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Exploring the synergistic effects of calcium chloride modification
on stem bark eucalyptus biochar for Cr(VI) and Pb(II) ions
removal: Kinetics, isotherm, thermodynamic and optimization
studies
Lukman Shehu Mustapha, Oluwatobi Victoria Obayomi, Muibat
Diekola Yahya, Sie Yon Lau, Kehinde Shola Obayomi
PII: S2589-014X(23)00370-5
DOI: https://doi.org/10.1016/j.biteb.2023.101699
Reference: BITEB 101699
To appear in: Bioresource Technology Reports
Received date: 8 September 2023
Revised date: 8 November 2023
Accepted date: 10 November 2023
Please cite this article as: L.S. Mustapha, O.V. Obayomi, M.D. Yahya, et al., Exploring
the synergistic effects of calcium chloride modification on stem bark eucalyptus biochar
for Cr(VI) and Pb(II) ions removal: Kinetics, isotherm, thermodynamic and optimization
studies, Bioresource Technology Reports (2023), https://doi.org/10.1016/
j.biteb.2023.101699
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© 2023 Published by Elsevier Ltd.
Exploring the synergistic effects of calcium chloride modification on stem bark eucalyptus
biochar for Cr(VI) and Pb(II) ions removal: Kinetics, isotherm, thermodynamic and
optimization studies
Lukman Shehu Mustapha a, Oluwatobi Victoria Obayomi b, Muibat Diekola Yahya a, Sie Yon
Lau c, Kehinde Shola Obayomi c, d *
a Department of Chemical Engineering, Federal University of Technology Minna Niger State,
Nigeria
b Department of Microbiology, Landmark University, Omu-Aran Kwara State, Nigeria
c Department of Chemical Engineering, Curtin University, CDT 250, 98009 Miri, Sarawak,
Malaysia
d Institute for Sustainable Industries and Liveable Cities, Victoria University, Werribee, VIC
3030, Australia
Corresponding author: Obayomikehindeshola@gmail.com
Department of Chemical Engineering, Curtin University, CDT 250, 98009 Miri, Sarawak,
Malaysia; Institute for Sustainable Industries and Liveable Cities, Victoria University, Werribee,
VIC 3030, Australia
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Abstract
In this study, stem bark eucalyptus was subjected to pyrolysis modification to produce modified
biochar. The modified biochar (MSBEB) was characterized using Fourier transform infrared
Spectroscopy (FTIR), Scanning electron microscopy coupled with energy-dispersive X-ray
spectroscopy (SEM-EDS), X-ray Diffraction (XRD), and Brunauer–Emmett–Teller (BET)
techniques. The impact of adsorption process variables such as temperature, adsorbent dosage,
pH and contact time on the metals ion removal efficiency were investigated. The optimum
conditions for maximum removal of Cr(VI) and Pb(II) ions were determined as 45.20 °C, 0.12
g/L, and 120.5 min through optimization process. The excellent performance of the modified
biochar can be attributed to its porous structure, surface area, surface chemistry and crystallinity
as supported by SEM, BET, FTIR and XRD analysis. However, based on the high correlation
coefficient (R2), low values of chi square (χ2) and sum of square error (SSE), the Freundlich
isotherm and Pseudo-second order kinetics provided the best fit, suggesting chemisorption as the
dominant mechanism. Thermodynamic analysis indicated an endothermic, spontaneous, and
feasible reaction.
Keywords: Steam Bark eucalyptus; Biochar; Tannery Wastewater; Adsorption; Optimization;
Desorption
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1. Introduction
The unselective release of industrial effluent containing high concentrations of heavy metals has
led to the contamination of water and land in recent times. The toxic nature, persistence, and
carcinogenic potential of several heavy metals arise from their ability to accumulate within
biological systems, leading to gradual damage to internal organs as time passes (Mu et al., 2023).
The industrial sectors, such as pesticides formulation, fertilizer production, textile production,
electroplating, mining, tanning, battery and wood processing extensively employ heavy metal
compounds. Consequently, the discharge of untreated industrial effluents containing these
compounds becomes a major contributor to soil contamination and water pollution, leading to
significant environmental degradation. The high solubility and ability to accumulate in biological
systems make chromium and lead a toxic heavy metal capable of causing damage to the lungs,
liver and kidneys. Furthermore, exposure to chromium and lead can result in asthma, skin
disorder, abdominal pain and eye irritation (Jung et al., 2022). Two forms of chromium are
present: {Cr(VI) and Cr(III)}, with the latter being a necessity for maintaining human health
(Swaroop et al., 2019). Hexavalent chromium Cr(VI) poses a greater hazard compared to
trivalent chromium (Cr(III)) due to its higher mobility in water, cell permeability, and water
solubility (Jiamin et al., 2023). Lead exposure can have detrimental effects on all organs within
human body. It is especially crucial to note that children under the age of six are particularly
vulnerable to the harmful consequences associated with lead exposure (Chowdhury et al., 2022).
Consequently, it is imperative to subject Cr(VI) and Pb(II) ions containing effluents to
appropriate treatment prior to their release into aquatic ecosystems. Reverse Osmosis (Seef et al.,
2023), membrane filtration (Yuhuan et al., 2023), photocatalysis (Xinyu et al., 2021), chemical
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precipitation (Begum et al., 2021), ion exchange (Hao et al., 2023), filtration (Ahmad et al.,
2023) and adsorption (Mustapha et al.,2023) techniques have been employed for the removal of
Cr(VI) and Pb(II) ions from wastewater. Amongst these methods, adsorption has demonstrated
superior attributes in terms of adaptability, cost-effectiveness, efficiency, and effectiveness for
the removal of contaminants from effluent (Ghasaq et al., 2023; Obayomi et al., 2023). Through
the process of mass transfer, it becomes feasible to eliminate a wide range of inorganic and
organic molecules, irrespective of their size or toxicity from polluted water. Notably, this process
does not generate intermediate byproducts or induce molecule fragmentation (Sapana et al.,
2022). Moreover, the occurrence of adsorption does not entail the production of substances that
could potentially be hazardous.
