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1. Inorganic Chemistry Communications 145 (2022) 110009
Available online 18 September 2022
1387-7003/© 2022 Elsevier B.V. All rights reserved.
Short communication
Sequential modifications of chitosan biopolymer for enhanced confiscation
of Cr(VI)
Vaishnavi Gomase a
, Ravin Jugade a,*
, Priyanka Doondani a
, D. Saravanan b
, Sadanand Pandey c
a
Department of Chemistry, R.T.M. Nagpur University, Nagpur 440033, India
b
Department of Chemistry, National College, Tiruchirappalli, Tamilnadu 620001, India
c
Department of Chemistry, College of Natural Science, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk 38541, Republic of Korea
A R T I C L E I N F O
Keywords:
Biopolymer
Chitosan
Crosslinking
Kinetics and isotherm study
Cr(VI) adsorption
A B S T R A C T
Chitosan was successively modified by crosslinking with oxalic acid and forming its composite with cotton-straw-
derived biochar. Cotton straw is a huge agricultural waste in central India region where cotton is a major
agriculture product while chitosan is the second largest available biopolymer. Native chitosan (Ch), oxalate
crosslinked chitosan (ChOx) and the Chitosan-Oxalate-Biochar composite (ChOxBC) were synthesized and
characterized by FT-IR, SEM, EDX, XRD, BET surface area analysis and pHpzc etc. In batch adsorption studies for
Cr(VI) removal, all three materials showed increased adsorption capacities from 2.062 mg g− 1
for Ch, 348.3 mg
g− 1
for ChOx, and 383.8 mg g− 1
for ChOxBC after just 60 min of adsorption. The enhancement in the adsorption
capacities have been attributed to better stability of crosslinked chitosan, stronger ionic interaction with
hydrogen chromate at pH 3.0 and enhanced surface area of the composite. In thermodynamic studies, the
spontaneity of processes was assessed across the three materials. The experimental data from kinetics studies
revealed that the Cr(VI) adsorption followed pseudo-second order kinetics. A study of columns confirmed that
adsorbents can be applied to large volumes of samples. The adsorbents can be regenerated and reused, which
makes the study more environmentally friendly.
1. Introduction
Water is foundation of our lives. Freshwater is of fundamental
importance for human health. But, due to pollutants present in water the
idealness of water is missing which leads to acute or chronic diseases.
Chromium is a potential contaminant occurring in water as a result of
natural and anthropogenic sources [1]. Chromium exists in two states,
trivalent and hexavalent state. Trivalent chromium is of dietary
importance for many organisms [2]. This however is applicable to only
trivalent chromium species. Hexavalent chromium is toxic and is
harmful for both flora and fauna. Trivalent chromium is an indispens­
able trace element for humans, where it helps eliminate glucose from the
blood and helps regulate fat metabolism [3]. Hexavalent chromium is
known for is negative impacts on health, it is a carcinogen. Higher levels
of hexavalent chromium in water can lead to severe health effects like
nausea, gastrointestinal distress, kidney and liver damage and is also
associated with lung and nasal cancer [4]. In addition to naturally
occurring in groundwater, the Cr (VI) is also found naturally in rocks and
as effluent from various industries such as electroplating and leather
manufacturing [5]. According to WHO guidelines, the permissible limit
of Cr(VI) in drinking water is 0.05 mg/L [6]. Various techniques such as
adsorption [7], coagulation/flocculation [8], membrane filtration [9],
photo degradation [10], ion exchange [11], electrochemical processes
[12], microbial process [13], etc. are the most common methods of
removing Cr(VI) exist. As the most efficient way to remove Cr(VI) from
water, adsorption offers high effectiveness, selectivity, low cost, and the
ability to reuse adsorbents. Various biosorbents are capable of adsorbing
hexavalent chromium out of which chitin, chitosan, starch and cellulose
are present in plentiful amount in nature. Biopolymer chitosan is the
second most abundant in nature and is very useful because of its
biocompatibility and biodegradability. Chitosan has D-glucosamine
units joined together by β-(1 → 4) glycosidic bond. However, it has
relatively low adsorption capacity for chromium, but when combined
with suitable chemical reagents to form cross-linked chitosan or com­
posite, the adsorption capacity has been reported to increase substan­
tially [14]. Chitosan has hydroxyl and amine groups that can be used for
crosslinking and composite formation. Recently, various crosslinking
agents are used such as ethylenediamine [15], β cyclodextrin [16],
* Corresponding author.
