Journal of Environmental Protection, 2011, 2, 729-735
doi:10.4236/jep.2011.26084 Published Online August 2011 (
Copyright © 2011 SciRes. JEP
Cr(VI) Ions Removal from Aqueous Solutions
Using Natural Adsorbents—FTIR Studies
Biswajit Singha, Tarun Kumar Naiya, Ashim Kumar Bhattacharya, Sudip Kumar Das
Department of Chemical Engineering, University of Calcutta, Kolkata, India.
Received March 22nd, 2011; revised May 6th, 2011; accepted June 15th, 2011.
The ability of eight natural adsorbents were investigated for adsorptive removal of Cr(VI) from aqueous solutions.
Various physico-chemical parameters such as pH, initial metal ion con centratio n, adsorbent dose level and equilibrium
contact time were optimized in batch adsorption technique. A detailed Fourier Transform Infrared spectra (FTIR) study
of adsorbents and Cr(VI) loaded adsorbents at the optimized condition was carried out to identify the different func-
tional groups that were responsible for the adsorption. The important functional groups like hydroxyl, alkene, aromatic
nitro, carboxilate anion, silicon oxide, sulphonic acid etc. were present in the natural adsorbent and were responsible
for the chemical adsorption of Cr(VI) from aqueous solutions. The sorption energy calculated from
Dubinin-Radushkevich isotherm indicated that the adsorption process were chemical in nature.
Keywords: FTIR, Chromium(VI), Rice Straw, Hyacinth Roots, Saw Dust
1. Introduction
Cr(VI) containing waste water discharged from various
industries, including mining, tanning, cement, production
of steel and other metal alloys, electroplating operations,
photographic material and corrosive painting industries
[1,2]. It is carcinogenic, mutagenic and toxic; thus, its
presence in the environment poses a significant threat to
aquatic life and as well as public health [3].
The maximum permissible limit of Cr(VI) for the dis-
charge to inland surface water is 0.1 mg/L and in potable
water is 0.05 mg/L [4,5]. The Ministry of Environment
and Forest (MOEF), Government of India has set mini-
mal national standards (MINAS) of 0.1 mg/L for safe
discharge of effluent containing Cr(VI) in surface water
[6]. To comply with this limit, industries have to treat
their effluents to reduce the Cr(VI) concentration in
wastewater to acceptable levels. In waste water treatment
various technologies are available such as chemical pre-
cipitation, ion exchange, electrochemical precipitation,
solvent extraction, membrane separation, concentration,
evaporation, reverse osmosis, emulsion per traction, ad-
sorption etc. [7]. Among these technologies, adsorption
is an user-friendly technique for the removal of heavy
metal. This process includes the selective transfer of sol-
ute components in the fluid phase onto the surface or
onto the bulk of solid adsorbent materials.
In recent years, several natural or agricultural wastes
[8-11] have been used for the removal of heavy metal
from industrial waste water. In general natural or agri-
cultural waste contains different functional groups like
hydroxyl, aldehyde, aliphatic acid, alkene, amide, aro-
matic nitro, silicate, sulphonate etc. The present paper
deals with the identification of functional groups which
are responsible for Cr(VI) ion adsorption in the eight
2. Experimental Methods
2.1. Preparation of Adsorbents
Rice straw, rice bran, rice husk, hyacinth roots, neem
bark, saw dust of teakwood origin, neem leaves and co-
conut shell were used as low cost natural or agricultural
wastes for Cr(VI) removal from aqueous solutions. All
the adsorbents were collected from local area near
Kolkota, West Bengal, India.
Rice straw, rice bran, rice husk and hyacinth roots
were boiled for 6 hr. to remove color materials. Coconut
shell was crashed in roll crusher and then grinded. Saw-
dust, neem bark, neem leaves and coconut shell were
treated with 0.1 N NaOH to remove lignin based color
materials followed by 0.1 N H2SO4. Finally all the ad-
sorbents were washed with distilled water several time
and dried at 105˚C for 6 hr to remove the adherent mois-
Cr(VI) Ions Removal from Aqueous Solutions Using Natural Adsorbents—FTIR Studies
ture. After drying, all the adsorbents were sieved to ob-
tain particle size of 250 - 350 μm prior to being used for
adsorption studies.
