Decolorization of dyes from textile wastewater using biochar: a review

Abstract

The textile industry is one of the largest in many low and middle-income countries, especially in Asia, second only to agriculture. Textile wastewater is discharged into the environment due to the lack of affordable and sustainable solutions to adsorb or remove the dye from the water. Biochar is generated by pyrolysis of organic material from plant waste in low-oxygen conditions, and is considered carbon-negative. Biochar for dye adsorption in textile wastewater effluent was proven to be highly effective. However, adsorption efficiency varies with experimental parameters, therefore there is a gap in application especially in small dye houses. Efforts should be made to find innovative and affordable solution to make the textile industry more sustainable, by developing methods for collection and reuse, recycle and upcycle of textile waste, by reducing the consumption of water, energy and chemicals and by developing methods for treatment of the textile wastewater.

Full text

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DECOLORIZATION OF DYES FROM TEXTILE WASTEWATER USING BIOCHAR: A REVIEW
Hadas Mamane*
School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University
Tel Aviv 69978, Israel, hadasmg@tauex.tau.ac.il
https://orcid.org/0000-0001-5321-4342
Shir Altshuler
School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University
Tel Aviv 69978, Israel
Elizaveta Sterenzon
School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University
Tel Aviv 69978, Israel
Vinod Kumar Vadivel
School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University
Tel Aviv 69978, Israel
https://orcid.org/0000-0003-4986-4929
Abstract
The textile industry is one of the largest in many low and middle-income countries, especially in Asia, second
only to agriculture. Textile wastewater is discharged into the environment due to the lack of affordable and
sustainable solutions to adsorb or remove the dye from the water. Biochar is generated by pyrolysis of organic
material from plant waste in low-oxygen conditions, and is considered carbon-negative. Biochar for dye
adsorption in textile wastewater effluent was proven to be highly effective. However, adsorption efficiency varies
with experimental parameters, therefore there is a gap in application especially in small dye houses. Efforts
should be made to find innovative and affordable solution to make the textile industry more sustainable, by
developing methods for collection and reuse, recycle and upcycle of textile waste, by reducing the consumption
of water, energy and chemicals and by developing methods for treatment of the textile wastewater.
Keywords
textile dye effluent; biochar; wastewater; sustainability; contamination; sorption
Introduction
Climate change, population growth, rising standards of living and uneven distribution of water are the main
causes for competition over water resources, water scarcity, poor water quality and variability of hydrological
events. Water is the core of sustainable development and unfortunately, water is not equally available and in
many areas around the world clean water is out of reach [1]. The environmental stress on water bodies is evident
in terms of not only quantity, but also quality. Globalization of industrialization has resulted in high pollution
of water resources worldwide. The major industries responsible for pollution are the dyeing industries, paper
industries, tanneries, metal-plating industry, mining operations, fertilizer industries, agricultural waste and
pesticides. The demand for water in the industrial sector is expected to increase by 283% during the first half
of the 21st century [2], resulting in increasing industrial wastewater discharge.
Discussion
The textile industry is one of the largest in many low and middle-income countries, especially in Asia [3]. The
textile industry is one of the cornerstones in economies of many countries [4]. For example, the Indian textile
industry according to the India Brand Equity Foundation (IBEF), it is the second largest industry, after agriculture,
providing employment to over 45 million people directly and 60 million people indirectly, and it contributes 14%
of the Indian total industrial production [5]. Humans are aesthetically interested in dyed textile, therefore the
use of dyes in textile will not be abandoned. Different types of dyes are used in a variety of industries including
the food industry, textile, tanneries, plastics and pharmaceuticals. Many products in those industries contain