In recent research, the utilization of bio-adsorbents derived from diverse biomasses rich carbon
such as peanut shell (Wannipha et al., 2022), paper mill sludge (Yusuff et al., 2022), almond
shell (Boulika et al., 2022), neem bark (Bhanupriya et al., 2023) and walnut shell (Sabrina et al.,
2023) have been explored for the purpose of removing Cr(VI) and Pb(II) from aqueous solutions.
The use of biochar, a valuable byproduct obtained from biomass pyrolysis, has generated
significant interest among researchers and industry due to its potential as an effective adsorbent
for the removal of contaminants from wastewater. This unique characteristic of biochar has led
to its exploration and consideration in the field of wastewater treatment. Its high adsorption
capacity, abundance active site and favorable textural properties contribute to its effectiveness
(Ibrahim et al., 2022). The interaction between biochar and heavy metals occurs through various
mechanisms, including the formation of organic functional groups (OFGs), the bonding of
exchange ions and precipitation with soluble minerals (Yang et al., 2021). The use of additional
organic component as bio-sorbents for the remediation of polluted water holds significant value
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and merit. Due to its rapid growth and wide distribution across various Nigerian states, the
eucalyptus tree yields substantial quantities of bark that are currently disposed as waste. In spite
of previous investigations, eucalyptus barks and leaves have been examined as viable bio-
sorbents for effective of adsorption of heavy metals and dyes from industrial wastewater or
aqueous solutions (Yusuff et al., 2022). The steam bark of eucalyptus trees holds significant
potential as valuable source material for producing biochar primarily due to its substantial
content of hemicellulose, pectin and cellulose component (Changgil et al., 2021). Gao et al.,
(2023) investigated the utilization of Calcium chloride (CaCl2) treated selenium-rich straw for
the purpose of removing cadmium (II) ion from aqueous solutions. The research findings
indicated that modified selenium rich straw exhibited significant efficiency as an adsorbent for
the removal of Cd (II) from wastewater. In a similar study, Changgil et al., (2021) reported the
removal efficiencies of phosphate ions (PO43-) by employing both pristine tangerine peel biochar
(TB) and CaCl2 chemically-activated tangerine peel biochar. In a noteworthy study published by
Ouakouak et al., (2016), a novel approach utilizing CaCl2 pretreated Algerian bentonite to
effectively remove toxic Cu(II) ion. Zanella et al., (2014) also showed that modified activated
carbon using CaCl2 served as good adsorbent for nitrate removal. Thus, the introduction of CaCl2
during the modification process of biochar can greatly enhance its surface area thereby leading to
its ability to adsorb various substance. However, to the best of the authors knowledge, reports on
the modification of stem bark eucalyptus biochar using CaCl2 as a modifying agent for the
adsorption of Cr(VI) and Pb(II) ions from tannery effluent have not been mentioned. Hence, the
application of CaCl2-modified biochar derived from stem bark eucalyptus offers a novel,
uncomplicated, highly efficient, and economically viable solution for metal ion adsorption. In
this experimental investigation, stem bark eucalyptus underwent pyrolysis at a temperature of
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500 oC, resulting in the production of biochar. Subsequently, the biochar underwent modification
using CaCl2 and was deployed as a sorptive material for eliminating Cr(VI) and Pb(II) from a
wastewater. This study aimed to examine the influence of pH, dosage of sorbent, time of contact
and temperature on the effectiveness of both metal ions removal using modified stem bark
eucalyptus biochar (MSBEB). Kinetic, isotherm and thermodynamics parameters were evaluated
and described. Additionally, this study explored the mechanism of adsorption-desorption
efficiency to assess the hexavalent chromium and divalent lead ions removal on the modified
stem bark eucalyptus biochar.
2.0 Materials and Method
2.1 Materials
The stem barks of eucalyptus (SBE) were retrieved from the desiccated waste container situated
at car park area of the school cafeteria University of Ilorin Secondary School, Kwara State. The
chemicals of analytical grade (Hydrochloric acid, Calcium chloride and Sodium hydroxide) were
purchased from Merck company (Germany) with > 99% purity. The effluent sample was
collected from Z-tannery production industry outlet in Challawa Kano State, Nigeria. While the
effluent was kept in a refrigerator at 4oC for further usage. The concentration of the heavy metals
present was analyzed using Atomic Absorption Spectrometer (AAS). The pH levels of the
tannery effluent when released directly into water bodies ranges from 3.5 to 12.5, which poses a
significant threat to the aquatic organism.
2.2 Preparation of modified stem barks eucalyptus biochar
The stem bark of eucalyptus (SBE) that was gathered underwent a thorough washing process
with clean water to eliminate particles of dirt, dried at 80 °C for 5 h in an oven, subsequently
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crushed into a powdered form using a manual process. Furthermore, the (SBE) powder was
passed through a sieve mesh with a size of 0.3 mm to attain the particles size desire. To generate
biochar, the BE powder that had been sieved was subjected to pyrolysis within a muffle furnace
in absence of atmospheric pressure. The process was carried out at 650 °C, for a duration of 2 hr,
with a rate of 6 °C per minute. After the pyrolysis process, the stem bark eucalyptus biochar
(SBEB) was cooled down and later placed into a glass flask for storage. To initiate the
modification of biochar synthesizes, 3.0 g of SBEB powder was added to 5 M CaCl2 solution in
a beaker. Following that, the mixture underwent stirring process for a duration of 4 h at a
temperature of 65 °C using a magnetic stirrer. Subsequently, the mixture was subjected to
filtration using filter paper. The modified form of biochar was subjected to successive rinsing
cycles with distilled water, aiming to eliminate impurities until a neutral pH of 7 was attained in
the solution. After that, the sample underwent an overnight drying at a temperature of 100 oC,
allowing for the complete removal of any remaining moisture and achieving a state of dryness.
