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ORIGINAL ARTICLE
Utilization of sugarcane bagasse/ZnCl
2
for sustainable production
of microporous nano-activated carbons of type I for toxic Cr(VI)
removal from aqueous environment
Ahmed El Nemr
1
&Rawan M. Aboughaly
2
&Amany El Sikaily
1
&Safaa Ragab
1
&Mamdouh S. Masoud
2
&
Mohamed Shafik Ramadan
2
Received: 20 September 2020 /Revised: 9 March 2021 /Accepted: 11 March 2021
#The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2021
Abstract
In this work, low-cost microporous nano-activated carbon (MNSAC) was prepared from sugarcane bagasse using chemical
activation with zinc chloride. The activated carbon prepared MNSAC was characterized using Brunauer-Emmett-Teller (BET),
thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) analyses, transmis-
sion electron microscopy (TEM), and energy-dispersive X-ray (EDX). MNSAC had BET, micropore analysis (MP), and t-plot
surface area 1174.1, 1322.2, and 1401.5 m
2
/g, respectively, and was essentially microporous. Batch experiment was used to
investigate the efficiency of MNSAC to remove toxic Cr(VI) ions from an artificial wastewater. Different adsorption behaviors
towards toxic Cr(VI) ions have been studied to optimize adsorption status such as pH, initial concentration, absorbent dose,
contact time, and temperature. Langmuir isothermal well fits experimental data compared to Freundlich isothermal model, which
indicates that Cr(VI) ion adsorption process may be monolayer adsorption. The maximum adsorption capacity (Q
m
)ofMNSAC
obtained from Langmuir isotherm model was 277.78 mg/g. The regeneration of MNSAC was studied and the maximum removal
of Cr(VI) ion was 93.61%, 82.38%, and 64.11% in the consequent three cycles. The pseudo-second-model kinetic well described
the experimental data of hexavalent chromium adsorption (R
2
> 0.9900) compared to the other kinetic models studied.
Thermodynamic parameters expose that adsorption process is endothermic, spontaneous, and appropriate in nature. A negative
ΔG° value of Cr(VI) ion adsorption was found, which confirmed the spontaneous probability of the adsorption process.
Keywords Activated carbon .Sugarcane bagasse .Hexavalent chromium .Adsorption .Thermodynamic .Zinc chloride
Highlights
Preparation of nano-activated carbon micropores from sugarcane bagasse
using ZnCl
2
1401.5 m
2
/g t-plot surface area has been obtained for micropores nano-
activated carbon
Prepared nano-activated carbon micropores were used for Cr(VI) ion
removal
Prepared activated carbon has Cr(VI) maximum adsorption capacity of
277.78 mg/g
*Ahmed El Nemr
ahmedmoustafaelnemr@yahoo.com; ahmed.m.elnemr@gmail.com
Rawan M. Aboughaly
r.aboghaly@gmail.com
Amany El Sikaily
dramany_mas@yahoo.com
Safaa Ragab
Safaa_ragab65@yahoo.com
Mamdouh S. Masoud
drmsmasoud@yahoo.com
Mohamed Shafik Ramadan
drmshafek@yahoo.com
1
Environmental Division, National Institute of Oceanography and
Fisheries (NIOF), Kayet Bey, Elanfoushy, Alexandria, Egypt
2
Department of Chemistry,Faculty of Science, Alexandria University,
Alexandria, Egypt
Biomass Conversion and Biorefinery
https://doi.org/10.1007/s13399-021-01445-6
1 Introduction
Today, obtaining enough clean water is one of the biggest
challenges facing the world due to the acceleration of indus-
trial development and the high rates of water pollution with
many toxic pollutants, including toxic heavy metal pollution,
so that pollution has become a major concern for chemists and
engineers working in the field of environmental pollution [1,
2]. The toxicity and accumulation of heavy metals across the
food chain pose a great danger to human and animal health
and thus cause a very important environmental problem [3–5].
Hexavalent chromium is the most dangerous species of chro-
mium compounds. Chromium has a valence state from II toVI
and is generally found in natural environments as trivalent
(Cr(III)) and hexavalent chromium (Cr(VI)), while the other
valence states are usually unstable and short-lived in the eco-
system [6–10]. Trivalent chromium is much less toxic than
hexavalent and is considered important for humans in trace
concentrations because it helps the human body to control
blood sugar levels [11], while hexavalent chromium is
reported as a carcinogenic and mutagenic agent, and it
is a powerful epithelial irritant and is toxic to many
plants, aquatic animals, and bacteria [12,13].
Hexavalent chromium also causes stomach upsets, ul-
cers, kidney and liver damage, and even death [14].
Removal of toxic hexavalent chromium has been studied
for many years using different methods such as chemical pre-
cipitation [15–17], filtration [18,19], electrochemical treat-
ment [20,21], reduction [22–24], ion exchange [25–28],
membrane technology [29–31], evaporation removal [32], re-
verse osmosis [33,34], and solvent extraction [35,36].
However, most of these methods have defects such as lack
of clarity including the incomplete removal of metals, their
high cost, and the production of toxic sludge and other
waste disposal products that may result in environmental
problems [37]. Therefore, other treatment methods as
well as modification and simplification of these methods
are still needed [9,38–43].
Adsorption is still the most economical process and widely
used in removing toxic pollutants from water. This process is
the most widely used to remove toxic metal ions from water.
Several reports have already been published about producing
low-cost adsorbents using cheaper and readily available ma-
terials [44–50].
Activated carbon can be used as an absorbent material for
different industrial applications, and activated carbon has a
great variety and wide range of applications in the industry
and has been proven to be an effective absorbent material in
removing a wide range of organic and inorganic pollutants
from different media because it possesses many features such
as porous structure and high thermal stability [51–54].
Activated carbon is categorized based on its size form into
four types: granular, powder, fibrous, and clothing, each of
them has its own application in addition to the advantages
and disadvantages inherent in treating polluted water [44,
55]. Therefore, the production of low-cost activated carbon
becomes a major goal for many researchers around the world
since commercial activated carbon is still very costly.
Adsorption can be an effective and varied method for re-
moving chromium, especially when using good methods of
carbon recycling and activation. This method may solve
sludge disposal problems and make the treatment system more
economical to use in wastewater treatment, especially when
low-cost adsorbents are used [16,56–58]. In view of previous
research, several studies have published the use of activated
carbon adsorbents produced from cheap and easily available
sources to reduce production costs for wastewater treatment
[59–61]. Having activated carbon on such features as high
surface area, small grain size, and chemical surface nature
made it possible for adsorbents to remove heavy metals from
polluted water [62–65].
Various agro-industrial and domestic wastes have been
used for preparation of activated carbons, for instance, luffa
cylindrical [66], tobacco petiole [67], Sakura waste [68],
worn tires [69], waste sludge [70], sugarcane bagasse
[71–76], Bermuda grass [77], and bovine bone [78].
They all showed a good performance in removing pol-
lutants from the aquatic environment.
The annual global production of sugarcane bagasse in 2018
was 1907 million metric tons [79,80]. Sugarcane residues are
used as fuel in boilers, to generate electricity and steam, or in
animal feed or as raw materials for the manufacture of paper
and cardboard, but the residue remains as a surplus which may
cause the disposal problem for millers. Among the uses is
sugarcane residue, which can be converted into activated car-
bon with good adsorption properties and this may lead to
alleviating the problems of disposal and management of these
secondary wastes, with the possibility of providing a high-
quality final product used to treat polluted water and this can
be done to contribute to the expansion of the activated carbon
market [81].