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several dyes from different chemical classes resulting in a complex wastewater [6]. Industrial dyes, in particular
used in the textile industry, have complex molecular structures, synthetic in origin and recalcitrant [7].
Most of the chemicals are added in the dyeing process where a color is added to the dye baths, the fabric
is immersed in the dye baths until the dye is fixed. In some cases, there is a need to add salt to the bath in addition
5 tons
of synthetic dye are produced annually, with 10 15% of it not ending up in the final product [4] thus eventually
further contaminating the environment.
Therefore the effluent also contains a large amount of recalcitrant unfixed dyes (as acid dyes, basic dyes, sulfur
dyes, chrome dyes, optical/fluorescent brightener and azoic dyes) as the dyes are not totally fixed to the fiber
of different textiles during the dyeing process (fibers as wool and nylon, cotton and viscose, polyester and
acrylic). The textile industry requires a large amount of water for the production process, and is also one of the
major producers of wastewater that can have carcinogenic and mutagenic compounds [9]. Wastewater from the
textile industry is frequently discharged directly into lakes and rivers without any proper treatment and often
these water sources are used by locals domestically [10,11]. Since the dyeing process uses a significant amount
of water, recycling used in the dyeing process can conserve water, however it requires treatment whether
recycled or discharged to the environment. Reference values for water reuse in textile industry, included COD
between 60- In addition to dyes,
lubricating oils and fibers [11].
Dyes cannot be removed through conventional treatment unit operations due to the complex characteristics of
the wastewater as high solubility, non-degradable nature, diversity and often changing speciation in water. When
industrial wastewater is discharged into natural water bodies it can result in hazardous effects on the living
systems because of the carcinogenic, mutagenic, allergenic and toxic nature of dyes [6]. This is a paradox as
current conventional and advanced methods for the removal or degradation of persistent and emerging textile
contaminants are limited, since they often involve intensive capital, lack of adaptive technological tools, social
barriers and emphasis on centralized systems. Consequently, to close the gap, there is a dire need for innovative
solutions and for widespread decentralized systems in the textile industry suitable for rural areas and capital-
challenged countries.
There are many treatment methods when dealing with textile wastewater. The different techniques can be based
on physical (sedimentation, filtration, floatation, coagulation, reverse osmosis, solvent extraction, adsorption,
incineration, and distillation), chemical (neutralization, reduction, oxidation, catalysis, ion exchange, electrolysis)
and biological (stabilization, aerated lagoons, trickling filters, activated sludge, anaerobic digestion) [12]. Physical
methods are very common methods in textile wastewater treatment due to their high color removal efficiency,
especially adsorption, filtration and membrane filtration. Other treatment processes as reverse osmosis,
nanofiltration and multiple effect evaporators are effective but expensive, while the common treatment
methods are non-destructive, lower in cost, time-consuming and less efficient [13].
Adsorption by porous materials is one of the most promising and affordable techniques for the removal
of dissolved pollutants, serving as an alternative to energy-intensive technologies [14]. Activated carbon is
a widely used adsorbent, but biochar, which is inexpensive, abundant and may have comparable adsorption
capacity, can be used as an affordable alternative [15-17]. However, the efficiency, local manufacturing,
availability and costs must be examined. Biochar is the result of low-temperature pyrolysis of carbon-rich
biomass (from agricultural and forestry wastes) under low-oxygen conditions [18]. As can be seen in Fig. 1, there
is an increase in the number of publications using the keywords: dye sorption, textile wastewater and biochar
(by google scholar).

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Fig. 1. Yearly number of publications using the keywords: dye sorption, textile wastewater and biochar
(by google scholar)
The biochar created is a stable carbon black solid that is highly porous with large surface area (Fig. 2.). More than
70 percent of its composition is carbon [19]. The chemical composition varies with feedstocks used
to make it and methods used to heat the biomass. The pyrolysis process varies as it can be done with different
conditions as burning temperature and burning time, reactor volume, other gasses and materials in the reactor
[20]. However, different thermochemical processes can be also used for biochar production [18]. Compared
to activated carbon, biochar can be created from various types of biomass, requires less energy in the production
process and consequently can provide a solution in the treatment of textile wastewater in poor income countries.
Fig. 2. Biochar particles image (left) and scanning electron microscopy (right, taken at Tel-Aviv University)