E-mail address: ravinj2001@yahoo.co.in (R. Jugade).
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
https://doi.org/10.1016/j.inoche.2022.110009
Received 30 May 2022; Received in revised form 3 September 2022; Accepted 13 September 2022

2. Inorganic Chemistry Communications 145 (2022) 110009
2
diethylenetriaminepentaacetic acid [17] and many more. We have re­
ported crosslinking and impregnation of chitosan for enhancement of Cr
(VI) adsorption using sulphate ions [18,19] and phosphate ions [20],
composite formation with zirconia [21] and red mud [22]. In all these
materials, enhancement in adsorption capacity was observed due to
ionic interaction with Cr(VI) species present at optimum pH. Recent
literature shows application of chitosan-biochar composites for the
removal of Cr(VI) [23–25]. However, these materials work at very low
pH and the adsorption capacities are poor.
In present study, we report two hierarchical modifications of chito­
san formed by crosslinking with oxalate ions and composite of the
crosslinked chitosan with biochar derived from cotton straw. The
resultant products showed excellent behaviour towards removal of Cr
(VI).
2. Materials and methods
2.1. Materials
Potassium Dichromate, diphenylcarbazide, oxalic acid, sodium hy­
droxide, liquor ammonia were obtained from from Loba Chemie Pvt. ltd.
Sisco Research Laboratories Pvt. ltd. supplied Chitosan with a deacety­
lation degree of 90 %. Analytical-grade reagents were all used without
further purification.
2.2. Modifications of chitosan
5 g of chitosan was taken and to it 100 mL 10 % (w/v) oxalic acid
solution was added. The solution was kept for overnight stirring till a gel
was obtained. Then the chitosan-oxalate gel was dripped in 15 %
ammonia solution with the help of a syringe to form beads. The chitosan-
oxalate beads were then washed several times with de-ionized water till
the filtrate had neutral pH. The beads were dried overnight at 50 ◦
C. This
compound was labelled as Ch-Ox.
The biochar was prepared using cotton plant residues, the cotton
stalks were washed thoroughly to remove adsorbed impurities and then
sun dried. The dried stalks were then dipped into 1 N phosphoric acid for
24 h. The phosphoric acid treated stalks were dried and were carbonized
at 600 ◦
C for 5 h using electric muffle furnace with flow through ni­
trogen at the rate of 150 cm3
/min. The cotton biochar then formed was
crushed to size 100 μm. After the biochar was washed, it was diluted
with distilled water until the pH of the washings was neutral. 2.5 g of
cotton biochar and 2.5 g chitosan were mixed using mortar and pestle. It
was then mixed with 300 mL of a 10 % oxalic acid solution. This mixture
was stirred overnight and chitosan-oxalate-biochar composite was ob­
tained by dropwise addition of 15 % ammonia solution to it. Deionized
water was used to wash and filter the biochar until the pH was neutral.
This composite was labelled as ChOxBC.
2.3. Batch adsorption experiments
For the batch adsorption experiments, 25 mL Cr(VI) solution of
desired concentration (10–200 mg/L) having fixed pH (2–10) were
equilibrated for with various doses (10–400 mg) of adsorbents (Chito­
san, ChOx or ChOxBC) for various time intervals (5–120 min) at desired
temperature (298–333 K). At equilibrium (qe), the concentration of Cr
(VI) adsorbed on the adsorbents was calculated as -.
qe =
C0 − Ce
W
× V
In addition, the percent removal capacity was calculated as follows:
% Removal =
C0 − Ce
C0
× 100
C0 and Ce are the initial and equilibrium concentration of Cr(VI)
solution in mg L-1
respectively. V is the volume of Cr(VI) solution in L
and W is the weight of the adsorbent in g. The Cr(VI) estimation was
carried out using diphenylcarbazide reagent spectrophotometrically.
