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Adsorptive Removal of CD (II) from Aqueous Solution using
Activated Carbon Derived from Mango Seed Coats
Jimoh Olabisi Faidah
1
, Animasahun Tobi Seun
2*
, Ali Abdallah Kolawole
3
1
Department of Chemistry, University of Ilorin, Ilorin, Kwara State, Nigeria
2
Department of Electrical/Electronics Engineering Technology, Federal Polytechnic Ayede, Oyo State,
Nigeria
3
Department of Electrical/Electronics Engineering, Kwara State University , Malete, Kwara State,
Nigeria
*
Corresponding Author
DOI:
https://doi.org/10.51583/IJLTEMAS.2026.150300103
Received: 27 March 2026; March: 02 April 2026; Published: 22 April 2026
ABSTRACT
Heavy metal contamination of water systems remains a critical environmental challenge due to its persistence,
toxicity and bio-accumulative nature. This study investigates the potential of activated carbon derived from
mango (Mangifera indica) seed coats as a low cost and sustainable adsorbent for the removal of cadmium
(Cd
2+
) ions from aqueous solutions. The adsorbent was prepared via carbonization followed by chemical
activation and characterized using Fourier Transform Infrared Spectroscopy (FTIR), which revealed the
presence of functional groups such as hydroxyl and carbonyl responsible for metal binding.
Batch adsorption experiments were conducted to evaluate the effects of operational parameters, including initial
metal ion concentration (10 to 50 mg/L) and adsorbent dosage (0.1 to 0.5 g) at an optimized pH of 5. The
results indicated that adsorption efficiency decreased with increasing Cd
2+
concentration, reaching a maximum
removal efficiency of about 81.00 % at 10 mg/L. Furthermore, increasing adsorbent dosage beyond 0.1 g
resulted in only marginal improvements due to possible site overlap and particle aggregation, identifying 0.1 g
as the optimal dosage.
Equilibrium data were best described by the Freundlich isotherm model, indicating heterogeneous surface
adsorption, while kinetic studies showed that the adsorption process followed pseudo second order kinetics,
suggesting chemisorption as the dominant mechanism. The adsorption process was governed by electrostatic
interactions, ion exchange, and surface complexation.
The study demonstrates that mango seed coat derived activated carbon is an effective, eco-friendly and
economically viable alternative for Cd
2+
removal. This work highlights the dual benefits of agricultural waste
valorization and sustainable water treatment, making it particularly relevant for resource limited environments.
Keywords: Activated carbon, Mango seed coat, Cadmium removal, Adsorption, Heavy metals, Wastewater
treatment, Low cost adsorbent, Biomass valorization.
INTRODUCTION
Water contamination by heavy metals has emerged as a critical global environmental issue, driven largely by
rapid industrialization, urban expansion and inadequate waste management practices. Among these
contaminants, cadmium (Cd(II)) is of particular concern due to its high toxicity, persistence and tendency to
bio-accumulate in living organisms. Even at trace concentrations, cadmium poses severe health risks, including
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renal dysfunction, skeletal damage and carcinogenic effects, thereby necessitating its effective removal from
contaminated water systems.
Conventional treatment technologies such as chemical precipitation, ion exchange, membrane filtration and
reverse osmosis have been widely employed for heavy metal removal. However, these methods are often
associated with significant drawbacks, including high operational costs, generation of secondary pollutants and
reduced efficiency at low metal concentrations. In this context, adsorption has gained considerable attention as
a preferred alternative due to its simplicity, cost effectiveness, high efficiency and operational flexibility.
Activated carbon remains one of the most effective adsorbents owing to its large surface area, well developed
pore structure and diverse surface functional groups. Despite its excellent performance, the widespread
application of commercial activated carbon is limited by its high production cost and dependence on
nonrenewable precursors. This limitation has driven increasing research interest toward the development of
low cost, sustainable adsorbents derived from agricultural wastes.
Agricultural by-products such as coconut shells, rice husks and fruit seeds have shown significant promise as
precursors for activated carbon production due to their high carbon content and inherent porosity. Among these,
mango (Mangifera indica) seed coats represent an abundant and underutilized biomass resource, particularly
in tropical regions. Rich in lignocellulosic components, mango seed coats can be effectively converted into
activated carbon with enhanced adsorption properties through appropriate activation processes.
Recent studies have demonstrated that biomass-derived activated carbons exhibit comparable or even superior
adsorption performance to commercial alternatives, particularly for the removal of heavy metals. However,
further investigation is required to optimize their preparation and evaluate their efficiency under varying
operational conditions.
Against this backdrop, the present study explores the adsorption potential of activated carbon prepared from
mango seed coats for the removal of Cd(II) ions from aqueous solutions. By integrating waste valorization with
environmental remediation, this work contributes to the development of sustainable and economically viable
water treatment technologies.