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The porosity and high surface area of biochar make it an excellent adsorbent of organic contaminants and heavy
metals in wastewater [17]. This has led to a growing interest in using biochar for water treatment, although most
research today focuses on its abilities for soil fertilization [15]. Adsorption of textile dyes was examined with
various biochar types [15,21,22]. For each type of dye, the biochar type, process parameters (temperature, pH,
agitation time) and wastewater quality influence the efficiency of the dye removal from the wastewater. Most
studies have focused on the removal of pure, highly concentrated dye solutions that do not represent the actual
effluent from real dye houses. Real effluent may contain lower dye concentrations and additional substances
as previously mentioned, such as salts, detergents, solids, and fiber residuals, which have a tremendous effect
There are numerous types of dyes and biochars and therefore the biochar type and dose should match the dye
type. The major anionic dyes are the acid and reactive dyes, and the most problematic ones are the brightly
colored, water-soluble acid dyes [23]. Acid dyes are a sub-group of anionic dyes and are called so, because they
are usually applied to the fibers in acid solutions [24]. Cationic dyes are dyes that can be dissociated into
positively charged ions in the aqueous solution, and the cationic dye dyes the fiber through the binding of its
cation ion [7,23]. Basic dye, are highly visible and have high brilliance and intensity of colors [25].
As mentioned above, the textile industry uses different colors, each with its own properties and characteristics.
Moreover, the biochar itself can vary based on the initial biomass from which it is produced. In order to find the
best biochar-dye combination, a summary of articles examining different combinations was made [26-43].
In these articles, the biochar efficiency in removing the dye from a mixture was calculated using mass balance
equation for dye adsorption on biochar [37].
Table 1 summarizes different characteristics from 1 articles that present the use of biochar in treatment
of textile wastewater [26-43]. The articles are catagorized to biochar properties, dye properties, experimental
conditions, dye adsorption experiments compared to a common biochar and modified biochar. All the
experiments presented were batch experiment in a controlled environment. From the table it is evident that
there are some differences between the treatment requirements for cationic dyes and anionic dyes. The
adsorption of the dye to the biochar depend on the characteristics of the solution, the type of the biochar and
the conditions in which the experiment was performed.
One of the critical conditions is the solutions pH, where the adsorption of cationic dyes will be maximized when
the pH of the solution is basic and anionic dyes in acidic pH, with preferred pH at . The pH of the solution has
a significant effect on the interaction of the dyes with the biochar in the adsorption process. For example, the
effects of the solutions pH were examined on the adsorption of three dyes, Methyl Orange, an anionic dye and
Malachite green & Methylene blue, both cationic dyes and three types of biochar, each produced with a different
solvent (acetone, ethanol and methanol) [30]. The adsorption of the cationic dye Malachite Green onto the
biochar increased with the increase in pH. The solution pH influenced the adsorption through dissociation
of functional groups on the active sites of the biochar. In another study, the adsorption of anionic dye Reactive
Red 141 onto a Pecan Nutshell based biochar occured mostly in an acidic environment with a low pH [32]. Under
acidic conditions, the biochar surface is positively charged whereas the Reactive Red 141 has several sulfonated
groups, which are negatively charged that are attracted to the biochar surface, increasing the dye removal
percentage.
The dye to biochar interaction depends on the temperature of the mixture, the interaction time, the steering
speed and more. Almost all the experiments showed that a temperature of 25-30oC is the optimal temperature
ocity in the mixture and
will reduce the adsorption [27]. This result is significant because it means the treatment will not require any
major heating or cooling systems and the treatment can be effective in real wastewater. The interaction time is
also a crucial parameter. With short interaction time, the dye will not be able to sufficiently interact with the
biochar, while with long interaction time, the binding between the dye and the biochar could become loose, and
the biochar can undergo modifications that can damage its functionality and even release material and pollutants
to the water and cause more damage [27,30,34]. Each biochar-dye combination has its own optimal interaction
on the other conditions regulating