2.4. Physicochemical characterization
The FT-IR spectra recorded with Bruker Alpha spectrometer in the
range of 500–4000 cm− 1
were used to explain the structural specifica­
tions of the adsorbents Chitosan, ChOx, and ChOxBC. A TESCAN VEGA 3
SBH scanner was used to record the surface morphology of the adsor­
bents. The XRD spectra of the samples were recorded using a Righaku-
Miniflex 300 X-ray diffractometer in the range of 3 to 90◦
. In a nitro­
gen atmosphere, the materials were thermally analyzed using a Shi­
madzu DTG 60 analyzer at a scan rate of 20 ◦
C min− 1
in a temperature
range of ambient to 600 ◦
C. During these studies, nitrogen gas flow was
maintained at 100 mL min− 1
. Quantachrome NOVA 2200e Surface Area
and Pore Size Analyzer was used to determine the surface area of the
adsorbent using BET (Brunauer-Emmet-Teller) surface area estimation
method.
3. Results and discussions
3.1. Characterization of Ch, ChOx and ChOxBC
3.1.1. FT-IR spectra
Fig. 1 shows the FT-IR spectra of Chitosan, ChOx and ChOxBC. A
broad band in the FT-IR spectrum of chitosan is observed at 3304 cm− 1
,
which corresponds to O–H and N–H stretching vibrations, 2864 cm− 1
corresponds to C–H stretching vibrations. The spectra shows peak at
1641 cm− 1
which corresponds to the presence of C–
–O group of amide
due to the traces of chitin (N-acetyl chitosan) and a stretch at 1585 cm− 1
which corresponds to the bending vibrations C–N of amide group.
Chitosan has a characteristic skeletal vibration of 1064 cm-1 due to the
C–O–C bond [26]. The IR spectrum of ChOx shows peaks of C–
–O and
C–N of amide shifted towards lower wavenumber at 1590 and 1473
cm− 1
indicating the ionic interaction between chitosan and oxalate ions.
The IR spectrum of ChOxBC shows similar bands as in chitosan and
ChOx, N–H and O–H stretching vibrations at 3625 and 3591 cm− 1
respectively. The peak of unhydrolysed amide functional group was
observed at 1579 cm− 1
[23]. At 1064 cm− 1
, the characteristic skeletal
vibration of the C–O–C peak can be observed. After adsorption of Cr
(VI), the shift in positions of characteristic peaks were observed in all the
three materials indicating interaction between functional groups of the
materials and the Cr(VI) species at the adsorption pH condition.
3.1.2. Surface morphology and elemental analysis
The native chitosan and the two successively modified adsorbents
were examined using SEM micrographs and EDX spectra. The Ch surface
was found to be relatively smooth and uniform (Fig. 2a) thereby the
surface area should be less. On crosslinking with oxalate, the ChOx
shows fluffy morphology with enhanced surface area while on com­
posite formation with biochar, the ChOxBC shows highly porous and
non-uniform surface. The surface morphologies of these three materials
are in agreement with the BET surface area results obtained in the next
section. The EDX spectra show the peaks for C, N, O along with a residual
calcium impurity in chitosan [27].
3.1.3. X-ray diffraction studies
As illustrated in Fig. 3a, the peak for Chitosan can be observed at 2θ
= 20.023◦
. This value is in agreement with the reported value and can be
assigned to (110) plane of chitosan [28]. In ChOx this peak has been
shifted to 20.07◦
and also it was observed that the peak becomes narrow
(Fig. 3b). Similar observation has been reported by Mi et al [29]. Due to
the interaction between protonated amine groups and oxalate ions, the
shifts are a result of a decrease in adsorbent crystallinity. Additional
peaks at 15.0◦
, 30.17◦
, 35.68◦
, 44.62◦
etc can be attributed to oxalate
moiety crosslinking chitosan chains [29]. Pure biochar showed two
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3. Inorganic Chemistry Communications 145 (2022) 110009
3
broad peaks in the range of 15-30◦
and 40-50◦
showing amorphous
carbon with disorderly stacked carbon rings with (002) and (101)
planes of carbon respectively (Fig. 3c) [30]. The ChOxBC shows all of the
peaks of ChOx as well as the overlapping broad peak of biochar that
leads to distortion as well as sharpening in the peak at 19.86◦
showing
formation of composite (Fig. 3d).