2.2. Reagents and Equipments
All the necessary chemicals used in the study were of
analytical grade and obtained from E. Merck Limited,
Mumbai, India. The pH of the solution was measured
with a EUTECH make digital microprocessor based pH
meter previously calibrated with standard buffer solu-
tions. UV-Spectrophotometer (U-4100 spectrophotome-
ter, Hitachi, Japan) was used to determine the Cr(VI)
content in standard and treated solutions after adsorption
experiments. FT-IR (Jasco FT/IR-670 Plus) studies were
carried out to determine the type of functional group re-
sponsible for Cr(VI) adsorption. The surface area was
measured on Micromeritics Surface Area Analyzer
(ASAP 2020). The moisture content determination was
carried out with a digital microprocessor based moisture
analyzer (Metteler LP16). The point of zero charge was
determined by solid addition method [12] and reported in
Table 1.
2.3. Preparation of Standard Cr(VI) Solution
The stock solution containing 1000 mg/L of Cr(VI) was
prepared by dissolving 3.73 g of A. R. grade K2CrO4,
2H2O in 1000 ml double distilled water. Required initial
concentration of Cr(VI) standards were prepared by ap-
propriate dilution of the above stock Cr(VI) solution.
2.4. Batch Adsorption Studies
Using the necessary adsorbents in a series of 250 ml
stopper conical flask containing 100 ml of Cr(VI) solu-
tion batch adsorption were carried out. pH of the solution
adjusted by adding 0.1 N HCl or 0.1 N NaOH solution as
required. Then the flasks were shaken for the desired
contact time in an electrically thermostated reciprocating
shaker with 120 - 125 strokes/minute at 30˚C. The time
required to reach the equilibrium was estimated by with-
drawing conical flask containing treated solution at
regular intervals of time (simultaneously 8 conical flasks
with same concentrations of all items). The content of
these flasks were filtered through filter paper (Whatman
no.1). UV-visible spectrophotometer was employed to
determine the remaining Cr(VI) concentration in the
sample solution using 1,5-diphenylcarbazide method as
laid down in standard methods for examination of water
and wastewater, APHA, AWWA, WEF, 1998 edition
[13]. All the investigations were carried out in triplicate
to avoid any discrepancy in experimental results with the
reproducibility and the relative deviation of the order of
±0.5% and ±2.5% respectively. The solution pH adjusted
to 1.0 ± 0.1 to 9.0 ± 0.1 under thermostated conditions of
30˚C ± 0.5˚C.
3. Results and Discussion
3.1. Optimum Operating Condition and Cr(VI)
Adsorption Mechanism
Metal sorption is depends on the solution pH. The range
of variables investigated to obtain the optimum condition
is shown in Table 2. In general adsorption of anion is
favored at pH < pHpzc. At very low pH, chromium ions
exist in the form of 4
, at higher pH up to 6 differ-
ent forms such as 2
Cr O
, 4, and HCrO2
Cr O
, coex-
ists, of which 4
predominates. As the pH increa-
ses equilibrium shifted form 4
HC to rO2
Cr O
[14]. At very low pH values, the surface of ad-
sorbent would be surrounded by the hydronium ions
which enhance the Cr(VI) interaction with binding sites
of the biosorbent by greater attractive forces. As the pH
increased, the overall surface charge on the biosorbents-
became negative and adsorption decreased [2]. The fol-
lowing equilibrium may be written for the Cr(VI) anions
present in aqueous solutions [9].
Table 1. Different physical characteristics of natural adsorbents.
