Biomass-derived activated porous carbon from Manilkara kauki L. bark as potential adsorbent for the removal of Congo red dye from aqueous solution

Abstract

This study explores preparation (thermally) and activation (chemically) activated carbon from Manilkara Kauki L. bark (MKB) for the removing of Congo red (CR) dye by batch adsorption method. The surface characteristics of the MKBC were analyzed using SEM and FTIR. Variable experimental parameters, including initial concentration, contact time, and pH, were used to test the MKBC adsorption capacity to remove CR dye. The dye solution pH was alkaline, which favored CR absorption. The Langmuir model well modeled the equilibrium adsorption data. It had an adsorption capability of 3.029 mgg⁻¹ in MKBC and 148.08 mgg⁻¹ in CAC. The best model for adsorption kinetics in pseudo-second order. As a result, the optimum MKBC can serve as a cost-effective and active adsorbent for removing dyes from industrial wastewater.

Full text

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Biomass Conversion and Biorefinery (2023) 13:9475–9485
https://doi.org/10.1007/s13399-023-03969-5
ORIGINAL ARTICLE
Biomass‑derived activated porous carbon fromManilkara kauki L. bark
aspotential adsorbent fortheremoval ofCongo red dye fromaqueous
solution
RajeswaranRamaraj1· BanumathiNagarathinam1· MuthirulanPandi2
Received: 23 November 2022 / Revised: 2 February 2023 / Accepted: 15 February 2023 / Published online: 1 March 2023
© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2023
Abstract
This study explores preparation (thermally) and activation (chemically) activated carbon from Manilkara Kauki L. bark
(MKB) for the removing of Congo red (CR) dye by batch adsorption method. The surface characteristics of the MKBC
were analyzed using SEM and FTIR. Variable experimental parameters, including initial concentration, contact time, and
pH, were used to test the MKBC adsorption capacity to remove CR dye. The dye solution pH was alkaline, which favored
CR absorption. The Langmuir model well modeled the equilibrium adsorption data. It had an adsorption capability of 3.029
mgg−1 in MKBC and 148.08 mgg−1 in CAC. The best model for adsorption kinetics in pseudo-second order. As a result, the
optimum MKBC can serve as a cost-effective and active adsorbent for removing dyes from industrial wastewater.
Keywords Manilkara kauki L. bark· CR dye· Batch adsorption· Isotherm and kinetic studies
1 Introduction
Many countries have adopted circular economy principles
to encourage waste materials to be used for various pur-
poses [1]. The vast majority of waste wood biomass gen-
erated by the wood processing industry has been used for
heat and power. It allows non-renewable energy sources
to be replaced with more sustainable, renewable ones [2,
3]. However, a large amount of the waste wood biomass is
still unexploited and poses a potential environmental risk.
Another risk to aquatic environments is synthetic dyes,
which are common industrial effluents [4]. Poorly treated
or untreated industrial effluents can significantly impact
the physico-chemical properties of the significant impact
water’s chemical and physical properties recipient’s flora or
fauna [5]. Synthetic dyes are toxic, carcinogenic, and muta-
genic. Because of their complex chemical structures and
application requirements, synthetic dyes are resistant to heat,
light, oxidation, and recalcitrance to microbial decay. Con-
ventional biological wastewater treatment systems are not
efficient in removing dyes. Furthermore, dyes are retained
in the environment [6].
There are many ways to get rid of synthetic dyes in water,
and it is improving usage, high efficiency, adaptability, and
adsorption, which has emerged as the most preferred tech-
nique for removing synthetic dyes from water, particularly
on a large scale. The most commonly used dye-removal
agent, activated carbon, removes cationic and mordant
dyes. However, its efficiency for dispersed and direct dyes,
vat dyes, and reactive dyes is lower [7]. The main problem
with commercially available activated charcoals is their high
price and difficulty in regeneration. Also, this is because
regeneration can be complicated and reduce adsorption.
The problem with commercially available activated carbons
made from coal, which can effectively remove many water
pollutants, is that coal is a fossil fuel. It is unlikely that it
will ever be a renewable resource. Many other materials,
especially lignocellulosic, are being explored as low-cost
adsorbents to remove dyes in their natural (unmodified) form
or modified (including biochar). Waste wood biomass is a
low-cost adsorbent because it is inexpensive, readily avail-
able throughout the year, and requires little to no processing
[4]. It can also be classified as a biosorbent because it is an
* Muthirulan Pandi
pmuthirulan@gmail.com
1 Department ofChemistry, Rani Anna Government College
forWomen, Affiliated toManonmaniam Sundaranar
University, Tirunelveli627008, India
2 Department ofChemistry, Lekshmipuram College ofArts
andScience, Affiliated toManonmaniam Sundaranar
University, Tirunelveli, Neyyoor629802, India