Aim of the Study
The aim of this study is to develop and evaluate activated carbon derived from mango (Mangifera indica) seed
coats as a low-cost and efficient adsorbent for the removal of Cd(II) ions from aqueous solutions.
Objectives of the Study
The specific objectives of this research are to:
1. Prepare activated carbon from mango seed coats using appropriate carbonization and activation
methods.
2. Characterize the prepared activated carbon in terms of its surface functional groups and adsorption
properties.
3. Investigate the effect of key operational parameters, including initial metal ion concentration and
adsorbent dosage, on the adsorption of Cd(II).
4. Evaluate the adsorption efficiency of the prepared adsorbent using batch experimental techniques.
5. Assess the potential of mango seed coat-derived activated carbon as a cost-effective and sustainable
alternative for heavy metal removal in water treatment.
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LITERATURE REVIEW
Heavy metal contamination of aquatic systems has become a major global environmental challenge due to its
persistence, toxicity and non-biodegradable nature. Among these pollutants, cadmium (Cd(II)) is particularly
hazardous because of its high solubility, mobility and strong tendency to bio-accumulate in living organisms.
Industrial activities such as electroplating, battery manufacturing, mining and fertilizer production have
significantly contributed to the release of cadmium into water bodies. Exposure to Cd(II), even at trace levels,
is associated with severe health effects, including kidney dysfunction, skeletal damage and carcinogenic risks,
thereby necessitating the development of effective and sustainable remediation strategies.
Various conventional methods, including chemical precipitation, ion exchange, membrane filtration and reverse
osmosis, have been employed for the removal of heavy metals from wastewater. However, these techniques
are often limited by high operational costs, energy requirements, and the generation of secondary pollutants
such as sludge. These limitations have led to increased research interest in adsorption as a more efficient and
economically viable alternative. Adsorption is widely recognized for its simplicity, high efficiency, and
flexibility in operation, particularly for the removal of trace-level contaminants. The process involves the
accumulation of metal ions onto the surface of an adsorbent through physical and chemical interactions, which
may include electrostatic attraction, ion exchange, and surface complexation.
The effectiveness of adsorption systems is typically evaluated using equilibrium isotherm and kinetic models.
The Langmuir isotherm assumes monolayer adsorption on a homogeneous surface with finite adsorption sites,
whereas the Freundlich isotherm describes multilayer adsorption on heterogeneous surfaces. In many cases
involving biomass-derived adsorbents, the Freundlich model provides a better representation due to the
inherent surface heterogeneity of such materials. Kinetic studies often indicate that the adsorption of heavy
metals follows a pseudo-second-order model, suggesting that chemisorption involving valence forces and
electron sharing plays a dominant role in the adsorption mechanism.
Activated carbon is widely regarded as one of the most effective adsorbents due to its large surface area,
welldeveloped pore structure, and abundance of functional groups capable of binding metal ions. These
characteristics enable activated carbon to remove a wide range of pollutants through mechanisms such as pore
filling, ion exchange, and chemical interaction with surface functional groups. Despite its effectiveness, the
application of commercial activated carbon is constrained by its high production cost and reliance on
nonrenewable raw materials such as coal and petroleum-based precursors. Consequently, there has been a
growing interest in developing low-cost and sustainable alternatives derived from agricultural wastes.
Agricultural by-products such as coconut shells, rice husks, banana peels, and fruit seeds have been extensively
investigated as potential precursors for activated carbon production. These materials are rich in lignocellulosic
components, including cellulose, hemicellulose, and lignin, which facilitate the formation of porous carbon
structures upon carbonization and activation. Among these, mango (Mangifera indica) seed coats have emerged
as a promising and underutilized biomass resource, particularly in tropical regions where mango consumption
generates large quantities of waste. The high carbon content and inherent porosity of mango seed coats make
them suitable for the production of activated carbon with enhanced adsorption properties.
Several studies have demonstrated the effectiveness of mango seed-derived adsorbents in the removal of heavy
metals from aqueous solutions. The adsorption performance is largely attributed to the presence of surface
functional groups such as hydroxyl, carboxyl, and carbonyl groups, which facilitate metal binding through
complexation and ion exchange mechanisms. In addition, the porous structure of the activated carbon enhances
diffusion and accessibility of metal ions to active adsorption sites. Experimental studies have reported high
adsorption capacities for Cd(II) and other heavy metals, confirming the strong affinity between mango
seedbased adsorbents and metal ions. The adsorption process is also influenced by various parameters,
including pH, contact time, adsorbent dosage, and initial metal concentration, with optimal performance
typically observed at moderately acidic to neutral pH conditions.
