Investigation of the removal of Ni(II) from aqueous solution using pomelo fruit peel

Physical sciences | Chemistry
Vietnam Journal of Science,
Technology and Engineering 7June 2021 • Volume 63 number 2
Introduction
In recent years, the expansion of many industries has 
promote a huge increase in the economy of a large number 
of developing countries. However, the governments in these 
countries are faced with significant environmental problems 
especially those related to heavy toxic metal pollution in 
the effluent of industrial zones. Ni(II) is one such heavy 
toxic metal, which has existed in the wastewater of many 
factories such as electroplating, mineral processing, 
batteries manufacturing, and so on [1, 2]. As claimed by 
the World Health Organization (WHO), the limit of Ni(II) 
concentration in water is 0.005 kg/m3 [2]. Hence, various 
physicochemical methods have been applied to eliminate 
Ni(II) from aqueous solutions including adsorption [1-4], 
precipitation [5, 6], ion-exchange [7, 8] and so on. Among 
them, adsorption is a promising method since it is simple, 
low-cost, and easily reused [9, 10].
The use of agricultural waste as biosorbents has attracted 
many scientists because they are abundantly available, 
environmentally friendly, and low cost. There are many 
biosorbents used to remove Ni(II) from aqueous solutions 
including Sophora japonica pod powder [11], Sargassum 
sp. [12], activated banana peel [13], modified plantain 
peel [14], and Citrus reticulata (fruit peel of orange) [15]. 
However, the utilisation of pomelo fruit peel (Citrus grandis) 
as a biosorbent to remove Ni(II) from aqueous solutions 
has been limited. In previous reports, the pomelo fruit peel 
was used to adsorb methylene blue [16], Cr(III) [16], Pb(II) 
[17], and Cd(II) [17]. The obtained results indicated that 
the pomelo fruit peel is a potential biosorbent to uptake 
heavy toxic metals and organic molecules from aqueous 
solutions. Therefore, in this work, the study is extended to 
Ni(II) adsorption onto the pomelo fruit peel. The pHsolution, 
adsorption time, and initial Ni(II) concentration, all of which 
affect the Ni(II) adsorption, are examined. Some common 
isotherm and kinetic models are fit to the experimental data 
to understand the nature of the uptake. 
Materials and methods
Preparation of biosorbent 
The biosorbent was prepared identical to the author’s 
previous studies [17]. Herein, the pomelo fruit peel was 
washed by deionised water several times after collection 
from the Vinh Cuu district, Dong Nai province, Vietnam. 
The material was then dried in an oven at 80oC within 24 
h, prior to cutting into small pieces about 0.5-1 mm in size. 
Finally, the biosorbent was stored in the oven. 
Investigation of the removal of Ni(II) 
from aqueous solution using pomelo fruit peel
Van phuc Dinh*
Duy Tan University
Received 8 September 2020; accepted 4 December 2020
*Email: dinhvanphuc@duytan.edu.vn
Abstract:
Pomelo fruit peel, an organic waste, was utilised as a biosorbent to remove Ni(II) from aqueous solutions. Some 
major factors influencing Ni(II) uptake such as pH, adsorption time, and initial Ni(II) concentration were examined. 
Several isotherm and kinetic models including the Langmuir, Freundlich, Sips, pseudo-first-order, pseudo-second-
order, and intra-diffusion models were fit to the experimental data. Results showed that the Ni(II) uptake obtained 
an equilibrium at pH=6 after 80 min at 303 K. The Sips isotherm model described the Ni(II) adsorption better than 
other models and the monoadsorption capacity calculated from the Langmuir model was 9.67 mg/g. The adsorption 
of Ni(II) followed pseudo-second-order kinetic models with three stages. 
Keywords: biosorption, isotherm models, Ni(II), pomelo fruit peel.
Classification number: 2.2
DOI: 10.31276/VJSTE.63(2).07-12
Physical sciences | Chemistry
Vietnam Journal of Science,
Technology and Engineering8 June 2021 • Volume 63 number 2
Chemicals
The Ni (II) ion was used as an adsorbate, which was 
prepared by dissolving a Ni(II) standard (1000 mg/l) in 
deionised (DI) water. The pH adjustment of the investigated 
solution was carried by using HNO3 and NaOH with 
different concentrations. All experimental chemicals used 
in this work were from Merck (Germany) and were in the 
analytical reagent grade.
Instruments
The pH meter (Martini instruments, Mi-15, Romania), 
with buffer solution values of 4.01±0.01, 7.01±0.01, and 
10.01±0.01, was used to determine the pHsolution values. 