9476 Biomass Conversion and Biorefinery (2023) 13:9475–9485
1 3
organic (biological) material. Biosorption is a subcategory
of adsorption where the adsorbent must be biologically
derived. The main components of wood biomass are cellu-
lose, hemicellulose, and lignin. Adsorptive removal of dyes
from water (and other contaminants) is mainly accomplished
through the interaction between dyes and specific functional
groups of lignocellulosic polymers [8].
Following the processing and finishing applications and
the release of enormous volumes of effluent into water bod-
ies, the dyeing industries are responsible for the largest use
of water during the manufacturing process. An anionic dye,
Congo red (CR), is part of the azo group that contains nitro-
gen-containing dyes. Congo red is widely used in the textile,
printing, and leather sectors, as well as paper, pharmaceuti-
cal, and food industries. Because of the abundance of N-N
on CR and its limited biodegradability, it is a concern for
environmental scientists. Azo dyes are frequently introduced
into the environment due to industrial waste. Discharging
highly colored dye effluents may cause damage to the water
bodies [9]. Many dyes can cause harm to aquatic life as well
as human beings. They are toxic, mutagenic, or carcino-
genic. It is therefore important to remove color synthetic
organic dyestuffs from waste effluents [8].
The elimination of dyes from wastewater has been accom-
plished by the application of plenty of different treatment
techniques, including photocatalysis, sonochemical/elec-
trochemical degradation, membrane types, ultrafiltration,
or adsorption/precipitation processes, integrated iron (III)
photo assisted-biological or integrated chemical-biological
degradation, solar photo-Fenton, and biological processes
also adsorption on activated carbon [10]. Among these
methods, adsorption is one of the most versatile and widely
used methods in water purification technologies due to its
low cost, better performance, easy operation, higher effi-
ciencies, and simple design. Efforts have been made to find
more efficient, environment-friendly, low-cost adsorbents to
remove pollutants [8].
Biomass-derived activated carbons (biochars) have
been attracting much attention because of their outstand-
ing physico-chemical characteristics, such as high specific
areas, large pore volumes, well-defined microporous struc-
tures, and low cost [4]. Furthermore, in the recent years,
there has been a particular focus on preparing activated car-
bons using several agricultural byproducts. There is also a
growing interest in using inexpensive activated charcoals
for wastewater treatment. Agricultural waste is considered
one of the best alternatives because of its low cost and high
adsorption efficiency. In addition, these materials are cheap,
eco-friendly, and viable to remove dyes from the aqueous
systems owing to their unique chemical composition, abun-
dant availability, renewable nature, and low cost. There-
fore, converting these raw materials into valuable adsor-
bents to eliminate pollutants from aqueous systems will be
a promising alternative to reduce water pollution [5]. M.
Kauki bark is an abundant genus of quick-growing and hardy
trees found almost everywhere on our planet. M. kauki tree
is available abundantly on our college campus. Hence, we
prepared activated carbon from the M. kauki tree’s bark to
remove CR dye.
Moreover, the bark is an essential byproduct of this tree,
generally found peeling off of tree trunks or on the ground
after being shed. The lignin and cellulose content of M.
Kauki bark is about 20 % and 45 % (w/w), respectively, and
both these compounds provide essential microporous struc-
tures for dye adsorption. Besides, to the best of our knowl-
edge, there were no reports of adsorption studies for the
removal dyes using M. kauki bark carbon (MKBC). There-
fore, this investigation aims to determine the preparation and
possibility of applying Manilkara kauki L. bark–activated
carbon, which is the alternative to commercial activated
carbon as an adsorbent for removing Congo red (CR) dye.
2 Materials andmethods
2.1 Chemicals andreagents
Congo red (CR) dye was purchased from Sigma-Aldrich
used as received. The physico-chemical characteristics,
application, toxicity, and chemical structure of CR dye
are listed in Table1. All the other chemicals are analyti-
cal grade and utilized in their unpurified as-received state.
Also, the entire experiments were done at room tempera-
ture, and double-distilled (DD) water was used throughout
the investigation.
2.2 Preparation, activation, andstorage
ofadsorbent
Manilkara kauki L. bark was collected from our college
campus (locally); the bark is sliced into little pieces and
water-cleaned prior to use. It is then dried and carbonized in
a muffle furnace at a temperature of 300–400°C. Afterward,
the carbonized materials were activated with 200g of carbon
and 600 ml 4N nitric solution for 120 min at 80°C. Next, it
was rinsed with boiling DD water to remove any acid (tested
using pH Paper) and metal ions. The activated adsorbent was
then dried in an oven at 120°C for 5h. It was then placed in
a sealed wide-mouth laboratory bottle and used to conduct
absorption findings. The adsorbents employed in the pre-
sent work were labeled as M. kauki bark carbon (MKBC)
and commercial activated carbon (CAC). Figure1 shows
the schematic representation of the MKBC preparation and
its application for CR dye removal by adsorption.