Sugarcane bagasse consists of α-cellulose (44.32%), hemi-
cellulose (30.28%), Klason lignin (21.51%), soluble lignin
(2.33%), ashes (2.46%), and extractives in acetone (2.48%)
[82]. The presence of α-cellulose, hemicellulose, and Klason
lignin biological polymers makes sugarcane bagasse rich in
hydroxyl and phenolic groups. This type of chemical compo-
sition can give absorbents with good properties [83]. In the
present work, microporous nano-activated carbon was pre-
pared from an economically viable material sugarcane ba-
gasse, by zinc chloride activation. The micropore sur-
face area and micropore volume from the N
2
gas
adsorption-desorption isotherm data were calculated.
The microporous nano-activated carbon (MNSAC) pre-
pared at optimum conditions was applied for the remov-
al of toxic Cr(VI) ions from the aqueous phase.
Biomass Conv. Bioref.
2 Materials and methods
2.1 Materials
Sugarcane bagasse was collected from the local market in
Alexandria, Egypt, and washed with tap water to remove dirt,
and then with distilled water and finally dried in the oven at 70
°C for 96 h. The dried sugarcane bagasse was milled and
sieved for size <100 μm for subsequent studies. 1000 mg/L
stock solution of Cr(VI) ions was prepared via dissolving of
potassium dichromate (K
2
Cr
2
O
7
) (2.83 g) in double-distilled
water (1 L). Zinc chloride (ZnCl
2
,MW136.30g,assay
99.5%), and starch (C
6
H
10
O
5
)
n
were obtained from
Universal Fine Chemicals PVT-LTD, Mumbai, India.
Sodium nitrate (NaNO
3
, MW 84.99 g) was obtained from E.
MERCK, Darmstadt. Iodine and sodium thiosulphate
(Na
2
S
2
O
3
.5H
2
O, MW 248.17 g, assay 99%), and potassium
dichromate (K
2
Cr
2
O
7
, MW 294.195 g) were obtained from
ADWIC, El-Nasr Pharmaceutical Chemical Company,
Egypt. Potassium iodide (KI) was obtained from Sigma-
Aldrich. Hydrochloric acid (HCl, M.W 36.46 g, Assay 30–
34%) was obtained from SD Fine-Chem Limited (SD FCL),
Mumbai, India, and 1,5-diphenylcarbazide as a reagent for
Cr(VI) was obtained from BDH chemicals LTD, England.
2.2 Activated carbon preparation
The dried sugarcane bagasse (800 g) was impregnated with
zinc chloride solutions including 400 g of zinc chloride in 1-L
distilled water (ratio 2:1 weight:weight). After a 24-h impreg-
nation period, the mixtures were dehydrated in 24 h at 110 °C
and then pyrolyzed in stainless steel 5 cm diameter and 60 cm
length at 900 °C for 30 min holding time under flow of nitro-
gen gas (100 mL/min). The activation temperature, holding
time, and impregnation ratio were selected depending on our
previous work [53,54]. The activated carbon was cooled to
room temperature and boiled in a solution of 3 M HCl, filtered
off, and washed with distilled water until the washing solution
contains no chloride. During carbonization and ZnCl
2
activa-
tion processes, volatile substances, cellulose and lignin in sug-
arcane bagasse were gradually pyrolyzed, which resulted in
the weight decrease of samples. The final activated carbon
product was dried at 105 °C overnight to give 31.5% yield
and then crushed and sieved to a particle size <100 μmand
kept in a bottle until use. This method gave the highest surface
area activated carbon with micropore structure.
2.3 Artificial wastewater preparation
All solutions and reagents were prepared using double-
distilled water and the pH of solution was adjusted using
0.1 M HCl or NaOH. The Cr(VI) ion adsorption experiments
were studied at room temperature (25±2 °C). Analytical-grade
reagents were used throughout this study. To confirm
the results, all trials were performed three times and
only average values were reported in this study with
standard deviation less than ±3.
2.4 Batch experiment
A batch adsorption experiment was employed to evaluate the
adsorption capability, thermodynamic, and kinetic parameters
of MNSAC. A series of Erlenmeyer flasks (250 mL) contain-
ing 100 mL of different initial concentrations of Cr(VI) ion
solution ranging between 100 and 850 mg/L and different
amounts of MNSAC ranging between 1.0 and 3.0 g/L were
shaken at 200 rpm for a certain time. The pH screening was
done first in order to gain the optimal pH. The sample pH was
adjusted to the desired values before and during the experi-
ment work with 0.1 M HCl or NaOH. The concentration of
Cr(VI) ions was determined by spectrophotometry
(SPEKOL1300 UV/Visible) using the method of 1,5–
diphenylcarbazide as chromogenic agent (λ
max
= 540 nm)
[8,9,38]. The adsorption capacities at equilibrium (q
e
)were
calculated from Eq. (1):
qe¼c0−ce
wVð1Þ
The effect ofpH was studied for MNSACby using 100 mL
of 100 ppm of initial Cr(VI) ion concentration using solution
pH ranging between 1 and 5.5. The removal (%) was calcu-
lated by Eq. (2):
Removal %ðÞ¼
C0−Ct
C0100 ð2Þ
The effects of adsorbent dose, the kinetics, and the iso-
therm studies for MNSAC were performed using various ini-
tial concentrations of Cr(VI) ion solution ranging between 100
and 850 mg/L applying different MNSAC doses ranging be-
tween 1.0 and 3.0 g/L at optimum pH value. The experiment
mixtures were shaken at 200 rpm, and the concentrations of
Cr(VI) ions were analyzed at different interval times.
2.5 Characterization of MNSAC
2.5.1 Bulk density test
A specified volume in a glass cylinder (10 mL) with a powder
MNSAC was dried in an oven at 80 °C overnight. MNSAC
(2.0 g) was inputted in a cylinder and tapped for 2 min to make
compact for the carbon in the cylinder and the bulk density
was calculated as g/mL following Eq. (3)[84]. Prepared
MNSAC showed higher bulk density (0.4219 g/mL) than
raw material sugarcane bagasse (0.1574 g/mL).
Biomass Conv. Bioref.
Bulk density ¼Weight of dry material gðÞ
Volume of packed dry material mlðÞ ð3Þ
2.5.2 Iodine number (IN)
IN is known as the amount of iodine adsorbed by 1.0 g of
carbon and it is considered as a relative indicator of porosity
and can be used for measuring micropore content. Different
weights of the MNSAC sample (0.05–0.20 g) were taken in a
250-mL conical flask, and 10 mL of 5% HCl was added and
the conical flask was swirled until the carbon become wet.
Then 100 mL of stock iodine solution (2.7 g of iodine and
4.1 g of potassium iodide in 1 L of double-distilled water) was
added to the reaction mixture and then shake for 5 min. 50 mL
of the samples was titrated with 0.1 M sodium thiosulphate
until the solution become pale yellow followed by addition of
starch indicator solution 1% (1 mL) and the titration was con-
tinued until the solution become colorless. A blank was pre-
pared without adding carbon. The iodine removed % by
MNSAC was calculated by applying Eq. (4)[84]. The maxi-
mum removal is 95.65% with 0.2 g of MNSAC (Fig. 1).
