applications. Iron species on the biochar are crucial in immobiliza-
and, thus, its performance in a specific
tion mechanisms application.
and redox The surface
reactions [180–182]. chemistry
They can enhance of thebiochar
biochar’scan ability to
also be altered by doping heteroatoms
retain essential such as N,
plant nutrients, P, as
such S, nitrogen
and metal andoxide from various
phosphorus, by formingsources
complexes
[120]. Some modifications haveions,
with nutrient beensuchproposed
as nitratesto and
improve the adsorption
phosphates capability
[183]. In addition, iron can ofhelp
bio-buffer
the pH of soils. It acts
char-based materials, which are described below.as a pH stabilizer, preventing extreme fluctuations in soil acidity or
alkalinity [184]. Iron can reduce or oxidize various metals and organics. In the presence of
iron, contaminants like arsenic or nitrate can undergo redox reactions that enhance their
2.6.2. Iron-Doped Biochar
removal.
Among the dopants Table 6 presents
used during diverse Fe-doped
biochar biochar samples
synthesis, one of from variouscommon
the most feedstocks,and
detailing
synthesis conditions and contaminant removal efficiencies. Remarkable examples include
effective has been iron and its species, like iron oxide (Fe2O3)6+[177–179].
peanut hulls, which achieve 98% removal of Cr through hydrothermal carbonization
The presence of iron and
(HTC), species in biocharbiochar,
oak wood/bark can promote several
which exhibits highproperties and
removal rates enhance
(>98%) for Pb and
its effectiveness in various applications.
Cd via pyrolysis Iron species
and impregnation on the biochar are crucial in immo-
processes.
bilization mechanisms and redox reactions [180–182]. They can enhance the biochar’s abil-
ity to retain essential plant nutrients, such as nitrogen and phosphorus, by forming com-
plexes with nutrient ions, such as nitrates and phosphates [183]. In addition, iron can help
buffer the pH of soils. It acts as a pH stabilizer, preventing extreme fluctuations in soil
acidity or alkalinity [184]. Iron can reduce or oxidize various metals and organics. In the
presence of iron, contaminants like arsenic or nitrate can undergo redox reactions that
Resources 2024, 13, 8 15 of 33

Table 6. Fe-doped biochar samples and their synthesis conditions and removal efficiencies.

Contaminant Adsorption Mechanism

Feedstock Fe Precursor Synthesis Conditions Ref.
(Removal, %) Proposal
HTC Rhodamine B
Pomelo peel FeCl3 Physical adsorption [185]
T = 200 ◦ C; t = 5 h (>95%)
Pyrolysis
Acid orange Complexation
Wheat straw FeSO4 T = 800 ◦ C; t = 1 h [186]
(98%) Magnetic interactions
Heating rate: 10 ◦ C/min
Pyrolysis
Rice straw bio- FeSO4 and As3+
T = 500 ◦ C; t = 1 h Electrostatic interactions [181]
mass FeCl3 (94%)
Heating rate: 10 ◦ C/min
HTC Cr6+ Chemisorption
Peanut hulls FeCl3 [187]
T = 220 ◦ C; t = 12 h (98%) Electrostatic interactions
Rice and wheat Pyrolysis As3+
FeCl3 Complexation [188]
husks T = 600 ◦ C; t = 1 h (>90%)
Oak wood and oak Pyrolysis and impregnation Cd (90%)
FeSO4 Electrostatic interactions [189]
bark T = 450 ◦ C; t = 5 h Pb (>98%)
Pyrolysis Hg0 Physical and chemical
Walnut shells FeCl3 [190]
T = 800 ◦ C; t = 1 h (15%) adsorption
Pyrolysis Cr6+ Ion exchange
Corncob Fe(NO3 )3 [178]
T = 600 ◦ C; t = 2 h (27–100%) Electrostatic interactions
Pyrolysis
p-nitrophenol
Wood wastes FeCl3 T = 600 ◦ C; t = 2 h Complexation [179]
(85%)
Heating rate: 5 ◦ C/min
long-chain per-
Pyrolysis
/polyfluoroalkyl Electrostatic interactions
Maize straw FeCl3 T = 500, 700, 900 ◦ C; t = 2 h [182]
substances Complexation
Heating rate: 5 ◦ C/min
(95–100%)
π–π interactions
HTC: Methylene blue Ion exchange
Date palm fronds FeSO4 [191]
T = 200 ◦ C; t = 3 h (45%) Hydrogen bond
interactions

