Studies on removal of chromium (VI) from water using

Q IWA Publishing 2010 Water Science & Technology—WST | 62.10 | 2010

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Studies on removal of chromium (VI) from water using chitosan coated Cyperus pangorei R. Malarvizhi, Y. Venkateswarlu, V. Ravi babu and S. Syghana Begum

ABSTRACT Environmental contamination by toxic heavy metals is a significant universal problem. The main objective of the study is to use a biodegradable materials like Cyperus pangorei and Chitosan as a composite biosorbent for the removal of Cr(VI) from water. The newly prepared biosorbent is characterized and the capacity of Cr(VI) removal of the biosorbent is carried out systematically by

R. Malarvizhi (corresponding author) Department of Chemistry, PRIST University, Vallam, Thanjavur, India E-mail: [email protected]

batch mode operations. The adsorption capacity of the biosorbent is examined by changing the parameters like biosorbent dose, varying the initial contact time, varying initial concentration of metal ion and pH of the metal ion solution to know the actual mechanism taking place during the initial sorption process. The experimental data obtained were fitted with the Freundlich, Langmuir and Redlich–Peterson isotherm models and the pseudo first order and the pseudo second order kinetic models. Equilibrium data were fitted very well to the Langmuir Isotherm model and pseudo second order kinetic model. Desorption of the metal ion is also carried out using different concentration of NaOH. Key words

| chitosan, Cyperus pangorei, desorption, langmuir model, Redlich–Peterson, sorption

Y. Venkateswarlu V. Ravi babu Department of Bio-technology, PRIST University, Vallam, Thanjavur, India S. Syghana Begum Department of Chemistry, Holy Cross College, Trichy 620 002, India

isotherm

INTRODUCTION The presence of heavy metals over permissible levels in

limit (ISI 1977) of Cr(VI) is 0.05 and 0.1 mg/L for potable

drinking water may cause adverse effect on human

and industrial discharge water respectively.

physiology. Among these, Chromium is one of the most

Normally, a wide range of physical and chemical

toxic metal, that is mixed into river and ground water

processes are available for the removal of heavy metal

through the effluent discharged from electroplating, metal

Cr(VI) from waste water such as oxidation, reduction,

finishing, leather tanning and chrome tanning industries

electro-chemical precipitation, ultra filtration, ion exchange

(Nomanbhay & Palanisamy 2005). Chromium exists mainly

and reverse osmosis (Benito & Ruiz 2002). These physico-

in two valance states, namely trivalent Chromium [Cr(III)]

chemical methods have several disadvantages and are not

and hexavalent Chromium [Cr(VI)], out of which, the later

economical because of high operational cost. Search for

one is of great concern due to its toxicity. It is an E.P.A

newer treatment technologies for removal of heavy

classified Group-A carcinogen based on its chronic effects.

metals from waste waters has directed attention to biosorp-

Chromium (VI) is generally considered thousand times

tion (Veglio & Beolchini 1997). In 1990s, a new scientific area

more toxic than Chromium (III), it has been reported to

developed that could help to remove and recover the heavy

be responsible for lung cancer, chrome ulcer, perforation

metals by biosorption. Metal-sequestering properties of non-

of nasal septum, brain and kidney damage (Mohan et al.

viable biomass provide a basis for a new approach to remove

1996). According to the Indian standards the permissible

heavy metals when they occur at low concentrations

doi: 10.2166/wst.2010.494

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R. Malarvizhi et al. | Studies on removal of chromium (VI) from water

(Volesky & Holan 1995). Biosorption is an effective and versatile method; it can be easily adopted in low cost to remove the heavy metals from large amount of industrial wastewater. Natural biopolymers are industrially attractive, because of their capability of lowering transition metal ion concentration to parts per billion concentrations. Abundant natural materials like chitosan, particularly of cellulosic

Water Science & Technology—WST | 62.10 | 2010

Preparation of Chitosan gel About 50 g of Chitosan is slowly added to 500 ml of 10% oxalic acid (w/w) with constant stirring. The mixture is also heated to 40 – 508C to facilitate mixing. At room temperature, the Chitosan-oxalic acid mixture formed whitish viscous gel (Nomanbhay & Palanisamy 2005).

nature have been suggested as potential biosorbents for heavy metals. Chitosan is the deacetylated form of chitin

Surface coating of ACPP with Chitosan

containing a chain of amino groups along the chitosan structure. Now researchers are looking at the ability of