Henceforth, the modified biochar derived from stem bark of eucalyptus will be referred to as
modified stem biochar eucalyptus bark (MSBEB).
2.3 Characterization of stem bark eucalyptus biochar
The modified adsorbent underwent comprehensive analysis to investigate its textural properties,
surface chemistry, composition of phases, morphological features, and elemental composition.
This analysis involved the utilization of advanced scientific techniques such as BET (Horiba
analyzer) which employed the use of nitrogen adsorption-desorption process so as to provide
valuable information about porosity and surface properties, FT-IR (ASD- NIR spectrometer),
XRD (Empyrean Alpha 1), SEM (JEOLs flagship FE- SEM UK). Additionally, proximate, and
ultimate analyses were also conducted.
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2.4 Adsorption equilibrium
In each experimental run, a predetermined quantity of the modified stem bark eucalyptus biochar
(MSBEB) sample was introduced into a 250 mL flask containing 100 mL of tannery effluent,
while water bath shaker was employed for mixture agitation at 250 rpm. Upon conclusion of the
experiment, the solution underwent centrifugation. The metals ion residual concentration was
evaluated using the atomic absorption spectrophotometer (Thermo- fisher scientific USA). The
determination of the removal percentage (Z,%) and the sorption capacity of metal ions was
carried out using the Eq (1) and (2) respectively.
(1)
(2)
In the given context, Co and Ce represents the initial and equilibrium concentration of Cr(VI) and
Pb(II)ions, while. M and V represents the mass of the adsorbent and volume of the selected metal
ions solution.
2.5 Isotherm Adsorption
The experimental data for the adsorption of hexavalent chromium and divalent lead ion by
MBEB was analyzed using different isotherm models, namely Langmuir, Freundlich and
Temkin. The non-linear form of the isotherm model equations is presented in Table S1. The
isotherm model’s parameter with (Qmax mg/g) is termed maximum adsorption capacity of metal
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ions, KL is termed as Langmuir isotherm constant (L/g). Freundlich constant is (KF), adsorption
intensity is represented as (n), while KT(L/g) and bT(KJ/mol) is denoted as Temkin constant.
Also, the R (8.314 J/mol.K) is referred to as universal gas constant. The Langmuir isotherm
model possesses a distinctive characteristic in the form of a dimensionless constant known as the
separation factor (RL). This factor allows for the prediction of the adsorption process pattern,
indicating when (0 < RL < 1) favorable, (RL > 1) unfavorable, (RL = 1) linear and (RL = 0)
irreversible. The Eq. (3) represent the separation factor.
=
(3)
2.6 Adsorption process kinetics
The adsorption rate of the MSBEB sample was evaluated by measuring the amount of Cr(VI)
and Pb(II) ion from the wastewater using batch studies. The measurements were conducted at
different time intervals between the MSBEB sample and the wastewater solution, until an
adsorption equilibrium state was achieved. The process described in this study investigates the
time-dependent changes in the adsorption rate of the targeted metal ions. The Pseudo first order,
pseudo second order and intraparticle diffusion models were employed to investigate the kinetics
data of the two metal ions onto developed MSBEB. The kinetic model equation is depicted in
Table S2.
The quantity of metal ion at time t is denoted as qt, while the adsorption volume at equilibrium is
qe. K1 represent the rate constant for the pseudo-first order reaction (min-1), K2 represent the rate
constant for pseudo-second-order reaction (g/mg.min), Kid denote intraparticle diffusion rate
constant (mg g-1 min1/2) and the intercept is termed as C (mg g-1).
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2.7 Adsorption thermodynamics
By employing temperature dependent adsorption isotherms, it becomes possible to estimate
various thermodynamics parameters of adsorption. These parameters include entropy change
(∆So), Gibb’s free energy and enthalpy change (∆Ho) which can be evaluated using Eq.
(4), (5) and (6) respectively.
(4)
(5)
(6)
, KJmol-1 signify Gibb’s free energy difference, kJ/mol, ∆Ho denoted for enthalpy change
while ∆So, (J mol -1 K-1) termed as entropy change, where the universal gas constant (R) (8.314
Jmol-1K-1), the thermodynamic equilibrium constant is represented with Keq, and T (K) denote
absolute temperature.
2.8 Investigation on Optimization Studies
The study utilized a central composites design (CCD), a subset of the response surface
methodology (RSM) along with various design methodologies. The primary focus was to
analyze the individual and combined effects of specific variables (adsorbent dosage, contact time
and temperature) on the removal of selected heavy metals aiming to optimize the overall
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performance. The experimental design employed in this study, known as central composites
design (CCD), consisted of three key operations: 2n axial runs (with 6 points), 2n factorial runs
(with 8 points), and six center runs (with 6 points). In total, 20 experimental runs was evaluated.
To establish a correlation between the independent and dependent variables, the quadratic model
equation presented in Eq. (7) that offered the optimal prediction capabilities.
Y = βo + ∑ βi Xi + ∑ βii Xi2 + ∑ βij XiXj (7)
The response variable (Y) in this study was predicted using a mathematical model. The model
consisted of various coefficients, including a constant term (βo), linear coefficients (βi),
quadratic coefficients (βii), and interaction coefficients (βij). The independent parameters in the
model were represented by Xi and Xj. The purpose of this model was to establish a correlation
between the independent and dependent parameters. Version 13.0 design expert statistical
software was utilized to conduct the regression analysis. The main objectives were to develop
equations that accurately represented the outcome and to evaluate the importance of these
equations using the experimental data. The variability of the responses was assessed using
statistical measures such as the Fisher's value (F-value), probability value (P-value) and the
correlation coefficient(R2). The independent factors coded levels and range values are outlined in
Table S3.
