la courbe d'étalonnage peut être configurée en mesurant ou en entrant jusqu'à 10 étalons ou en entrant K et B facteurs

1. Citation: Boutaleb, Y.; Zerdoum, R.;
Bensid, N.; Abumousa, R.A.; Hattab,
Z.; Bououdina, M. Adsorption of
Cr(VI) by Mesoporous Pomegranate
Peel Biowaste from Synthetic
Wastewater under Dynamic Mode.
Water 2022, 14, 3885. https://
doi.org/10.3390/w14233885
Academic Editor: Dipti
Prakash Mohapatra
Received: 29 October 2022
Accepted: 22 November 2022
Published: 28 November 2022
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4.0/).
water
Article
Adsorption of Cr(VI) by Mesoporous Pomegranate Peel
Biowaste from Synthetic Wastewater under Dynamic Mode
Yassira Boutaleb 1, Radia Zerdoum 2, Nadia Bensid 1, Rasha A. Abumousa 3,* , Zhour Hattab 1
and Mohamed Bououdina 3
1 Laboratory of Water Treatment and Valorisation of Industrial Wastes, Department of Chemistry, Faculty of
Sciences, Badji Mokhtar University, B.P.12, Annaba 23000, Algeria
2 Science and Technology Laboratory of Water and Environment, Department of Material Sciences, Faculty of
Science and Technology, University Mohammed-Cherif Messadia, Souk Ahras 41000, Algeria
3 Department of Mathematics and Sciences, College of Humanities and Sciences, Prince Sultan University,
Riyadh 11586, Saudi Arabia
* Correspondence: rabumousa@psu.edu.sa
Abstract: This study aims to eliminate hexavalent chromium Cr(VI) ions from water using pomegranate
peel (PGP) powder. Dynamic measurements are carried out to examine the influence of the operating
factors on the adsorption efficiency and kinetics. The analyzed PGP is found to be amorphous with
relatively high stability, contains hydroxyl and carboxyl functional groups, a pH of zero charge of
3.9, and a specific surface-area of 40.38 m2/g. Adsorption tests indicate that PGP exhibits excel-
lent removal effectiveness for Cr(VI) reaching 50.32 mg/g while the adsorption process obeys the
Freundlich model. The thermodynamic study favors the exothermic physical adsorption process.
The influence of operating parameters like the flow rate (1 to 3 mL/min), bed height (25 to 75 mm),
concentration (10 to 30 mg/L), and temperature (298 to 318 K) on the adsorption process are investi-
gated in column mode. To assess the performance characteristics of the column adsorption data, a
non-linear regression has been used to fit and analyze four different kinetic and theoretical models,
namely, Bohart-Adams, Thomas model, Clark, and Dose response. The obtained experimental results
were found to obey the Dose Response model with a coefficient of regression R2 greater than 0.977.
This study proved the excellent efficiency in the treatment of chemical industry effluents by using
cost-effect abundant biowaste sorbent. This research demonstrated great efficacy in the treatment of
chemical industrial effluents by using an abundant, cost-effective biowaste sorbent, thereby achieving
the UN SDGs (UN Sustainable Development Goals) primary objective.
Keywords: pomegranate peel; adsorption; fixed bed; batch; modeling
1. Introduction
The contamination of wastewater by chromium Cr(VI) is a major problem due to its
high toxicity and harmful effects on humans and animals [1]. The major effluents containing
such toxic and hazardous element arises from industrial processes such as metallurgy,
dyeing, and finishing industries [2]. Chromium exists in two major forms: Cr(III) and
Cr(VI). The first form has low solubility and low reactivity resulting in low mobility in the
environment and low toxicity in living organisms. However, Cr(VI) is a highly toxic [3]
and carcinogenic [4] heavy metal when its concentration exceeds the standard limit fixed
at 0.05 mg/L [5]. Its compounds include chromate (CrO4
2−), dichromate (Cr2O7
2−) and
chromic acid (H2CrO4). CrO4
2− is predominant in basic solutions, H2CrO4 is predominant
at pH < 1, while HCrO4
2−, and Cr2O7
2− are predominant at pH 2–6 [6].
Extensive studies were devoted toward the development of new cost-effective tech-
nologies for the removal of Cr(VI) metal ions from aquatic environments, like precipita-
tion [7], ion exchange [8], electro-coagulation [9], and adsorption [10–14]. Nevertheless,
most of the technique’s present drawbacks and disadvantages, primarily high energy
Water 2022, 14, 3885. https://doi.org/10.3390/w14233885 https://www.mdpi.com/journal/water

2. Water 2022, 14, 3885 2 of 20
consumption and cost. In contrast, adsorption is an eco-friendly, cost-effective, and widely
used technique for heavy metals remediation from aqueous solutions, which meets the UN
SDGs (UN Sustainable Development Goals) main aim [15–17].
In recent decades, green chemistry has emerged as a potential fabrication route and
attracted considerable attention in scientific research. Specifically, fruit wastes are being
investigated as potential natural sorbents for heavy metal ions, thanks to their biodegrad-
ability and cost-effectiveness, besides the presence of functional groups on adsorbent
particles’ surface that are responsible for the reduction and elimination of contaminants
and pollutants from aqueous media within a ‘sustainable development’ framework [18,19].
Valency, concentration, the ionic nature of substances in solution, pH of solution, and
binding groups of sorbent material are key sorption parameters. The removal mechanism
relies on the sorbate molecule’s size, concentration, affinity for the sorbent, bulk diffusion
coefficient, and pore size distribution [20,21]. Sorbent’s physicochemical characteristics and
sorption capacity determine the sorption’s efficacy.
Pomegranates (Punica granatum) are widely cultivated in north/tropical Africa, and
the Middle East, the Mediterranean Basin, the Caucasus region, the Indian subcontinent,
Central Asia, and the drier parts of Southeast Asia [22].
Punica granatum presents a great variety of therapeutic bioactive species such as
ellagitannins, proanthocyanidin compounds, phenolics, and flavonoids [23], which are
well-known to facilitate the adsorption process of metal ions. Several studies have been
conducted to highlight the biological and functional characteristics of pomegranate peel
as it contains large amounts of polyphenols such as ellagic acid, gallic acid, and tan-
nin acid [24]. Also, it possesses two important hydroxycinnamic acids and derivatives
of flavones [25]. It has been widely studied as an antioxidant [26], antimicrobial [27],
preparation of nanoparticles [28], anticancer activities [24], and extraction of bioactive
compounds [29]. Additionally, several works have proposed the use of low-cost wastes for
water and effluent treatment [30], particularly for the removal and biosorption of dyes [31],
uranium [32], and heavy toxic metal ions especially chromium [33,34].
Extensive studies were performed on the effectiveness of biomass and its regenera-
tion for Cr(VI) ions removal using pomegranates peel. Salam and Narayanan (2019) [19]
reported a maximum removal of 100% achieved in 3 min for a low concentration of 20
mg/L and an amount of biosorbent (RPP with 125 µm size) of 0.5 g/L. Yi et al. (2017) [35]
studied a modified Litchi peel as a biosorbent to eliminate Cr(VI), but the achieved capacity
is relatively low reaching only 7.05 mg/g for a longer contact time of 100 min at 303 K for a
concentration of 30 mg/g and an amount of biosorbent was 8 g/L. Rafiaee et al. (2020) [36]
used pomegranate peel (PGP) adsorbent powder functionalized by polymeric coatings
such as polypyrrole (PPy) and polyaniline (PANI) to reduce/remove Cr(VI)metal ions from
wastewater. The highest achieved capacity was 59.47% in 90 min obtained in the batch sys-
tem for 50 mg/L and biosorbent dose of 10 g/L. A new composite Anthracite/pomegranate
peel synthesized by Seliem et al. exhibited a relatively high efficiency of 275 mg/g but for a
longer time of 120 min at 25 ◦C and pH 3, the concentration of 70 mg/L and an amount of
biosorbent 0.05 g.
In this study, pomegranate peel (PGP) adsorbent has been prepared by low-cost
and eco-friendly physical and chemical processes. Structural, morphological, and physic-
ochemical studies are investigated. This research aims to study the surface chemistry
characteristics and evaluate the performance of this biomaterial for the elimination of toxic
and hazardous heavy metal ions. Indeed, particular emphasis is devoted to investigate the
adsorption of hexavalent chromium ions Cr(VI) from an aqueous solution under dynamic
mode rather than batch. Several operating parameters are examined, including the flow
rate, the solution pH, adsorbent dose, contact time, bed height, as well as the solution
temperature. To confirm the experimental data, several models have been applied for both
batch and dynamic measurements, subsequently a plausible mechanism is proposed.