About 500 mL of the Chitosan gel is diluted with water and

this amino group to adsorb heavy metal ions from industrial

heated to 40 – 508C. About 500 g of the ACPP is slowly

wastewaters and leachates. (Gerente et al. 2007). So far,

added to the diluted gel and mechanically agitated using

chitosan coated sorbents were used to remove the following

a rotary shaker at 250 rpm for 24 hrs. The gel coated ACPP

metals like, cadmium (Evans et al. 2002), iron (Wan Ngah

is then washed with distilled water and dried. The process

et al. 2005), nickel (Pradhan et al. 2005; Paulino et al. 2007;

is repeated for three times to form a thick coating of

Rana et al. 2009) lead (Paulino et al. 2007; Rana et al. 2009)

Chitosan on the ACPP surface. The Chitosan coated ACPP

and zinc (Rana et al. 2009) ions from aqueous solution.

is named as Chitosan coated Acid treated Cyperus pangorei

In the present work, Cyperus pangorei (Korai grass) is a monocot belonging to the family Cyperaceae is chosen as

powder (CACPP). The CACPP are then extensively rinsed with distilled water and dried.

precursor for deriving biosorbents and to improve the activity of the biosorbent from the plant biomaterial, it is treated with acid, and their surface modification is carried

Batch mode experiments

out with coating of the surface with natural biopolymers

A stock solution of 1,000 mg/L of Cr(VI) is prepared by

(Chitosan). Systematic batch-mode sorption studies are

dissolving 2.829 g of Potassium dichromate in distilled water

carried out in order to evaluate the sorption capacity of

and the pH of the solution is adjusted using 0.1 N HCl or 0.1 N

the newly prepared composite biosorbent.

NaOH. Various concentrations of chromium solution is prepared by diluting the stock solution. Batch mode experiments are conducted using screw capped closed containers in

MATERIALS AND METHODS Preparation of the composite biosorbent

an ORBITEK shaker at 250 rpm. A calibration graph of absorbance versus concentration of Cr(VI) is obtained using systronics photometer (model 104) at lmax 540 nm. The

The biomaterial is washed extensively in running tap water

residual Cr(VI) ion concentration is estimated by using

to remove dirt and other particulate matter. The washed

diphenyl carbozide as a complexing agent. Experiments are

plant material are cut into small pieces and allowed to dry

repeated at least three times and all the calculations are

in an oven at 708C for 24 hrs. The dried plant material are

performed in the Excel solver add in spread sheets.

powdered into required particle size (2 mm) and stored in

The influence of biosorbent dose on metal ion adsorption

desiccators and used for biosorption sorption studies. About

is investigated by performing experiments taking 100 mL of

50 g of the Cyperus pangorei powder is treated with 500 mL

50 mg/L of Cr(VI) solutions and equilibrating with varying

of 1 N HCl for 24 hrs and kept on the water bath (708C)

amounts of biosorbent (0.2 g to 0.8 g) after adjusting the

for half an hour. It is cooled and is neutralized with 250 mL

solution to pH 4.0, for 1 hr. at room temperature

of 1 N NaOH. The filtrate is separated and dried in the oven

(30 ^ 0.58C). Effect of pH on removal of metal is carried

for 4 hrs at 608C. Then the pretreated Cyperus pangorei is

out by taking 100 mL of 50 mg/L of Cr(VI) solutions at

named as Acid treated Cyperus pangorei powder (ACPP).

various pH for 1 hr. A complete isotherm study is carried out

R. Malarvizhi et al. | Studies on removal of chromium (VI) from water

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Water Science & Technology—WST | 62.10 | 2010

by taking 100 mL of different concentrations of Cr(VI)

99.5% for CACPP-Cr(VI) system. Increased in adsorbent

solutions (20 to 90 mg/L) by adding 50 mg of adsorbent

dose decreased the adsorption capacity. This is due to the

at pH 4 for 1 hr. After performing adsorption experiments

fact that as the adsorbent dose is increased, there is less

with 20 mg/L of Cr(VI) solution with 50 mg of adsorbent

commensurate increase in adsorption resulting from the

at the optimum pH 4, the spent adsorbent is washed with

lower adsorptive capacity of adsorbent. The second cause

distilled water to remove any unabsorbed metal ion. The

may be the aggregation/agglomeration of biosorbent

metal loaded adsorbent is then agitated with 100 mL of

particles at higher concentrations, which would lead to a

0.1 M NaOH, 0.5 M NaOH and 0.1 M HCl for 1 h to desorb

decrease in the surface area and an increase in the diffusion

the metal ion from the biosorbent.

path length (Manohar et al. 2002). From this study 50 mg of adsorbent dose is fixed for further studies.