The method of activation plays a crucial role in determining the physicochemical properties and adsorption
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performance of activated carbon. Physical activation involves high-temperature treatment with oxidizing gases
such as steam or carbon dioxide, while chemical activation employs activating agents such as potassium
hydroxide, phosphoric acid, or zinc chloride. Chemical activation is generally preferred for biomass-derived
materials due to its ability to produce higher surface area, improved pore structure, and enhanced surface
functionality at relatively lower temperatures. Studies have shown that chemically activated mango seed carbon
exhibits superior adsorption performance compared to physically activated counterparts, primarily due to the
increased availability of active sites and functional groups.
The adsorption of Cd(II) onto biomass-derived activated carbon is governed by multiple mechanisms, including
electrostatic attraction between positively charged metal ions and negatively charged adsorbent surfaces, ion
exchange processes, and chemical complexation with functional groups. In many systems, these mechanisms
occur simultaneously, contributing to the overall adsorption efficiency. Thermodynamic studies often indicate
that the adsorption process is spontaneous and, in some cases, endothermic, suggesting improved adsorption
capacity at elevated temperatures.
Despite the promising potential of mango seed-derived activated carbon, several challenges and research gaps
remain. Most existing studies have been conducted under controlled laboratory conditions using single-metal
systems, which do not fully represent the complexity of real wastewater containing multiple competing ions.
Additionally, there is limited research on large-scale applications, continuous flow systems, and long-term
adsorbent stability and regeneration. Variations in raw material composition and activation conditions also pose
challenges in achieving consistent performance.
Future research should focus on evaluating the performance of mango seed-based adsorbents in
multicomponent systems, optimizing regeneration and reuse processes, and scaling up production for industrial
applications. Furthermore, economic and life-cycle assessments are necessary to establish the feasibility and
sustainability of these materials in practical water treatment systems.
MATERIALS AND METHODS
Materials
All chemicals used in this study were of analytical grade and were used without further purification. Distilled
water was used throughout the experiment for solution preparation and dilution.
The primary adsorbate used was cadmium in the form of cadmium nitrate (Cd(NO₃)₂), which served as the
source of Cd(II) ions. Other reagents included hydrochloric acid (HCl) and sodium hydroxide (NaOH), which
were used for pH adjustment during the adsorption experiments.
Mango seed coats were collected locally and used as the raw material for the preparation of activated carbon.
Preparation of Activated Carbon
The mango seed coats were first washed with distilled water to remove dirt and soluble impurities, followed
by drying at room temperature and further oven-drying at 105 °C to eliminate moisture content. The dried
material was then crushed and sieved to obtain a uniform particle size.
Carbonization was carried out by heating the prepared biomass in a muffle furnace under limited oxygen
conditions at an elevated temperature (typically between 400 °C–600 °C) for a specified duration. The resulting
char was allowed to cool to room temperature.
Chemical activation was subsequently performed using a suitable activating agent (Orthophosphoric acid),
H
3
PO
4
, to enhance porosity and surface functionality. The impregnated sample was heated at 600 °C for 90
minutes, after which it was washed repeatedly with distilled water to remove residual chemicals until a neutral
pH was achieved. The final product was dried, ground, and stored in airtight containers for subsequent
characterization and adsorption experiments.
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Figure 1: Freshly Washed Mango Seed Coats
Figure 2: Dried Mango Seed Coats Characterization of the Adsorbent
The surface functional groups present on the prepared activated carbon were analyzed using Fourier Transform
Infrared Spectroscopy (FTIR). This technique was employed to identify key functional groups responsible for
metal ion binding.
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Preparation of CD(II) Solutions
Stock solution of Cd(II) ions was prepared by dissolving accurate quantity of cadmium nitrate (Cd(NO
3
)
2
) in
distilled water to obtain a concentration of 1000 mg/L for the metal ion. The stock solution serves as the primary
source for preparing working solutions of desired concentrations. From the stock, working solutions ranging
between 10 mg/L and 50 mg/L were prepared by serial dilution using distilled water. The pH of each solution
was adjusted using either 0.1 M HCl or 0.1 M NaOH, depending on the experimental requirement. The prepared
metal ion solutions were stored in polyethylene bottles and labeled accordingly to prevent contamination. All
solutions were freshly prepared and used within 24 hours to avoid hydrolysis or oxidation of metal ions.
Batch Adsorption Experiments
Batch adsorption studies were conducted to evaluate the removal efficiency of Cd(II) ions using the prepared
activated carbon. A known mass of adsorbent (0.1–0.5 g) was added to a series of conical flasks containing a
fixed volume of Cd(II) solution of known concentration.
The pH of the solution was maintained at approximately 5, as this was found to be optimal for cadmium
adsorption. The mixtures were agitated at a constant speed of 120 rpm using a mechanical shaker for a contact
time of 2 hours to ensure equilibrium was reached.
After adsorption, the mixtures were filtered, and the residual concentration of Cd(II) in the filtrate was
determined.