2.6.3. Nitrogen-Doped Biochar

Nitrogen (N) doping has attracted much attention as it can enhance the characteristics
of carbon-based materials [192]. Because of the significant electronegativity of N, electron
modulation can improve the surface polarity of biochar and generate unique electronega-
tivities [193]. Moreover, N-doping into the biochar matrix can alter its electronic structure,
enhancing its interaction with pollutants [194]. In addition, introducing N heteroatoms
into the ordered sp2 -hybridized graphite structure can modify the electrical charges of
the original electron network due to the difference in electronegativity. Thus, unbalanced
charged areas throughout the carbon structure can result in an electroactive state that may
be used for various practical purposes. Likewise, it has been found that N-doping can
improve the catalytic activity of nanocarbons, favor nanomaterial dispersion, and increase
the detection limit of sensors [195].
The most common synthesis method for N-doped carbonaceous materials is the
thermal decomposition of an inherent N-rich precursor [192]. It involves chemically pre- or
post-treating biochar with ammonia, urea, melamine, or an N-containing organic polymer
to add exogenous nitrogen into the carbon structure [196].

2.6.4. Phosphorus-Doped Biochar

Another way to enhance the biochar properties and performance in a specific ap-
plication is by doping phosphorus species into the biochar structure. Including these
Resources 2024, 13, 8 16 of 33

phosphorous-based dopants aims to improve the pollutant removal capacity of biochar.

Phosphoric acid (H3 PO4 ) is a typical dopant that enhances biochar properties. Fan
et al. [197], have prepared a series of novel N- and phosphorus-enriched biochar nanocom-
posites via co-pyrolysis with different ammonium polyphosphate (APP) weight ratios.
They used the mixture of phosphorus and N dopants to improve the Pb2+ adsorption on
the APP-doped biochar, observing that the Pb2+ removal efficiency of this last sorbent
(723.6 mg/g) was significantly enhanced compared to that of the unmodified biochar
(264.2 mg/g).

2.6.5. Composites for Biochar

Adding a composite material to the biochar structure can be an interesting enhance-
ment strategy to improve its environmental remediation efficiency. Metal composites (e.g.,
Fe2 O3 and iron sulfide), minerals (e.g., kaolinite), and layered double hydroxides (LDH)
have been the typical composites used to promote the performance of the carbonaceous
material during soil remediation (resulting in fertility improvements) and wastewater
treatments [198]. LDHs are anionic clay minerals made up of positively charged metal
hydroxide layers and anions in the interlayer gap to neutralize charge [199]. In pollutant
adsorption, various LDH-biochar composites with divalent and trivalent metal cations (e.g.,
Mg-Al, Mg-Fe, Zn-Al, Ca-Al, and Ni-Fe) have been frequently used [200].

2.6.6. Other Additives for Biochar Modification

Other additives, such as phosphorus, zinc, and calcium species, can be added to
biochar during its synthesis to improve its properties and broaden its applicability [201].
For example, adding calcium oxide to biochar followed by a heating process at 450 ◦ C can
generate a more stable carbonaceous material with fewer oxygen functional groups [202].
According to Li et al. [203], adding mineral additives to biochar promotes carbon retention
and the stability of the solid in terms of carbon sequestration. They studied the use of
kaolin, calcite (CaCO3 ), and calcium dihydrogen phosphate [Ca-(H2 PO4 )2 ] as additives
in biochar obtained from rice straw biomass. These three minerals are frequently used to
enhance soil quality and remediate soil and water pollution [204]. Likewise, adding these
chemicals to biochar can improve the stability of the biochar-based material and, thus, its
efficiency in removing pollutants [8].