Adsorbents Surface area (m2/g) Moisture content (%) Point of zero charge Ash content (%)
Rice straw 1.21 7.26 6.85 9.40
Rice bran 0.12 10.68 6.10 11.72
Rice husk 0.54 9.02 6.05 11.80
Saw dust 3.85 8.63 3.90 12.35
Neem bark 3.47 9.23 4.50 10.62
Hyacinth roots 5.78 11.25 6.59 10.74
Neem leaves 0.57 8.33 6.94 13.58
Coconut shell 0.52 6.16 6.62 9.23
Copyright © 2011 SciRes. JEP
Cr(VI) Ions Removal from Aqueous Solutions Using Natural Adsorbents—FTIR Studies731
Table 2. Range of variables for batch experiment.
Adsorbent Initial pH Initial Cr(VI) concentration (mg/L) Contact time (min) Adsorbent dosage (g/L)
Rice straw 1 - 9 5 - 300 0 - 420 2.5 - 12.5
Rice bran 1 - 9 5 - 300 0 - 420 2.5 - 12.5
Rice husk 1 - 9 5 - 300 0 - 420 2.5 - 12.5
Saw dust 2 - 8 3 - 300 0 - 300 2.5 - 30.0
Neem bark 2 - 8 3 - 300 0 - 300 2.5 - 30.0
Hyacinth roots 1 - 9 5 - 300 0 - 420 2.5 - 12.5
Neem leaves 1 - 9 5 - 300 0 - 300 2.5 - 12.5
Coconut shell 1 - 9 5 - 300 0 - 360 2.5 - 12.5
244 1
HCrOHCrOH 1.21k
  (1)
2724 2
CrOHO2HCrO 35.5k
  (2)
44 3
HCrOCrOH 310k
 
Adsorption of Cr(VI) was not significant at pH values
more than 6 due to dual complexation of the anions
( , and ) to be adsorbed on the sur-
face of the adsorbents, of which predominates [15].
The optimum adsorbent dosage, equilibrium contact time,
optimum initial Cr(VI) ion concentration and maximum
adsorption capacities of different natural or agricultural
waste adsorbent using Langmuir adsorption isotherm
model [16] were experimentally determined in batch
process and the results are shown in Table 3.
max max
Equation (4) represents the Langmuir adsorption model
where Ce is the concentration of Cr(VI) in solution at
equilibrium (mg/L), qe is the amount adsorb per gram of
the adsorbent at equilibrium, qmax is the maximum ad-
sorption capacity (mg/g) and b is the Langmuir constant
(L/mg). Linear plots of Ce/qe vs. Ce were employed to
determine the value of qmax (mg/g). The maximum ad-
sorption capacity along with correlation coefficient (r2)
obtained were listed in Table 3.
3.2. Calculation of Sorption Energy
The Dubinin-Radushkevich [17] isotherm model was
used to predict the nature of adsorption processes as
physical or chemical by calculating sorption energy. The
linear from of the model is described as,
ln ln
abs m
 (5)
where Cabs is the amount of Cr(VI) adsorbed onto ad-
sorbent surface (mol/g) and Xm represents the maximum
adsorption capacity of adsorbent (mmol/g),
is con-
stant related to sorption energy. The Polanyi potential [18]
which is equal to,
ln 1
R is the ideal gas constant in kJ/mol/K and T is the
temperature in Kelvin. From the plot of lnabs
C vs. 2
gave a straight line from which the values of
for all the adsorbents were calculated. Using the
value of
, the mean sorption energy, E, is evaluated as
The mean sorption energy, E, which indicated the in-
formation about adsorption mechanism. If E 8 kJ/mol,
the adsorption process was physical in nature and in the
ranges from 8 to 16 kJ/mol, it was chemical in nature
[19-21]. The estimated values of E were 14.142 kJ/mol,
13.921 kJ/mol, 13.558 kJ/mol, 9.341 kJ/mol, 9.205
kJ/mol, 12.845 kJ/mol, 14.712 kJ/mol and 15.394 kJ/mol
for rice straw, rice bran, rice husk, saw dust, neem bark,
hyacinth roots, neem leaves and coconut shell respec-
tively which suggested the adsorption process was che-
mical in nature i.e. the indication of chemical bond for-
mation between metal ion species and the functional
group of the adsorbents.