3. Water 2022, 14, 3885 3 of 20
2. Materials and Methods
The pomegranate (Punica granatum) peels (PGP) were collected from fresh juice ven-
dors at a local market. To remove the impurities, the collected peels were thoroughly
washed with tap water and rinsed using distilled water. Afterward, the obtained PGP
product was dried at 100 ◦C for 24 h, grinded into a powdered sample then sieved to a
particle size <500 µm.
The used chemicals were purchased from Merck Company. A stock solution of
1000 mg/L was prepared using potassium dichromate K2Cr2O7, then different concentra-
tions of chromium solution were obtained by consecutive dilution. Adsorption experi-
ments in batch and dynamic modes were performed using a UV-vis spectrophotometer
JENWAL 7315 (England) at maximum absorption of 540 nm after complexation with
1,5-diphenylcarbazide.
2.1. Characterization of the Powdered PGP
To evaluate the morphology and chemical composition of PGP powder, a Quanta 200
FEI scanning electron microscope (SEM) coupled with energy-dispersive X-ray spectroscopy
(EDX) was used. Fourier-transform infrared (FTIR) spectroscopy was used to identify
the functional groups of PGP. FTIR spectrometer (IR Affinity-1S) equipped with a single
reflection ATR, operating in the wave number range 400–4000 cm−1 acquired over 50 scans
with a resolution of 4 cm−1. The phase stability of PGP was checked by X-ray diffraction
(XRD) using a Rigaku Ultima IV instrument equipped with a CPS detector and CuKα
radiation (λ = 1.5418 Å). A Thermogravimetric analyzer TGA-2050 equipment was used to
analyze the thermal degradation profile of the PGP adsorbent up to 600 ◦C with a heating
rate of 10 ◦C/min. The PGP the Brunauer Emmett Teller (BET) specific surface area and the
average pore size were computed by recording Nitrogen adsorption-desorption curves at
70 K using a Quanta chrome NOVA 2200 E BET analyzer.
2.2. Biosorption of Cr(VI) on PGP—Batch and Dynamic Experiments
2.2.1. Batch Studies
The adsorption isotherms were performed at varying Cr(IV) concentrations and the
following operating conditions: m = 1.0 g/L, pH = 2 ± 0.2, T = 20 ◦C, stirring speed 50 rpm,
contact time 2 h. After equilibrium, the adsorbent was separated from the suspension by a
membrane filter (0.45 µm).
The concentration of PGP was determined by analyzing the supernatant. The amount
of the adsorbate, qe (mg/g), was estimated using Equation (1):
qe =
C0 − Ce
m
× V (1)
where C0/Ce represents the initial/equilibrium concentrations of Cr (VI) (mg/L), V is the
solution volume (L), and m is the PGP sorbent mass(g).
Langmuir (Equation (2)) and Freundlich (Equation (3)) models were used to fit the
equilibrium experimental adsorption data by means of the following expressions:
qe =
qmKLCe
1 + KLCe
(2)
qe = KfCe
1
n (3)
where qe is the amount of Cr(VI) adsorbed onto PGP at equilibrium (mg/g), Ce is the Cr(VI)
concentration in solution (mg/g), qm is the PGP capacity of the adsorbed Cr(VI) expressed
per unit mass of adsorbent (mg/g), KL is the Langmuir constant (L/mg), while Kf and n
correspond to Freundlich adsorption constant (mg 1−1/n L1/n) and intensity, respectively.

4. Water 2022, 14, 3885 4 of 20
2.2.2. Dynamic Studies
The tests were conducted in dynamic mode. The performance of a fixed column was
investigated using breakthrough curves, presented by the concentrations of pollutants vs.
the time profile curve in a fixed bed column.
Packed-bed adsorption experiments were carried out in a cylindrical glass column
(height = 35.0 cm, internal diameter = 11.0 cm) at 20 ◦C and an initial pH of 2 ± 0.2. Em-
ploying a peristaltic pump (ISMATEC A39494), the PGP dose (0.105, 0.210, and 0.315 g) cor-
responding to (25, 50, and 75 mm) bed heights and varying flow rates (1, 2, and 3 mL/min)
were used for the removal of Cr(VI) (10, 20, and 30 mg/L). Using the breakthrough curves
(breakthrough time—tb and exhaustion time—te), it is possible to obtain various informa-
tion about the system such as the treated effluent volume (Veff in mL; Equation (4)), the
total quantity of the adsorbed Cr(VI) (qtot in mg; Equation (5)), the amount of adsorbate that
passes through the glass column(Wtotal in mg; Equation (6)), the percentage of adsorbate
uptake(R in%; Equation (7)), and the uptake (qeq(exp) in mg/g; Equation (8)):
Veff = Fte (4)
qtot =
FA
1000
=
F
1000
Z t=total
t=0
(C0 − Ct)dt (5)
Wtot =
C0Dttotal
1000
(6)
R% =
qtotal
Wtotal
× 100 (7)
qeq(exp) =
qtotal
m
(8)
where F (mL/min) represents the flow rate, te (in min) the exhaustion time, C0 (mg/L) the
initial adsorbate concentration, Ct (mg/L) the column exit concentration of Cr(VI) at time t,
and m (g) the biosorbent quantity.
The area under the breakthrough curve determined by integrating the Cad vs. t
curve will be used to calculate the total amount of PGP adsorbed (maximum column
capacity) [37]. The flow rate indicates the column’s empty bed contact time (EBCT), as
given by Equation (9):
EBCT =
Bed Volume (mL)
Flow Rate(mL/min)
(9)
Herein, it is important to mention that once the PGP sorbent becomes saturated, the
fixed bed operation was stopped. The adsorption experiments were repeated at least in
two replicates in order to estimate the error bars of the measurements.
2.3. Biosorption Modeling in Continuous Systems
The Bohart-Adams (BA), Clark, modified Dose response, and Thomas models were
adopted to characterize the breakthrough curves obtained in the continuous biosorption
studies. All related information is reported in Table 1.

