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Life Science Archives (LSA)

ISSN: 2454-1354

Volume – 1; Issue - 1; Year – 2015; Page: 6 - 12

© 2015 Published by JPS Scientific Publications Ltd. All rights reserved

Research Article

STUDIES ON THE REMOVAL OF Cr (VI) FROM WASTEWATER BY

STISHOVITE - TiO2 NANOCOMPOSITE

V. T. Priya1* and V. Venkateswaran

2

1Research scholar, Research and Development Centre, Bharathiyar university, Coimbatore, Tamil Nadu,

India. 2Principal, Sree Saraswathi Thyagaraja College, Pollachi, Tamil Nadu, India.

Abstract

The adsorption behavior of Cr(VI) from aqueous solution onto Stishovite-TiO2 nanocomposite was

investigated as a function of parameters such as initial metal concentration, contact time, pH

and temperature.

The Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms. The

pseudo-second-order kinetic, Elovich, intraparticle diffusion models were used to describe the kinetic data

and rate constants were evaluated. The final result shows that the adsorption process under study could be

best described by the pseudo second order equation. The experimental data fitted well with both Langmuir

and Freundlich isotherms. Thermodynamic parameters reveals that the adsorption to be spontaneous and

endothermic.

Article History Received : 05.02.2015

Revised : 18.02.2015

Accepted : 24.02.2015

Keywords: Stishovite-TiO2 Nano composite, Cr (VI), Adsorption kinetics and Isotherm

1. Introduction

Pollution by heavy metals is caused by

many industrial wastewaters such as those let out

by metal plating units, mining operations, battery

manufacturing, paints and pigments and the glass

production industries. Heavy metals are not

biodegradable and their presence in streams and

lakes leads to biomagnifications in living

organisms, causing health problems in animals,

plants and human beings (Mehmet et al., 2006).

Several clay materials, either in their

natural form or in modified forms had been

utilised as adsorbents for water and wastewater

treatments because of their low cost, availability in

abundance in all parts of the world, high

* Corresponding author: V. T. Priya

Tel.: +91-9952215776

E-mail: [email protected]

adsorption capacities high surface area they

provide and large ion exchange potentials.

Reports on the use of clays and polymers

are available in literature. Several clay minerals

like sodium montmorillionite (Abollino et al.,

2003), hydroxy apatite (Badillo-Almaraz and Ly,

2003), siderite, fugirite and magnetite (Johnson

and Sherman, 2008), rectorite (Zili Liu et al.,

2012), vermiculite (Machado et al., 2006) had

been effectively used as adsorbents. But, the

studies on the use of both of them in combination

in the form of a composite are not that much

available. Hence, it was thought worthwhile to

investigate the efficiency of Stishovite-TiO2

nanocomposite as an adsorbent in removing Cr

(VI) from aqueous solutions and the results are

summarized in the following sections.

V. T. Priya /Life Science Archives (LSA), Volume – 1, Issue – 1, Page – 6 to 12, 2015 7


2. Methodology

2.1. Preparation of Stishovite-titanium-di-oxide

nanocomposite

Stishovite (3g) was allowed to swell in 15

ml of water-free alcohol and stirred for 2 hours at

25 °C to get a uniform suspension. At the same

time, the titanium dioxide (3 g) was dispersed into

water-free alcohol (15 ml). Then, the diluted

titanium dioxide was slowly added into the

suspension of Stishovite and stirred for a further 5

hours at 25 °C. Finally, 5 ml alcohol mixed with

0.2 ml deionized water was slowly added. The

stirring was continued for another 5 hours at 25 °C

and the resulting suspension was kept overnight in

a vacuum oven for 6 hours at 80°C.

2.2. Determination of Chromium (VI) (APHA

et al., 1998)

Hexavalent Chromium was determined

spectrophotometrically by diphenyl carbazide

method. To a series of standard solutions of Cr

(VI) (5.50 µg), 3 ml of 2 N sulphuric acid, 2 drops

of phosphoric acid and 0.5 ml of diphenyl

carbazide solution (0.5% in acetone) were added

and made up to 25 ml with doubly distilled water.

After few minutes the absorbance was measured

at 540 nm against a reagent blank. A calibration

graph with absorbance vs Chromium (VI)

concentration was prepared. The concentration in

the sample was determined using the calibration

graph.

2.3. Preparation of Chromium (VI) stock

solution (1000 ppm)

A stock solution of (1000 mg/L) of

Chromium (VI) was prepared by dissolving 5.658

g of dried potassium dichromate (K2Cr2O7.H2O) in

doubly distilled water and making up to 1000 ml.

2.4. Characterization of adsorbent

Physico-chemical characteristics of the

adsorbents were studied as per the standard testing

methods. The XRD pattern of Stishovite - TiO2

nanocomposite (Fig.1) showed characteristic

peaks at 28° confirming the presence of Stishovite

- TiO2 phase in the nanocomposite. The surface

morphology of the adsorbents was visualized via

scanning electron microscopy (SEM) (Fig.2). The

diameter of the composite range was 50 µm.