Determination of Residual Metal Concentration
The concentration of cadmium ions before and after adsorption was analyzed using Atomic Absorption
Spectroscopy (AAS). This technique provided accurate and reliable quantification of metal ion concentrations
in solution.
The percentage removal of Cd(II) was calculated using the following expression:
where:
C
o
= initial concentration of Cd(II) (mg/L)
C
e
= equilibrium concentration of Cd(II) (mg/L)
The adsorption capacity at equilibrium, q
e
(mg/g), was calculated as:
where:
C
o
= initial concentration (mg/L)
C
e
= equilibrium concentration (mg/L) V = volume of solution (L) m = mass of adsorbent (g)
Adsorption Isotherm Models
To evaluate the adsorption behavior, equilibrium data were analyzed using Langmuir and Freundlich isotherm
models.
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Langmuir Isotherm
The Langmuir model assumes monolayer adsorption on a homogeneous surface and is expressed as:
where:
q
e
= adsorption capacity at equilibrium (mg/g)
q
max
= maximum adsorption capacity (mg/g)
K
L
= Langmuir constant (L/mg)
C
e
= equilibrium concentration (mg/L)
The separation factor R
L
is given by:
Indicating the favorability of adsorption.
Freundlich Isotherm
The Freundlich model describes adsorption on heterogeneous surfaces and is expressed as:
where:
K
F
= adsorption capacity constant
n = adsorption intensity
If the value of n > 1, it indicates favorable adsorption.
Adsorption Kinetics
To investigate the adsorption mechanism, kinetic models were applied.
Pseudo-First-Order Model
Where:
q
t
= amount of metal ion adsorbed at time t (mg/g)
q
e
= amount adsorbed at equilibrium (mg/g)
k
1
= pseudo-first-order rate constant (min⁻¹)
t = contact time (min)
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Pseudo-Second-Order Model
where:
k
2
= pseudo-second-order rate constant (g/mg·min)
This model assumes chemisorption as the rate-limiting step.
Effect of Operational Parameters
The influence of key parameters on adsorption efficiency was investigated as follows:
1. Initial Metal Ion Concentration: Varied between 10–50 mg/L to study its effect on adsorption
capacity.
2. Adsorbent Dosage: Varied from 0.1–0.5 g to determine the optimal dosage for maximum removal
efficiency.
All experiments were conducted under controlled conditions, and results were analyzed to determine the
adsorption performance of the prepared activated carbon.
Model Evaluation
The correlation coefficient (R²) was used to evaluate the goodness of fit of the pseudo-first-order and pseudo-
second-order kinetic models. The model with the higher R² value was considered to provide a more accurate
representation of the adsorption kinetics of Cd(II) ions onto the prepared activated carbon, consistent with
approaches reported in similar studies (Zhang et al., 2020; Zakaria et al., 2023).
RESULTS AND DISCUSSION
Characterization of Activated Carbon
Figure 3: FTIR of Carbonized Sample
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Figure 4: FTIR of Activated Carbon
Fourier Transform Infrared Spectroscopy (FTIR) analysis was conducted to identify the surface functional
groups present on the prepared activated carbon. The FTIR spectrum of the carbonized and activated samples
revealed the presence of key functional groups responsible for metal ion adsorption. Broad absorption bands
observed around 3200–3600 cm⁻¹ were attributed to hydroxyl (–OH) stretching vibrations, indicating the
presence of alcohol and phenolic groups. Peaks observed near 1700 cm⁻¹ correspond to carbonyl (C=O) groups,
while those around 1000–1200 cm⁻¹ are associated with C–O stretching vibrations.
The presence of these oxygen-containing functional groups confirms that the activated carbon surface possesses
active binding sites capable of interacting with Cd(II) ions through mechanisms such as complexation and ion
exchange. Chemical activation enhanced the intensity and availability of these functional groups, thereby
improving the adsorption potential of the material.
AAS Analysis
The atomic absorption spectroscopy (AAS) results for cadmium (Cd
2+
) concentrations after treatment with
activated carbon under varying conditions are presented in Table 1 and 2.
Two experimental sets were studied:
Set A1- A5 : Cd
2+
concentrations varied from 10 to 50 ppm with constant 0.5 g adsorbent.
Set B1- B5 : Adsorbent dosages varied from 0.1 to 0.5 g at a fixed Cd
2+
concentration of 50 ppm.
All experiments were conducted at constant pH 5.
Table 1: AAS Analysis Result of % Cadmium Removal Vs Initial Concentration
Initial Cd (ppm)
Final Cd (ppm)
% Removal
A1
10
1.900
81.00%
A2
20
7.500
62.50%
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A3
30
13.500
55.00%
A4
40
20.500
48.80%
A5
50
27.500
45.00%
Figure 5: Graph of % Cd Removal Vs Initial Concentration
As shown in Table 1 and the Figure 5, Cd
2+
removal decreased with increasing initial concentration, from 81.00
% at 10 ppm to 45.00 % at 50 ppm. This is due to the limited number of active sites on the activated mango
seed coat surface. At lower concentrations, the ratio of available functional groups—hydroxyl (–OH) and
carbonyl (C=O), as identified by FTIR—to metal ions is high, enabling efficient capture.