3. Main Uses of Biochar

3.1. Biochar for Soil Remediation (Crop Improvement)
The presence of biochar can enhance soil characteristics and, at the same time, increase
crop biomass and improve disease resistance. Biochar may improve soil fertility [205], soil
quality (e.g., pH [40], cation exchange capacity (CEC), and water holding capacity [206]),
and plant development [207,208]. Recently, biochar has been used to treat soil contaminated
with heavy metals and organic contaminants [43]. Figure 5 shows the primary mechanisms
for the remediation of contaminated soils containing heavy metals and organic pollutants
using biochar. As seen here, precipitation, electrostatic interaction, and ion exchange are
the most common mechanisms that describe soil remediation using biochar.

3.2. Biochar to Remove Pollutants in Water and Wastewater

Sources of water pollution can be classified in different ways. The most important
sectors that generate wastewater or contribute to its pollution are domestic, agricultural,
and industrial [209,210]. The word “contaminant,” according to the Safe Drinking Water
Act, is defined as any physical (sediment or organic material suspended in the water),
chemical (nitrogen, bleach, salts, pesticides, metals), biological (bacteria, viruses, protozoa,
and parasites), or radiological (cesium, plutonium, and uranium) substance or species
in water [211]. Some pollutants in drinking water may be dangerous or toxic at specific
concentrations in drinking water, while others can be innocuous. The presence of pollutants
does not always imply that the water is unsafe to drink. However, it is necessary to
 Act, is defined as any physical (sediment or organic material suspended in the w
chemical (nitrogen, bleach, salts, pesticides, metals), biological (bacteria, viruses, p
zoa, and parasites), or radiological (cesium, plutonium, and uranium) substance or sp
Resources 2024, 13, 8 in water [211]. Some pollutants in drinking water may be dangerous or toxic at sp
17 of 33
concentrations in drinking water, while others can be innocuous. The presence of p
tants does not always imply that the water is unsafe to drink. However, it is necessa
consider
consider how
how to remove
to remove them.them.
Table 7Table
shows7the
shows the sub-classifications
sub-classifications and various
and various sources of sou
water pollution.
of water pollution.

Figure 5. Main mechanisms and description for remediation using biochar of contaminated soils with
(a) heavy metals and (b) organic pollutants. (Reprinted from [212], with permission from Elsevier).
© 2015 Elsevier Ltd.
Resources 2024, 13, 8 18 of 33

Table 7. Sources of wastewater generation [143].

Agricultural Source Domestic Source Industrial Source

Poultry waste Washing/laundry Fertilizer
Piggery waste Shower Pulp and paper
Silage liquor Kitchen Textile, tanneries
Dairy farming waste Toilet Dye processing
Vegetable waste Septic tank Food processing
Firewater School Petrochemical/oil industry
Sediment run-off Hospitals Crude oil extraction/refinery
Nutrient run-off Hotels/restaurant Metallurgical industry
Commercial fertilizer Small business activities Plastics/polymer industries

Biochar’s physical and chemical properties are influenced by the feedstock, synthesis
method, and activation procedures, and its adsorption capacities are also influenced.
This carbonaceous material can be used to remove a wide range of organic (agricultural),
inorganic (toxic gases in the oil industry), and microbiological (pathogen) pollutants [213].
Table 8 describes some examples of types of contaminants in water that can be removed
using biochar-based materials.

Table 8. Frequent pollutants of water generated in industrial areas [145].

Type of Pollutant
Organic Inorganic Microbial
Dye, humid substances Heavy metals Bacteria
Phenolic compounds Inorganic ions Mushrooms
Petroleum surfactants Pb2+ Salmonella
Pesticides, pharmaceuticals Zn2+ Enterococcus faecalis
Compounds Cd2+

In addition to classifying the origin of wastewater, it is necessary to subdivide the types

of contaminating agents in water and their effects on the hydric fluid. Table 9 describes the
main contaminating agents, such as organic and inorganic agents, and their environmental
effects.

Table 9. Polluted water and its environmental effects [146–149].