2.9 Recovery and Reusability evaluation
The ability to reuse adsorbents helps reduce operational expenses and minimize waste
generation. To evaluate its stability, the utilized MSBEB was subjected to reusability testing. In
this context, 0.5 g of MSBEB loaded with selected metal ions was introduced into a flask
containing 50 mL of 0.1M HCl solution. The mixture was stirred at 80 °C for a duration of 100
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min. Afterward, a set of adsorption-desorption experiments was performed to investigate the
process further. The efficiency of desorption was quantified by dividing the mass of regenerated
metal ions by the mass of metal ions initially adsorbed (Dim et al., 2021).
3.0 Results and Discussion
3.1. Investigation of adsorbent performance
3.1.1 Physical and Chemical Properties
The result depicted in Table 1 shows the physical and chemical properties of both SBEB and
MSBEB, based on the results obtained, it can be seen that the incorporation of the modifying
agent (CaCl2) onto SBEB was successful, leading to an alteration in the chemical compositions.
The MSBEB exhibited larger specific surface area, pore volume, and pore diameter compared to
SBEB. This observation suggests that the surface of the modified biochar under investigation
contained a significant number of active sites. Furthermore, these results provide additional
evidence of the modification of SBEB (Yusuff et al., 2022).
Table 1: physiochemical characterization of SBEB and MSBEB
SBEB
MSBEB
Elemental Analysis
H
2.60
1.06
O
37.1
22.3
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N
28.3
34.7
C
43.4
58.2
C/O
1.26
3.48
C/H
1.40
3.44
C/N
1.79
1.45
Physical Parameters
Specific surface area (m2/g)
56.3
323.4
Pore volume (cm2/g)
0.02
0.26
Pore Diameter (Å)
12.4
38.7
Bulk Density (g/cm3)
0.372
0.702
Ash content (%)
8.5
2.03
Fixed Carbon content
12.08
32.75
Moisture content
9.2
4.5
According to result in Table 1, the MSBEB sample demonstrated a higher fixed carbon content
of 32.75% compared to the SBEB sample, which had a carbon content of 12.08%. The
composition of oxygen and hydrogen in MSBEB decreased from 37.1% to 22.3% and 2.60% to
1.06%, respectively. This reduction is likely attributed to occurrence of chemical reaction after
introduction of CaCl2 resulting in formation of water and (CO2) leading to elimination of O and
H from the matrix of biochar (Yucan et al., 2023). The observation is similar to a report in
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literature on modification of biochar (Yusuff et al., 2022). The C/O and C/H ratios of MSBEB
were found to be higher than those of SBEB, suggesting that the incorporation of CaCl2 onto
SBEB resulted in an increase in functional groups of O attached. Moreover, the modification
process improved the polarity and hydrophilicity of the modified biochar, consequently enhanced
its adsorption of metal ions. These findings align with the reported results for Leucocephala seed
pod (Yusuff et al., 2019) and corn- carb (Deng et al., 2022) confirming the consistency between
the observations. Furthermore, the data presented in Table 1 indicate that the MSBEB exhibited a
high bulk density of 0.702 g/cm3. This suggests that it has the potential to function as an efficient
adsorbent for the elimination of selected metals ions from wastewater, surpassing the
performance of the SBEB. The result obtained agrees with similar work carried out by Adeolu et
al., (2019) and Abdulrazak et al., (2017). On the other hand, the MSBEB exhibited a lower ash
content of 2.03% compared to SBEB, which had a higher ash content of 8.5%. This discrepancy
suggests that SBEB contained a significant amount of inorganic minerals which is good for its
efficiency (Adeolu et al., 2019). Also, the MSBEB exhibited higher surface area of 323.4 m2/g
compared to 56.3 m2/g of SBEB which could be attributed to the modification.
3.1.2 X-ray diffraction analysis (XRD)
The X-ray plots of the SBEB and MSBEB as depicted in Fig. S1 exhibited a broad intensity peak
spanning 2θ angles of 10-30°. This peak corresponds to the presence amorphous carbon structure
aligning with the (002) plane of carbon (Dechakhumwat et al., 2020). However, the absence of
the sharp peaks in MSBEB suggests that no discrete mineral peaks were noticed in the sample
(Vunain et al., 2019). Similar observations were reported by other researchers who conducted
experiments on activated carbons derived from agricultural waste materials. (Joel et al., 2022 and
Pongener et al., 2019).
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3.1.3 Morphological Analysis (SEM)
The SEM image in Fig. S2 illustrates the SBEB and MSBEB as examined in the study. Fig.
S2(a) reveals that the image depicts a disorganized and rough surface structure of the SBEB
(Stem bark eucalyptus biochar). The inability to identify the pore structure of the biochar was
attributed to the presence of deposited irregular fragments on its surface. On the other hand, Fig.
S2(b) revealed that MSBEB surface become smother and flatter due to modification by CaCl2
(Feng et al., 2021).
3.1.4 FT-IR Analysis
The alterations in functional groups on the biochar were elucidated by analyzing the Fourier
transform infrared (FT-IR) spectra of (SBEB) and (MSBEB) as depicted in Fig. S3. The
appearance of peak at 3415 cm-1 in the spectrum indicated the presence of the –OH group. Also,
the peak observed at 1685 cm-1 revealed the asymmetric vibration of C–H bonds. The stretching
vibration of the C=O bond was observed at 1017 cm-1. Furthermore, Following the modification
with (CaCl2), a noticeable change in the peak at 2285 cm-1 was observed, indicating a C=O
vibration. This alteration provides evidence that the Ca ions are chemically bonded with acid
group.