5. Water 2022, 14, 3885 5 of 20
Table 1. The models used for modelling the adsorption data.
Model Equation and Corresponding Plot Parameters References
Bohart-Adams
Ct
C0
= exp
KBA C0t − KBA N0
Z
U
Ct
C0
vs. t
(10)
Ct
C0
: final adsorbent concentration
initial adsorbent concentration
KBA (L/mg.min): sorption rate coefficient
N0 (mg/L): fixed bed sorption capacity per unit volume
Z(mm): bed–depth in the column
U (mm/min): linear velocity,
t(min): contact time
[38]
Thomas
Ct
C0
= 1
1+exp
Kthqthm
D −kthC0t
Ct
C0
vs. t
(11)
Ct
C0
: final adsorbent concentration
initial adsorbent concentration
m(g): adsorbent mass
Kth (L/mg.min): kinetic constant of Thomas model
qth (mg/g): adsorption capacity
t (min): contact time
[39]
Modified Dose Response
Ct
C0
= 1 − 1
1+
Veff
b
a
q0 = b C0
m
Ct
C0
vs. t
(12) Ct
C0
: final adsorbent concentration
initial adsorbent concentration
Veff (mL): volume of the effluent
q0 (mg/g): adsorption capacity
a and b 6= 0: constant values in Dose Response model
m (g): adsorbent mass
[40]
(13)
Clarck
Ct
C0
=
1
1+Ae−rt
( 1
n−1 )
Ct
C0
vs. t
(14)
A and r (min−1): constant of the Clarck model.
t(min): contact time
n: constant value of Freundlich model
[41]

6. Water 2022, 14, 3885 6 of 20
3. Results and Discussion
3.1. Characterizations
The SEM images of PGP powder at the magnification (×10,000), as shown in Figure 1a,b,
manifest flat and smooth surface texture of the PGP particles before Cr(VI) uptake. The
corresponding elemental composition as determined by EDX reveals the main constituent’s
carbon and oxygen with minor amounts of Al, Si, Cl, P, K, Ca, Mg, Cu, and Fe, see
Figure 1d. This indicates that the two major elements forming PGP are carbon (51.40%) and
oxygen (46.21%).
Water 2022, 14, x FOR PEER REVIEW 6 of 24
3. Results and Discussion
3.1. Characterizations
The SEM images of PGP powder at the magnification (×10,000), as shown in Figure
1a,b, manifest flat and smooth surface texture of the PGP particles before Cr(VI) uptake.
The corresponding elemental composition as determined by EDX reveals the main con-
stituent’s carbon and oxygen with minor amounts of Al, Si, Cl, P, K, Ca, Mg, Cu, and Fe,
see Figure 1d. This indicates that the two major elements forming PGP are carbon (51.40%)
and oxygen (46.21%).
Figure 1. SEM images (a,b) before and (c) after Cr (VI) adsorption; EDX spectra (d) before and (e)
after adsorption by PGP.
The TGA analysis has been carried out to investigate the thermal properties of PGP,
see Figure 2A. The mass loss reflects three main stages. The first stage (30–100 °C) corre-
sponds to a weight loss of 10 wt.% owing to the evaporation of physisorbed water onto
PGP. The second stage (150–250 °C) demonstrates a weight loss of 45 wt.% and is associ-
ated with the thermal decomposition of hemicellulose and pectin [25]. While the third
stage (250–600 °C) manifests an exothermic peak with a considerable weight loss of 30
wt.%, most probably originating from the separation of lignin and decomposition of the
cellulose material [42,43]. The total mass loss at 600 °C is extremely high reaching 85 wt.%,
hence confirming the organic nature of the PGP.
Figure 1. SEM images (a,b) before and (c) after Cr (VI) adsorption; EDX spectra (d) before and (e); after
adsorption by PGP.
The TGA analysis has been carried out to investigate the thermal properties of PGP, see
Figure 2A. The mass loss reflects three main stages. The first stage (30–100 ◦C) corresponds
to a weight loss of 10 wt.% owing to the evaporation of physisorbed water onto PGP. The
second stage (150–250 ◦C) demonstrates a weight loss of 45 wt.% and is associated with the
thermal decomposition of hemicellulose and pectin [25]. While the third stage (250–600 ◦C)
manifests an exothermic peak with a considerable weight loss of 30 wt.%, most probably
originating from the separation of lignin and decomposition of the cellulose material [42,43].
The total mass loss at 600 ◦C is extremely high reaching 85 wt.%, hence confirming the
organic nature of the PGP.

7. Water 2022, 14, 3885 7 of 20
Water 2022, 14, x FOR PEER REVIEW 7 of 24
Figure 2. (A) TGA/DTA curves; (B) X−ray diffraction pattern; (C) FTIR spectra before and after ad-
sorption; (D) N2 adsorption−desorption isotherms of PGP.
The crystalline structure of PGP has been investigated by X−ray diffraction. As
shown in Figure 2B, no characteristic pattern of crystalline phase(s) can be observed while
a broad halo appears in the 2θ range (10−50°). This reflects the amorphous structure of
PGP powder, in good agreement with the literature [44].
Figure 2C displays the FTIR spectra of PGP powder before and after Cr(VI) absorp-
tion. The bands in the range 2800–3420 cm−1 are ascribed to −OH of carboxylic acid found
in the benzene compound [28]. The bands located at 2850 and 2930 cm−1 correspond to
C−H aliphatic (CH3) or (−CH2), while the band at 1593 cm−1 is assigned to the aromatic
rings’ stretching of phenolic compounds (C−C and C=C aromatic vibrations). In addition,
hydroxyl functional groups are defined by the absorption peak located at 1480 cm−1 and
attributed to the OH of alcohols or in-plane vibration of the OH bond in carboxylic groups.
Moreover, the absorption band at 1050 cm−1 indicates the C-O stretching of primary alco-
hol groups. Furthermore, after the adsorption of Cr(VI), a relatively significant shift of the
main band from 3200 to 3400 cm−1 is observed, hence confirming the occurrence of elec-
trostatic interaction between PGP adsorbent and adsorbate. The band at 900 cm−1 indicates
that the Cr-sorbet PGP could be due to the formation of Cr(OH)3, which suggests the re-
duction of Cr(VI) to Cr(III).
Figure 2. (A) TGA/DTA curves; (B) X−ray diffraction pattern; (C) FTIR spectra before and after
adsorption; (D) N2 adsorption−desorption isotherms of PGP.
The crystalline structure of PGP has been investigated by X−ray diffraction. As shown
in Figure 2B, no characteristic pattern of crystalline phase(s) can be observed while a broad
halo appears in the 2θ range (10−50◦). This reflects the amorphous structure of PGP
powder, in good agreement with the literature [44].
Figure 2C displays the FTIR spectra of PGP powder before and after Cr(VI) absorption.
The bands in the range 2800–3420 cm−1 are ascribed to −OH of carboxylic acid found
in the benzene compound [28]. The bands located at 2850 and 2930 cm−1 correspond to
C−H aliphatic (CH3) or (−CH2), while the band at 1593 cm−1 is assigned to the aromatic
rings’ stretching of phenolic compounds (C−C and C=C aromatic vibrations). In addition,
hydroxyl functional groups are defined by the absorption peak located at 1480 cm−1 and
attributed to the OH of alcohols or in-plane vibration of the OH bond in carboxylic groups.
Moreover, the absorption band at 1050 cm−1 indicates the C-O stretching of primary alcohol
groups. Furthermore, after the adsorption of Cr(VI), a relatively significant shift of the main
band from 3200 to 3400 cm−1 is observed, hence confirming the occurrence of electrostatic
interaction between PGP adsorbent and adsorbate. The band at 900 cm−1 indicates that the
Cr-sorbet PGP could be due to the formation of Cr(OH)3, which suggests the reduction of
Cr(VI) to Cr(III).
As shown in Figure 2D, the PGP N2 adsorption-desorption isotherm exhibits a type IV
characteristic, in accordance with the International Union of Pure and Applied Chemistry
(IUPAC) classification. The characteristic BET parameters given in Table 2, indicate a
specific surface of 40.38 m2/g, an average pore diameter of 32.13 Å, and a pore volume of