Removal% ¼mL of Na2S2O3used blankðÞ−mL of Na2S2O3used sampleðÞ
mL of Na2S2O3used blankðÞ 100
ð4Þ
2.5.3 Batch test for Methylene blue number (MBN)
The MBN is the amount of methylene blue adsorbed on the
activated carbon under specified conditions and is known as
an indication of the mesopore structure of activated carbon
and it is a good indicator for the uptake of larger particles from
water. 100 mL of MB dye solutions with initial concentrations
of 50–200 mg/L was placed in 300-mL conical flasks contain-
ing 0.1 g of MNSAC and shake at room temperature for 24 h
to reach equilibrium. The concentrations of MB dye were
analyzed in the supernatant solution before and after adsorp-
tion process using an analytical Jena (SPEKOL300) spectro-
photometer at λ
max
665 nm. The amount of adsorption at
equilibrium, q
e
(mg/g) was calculated by Eq. (1)[84,85].
The MBN is the maximum amount of MB dye adsorbed by
0.1 g of MNSAC sample [69]. The maximum MBN was
199.94 mg/g (Fig. 2).
2.5.4 Determination of zero point of charge pH
ZPC
To determine the pH
ZPC
(pH at which charge of the solid is
zero) value of MNSAC, 0.1 g of sample was introduced into
100 mL of 0.1 M NaNO
3
solution with initial pH values rang-
ing between 3 and 10 was shaken for 24 h. After that, the
sample was separated from the solution and equilibrium pH
values of the solution were measured. The difference between
initial and equilibrium pH was plotted against initial pH of the
solution (Fig. 3). The point at which the graph crossed the x-
axis noted as pH
ZPC
value [85–87]. The pH
ZPC
value of
MNSAC was found to be basic pH 7.23 (Fig. 3).
2.5.5 Physical characterization
TGA of the raw precursor (sugarcane bagasse) was carried out
by a thermogravimetric analyzer (SDT Q600 V20.5 Build 15),
where the sample was taken in silica crucible and subjected to
pyrolysis under N
2
flow (100 mL/min) from 50 to 900 °C with
heating rate of 20 °C/min.
The morphology of the surface and pore size distribution of
the optimized sample were observed by a scanning electron
microscope (Quanta 250 FEG) [88]. The MNSAC particle
size was obtained by TEM using JEOL 100CX II instrument
at 100 kV. Alcohol was used as a solvent to spread the sample
on the grid for TEM analysis. The porous characteristics of
MNSAC were determined from N
2
gas adsorption isotherm
data performed at 77 k [89,90]. Before BET surface analysis,
the MNSAC sample was degased for 3 h at 300 °C under N
2
gas. Surface area of the samples was determined by applying
0
10
20
30
40
50
60
70
80
90
100
050100150200
Removal of iodine %
MNSAC (mg)
Fig. 1 Iodine number removal % using MNSAC samples (50–200 mg) at
room temperature
49.92
99.95
149.90
199.94
0
50
100
150
200
250
50 100 150 200
qe(mg/g)
MNSAC dose (mg/L)
Fig. 2 Methylene blue number using (50–200 mg/L initial concentration
of MB and 1.0 g/L of MNSAC) at room temperature
Biomass Conv. Bioref.
BET (Brunauer-Emmett-Teller) (S
BET
) and the total pore vol-
ume (V
T
) was determined by the volume of N
2
adsorbed at
high relative pressure [91]. The micropore surface area and
micropore volume (V
m
) were determined by using (BELL
SORP MINI II, Japan). The pore size distribution (PSD) was
estimated by MP (micropore) analysis method.
2.5.6 Functional group analysis
The Fourier transform infrared (FTIR) spectrometry was
applied in the characterization of the surface functional
groups of the prepared MNSAC. To obtain the observed
absorption spectra, the MNSAC was grounded to a very
fine powder and then dried at 70 °C overnight. The
surface functional groups of the MNSAC were estimat-
ed by a Bruker VERTEX 70v FTIR spectrometer con-
nected to a Platinum ATR V-100 model in the wave-
number range (400–4000 cm
−1
)[53,54].
2.5.7 Particle size analysis
The particle size of the MNSAC powder activated carbon
samples was analyzed using Malvern Instruments Ltd.,
Master Seizer 3000 (UK). Samples were dispersed in wa-
ter under stirring using an effective magnetic stirrer (2900
RPM) and ultrasonic effect before feeding into the instru-
ment. The laser diffraction technology relies that the laser
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
23456789
pHf -pH
i
pHi
Fig. 3 Zero point of charge (pH
ZPC
) of MNSAC dose (100 mg) with
initial pH values ranging between 3 and 10
Fig. 4 Thermal gravimetric
analyses (TGA) under N
2
of a
sugarcane bagasse and bsugar-
cane bagasse/ZnCl
2
Biomass Conv. Bioref.
beam passing through particles will scatter at an angle
directly related to the size of the particles [47,92].
2.6 Adsorption isotherm
Batch adsorption experiments were carried out at room
temperature by agitating MNSAC doses (0.05, 0.075,
0.1, 0.125, and 0.3 g) with 100 mL of Cr(VI) (200–850
mg/L) for 3 h at pH 1.0 and then the reaction mixture was
analyzed for the residual Cr(VI) ion concentration [7,8,
10]. The Cr(VI) ion concentrations during and at equilib-
rium adsorption process were measured following to stan-
dard method reported by Gilcreas et al. [93] using a UV-
vis spectrophotometer (SPEKOL1300 UV/Visible spec-
trophotometer) with glass cells at wavelength λ
max
540
nm. The amount of Cr(VI) ions adsorbed onto MNSAC
(q
e
, mg/g) was calculated by the mass balance relationship
Eq. (1). The percentage of removed Cr(VI) ions (R%) was
calculated by Eq. (2). The effect of pH was studied for
MNSAC by using 100 mL of 100 mg/L of initial Cr(VI)
ion concentration using solution pH ranging between 1
and 6. The percentage of removal was calculated by Eq.
(2). The kinetics and the isotherm studies for MNSAC
were performed using various initial concentrations of
Cr(VI) ion solution (200–850 mg/L) using different doses
(1.0–3.0 g/L) at pH 1.0. The reaction mixture was shaken
at 200 rpm, and the Cr(VI) ion concentration was ana-
lyzed at different interval times at room temperature (25
±2 °C).
3 Results and discussion
3.1 Materials characteristics
3.1.1 Thermogravimetric analysis
The TGA weight-loss curves for sugarcane bagasse (Fig.
4a) and for sugarcane bagasse/ZnCl
2
under a N
2
atmo-
sphere are distinctly different (Fig. 4b). The bagasse pre-
sents two continuous regions of weight loss from 200 to
400 °C and from 442 to 730 °C, with a dTGA/dt peak at
approximately 350 °C. The weight-loss processes of sug-
arcane bagasse occurring below 400 °C are due to the
dehydration and release of volatile materials from the
sample. The weight losses at temperatures less than 100
°C for sugarcane bagasse and sugarcane bagasse/ZnCl
2
are due to water evaporation. The complex thermal
Fig. 5 Scanning electron
microscopy images of a
sugarcane bagasse, bsugarcane
bagasse/ZnCl
2
,bprepared
activated carbon (MNSAC), and
dTEM of MNSAC
Biomass Conv. Bioref.
response of sugarcane bagasse/ZnCl
2
in Fig. 4b is consis-
tent with results reported for the ZnCl
2
activation of other
biomass materials. The first weight-loss step from 36.37
to 136.42 °C (16.53% loss) is due to the dehydration of
the material which is promoted by the nature of ZnCl
2
Lewis acid and the loss of volatile compounds of the
sugarcane bagasse. The second weight-loss step from
163.24 to 348.90 °C (27.51% loss) includes the volatili-
zation of ZnCl
2
. The third weight-loss step at 388.09 to
617.07 °C (24.77% loss) includes carbon mass-loss via
active pyrolysis and aromatic condensation reactions.
The fourth weight-loss step from 617.07 to 885.24 °C
(18.88% loss) includes mass-loss due to loss of oxygen,
function groups, and aromatic formation [53,54].