3.3. FTIR Analysis for Cr(VI) Adsorption
Fouier transform infrared spectra (FTIR) was used to inves-
tigate the changes in vibration frequency in the functional
groups of the adsorbents due to Cr(VI) adsorption. Each
fresh and Cr(VI) loaded adsorbents were mixed sepa-
rately with KBr of spectroscopic grade and made in the
form of pellets at a pressure of about 1 MPa. The pellets
were about 10 mm in diameter and 1 mm thickness. Then
the adsorbents were scanned i the spectral range of 4000 - n
Copyright © 2011 SciRes. JEP
Cr(VI) Ions Removal from Aqueous Solutions Using Natural Adsorbents—FTIR Studies
Table 3. Optimum operating condition obtaining in the batch process.
Langmuir isotherm model
Adsorbent Initial pH Initial Cr(VI) concentration (mg/L)Contact time (min)Adsorbent dosage (g/L) qmax (mg/g) r2
Rice straw 2 25 180 10 12.17 0.9801
Rice bran 2 25 300 10 12.34 0.9476
Rice husk 1.5 25 360 10 11.39 0.9869
Saw dust 3 50 240 10 20.70 0.9963
Neem bark 3 50 240 10 19.60 0.9959
Hyacinth roots 2 25 240 10 15.28 0.9790
Neem leaves 2 25 240 10 15.95 0.9652
Coconut shell 2 25 240 10 18.69 0.9530
400 cm1. Figures 1-3 show the FTIR spectra of rice
straw, hyacinth roots and coconut shell respectively.
Similar type of spectra also occurred for other adsorbents.
These spectra indicated a number of absorption peaks
showing the complex nature of the adsorbent. The func-
tional group is one of the key factors to understand the
mechanism of metal binding process on natural adsorb-
Tables 4 and 5 represented the shift in the wave num-
ber of dominant peak associated with the fresh and Cr(VI)
loaded adsorbents in the FT-IR plots. These shifts in the
wave length showed that there was metal binding process
taking place at the surface of the adsorbents [12,22]. The
spectra display a number of absorption peaks, indicating
the complex nature of the natural adsorbents. There was
a clear shift from wave number of 3348.78 cm1 (rice
straw) to 3417.24 cm1 (metal loaded rice straw), 3342.03
cm1 (rice bran) to 3328.53 cm1 (metal loaded rice bran)
and 3385.42 cm1 (rice husk) to 3421.10 cm1 (metal
loaded rice husk), 3297.75 cm1 (neem bark) to 3266.82
cm1 (metal loaded neem bark), 3328.53 cm1 (hyacinth
Figure 1. FTIR spectra of (a) rice straw and (b) Cr(VI)
loaded rice straw.
Figure 2. FTIR spectra of (a) hyacinth roots and (b) Cr(VI)
loaded hyacinth roots.
Figure 3. FTIR spectra of (a) coconut shell and (b) Cr(VI)
loaded coconut shell.
root) to 3305.39 cm1 (metal loaded hyacinth roots)
which indicate surface -OH group is one of the functional
group responsible for adsorption Cr(VI) on rice straw,
rice bran, rice husk, neem bark and hyacinth root.
Though surface -OH present as functional group in saw
dust but it is not responsible for metal binding in case of
adsorption of Cr(VI) on saw dust. This can be inferred
Copyright © 2011 SciRes. JEP
Cr(VI) Ions Removal from Aqueous Solutions Using Natural Adsorbents—FTIR Studies733
Table 4. Wave number (cm1) for the dominant peak from FT-IR for Cr(VI) adsorption.