Water Pollution Effects on Humans and

Source of Pollution Polluting Agent
Type Environment Damage
Human and animal Infectious agents Can cause waterborne
Municipal
wastes (pathogens) diseases
Sewage, animal Oxygen-demanding Deplete oxygen needed
Agricultural
feedstocks waste by aquatic species
Add toxins to aquatic
Industrial Oil, gasoline, plastics Organic chemicals
systems
Acids, salts, metal Add toxins to aquatic
Industrial Inorganic chemicals
compounds systems
Nitrates and Plant nutrients, Cause excessive growth
Agricultural
phosphates fertilizers. agricultural run-off of algae
Sediments, Disrupt photosynthesis,
Agricultural Soil, silt
suspended solids food webs
Discharge heated water
Radioactive Make species vulnerable
Thermal Nuclear power plant
pollutants, heat to diseases
discharges
Resources 2024, 13, 8 19 of 33

Biochar treatment has a high potential for wastewater treatment [47,214]. Compared
to conventional low-cost technologies (such as sand filtration, boiling, sun disinfection,
and chlorination), water treatment using biochar has numerous potential advantages:
(1) biochar is a low-cost and renewable adsorbent made from readily available bioma-
terials and skills, making it suitable for low-income communities; (2) existing methods
primarily remove pathogens, whereas biochar removes chemical, biological, and physical
contaminants; and (3) biochar preserves the organoleptic properties of water, whereas exist-
ing methods generate carcinogenic by-products (e.g., chlorination) and increase chemical
contaminant concentrations (e.g., boiling) [213].
Biochar has been widely explored as an adsorbent for removing contaminants from
wastewater due to its unique features, such as a large surface area, well-distributed pores,
and a high abundance of surface functional groups [215]. The oxygenated functional groups
(OFGs) in biochar are essential active sites for removing contaminants from the water via
interfacial adsorption/redox reaction [214,216].

3.2.1. Mechanisms to Remove Pollutants from Water with Carbonaceous Materials

Low-cost biochar has emerged as the substitute for activated carbon for the removal
of organic pollutants such as volatile organic compounds, aromatic dyes, hydrocarbons,
agrochemicals, and others. Regarding inorganic contaminants, biochar has been success-
fully used for the removal of sulfides, ammonia, nitrates, phosphate, and heavy metals [92].
The application of biochar as an efficient contaminant remover depends on its remarkable
characteristics, e.g., high specific surface area, cation exchange capacity, active functional
groups, microporosity, and electrostatic interactions, among others. These properties gov-
ern the binding of polar compounds on the surface of biochar, which immobilizes the
contaminants. Because of all this, biochar has been proposed in many reports as an efficient
adsorbent to remove different types of organic and inorganic contaminants from water
and soil in the near future. The adsorption of inorganic pollutants on biochar results
from stoichiometric ionic exchange, electrostatic attraction, surface precipitation, surface
sorption, and complexation [217]. In this sense, the adsorptive capabilities of biochar are
influenced by various factors, including hydrophobicity, alkalinity, ion exchange capacity,
and elemental compositions [218]. Surface functionality can also alter the biochar sorp-
tion capacity [219]. Rajapaksha et al. [220] have reported a mechanism of contaminant
removal in water through the strong interaction between organic compounds and carbon
membranes. A recent report [92] has summarized these processes to remove inorganic
contaminants as a combined effect of several types of interactions, such as electrostatic
interactions due to a high dependency on the point of zero charge, surface sorption because
of the diffusion of the metal ions onto the pores of the sorbent, and chemical bonds with
active functional groups. Also, via cation exchange as a result of the replacement of positive
charges on the surface of biochar by metal ions, complexation takes place because of the
oxygen functional group (for example, carboxyl and phenolic) with high efficiency of
binding heavy metal ions. On the other hand, the removal of organic contaminants can also
be connected with the combination of different interactions. These interactions are mainly
hydrophobic interactions, pore filling, partitioning, electron donor and acceptor, and elec-
trostatic interactions. The contaminants can be attracted to the carbonaceous membranes
(e.g., graphene and biochar) through intermolecular forces, such as non-covalent bonds,
hydrogen bonds, van der Waals forces, π–π stacking, and hydrophobic interactions. The
mechanism of removal of contaminants by carbonaceous materials is illustrated in Figure 6.
OR PEER REVIEW 20 of 33
Resources 2024, 13, 8 20 of 33

Figure 6. Mechanism Figure

of pollutant removal
6. Mechanism of from water
pollutant usingfrom
removal granular carbonaceous
water using materials (Re-
granular carbonaceous materials
printed from [221]). ©(Reprinted
2020 by the
fromauthors.
[221]). © 2020 by the authors.