3.1.4. Thermal studies
For different adsorbent materials, thermogravimetric and
differential thermal analyses were conducted in order to determine their
thermal stability. The TGA curves of Ch, ChOx as well as ChOxBC
(Fig. 4a) showed initial weight loss between 40 and 100 ◦
C which cor­
responds to loss of moisture and adsorbed water [22]. The second
degradation in the range of 250-350℃ can be attributed to degradation
of polysaccharide chains. The second degradation of the range 250-
350℃ occurs via degradation of polysaccharide chains. The total weight
loss in this region for Ch and ChOx was found to be about 65 % while the
weight loss of about 50 % was observed in ChOxBC indicating that
Fig. 1. FT-IR spectra of (a) Ch (b) ChOx (c) ChOxBC before adsorption of Cr(VI) and (d) Ch (e) ChOx (f) ChOxBC after adsorption of Cr(VI).
Fig. 2. SEM images and EDX spectra of (a) Ch (b) ChOx and (c) ChOxBC.
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4. Inorganic Chemistry Communications 145 (2022) 110009
4
ChOxBC is more thermally stable due to incorporation of biochar in the
matrix of the biopolymer.
The DTA curves of all the three materials (Fig. 4b) showed endo­
thermic peak in vicinity of 100 ◦
C corresponding to initial moisture loss
and exothermic peak in vicinity of 300 O
C corresponding to
decomposition of the polymer [31]. The decrease in sharpness of the
exothermic peak is a clear indication that the thermal decomposition
becomes sluggish when the thermal stability of the material goes on
increasing with incorporation of crosslinker as well as biochar.
3.1.5. Surface area analysis
We assessed the surface area and pore volume of the adsorbents Ch,
ChOx, biochar, and ChOxBC in a nitrogen adsorption–desorption
experiment using the BET surface area and pore volume analyser. From
the results (Table 1), it is seen that the native chitosan has negligible
surface area as well as porosity. In contrast, the surface area and pore
volume of ChOx increased due to crosslinking. Biochar was found to
have substantially high surface area of 405 m2
/g with extremely porous
nature which imparted better surface properties to ChOxBC thereby
improving the surface area of the composite to 75.143 m2
/g with a pore
volume of 5.185 × 10-2
cm3
g− 1
. This material, which has a large surface
and pore dimension increase, has become an excellent absorbent of Cr
(VI) ions [32]Table 2..
4. Adsorption experiments
4.1. Screening experiments
In order to compare the adsorption efficiencies of the native chito­
san, biochar, ChOx and ChOxBC, various screening experiments were
carried out with each of these materials. Apart from pure chitosan and
biochar, the mixtures of chitosan, oxalate and biochar in varying pro­
portions were prepared as explained in the Materials and Methods sec­
tion for ChOxBC. Eight solutions of Cr(VI) having concentration of 50
mg/L were taken and added with 100 mg of the different composites.
The percentage adsorption of each composition was recorded after 60
min of contact time. The comparison (Fig. 5) clearly shows that native
chitosan as well as the biochar have very low adsorption tendency to­
wards Cr(VI). After crosslinking with oxalate, the percentage adsorption
increased from 11.93 % for native chitosan to 84.33 % in 1:2 chitosan-
oxalate. Hence, this compound was selected as one of the adsorbents.
Further incorporation of biochar into the ChOx matrix enhanced the
adsorption capacity to 91.43 % in 1:2:1 chitosan-oxalate-biochar com­
posite. Hence, this composite was used as second variant of the material.
The only slight increase in adsorption percentage from 84.33 % to 91.43
% is a clear indication that not the van der Waal’s but the ionic forces are
the chief reason for adsorption.
4.2. pH point of zero charge (pHPZC)
In pHpzc, the adsorbent’s charge is zero on its surface. The pHpzc of
Chitosan, ChOx and ChOxBC were determined by the reported method
[33]. We prepared 50 mL of 0.1 M NaCl solution and adjusted the pH
between 2.0 and 10.0. For pH adjustment, HCl and diluted NaOH were
added. A 100 mg adsorbent was added to each system and the solutions
were stirred for 24 h. A pH value was determined from the filtrate after
the solutions were filtered. From the plot of ΔpH versus initial pH, the
pHPZC of the three adsorbents (Chitosan, ChOx, ChOxBC) were
determined.