2.5. Batch adsorption experiments

Entire batch mode experiments were

carried out in the temperature range 301 K to 317

K by taking 50 ml of the respective metal solution

and known amount of the adsorbent in a 100 ml

conical flask. The flasks were agitated for pre

determined time intervals in a thermostat attached

with a shaker at the desired temperature. The

adsorbent and adsorbate were separated by

filtration. Studies on the effects of agitation time,

pH, initial metal concentration, adsorbent dose and

temperature were carried out by using known

amount of adsorbent and 50 ml of metal solution

of different concentrations. Metal solution (50 ml)

with different amounts of adsorbent was taken to

study the effect of adsorbent dosage on the

removal of metals.

Figure - 1: XRD analysis of Stishovite - TiO2

Composite

Figure – 2: SEM of Stishovite - TiO2

Nanocomposite



3. Results and Discussion

3.1. Effect of agitation time and initial dye

concentration

The effect of initial metal concentration

and contact time for the removal of Cr (VI) is

shown in Fig.3. For this study 50ml of 10 to 40

mg/L of metal solution was agitated with 100mg

of adsorbent. The extent of removal of metal was

faster in initial stages, then showed decreasing

pattern and finally became constant showing the

attainment of equilibrium. The extent of removal

was found to be 86.58%. The curves obtained are

single and smooth, indicating monolayer coverage

on the adsorbent surface (Namasivayam and

Periasamy, 1993).

Figure - 3: Effect of initial dye concentration

3.2. Effect of adsorbent dosage

The effect of adsorbent dosage on the

metal removal was studied by keeping all other

experimental conditions constant except that of

adsorption dosage. The amount adsorbed per unit

mass of the adsorbent decreased with increase in

adsorbent concentration (Fig.4). The decrease in

unit adsorption with increasing dose of adsorbent

may basically be due to the fact that adsorption

sites remaining unsaturated during the adsorption

process.

3.3. Effect of pH

Adsorption experiments were carried out at

various pH values ranging from 6 to 11

maintaining the required pH by adding necessary

amount of dilute hydrochloric acid and sodium

hydroxide solutions. A pH meter calibrated with

4.0 and 9.0 buffers were used. The Figure - 5

indicates that maximum metal removal had

occurred in basic medium. It was observed that as

the pH increases the sorption capacity also

increases.

Figure - 4: Effect of adsorbent dosage

3.4. Effect of temperature

Temperature has an important effect on the

adsorption process. Fig.6 shows effect of different

temperatures on the removal of Cr (VI) by the

nanocomposite. The amount of metal adsorbed

increases with increasing temperature from 301K

to 317K indicating the adsorption process to be

endothermic. This may be due to the fact that as

the temperature increases, rate of diffusion of

adsorbate molecules across the external boundary

layer and internal pores of adsorbent particle

increase.

Figure – 5: Effect of pH



Figure – 6: Effect of temperature

3.5. Adsorption isotherms

Langmuir and Freundlich isotherms were

used to determine the amount of metal adsorbed

and its equilibrium concentration.

3.6. Langmuir isotherm

In linear form the Langmuir model

(Langmuir, 1916) is usually expressed as,

Where Ce is equilibrium concentration of

metal (mg/L), qe is the amount of metal adsorbed

at equilibrium (mg/g), Q0 and b are the Langmuir

constants correlated to adsorption capacity and

rate of adsorption, respectively. A linear plot of

Ce/qe vs Ce is shown in Fig.7. The values of Q0 and

b were calculated from the slope and intercept of

the plots and the values are given in Table 1.

These values indicate that the maximum

monolayer adsorption capacity of Stishovite-TiO2

nanocomposite was 12.33 mg/g. The crucial

features of the Langmuir isotherm was examined

by the dimensionless constant separation term

(RL) to determine high affinity adsorption.

RL was calculated as follows:

RL = 1/ (1+bCo), Where Co is initial metal

concentration (mg /L).

The nature of adsorption if, RL > 1

Unfavourable, RL = 1 Linear, RL = 0 Irreversible,

0 < RL < 1 Favourable. In the present study, the

RL values were less than one in the concentration

range studied, which shows that the adsorption

process was favourable.

Table – 1: The values of Langmuir constantQ0

and b in addition to RL constantQ0 and b in

addition to RL

Figure - 7: Langmuir isotherm

3.7. Freundlich isotherm (Freundlich, 1906)

The Freundlich isotherm can be

represented in its logarithmic form as,

)2(........................................log1

loglog efe Cn

Kq

Where Kf and n are Freundlich constants

representing adsorption capacity and intensity of

the adsorbent respectively. The plot of log qe vs

Conc. of

metal

(mg/L)

Chromium (Stishovite - TiO2

nanocomposite)

RL b Q0(mg/g) R

2

20 0.093

- -

- 40 0.049 - - -

60 0.033 0.486 12.330 0.9694

80 0.025 - - -

100 0.020 - - -

120 0.017 - - -

)1(............................................1

00 Q

C

bQq

C e

e

e



log Ce shown in Fig.8 indicates that the adsorption

of Cr (VI) fit into the Freundlich isotherm. The

Freundlich constants (Kf and 1/n) are given in

Table - 2. The value of 1/n was less than one

which indicates a favorable adsorption (Bell,

1998).