8. Water 2022, 14, 3885 8 of 20
0.00277 cm3/g. The distribution of pore diameter reveals that the adsorbent is mesoporous
in nature, in good agreement with the literature [45].
Table 2. BET characteristics of the PGP powder.
BET Surface Area
(m2/g)
Total Pore Volume
(cm3/g)
Average Pore Diameter
(Å)
Particle Density
(g/cm3)
PGP 40.38 2.77 × 10−3 32.13 0.25
The point of zero charge (pHpzc) of the PGP has been also determined. NaCl (0.01 N)
solution is added to a series of flasks, and the initial pH is adjusted by the addition of
NaOH (0.01 N, 0.1 N) and/or HCl (0.01 N, 0.1 N). After the pH adjustment, an amount of
adsorbent (PGP) is mixed with aliquots of 15 mL. The containers have been placed and
subjected to rigorous magnetic stirring for 48 h at 25 ◦C. The pHpzc value is then obtained
by intersecting the (pHfinal−pHinitial) vs. pHinitial curve with the bisector (Figure 3a). The
pHpzc value of PGP powder is found to be 3.9. This value suggests the presence of acid
functional groups on the PGP particles’ surface, which corroborates with FTIR analysis,
and indicates that PGP can adsorb Cr(VI) by electrostatic interaction.
Water 2022, 14, x FOR PEER REVIEW 8 of 24
As shown in Figure 2D, the PGP N2 adsorption-desorption isotherm exhibits a type
IV characteristic, in accordance with the International Union of Pure and Applied Chem-
istry (IUPAC) classification. The characteristic BET parameters given in Table 2, indicate
a specific surface of 40.38 m2/g, an average pore diameter of 32.13 Å , and a pore volume
of 0.00277 cm3/g. The distribution of pore diameter reveals that the adsorbent is mesopo-
rous in nature, in good agreement with the literature [45].
Table 2. BET characteristics of the PGP powder.
BET Surface
Area
(m2/g)
Total Pore
Volume
(cm3/g)
Average Pore
Diameter
(Å)
Particle Density
(g/cm3)
PGP 40.38 2.77 × 10−3 32.13 0.25
The point of zero charge (pHpzc) of the PGP has been also determined. NaCl (0.01
N) solution is added to a series of flasks, and the initial pH is adjusted by the addition of
NaOH (0.01 N, 0.1 N) and/or HCl (0.01 N, 0.1 N). After the pH adjustment, an amount of
adsorbent (PGP) is mixed with aliquots of 15 mL. The containers have been placed and
subjected to rigorous magnetic stirring for 48 h at 25 °C. The pHpzc value is then obtained
by intersecting the (pHfinal−pHinitial) vs. pHinitial curve with the bisector (Figure 3a). The
pHpzc value of PGP powder is found to be 3.9. This value suggests the presence of acid
functional groups on the PGP particles’ surface, which corroborates with FTIR analysis,
and indicates that PGP can adsorb Cr(VI) by electrostatic interaction.
Water 2022, 14, x FOR PEER REVIEW 9 of 24
Figure 3. (a) Plot for the determination of point zero charge of the PGP powder, (b) effect of pH on
the adsorption capacity of Cr (VI) by PGP powder and (c) speciation diagram of Cr vs. pH (T = 20 ±
2°C, C0 = 20 mg/L, H= 2 cm, D= 2 mL/min).
3.1.1. Effect of pH Value
The pH effect has been investigated between 2 and 12. It is observed that a marked
removal rate of Cr(VI) ions is achieved at lower pH 2, then declines gradually with in-
creasing the pH value. At acidic medium, the predominant species of chromium are di-
chromate (Cr2O72−) and hydrogen chromate (HCrO4-) meanwhile the surface of PGP parti-
cles becomes highly protonated, thereby favoring the adsorption and elimination of
Cr(VI) metal ions. Under this condition, the phenolic groups (−OH) of the adsorbent easily
exchange with the chromium species. Furthermore, the removal efficiency decreases sig-
nificantly by increasing the pH to more than 4. The effect of pH value on the adsorption
mechanism may be attributed to the presence of different functional carboxylic COO− and
hydroxylic OH- groups onto the surface of PGP particles, hence favoring different possible
interactions with Cr(VI) ions and other ionic species present in the solution. These results
corroborate with zeta potential measurements which revealed a positive surface charge of
PGP for pH smaller than 3.9 and a negative charge at pH greater to 3.9.
The rise in pH value causes a reduction in the positive surface of the adsorbent [46].
In fact, this decrease is directly associated with the electrostatic forces existing between
PGP and chromium solution, as consequence a marked reduction in the sorption capacity
is observed [6]. Furthermore, as the pH value increases, a competition between OH- and
2− −
Figure 3. (a) Plot for the determination of point zero charge of the PGP powder, (b) effect of pH on the
adsorption capacity of Cr (VI) by PGP powder and (c) speciation diagram of Cr vs. pH (T = 20 ± 2 ◦C,
C0 = 20 mg/L, H = 2 cm, D = 2 mL/min).
3.1.1. Effect of pH Value
The pH effect has been investigated between 2 and 12. It is observed that a marked
removal rate of Cr(VI) ions is achieved at lower pH 2, then declines gradually with increas-
ing the pH value. At acidic medium, the predominant species of chromium are dichromate
(Cr2O7
2−) and hydrogen chromate (HCrO4
−) meanwhile the surface of PGP particles be-
comes highly protonated, thereby favoring the adsorption and elimination of Cr(VI) metal
ions. Under this condition, the phenolic groups (−OH) of the adsorbent easily exchange
with the chromium species. Furthermore, the removal efficiency decreases significantly by

9. Water 2022, 14, 3885 9 of 20
increasing the pH to more than 4. The effect of pH value on the adsorption mechanism may
be attributed to the presence of different functional carboxylic COO− and hydroxylic OH−
groups onto the surface of PGP particles, hence favoring different possible interactions
with Cr(VI) ions and other ionic species present in the solution. These results corroborate
with zeta potential measurements which revealed a positive surface charge of PGP for pH
smaller than 3.9 and a negative charge at pH greater to 3.9.
The rise in pH value causes a reduction in the positive surface of the adsorbent [46].
In fact, this decrease is directly associated with the electrostatic forces existing between
PGP and chromium solution, as consequence a marked reduction in the sorption capacity
is observed [6]. Furthermore, as the pH value increases, a competition between OH−
and CrO4
2− as predominant ions occurs in the basic medium [47]. It was concluded that
HCrO4
− was adsorbed through electrostatic interaction to reduce to trivalent chromium
Cr(III) [34]. The optimum pH of 2 will be utilized for the following experiments. The
as-obtained results are found to be close to the results reported in a previous study using
pomegranate peel for the elimination of hexavalent chromium [12]. The predominant form
of Cr(VI) vs. pH (Figure 3a) is found to be HCrO4
− at the pH of 3.0 and would change to
CrO4
2− and Cr2O7
2− when the pH 6.5. Compared with CrO4
2−, HCrO4
− is known to be
more easily adsorbed on the surface active site owing to its low adsorption free energy [45].
3.1.2. Effect of Contact Time
Figure 4a shows the effect of contact time on Cr(VI) removal from aqueous solution
onto PGP adsorbent. Based on the plot, the adsorption processes are almost accomplished
during the first time and reach equilibrium within 2 h (removal percentage = 92%). After a
certain period of time, the reaction slows down as the process reaching the steady state,
because the number of vacant sites decreases due to the repulsive forces occurring between
the solute molecules and the solid adsorbent phase [20].
Water 2022, 14, x FOR PEER REVIEW 10 of 24
3.1.2. Effect of Contact Time
Figure 4a shows the effect of contact time on Cr(VI) removal from aqueous solution
onto PGP adsorbent. Based on the plot, the adsorption processes are almost accomplished
during the first time and reach equilibrium within 2 h (removal percentage = 92%). After
a certain period of time, the reaction slows down as the process reaching the steady state,
because the number of vacant sites decreases due to the repulsive forces occurring be-
tween the solute molecules and the solid adsorbent phase [20].
Figure 4. (a) effect of contact time on Cr(VI) removal, (b) Langmuir and Freundlich plots for the
adsorption on PGP at different concentration of Cr(VI) (pH = 2 ± 0.2, T = 20 ± 2 °C, m = 1.0 g/L,
agitation speed = 50 rpm).
3.2. Adsorption Isotherms
The non-linear Freundlich and Langmuir isotherms are shown in Figure 4b. It is no-
ticed that the equilibrium data are well described by Freundlich isotherm compared to
Langmuir isotherm. The model parameters, R2 values and 𝜒2
are presented in Table 3. The
obtained results indicate that the regression coefficients for Cr(VI) metal ions adsorption
with Freundlich isotherm are relatively high (0.994) compared to Langmuir (0.990) iso-
Figure 4. (a) effect of contact time on Cr(VI) removal, (b) Langmuir and Freundlich plots for the
adsorption on PGP at different concentration of Cr(VI) (pH = 2 ± 0.2, T = 20 ± 2 ◦C, m = 1.0 g/L,
agitation speed = 50 rpm).