3.2 Influence of parameters on adsorption process
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The study investigated the influence of contact time on the uptake of Cr(VI) and Pb(II), and the
finding is illustrated in Figure 1(a). As the contact time increased, the percentage removal of
hexavalent chromium and divalent lead ion concentration exhibited an initial rise followed by a
gradual decline until reaching equilibrium at 100 min. Initially, during the process of adsorption,
the removal rate of these selected metals ions was more significant due to the presence of an
ample number of active sites on the adsorbent. However, as the process progressed, the rate of
removal efficiency gradually decreased due to the inward movement of anions into the pores of
the MSBEB sample (Yusuff et al., 2022; Mustapha et al., 2023).
The impact of MSBEB dosage on the removal of Cr(VI) and Pb(II) from aqueous solution is
showed in Fig. 1(b&c) where the adsorbent dosage was varied between 0.1 to 0.6 g/L with
contact time of 100 min . The graph demonstrates that higher MBEB dosage resulted in
increased removal efficiency of selected metal ion. Conversely, as the dosage of the adsorbent
increased, the adsorption capacity exhibited a decrease. The reason behind this observation was
that with an increase in the adsorbent dosage, a greater number of sorption sites became
accessible for the two metals ion removal. However, the adsorption capacity of the adsorbent
decreased because a larger proportion of active sites remained unsaturated during the removal of
the adsorbate from the aqueous solution (Obayomi et al., 2023).
The study examined the significance of solution pH on the efficiency of selected metals ion
removal using MSBEB. The solution pH was varied within between 2 to 8. Fig. 1(d)
demonstrated a positive correlation between solution pH and the removal efficiency of Cr(VI)
and Pb(II). The removal efficiency of Cr(VI) progressively increased from 56.04% to 74.17% at
pH 5, while Pb(II) ion also show a similar trend of 43.02% to 70.46% respectively. The results
indicated a significant increase in efficiency removal efficiency of hexavalent chromium and
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divalent ion when the pH was raised from 2 to 6. However, further increasing the pH above pH 6
for the two metal ions had minimal impact on the efficiency of Cr(VI) and Pb(II) removal which
could be attributed to electrostatic attraction that occur during adsorption process (Dahlan et al.,
2023; Nazir et al., 2022).
The study also investigated the impact of temperature, ranging from 20 to 70 oC, with adsorbent
dosage of 0.3 g/L, contact time of 100 min and pH of 6 on the percentage removal of selected
metals ions. As depicted in Fig. 1(e), the percentage removal of Cr(VI) and Pb(II) exhibited an
increase with increase in temperature which could be as a result of endothermic nature of the
adsorption process. This observation aligns with similar results obtained in the adsorptive
removal of Cr(VI) using modified water hyacinth-based biochar (Liu et al., 2023), Tangerine
peel biochar and (Changgil et al., 2021).
The evaluation of point of zero charge (pHpzc) for the MSBEB was conducted using a 0.01 M
potassium nitrate (KNO3) solution. The pH of the adsorbent was adjusted by gradually adding
0.1 M NaOH or 0.1 M HCl. The pHpzc was then determined to evaluate the pH properties of the
adsorbents. The findings depicted in Fig. 1(f) demonstrated that both Cr(VI) and Pb(II) ion
exhibit a comparable point of zero charge, which is indicated by a pHpzc value of 7.2. At this
stage, MSBEB possess a surface charge that is electrically neutral. When the pH of the solution
is below the pHpzc, the adsorbent surface undergoes protonation due to the increased presence of
H+ ions. This protonation process enhances the adsorbent ability to remove contaminants,
particularly anions, thereby favoring their removal from the solution. When the pH of the
solution exceeds the pHpzc, the adsorbent surface undergoes deprotonation as a result of the
presence of OH− ions. This process leads to a negative charge on the surface of adsorbent,
making it favorable for the adsorption of pollutants such as cations (Zaimee et al., 2021).
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Fig. 1. Plots of (a) contact time, (b and c) adsorbent dosage (d) solution pH and (e)
temperature and (f) point of zero charge for MSBEB.
3.3 Isotherm adsorption
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A merged graph illustrating the relationship between the amount of adsorbed metal ions and the
concentration of metal ions at equilibrium is depicted in Fig. 2. The graphs include data points
from the experimental, Langmuir, Freundlich model and Temkin. Fig. 2 plots demonstrated a
nonlinear correlation, allowing model parameters determination, while Table 2 also display the
constant value estimated. The selection of the most suitable model was based on the coefficient
of correlation (R2) smallest value of ᵪ2 and SSE. With an R2 value of (0.999 and 0.998), ᵪ2
(1.05x10-6 and 1.39 x10-6), SSE (5.13 x10--3 and 3.76 x10-5) for hexavalent chromium and
divalent lead respectively, indicating a high degree of correlation, the experimental data
exhibited a strong fit with Freundlich model. Consistence finding have been reported regarding
biochar modification (Gorzin et al., 2018). Based on the values of the separation factor (RL =
0.017 and 0.106) and the exponent (n = 7.57 and 4.81) for the two metal ions, it can be inferred
that the adsorbate-adsorbent system exhibited favorable conditions and a significant adsorption
capacity (Yusuff et al., 2022; Mustapha et al., 2023).