RESULTS AND DISCUSSION

Effect of pH and mechanism of adsorption

Characterization of the adsorbent

The effect of pH on the removal of Cr(VI) ions on CACPP

The pH of the biosorbent is found to be 3.9 for CACPP, due

is shown in Figure 3. The increased in pH of the metal

to the attachment of acidic functional groups. The presence

solution decreased the adsorption capacity and the initial

of these acidic groups is confirmed by the FTIR spectral

adsorption is mainly due to the ion exchange mechanism.

analysis of CACPP (Figure 1). The amount of Chitosan

At lower pH the biosorbent is positively charged due to

coated is determined to be about 26% by weight. The other

protonation and dichromate ion exists as anion leading

important characters like moisture content, bulk density,

to an electrostatic attraction between them. As initial pH

ash content, water soluble matter, acid soluble matter are

of the metal solution increased, deprotonation starts and

carried out by adopting ISI procedures I. The physico-

there by results in decrease of adsorption capacity.

chemical characteristic of CACPP is summarized in Table 1.

H2 CrO4 ¼ Hþ þHCrO12 4

ð1Þ

22 þ HCrO12 4 ¼ H þ CrO4

ð2Þ

Effect of biosorbent dose

2HCrO12 4

ð3Þ

The change in adsorption capacity with varying adsorbent

12 Cr(VI) ions forms stable complexes like Cr2O22 7 , HCrO4 ,

dose is shown in Figure 2. On increasing the adsorbent dose

12 CrO22 4 and HCr2O7 based on the pH of the metal solution.

the percentage removal of Cr(VI) increased from 96.2 to

The fraction of any particular species depends on the metal

¼

Cr2 O22 7

þ H2 O

CACPP

50

% Transmittance

40 895.08.30.86 30 516.99.18.94 20 1059.05.10.38 2920.58.20.29 1620.40.5.624

10

1319.47.9.788 4,000

3,500

3,000

2,500

2,000

Wavenumber cm–1 Figure 1

|

FTIR spectrum of CACPP.

1,500

1,000

500

R. Malarvizhi et al. | Studies on removal of chromium (VI) from water

2438

|

Characteristics of CACPP

157

Parameter

CCACPP

pH

3.9

Moisture content (%)

7.0

Bulk density (g/cc)

0.16

Ash content (%)

3.0

Water soluble matter (%)

10

Acid soluble matter (%)

36

ion concentration and pH of the solution (Udaybhaskar et al. 1990). The decrease in adsorption with increase of pH may

156 Adsorption capacity (mg/g)

Table 1

Water Science & Technology—WST | 62.10 | 2010

155

154

153

152

151 0

be due to the decrease in electrostatic force of attraction

2

between the metal ions and the sorbent. At lower pH range the Cr2O22 7 ions are more in solution. At very lower pH value the surface of sorbent would also be surrounded by the hydronium ions which enhanced the chromium ions

4

6

8

pH Figure 3

|

Effect of pH for CACPP-Cr(VI) system.

precipitating from the solution, so making the sorption studies is impossible.

interaction with the binding sites of the biosorbent by greater attractive forces. A sharp decrease in adsorption above the pH 4.0 may be due to the occupation of the adsorption sites by the anionic species

HCrO12 4 ,

Cr2O22 7 ,

CrO22 4 etc., which retards the approach of such ions further towards sorbent surface (Do¨nmez & Aksu 2002). At low pH (below 4) the amine group on Chitosan is protonated to varying degree. The NHþ 3 group on the Chitosan is the main reason for Cr(VI) adsorption. With increase in pH from 5 to

Adsorption isotherm models In this study a non-linear method of 3 widely used isotherms, the Langmuir (Langmuir 1918), Freundlich (Freundlich 1906) and Redlich – Peterson (1959) models are used. The equilibrium adsorption capacity increased from 99.0 to 291.5 mg/g, while increasing the initial Cr(VI) ion concentration from 30 to 90 mg/L. The observed trend in

9 the degree of protonation of Chitosan functional groups

adsorption capacity may be due to the increase in driving

decreased gradually hence the removal is decreased. At pH

force offered by the concentration gradient as the initial

greater than 7.0 insoluble Chromium hydroxide starts

concentration was increased.

100

tration study were modeled with varies isotherm equations

400

99

discussed in the proceeding paragraphs. Fitted curves of each

350

98

model with the experimental data are illustrated in Figure 4.