3.2.2. Biochar Used at Medium and Large-Scale in Water Filtering Process

3.2.2. Biochar Used at Medium and Large-Scale in Water Filtering Process
The global demand for safe and quality drinking water has become increasingly im-
The global demand
portantfor duesafe and
to the quality
growing worlddrinking
population water has become activities.
and anthropogenic increasingly Waterim-pollution
portant due to the growing world population and anthropogenic activities. Water pollu- is an
by synthetic organic compounds, such as pesticides, medicines, and fuel components,
increasing concern worldwide because these chemicals can bioaccumulate in the human
tion by synthetic organic compounds, such as pesticides, medicines, and fuel components,
body, causing cancer and other disorders. In recent years, many researchers have focused
is an increasing concern
on theworldwide
applications of because
biochar as these chemicals
a potential can bioaccumulate
and efficient adsorbent to remove in the hu-
contaminants
man body, causingfrom cancer and solution.
aqueous other disorders.
Due to its In recent years,
remarkable many
properties, researchers
numerous reportshave
have been
published confirming
focused on the applications of biochar theasmany advantages
a potential of biochar
and efficient for adsorbent
environmental to uses,
removeand it has
been widely studied in removing both organic and inorganic contaminants [64]. As an
contaminants from aqueous solution. Due to its remarkable properties, numerous reports
efficient adsorbent, it has been used to immobilize heavy metal ions, even as a catalyst for
have been published theconfirming
degradation the many advantages
of complex organic compounds. of biochar for environmental
Nevertheless, the industrial uses,
application
and it has been widely studied
of these in removing
carbonaceous materialsboth organic
requires and inorganic
significant contaminants
infrastructure expenditures[64]. [222,223].
As an efficient adsorbent, it has been used to immobilize heavy metal ions, even as a cat- be-
Considering this fact, creating filters with carbonaceous materials at different scales
comes an
alyst for the degradation of excellent
complex option to mitigate
organic or reduce Nevertheless,
compounds. aquatic pollutionthe at different
industrialscales.
ap-The use
of biochar filters has been suggested as an option to replace both treatments of drinking wa-
plication of these carbonaceous materials requires significant infrastructure expenditures
ter: the conventional treatment (e.g., coagulation-flocculation, filtration, and chlorination)
[222,223]. Considering thisadvanced
and the fact, creating
treatment filters with carbonaceous
(e.g., membrane materials
filtration, ozonation, andat different [3].
biofiltration)
scales becomes an excellent optionhave
Some authors to mitigate
also compared or reduce aquaticofpollution
the advantages using biocharat different
for water treat-
ment to low-cost methods [213]. They consider that biochar
scales. The use of biochar filters has been suggested as an option to replace both treatments treatment has several merits
compared to methods such as sand filtration, boiling, solar disinfection, or chlorination
of drinking water: the conventional treatment (e.g., coagulation-flocculation, filtration,
because, although some methods remove pathogens, biochar removes chemical, biological,
and chlorination) andandthe advanced
physical treatment
contaminants. (e.g., itmembrane
Moreover, maintains the filtration,
organolepticozonation,
propertiesand of water,
biofiltration) [3]. while other treatments, such as chlorination, might produce carcinogenic by-products [213].
Recentalso
Some authors have workcompared
has focusedthe on using engineering
advantages biologically
of using biochar enhanced biochar
for water (BEB) for
treat-
biological water treatment [210], focusing on the scope,
ment to low-cost methods [213]. They consider that biochar treatment has several merits potential benefits, and challenges
of sustainable water treatment using BEB. The work examines BEB’s dynamic and complex
compared to methods such as sand filtration, boiling, solar disinfection, or chlorination
biofilm–biochar interactions in water treatment. The authors also suggest the use of BEB
because, although some
insteadmethods remove
of biological pathogens,
activated carbon (BAC) biochar
in theremoves chemical,
tertiary treatment biologi-water
of drinking
cal, and physical contaminants. Moreover, it maintains the organoleptic properties of wa-
due to the immobilization of microbes on the surface facilitating contaminants removal
via a combined
ter, while other treatments, such as adsorption and biodegradation
chlorination, might produce process, on the basis
carcinogenic that the biofilms
by-products
can degrade and remove a wide range of organic, inorganic, and biological waterborne
[213]. Recent work has focused on using engineering biologically enhanced biochar (BEB)
contaminants.
for biological water treatment
Inexpensive [210], focusingbiochar
and available on the andscope,
woodchipspotential
were usedbenefits, and chal-
for anaerobic wastewater
lenges of sustainable water treatment
filtration, using BEB.
and their suitability The work
was evaluated examines
compared BEB’sasdynamic
to gravel a standardand reference
complex biofilm–biochar interactions in water treatment. The authors also suggest the use
of BEB instead of biological activated carbon (BAC) in the tertiary treatment of drinking
water due to the immobilization of microbes on the surface facilitating contaminants re-
moval via a combined adsorption and biodegradation process, on the basis that the bio-
films can degrade and remove a wide range of organic, inorganic, and biological water-
Resources 2024, 13, 8 21 of 33