The material’s morphology was examined by ultrahigh 
resolution SEM (S-4800), whereas the bonding in the 
materials’ structure was found out by Fourier-transform 
infrared (FT-IR) spectroscopy that was conducted on a 
Tensor 27 (Bruker, Germany).
In order to determine the Ni(II) concentration before and 
after the uptake, an atomic absorption spectrophotometer 
(Shimadzu AA-7000, Japan) was used.
Batch adsorption study
The Ni(II) batch adsorption onto the pomelo fruit peel 
was carried on IKA magnetic stirrers with a RT 10 P heater. 
Herein, 0.1 g of the synthesised material was placed into 
100 ml flasks together with 50 ml of Ni(II) aqueous solution. 
These flasks were stirred at a constant rate of 150 rpm. The 
factors affecting the uptake including pH (2-6), adsorption 
time (10-240 min), and Ni(II) initial concentration (5-50 
mg/l) were examined. 
The percentage of the Ni(II) uptake (% removal) and 
adsorption capacity, Qe, (mg/g) were determined based on 
the following equations:
o e
o
(C -C )% Removal = .100% ,
C
 (1)
o e
e
(C -C ).VQ = ,
m
 (2) 
where the Ni(II) concentration in the aqueous solution 
before and after the adsorption are symbolised Co (mg/l) and 
Ce (mg/l), respectively, V is the volume (l) of metal solution, 
and m is the mass (g) of the material used.
Adsorption isotherm and kinetic models
In this report, some common adsorption isotherm and 
kinetic models are fit to the experimental data [17, 18 ... d 1B show SEM images 
of pomelo fruit peel at 1.00k and 10.0k magnifications. As 
seen in these images, the adsorbent surface is very rough, 
porous, and heterogeneous. These properties are favourable 
for the heavy metal ion adsorption. The elemental 
composition of this material was determined by energy-
dispersive X-ray spectroscopy (EDX), which is presented 
in Fig. 1C. The results confirm that the weight percentages 
of carbon and oxygen were 47.41 and 52.59%, respectively. 
Point of zero charge (pHPZC): pHPZC is the pH value of 
the solution when the material’s surface charge is neutral. 
Indeed, if pHsolution is less than pHPZC, the material surface is 
positively charged. In contrast, the material’s surface charge 
is negative when pHsolution>pHPZC. Fig. 1D presents the pHPZC 
of the pomelo fruit peel in this study, which was determined 
to be 4.6. 
FT-IR spectrum: Fig. 2 depicts the vibrations of 
characteristic groups in the pomelo fruit peel. As seen in this 
figure, the vibrations of the O-H groups of pectin, cellulose, 
and lignin are recorded at 3246 cm-1, while the vibrations of 
the C-H bonds in the CH2 and CH3 groups are assigned to 
wavenumbers 2924 cm-1 and 2851 cm-1, respectively. The 
wavenumbers 1747 cm-1 and 1643 cm-1 are related to the 
C=O groups [19]. Finally, the wavenumbers 1107 cm-1 and 
1026 cm-1 confirm the C-O group’s stretching vibrations in 
the lignin structure of pomelo fruit peel [16].
Physical sciences | Chemistry
Vietnam Journal of Science,
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Fig. 2. FT-IR spectrum of pomelo fruit peel.
Factors affecting the removal of Ni(II)
pHsolution: pHsolution directly affects the removal of Ni(II) 
due to its effects on the formation of different complexes of 
Ni(II) and the surface charge of materials. Fig. 3A indicates 
that the uptake of Ni(II) rises rapidly when pHsolution is 
increased from 2 to 4. In the next stage, there is a slight 
increase in the adsorption prior to obtaining the maximum 
at pH=6. The increase in pHsolution from 2 to 6 leads to a 
change in material surface charge from positive to negative. 
At pHsolution>pHPZC=4.6, the material’s surface charge is 
negative, which leads to a rise in Ni(II) adsorption due to 
the electrostatic attraction between Ni(II) cations and the 
negatively-charged material surface [20, 21]. However, the 
author observed that nickel (II) hydroxide can be formed 
at pHsolution>6. Therefore, pH=6 is chosen for further 
experiments.
The adsorption time: the influence of the adsorption 
time on the Ni(II) biosorption by pomelo fruit peel is 
indicated in Fig. 3B. The uptake rate of Ni(II) significantly 
increases prior to reaching equilibrium at 80 min and then 
remained stable. Therefore, the optimal adsorption time was 
determined to be 80 min.
Fig. 1. (A, B) SEM images at different magnifications, (C) the EDX spectrum, and (D) pHPZC of the pomelo fruit peel.