97

The fitted parameters of the isotherm models obtained are

96

presented in their respective equations in Table 2. Within

300 250 Adsorption capacity % Removal

200

95 94

150

93

100

92

50

91

0

90 0

0.4 0.8 1.2 1.6

2

2.4 2.8 3.2 3.6

Dose (g/L) Figure 2

|

Effect of dose for CACPP-Cr(VI) system.

4

% Removal

Adsorption capacity (mg/g)

The results obtained from varying initial metal concen450

the concentration range studied, the equilibrium data fit very well with Langmuir model. Both Langmuir and Redlich-Peterson models have very high correlation coefficients (0.990). Therefore, it is assumed that the adsorption of Cr(VI) ions takes place through monolayer on homogenous adsorption sites on the chitosan coated Cyperus pangorei. The similar types of results were reported by other researchers used chitosan coated biosorbent for the heavy metal removal from water (Wan et al. 2007).

R. Malarvizhi et al. | Studies on removal of chromium (VI) from water

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Water Science & Technology—WST | 62.10 | 2010

systems the equilibrium is achieved somewhat slowly.

350

Process performance and ultimate costs of an adsorption 300

system depend upon the effectiveness of the process design and the efficiency of process operation. The efficiency of

qe (mg/g)

250

process operation requires an understanding of the kinetics 200

of uptake or the time dependence of the concentration

150

Cr(VI)/CACCP

distribution of the solute in both bulk solution and solid

Redlich-Peterson

adsorbent and identification of the rate determining step.

Langmuir

100

The removal of Cr(VI) increased with agitation time and

Freundlich

attained equilibrium within 15 min. While increasing the

50

Cr(VI) ion concentration from 20 to 40 mg/L, the amount 0 0

0.5

1

1.5

2

of Cr(VI) adsorbed increased from 67.8 to 137.1 mg/g for

2.5

CACPP. This indicates that the rate of adsorption is likely

Ce (mg/L)

|

Figure 4

to depend on precise experimental conditions such as

Fitted isotherms for CACPP-Cr(VI) system.

The important factor that obtained from Langmuir

adsorbent/adsorbate ratio and agitation.

model is KR from this value one can predict whether the sorption system is favourable (Hall et al. 1966) or not. According to Hall et al., the essential features of the

Kinetic modeling

Langmuir isotherm can be expressed in terms of a

A suitable kinetic model is desired to examine the mech-

dimensionless constant separation factor which is defined

anisms such as mass transfer and chemical reaction. Many

by the following relationship. 1 KR ¼ ð1 þ Ka C 0 Þ

models including the homogenous surface diffusion, pore ð4Þ

diffusion and heterogeneous diffusion models have been applied in batch reactors to describe the transport of solute

where KR is a dimensionless separation factor, C0 is the

species inside the adsorbent particles (Zhou & Martin 1995),

initial metal ion concentration and Ka is Langmuir constant.

however the mathematical complexity of these models

The value of KR, explains the feasibility of the reac-

makes them inconvenient for practical use (Cheung et al.

tions. (KR . 1—unfavourable, KR ¼ 1—linear, 0 , KR , 1—

2001). Any kinetic or mass transfer representation is likely to

favourable, KR ¼ 0—Irreversible). In this present study the

be global. From a system design viewpoint, a lumped

KR values obtained for both systems lie between one to zero.

analysis of kinetic data is therefore sufficient for practical

Higher the Ka value, stronger is the affinity between the

operation (Wu et al. 2001) Hence identification of a

biosorbent surface and the metal ions.

simplified kinetic equation, which can represent the mass transfer precisely for the sorption systems studied, assumes importance.

Adsorption kinetics

The kinetic results are initially fitted with Lagergren’s Kinetic studies are valuable for interpreting mechanisms of homogenous reactions, but the interpretation of the kinetics

pseudo first order kinetic Equation (Lagergren 1898) which is given below:

of heterogeneous reactions has been less successful because their analysis is particularly difficult. In micro porous Table 2

|

qt ¼ qe ½1 2 exp ð2k1 tÞ


ð5Þ

Fitted isotherm parameter for CACPP-Cr(VI) system

Non-linear Langmuir Cr(VI)-CACCP System

Non-linear Freundlich

Non-linear Redlich-Peterson

qm (mg/g)

Ka (dm3/mg)

r2

1/n

KF (dm3/g)

r2

g

B (dm3/mg)

A (dm3/g)

r2

291.5

1.265

0.990

0.468

207.23

0.969

0.981

1.32

306.6

0.990

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Water Science & Technology—WST | 62.10 | 2010