3.1.2 Electron microscopy (SEM and TEM)
The raw materials of sugarcane bagasse, sugarcane ba-
gasse/ZnCl
2,
and the prepared activated carbons were
examined by scanning electron microscopy (SEM) to
analyze the surface physical morphology of the raw
and its adsorbents. SEM micrographs of the sugarcane
bagasse, sugarcane bagasse/ZnCl
2
, and MNSAC are pre-
sented in Fig. 5a–c. From the SEM images, pores of
different sizes and shapes could be observed. In activat-
ed carbons, a well-developed porous surface was ob-
served at higher magnification and the dark area is con-
sidered micropore. The SEM micrograph of MNSAC
showed that it has an external surface full of cavities,
Fig. 6 EDX of asugarcane
bagasse, bsugarcane bagasse/
ZnCl
2
,cprepared activated
carbon
Biomass Conv. Bioref.
which resulted from the evaporation of ZnCl
2
during
carbonization forming space that known as pores have
diameter in micrometer (μm) range [88]. These pores
are considered as channels to the microporous network.
MNSAC was characterized by transmission electron mi-
croscopy (TEM) to measure its particle size. 25 mL of
ethanol was used as a solvent to spread the sample (5
mg) on the grid and the image is presented in Fig. 5d.
The TEM analysis proved that the MNSAC particle size
ranged between 10.4 and 33.3 nm.
3.1.3 Energy-dispersive X-ray (EDX) quantification
Chemical compositions of the samples were analyzed with
EDX. This analysis was carried out for the raw materials of
sugarcane bagasse, sugarcane bagasse/ZnCl
2
, and the pre-
pared activated carbons were examined to elements percent
as C, O, N, and Si (Fig. 6a–c).The sugarcane bagasse gave the
value of C = 57.40%, O = 37.13%, and Si = 3.31%, where the
element in sugarcane bagasse/ZnCl
2
(C = 30.92%, O =
20.87%, Zn = 24.94%, Cl = 22.82%) and MNSAC consist
of C = 87.69%, O =10.92%, and Si = 0.57%.
3.1.4 Fourier transform infrared (FTIR) spectroscopy
The surface chemistry of raw sugarcane bagasse, sugarcane
bagasse/zinc chloride, and MNSAC samples was analyzed by
FTIR (Fig. 7). FTIR bands can be classified into four regions
of the following spectra: 4000–2000, 2000–1300, 1300–900,
and 900–600 cm
−1
. The first region usually assigned to dehy-
dration and aliphatic units, mostly free O-H, H bonded OH,
adsorbed H
2
O, symmetric, and asymmetric stretching in CH-,
CH
2
-, or CH
3
-bonds. The second range (2000–1300 cm
−1
)
comprises the most important oxygen functionalities charac-
terized by the presence of C=O and N-O containing structures
in carbonyls, lactones, aldehydes, and carboxylic radicals.
Absorption in the third range, which appears within 1300–
900 cm
−1
as a broad and strong band, is currently assigned
to various C-O single bonds such as those in ethers, esters,
phenols, and hydroxyl groups. Moreover, shoulder bands at
lower wave numbers (830, 760, 670, and 600 cm
−1
)mightbe
associated with out-of-plane bending modes of C-H as in
78%
81%
84%
87%
90%
93%
96%
99%
400140024003400
Transmiance [%}
Wavenumber cm–1
(a)
52%
57%
62%
67%
72%
77%
82%
87%
92%
97%
400140024003400
Transmiance [%}
Wavenumber cm–1
(b)
99%
99%
99%
99%
100%
100%
100%
400140024003400
Transmiance [%}
Wavenumber cm–1
(c)
Fig. 7 FTIR of asugarcane
bagasse, bsugarcane bagasse/
ZnCl
2
,cMNSAC
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0.01 0.10 1.00 10.00 100.00 1000.00 10000.00
Volume Density (%)
Size classes (μm)
Fig. 8 Particle size for activated carbon from sugarcane bagasse
(MNSAC), Dv (10): 0.0229 μm, Dv (50): 0.0968 μm, and Dv
(90): 12 μm
Biomass Conv. Bioref.
benzene derivatives [53,54]. In the case of raw material, FTIR
of the sugarcane bagasse showed broad absorption peaks
around 3338.19 cm
−1
due to the hydroxyl groups of cellulosic
materials. The peak observed around 3000 cm
−1
is considered
for CH
3
group; the peaks around 1241 cm
−1
are due to C-O
stretching of COOH and the peak at 1033.5 cm
−1
is due to C-
H aromatic, where the peak at 558 cm
−1
represented the C-C
stretching vibrations (Fig. 7a).
FTIR of sugarcane bagasse impregnated with zinc chloride
showed intense absorption peaks around 3340 cm
−1
, which is
considered for hydroxyl groups. The peak observed at 1623
cm
−1
is due to the C=C stretching of the aromatic C-C bond
and the peak observed at 1032.93 cm
−1
is considered for C-H
aromatic, where the peak at 556.71 cm
−1
represented C-C
stretching vibrations (Fig. 7b). The FTIR of MNSAC indi-
cates that the broad peak around 3327.39 cm
−1
is related to
stretching vibrations of the O-H group. The peaks at 2352
cm
−1
are due to CO
2
adsorption from the air and the peak
observed at 1791 cm
−1
is due to the stretching vibration of
carboxyl group bond and may be assigned to carboxylic acids
or to their esters. The peak observed at 1530 cm
−1
is due to
C=C stretching vibrations of aromatic C-C bond and the peak
observed at 1099 cm
−1
is considered for the C-O-C bond. The
peak observed at 617 cm
−1
is assigned to C-C stretching vi-
brations and the peak observed at 468 cm
−1
is due to metal
oxide (Fig. 7c).
3.1.5 Particle size analysis
The results of the particle size distribution analysis of
the MNSAC obtained at 2700-rpm rotation speed under
ultrasonic are presented in Fig. 8.TheMNSACanalysis
showed two narrow particle size distribution peaks with
spam 12.91 (the span is the width of distribution, the
narrower the distribution, the smaller the span becomes).
The particle size distribution analysis showed that about
10% of the MNSAC represented a particle size below
22.9 nm. However, 50% of the sample reported particle
size less than 96.8 nm and the rest of the sample has
particle size less than 12 μm.
3.1.6 Surface area, pore distribution, and porosity
Adsorption isotherm was performed to study the physi-
cal properties of MNSAC at 77 K of N
2
.Thecurveof
nitrogen adsorption gives qualitative information on the
mechanism of adsorption and the structure of the pores
of MNSAC (Fig. 9). MNSAC exhibited adsorption-
desorption isotherm that following type I as classified
by the International Union of Pure and Applied
Fig. 9 aAdsorption-desorption
analysis of MNSAC, bBET sur-
face area curve of MNSAC, cMP
surface area curve of MNSAC,
and dsurface area curve from t-
plot of MNSAC
0
10
20
30
40
50
60
70
80
0123456
Removal %
pH
Fig. 10 Effect of pH ranging between 1 and 5.5 on Cr(VI) ion adsorption
by MNSAC
Biomass Conv. Bioref.
Chemistry (IUPAC) [94]. This analysis proved that the
MNSAC is micropore (mean pore diameter less than 2
nm). The results agree with the porosity analysis obtain-
ed from t-plot which had higher porosity of 99.18%.