9477Biomass Conversion and Biorefinery (2023) 13:9475–9485
1 3
2.3 Instrumental studies
The as-prepared MKBC adsorbent was ground and sieved
using a Jayanth brand (India) mechanical sieve. This pro-
duced consistent, discrete particles of different sizes (90
microns). Systronics digital pH monitor (model: 335) was
used to measure pH. The FTIR spectra of the adsorbent
material were obtained in KBr pellets using a Spectrom-
eter (Model BIORAD WIN IR). SEM photographs of
the adsorbent material were taken by JEOL Instrument
(Model JSM – 5300).
2.4 Batch adsorption experiments
The adsorption experiments were performed in a batch pro-
cess. Adsorption experiments were carried out by adding a
fixed amount of adsorbents into 250-ml stopper flasks con-
taining a definite volume (100 ml in each case) of different
initial concentrations of dye solution at room temperature
(30±1°C). The flasks were placed in a mechanical shaker
and agitation was provided at 130 rpm for the required time
to ensure equilibrium was reached. At time t = 0 and equi-
librium, the absorbance of the dye solution was determined
Table 1 Physico-chemical characteristics and application CR dye
Dye Congo red
CI No. 22120
Characteristics Anionic, water soluble, red orange color
Formula C32H22N6Na2O6S2; MW : 696.67
Structure
λmax 498nm
IUPAC name Disodium 4-amino-3-[4-[4-(1-amino-4-sulfonato-naphthalen-2-yl)diazenylphenyl]phenyl]diazenyl-naphthalene-1-sulfonate
Applications Most common dye found in the textile industry.
Applied as a pH indicator in the diagnosis of amyloidosis.
• As a laboratory aid in testing for free HCl in gastric content
Toxic effects Known to be metabolized into benzidine which is a human carcinogen and mutagen. Carcinogenic and neurotoxic effects
and had the ability to cause several human diseases. Causing respiratory tract infection, skin, gastrointestinal tract irrita-
tion and eye infections. Causing liver and thyroid damage, and eye and skin irritations.
Fig. 1 Schematic representa-
tion of the MKBC preparation
and its application for CR dye
removal