Adsorption rate constant 80

As the pseudo second order model gave better kinetic fits, the interpretation on rate constants will be mainly with

qt (mg/g)

60

those obtained by using this model. It is evident that the rate constant decreases with increase in Cr(VI) concentration. This trend in rate constant values may be attributed

40

to decrease in the readily available vacant adsorption sites

Cr(VI)/CACCP 40 mg/L Cr(VI)/CACCP 30 mg/L Cr(VI)/CACCP 20 mg/L 1st order model 2nd order model

20

as the Cr(VI) concentration is increased. In other words, once the easily available sites are occupied the Cr(VI) molecules, then the remaining Cr(VI) in solution find

0 0

4

remote adsorption sites inside the pores of the adsorbent

8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

t (min)

|

Figure 5

with difficulty, which makes the rate of adsorption to decrease as the Cr(VI) concentration is increased. Weber

Fitted kinetic models for CACPP-Cr(VI) system.

where qe and qt are the amounts of metal adsorbed at

(1967) reported that the relationship between soluble solute

equilibrium and at time t (mg/g), respectively and k1 is the

concentration and the rate of adsorption helps to describe

first order rate constant (L/min). Similarly the data are

the mechanism of adsorption taking place. In cases of strict

fitted with pseudo second order kinetic Equation (Ho &

surface adsorption, a variation of rate should be pro-

McKay 2000) given below:

portional to the first power of concentration. However,

qt ¼

t


1=k2 q2e þ t=qe

when pore diffusion limits the adsorption process, the


ð6Þ

where k2 is the second order rate constant (g/mg min), is applied to the kinetic results to compare the fitted curves with that obtained for pseudo first order equation and identify the equation, which defines the whole range of adsorption period precisely. The product, k2q2e , in above equation represents the initial adsorption rate (h). The experimental results obtained for agitation time study are modeled with pseudo first order and pseudo second order and the results are depicted in Figure 5. The fitted parameters obtained by nonlinear optimization method along with their respective r 2 and qe values are presented in Table 3 for CACPP-Cr(VI) system at different metal concentrations. It is

relationship between initial solute concentration and the rate of reaction will not be linear (Komoto 1956). In this study we observed the non linear relation between the initial concentration of the metal ions and the rate constant values. The third step is very rapid and does not contribute to the rate of adsorption. Hence, film and pore transports are also play a major role in controlling the rate of adsorption from solution along with chemisorption. In general, these two steps either singly or in combination limit the overall rate of adsorption. Desorption studies

evident from the tables that pseudo second order model

Desorption studies are carried out to recover the Cr(VI)

defines the experimental data more precisely than pseudo

adsorbed onto adsorbents which would help to regenerate

2

and recycle the spent biosorbent. About 70 and 80% of

first order model based on the higher r values. Table 3

|

Fitted kinetic parameters for CACPP-Cr(VI) system

1st order

2nd order k1 (1/min)

r2

65.5

0.456

0.819

Cr(VI)/CACCP 30 mg/L

94.1

0.307

Cr(VI)/CACCP 40 mg/L

124.5

0.178

Varying initial concentration of Cr(VI) ions

Cr(VI)/CACCP 20 mg/L

q1 (mg/g)

k (g/mg min)

h (mg/g min)

r2

67.8

0.01564

71.9

0.941

0.952

101.5

0.00488

50.3

0.994

0.928

137.1

0.00193

36.4

0.971

qe (mg/g)

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R. Malarvizhi et al. | Studies on removal of chromium (VI) from water

Cr(VI) is recovered from the spent CACPP by using 0.1 and 0.5 M NaOH solutions.

CONCLUSION In this work, an efficient and low cost composite biosorbent is derived from the waste biomasses. Its adsorption capacity is improved by coating it with chitosan. The study about the pH, confirmed the ion exchange process is responsible for the initial adsorption of metal ions on the surface of the biosorbent followed by the protonation of amine group in Chitosan coating. From the fitted isotherm, the removal of Cr(VI) ions by the chitosan coated Cyperus pangorei follows monolayer adsorption, since the r 2 (0.990) value for the Langmuir and the Redlich- Peterson model is same and more compared to Freundlich model. From the kinetic data it is cleat that the adsorption capacity (67.8 to 137.1 mg/g) of the newly prepared biosorbent is increased while increasing the metal ion concentration from 20 to 40 mg/L. The maximum adsorption capacity is 291.5 mg/g. The spent biosorbent can be regenerated using 0.5 M NaOH.

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