The BET (Fig. 9b), MP (Fig. 9c), and t-plot (Fig. 9d) surface
area obtained for the sample are 1174.1, 1322.2, and 1401.5 m
2
/
g, respectively. The sugarcane bagasse chemically activated by
ZnCl
2
developed an activated carbon has better pore characteris-
tics compared to the other samples prepared without any activa-
tion. Total pore volume of the activated carbon was calculated by
the nitrogen adsorption amount at P/P
0
value of 0.990. The vol-
ume increased as a function of the activation temperature which
is 0.5274 cm
3
/g. The pore diameter was calculated from the BET
surface area which equals 0.7 nm and total pore volume on the
assumption of the cylindrical shape of the micropore. The mean
pore diameter was 1.7966 nm [95,96].
3.2 Batch sorption studies
3.2.1 Effect of pH on Cr(VI) ion adsorption
The effect of pH on the chromium ion adsorption onto
MNSAC was obtained at 100 mg/L initial Cr(VI) ion
concentration with 0.5 g of MNSAC at 25±2 °C for 3-h
equilibrium time. The initial pH values were adjusted to
1, 2, 3, 4, and 5.17 with 0.1 M HCl or NaOH. The
suspensions were shaken using a shaker at 200 rpm for
3 h followed by Cr(VI) ion adsorbed measurement (Fig.
10)[93].
3.2.2 Effect of MNSAC dose and contact time
The effect of MNSAC dose on the uptake of Cr(VI) ions was
investigated using MNSAC concentrations of 1.0, 1.5, 2.0,
2.5, and 3.0 g/L. The experiments were achieved by shaking
known concentrations of Cr(VI) ions with the above MNSAC
doses for 3.5 h and the amount of Cr(VI) ion adsorbed deter-
mined. The results show that the equilibrium time required for
the adsorption of Cr(VI) ions on activated carbon is almost 3.5
h. The largest amount of Cr(VI) ions was attached to the
MNSAC within the first 60 min of contact time. It was found
that the Cr(VI) ion removal increased with increasing contact
time (Fig. 11).
3.2.3 Effect of Cr(VI) ion concentration on adsorption process
The metal ion initial concentration provides an important fac-
tor to overcome all mass transfer resistances of metal ions
between the aqueous and solid phases represented the effect
of different initial concentrations of Cr(VI) ions (Fig. 12)[38].
The results indicate that the removal ratio of Cr(VI) ions de-
creased with increasing the initial concentration of Cr(VI)
ions. This can be attributed to the fact that all sorbents have
a limited number of active sites that will occupy a certain
amount of pollutants. An increase in the initial chromium
concentration leads to a decrease in the initial rate of external
diffusion and an increase in the rate of chromium propagation
within the adsorbent particles. In the process of adsorption of
hexavalent chromium ions, the first chromium ions must face
the effect of the boundary layer and then spread from the
boundary layer into the adsorbent surface and then spread into
the porous structure of the adsorbent. The amount of Cr(VI)
ions adsorbed on MNSAC at a lower initial concentration of
chromium was smaller than the corresponding amount when
higher initial Cr(VI) ion concentrations were used. However,
at low initial chromium concentrations, the removal % of
40
50
60
70
80
90
100
0.0 1.0 2.0 3.0 4.0
Removal %
MNSAC dose (g/L)
(a)
0
20
40
60
80
100
0 50 100 150 200 250
Removal %
Time (min)
(b)
1 .0g/l 1.5 g/l
2.0g/l 2.5 g/l
3.0 g/l
Fig. 11 Effect of aMNSAC dose
and bcontact time on the removal
% of Cr(VI) ions (300 mg/L) on
different MNSAC doses at pH 1.0
and room temperature
0
20
40
60
80
100
120
0 200 400 600 800 1000
Removal %
Cr(VI) Cocentraons (mg/L)
1.0 g/L
1.5 g/L
2.0 g/L
2.5g g/L
3.0 g/L
Fig. 12 Effect of different Cr(VI) initial concentrations of ions and
different MNSAC doses on the removal % of Cr(VI) ions at room
temperature
Biomass Conv. Bioref.
chromium was high while at higher initial concentrations the
removal % was low, which clearly indicates that the adsorp-
tion of Cr(VI) ions from its aqueous solution depends on its
initial concentration (Fig. 12).
3.2.4 Thermodynamic study
The effect of temperature on Cr(VI) ion adsorption on
MNSAC was performed at temperatures ranging between 20
and 60 °C using 3.0 g/L of MNSAC and 300 mg/L of Cr(VI)
ion initial concentration at pH 1.0 (Fig. 13a). The adsorption
rate of Cr(VI) ions was high at 60 °C while the rate of Cr(VI)
ion adsorption was low at 20 °C. A rapid increase in adsorp-
tion was observed initially within the first 30 min at different
temperatures and then a gradual increase was observed with
time. The adsorption of Cr(VI) ions increased from 93.61 to
99.79 % with the rise in temperature from 20 to 60 °C. The
increase in the adsorption percentage of Cr(VI) ions by
MNSAC indicates an endothermic process. At higher temper-
atures, the rate of diffusion of solute within the pores of the
adsorbent increases since diffusion is an endothermic process.
Thus, the adsorption percentage of Cr(VI) ions increases as
the rate of diffusion of Cr(VI) ions in the external mass trans-
port process increases with temperature [97]. The high remov-
al % of Cr(VI) ions by MNSAC may be attributed to the high
diffusion rate of Cr(VI) ions into the MNSAC pores as the
surface area and pore volume of MNSAC were large.
Moreover, at low temperatures, the kinetic energy of Cr(VI)
ion species is low, and hence contact between the metal ions
and the active sites of MNSAC is insufficient, which led to
reduce efficiency of adsorption. Adsorption increase with
temperature may also be due to an increase in the number of
adsorption sites created as a result of breaking some internal
bonds near the edge of the active surface sites for absorption.
The nature of the adsorptionof Cr(VI) ions on the prepared
MNSAC was predicted by estimating the thermodynamic pa-
rameters. The changes in thermodynamic parameters such as
free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) are cal-
culated from Eq. (5)[98].
ΔGo¼−RT lnKCð5Þ
where K
C
constant can be calculated from Eq. (6).
KC¼qe
Ceð6Þ
The K
C
values were used to determine the ΔG°, ΔH°(kJ/
mol), and ΔS° (kJ/mol) as a function of temperature Eq. (7).
lnKc¼ΔH°
RT þΔS°
Rð7Þ
ΔH°andΔS° are obtained from the slopes and intercepts of
the plots of ln K
C
against 1/TasshowninFig.13b
[99]. The ΔG° shows the degree of the spontaneity of
the adsorption process. The higher negative value of
ΔG° reflects the more energetically favorable adsorption
and the adsorption is feasible and thermodynamically
spontaneous [100]. The increase in a negative value of
ΔG° with increase of temperature proved that the ad-
sorption of Cr(VI) ions on MNSAC increased with the
rise in temperature. The positive values of ΔH° con-
firmed the endothermic nature of the MNSAC for
Cr(VI) ion adsorption in the studied range 30–60 °C.
The positive values of ΔS° confirmed the randomness
of the adsorption process [101,102]. The thermodynam-
ic parameters for adsorption of Cr(VI) ions on MNSAC
are presented in Table 1.
3.3 Kinetics studies
Kinetic models have been used to test experimental data and
to determine the adsorption mechanism and potential rate-
control step that includes mass transfer and chemical reaction.