Functional Groups Rice straw Cr(VI) loaded
rice straw Rice bran Cr(VI) loaded
rice bran Rice huskCr(VI) loaded
rice husk Saw dust Cr(VI) loaded
saw dust
Surface O-H
stretching 3348.78 3417.24 3342.03 3328.53 3385.42 3421.10 3335.10 3328.53
Aliphatic C-H
stretching 2918.73 2916.81 2924.52 2924.52 2925.48 2925.48 2917.70 2920.66
Aldehyde C-H
stretching x x 2854.13 2854.13 2854.13 2854.13 x x
Aliphatic acid C=O
Stretching x x 1709.59 1713.44 x x x x
group like alkene 1644.09 1633.41 1655.59 1644.02 1654.62 1638.23 x x
Amide C-O
stretching x x x x x x 1594.04 1593.88
Aromatic C-NO2
stretching 1512.88 1505.17 1546.63 1514.81 1515.77 1509.99 x x
Carboxilate anion
C=O stretching 1321.00 1371.14 x x x x x x
Si-O stretching 1072.66 1058.73 1079.94 1055.84 1098.26 1075.12 x x
Sulphonic acid S=O
stretching x x x x x x 1033.60 1031.73
S-O stretching x x x x x x 693.28 651.82
Table 5. Wave number (cm1) for the dominant peak from FT-IR for Cr(VI) adsorption.
Functional Groups Neem bark Cr(VI) loaded
neem bark
Cr(VI) loaded
hyacinth rootsNeem leavesCr(VI) loaded
neem leaves
Cr(VI) loaded
coconut shell
Surface O-H
stretching 3297.75 3266.82 3328.53 3305.39 x x x x
Aliphatic C-H
stretching x x 2924.52 2923.88 2920.28 2910.16 x x
Phosphite ester
group x x x x x x 2353.97 2358.78
Aliphatic acid C=O
Stretching x x 1713.44 1713.44 1715.83 1715.67 1717.73 1715.75
group like alkene x x 1644.02 1633.41 x x x x
Amide C-O
stretching 1606.40 1603.52 x x x x x x
Aromatic C-NO2
stretching x x 1514.81 1505.17 1515.46 1515.80 1507.22 1507.19
Alkane group
stretching x x x x 1455.88 1455.98 1472.91 1456.25
-SO3 stretching x x x x x x 1236.10 1226.83
Sulphonyl chlorides
stretching x x x x 1163.39 1162.00 x x
Sulphonic acid
S=O stretching 1032.91 1034.84 1055.84 1035.59 x x 1031.37 1032.23
S-O stretching 756.92 658.57 x x x x x x
Copyright © 2011 SciRes. JEP
Cr(VI) Ions Removal from Aqueous Solutions Using Natural Adsorbents—FTIR Studies
from the peak shift (Tables 4 and 5). Aliphatic C-H
stretching may be responsible for Cr(VI) adsorption onto
neem leaves as wave number shift from 2920.28 cm1 to
2910.16 cm1. Unsaturated group like alkenes present
may also responsible for adsorption of Cr(VI) on rice
straw, rice bran, rice husk and hyacinth root which is
inferred from the shift of the peak more than 10 cm1.
Aromatic nitro, C-NO2 stretching was found to have
major shift of wave number from 1546.63 cm1 to
1514.81 cm1 for the adsorption of Cr(VI) on rice bran.
There were also minor shift of peak for the adsorption of
Cr(VI) on rice straw, rice husk, hyacinth roots, neem
leaves and coconut shell. So the aromatic nitro groups
are responsible for adsorption of Cr(VI) on rice bran not
for the adsorption on other adsorbents. Alkane group was
only responsible for Cr(VI) adsorption onto coconut shell
as indicated in Table 5.
FT-IR spectrum of rice straw also showed intense
bands around 1321.00 cm1 which shifted to 1371.14
cm1 for Cr(VI) loaded rice straw. This is to be attributed
that the carboxylate anion are responsible for the adsorp-
tion on rice straw. At 1072.66 cm1 (rice straw), 1079.94
cm1 (rice bran) and 1098.26 cm1 (rice husk) bands can
be assigned Si-O stretching. Major shift of these band
indicated that Si-OH group is responsible for adsorption.