At higher concentrations, these groups become increasingly occupied, leading to surface saturation. This
behavior aligns with the Freundlich isotherm (R
2
= 0.9834), confirming adsorption on an energetically
heterogeneous surface. FTIR spectra support this, showing that Cd
2+
ions interact with multiple binding sites
of varying energy rather than a uniform monolayer.
Table 2: % Cadmium Removal Vs Adsorbent Dosage
Sample
Adsorbent Dosage
Final Cd (ppm)
% Removal
B1
0.1
9.304
81.39%
B2
0.2
9.447
81.11%
B3
0.3
9.526
80.95%
B4
0.4
9.070
81.86%
B5
0.5
9.489
81.02%
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Figure 6: Graph of % Cd Removal Vs Adsorbent Dosage
The data in Table 2 and Figure 6 indicate that increasing the adsorbent dosage from 0.1 g to 0.5 g did not
significantly alter the Cd
2+
removal efficiency, which remained stable at approximately 81 %. This suggests
that at the initial concentration of 50 ppm, a dosage of 0.1 g already provides an abundance of the hydroxyl (–
OH) and carbonyl (C=O) functional groups identified via FTIR to satisfy the available metal ions.
The lack of further removal at higher dosages implies that the system reached an equilibrium limit where the
metal ion concentration, rather than the number of active sites, became the limiting factor. Consequently, 0.1 g
is identified as the optimum dosage for high-efficiency removal, demonstrating the high affinity and economic
viability of the activated mango seed coat for cadmium remediation.
Adsorption Isotherm
At constant mass of 0.5 g of absorbent and volume of 0.1 L, the absorption capacity at equilibrium was
evaluated below.
Table 3: Absorption Capacity at Equilibrium
Value 1
Value 2
Value 3
20
7.50
2.500
30
13.50
3.300
40
20.50
3.900
50
27.50
4.500
Langmuir Isotherm Model
Table 4: Langmuir Isotherm Results
C
e
(mg/L)
q
e
(mg/g)
C
e
/q
e
(L/g)
1.900
1.620
1.173
7.500
2.500
3.000
13.500
3.300
4.091
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20.500
3.900
5.256
27.500
4.500
6.111
Plot of
Slope:
Intercept:
Figure 7: Langmuir Linear Plot
The Langmuir linear plot of
C e
versus C
e
yielded a strong correlation coefficient (R
2
= 0.9650), indicating
thatq
e
the adsorption of Cd
2+
follows a predictable monolayer pattern on the activated mango seed coat. From
the regression equation y = 0.182x + 1.347, the calculated maximum monolayer capacity (q
max
) is 5.49 mg/g
and the Langmuir affinity constant (K
L
) is 0.135 L/mg. These positive constants confirm that the adsorbent
possesses a finite number of specific, high-affinity binding sites, likely the hydroxyl and carbonyl groups
identified in the FTIR spectra. Furthermore, the dimensionless separation factor (R
L
) values for the studied
concentration range (10–50 ppm) were found to be between 0.12 and 0.42. Since these values fall within the
range of 0 < R
L
< 1, the adsorption process is mathematically confirmed to be highly favorable. While the
surface exhibits heterogeneity, the Langmuir model successfully characterizes the saturation limit of the
material.
Freundlich Isotherm Model
Table 5: Freundlich Isotherm Result
C
e
(mg/L)
q
e
(mg/g)
log C
e
log q
e
1.900
1.620
0.279
0.210
7.500
2.500
0.875
0.398
13.500
3.300
1.130
0.519
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20.500
3.900
1.312
0.591
27.500
4.500
1.439
0.653
Plot of log q
e
Vs log C
e
Slope:
Intercept: log K
F
Figure 8: Freundlich Linear Plot
The Freundlich linear plot of log q
e
versus log C
e
yielded a strong correlation coefficient of R
2
= 0.9834,
confirming that the adsorption of Cd
2+
occurs on an energetically heterogeneous surface. From the regression
equation y = 0.352x + 0.127, the calculated adsorption intensity (
) is 0.352.
Since
is between 0 and 1, the process is mathematically confirmed to be highly favorable, with a calculated
n value of 2.84. The intercept corresponds to a Freundlich capacity constant (K
F
) of 1.34 mg/g.
These results are in excellent agreement with the FTIR analysis, as the multiple functional groups (hydroxyl
and carbonyl) provide a variety of binding sites with different energy levels. This validates the multilayer
adsorption behavior typical of biomass-derived activated carbon.