0
20
40
60
80
100
0 50 100 150 200
Removal %
Time (min)
(a)
20 °C
30 °C
40 °C
50 °C
60 °C
y = -0.9146x + 6.8616
R² = 0.9429
3.7
3.8
3.9
4.0
4.1
4.2
2.9 3.1 3.3 3.5
ln Kc
1/T ×10
–3 (K-1)
(b)
Fig. 13 aEffect of temperature
on Cr(VI) ion adsorption by
MNSAC (pH 1, Cr(VI) ion
concentration, 300.0 mg/L) and b
plot of ln K
c
vs. 1/Tfor adsorption
of Cr(VI) ions (300 mg/L) on
MNSAC (3.0 g/L)
Table 1 Thermodynamic parameters for adsorption of Cr(VI) ions on
MNSAC
ΔG°ΔH°ΔS°
30 °C 40 °C 50 °C 60 °C
−9682 −10252 −10823 −11393 7603.65 57.047
Biomass Conv. Bioref.
Adsorption kinetics is expressed as the rate of solute removal
using solid materials. These models include pseudo-first-or-
der, pseudo-second-order, intraparticle diffusion, and film dif-
fusion models [51]. Lagergren proposed a pseudo-first-order
kinetic model that was applied successfully to describe the
kinetics of many adsorption systems [103]. The integral form
of the model equation is expressed as in Eq. (8).
log qe−qt
ðÞ¼log qe
ðÞ−k1
2:303tð8Þ
The values of k
1
and q
e
were calculated from the slopes and
intercepts of log (q
e
−q
t
) against the tplots (Fig. 14a) and the
calculated data are summarized in Table 2. The lower corre-
lation coefficients (R
2
) obtained suggest that the adsorption of
Cr(VI) ions on MNSAC does not follow the pseudo-first-
order kinetics model, which can be attributed to the boundary
layer control of the absorption of Cr(VI) ions in the primary
stages [38,104]. The adsorption kinetics may be described
also by a pseudo-second-order kinetic model which can be
presented by the linearized integral form in Eq. (9)[51,105].
t
qt
¼1
k2q2
e
þ1
qe
tðÞ ð9Þ
The initial adsorption rate, h(k
2
q
e2
)[105], has been widely
used for evaluation of the adsorption rates [106]. From the slopes
and intercepts of the linear plots obtained by plotting t/q
t
versus t
(Fig. 14b), the values of the pseudo-second-order rate constants
q
e
and k
2
are calculated and reported in Table 2. The experimen-
tal adsorption results of Cr(VI) ions by MNSAC showed the
higher applicability to the pseudo-second-order equation with
R
2
values very close to one. The adequate fitting of the plots
confirmed that the adsorption of Cr(VI) ions by MNSAC follow-
ed pseudo-second-order kinetic model (Table 2). Weber and
Morris model is a widely used intraparticle diffusion model to
predict the rate-controlling step [107,108]. When mass transfer is
the controlling step, it is important to identify the diffusion mech-
anism. According to intraparticle diffusion model, the initial rate
of diffusion is given by Eq. (10).
qt¼Kdif t1=2þCð10Þ
The values of K
dif
(mg/g min
1/2
)weredeterminedfromthe
slopes of respective plots of qversus t
1/2
(Fig. 14c).
Adsorption of metal ion at an active site on the solid-phase
surface could also occur and could have chemical reaction
such as ion exchange, complexation, and chelation [38,
109]. The results of interaparticle diffusion and Film diffusion
analyses were presented in Table 3.
The heavy metal adsorption is usually by either the
intraparticle or liquid-phase mass transport rates. There
is a possibility that the diffusion within the particles is
the step of controlling the adsorption rate if the adsorption
experiment is carried out by the batch system with con-
tinuous rapid stirring. In the case of the involvement of
intraparticle diffusion in the adsorption process, the plot
-2.5
-1.5
-0.5
0.5
1.5
0 50 100 150 200 250
Log(qe-qt)
Time(min)
(a)
100 mg/L
200 mg/L
300 mg/L
500 mg/L
700 mg/L
850 mg/L
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 50 100 150 200 250
t/qt(min.g/mg)
Time (min)
(b)
100 mg/L
200 mg/L
300 mg/L
500 mg/L
700 mg/L
850 mg/L
20
40
60
80
100
120
140
160
1.00 6.00 11.00 16.00
qt(mg/g)
Time1/2 (min)1/2
(c)
100 mg/L 200 mg/L
300 mg/L 500 mg/L
700 mg/L 850 mg/L
0
2
4
6
8
0100200300
-ln(1-F)
Time (min)
(d)
100 mg/L
200 mg/L
300 mg/L
500 mg/L
700 mg/L
850 mg/L
Fig. 14 Kinetics plots apseudo-
first-order, bpseudo-second-
order, cintraparticle diffusion, d
film diffusion kinetic models for
adsorption of Cr(VI) ions for
adsorbent dose 3.0 g/L
Biomass Conv. Bioref.
Table 2 Pseudo-first- and second-order parameters for the adsorption of Cr(VI) by MNSAC
First-order kinetic model Second-order kinetic model
Carbon conc. Cr(VI) (mg L
−1
)q
e
(exp.) k
1
×10
3
q
e
(calc.) R
2
k
2
×10
3
q
e
(calc.) HR
2
1.0 g/L 100 86.39 36.16 77.68 0.974 0.52 101.01 5.32 0.992
200 131.20 11.05 70.65 0.991 0.42 136.99 7.86 0.997
300 155.19 7.3700 41.24 0.801 0.51 156.25 12.45 0.989
500 168.57 4.51 397.47 0.700 0.36 181.82 12.01 0.991
700 233.83 9.21 88.7 0.948 0.30 232.56 16.42 0.997
850 265.94 3.92 82.81 0.935 0.52 232.56 27.86 0.997
1.5 g/L 100 66.52 95.11 68.47 0.982 3.14 70.42 15.57 1.000
200 130.83 8.52 32.78 0.880 0.76 123.46 11.60 0.996
300 123.91 11.05 52.60 0.955 0.45 128.21 7.39 0.995
500 159.70 11.28 52.23 0.916 0.49 163.93 13.11 0.996
700 175.54 4.61 19.85 0.922 1.48 172.41 43.86 0.997
850 208.57 4.38 79.14 0.960 0.68 172.41 20.20 0.995
2.0 g/L 100 49.77 49.51 3.96 0.818 19.70 50.51 50.25 1.000
200 97.93 15.43 9.04 0.963 1.87 100.00 18.66 1.000
300 96.54 13.82 23.67 0.822 1.43 98.04 13.77 0.999
500 136.62 12.44 55.14 0.954 0.47 140.85 9.33 0.996
700 174.06 17.75 50.79 0.716 0.59 175.44 18.02 0.992
850 175.38 11.75 66.25 0.946 0.39 181.82 12.84 0.993
2.5 g/L 100 39.85 46.06 1.53 0.919 66.49 40.00 106.38 1.000
200 78.95 18.65 13.82 0.943 0.47 80.00 3.02 1.000
300 96.54 15.43 55.07 0.898 0.55 117.65 7.55 0.997
500 128.30 16.58 51.20 0.909 0.61 126.58 9.79 0.997
700 154.17 14.05 44.73 0.961 0.63 158.73 15.87 0.999
850 184.57 28.79 202.63 0.847 0.24 192.31 8.97 0.989
3.0 g/L 100 33.26 38.69 0.66 0.866 155.17 33.33 172.41 1.000
200 66.29 55.73 21.74 0.9649 6.03 67.57 27.55 1.000
300 95.19 21.19 33.37 0.988 1.25 97.09 11.78 0.994
500 126.07 20.96 89.89 0.877 0.37 131.58 6.40 0.992
700 166.47 18.42 59.76 0.965 0.60 153.85 14.21 0.999
850 157.02 21.65 95.61 0.822 0.44 163.93 11.95 0.996
Biomass Conv. Bioref.