10. Water 2022, 14, 3885 10 of 20
3.2. Adsorption Isotherms
The non-linear Freundlich and Langmuir isotherms are shown in Figure 4b. It is
noticed that the equilibrium data are well described by Freundlich isotherm compared to
Langmuir isotherm. The model parameters, R2 values and χ2 are presented in Table 3. The
obtained results indicate that the regression coefficients for Cr(VI) metal ions adsorption
with Freundlich isotherm are relatively high (0.994) compared to Langmuir (0.990) isotherm.
Table 3. Langmuir and Freundlich isotherm models constants and determination coefficients for the
adsorption of Cr(VI) by PGP powder.
Langmuir Freundlich
qm(exp)
(mg/g)
qm(theo)
(mg/g)
KL
(L/mg)
R2 χ2 n
Kf
(mg1−1/n.L1/n)
R2 χ2
50.32 38.29 ± 2.58 0.08 ± 0.01 0.990 0.528 3.00 ± 0.07 4.49 ± 0.28 0.994 0.33
The value of 1/n for all adsorption curves is found to be less than unity, which reflects
the favorable exchange characteristics between Cr(VI) and PGP. The presence of hydroxyls
and carboxyls on the surface of PGP particles produces an irregularity in the distribution
energy over the surface of the adsorbent and therefore confirms the suitability of the
Freundlich isotherm.
Furthermore, the batch mode’s isotherm constants are significantly greater than the fixed
bed mode. This may be due to the influence of the solution flow rate which is equal to zero in
the batch mode, i.e., the contact time between Cr(VI) and PGP approximates the infinity [48].
3.3. Influence of Parameters on Fixed Bed
3.3.1. Flow Rate
The breakthrough curves have been recorded at varying flow rates (1, 2, and 3 mL/min),
while the remaining operating parameters are kept constant, i.e., inlet Cr(VI) concentration
20 mg/Land bed-depth 50 mm, see Figure 5 and the computed parameters are summarized
in Table 4. It is observed that the sorption rate is reduced with the increase in the flow rate.
The empty bed contact time (EBCT) decreases significantly from 108 up to 38 min and the
exhaust time (equivalent to 90% of the Cr (VI) concentration) also decreases markedly from
550 to 168 min. The flow rate is found to have a relatively moderate effect on the adsorption
capacity; as the flow rate increases, the amount of adsorbed Cr(VI) (qexp) decreases slightly
from 28.67 to 24.85 mg/g. The observed decline in the adsorption capacity can be attributed
to the shorter contact time for the adsorbent to interact with the solution, which is in
agreement with the previous research findings [39].
Water 2022, 14, x FOR PEER REVIEW 11 of 24
The value of 1/n for all adsorption curves is found to be less than unity, which reflects
the favorable exchange characteristics between Cr(VI) and PGP. The presence of hydrox-
yls and carboxyls on the surface of PGP particles produces an irregularity in the distribu-
tion energy over the surface of the adsorbent and therefore confirms the suitability of the
Freundlich isotherm.
Furthermore, the batch mode’s isotherm constants are significantly greater than the
fixed bed mode. This may be due to the influence of the solution flow rate which is equal
to zero in the batch mode, i.e., the contact time between Cr(VI) and PGP approximates the
infinity [48].
3.3. Influence of Parameters on Fixed Bed
3.3.1. Flow Rate
The breakthrough curves have been recorded at varying flow rates (1, 2, and 3
mL/min), while the remaining operating parameters are kept constant, i.e., inlet Cr(VI)
concentration 20 mg/Land bed-depth 50 mm, see Figure 5 and the computed parameters
are summarized in Table 4. It is observed that the sorption rate is reduced with the in-
crease in the flow rate. The empty bed contact time (EBCT) decreases significantly from
108 up to 38 min and the exhaust time (equivalent to 90% of the Cr (VI) concentration)
also decreases markedly from 550 to 168 min. The flow rate is found to have a relatively
moderate effect on the adsorption capacity; as the flow rate increases, the amount of ad-
sorbed Cr(VI) (qexp) decreases slightly from 28.67 to 24.85 mg/g. The observed decline in
the adsorption capacity can be attributed to the shorter contact time for the adsorbent to
interact with the solution, which is in agreement with the previous research findings [39].
Figure 5. Cont.