Fig. 2. Isotherm model curves for Cr(VI) and Pb(II) ions onto MSBEB
Table 2. Isotherm models for adsorption of Cr(VI) and Pb(II) ion onto MSBEB
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Isotherm model
Equation
Parameter
Adsorbate Cr(VI)
Adsorbate Pb(II)
Langmuir
Qmax(mg/g)
238.12
175.02
RL(L/g)
0.017
0.106
R2
0.902
0.937
ᵪ2
3.1 X10-4
2.1 X10-5
SSE
1.8 X10-5
1.1 X10-3
Freundlich
KF(mg/g)(L/g)
0.046
0.036
1/n
0.132
0.208
R2
0.999
0.998
ᵪ2
1.1X10-6
1.4 X10-6
SSE
5.1 X10-3
3.7 X10-5
Temkin
KT(L/g)
87.01
121.53
bT(J/mol)
47.35
67.12
R2
0.853
0.702
ᵪ2
0.041
0.038
SSE
1.2 X10-2
2.9 X10-2
3.4 Kinetics adsorption
The plot in Fig. 3 shows the Pseudo- first order, Pseudo second order and Intraparticle diffusion
model curves. This plot helps in evaluating the rate constant of (K1 and K2) as well as predicted
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amount of metal ions adsorbed at equilibrium. Table 3 presents the estimated parameters and
their corresponding values. The findings indicated a lack of correlation between the predicted qe
and the experimental qe in the pseudo-first-order kinetics. Also, the value of R2 value for the
pseudo-first-order kinetics and intraparticle was smaller compared to pseudo-second-order.
Hence, the data was analyzed using the pseudo-second-order kinetic model. The results
demonstrated a strong agreement between the predicted qe and the experimental qe, as indicated
by the high value of R2 (0.999 and 0.991) for Cr(VI) and Pb(II). The smaller value of ᵪ2
(9.03×10-4 and 0.0058) and SSE (2.80×10-5 and 0.023) provide additional evidence supporting
the fitness of the pseudo-second order model. Also, the observed decrease in the rate constant
(K2) provides provide information confirmation that the Cr(VI) and Pb(II) ion reached
equilibrium more rapidly at lower concentration.
Fig. 3. Plots of Kinetic model plots for Cr(VI) and Pb(II) adsorption onto MSBEB
Table 3. Kinetic models for adsorption of Cr(VI) and Pb(II) ion onto MSBEB
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Kinetic model
Equation
Parameter
Adsorbate
Adsorbate
Cr(VI)
Pb(II)
Pseudo-first order
qe cal (mg/g)
0.0731
0.1370
qe exp (mg/g)
0.0258
0.0514
K1 (min-1)
0.1631
0.1910
SSE
2.23×10-3
0.0073
R2
0.951
0.932
ᵪ2
0.0306
0.0534
Pseudo-second order
qe cal (mg/g)
0.0311
0.4022
qe exp
0.0258
0.0514
K2 (g/mg min)
0.0537
0.0391
SSE
2.80×10-5
0.023
R2
0.999
0.991
ᵪ2
9.03×10-4
0.0058
Intraparticle diffusion
Kid (mg/gmin1/2)
0.0501
0.0207
C
0.0210
0.0306
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SSE
2.04×10-4
1.401
ᵪ2
3.1×10-3
2.0×10-2
R2
0.82618
0.738
3.5. Adsorption Thermodynamics study
Table 4 presents the thermodynamics analysis of the sorption of the selected metal ions onto
MBEB. By plotting log
against
and analyzing the resulting linear graph, the enthalpy (∆Ho),
and entropy (∆So) values were determined. The observation negative value for Gibbs free (∆Go)
during the adsorption process indicates that the sorption of hexavalent chromium and divalent
lead ions onto the sorbent is spontaneous and feasible. The adsorption process exhibited an
endothermic nature, as evidenced by positive value of (∆Ho). The positive entropy value (∆So)
signifies that the adsorption process results in an enhanced degree of freedom and randomness at
the adsorbent-adsorbate interface (Mustapha et al., 2023).
Table 4. Parameters for thermodynamics adsorption of Cr(VI) and Pb(II) ion onto MSBEB
Adsorbate
Temp (0K)
∆So(KJmol-1K-1)
∆Ho(kJmol-1)
∆Go(kJmol-1)
Cr(VI)
293
0.1109
42.052
-2.8403
303
-3.0924
313
-3.7262
323
-3.9221
Pb(II)
333
293
303
0.2047
86.33
-4.0197
-1.8702
-2.6048
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313
323
333
-2.9151
-3.3173
-3.7102
In Table 5, a comprehensive comparison is made between MSBEB adsorption capacities and
other reported adsorbents for selected metal ions removal from wastewater. The comparison
takes into account various optimum parameters. The data provided in the Table 5 illustrates that
MSBEB produced exhibited a superior maximum adsorption capacity for hexavalent chromium
and divalent lead ions having 238.12 mg/g and 175.02 mg/g compared to other adsorbent listed.
These findings lead to the conclusion that MSBEB demonstrated high removal efficiency as an
adsorbent for the removal metals from wastewater. The outcome of excellent removal efficiency
could be attributed to presence of CaCl2 as modifying agent during the process of adsorption.