of q
t
against t
1/2
should give a linear relationship, and the
intraparticle diffusion would be the rate-controlling step if the
regression line passing through the origin [38,109]. The shape
in Fig. 14c confirms straight lines that do not pass through the
origin with the correlation coefficients that ranged between low
and high without a specific meaning. This indicates a certain
degree of control over the boundary layer and this also shows
that intraparticle diffusion within the particles is not only the rate-
control step to adsorb Cr(VI) ions on the MNSAC but also other
processes that may control the adsorption rate. When the trans-
port of the solute molecules from the liquid-phase to the solid-
phase boundary plays a most significant role in adsorption, the
liquid film diffusion model can be applied by Eq. (11)[110].
ln 1−FðÞ¼−KFD tðÞ ð11Þ
3.4 Adsorption isotherm study
The successful representation of the dynamic separation of the
adsorption of a solution by adsorbent is based on a good
description of the two-stage balance. Adsorption equilibrium
is achieved when the amount of dissolved substance on the
adsorbent is equal to the adsorption amount. Adsorption iso-
thermal equals the interaction between adsorption and adsorp-
tion, which is crucial for designing an adsorption process.
Langmuir and Freundlich are the most widely used models
to describe experimental adsorption data. These isotherms
were used to study the adsorption process of Cr(VI) ions on
MNSAC at different conditions and parameters by depicted
by plotting solid-phase concentration (q
e
) against liquid-phase
concentration (C
e
)ofsolute[111]. Langmuir model assumes
that the adsorption occurred as a monolayer with no
Table 3 Comparison of the
kinetic model intraparticle
diffusion and film diffusion for
prepared MNSAC
Carbon conc. Cr(VI) (mg L
−1
) Intraparticle diffusion Film diffusion
K
dif
CR
2
K
FD
CR
2
1 g/L 100 7.47 16.67 0.978 0.036 0.106 0.974
200 4.23 68.71 0.940 0.014 0.701 0.981
300 3.30 103.16 0.887 0.009 1.301 0.955
500 5.14 1.51 0.929 0.051 −2.577 0.920
700 6.74 128.73 0.887 0.009 0.931 0.931
850 5.55 157.56 0.790 0.004 1.129 0.957
1.5 g/L 100 3.48 38.41 0.790 0.096 −0.006 0.987
200 2.49 83.07 0.925 0.009 1.240 0.969
300 3.71 67.62 0.953 0.012 0.792 0.967
500 3.30 108.12 0.972 0.010 1.202 0.927
700 1.50 149.15 0.774 0.005 2.180 0.922
850 3.86 118.65 0.930 0.004 0.969 0.960
2 g/L 100 0.07 45.02 0.354 0.027 3.991 0.978
200 1.43 78.14 0.854 0.015 1.647 0.976
300 1.59 73.41 0.926 0.014 1.405 0.911
500 3.68 82.20 0.993 0.011 0.994 0.982
700 2.53 130.42 0.894 0.012 1.619 0.942
850 5.40 100.01 0.975 0.012 0.954 0.930
2.5 g/L 100 0.25 37.75 0.617 0.045 3.197 0.968
200 0.80 68.45 0.617 0.106 2.534 0.968
300 3.05 67.33 0.979 0.010 1.064 0.982
500 2.96 79.22 0.969 0.016 0.801 0.945
700 3.69 104.10 0.923 0.014 1.237 0.961
850 8.87 65.18 0.965 0.030 0.052 0.984
3 g/L 100 0.13 32.20 0.483 0.031 4.303 0.943
200 1.01 55.70 0.712 0.061 0.845 0.983
300 2.23 65.51 0.959 0.021 1.031 0.988
500 4.84 56.97 0.986 0.019 0.480 0.951
700 3.62 97.43 0.942 0.019 0.931 0.985
850 3.92 102.26 0.957 0.017 0.627 0.957
Biomass Conv. Bioref.
transmigration of adsorbate in the surface plane of the adsor-
bent [112,113]. Therefore, the Langmuir isotherm model was
chosen for estimation of the maximum adsorption capacity
(Q
m
) corresponding to complete monolayer coverage on the
sorbent surface. The linear form of the Langmuir equation can
be shown in Eq. 12, which gives different ways for parameter
estimation [114,115].
Ce
qe
¼1
KaQm
þ1
Qm
Ceð12Þ
The plots of C
e
/q
e
versus C
e
are represented in Fig. 15a and
the data obtained from the plots are summarized in Table 4.
The constants were evaluated from the slope 1/Q
m
and inter-
cept 1/K
a
Q
m
, where 1/Q
m
gives the theoretical monolayer
saturation capacity Q
0
. Freundlich isotherm is the most impor-
tant multisite adsorption isotherm on heterogeneous surfaces
and is characterized by the heterogeneity factor 1/n, and is
represented by linear Eq. (13).
log qe¼logKFþ1
nlogCeð13Þ
where K
F
is a constant indicator of the relative adsorption
capacity of the adsorbent and 1/nis a constant indication of the
adsorption intensity of Cr(VI) on the heterogeneity of the ab-
sorbent or surface material, to become more homogeneous as
its value approaches zero. The value of ngives an indication
of the adsorption preference; when the value of nfalls between
2 and 10, this represents good adsorption, when the value of n
falls between 1 and 2, the adsorption becomes moderate, and
when the value of n<1, the adsorption becomes weak [116].
The intercept log K
F
is a measure of adsorption capacity, and
the slope 1/nis the adsorption intensity. The Freundlich max-
imum adsorption capacity can be determined following
Halsey [117] Eq. (14).
Qm¼KFC1=n
0ð14Þ
The values of K
F
and nwere calculated from the intercept
and slope of the plots Log q
e
against Log C
e
(Fig. 15b). The
isotherms were found to be linear as evidenced from correla-
tion coefficients obtained in the range of 0.9276–0.957 for
prepared MNSAC. Langmuir isotherm model fits well the
adsorption process than the Freundlich isotherm suggesting
the homogeneous nature of the MNSAC (Table 5)[122].
3.5 Regeneration of MNSAC
Adsorption by activated carbon is a widely used technology
and the economy of this technology greatly depends on the
reuse of activated carbon. There are different regeneration
techniques that may be used for the regeneration of activated
carbon such as thermal, chemical, and wet air oxidation
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 100 200 300 400 500 600
Ce/qe
Cemg/L
(a)
1.0 g/L
1.5 g/L
2.0 g/L
2.5 g/L
3.0 g/L
1.5
1.7
1.9
2.1
2.3
2.5
-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0
Log qe
Log Ce
(b)
1.0 g/L
1.5 g/L
2.0 g/L
2.5 g/l
3.0 g/l
Fig. 15 Isotherm models a
Langmuir isotherm model and b
Freundlich isotherm model for the
Cr(VI) ion adsorption data at
room temperature
Table 4 The Langmuir and
Freundlich isotherm models
constants obtained for MNSAC
Isotherm models Isotherm parameters Activated carbon concentrations
1.0 g/L 1.5 g/L 2.0 g/L 2.5 g/L 3.0 g/L
Langmuir Q
m
(mg/g) 277.78 200.00 178.57 166.67 156.25
K
a
×10
3
11.21 30.45 29.69 63.56 88.03
R
2
0.9204 0.969 0.966 0.9664 0.9909
Freundlich 1/n 0.276 0.132 0.152 0.186 0.194
K
f
40.626 79.104 67.873 56.416 52.759
R
2
0.937 0.929 0.957 0.926 0.951
Biomass Conv. Bioref.
regeneration are well known. The thermal regeneration pro-
cess needs to maintain temperature as high as 800 °C, which
leads to high-energy consumption. It typically involves some
loss of carbon and some changes in adsorption characteristics
of carbon itself caused due to the enlargement of pores during
the reactivation process. Regeneration by wet air oxidation
requires high-pressure oxygen (0.1–1.0 MPa) that limits its
application. The chemical regeneration technique looks more
economic and was used for the regeneration of many
exhausted carbons. The advantages of this process include
the following: performance at room temperature, less energy,
no additional pressure, and the used chemical can be recov-
ered. The prepared MNSAC was regenerated by washing with
0.1 M NaOH followed by 1 M HCl and then washed with hot
distilled water. The maximum removal of chromium in the 1st
cycle is 93.61%, 2nd cycle is 82.38%, and 3rd cycle is 64.11%
for prepared MNSAC (Fig. 16). The reusability decreases
with each cycle may be attributed to the efficiency of the
regeneration process and to the possible pore blocking.
