Cr(VI) Ions Removal from Aqueous Solutions Using Natural Adsorbents – FTIR Studies

doi:10.4236/jep.2011.26084

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Journal of Environmental Protection, 2011, 2, 729-735

doi:10.4236/jep.2011.26084 Published Online August 2011 (http://www.SciRP.org/journal/jep)

729

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.

Email: [email protected]

Received March 22nd, 2011; revised May 6th, 2011; accepted June 15th, 2011.

ABSTRACT

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

bio-adsorbents.

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

730

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

HCrO



, at higher pH up to 6 differ-

ent forms such as 2

Cr O



, 4, and HCrO2

310

Cr O



, coex-

ists, of which 4

HCrO



predominates. As the pH increa-

ses equilibrium shifted form 4

HC to rO2

CrO



and

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

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



 



(3)

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.

CrO2

Cr OOH

OH

max max

qqbq



(4)

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

RT C













(6)

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



and

for all the adsorbents were calculated. Using the

value of



, the mean sorption energy, E, is evaluated as



 (7)

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

Cr(VI) Ions Removal from Aqueous Solutions Using Natural Adsorbents—FTIR Studies

732

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 cm−1. 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-

ents.

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 cm−1 (rice

straw) to 3417.24 cm−1 (metal loaded rice straw), 3342.03

cm−1 (rice bran) to 3328.53 cm−1 (metal loaded rice bran)

and 3385.42 cm−1 (rice husk) to 3421.10 cm−1 (metal

loaded rice husk), 3297.75 cm−1 (neem bark) to 3266.82

cm−1 (metal loaded neem bark), 3328.53 cm−1 (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 cm−1 (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

Cr(VI) Ions Removal from Aqueous Solutions Using Natural Adsorbents—FTIR Studies733

Table 4. Wave number (cm−1) 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

Unsaturated

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

Sulphonate

S-O stretching x x x x x x 693.28 651.82

Table 5. Wave number (cm−1) for the dominant peak from FT-IR for Cr(VI) adsorption.

Functional Groups Neem bark Cr(VI) loaded

neem bark

Hyacinth

roots

Cr(VI) loaded

hyacinth rootsNeem leavesCr(VI) loaded

neem leaves

Coconut

shell

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

Unsaturated

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

Sulphonate

S-O stretching 756.92 658.57 x x x x x x

Cr(VI) Ions Removal from Aqueous Solutions Using Natural Adsorbents—FTIR Studies

734

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 cm−1 to

2910.16 cm−1. 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 cm−1.

Aromatic nitro, C-NO2 stretching was found to have

major shift of wave number from 1546.63 cm−1 to

1514.81 cm−1 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 cm−1 which shifted to 1371.14

cm−1 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 cm−1 (rice straw), 1079.94

cm−1 (rice bran) and 1098.26 cm−1 (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

cm−1 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 cm−1 to

1035.59 cm−1. 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

cm−1 to 658.57 cm−1 and 693.28 cm−1 to 651.82 cm−1

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

simultaneously.

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

follows

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

adsorbents.

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

16/4/2009.

The authors acknowledge to AICTE for financial sup-

port (Project No.- F. No.:8023/BOR/RID/RPS-72/2008-

09).

Cr(VI) Ions Removal from Aqueous Solutions Using Natural Adsorbents—FTIR Studies735

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