Table 5: Literature review of maximum adsorption capacity comparison with other adsorbents
Adsorbent
Adsorption capacity
(mg/g)
Metal
References
Flamboyant pod activated carbon
16.13
34.48
Cr(VI)
Pb(VI)
Mustapha et al.,2017
Adsorption using c-phenylcalix
14.31
60.97
Cr(VI)
Pb(II)
Jumina et al., 2020
Cobalt ferrite supported activated
carbon
6.27
Cr(VI)
Muhammed et al.,
2020
Electro-coagulation
3.56
342.16
Cr(VI)
Pb(II)
Lokendra et al., 2023
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Orange peel activated
65.14
Pb(II)
Afolabi et al., 2021
Nitrogen enriched Chitosan
20.04
75.43
Cr(VI)
Pb(II)
Fatma et al.,2021
Ornge peel cellulose
Amide Biochar
Rice husk
Rutile Phase titanium nanoparticles
Modified bark eucalyptus
4.90
50.10
229.8
11.39
0.019
-
238.12
175.02
Cr(VI)
Pb(II)
Cr(VI)
Cr(VI)
Pb(III)
Pb(II)
Cr(VI)
Pb(II)
Rahman et al.,2023
Ashraf et al., 2023
Mitra et al., 2019
Mary et al, 2021
This Study
3.6. Response Surface methodology (RSM) for statistical modelling
3.6.1. Development of ANOVA (analysis of variance) model
The study investigated the combined effects of contact time, adsorbent dosage, and temperature
on the adsorption of Cr(VI) and Pb(II) ion using MSBEB materials. This investigation was
carried out using central composite design (CCD) methodology. The recorded experimental
responses can be found in Table S4. The results in Table S4 indicate that the highest percentage
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of Cr(VI) removal was (98.02%) and Pb(II) (92.73%) at run 7. This run involved a temperature
of 45°C, an adsorbent dosage of 0.1 g/L, and a contact time of 120 min. On the other hand, the
least metal adsorption was observed at run 20 for Cr(VI) (73.31%) and Pb(II) (66.97%). The
equations depicting the final empirical models for the selected metal ions removal by MSBEB
including the coded factors and insignificant terms, are presented in Eq. (8) and (9).
YCr(VI) = 90.83 +1.74A – 2.44B + 3.53C + 0.78AB – 3.54AC + 4.91BC + 1.49A2 -3.13 B2 +1.05
C2 (8)
YPb(II) = 75.15 +5.12A + 3.56B -1.666 - 4.70AB - 6.16AC + 4.94BC - 2.05A2 + 1.86B2 - 1. 42C2
(9)
To evaluate the model's significance and reliability, the ANOVA was utilized. The ANOVA
results indicated that the polynomial quadratic model was the optimal selection for both
responses. By dividing the sum of squares (SS) of each source of variation by the degrees of
freedom (df), the mean squares (MS) were computed in Tables S5 and S6. Terms within the
model were deemed significant if their corresponding p-values were less than 0.05. Conversely,
terms with p-values greater than 0.05 were considered insignificant. The study demonstrated p-
values of less than 0.0001, indicating both the validity and statistical significance of the model.
By employing CCD ANOVA, the P-value and F- value for sorption of the two selected metal
ions onto MSBEB were determined as 20.11, <0.0001, and 11.63, <0.0001, respectively. The
significance of the model was confirmed by the corresponding P-values and F-values calculated,
indicating that the quadratic model equation successfully accounted for the variation in
responses. In the Cr(VI) ion adsorption model using MSBEB, the terms A, B, C, AB, AC, BC,
B2, and C2 were found to be significant, whereas the term C2 was deemed insignificant. While
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the significant terms contributing to Pb(II) removal were similar to, where the term A2 and C2
was determined to be insignificant. Also, a higher R2 value of 0.9964 and 0.9818 for both metal
ions were obtained, indicating that the selected model can predict approximately 98.50% and
98.27% of the variation in response. Additionally, the model's reliability is indicated by the
consistency between the adjusted R2 and predicted R2 values for both metals, with a difference of
less than 0.2 in terms of the selected metal ions removal (Abbasi et al., 2023). The precision of
the model, as indicated by the adequate precision values, was 80.31 for Cr(VI) and 31.61 for
Pb(II). Furthermore, the coefficient of variation (CV) values were 0.49% for Cr(VI) and 1.98%
for Pb(II). The model selected exhibited high precision and notable reproducibility, as evidenced
by the adequate precision values of greater than 4.0 and the CV values of less than 10% obtained
for both composites (Jung et al., 2019).
3.6.2. Three dimensional plots: Impact of interacting variables.
The response surface plots were generated using the selected quadratic regression models to
depict the independent and interactive influences on the uptake of the two metals ions onto
MSBEB. Fig. 4 demonstrates the interactive impact of the investigated independent process
factors (time, adsorption dosage, and temp) on the uptake of Cr(VI) and Pb(II) ions by MSBEB.
In Fig. 5a, the interactive effect of adsorbent dosage and time on the removal uptake of Cr(VI)
ions is presented, focusing on a specific temperature of 35 0C. The observed trend in Fig. 4(a)
reveals a negative correlation between the removal efficiency and interaction of adsorbent
dosage and time. This observation signifies that a minimal quantity of adsorbent can adsorbed a
substantial amount of Cr (VI) ions, indicating high adsorption efficiency (Gorzin et al., 2018).
Fig. 4(b) demonstrates a positive correlation between the efficiency removal of Cr(VI) ions when
there is an increase in contact time. The plot in Fig. 4(b) also indicated that achieving maximum
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removal uptake of Cr(VI) ions required some amount of heat. Furthermore, Fig. 4(c) demonstrate
the combined influence of temp and adsorbent dosage on the removal efficiency of Cr(VI) ions
at a time of 65 min. The plot indicates that there is an upward trend in the removal efficiency as
the temperature increases, reaching its peak at 45 oC, this observation implies that the
temperature needed to reach adsorption equilibrium, where the removal efficiency is at its
highest, corresponds to the maximum value of the temperature while an opposite trend appears
between removal efficiency and adsorbent dosage.
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Fig. 4: 3D response plots for Cr(VI) (a-c): and Pb(II) (d-f). Impact of dosage and time,
temperature and time and temp and dosage.
Fig. 4(d) illustrates the graphical representation of the relationship between the dosage of the
adsorbent and the adsorption time with respect to the removal efficiency of Pb(II) ions. The
findings demonstrated a decline in the efficiency removal of Pb(II) ions as the time was raised,
whereas an elevation in adsorbent led to an enhancement in the removal efficiency of Pb(II) ions.