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the environment. Mubarak et. al. showed an increase in removal effecincy with contact time of methylene blue
(cationic) and orange-G (anionic) onto Empty fruit bunch based biochar [36]. Another factor that can affect the
interaction is the rotation speed of the mixture. A high rotation speed can lead to a high shear stress that can
break the bindings between the biochar and the dye and can even damage the biochar particles and thus lower
its efficiency. The most common rotation speed as presented in Table 1, is ~130-150 rpm.
Several studies attempted to increase an existing biochar activity by performing different modifications on it as
Iron impregnation biochar, addition of cationic surfactant to the biochar surface, inserting magnetic formation
to the biochar and more. These new characteristics provide the biochar with a stronger affinity and interaction
between the biochar and the dye. One example of the benefits of these modifications used Rice Husk derived
biochar mixed with a solution containing an aqueous phase reduction of ferrous iron (FeSO47H2O) called nZVI,
resulting in attachment of nanoscale zero-valent iron particles to the biochar creating a modified biochar
(nZVI/BC) to adsorb the anionic dye Methyl Orange. In this experiment, three types of biochar various theoretical
mass ratios of nZVI/BC at 1:3, 1:5, and 1:7 [28]. The nZVI/BC 1:5 adsorbed almost 100% of the dye in the mixture
compared to the other modified biochars that peaked at 90% and the non-modified biochar that reached only
10%. Although there may be an advantage in using modified biochar, there might be a problem after extended
usage. For example, high concentrations of oxygen can convert the Fe0 molecules into ferrous or ferric oxide
leading to a passivation layer forming on the nZVI surface.
Table 1. A summary of articles presenting biochar source, postproduction modifications, dye type, optimal conditions and
experimental results
Biochar
Source
Biochar
preparation
Postproduction
Modifications
Dye
Optimal Condition Results Refere
nce
pH Temp
(oC)
RPM Contact
time
(min)
Spent
mushroom
substrate
(SMS)
Pyrolysis at
450oC for 4
hours
200 cm3
STP/min of
N2 was fed into the
reactor, and steam
was used as the
activation agent at
2 h.
Modification goals:
Enhancement of
textural properties.
Congo Red
(CR)
(anionic)
Crystal
Violet (CV)
(cationic)
CR:4
CV:6
30 150 CR:750
CV:1000
Color and COD
removal
efficiencies up
to 99.6% and
67.7% for CV.
Color and COD
removal
efficiencies up
to 10.3% and
23.7% for CR.
[41]
Rice husk
(RHB) and
Coir pith
(CPB)
Pyrolysis at
700oC for 5
hours
RHB or CPB were
added to a solution
containing Fe
(NO3)3 2O
dissolved in water.
The mixture was
oven dried at 105
followed by
calcination at 500
Modification goals:
Provide the biochar
with oxidizing
properties.
Acid Red 1
(AR1)
3 30-50
150 120
Maximum dye
removal for Fe-
RHB was 97.6%.
Maximum dye
removal for Fe-
CPB was 99.1%
[27]
Rice husk Pyrolysis at
500oC
Biochar was mixed
with HCl for
demineralization
then mixed with
different mass
ratios of nZVI.
Methyl
orange
(anionic)
4 - 15 Maximum dye
removal for
nZVI at 5:1 was
98.5%.
Removal
capacity of
97.8,
[28]

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Modification goals:
increase dye
adsorption by
transforming it to
low molecular
weight products
through
destruction of its
N=N bonds.
306.7, 605.0,
and 709.1
mg/g for initial
concentrations
of 60, 200, 400,
and 600 mg/L,
respectively.
Bael shell
(BS)
Pyrolysis at
500oC for 3
hours
The biochar did n
ot
undergo any
special
modifications
Patent
blue (PB)
(Anionic)
2.7 - 110 60 Maximum dye
removal was
74% (3.7 mg/g)
[21]
Carboxym-
ethyl
cellulose
(CM) from
raw chicken
manure
Pyrolysis at
600oC for 2
hours
The biochar did not
undergo any
special
modifications
Methyl
orange
(anionic)
6.5 25 150 30 Almost 100% dye
removal
[29]
Sewage
sludge (SS)
with
acetone as
the solvent
Liquefaction
at 260-280
oC
The biochar did not
undergo any
special
modifications
Methyl
orange
(MO)
(anionic)
Malachite
green
(MG)
(cationic)
Methylene
blue (MB)
(cationic)
Acetone based
solution gave
53.12%
removal.
The bio-chars
were only
effective on
cationic MG
and MB with
removal of 10
40 mg/g and
15 45 mg/g
[30]
Eichhornia
crassipes-
molasses
Pyrolysis at
400oC for 5
hours
The biochar did not
undergo any
special
modifications
Methylene
blue (MB)
(cationic)
8 25 125 30 Maximum
adsorption
capacity of
44.13 mg/g
[31]
Pecan
nutshell
Pyrolysis at
800oC for 1
hour
The biochar did not
undergo any
special
modifications
Reactive
Red 141
(RR-141)
(anionic)
2-3 25 250 80
Increased initial
dye
concentration
provided an
increase
adsorption
from 40 to 130
mg/g
85% dye
removal. 80%
of saturation
was attained
within 10 min
[32]
Palm Kernel
Shell (PKS)
Pyrolysis at
350oC in a
rotary kiln
for 20 min
The biochar did not
undergo any
special
modifications
Crystal
Violet (CV)
(cationic)
6 25 100 30 maximum
adsorption
capacity of
24.45 mg/g
[33]