The pHPZC of Chitosan, ChOx and ChOxBC were 7.9, 7.6 and 7.8
respectively (Fig. 6a). When the solution pH is less than pHPZC, the
charge on the adsorbent surface is positive and thus it will take up
negative ions and therefore at low pH the chromium adsorption is very
effective [34].
4.3. Effect of initial solution pH
Optimization of pH is the most crucial parameter to evaluate the
adsorption capacity of an adsorbent. The effect of pH was studied for all
the three adsorbents Chitosan, ChOx, ChOxBC. The pH were adjusted
from 3.0 to 10.0 for 25 mL of 50 mg/L Cr(VI) solution, each added with
Fig. 3. X-ray diffractograms of (a) Ch (b) ChOx (c) biochar (d) ChOxBC.
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5. Inorganic Chemistry Communications 145 (2022) 110009
5
100 mg adsorbent and stirred for contact time of 60 min. Maximum
adsorption was found at pH 3.0 ± 0.1 then decreased with increase in
the pH for all the three adsorbents. At this pH, chromium exists as Cr2O7
2-
and HCrO4
-
while the adsorbent surface is positively charged as the –NH2
groups being protonated. But, pure chitosan has high solubility at low
pH, thus the experiments with chitosan for optimization of parameters
were carried out at pH 5.0. For the other two adsorbents, pH 3.0 was
selected due to highest efficiency under this condition (Fig. 6b). The
results obtained for effect of pH on adsorption capacity are in resem­
blance with that obtained in earlier studies [18–20].
4.4. Effect of adsorbent dose
In order to determine the effect of an adsorbent dose on the removal
of Cr(VI), increasing the dose from 10 mg to 400 mg at room tempera­
ture (298 K) for 50 mg/L Cr(VI) solution for 60 min at pH 3.0 was
studied. With increased adsorbent dose, the rate of adsorption increased.
This is due to increase in available surface area as well as a greater
number of ionic groups available for electrostatic interaction with the
adsorbate ions. Almost constant adsorption was observed after 100 g.
This can be attributed to the fact that the surface saturation takes place
for given concentration of Cr(VI) at this stage. Also, as the pHpzc values
were above 7 for all the three adsorbents, the pH shifts toward higher
values due to increase in adsorbent dose. This reduces the adsorption
efficiency and compensates for enhancement due to the added dose
[21]. There was no significant increase in the adsorption for all the three
adsorbents above 100 mg dose. Therefore, this dose was fixed for batch
adsorption studies (Fig. 6c).
4.5. Effect of contact time
In order to determine the effect of contact time on chromium (VI)
adsorption, the contact time was varied from 5 to 120 min at a pH of 3.0
and a starting concentration of 50 mg/L adsorbate filled with 100 mg
adsorbent. A significant increase in adsorption efficiency was observed
with time and equilibrium was reached in 60 min. Similar trend was
followed by all the three adsorbents Chitosan, ChOx and ChOxBC. This is
pretty obvious as there are more vacant adsorption sites on the adsor­
bent initially which are eventually occupied by the Cr(VI) ions with
increase in time (Fig. 6d). The adsorption reacheed to equilibrium in
about 60 min. Further adsorption period did not increase the adsorption
percentage.
4.6. Initial Cr(VI)concentration
A range of initial Cr(VI) concentrations were used in the adsorption
studies, ranging from 10 mg/L to 500 mg/L. The study was carried out at
fixed contact time, adsorbent dose and pH as optimized previously. The
adsorption was more than 90 % till 50 mg/L Cr(VI) concentration and
then decreased rapidly due to saturation of adsorbent surface. When all
the adsorbent surface cites are occupied by the adsorbate ions, further
adsorption is not possible and leads to surface saturation, in accordance
with previous reports [22]. Also, it was observed that the adsorption
capacities of both ChOx and ChOxBC were very close to each other upto
50 mg/L concentration. However, with increase in concentration of Cr
(VI), the adsorption efficiency of ChOx decreased more rapidly as
compared to ChOxBC. Therefore, the initial concentration of Cr (VI) was
set to 50 mg/L for all the studies (Fig. 6e).
4.7. Effect of temperature
At a starting concentration of Cr(VI) of 50 mg/L, and a pH of 3.0, an
adsorption period of 60 min in 303 K and 333 K temperatures was
studied to determine the effect of temperature on the removal of Cr(VI).