Tables 4 and 5 also indicated that the minor shift for
the other band (aldehyde C-H stretching, phosphate ester
group, aliphatic carboxylic acids) which showed that
these groups were not involved in the adsorption process.
SO3 stretching were found be to responsible for Cr(VI)
adsorption onto coconut shell.
The peak at 1033.60, 1032.91, 1055.84 and 1031.37
cm1 for saw dust, neem bark, hyacinth roots and coconut
shell can be assigned to the S=O stretching mode of sul-
phonic acid group. S=O stretching was slightly shifted by
Cr(VI) adsorption on saw dust, neem bark and coconut
shell while the adsorption on hyaceinth root resulted in a
large shift of functional group from 1055.84 cm1 to
1035.59 cm1. This would be imply that S=O stretching
of sulphonic acid group is available for the adsorption of
Cr(VI) on hyacinth root, however not involved on other
adsorbents used in our study.
The observation for the sulphonate group revealed that
S-O stretching was highly occurred by Cr(VI) adsorption
on saw dust and neem bark. The S-O stretching group
was observed to shift clearly from wave number 756.92
cm1 to 658.57 cm1 and 693.28 cm1 to 651.82 cm1
neem bark and saw dust respectively. This indicated that
there is a high potential of S-O stretching group from
sulphonate involved with Cr(VI) binding on neem bark
and saw dust. So S-O stretching was only associated with
the adsorption of Cr(VI) on neem bark and saw dust but
not for other adsorbents.
Crystal radius of Cr(VI) is 0.52 Å. It is moderately
large ion, fit into the binding site of the natural adsorb-
ents and bind to several group present in the adsorbents
4. Conclusions
In this study batch adsorption experiments for the re-
moval of Cr(VI) from aqueous solutions has been carried
out using eight different natural adsorbents. The adsorp-
tion characterestics have heen examined at different pH
values, initial metal ion concentrations, contact time and
different adsorbent dosages. FTIR analysis confirmed the
existence of different functional groups responsible for
the adsorption. The obtained results are summarized as
1) The optimum pH for the removal of Cr(VI) was
found to be 1.5 for husk and 2 for other adsorbents.
2) Maximum uptake was obtained at adsorbent dosage
of 10 g/L for all the adsorbents.
3) The equilibrium time for the adsorption of Cr(VI)
from aqueous solutions were varied from 3 hr to 6 hr for
adsorbents used.
4) The maximum monolayer adsorption capacities by
the adsorbents were measured using Langmuir adsorp-
tion isotherm.
5) Sorption energy calculated from Dubinin-Raduske-
vich (D-R) shows the chemisorptions process for all the
6) FTIR studies indicated the following functional
groups were responsible for adsorption,
a) Rice straw—Surface hydroxyl, unsaturated group
like alkene, Carboxilate anion, Silicate groups;
b) Rice bran—Surface hydroxyl, unsaturated group
like alkene, Aromatic nitro, Silicate groups;
c) Rice husk—Surface hydroxyl, unsaturated group
like alkene, Silicate groups;
d) Saw dust—Sulphonate groups;
e) Neem bark—Surface hydroxyl, Sulphonate groups;
f) Hyacinth roots—Surface hydroxyl, Unsaturated
group like alkene, Sulphonic acid groups;
g) Neem leaves—Aliphatic group;
h) Coconut shell—Alkane, -SO3 groups.
5. Acknowledgements
Biswajit Singha wishes to thanks the University of Cal-
cutta for the Project Fellow (UPE/Science & Technol-
ogy), Ref. No. UGC/489/Fellow UPE (SC/T), dated the
The authors acknowledge to AICTE for financial sup-
port (Project No.- F. No.:8023/BOR/RID/RPS-72/2008-
Copyright © 2011 SciRes. JEP
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