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Switchgrass
Biochar
modified
by cationic
surfactant
(SB-TTAB)
Pyrolysis at
450o
C for 20
minutes
Biochar added to a
solution of
tetradecyltrimethyl
ammonium
bromide (TTAB) in
ethanol.
Modification goals:
binding a cationic
Reactive
Red (RR-
195A)
(anionic)
5 25 150 40 Dye removal
from different
contaminated
solutions:
conc.10/30/50
mg/L)
Tap water %:
10 - 96.61
[34]
surfactant to the
biochar to form
micelle like
structures which
can solubilize dye
within this
structure and
increase the
biochar
capabilities.
30 - 98.82
50 - 98.76
Raw water %:
10 - 98.56
30 - 97.79
50 - 99.26
Wastewater %:
10 - 100.00
30 - 94.83
50 - 94.24
Sea water %:
10 - 92.96
30 - 92.60
50 - 90.98
Kappaphy-
cus alvarezii
seaweed
Pyrolysis at
350oC for 2
hours
The biochar did not
undergo any
special
modifications
Reactive
blue 4
(RB4)
(anionic)
Reactive
orange 16
(RO16)
(anionic)
2-3 30 180 60
Around 90% of
total reactive
dye sorption
occurred within
the first 60 min
of contact
uptake of 0.324
mmol/g for RB4
uptake of 0.140
mmol/g for
RO16
[35]
Empty fruit
bunch (EFB)
microwave
at 800W for
30 min.
EFB particles were
treated chemically
by (FeCl3) before
pyrolysis. Flow of
nitrogen gas
provided iron oxide
magnetite
formation to the
chemical treated
EFB. Modification
goals: biochar with
magnetic features
enable the dye to
be separated by
magnetic
separation
techniques
Methylene
blue (MB)
(cationic)
Orange-G
(OG)
(anionic)
MB:
Both
2&10
OG:2
25 120 120 Maximum
adsorption
capacity of
96.68% (31.25
mg/g) for MB
Maximum
adsorption
capacity of
90.76% (32.36)
mg/g for
Orange-G
[36]
Empty fruit
bunch (EFB)
Pyrolysis at
00oC
The biochar did not
undergo any
special
modifications
Methylene
blue (MB)
(cationic)
- 30 150 250 Dye removal of
91%, 90%, 49%
and 36% for 50,
100, 200 and
300 mg/L
respectively
The EFB
biochar has a
maximum
sorption of
55.25 mg/g.
[37]