Adsorption Kinetics
Understanding the kinetics of adsorption is crucial for designing treatment systems and optimizing contact
time. Kinetic studies help to elucidate the rate-controlling mechanisms of the adsorption process whether they
are governed by chemical reaction, surface diffusion, or pore diffusion (Abd-Talib et al., 2020). In this study,
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the experimental data were fitted to two widely used kinetic models: the pseudo first order and pseudo second
order models.
The results indicated that the pseudo-second-order model provided a superior fit, suggesting that chemisorption
is the dominant mechanism. This implies that Cd
2+
ions interact with the activated mango seed coat through
chemical bonding, likely involving electron sharing or exchange with the hydroxyl and carbonyl functional
groups identified via FTIR.
Table 6: Adsorption Kinetics Data
Time (t,min)
q
t
(mg/g)
5
2.650
10
3.240
30
3.980
60
4.310
120
4.500
Figure 9: Adsorption Kinetics Curve
The kinetic plot of adsorption capacity (qₜ) as a function of contact time (t) is presented above, illustrating the
uptake behavior of Cd²⁺ ions onto mango seed-derived activated carbon. The curve shows a rapid increase in
adsorption capacity at the initial stages, followed by a gradual approach to a constant equilibrium value (qₑ) of
4.500 mg/g.
At the initial stage (0–10 minutes), a steep rise in qₜ is observed, indicating a rapid adsorption rate. This can be
attributed to the abundance of available active sites on the adsorbent surface, particularly hydroxyl and carbonyl
functional groups, as well as a high concentration gradient that drives Cd²⁺ ions toward the surface.
As the contact time increases (10–60 minutes), the rate of adsorption begins to decline. This transition phase is
associated with the progressive occupation of active sites and the onset of diffusion limitations, as Cd²⁺ ions
penetrate into the internal pore structure of the activated carbon.
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At longer contact times (around 120 minutes), the curve reaches a plateau, indicating that equilibrium has been
attained. At this stage, the adsorption capacity stabilizes at 4.500 mg/g, suggesting that the available adsorption
sites are saturated and no significant further uptake occurs. This plateau represents the maximum adsorption
capacity under the given experimental conditions (initial concentration of 50 mg/L and adsorbent dosage of 0.5
g).
Overall, the kinetic profile demonstrates a typical adsorption behavior characterized by an initial rapid uptake
followed by a slower approach to equilibrium, confirming the efficiency of the adsorbent in removing Cd²⁺
ions from solution.
Pseudo-First-Order Model
Table 7: Pseudo First Order (m = 0.5 g, q
e
= 4.500 mg/g)
Time (t,min)
q
t
(mg/g)
(q
e
– q
t
)
log(q
e
– q
t
)
5
2.650
1.850
0.267
10
3.240
1.260
0.100
30
3.980
0.520
-0.284
60
4.310
0.190
-0.271
120
4.500
0.000
undefined
A plot of log(q
e
- q
t
) versus t allows the determination of k
1
and q
e
. However, studies have shown that this model
often underestimates q
e
and fits best only during the early stages of adsorption (Zhang et al., 2020).
The pseudo first order model, proposed by Lagergren, assumes that the rate of occupancy of adsorption sites is
proportional to the number of unoccupied sites. Its linear form is given by:
Slope:
Intercept:
Figure 10: Plot of Pseudo First Order Kinetic Model
The Pseudo First Order linear plot of log(q
e
− q
t
) versus time (t) yielded a correlation coefficient (R² = 0.9864),
providing an initial mathematical framework for the adsorption kinetics of Cd²⁺ on the activated mango seed
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coat. From the regression equation y = −0.0175x + 0.3001, the calculated adsorption rate constant (k
1
) is found
to be 0.0403 min⁻¹, and the theoretical equilibrium adsorption capacity (q
e
, calc) is 1.996 mg/g.
While the correlation coefficient indicates a relatively strong linear relationship, a significant discrepancy exists
between the experimental equilibrium capacity (q
e
, exp = 4.500 mg/g) and the calculated value (q
e
, calc = 1.996
mg/g). This large deviation suggests that the Pseudo First Order model, which typically characterizes a process
controlled by physical diffusion or physisorption, does not fully describe the rate-limiting step of this system.
Instead, the kinetic behavior points toward a more complex interaction. The failure of this model to accurately
predict the saturation limit (q
e
) confirms that the removal of cadmium is not merely a surface-layer physical
trapping. Rather, it is likely governed by chemisorption, where the rate is determined by chemical bonding and
ion exchange with the high-affinity functional groups specifically the hydroxyl and carbonyl groups previously
identified. Consequently, the Pseudo Second Order model is required to more accurately characterize the
definitive chemical nature of the Cd²⁺ uptake.