11. Water 2022, 14, 3885 11 of 20
Water 2022, 14, x FOR PEER REVIEW 12 of 24
Figure 5. Comparison of the experimental and predict breakthrough curves obtained at different
(A) flow rights, (B) bed heights, and (C) concentrations according to the studied models for Cr(VI)
adsorption by PGP powder (T = 20 ± 2 °C, pH = 2 ± 0.2).
Table 4. Total removal percentage of Cr(VI), column uptake capacity, and column data parameters
obtained at different bed heights, concentrations, and flow rates (T = 20 ± 2 °C, pH = 2 ± 0.2).
F
(mL/min)
H
(mm)
C0
(mg/L)
tb
(min)
te
(min)
Veff
(L)
Area 𝐀′
(mg/min.L)
𝐪𝐭𝐨𝐭
(mg)
𝐖𝐭𝐨𝐭
(mg)
Total
Removal
(%)
𝐪𝐞𝐪(𝐞𝐱𝐩)
(mg/g)
1 50 20 108 700 0.70 6087 6.08 14.00 43.47 28.67
2 50 20 45 320 0.64 2803 5.60 12.80 43.75 26.73
3 50 20 36 204 0.61 1746 5.23 12.24 42.79 24.85
2 25 20 24 201 0.40 1083 2.16 8.04 20.57 20.62
2 75 20 78 430 0.86 4187 8.37 17.20 48.66 26.62
2 50 10 72 385 0.77 1924 3.84 7.70 49.87 18.28
2 50 30 27 270 0.54 3151 6.30 16.20 38.88 30.00
3.3.2. Bed Height
The influence of varying the bed-height from 25 to 75 mm on the behavior of the
breakthrough curves has been investigated at a fixed adsorbent flow rate of 2 mL/min and
an inlet Cr(VI) concentration of 20 mg/L. The recorded breakthrough curves are displayed
in Figure 5B and the computed values are given in Table 4. It is clearly observed that as
the bed height increases, the Cr(VI) removal efficiency and qeq(exp) increase simultane-
ously, from 20.57 to 48.66% and 20.62 to 26.62 mg/g. Consequently, while investigating
the effect of PGP bed height on the adsorption of 20 mg/L Cr(VI) ions, the breakpoint
value has been fixed at the point where
Ct
C0
is equal to 0.076, 0.090, and 0.100 for a fixed
bed height of 25, 50 and 75 mm respectively. It is found that with the increase of the bed
height, the breakthrough curve slope decreases, resulting in a broader mass transfer [39].
For this reason, the specific surface area of the PGP biosorbent increases, consequently
providing a large number of binding active sites thereby a higher adsorption rate of the
column [49]. Also, it is observed that the breakthrough time increases significantly from
24 to 78 min with the increase in the bed height. This can be associated with the occurrence
of rapid and larger mass transfer in the fixed bed mode [49].
Figure 5. Comparison of the experimental and predict breakthrough curves obtained at different
(A) flow rights, (B) bed heights, and (C) concentrations according to the studied models for Cr(VI)
adsorption by PGP powder (T = 20 ± 2 ◦C, pH = 2 ± 0.2).
Table 4. Total removal percentage of Cr(VI), column uptake capacity, and column data parameters
obtained at different bed heights, concentrations, and flow rates (T = 20 ± 2 ◦C, pH = 2 ± 0.2).
F
(mL/min)
H
(mm)
C0
(mg/L)
tb
(min)
te
(min)
Veff
(L)
Area A’
(mg/min.L)
qtot
(mg)
Wtot
(mg)
Total Re-
moval
(%)
qeq(exp)
(mg/g)
1 50 20 108 700 0.70 6087 6.08 14.00 43.47 28.67
2 50 20 45 320 0.64 2803 5.60 12.80 43.75 26.73
3 50 20 36 204 0.61 1746 5.23 12.24 42.79 24.85
2 25 20 24 201 0.40 1083 2.16 8.04 20.57 20.62
2 75 20 78 430 0.86 4187 8.37 17.20 48.66 26.62
2 50 10 72 385 0.77 1924 3.84 7.70 49.87 18.28
2 50 30 27 270 0.54 3151 6.30 16.20 38.88 30.00
3.3.2. Bed Height
The influence of varying the bed-height from 25 to 75 mm on the behavior of the
breakthrough curves has been investigated at a fixed adsorbent flow rate of 2 mL/min and
an inlet Cr(VI) concentration of 20 mg/L. The recorded breakthrough curves are displayed
in Figure 5B and the computed values are given in Table 4. It is clearly observed that as the
bed height increases, the Cr(VI) removal efficiency and qeq(exp) increase simultaneously,
from 20.57 to 48.66% and 20.62 to 26.62 mg/g. Consequently, while investigating the effect
of PGP bed height on the adsorption of 20 mg/L Cr(VI) ions, the breakpoint value has
been fixed at the point where Ct
C0
is equal to 0.076, 0.090, and 0.100 for a fixed bed height
of 25, 50 and 75 mm respectively. It is found that with the increase of the bed height, the
breakthrough curve slope decreases, resulting in a broader mass transfer [39]. For this
reason, the specific surface area of the PGP biosorbent increases, consequently providing a
large number of binding active sites thereby a higher adsorption rate of the column [49].
Also, it is observed that the breakthrough time increases significantly from 24 to 78 min
with the increase in the bed height. This can be associated with the occurrence of rapid and
larger mass transfer in the fixed bed mode [49].
3.3.3. Initial Concentration
Figure 5C illustrates the breakthrough curves of different inlet Cr(VI) concentra-
tions onto PGP adsorbent obtained at a constant bed depth (50 mm) and fixed flowrate

12. Water 2022, 14, 3885 12 of 20
(2 mL/min), the corresponding data are given in Table 4. It can be observed that the value
of Ct
C0
reaches 0.010, 0.039, and 0.067 min in the intervals 365, 300, and 230 min when
the Cr(VI) inlet concentration varies from 10 to 30 mg/L. This indicates that the value of
the breakthrough time (tb) is slightly reduced with the rise in the inlet concentration of
Cr(VI). The maximum adsorption capacity at 10, 20, and 30 mg/L Cr(VI) is found to be
18.32, 26.73 and 30.01 mg/g, respectively. In contrast, the adsorption efficiency is reduced
by almost 22% (from 49.87 to 38.88%). Furthermore, it is noted that when the Cr(VI) con-
centration in the solution is high, the rate of Cr(VI) ions for the adsorptive sites has a more
pronounced trend. Indeed, a higher concentration of Cr(VI) in the solution provides a
random diffusion path [50] for sorbate ions onto the surface of PGP adsorbent. This may
be explained by the enhanced driving force and the high mass-transfer flux from Cr(VI)
solution toward PGP [51]. However, at lower inlet Cr(VI) concentration, the breakthrough
curves become dispersed, and the binding sites tend to be slowly saturated. Indeed, similar
results were reported for the biosorption of Cr(VI) by chemically surface-modified Lantana
Camara sorbent [39].
3.3.4. Temperature
The influence of the medium temperature has been also investigated. The break-
through curves recorded for different bed heights at various temperatures are shown in
Figure 6, and the results of Dose Response Model are given in Table 5. It is noted that
with the rise of temperature, the adsorption rate of Cr(VI) ions decreases whereas the rate
constant increases. Also, it is noticed that the adsorption rate is clearly variable at the
same bed height. This is maybe due to the desorption process occurring following the rise
in the thermal energy and the weakening of the interactions of the intermolecular bonds
between solution ions and active sites of the PGP adsorbent. This signifies that the PGP
particles’ surface is exothermic in nature and that the adsorption mechanism is favored at
low temperatures. The above results are in accordance with the results obtained during the
thermodynamic study in batch mode; the exothermic reaction of Cr(VI) ions adsorption onto
PGP will be confirmed. This result has been previously reported by M. Zamouch et al. [21].
Water 2022, 14, x FOR PEER REVIEW 14 of 24
Figure 6. Effect of bed height on the breakthrough curves at different temperatures: (a) 303 K and
(b) 313 K (C0 = 20 mg/L, H = 50 mm, D = 2 mL/min, pH = 2 ± 0.2).3.4. Adsorption Thermodynamics.
The adsorption Cr(VI) ions by PGP thermodynamic parameters in the temperature
range 298–318 K, have been determined using Van’t Hoff equations [52]:
∆G°
= −RTLnKC (15)
∆G°
= ∆H°
− T∆S° (16)
KC =
qe
Ce
(17)
LnK =
∆S°
−
∆H°
(18)
Figure 6. Effect of bed height on the breakthrough curves at different temperatures: (a) 303 K and
(b) 313 K (C0 = 20 mg/L, H = 50 mm, D = 2 mL/min, pH = 2 ± 0.2).3.4. Adsorption Thermodynamics.