material [224]. Filters were fed with raw sewage from a municipal full-scale wastewater
treatment plant in Germany at room temperature. The performance of the biochar filters
was much better over the experiment compared to woodchip and gravel filters concerning
chemical oxygen demand, total organic carbon, turbidity, and fecal indicator bacteria
removal efficiency, showing the superior properties of biochar for wastewater treatment.
Advanced oxidation processes are proven to be efficient in water treatment (reduction of
toxic, organic pollutants) and elimination of emerging concerns like pollutants (toxins,
pesticides, dyes, etc.) and include UV/O3 , UV/H2 O2 , Fenton, photo-Fenton, nonthermal
plasmas, sonolysis, photocatalysis, radiolysis, supercritical water oxidation processes,
etc. [225]. In this point, it is very important to mention that advanced oxidation can be
achieved using biochar because of the radical groups, mainly hydroxyl radical, introduced
by chemical treatments such as acid or alkali hydrolysis. Biochar functionalized with
hydroxyl groups enhances soil structure and reduces soil erosion, facilitates water and
nutrient retention, etc.

4. Potential Drawbacks and Future Perspectives of the Use of Biochar-Based Materials

In recent years, biochar has gained significant attention as a promising alternative
to mitigate environmental and climate change issues through efficient and inexpensive
water treatment and soil amendment methods. However, despite its advantages, using this
carbonaceous material can generate long-term drawbacks, especially for soil health and
ecosystems [25,226]. One of the potential drawbacks of biochar is related to the synthesis
conditions and the use of chemicals to improve its physicochemical properties [115]. For
example, for the pyrolysis process, it is necessary to reach high temperatures (above 400 ◦ C)
under anoxic atmospheres [58,79,90,221]. To avoid these conditions, biochar-based materi-
als can be prepared using HTC. However, it is necessary to use a large amount of water
to carry out the washing cycles of the carbonaceous material. In addition, hydrothermal
reactors may be used by HTC to obtain biochar, which, until now, has only been used at a
laboratory scale. Due to this, the HTC of biomass has not been able to be used on a large
scale. Another potential drawback related to biochar synthesis is the available feedstock.
Although biochar can be produced from any biomass, the process yield is generally less
than 60% [119,133]. For this reason, a more significant amount of biochar would be needed
to obtain a considerable amount of biomass, which, in many cases, can be used directly as
fertilizer or organic fertilizer.
On the other hand, some drawbacks are related directly to the biochar application. One
of them is the generation of sludge containing biochar with contaminants after adsorption
processes, which, sometimes, must be treated or incinerated, generating a large amount of
gases that can harm the environment [2]. Likewise, the efficiency of biochar is sometimes
lower than that of other types of adsorbents, like zeolites [227–230], clays [32,50,231,232],
and hydrogels [233–235], which can be even cheaper than the carbonaceous materials.
When biochar is used as a soil amendment, its application can affect the soil biota
because the carbonaceous material can potentially alter essential biogeochemical processes,
like nutrient cycling and decomposition [34,104]. According to Han et al. [41], high biochar
concentrations can inhibit the growth of specific microbial communities and favor others,
generating considerable changes in microbial diversity and function in soils. In addition,
biochar can influence nutrient dynamics and their interactions in soils. Its presence provides
a porous surface for nutrient adsorption and immobilization, making it less accessible to
plants [39,226]. Moreover, biochar may interact with pesticides and fertilizers, altering their
bioavailability and potential environmental effects [34,236]. Furthermore, the presence
of biochar in soils could modify their water-holding capacity, aeration, root penetration,
and aggregate stability [39,104]. For all these reasons, it is necessary to pay attention to
the type and amount of biochar used for soil remediation and not to considerably alter its
physicochemical properties.
Even though biochar is obtained from residual biomass, it can be produced from con-
taminated raw materials. In this sense, when this type of biochar is used in soil remediation,
Resources 2024, 13, 8 22 of 33