9478 Biomass Conversion and Biorefinery (2023) 13:9475–9485
1 3
by the photo colorimeter at λmax (498nm) value of the dye.
The amount of adsorption at equilibrium, qe (mg/g), was
calculated by
where Ci and Ce were the initial and equilibrium concen-
tration (in mgl−1 or ppm) of dye, respectively, and m is the
weight of AC (in gl−1). The dye removal percentage can be
calculated as follows:
The effect of the dose of adsorbents on the amount of CH
adsorbed was obtained by adding different amounts of adsor-
bents by keeping other parameters constant. In order to study
the kinetics/dynamics of the adsorption of dyes, the adsorp-
tion experiments were conducted by varying the contact time
(range: 5–60 min.) at the fixed optimum initial concentration
of dyes with a fixed dose of adsorbent and solution pH. The
dye concentrations were measured at different time intervals.
In order to analyze the adsorption data, the Freundlich and
Langmuir adsorption isotherms were employed.
Freundlich adsorption isotherm in its linearized form is
given by the equation,
where qe = (x/m) is the amount of dye adsorbed (in mg
g−1) per unit mass of the adsorbent at equilibrium; m is
the mass of the adsorbent (in gl−1), K and 1/n are the Fre-
undlich constants, which are the measures of adsorption
capacity (in mg g−1) and intensity of adsorption, respec-
tively. The value of 1/n [0 < (1 / n) < 1] is a fraction, so
that the value of n is a whole number (n > 1), which is the
order of adsorption. A plot of qe versus Ce was found to
be an exponential one. However, when log qe was plotted
against log Ce values, straight line plots were obtained,
with log K values as the intercept and (1/n) value as the
slope. Freundlich constants were calculated by employing
linear regression analysis and reported.
The expression gives Langmuir adsorption isotherm
where qe is the amount of dye adsorbed (in mg g−1) at
equilibrium contact time (i.e., time for maximum adsorp-
tion); Ce is the equilibrium concentration (in ppm) of dye;
(1/a) is the slope and (1/ab) is the intercept and a and b are
the Langmuir constants indicating respectively the mon-
olayer adsorption capacity (mg g−1) and adsorption energy
(gl−1). A plot of (Ce/qe) versus Ce was found to be linear with
(1/ab) value as the intercept and (1/a) value as the slope.
This plot is known as the Langmuir adsorption isotherm
(1)
Amount adsorbed (
q
e)
=
(
C
i
−C
e)
∕
m
(2)
Percentage removal
=100
(
C
i
−C
e)
∕C
i
(3)
Log qe=log K +(1∕n)log Ce
(4)
(
C
e
∕q
e)
=(1∕ab)+
(
C
e
∕a
)
plot. The applicability of the Langmuir isotherm indicates
the formation of a unimolecular layer and the nature of the
adsorption process. Furthermore, the essential characteris-
tics of the Langmuir isotherm can be described in terms of
a dimensionless constant, viz., separation factor or equilib-
rium parameter, RL, which is defined by the equation.
where b is the Langmuir constant (in gl−1) and Ci is the
optimum initial concentration of dye (in ppm). The values
of RL indicate the nature of the adsorption process and the
shape of isotherm. The value of RL denotes an unfavora-
ble (RL > 1), linear (RL = 1), favorable (0 < RL < 1), and
irreversible (RL = 0) process of adsorption.
For the kinetic study of the adsorption process under
consideration, the following first-order kinetic equations
Natarajan and Khalaf, Lagergren, Bhattacharya, and Ven-
kobachar were employed.
where Ci and Ct are, respectively, the concentration of
dye (in ppm) at time zero and at time t. The qe and qt are
the amount of dye adsorbed per unit mass of the adsorbent
(in mg g−1), in turn, at equilibrium time and at time t, and
qmax is the maximum adsorption capacity (in mg g−1); k
and kad are the adsorption rate constants (in min−1) and
U(t) = [(Ci - Ct)] / (Ci – Ce)]; Ce = concentration of dye
(in ppm) at equilibrium (optimum) contact time. The lin-
ear graphical plots between the values of (i) 1/qt and 1/t,
(ii) log (Ci / Ct) and time, (iii) log (qe-qt) and time, and
(iv) log [1-U(t)] and time indicate the applicability of the
above kinetic equations, and the first-order nature of the
adsorption process.
The intra-particle diffusion process is often the rate-
limiting step in many adsorption processes. The equation
gives the intra-particle diffusion model:
where qt is the amount of dye adsorbed (in mgg−1) at time
t; and c and kp are the intercept and intra-particle diffusion
rate constant (unit: mg g−1 min0.5), respectively. The value
of intercept (c) gives an idea about boundary layer thickness,
(5)
RL
=1∕
(
1+bC
i)
(6)
First − order equation ∶ (
1∕q
t)
=
(
k∕q
max)
(1∕t)+
(
1∕q
max)
(7)
Natarajan and khalaf equation ∶
log
(
C
i
∕C
t)
=(k∕2.303)
t
(8)
Lagergen equation ∶
log
(
q
e
−q
t)
=log q
e
−
(
k
ad
∕2.303
)t
(9)
Bhatt acharya and Venkobachar ∶
log [1−U(t)]=−
(
k
ad
∕2.303
)t
(10)
qt
=k
p
t
1∕2
+
c

9479Biomass Conversion and Biorefinery (2023) 13:9475–9485
1 3
i.e., the larger the intercept, the greater the boundary layer
effect. Further confirmation of the presence of intra-parti-
cle diffusion could be obtained from the linear relationship
between the values of the log (% removal) and log (time).
where c = intercept and m = slope.
3 Results anddiscussion
3.1 Surface characterization analyses withFTIR
andSEM
For the production of activated charcoal, the raw material
selection is based on carbon content, sulfur content, availa-
bility, shelf life, and cost [4]. Depending on the raw material,
it can be either vegetable, animal, or mineral. The process of
producing activated carbon can be done by chemical activa-
tion or physical activation [7]. The activation and carboniza-
tion stages are the most common steps in physical activa-
tion [8]. During the activation stage, oxidizing chemicals
like steam and carbon dioxide are utilized to create pores
and canals in the material. Chemical activation is when a
chemical activator, either dry or in solution, is mixed with
the raw material [11]. Chemical activators are capable of
decomposing the structure and creating micropores. This
study produced activated carbons using M. kauki L. bark
through carbonization by chemical and physical activation.
The surface characteristics were also done (Fig.2).
(11)
Log (% removal)=c+m log (d)
Figure2A depicts the FTIR spectral results of MKBC.
The broad band at 3430cm−1 corresponds to chemisorbed
water molecules’ stretching and bending modes (-OH
stretching) over the MKBC surface. The peak at 2928cm−1
corresponded to asymmetrical C–H stretching vibrations of
the methyl (–CH3–) and methylene (–CH2–) groups. The
band at 1626cm−1 corresponds to carbonyl (--C=O and
-C-O) stretching vibration of phenolic ester, carboxylic
acid, and conjugated ketonic structures for activated carbon.
The peaks at 1361cm−1 indicated - C=C- groups present
in carbon and the region between 600 and 900 cm−1 con-
tains various bands related to aromatic, out-of-plane C-H
bending with different degrees of substitution. The FTIR
spectra of MKBC indicated the presence of various surface
functional groups like -C=O, -OH, -COOH, -C=C-, and
-S=O, and these surface functional groups are responsible
for the adsorption of GR dye. In Fig.2B, SEM image of
MKBC revealed carbon’s surface texture and porosity. The
MKBC surface was covered with large pores that formed a
honeycomb-shaped pattern with a large surface area due to
its well-developed pores. The pores of the adsorbents sur-
round or engulf the adsorbed dye molecules [7, 12].
3.2 Effect ofinitial CR dye concentration
andadsorption isotherm studies
The adsorption potential of activated porous carbon formed
from biomass is determined by the surface porosity and the
chemical reactivity of the functional groups on the surface
[13], which is responsible for the adsorption of dye mol-
ecules over their surface [8]. The percentage of CR dye
4000 3500 3000 2500 2000 1500 1000 500
80
90
100
536.11
3
1052.943
1364.3904
1631.483
3430.7427
536.11
3
1052.943
1364.3904
1631.483
3430.7427
-C-C-
-S=O
-CH
-C=O
-C=O
-CH
-OH
T%
Wavenumber (cm
-1
)
4000 3500 3000 2500 2000 1500 1000 500
80
90
100
-C-C-
-S=O
-CH
-C=O
-C=O
-CH
-OH
T%
A
A
Fig. 2 FTIR (A) and SEM (B) of MKBC