3.6 Comparison with previous studies
To determine the efficiency of the prepared activated carbon
and the activation method used, a comparative analysis was
performed with other reported literature of the activated car-
bon in terms of absorption capacity of Cr (VI) ions, and the
results are reported in Table 5. Here, activation methods and
carbon sources are also performed as a comparison given be-
cause these processes are considered standard physical and
chemical activation procedures to produce activated carbon
[8,38,39,75,118–121]. Activated carbon from pomegranate
peel was carbonized with 50% of the H
2
SO
4
using
dehydrating process and used as an adsorbent of chromium
(VI) ions to give a Q
m
of 35.2 mg/g [8], while when applying a
similar process, to date palm seeds, an active carbon was pro-
duced and applied to Cr(VI) adsorption to give a Q
m
of 120.48
mg/g [38]. On the other hand, application of dehydrating
method to red algae Pterocladia capillacea gave activated
carbon with low adsorption capacity of chromium (VI) ions
(Q
m
= 12.85 mg/g). Based on Table 5, it is shown that the
absorption of chromium (VI) ions using untreated powder
such as sugarcane bagasse [75], neem powder [118],
Eucalyptus bark [119], and rubber tree bark [120]gavea
low adsorption capacity of chromium (VI) ions. Thus, the
chemical functionalization of activated carbon derived from
the melon peels gave a high adsorption capacity of chromium
(VI) ions [121]. Therefore, MNSAC-prepared activated car-
bon revealed that the proposed activation method under opti-
mal conditions (this study) is among the best maximum ab-
sorption capabilities of chromium (VI) ions from water. All
the adsorbents were used at acidic pH values ranging between
1 and 2; therefore, a neutralization stepshouldbe added before
discharge of the treated water.
4 Conclusion
In this study, chemical activation using ZnCl
2
method was
applied to prepare microporous nano-activated carbon from
sugarcane bagasse. High specific surface area with type I
was obtained by using zinc chloride impregnation in a ratio
of 2:1 of sugarcane bagasse to ZnCl
2
. The MNSAC prepared
at activation temperature of 900 °C under a flow rate of nitro-
gen of 100 cm
3
/min revealed a high surface area (≥1174 m
2
/
g) which showed a total pore volume of 0.5067 cm
3
/g and
micropore surface area of 1302.4 m
2
/g. Hence, MNSAC,
which is a carbon-rich material, revealed high surface area
and porous structures making it suitable as an adsorbent for
the removal of hexavalent chromium from an aqueous solu-
tion. About 93.61% of hexavalent chromium was removed
from an aqueous solution of at 300-mg/L concentration by
Table 5 Comparison of maximum adsorption capacity (Q
m
)and
maximum removal percentage (MR) of hexavalent chromium by differ-
ent adsorbents
Adsorbent Q
m
(mg/g) MR (%) Ref.
MNSAC 277.78 100 This work
Pomegranate husk activated carbon 35.20 100 [8]
Date palm seed activated carbon 120.48 100 [38]
Red alga Pterocladia capillacea 12.85 58 [39]
Sugarcane bagasse 2.92 97 [75]
Neem powder 45.45 95 [118]
Eucalyptus globulus 45.00 41 [119]
Rubber 43.86 100 [120]
Melon-B biochar 72.46 69 [121]
Melon-BO-NH
2
biochar 123.46 98 [121]
Melon-BO-TETA biochar 333.33 99 [121]
0
10
20
30
40
50
60
70
80
90
100
050100150200250
Removal %
Time (min)
1st cycle
2nd cycle
3rd cycle
Fig. 16 Adsorption % of 300 mg/L Cr(VI)ion concentration and 3.0 g/L
MNSAC dose for three-cycle regeneration of MNSAC using 0.1 M
NaOH followed by 1 M HCl
Biomass Conv. Bioref.
using 3.0 g/L of MNSAC at room temperature. Results of
thermodynamic experiments showed that adsorption of
Cr(VI) ions increased with increasing temperature. Results
of determination of the thermodynamic parameters indicated
that adsorption process is spontaneous in nature and endother-
mic, and also, increasingof the temperature favors the adsorp-
tion of Cr(VI) ions onto MNSAC. Different kinetic and iso-
therm models were investigated for Cr(VI) ion adsorption by
MNSAC at room temperature. The maximum monolayer ad-
sorption capacity of Cr(VI) ions by MNSAC was 277.78 mg/
g. When regenerating this carbon three times, the adsorption is
applicable indicated the economic value of MNSAC.
Abbreviation ΔG°, Gibb’s free energy change; ΔH°, Standard enthalpy
change (kJ/mol); ΔS°, Standard entropy change (kJ/mol); BET, Brunauer-
Emmett-Teller analysis; C, The intercept of intraparticle diffusion; C
0
,Initial
Cr(VI) concentration (mg/L); C
t
, Cr(VI) concentration at time t(mg/L); DTG,
Deferential thermal analysis; EDX, Energy-dispersive X-ray analysis; F,
Fractional attainment of equilibrium (F=q
t
/q
e
); FTIR, Fourier transform infra-
red; h, Initial adsorption rate; HCl, Hydrochloric acid; IUPAC, International
Union of Pure and Applied Chemistry; k
1
, Pseudo-first-order rate constant
(L/min); k
2
, Pseudo-second-order rate constant (g/mg/min); K
2
Cr
2
O
7
,
Potassium dichromate; K
C
, The equilibrium constant; K
dif
, Rate constant of
intraparticle diffusion (mg/g/min
1/2
); K
F
, Freundlich relative adsorption capaci-
ty; K
FD
, Film diffusion rate constant; K
L
, Langmuir adsorption equilibrium
constant (L/mg); MB, Methylene blue; MNSAC, Microporous nano-activated
carbon from sugarcane bagasse; MP, Micropore analysis; MW, Molecular
weight; pH
ZPC
, Zero point of charge; PSD, Pore size distribution; q
e
,The
adsorption capacities at equilibrium (mg/g); Q
m
, Langmuir maximum mono-
layer adsorption (mg/g); Q
mF
, Freundlich maximum adsorption capacity (mg/
g); q
t
, The adsorption capacities at time t(mg/g); R
2
, Correlation coefficients;
S
BET
, Specific surface area (m
2
/g); SEM, Scanning electron microscopy; t,Time
(min); T, Absolute temperature; t
1/2
, The square root of time (min
1/2
); TEM,
Transmission electron microscopy; TGA, Thermogravimetric analysis; V,
Volume (L); V
m
, Micropore volume; V
T
, Total pore volume (cm
3
/g); W,The
mass of adsorbent (g); ZnCl
2
, Zinc chloride
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