Pseudo-Second-Order Model Table 8: Pseudo Second Order (q
e
= 4.500mg/g)
Here is your data organized into a clear table:
Time (t, min)
qt (mg/g)
y = t/qt (min·g/mg)
5
2.650
1.8868
10
3.240
3.0864
30
3.980
7.5377
60
4.310
13.9211
120
4.500
26.6667
This model typically gives a better fit for systems involving chemical bonding, especially in biosorbents
functionalized with oxygen-containing groups (Abd-Talib et al., 2020; Zakaria et al., 2023). A plot of t/q
t
versus
t yields a straight line, from which q
e
and k
2
can be determined.
The pseudo-second-order model assumes that chemisorption is the rate-limiting step, involving valence forces
through electron sharing or exchange between adsorbent and adsorbate (Abubakar et al., 2025). The linear form
of the equation is:
Slope:
Intercept:
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Figure 11: Plot of Pseudo Second Order Kinetic Model
The Pseudo Second Order linear plot of t/q
t
versus time (t) yielded an exceptionally high correlation coefficient
(R² = 0.9999), indicating that the adsorption of Cd²⁺ perfectly fits this kinetic model. From the regression
equation y = 0.2149x + 0.9510, the calculated equilibrium adsorption capacity (q
e
, calc) is 4.654 mg/g, which
shows excellent agreement with the experimental value (q
e
, exp = 4.500 mg/g). Additionally, the calculated
second-order rate constant (k
2
) is 0.0485 g/mg·min.
This near-perfect alignment between the theoretical and experimental data points confirms that the rate-limiting
step in the removal of cadmium is not physical diffusion, but rather chemisorption. The high R² value and the
accurate prediction of the saturation limit indicate that the process involves the sharing or exchange of electrons
between the Cd²⁺ ions and the adsorbent surface.
Specifically, this chemical interaction is facilitated by the specific, high-affinity binding sites primarily the
hydroxyl and carbonyl groups previously identified. Because the adsorption rate depends on the capacity of
the adsorbent and not just the concentration of the solution, the Pseudo Second Order model successfully
characterizes the definitive chemical uptake and the stability of the metal–carbon complex on the activated
mango seed coat.
Adsorption Efficiency Effect of Initial Cd(II) Concentration
The observed decrease in Cd
2+
removal from 81.00 % to 45.00 % as initial concentration increased (10–50
ppm) is a classic characteristic of adsorbent saturation. At the lower concentration of 10 ppm, the ratio of
available surface area to the total number of metal ions in the solution is at its highest. This provides the Cd
2+
ions with maximum access to the hydroxyl (–OH) and carbonyl (C=O) functional groups identified in the FTIR
analysis.
As the concentration increases to 50 ppm, the absolute amount of cadmium adsorbed per unit mass (q
e
) actually
increases due to the higher concentration gradient, which acts as a powerful driving force for mass transfer.
However, because the adsorbent dosage was kept constant at 0.5 g, the total number of active binding sites is
fixed. Once these high-energy sites are occupied, the percentage of the total ions removed naturally drops.
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This behavior confirms that the adsorption process is "site-limited." The fact that the removal does not drop to
zero even at 50 ppm supports the Freundlich model findings (R
2
= 0.9834), suggesting that the activated mango
seed coat possesses an energetically heterogeneous surface. This means that after the most reactive sites are
filled, secondary sites with different energy levels continue to contribute to the 45 % removal observed at
higher concentrations.
In conclusion, the activated mango seed coat is most efficient in dilute conditions, achieving an optimal 81.00%
removal. While higher concentrations increase the loading capacity (q
e
) via an enhanced driving force, the
declining removal percentage at 50 ppm confirms the eventual saturation of the surface functional groups. This
highlights the material's high affinity for cadmium and its suitability for treating metal-contaminated
wastewater within the studied concentration range.
Effect of Adsorbent Dosage
The investigation into adsorbent dosage (0.1 g to 0.5 g) revealed that Cd
2+
removal remained nearly constant
at approximately 81 % after the initial 0.1 g, identifying it as the optimal dosage for balancing efficiency and
material usage. This plateau is attributed to the aggregation of adsorbent particles at higher masses, which
causes the overlapping of hydroxyl and carbonyl active sites and reduces the effective surface area available
for metal interaction.
In conclusion, while increasing the mass provides more total sites, the marginal improvement beyond 0.1 g
confirms that the system reached an equilibrium limit, making the lower dosage the most economically viable
choice for cadmium remediation using activated mango seed coat.
Adsorption Isotherm Analysis
The equilibrium adsorption of Cd
2+
onto activated mango seed coat was evaluated using both the Langmuir and
Freundlich models to determine the nature of the surface interactions. The experimental data demonstrated a
superior mathematical fit to the Freundlich model (R
2
= 0.9834), indicating that the adsorption process occurs
on an energetically heterogeneous surface rather than a uniform one. This is physically supported by the FTIR
results, which identified multiple functional groups—including hydroxyl (–OH) and carbonyl (C=O)—that
provide a range of binding sites with varying affinities. The calculated value of n = 2.84 (1 < n < 10) confirms
that the process is highly favorable under the studied conditions.