13. Water 2022, 14, 3885 13 of 20
Table 5. Dose response parameters for Cr(VI) adsorption by PGP in fixed bed (C0 = 20 mg/L,
pH = 2 ± 0.2, D = 2 mL/min).
T (K)
Z
(mm)
a
b
(mL)
q0
(mg/g)
qexp
(mg/g)
Total Removal
(%)
R2 χ×104 SD
303
25 2.24 ± 0.04 52.85 ± 0.51 10.06 10.15 38.00 0.997 2.76 0.01
50 2.75 ± 0.07 138.32± 1.47 13.17 13.38 44.00 0.996 5.94 0.02
75 2.92± 0.08 194.12 ± 2.12 13.98 13.93 44.12 0.996 5.20 0.02
313
25 1.28± 0.10 20.96 ± 1.67 3.99 3.89 30.06 0.927 38.50 0.06
50 2.20 ± 0.14 51.85 ± 1.65 4.93 4.90 33.09 0.977 27.80 0.05
75 3.00 ± 0.11 98.13 ± 1.38 6.23 6.20 34.10 0.994 9.17 0.03
The adsorption Cr(VI) ions by PGP thermodynamic parameters in the temperature
range 298–318 K, have been determined using Van’t Hoff equations [52]:
∆G
◦
= −RTLnKC (15)
∆G
◦
= ∆H
◦
− T∆S
◦
(16)
KC =
qe
Ce
(17)
LnKC =
∆S
◦
R
−
∆H
◦
RT
(18)
where ∆G
◦
represents the Gibbs free energy (kJ/mol), R the gas constant (8.314 J/mol.K),
T the absolute temperature (K), KC the sorption distribution coefficient (L/g), ∆H◦ the
enthalpy (kJ/mol.), and ∆S
◦
the entropy (kJ/mol.K).
To better elucidate the adsorption process mechanism of Cr(VI) by PGP biosorbent,
the respective thermodynamic plots are depicted in Figure 7 while the computed fitting
parameters are given in Table 6. The negative values of ∆H◦, ∆S◦, and ∆G◦ demonstrate
that the adsorption process is spontaneous and exothermic, without the need for any
external power source. Also, it signifies the randomness of adsorption of Cr(VI) onto
PGP. The negative value of ∆S◦ corresponds to a decrease in the freedom of the adsorbed
chemical species and describes the randomness of chemical species movement at the
adsorbent-adsorbate interface during the sorption process. The negative values of ∆G◦ and
high negative values of ∆S◦ with the rise of temperature manifest the favorable nature of
the process. Whereas the observed decrease in ∆G◦ values with temperature signifies that
the adsorption process occurs favorably at lower temperatures [53]. Similar results were
also observed in the literature for Cr(VI) adsorption onto different adsorbents [6,54].
Water 2022, 14, x FOR PEER REVIEW 15 of 24
Figure 7. Plots of LnKC vs. 1/T for the estimation of the thermodynamic parameters for the adsorp-
tion of Cr(VI) on PGP (pH = 2 ± 0.2, m = 1.0 g.L−1, C0 = 20 mg/L).
Table 6. Thermodynamic parameters calculated for the adsorption of Cr(VI) onto PGP.
Sample
∆𝐇°
(kJ/mol)
∆𝐒°
(J/mol.K)
∆𝐆°
(kJ/mol)
298 K 308 K 318 K
PGP −44.69 −139.28 −3.98 −2.30 −0.48
3.4. Modeling of Column Data
The fitted curves are shown in Figure 5 and the models’ corresponding calculated
parameters are presented in Table 7. The nonlinear plot
Ct
vs. t has been fitted in the ini-
Figure 7. Plots of LnKC vs. 1/T for the estimation of the thermodynamic parameters for the
adsorption of Cr(VI) on PGP (pH = 2 ± 0.2, m = 1.0 g.L−1, C0 = 20 mg/L).

14. Water 2022, 14, 3885 14 of 20
Table 6. Thermodynamic parameters calculated for the adsorption of Cr(VI) onto PGP.
Sample
∆H
◦
(kJ/mol)
∆S
◦
(J/mol.K)
∆G
◦
(kJ/mol)
298 K 308 K 318 K
PGP −44.69 −139.28 −3.98 −2.30 −0.48
3.4. Modeling of Column Data
The fitted curves are shown in Figure 5 and the models’ corresponding calculated
parameters are presented in Table 7. The nonlinear plot Ct
C0
vs. t has been fitted in the
initial part of the curve (0–0.2) to compute the Bohart–Adams KBA and sorption capacity N0
parameters. It is found that the value of KBA increases from 4.16 × 10−3 to 7.92 × 10−3 with
increasing the flow rate but decreases from 11.28 × 10−3 to 3.28 × 10−3 with the increase
in the initial Cr(VI) concentration. Also, the value of N0 of the model is found to increase
with increasing both Cr(VI) concentration and bed depth. This can be associated with the
prevalence of external mass transfer activities occurring during the adsorption process
within the column at the initial stage [55]. This corroborates with the results published
by Han et al. [45] when employing green synthesized nanocrystalline chlorapatite for the
sorption of Cr(VI).
The Thomas model parameters namely kinetic coefficient Kth and sorption capacity
qth, have been determined using a nonlinear regression for bed height, flow rate, and Cr(VI)
concentration. From Table 7, it is noted that the value of Kth increases with increasing
both bed height and flow rate. Also, the value of qth increases markedly (from 15.75 to
25.39 mg/g) with the rise of Cr(VI) concentration (from 10 to 30 mg/L), which is attributed
to the high driving force between Cr(VI) ions and PGP sorbent. However, the increase in
the Kth value with the increase in the flow rate, signifies that the kinetics of the overall
process is dominated by the external mass transfer mechanism [56]. This suggests that
the Thomas model is more favorable for describing the adsorption processes, because the
limiting steps do not proceed through external/internal diffusion [37]. Similar results are
found in the literature when using different sorbate-sorbent systems [54].
According to the results given in Table 7, the rate of mass transfer (r) increases gradu-
ally with the rise of both flow rate and Cr(VI) concentration but decreases with the increase
in bed height. Furthermore, the increase in the initial Cr(VI) concentration results in the
enhancement of the driving force, favoring a better mass transfer of the adsorbate onto PGP
particles’ surface. Nevertheless, with the rise in the bed height, an increase in the particles’
number alongside a reduction in the mass transfer rate occur concurrently [57].
The fitting results of the modified Dose Response parameters are also examined. The
predicted breakthrough curves illustrated in Figure 5 evidenced a good agreement with
the measurements. The obtained high value of the regression coefficient (R2) satisfies the
validity of the model. The values of R2 obtained for different operating conditions are
higher than 0.977 and χ2 smaller than 2.98 × 10−3, thus providing a better fitting for the
Dose Response model compared to Thomas, Clarck, and Bohart-Adams models.

15. Water 2022, 14, 3885 15 of 20
Table 7. Parameters of various models for Cr(VI) adsorption by PGP using non-linear regression. Experimental fixed conditions: T = 20 ± 2 ◦C, pH = 2 ± 0.2.
Parameters Thomas Model Dose Response Model
F
(mL/min)
H
(mm)
C0
(mg/L)
qecal
(mg/g)
Total Removal
(%)
KThX103
(L/mg)
R2 χ2
×103
±SD
× 103 a
b
(mL)
q0
(mg/g)
R2 χ2
×103
±SD
×103
1 50 20 25.69 ± 0.37 43.47 0.59 ± 0.02 0.976 3.16 56.22 2.73 ± 0.07 269.78 ± 2.98 27.79 0.988 1.47 38.38
2 50 20 21.59 ± 0.54 43.75 1.16± 0.07 0.953 5.85 76.50 2.46 ± 0.07 226.74 ± 2.73 23.26 0.988 1.45 38.05
3 50 20 21.52 ± 0.34 42.79 2.04 ± 0.09 0.976 3.11 55.80 2.94 ± 0.06 226.04± 1.80 22.86 0.994 0.71 26.73
2 25 20 16.63 ± 0.44 20.57 3.70 ± 0.28 0.970 4.50 67.05 3.07 ± 0.11 87.34± 1.12 17.57 0.993 1.02 31.89
2 75 20 22.77 ± 0.56 48.66 0.72 ± 0.03 0.956 5.72 75.63 2.49± 0.08 358.64 ± 5.95 26.02 0.979 2.71 52.09
2 50 10 15.75 ± 0.42 49.87 1.53 ±0.09 0.952 6.34 79.63 2.50 ± 0.09 330.88 ± 5.75 18.02 0.977 2.98 54.57
2 50 30 25.39 ± 0.45 38.88 0.95 ±0.04 0.972 3.19 56.48 2.35 ± 0.06 177.37 ± 1.84 27.22 0.991 0.99 31.53
Clark model Bohart-Adams model
D
mL.min−1
H
(mm)
C0
(mg/L)
A
r × 103
(min−1)
R2 χ2
×103
±SD
×103
KBA×103
(mL/mg.min)
N0
(mg/L)
R2 χ2
×103
±SD
×103
1 50 20 535.29 ± 139.02 17.31 ± 0.85 0.964 4.72 68.69 4.16 ± 0.36 6024.43 ± 88.76 0.900 0.26 16.32
2 50 20 184.20 ± 56.80 33.24 ± 2.64 0.933 8.44 91.87 4.24 ± 0.11 6689.07 ± 218.38 0.914 0.32 17.99
3 50 20 298.95 ± 89.87 56.11 ± 3.49 0.962 4.99 70.65 7.92 ±0.11 6839.09 ± 209.58 0.923 0.40 20.21
2 25 20 383.93 ± 188.25 103.12 ± 10.26 0.933 6.66 81.63 10.30± 1.52 6222.88 ± 213.18 0.946 0.37 19.39
2 75 20 206.66 ± 52.18 19.67 ± 1.16 0.941 7.76 88.12 3.30± 1.52 6721.11 ± 91.34 0.940 0.20 14.32
2 50 10 186.00 ± 42.24 20.76 ± 1.31 0.936 8.52 92.29 11.28 ± 1.16 4208.87 ± 73.41 0.923 0.27 16.49
2 50 30 154.18 ± 37.66 40.68 ± 2.51 0.957 4.91 70.10 3.28 ± 0.43 7191.59 ± 323.61 0.922 0.48 21.94