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Spirulina
platensis
algae
Pyrolysis at
50oC for 2
hours
The biochar did not
undergo any
special
modifications
Congo red
dye (CR)
(anionic)
2-7 30 120 15 75-
80% dye
removal
For different
initial dye
concentrations.
:
30,50,70,90,20
0mg/l
[38]
Chicken
bones (CBB)
after
modificatio
n-MCBB
Pyrolysis at
500oC for 2
hours
Powdered CBB
subjected to co-
precipitation with a
mixture of Fe3+ and
Fe2+ salts. The CBB
was added into a
solution containing
FeSO47H2O and
FeCl36H2O.
Modification goals:
biochar with
magnetic
properties f
or rapid
sorption and a
convenient
recovery.
Rhodamin
e-B (RB)
(basic dye)
10 50 150 120 88.5% dye
removal for
40mgL-1. (36.2
mg/g) after 120
minutes.
Approximately
96.5 mg/g of
RB was
adsorbed at pH
10 within 180
min and
reduced to
68.5 mg/g in
the presen
ce of
0.5 g NaCl.
[39]
Pulp and
paper
sludge (PPS)
Pyrolysis at
108oC for 2
hours
PPS soaked in a
FeCl3.6H2
O solution
and dried before
being pyrolyzed.
Modification goals:
Reduce porosity
and decrease in
pore volume as a
result of
nanoparticle
impregnation that
will lead to a rapid
dye diffusion into
the active sites
when the particles
organically detach
from the biochar
into the solution
Methylene
blue (MB)
(cationic)
12 - - 40 Impregnating
PPS with Fe2O3
increased
maximum
adsorption
capacity of the
ad
sorbent by
more than 50%
saturation
points for BC
(97%) and NC
(98%) both
occurred at 5
g/L adsorbent
The maximum
adsorption
capacities
calculated
33
and 50 mg/g for
BC and NC
[40]
Korean
cabbage
(KC)
Pyrolysis at
500oC for 1
hour
The biochar did not
undergo any
special
modifications
Congo red
(CR)
(anionic)
crystal
violet (CV)
(cationic)
CV:11
CR:7
30 150 1400 maximum
adsorption
values:
CR on KC:
95.81mg/g
CV on KC:
1304mg/g
[41]
Sugarcane
bagasse
(SCB)
Pyrolysis at
400,600,800
oC for 1
hour.
Best results
for the 800
oC biochar
The biochar did not
undergo any
special
modifications
malachite
green
(MG)
(anionic)
7.5 60 10,000
51.89 removal
of dye
(conc.:500
mg/L) for SCB
prepared at
800oC
[42,43]

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Impact
The sustainability of the textile industry should be addressed across all sectors from fashion designers,
manufacturers, product developers and the consumers. The process residuals as waste and wastewater
generated requires innovative and affordable technologies and processes for collection and reuse, recycle and
upcycle of textile waste (clothing and other textiles), for reducing the consumption of water, energy and
chemicals and for treatment of the textile wastewater both in treatment plants and in small dye house industries.
The environmental law for the dyeing effluent is in many cases very stringent and this necessitates the need for
efficient treatment methods that must follow Zero Liquid Discharge (ZLD) either in common or in non-common
treatment plants; however in practice due to the treatment costs it is not always practiced. In addition, even
when plants are set for the ZLD, they are still generating hundred tons of hazardous solid waste per day as sludge
(residual dyes and waste salts). Efforts should be made also on recovery of the dyes, and other organics from the
wastewater before their discharge on to the soil and water bodies, in addition to efficient water treatment by
combining novel hybrid membranes and nanotechnologies.
Tamil Nadu is in southern part of India and is engaged in textile processes especially the cotton textile industry.
Real textile wastewater effluent from dye houses located in Coimbatore, Tamil Nadu, southern India, was
examined for acid-dye removal from wastewater generated when dyeing silk filaments for production of soft silk
sarees. The used dye solution from the dye houses is often discharged to the drainage or into the environment
due to lack of affordable solutions. These dyes can potentially cause serious environmental damage and health.
In our study, optimal conditions were demonstrated for filtration followed by high dye adsorption onto pine
derived biochar (both in batch and column studies), and recommendations were suggested for reuse of the water
back to the dye houses and for recovery of the biochar post use.
Conclusions
Different types of biochar were effective in adsorption of dyes from the textile wastewater effluent. Parameters
that affect the process are temperature, rotation and mixing speed of the biochar with the dye in batch tests,
and reaction time. Another important parameter is the pH of the biochar-dye suspension. Basic environment (pH
higher than 7) was proven to be ideal for cationic dyes, where acidic environment (pH below 7) was proven to
be ideal for anionic and acid dyes, with the optimal pH being between 3-4. Different post-production
modifications to the biochar can increase the efficiency of the adsorption process and thus improve the entire
treatment process; however, the long-term use and reuse of the modified biochar should be monitored.
Conflict of interest
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