9480 Biomass Conversion and Biorefinery (2023) 13:9475–9485
1 3
removal by CAC or MKBC is affected by the initial concen-
trations of the dye, as shown in Fig.3. Therefore, adsorption
studies of CR dye on CAC or MKBC were performed at a
fixed dosage of adsorbent (4gl−1 CAC or MKBC) at various
initial dye concentrations (range: 100 to 200 ppm in CAC
and 15–24 ppm in MKBC) and for a contact time of 30 min.
MKBC and CAC, pH (7.1), fixed particle size (90 microns
for CAC and MKBC), and temperature 30±1°C.
The range of removal percentage of CR dye observed is
98.00–97.50 for CAC and 80.00–67.83 for MKBC. How-
ever, the percentage of CR dye removed decreases exponen-
tially when the concentration of CR is increased. This could
be because there are fewer functional sites available on the
surface of the adsorbent than the large number required to
adsorb a high initial concentration of CR dye [14–16].
The investigation of adsorption thermometry has been
significant in wastewater treatment because it gives an esti-
mate of the adsorption capability of the adsorbents, therefore
equilibrium data for removing CR dye by adsorption of a
CAC and MKBC at 30±1°C. The Freundlich and Langmuir
isotherms were both fitted using the information in Table2.
where the Freundlich constants K and 1/n repre-
sent, respectively, the adsorption capacity and intensity
measurements.
At equilibrium, qe is equal to (x/m), or the quantity of dye
adsorbed per mass of the adsorbent (in x=(Ci-Ce), Ci and Ce
are the initial and final equilibrium dye concentrations, in ppm)
and m=mass of adsorbent respectively, in gl−1. Qo is monolayer
Freundlich isotherm = log (x∕m)=log K+1∕nlog Ce
Langmuir isot her m = Ce∕qe=1∕Qob+Ce∕Qo
adsorption capability in (mgg−1), and b is the Langmuir con-
stant relating to the energy for adsorption (in Lmg−1).
The Freundlich and Langmuir isotherms were applied in
order to find a fitting solution for the data obtained from the
adsorption tests in which the initial concentration was altered
in Fig.4. These isotherm plots were determined to be linear,
which was demonstrated by values that were quite close to
unity. This demonstrates that these adsorption isotherms may
be used to remove CR dye from CAC and MKBC [14–16].
Furthermore, Langmuir isotherm’s essential character-
istics could be illustrated using the separation factor RL—
which was defined by Weber, Chakravarthy etal. [14].
Ci refers to the Congo red dye’s optimum initial concen-
tration (ppm), while b is the Langmuir constant, Lmg−1.
The separation factor, RL, provides insight into both the
contours of the isotherm and also dynamics of the adsorp-
tion process and in this experimental study, the RL values
were computed and listed as RL> 1 (unfavorable), RL = 1
(linear), 0 < RL< 1 (favorable), and RL = 0 (irreversible).
The results of the correction analysis, viz., correlation coef-
ficient, Freundlich and Langmuir constants, and adsorption
capacities (Q, (1/n), log K and R), are presented in Table2.
In addition, Table2 shows that the statistical analysis of
adsorption data shows that both the Freundlich and Lang-
muir isotherms can be applied and that correlations are sta-
tistically significant. Furthermore, the values of R and L are
fractional, in the range 0 to 1, respectively (0.068 for CAC,
0.108 for MKBC), which indicates that the adsorption pro-
cess has been favorable. Finally, the following equation is
RL
=1∕
(
1+bC
i)
10 12 14 16 18 20 22 24 100 120 140 160 180 200 220
65
70
75
80
97.0
97.5
98.0
)%(lavomerruoloC
CAC
MKBC
Initial CR dye Concentration (ppm)
Fig. 3 Effect of initial concentration on the adsorption of CR dye
onto MKBC and CAC
Table 2 Characteristics of the Freundlich isotherm and Langmuir isotherm
Correlation analysis CAC MKBC
Freundlich isotherm
Correlation coefficient 0.998 0.983
Slope 0.736 0.293
Intercept 1.170 0.337
K14.81 2.173
Δq (%) 0.004 0.012
between the values of the log (%
removal
Correlation coefficient 0.985 0.994
Slope 0.007 0.199
Intercept 0.070 0.428
Qo148.08 35.029
B0.097 0.464
RL0.068 0.108
Δq (%) 0.009 0.027