The adsorption started decreasing with increase in temperature for all
the three adsorbents Chitosan, ChOx and ChOxBC probably due to
Fig. 4. (a) TGA curves and (b) DTA curves of Ch, ChOx and ChOxBC.
Table 1
Surface Parameters of samples.
Material Surface Area (m2
/g) Pore volume (cm3
g¡1
)
Ch 0.013 1.263 × 10-3
ChOx 0.695 3.421 × 10-3
Biochar 405.750 2.422 × 10-1
ChOxBC 75.143 5.185 × 10-2
Table 2
Isotherm parameters for the adsorption of Cr(VI).
Isotherm Parameter Ch ChOx ChOxBC
Langmuir qm (mg/g) 2.062 348.3 383.8
b (L/mg) 0.015 0.068 0.069
RL 0.574 0.227 0.224
R2
0.955 0.719 0.844
Freundlich KF (mg1-1/n
/g/L) 0.093 23.838 20.152
N 1.732 2.983 2.321
R2
0.901 0.937 0.969
Fig. 5. Comparison of various adsorbents for the removal of Cr(VI).
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6. Inorganic Chemistry Communications 145 (2022) 110009
6
decrease in force of attraction between adsorbent and adsorbate mole­
cule. Such temperature effect is a common observation whenever the
adsorption is exothermic in nature [19]. The decrease in adsorption
efficiency with respect to temperature was found to be more pronounced
in ChOxBC as compared to ChOx. At room temperature, ChOxBC had
higher adsorption efficiency while at higher temperature, ChOx was
better (Fig. 6f). This was a clear indication that BC leads to physisorption
while ChOx has electrostatic interaction.
4.8. Isotherm studies
Langmuir and Freundlich isotherms have been used to model the
adsorption process in order to gain a better understanding of the in­
teractions between adsorbate and adsorbent. In the adsorption experi­
ments, 25 mL of Cr(VI) solution was equilibrated with 100 mg
adsorbents at pH 3.0 for 60 min. Fig. 7 illustrates the correlation be­
tween experimental values and the isotherm model and the derived
parameters are listed in Table 1. The values of R2
indicate that Langmuir
model fits well for Chitosan indicating monolayer adsorption on ho­
mogeneous surface while Freundlich model explains the observed pa­
rameters well for ChOx and ChOxBC, indicating multilayer adsorption
on heterogeneous surfaces. The RL < 1 and n greater than 1 indicate
favourable adsorption of Cr(VI) on the adsorbent surface.
4.9. Kinetics of adsorption
In order to study the kinetics of adsorption, pseudo-first-order and
pseudo-second-order models were applied to the Cr(VI) adsorbed on the
adsorbent. We conducted our experiments at 50 mg/L concentration at
pH 3.0, and we equilibrated the system for varying times from 5 to 120
Fig 6. (a) pHpzc (b) effect of solution pH (c) effect of adsorbent dose (d) effect of adsorption time (e) effect of initial Cr(VI) concentration and (f) effect of tem­
perature on adsorption efficiency (General conditions: pH = 3.0, adsorbent dose = 100 mg, adsorption time = 60 min, Cr(VI) concentration = 50 mg/L and tem­
perature = 303 K).
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7. Inorganic Chemistry Communications 145 (2022) 110009
7
min with 100 mg adsorbent. It was shown that the correlation co­
efficients for pseudo-second-order reactions were close to unity, thus
describing best the adsorption of Cr(VI) by Ch, ChOx, as well as ChOxBC.
In the Weber-Morris model, it was studied whether intra-particle
diffusion is a rate-determining step. Qt versus t1/2
did not pass
through the origin, which indicates that diffusion was not simply a rate-
determining process. Results are summarized in Table 3 and illustrated
in Fig. 8. Table 4..
4.10. Thermodynamics of adsorption
Gibbs free energy change (ΔG) was determined from equilibrium
constant K. The enthalpy change (ΔH) and entropy change (ΔS) were
determined from slope and intercept of the plot of ln K against 1/T (vant
Hoff plot). The adsorption of Cr(VI) on native chitosan was found to be
non-spontaneous with positive value of Gibb’s free energy while the
ChOx and ChOxBC is spontaneous over the entire temperature range.