Physical sciences | Chemistry
Vietnam Journal of Science,
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Isotherm studies
The plots of several common isotherm models including 
Langmuir, Freundlich, and the Sips models are presented in 
Fig. 4A. The nonlinear isotherm parameters of these models 
are listed in Table 2. According to calculated RMSE and 
χ2 values, the experimental data had a better fit with the 
Sips model than the others as determined by the smallest 
RMSE and χ2 values. The main reason is that the Langmuir 
and Freundlich models are constrained by the adsorbates’ 
concentration, while the Sips model combines these 
models and overcomes this problem [18]. Furthermore, the 
Langmuir maximum monolayer adsorption capacity was 
9.67 mg/g, which is higher than other biosorbents such as 
hazelnut shell, fly ash, rice husk, banana peel, and doum 
palm (Hyphaene thebaica L.) (Table 3). The n value (n=2.67) 
evaluated from the Freundlich model ranges from 1 to 10 
and indicates how favourable conditions are for adsorption 
[18, 22]. However, the Ni(II) adsorption capacity is lower 
than Pb(II), Cd(II), and Cr(III) when the same pomelo fruit 
peel is used [16, 17]. This shows that the pomelo fruit peel 
is a potential material for removing heavy metals from 
aqueous solutions. 
Table 2. Parameters of nonlinear isotherm models at temperature 
of 303 K.
Isotherm models parameters
Langmuir
KL (l/mg) 0.1891
Qm (mg/g) 9.67
RMSE 0.2625
R2 0.9854
c2 0.1413
Freundlich
n 2.67
KF [(mg/g).(l/mg)1/n] 2.48
RMSE 0.3752
R2 0.9701
c2 0.2519
Sips
Qs (l/g) 2.25
as (l/mg) 0.1938
bs 0.7667
RMSE 0.1975
R2 0.9917
c2 0.0428
Fig. 3. Plots of the effects of (A) pHsolution and (B) adsorption time on Ni(II) adsorption.
Fig. 4. Plots of (A) isotherm models and (B) kinetic models of the Ni(II) adsorption onto pomelo fruit peel.
Physical sciences | Chemistry
Vietnam Journal of Science,
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Table 3. Maximum adsorption capacities of several biosorbents 
for the Ni(II) uptake from aqueous solutions [23-27].
Biosorbents Adsorptive condition
Adsorption 
capacity (mg/g)
References
Doum palm (Hyphaene 
thebaica L.) pH=7.00, t=120 min 3.24 [1]
Banana peel pH=6.89, t=24 h 6.88 [23]
Rice husk pH=6.00, t=120 min 8.86 [24]
Fly ash pH=8.00, t=60 min 0.03 [25]
Hazelnut shell pH=7.00, t=180 min 7.18 [26]
Cone biomass of Thuja 
orientalis pH=4.00, t=7 min 12.42 [27]
Brown algae 
Sargassum sp. pH=6.00, t=90 min 50.97 [12]
Pomelo fruit peel pH=6.00, t=80 min 9.67 This study
Kinetic studies
Figure 4B and Table 4 present the plots of the kinetic 
models and non-linear parameters, respectively. Clearly, the 
pseudo-second-model fit to the experimental data is better 
than the pseudo-first-order model owing to the small RMSE 
and c2 values. However, both models cannot describe the 
mass transfer of cations onto the material’s surface. The 
intra-diffusion model is therefore applied to determine the 
Ni(II) adsorption kinetic onto pomelo fruit peel. As seen 
from the plot of Qe versus t1/2 in Fig. 4B, the removal of 
Ni(II) includes three stages. Firstly, Ni(II) cations are steeply 
transferred from the solution to the material’s surface within 
about 20 min. In the next stage, the Ni(II) uptake more 
gradually occurs from 20 to 80 min, prior to obtaining the 
equilibrium in the last stage. From the nonzero C value 
calculated from the intra-diffusion model, the Ni(II) uptake 
follows not only the intra-diffusion process but also two or 
more different mechanisms [28, 29].
Conclusions
The Ni(II) adsorption onto pomelo fruit peel was 
investigated. The results showed that the Ni(II) uptake 
reached equilibrium at pH=6.00 after 80 min at 303 K. 
Kinetic studies showed that the Ni(II) uptake was controlled 
by various mechanisms. The Langmuir maximum adsorption 
capacity was 9.67 mg/g, which was higher than some other 
biosorbents. Therefore, pomelo fruit peel can be used as a 
promising, eco-friendly, and low-cost material to eliminate 
Ni(II) from the effluent. 
ACKNOWLEDGEMENTS
This research is funded by Vietnam National Foundation 
for Science and Technology Development (NAFOSTED) 
under grant number 103.02-2018.368.
COMPETING INTERESTS 
The author declares that there is no conflict of interest 
regarding the publication of this article. 
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