16. Water 2022, 14, 3885 16 of 20
3.5. Adsorption Mechanism
The SEM image of PGP after the adsorption of Cr(VI) metal ions (Figure 1d) reveals
that the particles’ surface becomes more rough and uneven. This confirms that the pores of
the PGP sorbent are occupied by the ions of the adsorbate. Furthermore, the EDX spectrum
(Figure 1e) of PGP after adsorption reveals the presence of Cr ions in the adsorbent, where
its content reaches 3.64 wt.%. The presence of 1.5 wt.% K, maybe a residue belonging to the
potassium dichromate (K2Cr2O7) used during the preparation of Cr(VI) aqueous solutions.
Through the analytical results of EDX (Figure 1e) and FTIR (Figure 2C) spectra, as well
as the data obtained from modeling of the equilibrium isotherms and kinetics, a plausible
mechanism related to the adsorption of Cr(VI) onto PGP is illustrated in Figure 8. It is
noted that the process is primarily controlled by electrostatic interaction between negatively
charged functional groups present at the PGP particles’ surface and the positively charged
Cr(VI) metal ions. Meanwhile, the surface adsorption via diffusion to the pores of PGP
improves the mass transfer and provides an adsorption pathway besides an area responsible
for the reduction of Cr(VI) metal ions to less toxic Cr(III).
Water 2022, 14, x FOR PEER REVIEW 19 of 24
Figure 8. Plausible mechanism for the adsorption of Cr(VI) by PGP powder.
3.6. Comparison with Other Sorbents in the Literature
To evaluate the sorption efficacy of PGP toward Cr(VI), a comprehensive comparison
with other materials such as solvent impregnated resin (SIR), pomegranate peel powder
(PGP) subjected to polyaniline (PANI), and polypyrrole (PPy) surface functionalization
Figure 8. Plausible mechanism for the adsorption of Cr(VI) by PGP powder.

17. Water 2022, 14, 3885 17 of 20
3.6. Comparison with Other Sorbents in the Literature
To evaluate the sorption efficacy of PGP toward Cr(VI), a comprehensive comparison
with other materials such as solvent impregnated resin (SIR), pomegranate peel powder
(PGP) subjected to polyaniline (PANI), and polypyrrole (PPy) surface functionalization and
modified Lantana Camara, is performed as illustrated in Table 8. It can be observed that the
proposed biosorbent exhibits a higher adsorption capacity than the previously reported
sorbents; it is even higher than pomegranate peel activated with H2SO4. In addition, the
obtained batch capacity is more significant than that of SIR and PGP for Cr(VI) removal.
Nevertheless, the existence of some differences is more probably attributed to several
factors, primarily the active sites and functional groups formed at the particles’ surfaces of
the sorbents, besides the microstructure nature (surface area, porosity, agglomeration level)
and the chemical formulation (intrinsic properties playing key role in terms of selectivity)
of the prepared materials.
Table 8. A comparison of adsorption capacity or removal percentage of Cr(VI) with different biosorbents.
Adsorbent Qmax (mg/g)/R (%) Operating Conditions References
Pomegranate peel
(Batch study)
22.87 mg/g pH 2, dose 300 mg/L, contact time 30 min [12]
Solvent impregnated resin (SIR)(Batch study) 28.2 0 mg/g pH (0.3–2), dose 1 mg/L, contact time 30 min [54]
Pomegranate peel (PGP) powder modified by
polyaniline (PANI) and polypyrrole (PPy) (Batch study)
47.59% pH 1, dose 10 mg/L, contact time 90 min [36]
Nanocrystalline chlorapatite (ClAP)
(Batch study)
63.47 mg/g pH 3, dose 0.1 mg/L, contact time 25 min [45]
Pomegranate peel (PGP) activated (PGP-1N H2SO4)
(Batch study)
28.28 mg/g pH 3, dose 4 mg/L, contact time 3 h [6]
Modified Lantana Camara
(Column study)
362.80 mg/g
pH 1.5, adsorbent bed height 40 mm,
Breakthrough time 1250 min
[39]
Pomegranate peel
(Column study)
(Batch study)
30.00 mg/g
50.32 mg/g
pH 2, adsorbent bed height 50 mm,
Breakthrough time 27 min
pH 2, dose 1 mg/L, contact time 24 h
This study
4. Conclusions
In the present study Pomegranate peel has been used as cost-effective biosorbent
for the removal of hazardous Cr(VI). The adsorption mechanism of Cr(VI) adsorbate by
PGP nanosorbent is determined and the physicochemical proprieties of PGP are presented.
Both batch and dynamic studies are performed, and the isotherms and thermodynamic
results are analyzed. It is found that the adsorption process occurs through an exothermic
mechanism. Several operating parameters (pH, initial Cr(VI) concentration, temperature,
bed depth, and flow rate) are found to significantly affect the Cr(VI) uptake through a fixed-
bed column. The maximum uptake of 43.78 mg/g is achieved at pH 2 with a breakthrough
time. The experimental data in dynamic mode has been fitted using four theoretical models
namely Bohart-Adams, Clark, Dose Response, and Thomas. The Dose Response model
successfully predicts the breakthrough curves for Cr(VI) removal under optimum operating
conditions (flow rate, bed depth, Cr(VI) concentration) with R2 0.97. The equilibrium data
were found to obey Freundlich isotherm, thereby confirming the heterogeneous sorption of
Cr(VI) onto PGP. The negative values of ∆H◦, ∆G◦, and ∆S◦ indicate an exothermic and
spontaneous process.
Author Contributions: Conceptualization: Z.H.; Methodology: Y.B., N.B.; Formal analysis and
investigation: R.Z.; Writing—original draft preparation: R.Z., M.B.; Writing—review and editing:
R.Z., M.B., R.A.A.; Resources: R.Z.; Supervision: M.B., R.A.A. All authors have read and agreed to
the published version of the manuscript.
Funding: This research was funded by Algerian Ministry of Higher Education, grant number
B00L01UN230120200002.

18. Water 2022, 14, 3885 18 of 20
Institutional Review Board Statement: Not applicable.
Data Availability Statement: Data is available upon formal request to authors.
Acknowledgments: The authors are thankful to Prince Sultan University for their support for paying
the article processing charge of this publication.
Conflicts of Interest: The authors have no competing interest to declare that are relevant to the
content of this article.
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