The values of ΔH show that the process is exothermic for all the three
adsorbents while the negative values of ΔS show that the system goes
from more random solution phase to less random adsorbent surface. As a
result, the process is driven primarily by enthalpy.
4.11. Column studies
Using column studies, the adsorbent’s capacity for larger samples
was examined. The adsorbent was packed in a glass column with a
diameter of 10 mm, and a pH 3.0 solution of Cr(VI) was pumped through
it at a flow rate of 5 mL min− 1
. Despite Chitosan’s low adsorption ca­
pacity, 50 mg/L Cr(VI) solution was passed through it, while 200 mg/L
solutions were used for ChOx and ChOxBC. Each 25 mL eluent was
collected and the Cr(VI) concentration was determined spectrophoto­
metrically. The volume corresponding to 10 % of inlet concentration is
termed as breakthrough volume while the volume corresponding to 90
% of inlet concentration is termed as exhaustion volume. According to
the following equations, breakthrough capacity, exhaustion capacity,
and column utilization were calculated.
Fig 7. (a-c) Langmuir and (d-f) Freundlich isotherm models for Ch, ChOx and ChOxBC respectively.
Table 3
Kinetics parameters for the adsorption of Cr(VI).
Rate Model Parameter Ch Ch-Ox Ch-Ox-BC
Pseudo-first order K1 0.0036 0.054 0.048
R2
0.912 0.981 0.936
Pseudo-second order K2 0.153 0.014 0.055
R2
0.999 0.999 0.999
Intraparticle diffusion Kint 0.058 0.906 0.198
Intercept 38.5 40.7 47.4
R2
0.949 0.780 0.889
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8. Inorganic Chemistry Communications 145 (2022) 110009
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Fig 8. (a) Pseudo-first order (b) Pseudo-second order and (c) Intraparticle diffusion models for adsorption of Cr(VI) on Chitosan, ChOx and ChOxBC.
Table 4
Thermodynamic parameters.
Adsorbent ΔG (kJ/mol) ΔH
(kJ mol− 1
)
ΔS
(J mol− 1
K− 1
)
303 K 313 K 323 K 333 K
Ch 4.452 5.207 6.816 9.138 − 42.816 − 154.70
Ch-Ox − 4.909 − 3.899 − 3.413 − 3.181 –22.206 − 57.69
Ch-Ox-BC − 6.350 − 2.574 − 1.739 − 1.281 − 55.366 − 164.64
Breakthrough capacity
(
mg g− 1
)
=
breakthrough volume (L) × Inlet concentration (mg L− 1
)
weight of the adsorbent (g)
Exhaustion capacity (mg g− 1
) =
exhaustion volume (L) × inlet concentration (mg L− 1
)
weight of the adsorbent (g)
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9. Inorganic Chemistry Communications 145 (2022) 110009
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Degree of column utilization =
breakthrough volume
exhaustion volume
× 100
It can be observed from Fig. 9 and Table 5 that ChOxBC is an
excellent adsorbent even in column mode with excellent capacity to
treat flowing solution containing high concentration of Cr(VI). The high
exhaustion capacity of 400 mg g− 1
makes ChOxBC a supersorbent for Cr
(VI).
4.12. Effect of diverse ions
As the effluents of real water samples contain diverse cations and
anions, these can compete for the available adsorption sites on the
adsorbent with the Cr2O7
2-
ions. Hence, it is important to carry out batch
adsorption experiments in the presence of other ions. Usually Cl-
, NO3
–
,
SO4
2-
, PO4
3-
are present in real water samples and so they were tested for
the interference in the removal of Cr(VI). An equal concentration of the
interfering ion and a concentration of Cr(VI) was used in the experi­
ments. With the adsorbent ChOx, there was not much interference of
ions except for NO3
–
ions which greatly affected the % removal of the
chromium ions. Whereas, in the case of adsorbent ChOxBC there was
hardly any effect of the interfering ion on the removal of chromium. A
marginal decrease in adsorption efficiency can be explained by compe­
tition for available sites on the surface of the adsorbent in the presence of
interference ions (Fig. 10a).