From 3D model graph, the decrease in removal efficiency of Pb(II) ions with increasing
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adsorbent dosage can be attributed to the presence of remaining active sites that are not fully
utilized during the adsorption process. The graphical representation in Fig. 4(e) demonstrates that
an upward trend exists between removal efficiency and interaction between temperature and
time. Initially, the rapid adsorption rate of Pb(II) ions on the adsorbent can be attributed to the
abundant availability of adsorption sites (Obayomi et al., 2023). The observed enhancement in
removal efficiency with rising temperature indicated the endothermic nature of Pb(II) ions
adsorption, emphasizing the increased affinity of the adsorbent for Pb(II) ions at higher
temperatures. Fig. 4(f) illustrates an increase in Pb(II) ions uptake as the adsorbent dosage is
increased. It can be inferred that the optimal adsorption of cadmium ions in the solution can be
achieved by using a dosage of 0.50 g/L MSBEB. This finding highlights the dominant influence
of adsorbent as the most critical parameters among the variables examined. Also, Fig. S4 depict
the plots of predicted vs actual of the two metals ion which it can be observed that there is robust
relationship between the observed and predicted values of Cr(VI) and Pb(II) ion efficiency
removal as obtained by Eq.(8) and (9). Furthermore, the results suggest that the quadratic models
selected was sufficient in predicting the response variables for the experimental data.
3.7. Optimization Studies
Achieving optimal efficiency removal of the two selected metal ions onto MSBEB is crucial as it
addresses both economic and environmental concerns. Nevertheless, the response to the
simultaneous maximization hexavalent chromium and divalent lead ions removal faces a
significant hurdle as each individual parameter exerts conflicting impacts. To ascertain the
suitability of the design, compatibility, and economic feasibility assessment, a process of
numerical optimization was implemented employing (Design Expert 13.0) statistical tools.
Within the desirability function, responses are assigned values ranging from 0 to 1, wherein a
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value of "0" indicates an undesirable outcome, while a value of "1" represents a preferred or
optimal response. To determine the optimal conditions, the outcome of independent variables
was defined as being "within the range of experimental limits." Meanwhile, maximization of
both output variables was set as the fundamental objective. The obtained results indicated that
the sorption of the selected metal ions onto MSBEB adsorbent achieved removal rates of 98.02%
and 92.73%, while the predicted values were 97.82% and 91.7%. The examination of the
experimental and predicted findings exhibited a discrepancy of less than 0.2%. Furthermore,
Table S7 shows the results were attained through the Utilization of optimum conditions, which
involved a temperature of 45.20 oC, an adsorbent dosage of 0.12 g/L, and a contact time of 120.5
min. These specific parameters were instrumental in achieving the reported outcomes.
3.8. Recoverability and reusability studies
The plots Fig. 5 depicts the results observed for the MSBEB adsorbent across five adsorption-
desorption processes conducted under optimum conditions. It reveals a consistent downward
trend, indicating a decline in adsorption capacity with each cycle. Subsequent to the fifth
generated adsorbent, there was a notable decrease in the removal percentage dropping from
89.79% to 38.60% for Cr(VI) and the same trend for Pb(II) ion with 86.01% to 52.39%
respectively. The results clearly indicated that MSBEB served as an effective adsorbent,
demonstrating its capability to be utilized and regenerated for a five minimum cycles.
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Fig. 5. Adsorption- Desorption efficiency plots for (a) Cr(VI) and (b) Pb(II) adsorption on
MSBEB
4.0 Conclusion
In a world grappling with increasing water pollution and environmental degradation, the study of
wastewater treatment emerges as a beacon of hope. An investigation was conducted to assess
MSBEB effectiveness in removing hexavalent chromium and divalent lead ions from
wastewater. The effective utilization of modified biochar obtained from bark eucalyptus as an
adsorbent exhibited good performance in removing Cr(VI) and Pb(II) from aqueous solutions. At
operating 0.12 g/L of MBEB, 120.5 min contact time, and a temperature value of 45.20 oC, the
adsorbent demonstrated an exceptional capability to adsorb 98.02% of Cr(VI) and 92.73% of
Pb(II) ions. SEM and FTIR analyses revealed notable structural changes in the adsorbent and
distinctive alterations in spectral peaks following the modification of the bark eucalyptus
biochar. Maximum uptake capacity for Cr(VI) and Pb(II) ions were 238.12 and 175.02 mg/g
respectively. The adsorbent equilibrium data exhibited a strong correspondence with Freundlich
model indicating surface pores heterogeneity. From the analysis conducted on adsorption
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kinetics and it was confirmed that pseudo-second order model fit well to the experimental data.
Also, the thermodynamic parameters indicated an endothermic, spontaneity and feasible reaction
process. Finally, the adsorption-desorption study revealed the adsorptive efficiency of the
adsorbent after reuse.
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Author Contributions
Lukman Shehu Mustapha: Conceptualization, methodology, and writing original draft
preparation. Oluwatobi Victoria Obayomi: Methodology, and writing original draft preparation
Muibat Diekola Yahya: Supervision, Reviewing and Editing. Sie Yon Lau: Reviewing and
Editing. Obayomi Kehinde Shola: Reviewing and Editing.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐ The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
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Graphical abstract
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Highlights
Novel synthesis of eco-friendly modified biochar from eucalyptus bark.
Various techniques such as FTIR, SEM, BET and XRD was utilized
Optimal conditions are 45.20 oC, 0.12 g/L, and 120.5 min.
Pseudo–second order kinetic and Freundlich isotherm fit the adsorption process.
The reusability of the adsorbent was confirmed.
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