contaminants from the carbonaceous materials may lead to the introduction of pollutants
into the soil, generating potential environmental and human health risks [237,238]. There-
fore, careful selection of raw materials and monitoring pollutant levels are significant to
ensure the environmental safety of biochar application.
There is scarce information about long-term data on the biochar effects on soil health,
water, and ecosystems [239,240]. Hence, conducting long-term field research studies may
be crucial to evaluate the cumulative effects of biochar application and identify potential
unintended consequences. Considering the drawbacks of the synthesis and applications of
biochar, it is necessary to develop alternatives to improve its use, considering the principles
of sustainable development. Among them, one can consider:
• Optimizing biochar synthesis methods and their physicochemical properties: Research
studies are necessary to optimize the production processes for a carbonaceous material
with tailored properties for specific applications, avoiding the use of complicated
synthesis conditions and hazardous chemicals, as well as the generation of residues.
This includes understanding the effects of the type of raw materials, temperature
conditions, and post-synthesis treatments on the biochar properties and their efficiency
in the corresponding application.
• Understanding the complex interactions between biochar and water bodies and soil
biota is essential for predicting and mitigating potential negative impacts. This in-
cludes identifying microbial communities sensitive to biochar and developing alterna-
tives to minimize disruptions to soil biodiversity and water bodies [63,104].
• To quantify and analyze the effects of biochar on the availability, retention, and interac-
tions of soil nutrients with fertilizers and pesticides [60,69]. It enables the development
of biochar-nutrient management strategies to optimize nutrient use efficiency and
minimize environmental risks.
• To evaluate the long-term effects of using biochar on soil health, crop productivity,
and ecosystem services. This fact may provide enough data for sustainable biochar
management practices [239,240].
• Developing novel standardized biochar characterization and assessment protocols to
compare biochar-based materials produced from different raw materials with specific
synthesis methods and conditions [241,242]. This, in turn, could facilitate the devel-
opment of evidence-based recommendations for using different types of biochar in
specific applications.
• To address social, economic, and policy considerations necessary to ensure the sustain-
able application of biochar-based technologies [243]. It includes identifying potential
socioeconomic implications, assessing the costs and benefits of biochar use, and devel-
oping supportive policies that promote the sustainable production and utilization of
this carbonaceous material [226].
Therefore, although biochar has potential applications in improving soil and water
properties, mitigating climate change, and remediating contaminated environments, its
potential drawbacks must be identified and addressed. Future research efforts should opti-
mize industrial biochar production conditions and methods, understand its interactions
with water and soil biota and nutrient dynamics, evaluate its long-term effects on ecosys-
tems, develop standardized characterization protocols, and address social, economic, and
political considerations. By considering these challenges and understanding the long-term
effects of using biochar, it will be possible to ensure its use as a sustainable and responsible
application to mitigate environmental problems, maximizing its benefits and minimizing
its potential drawbacks.