4.13. Regeneration and reusability of adsorbents
Several reagents were tried for the regeneration of the adsorbents,
including sodium chloride, sodium hydroxide, sodium sulphate, and
sodium carbonate, and studied the desorption mechanisms of two ad­
sorbents (ChOx, ChOxBC). A solution of 5 % sodium hydroxide (w/v)
produced the best results. Five adsorption–desorption cycles were con­
ducted using the adsorbent ChOx and ChOxBC, and decreasing adsorp­
tion efficiency was observed after each cycle. The adsorption efficiency
was more than 80 % for adsorbent ChOx and more than 90 % for
ChOxBC after first regeneration which went on reducing after each
successive cycle (Fig. 10b). In order to see if oxalate ions leach out into
the solution after NaOH treatment, the leachates were treated with
saturated solution of calcium chloride. It was observed that there is no
white precipitate (calcium oxalate) formation, indicating that the oxa­
late ions did not leach out from the composites.
4.14. Comparison with reported materials
A comparative account of ChOx and ChOxBC with chitosan based
materials has been presented in Table 6. According to the table, the
reported materials were more efficient than other materials.
Table 5
Fixed bed column parameters.
Adsorbent Ch ChOx ChOxBC
Inlet CrVI
concentration (mg L-1
) 50 200 200
Breakthrough volume (mL) 75 175 475
Exhaustion volume (mL) 375 525 1000
Breakthrough capacity (mg g− 1
) 7.5 70 190
Exhaustion capacity (mg g− 1
) 37.5 210 400
Degree of column utilization (%) 20.0 33.3 47.5
Fig 10. (a) Effect of diverse ions on adsorption efficiency (b) Efficiency of regenerated materials.
Table 6
Comparison with other adsorbents.
Adsorbents Adsorption
capacity (mg/g)
References
Chitosan flakes 22.09 [35]
Ethylene diamine cross-linked chitosan 48.70 [15]
Epichlorohydrin cross-linked chitosan 50 [36]
Ionic liquid impregnated chitosan 63.69 [37]
Cross-linked magnetic chitosan beads 69.40 [38]
Glutaraldehyde cross-linked chitosan
coated bentonite clay capsule
106.44 [39]
Chitosan coated charcoal 154 [40]
Sulphate cross-linked chitosan 157 [18]
Chitosan-[BMIM][OAc] composite 125.63 [41]
Magnetic zeolite/chitosan composites 16.96 [42]
Glycine Modified Cross-linked- chitosan
Resin
78.0 [43]
Oxalate cross-linked chitosan 348.3 Present
study
Chitosan-Oxalate-Biochar composite 383.8
Fig 9. Fixed bed column studies using 50 mg/L Cr(VI) solution for Ch and 200
mg/L Cr(VI) solution for ChOx and ChOxBC with 0.5 g adsorbent.
V. Gomase et al.

10. Inorganic Chemistry Communications 145 (2022) 110009
10
5. Conclusion
Chitosan was used for Cr(VI) adsorption and was found to have
2.068 mg g− 1
adsorption capacity and therefore it was cross-linked with
oxalate ions to give greater adsorption capacity. The protonated amine
group of chitosan was found to have ionic interaction with the oxalate
ions. The ChOx adsorbent had much higher adsorption capacity of
348.3 mg g− 1
which further improved to 383.8 mg g− 1
when ChOx
formed a composite with biochar. As the central India region is producer
of more than 35 lakh cotton bales per year can provide a constant source
of cotton straw for biochar production. Thus, ample availability of chi­
tosan and biochar, excellent adsorption capacity towards Cr(VI),
regeneration and reusability of the material and excellent results in
column studies can make this material a commercially workable mate­
rial for water treatment. As compared to reported materials in literature,
both of these materials were found to have better adsorption capacities.
Hence, they can be used in water treatment processes for tannery ef­
fluents and wastewater from electroplating industries.
Funding: Not applicable.
Availability of data and materials: All the original data included in
this study are available upon request by contact with the corresponding
author.
CRediT authorship contribution statement
Vaishnavi Gomase: Data curation, Formal analysis, Writing –
original draft. Ravin Jugade: Conceptualization, Funding acquisition,
Investigation. Priyanka Doondani: Methodology, Project administra­
tion. D. Saravanan: Resources, Software, Supervision. Sadanand
Pandey: Validation, Visualization, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
Data will be made available on request.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.inoche.2022.110009.
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