9481Biomass Conversion and Biorefinery (2023) 13:9475–9485
1 3
used to calculate a normalized standard deviation (q (%)) in
order to compare each model (isotherm) better.
where the experimental value can be found in the super-
scripts-exp, while the calculated value can be found in the
superscripts and superscripts-exp for qt viz. The amount of
dye adsorbed at various times t, where t is the many obser-
vations. Low values of ∆q (%) suggest that Freundlich’s
adsorption isotherm is the most excellent way to explain dye
adsorption. The large particles absorb less because they have
a smaller surface area than the volume. Small particles have
more surface area and are more susceptible to adsorption.
3.3 Effect ofcontact time andkinetic studies
The vast surface area that was accessible to absorb the dye
contributed to the high clearance rates that were seen at
the beginning of the contact time. The removal of CR dye
by adsorption was found to be rapid at the initial period of
contact time and then become slow and stagnant with the
increase in contact time. The boundary layer resistance will
be affected by the rate of adsorption and an increase in con-
tact time will reduce the resistance and increase the adsorp-
tion’s mobility in the adsorption system [17]. Similar find-
ings have been reported in the scientific literature in the past
on the topic of dye removal. Also, the contact time is a major
influence on the adsorption process. A good contact time
allows the adsorbent to be used to treat water contamination.
Therefore, contact time is crucial in an adsorption system.
This is independent of any other experimental parameters.
𝛥
q(%)=100 ×

qt
exp.–qt
cal.

∕qt
exp.

2

∕(n−1)
1∕2
The study of adsorption kinetics is crucial because it can
determine the rate and mechanism of adsorption. Batch-type
studies were carried out to investigate the kinetics of CR
dye adsorption [18]. There was a range in the contact time
(5–40 min for CAC, and 5–30 min for MKBC) and initial
concentration of CR dye (4gl−1 of MKBC and 140 ppm
respectively), in addition to the dose of adsorbent (4gl−1) of
both CAC) at 30 °C (Fig.5).
The numbers for the percentage of elimination and the
amount adsorbed increase exponentially with increasing
contact time. After a short contact time, the percentage of
CR dye removed quickly. However, it became slower and
more stagnant with increasing contact time. Adsorption of
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.4
0.6
1.4
1.6
1.8
(A)
CAC
MKBC
q(gol
e
)
log(C
e
)
1.0 1.52.0 2.53.0 3.54.0 4.55.0 5.56.0 6.57.0 7.58.0
0.0
0.1
1.0
1.2
1.4
1.6
1.8
2.0
(B)
CAC
MKBC
Ce/qe
C
e
Fig. 4 Freundlich (A) and Langmuir (B) isotherm plots for CR dye adsorption onto MKBC and CAC
0510 15 20 25 30 35 40 45
50
55
60
65
70
75
95
100
)%(lavomeregatnecreP
Contact time (min.)
CAC
MKBC
Fig. 5 Effect of contact time on the adsorption of CR dye onto
MKBC and CAC