5. Conclusions and Final Remarks

Biochar emerges as a product with high environmental value, which has a low cost and
is suitable for wastewater purification and soil amendment. This carbonaceous material has
a significant adsorption capacity for heavy metals and other industrial pollutants in polluted
water bodies. Biochar’s synthesis, activation, modification, and thermochemical treatment
Resources 2024, 13, 8 23 of 33

processes are crucial in obtaining the desired physicochemical properties. All these changes
can alter the material’s structure and, thus, the pollutant removal performance obtained
during the treatment. Biochar is typically used as an adsorbent. However, it can be used in
other wastewater treatment technologies, like advanced oxidation processes. Combining
primary and secondary treatment processes with biochar makes it possible to achieve high
removal percentages of any pollutants, toxins, or impurities in industrial wastewater. On
the other hand, current research has shown that adding dopants, additives, or composites
to biochar can improve its physicochemical, molecular, and structural properties. Likewise,
physicochemical or biological modifications of biochar can change its structural properties
(e.g., specific surface area and pore structure) or modify the type and concentration of the
surface oxygen functional group content, enhancing its performance in pollutant removal.
The type of raw material is essential to produce biochar-based materials with particular
physicochemical properties and characteristics and, therefore, with a specific use. Among
the raw materials used to obtain biochar, wood materials have been widely studied because
their high content of lignin, cellulose, and hemicellulose has allowed the production of
carbonaceous materials with high carbon content and high adsorption character. The
widely used technique to produce biochar has been the pyrolysis process since this method
favors high yields in biochar production. This process converts biomass into biochar with a
high fixed carbon content and stability, where 500–800 ◦ C is the ideal range for pyrolysis
temperature in biochar production. On the other hand, HTC is also an interesting synthesis
method of biochar because it has many advantages compared to others. Among them,
the operating temperature is lower than pyrolysis, it is not necessary to control the inert
atmosphere during the carbonization, and the generation of functional groups at the biochar
surface can be controlled by varying the operating conditions.
Biochar can be used to treat agricultural and industrial wastewater. However, strate-
gies to maximize its adsorption capability and stability must be developed. Thus, in
recent years, the use of precursors, dopants, or additives during biochar synthesis has been
widely studied to improve the capture of chemical molecules that are difficult to treat using
conventional methods. Furthermore, modification and activation processes have been
considered promising alternatives to enhance biochar’s physicochemical and structural
properties and, therefore, its performance in removing pollutants in soils and water bodies.
Most current scientific investigations on biochar and its applications have been performed
at a laboratory scale because small-scale trials and studies should be conducted before
carrying out biochar applications on an industrial scale. Biochar is a promising alternative
to mitigate environmental issues since it improves the quality of water and soil, allows the
use of biomass waste, and adds value to it, meeting the aim of the circular economy.

Author Contributions: Conceptualization, B.D., M.R. and C.N.-C.; methodology, B.D. and C.N.-C.;
validation, P.E.O., A.S.-M. and E.B.-G.; formal analysis, P.E.O., A.S.-M., E.B.-G., M.R. and C.N.-C.;
investigation, B.D., M.R. and C.N.-C.; data curation, B.D., M.R. and C.N.-C.; writing—original draft
preparation, B.D. and C.N.-C.; writing—review and editing, P.E.O., A.S.-M., E.B.-G., M.R. and C.N.-C.;
visualization, M.R. and C.N.-C.; supervision, M.R. and C.N.-C. All authors have read and agreed to
the published version of the manuscript.
Funding: The Ecuadorian Corporation for the Development of Research and the Academy (CEDIA)
funded this research, project number CEPRA XVI-2022-13 BioFe.
Data Availability Statement: The data supporting this study’s findings are available from the
corresponding authors (M.R. and C.N.-C.) upon reasonable request.
Acknowledgments: The authors acknowledge the support of the School of Chemical Sciences and
Engineering at Yachay Tech University.
Conflicts of Interest: The authors declare that this study received funding from the Ecuadorian
Corporation for the Development of Research and the Academy (CEDIA). The funder was not
involved in the study design, collection, analysis, interpretation of data, the writing of this article or
the decision to submit it for publication.
Resources 2024, 13, 8 24 of 33

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