While the Langmuir model was statistically secondary (R
2
= 0.9650), it successfully estimated a maximum
monolayer adsorption capacity (q
max
) of 5.34 mg/g, representing the theoretical saturation point. The calculated
separation factor (R
L
) values remained between 0 and 1, further validating the favorability of the adsorption.
In conclusion, while the Langmuir model provides a useful saturation limit, the superior fit of the Freundlich
model more accurately describes the real-world chemistry of the activated mango seed coat, where diverse
functional groups interact with Cd
2+
ions across multiple energy levels in a multilayer arrangement.
Adsorption Mechanism
The adsorption of Cd(II) onto mango seed-derived activated carbon is governed by multiple mechanisms,
including:
1. Electrostatic attraction between Cd(II) ions and negatively charged adsorbent surfaces
2. Ion exchange involving replacement of surface ions with Cd(II)
3. Surface complexation with functional groups such as –OH and –COOH
4. Pore diffusion into the internal structure of the adsorbent
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Reusability of the Adsorbent
The reusability of the activated carbon was evaluated through adsorption–desorption cycles. The results showed
that the adsorbent retained a significant portion of its adsorption capacity after reuse, although a gradual decline
in efficiency was observed.
This decrease may be due to incomplete desorption of Cd(II) ions or slight structural degradation of the
adsorbent during regeneration. Nevertheless, the ability to reuse the adsorbent demonstrates its potential for
cost-effective and sustainable application in water treatment.
Comparison with Other Adsorbents
The adsorption performance of mango seed-derived activated carbon compares favorably with other low-cost
adsorbents reported in the literature. Its relatively high removal efficiency, combined with its low cost and
abundance, makes it a competitive alternative to commercial activated carbon.
The results further highlight the potential of agricultural waste materials as effective adsorbents for heavy metal
removal, contributing to both environmental remediation and waste management.
Summary of Findings
Results of this study demonstrate that:
● Mango seed coat-derived activated carbon possesses functional groups suitable for Cd(II) adsorption
● Adsorption efficiency decrease with increase in initial metal concentration
● Optimal adsorbent dosage was observed at 0.1 g
● The Freundlich isotherm best describes the adsorption process
● Adsorption follows pseudo-second-order kinetics, indicating chemisorption
● The adsorbent shows good potential for reuse
Overall, these findings confirm that activated mango seed coat is a highly effective, low-cost, and sustainable
alternative to commercial adsorbents for Cd
2+
removal, offering a promising solution for both environmental
remediation and agricultural waste management.
CONCLUSION
This study successfully demonstrated the potential of activated carbon derived from mango (Mangifera indica)
seed coats as an efficient and low cost adsorbent for the removal of Cd
2+
ions from aqueous solutions. FTIR
analysis confirmed that the prepared adsorbent possesses favorable physicochemical properties, specifically
the presence of hydroxyl and carbonyl functional groups capable of strong heavy metal sequestration.
Batch adsorption experiments revealed that while the absolute adsorption capacity (q
e
) increased with
concentration due to a stronger driving force, the percentage removal decreased from 81.00 % to 45.00 % as
initial Cd
2+
concentration rose from 10 to 50 mg/L. Furthermore, an adsorbent dosage of 0.1 g was identified
as optimal, providing maximum removal efficiency while maintaining economic material usage.
The equilibrium data were best described by the Freundlich isotherm model (R
2
= 0.9834), indicating multilayer
adsorption on an energetically heterogeneous surface. Kinetic studies confirmed that the process follows
pseudo second order kinetics, suggesting that chemisorption—driven by electrostatic attraction, ion exchange,
and surface complexation—is the dominant mechanism.
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With demonstrated reusability and high stability, this mango seed derived activated carbon represents a
sustainable and economically viable alternative to conventional adsorbents. These findings highlight a
promising path for environmental remediation that simultaneously addresses agricultural waste management
through a circular economy approach.
RECOMMENDATIONS
Based on the findings of this study, the following recommendations are proposed:
1. Further studies should investigate the adsorption performance of the adsorbent in multi-component
systems to simulate real wastewater conditions.
2. Advanced characterization techniques such as SEM, BET surface area analysis, and XRD should be
employed to provide deeper insight into the structural properties of the adsorbent.
3. Column adsorption studies should be conducted to evaluate the performance of the adsorbent under
continuous flow conditions.
4. Regeneration and reuse studies should be optimized to enhance the long-term economic viability of the
adsorbent.
5. Scale-up studies and cost-benefit analyses should be carried out to assess the feasibility of industrial
application.
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