9482 Biomass Conversion and Biorefinery (2023) 13:9475–9485
1 3
CAC and MKBC to remove dye from aqueous solutions can
be broken down into the following four steps in order to
figure out how the process works: i. The dye moves from the
bulk solution to the surface of the adsorbent while it is being
adsorbed. ii. Diffusion of the dye outward from the absor-
bent via the boundary layer and onto the absorbent surface.
iii. The dye is absorbed in active spots on the surface of the
uppermost layer of material. iv. Intra-particle diffusion [19].
The adsorbent particles take the dye from their outermost
surface into their innermost pores.
Both the rate of adsorption and an increase in the length
of time that the two are in touch with one another will affect
the boundary layer resistance. Furthermore, increasing the
time that the two are in contact with one another will have an
even greater effect. This reduces the adsorbate’s resistance
during the adsorption process while increasing its mobil-
ity. Because CR dye is rapidly absorbed in active sites, the
adsorption rate is determined by either the liquid phase mass
transfer or the intra-particle mass transport rate [20, 21].
3.4 Adsorption kinetic studies
An adsorption curve is a curve that explains the phenom-
enon of chemical retention or release (or mobility) in aque-
ous porous media, aquatic habitats, or solid phases under
constant temperature and pH. This type of curve is also
known as an adsorption isotherm. Activated carbon biomass
was employed in this study to investigate whether or not
adsorption can be utilized to remove CR dye from aqueous
solutions. The design of an efficient wastewater treatment
system is dependent on the effectiveness of adsorption iso-
therms. Langmuir’s and Freundlich’s adsorption isotherms
were used to assess adsorption’s feasibility for removing CR
dye from aqueous solutions [22]. Langmuir’s adsorption
isotherm assumes a uniform, homogeneous, and identical
surface on which adsorptions occur. It is used to describe
the monolayer processes of adsorption.
The batch-type adsorption experiments of CR dye were
conducted to verify the kinetics of the dye’s adsorption
[23]. They included adjusting the contact period, deter-
mining the optimal initial concentration of the dye, and
maintaining a constant dosage of adsorbent (4 g per liter
of CAC and 30 °C for MKBC). There is a correlation
between the resistance to boundary layers and the adsorp-
tion rate. Therefore, a decrease in contact time will reduce
the mobility of adsorbate [CR] within the adsorption pro-
cedure. We examined the application of this model to sev-
eral different first-order equations, including Lagergren,
Natarajan-Khalaf, and Bhattacharya-Venkobachar, in order
to determine the order and nature of adsorption kinetics
in this investigation [22, 23]. Table3 contains the con-
stant rate values calculated from the rate equations. Fig-
ure6 depicts the linear kinetic plots that were generated,
the minimum and maximum first-order rate constants (k
min−1). These are for the CR-CAC (8.82–8.93 min−1) and
the CR-MKBC (1.94–11.36 min−1) systems. This means
that MKBC removes the most CR dye while CAC removes
it at its maximum.
There is also the potential for intra-particle diffusion,
whereby adsorbent molecules are able to diffuse from the
outer surface into the inner pores of the adsorbent material.
This happens in the opposite direction to the adsorption at the
outer surface. In the diffusion-controlled adsorption process,
the amount of solute adsorbed varies [24]. Intra-particle dif-
fusion model/equation is of the form. A linear relationship
existed when (x/m) was plotted versus t1/2 data. This indicates
that intra-particle diffusion is the important rate-limiting step
of adsorption process in experimental conditions. The intra-
particle diffusion rate constants are the minimum and maxi-
mum values (Kp, mgg−1min−0.5) that are noted for CR-CAC
and MKBC as 0.037and 0.183, respectively.
where Kp is the intra-particle diffusion coefficient (mg g−1
min−0.5), and C is the constant (intercept). For CR-MKBC
(33.310) and CR-CAC (8.90), respectively, the minimum and
maximum intercept (C) values for intra-particle diffusion
plots have been noted. For CAC, the correlation between
log (% removal) and log (time), respectively, is 0.993; for
MKBC, it is 0.958. The plots for the corresponding plots
are linear [14–16].
(
x∕m)=K
p
t
1∕2
+
C
Table 3 Kinetic models with CAC and MKBC
Kinetic models CAC MKBC
Natarajan-Khalaf
Correlation coefficient (r) 0.936 0.971
102 k (min−1) 8.820 1.904
Δq (%) 72.01 57.15
Lagergren equation
Correlation coefficient (r) 0.984 0.940
k (min−1) 8.930 11.36
Δq (%) 8.804 7.468
Bhattacharya and Venkobachar
Correlation coefficient (r) 0.984 0.940
k (min−1) 8.930 11.36
Δq (%) 11.53 7.881
Intra-particle diffusion model
Correlation coefficient (r) 0.967 0.980
Kp (mgg−1 min−0.5) 0.037 0.183
Δq (%) 33.28 47.12
Intercept 0.890 33.10
Log(%R) vs Log (time)
Correlation coefficient 0.993 0.958
Δq (%) 56.15 112.88

9483Biomass Conversion and Biorefinery (2023) 13:9475–9485
1 3
3.5 Effect ofinitial pH ofdye solution
This study examines the impact of an operating parameter,
the initial pH on water performance. It is essential to deter-
mine the adsorption rate by determining the adsorbate’s ini-
tial concentration. This makes it an important consideration
for successful adsorption. CR dye was chosen as the model
dye in this study and its degradation can produce toxic com-
pounds. The adsorption rate of CR dye on CAC and MKBC
was determined at various initial pH values [25]. The pH has
a key role in affecting the dyes adsorption process because of
the dissociation of functional groups on the active sorption
sites, the degree of ionization of the material in solution,
and could also change the surface charge of the employed
adsorbents[26, 27]. The zero point charge (pHzpc) of CAC
is 7.25 (reported by Emerck, India) and the zero point charge
of MKBC is 6.2 (determined by standard methods). In an
acidic medium (pH > 6.2), the surface of CAC is positively
charged, whereas it is negatively charged under an alkaline
medium (pH < 6.2). The adsorbent surface is positively
charged (pHzpc < pH), resulting in the increased adsorption
of the anionic dyes and the reverse is observed when pHzpc
> pH. Congo red is negatively charged due to the sulfonated
groups, which are ionized in water; their electrostatic attrac-
tion to the surface of the adsorbent is favorable in acidic
solution and forbidden in alkaline media due to the columbic
repulsion between the negatively charged surface of CAC
and MKBC and the negatively charged dye molecules.
The percentage removal of dye linearly increases with the
decrease in initial pH for the adsorption of CR dye for both
CAC and MKBC adsorbents (Fig.7), which indicates that
the acidic pH is found to be more suitable for the removal
5101520253035
0.4
0.6
1.4
1.6
(A)
C(gol i/Ct)
Contact time (min.)
CAC
MKBC
5101520253035
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
(B)
q(gol+2
e
-q
t
)
Contact time (min.)
CAC
MKBC
51015202530
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
(C)
CAC
MKBC
Contact time (min.)
])T(U-1[gol+2
2.02.5 3.03.5 4.04.5 5.05.5
0.0
0.5
1.0
1.5
2.0
33.0
33.2
33.4
33.6
33.8
34.0
34.2
34.4
34.6
34.8
35.0
(D)
CAC
MKBC
)L/gm(debrosdAtnuomA
T
1/2
Fig. 6 Natarajan-Khalaf (A), Lagergren (B), Bhattacharya-Venkobachar (C), and intra-particle diffusion (D) kinetic plots for CR dye adsorption
onto MKBC and CAC

9484 Biomass Conversion and Biorefinery (2023) 13:9475–9485
1 3
of CR dye. This study showed that the solution pH strongly
influences the degradation rate of CR. Similar results were
reported for the adsorption of alizarin cyanin green dye from
aqueous solutions onto activated palm ash [28] and also the
adsorption of acid yellow 36 on activated carbons prepared
from sawdust and rice husk [29].
Only if comprehensive mechanistic investigations were
conducted out would it be possible to explain how the effect
of pH variation on the amount of dye removed changed over
time [14–16]. The research study has shown M. Kauki–based
activated porous carbon postured as an excellent possible
material for wastewater removal of Congo red dye.
4 Conclusion
The first time we prepared and utilized the activated carbon
from Manilkara Kauki L. towards the removal of CR dye on
CAC and MKBC by batch adsorption technique. FTIR and
SEM studies confirm that the MKBC possessed excellent
structural and morphological features. Adsorption studies
indicated that the percentage removal of CR dye on these
adsorbents (CAC and MKBC) is found to decrease with an
increase in initial concentration and initial pH of dye solu-
tion but increases exponentially with an increase in contact
time. The Langmuir and Freundlich isotherm models were
tested and found to be applicable. Modeling adsorption data
is done using a variety of first-order kinetic equations such
as Lagergren, Bhattacharya, Natarajan-Khalaf, and Ven-
kobachar equations. The intra-particle diffusion model is
valid. This means that the CR dye adsorption onto CAC
and MKBC can be first ordered, with intra-particle diffusion
being one of the rate-determining factors. The study revealed
that the CR dye adsorption on CAC and MKBC can be used
to treat effluents containing the dye in neutral or slightly
acidic mediums. MKBC is more economical even though it
has a slightly lower adsorption rate than CAC.
Acknowledgements The authors thankfully acknowledged the Depart-
ment of Chemistry, Rani Anna Government College for Women and
Department of Chemistry, Lekshmipuram College of Arts and Science,
Affiliated to Manonmaniam Sundaranar University, Tirunelveli, India,
for the research facilities to accomplish this experimental study.
Author contribution Rajeswaran Ramaraj: conceptualization, method-
ology, data analysis, manuscript writing, and editing. Banumathi Naga-
rathinam: methodology, proofreading, editing. Tanabe Shuji: resources
and methodology. Muthirulan Pandi: funding acquisition, resources,
supervision, proofreading, editing.
Availability of data and materials Not applicable.
Declarations
Ethical approval Not applicable.
Conflict of interest The authors declare no competing interests.
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