Table – 2: The values of Freundlich constants

Kf and n

Metal

Kf

[mg1-1/n

L1/n

g-1

]

n

(mg/g) R

2

Chromium

with Stishovite

–TiO2

nanocomposite

PFR

8.831 1.729 0.9927

3.8. Kinetics of adsorption

In order to investigate the mechanism of

adsorption of Cr (VI) by the nanocomposite the

pseudo first order, pseudo second order and

Elovich models were considered.

The experimental data does not fit with pseudo

first order kinetic model.

Figure - 8: Freundlich isotherm

3.9. Pseudo second order kinetics

The pseudo second order chemisorptions

kinetic rate equation was expressed as (Ho et al.,

1999 and Ozacar, 2003).

)3.(.......................................2 Eqqqkdt

dqte

t

Here qe and qt were the adsorption capacity at

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

k2, the pseudo-second order rate constant

(g/mg/min). On integrating the Eq.3, for the

boundary conditions t=0 to t=t and qt = qe

)4.(.......................................

112 Eqtk

qqq ete

which is the integrated form of pseudo – second

order reaction. Eq.4 can be rearranged to obtain

)5.(.....................................11

2

2

Eqtqqkq

t

eet

Compared to Eq.3 and Eq.4 had an

advantage that k2 and qe can be obtained from the

intercepts and slope of the plot of (t/ qt) vs t and

there was no need to know any parameter

beforehand. The linearity of the plots (Fig.9)

clearly indicated that the adsorption process

followed pseudo second order kinetics.

Figure – 9: Pseudo second order kinetics

3.10. Elovich kinetic model (Low, 1960)

The Elovich equation is mainly applicable

for chemisorption processes involving

heterogeneous adsorbing surfaces. The Elovich

model in its integrated form can be

)6.(...........................).........ln(1

ln1

Eqabb

tb

qt

Where ‘a’ is the initial adsorption rate

(mg/g min) and ‘b’ is related to the extent of



surface coverage and the activation energy for

chemisorptions (g/mg). A plot (Fig.10) of qt vs ln

t is a straight line, as expected, with a slope of 1/b

and an intercept log 1/b ln (ab) with good

correlation coefficients confirming the

chemisorptive nature of adsorption.

Figure - 10: Elovich kinetic model

3.11. Thermodynamic of Adsorption

Thermodynamic parameters like ΔH0 and

ΔS0 were evaluated using Van’t Hoff’s equation

lnKc = ΔS0/R – ΔH

0/R ……………….Eq. (7)

Where KC is the Langmuir equilibrium

constant, ΔH0

(25.921 kJ/mol) and ΔS0

(3.118

kJ/mol), are the standard enthalpy and entropy

changes of adsorption respectively and their

values are calculated from the slopes and

intercepts respectively of the linear plot of ln Kc

vs 1/T. The free energy changes for the adsorption

process ΔG0 (kJ/mol) (7.805 – 8.22 at 301 K – 317

K) are derived using the relation

ΔG0 = ΔH

0 - T ΔS

0 …………….........Eq. (8)

Negative free energy change and positive

entropy change of adsorption indicate that the

adsorption process is favourable and spontaneous

in nature. The endothermic nature of adsorption is

confirmed by the positive value of ΔH0.

3.12. Desorption studies

Desorption studies with acetic acid

revealed that the regeneration of adsorbent was

not satisfactory, which confirms the chemisorptive

nature of adsorption.

4. References

1) Abollino, O., Aceto, M., Malandrino, M.,

Sarzanini, C and Mentasti, E., 2003.

Adsorption of heavy metals on Na-

montmorillonite. Effect of pH and organic

substances, Water Res., 37, 1619-1627.

2) APHA., AWWA., WPCF., 1998. Standard

methods for the examination of water and

wastewater, American Public Health Ass., 18

Ed., Washington DC.

3) Badillo-Almaraz, V.E and Ly, J., 2003.

Calcium sorption on hydroxy apatite in

aqueous solutions. Reversible and

nonreversible components, J. Colloid Interface

Sci., 258, 27-32.

4) Bell, T. R. K., 1998. Mass Transfer

Operations, 10th Ed., McGraw-Hill, New

York.

5) Freundlich, H.M.F., 1906. Uber die adsorption

in losungen. Zeitschriftsfur physialische

Chemie., 57, 385-471.

6) Ho, Y.S., McKay, G., 1999. Pseudo-second

order model for sorption processes, Process

Biochem., 34, 451-465.

7) Johnson, J and Sherman, D.H., 2008. Sorption

of As (III) and As (V) to siderite, green rust

(fougerite) and magnetite, implications for

arsenic release in anoxic ground waters, Chem.

Geol., 255, 173-181.

8) Langmuir, I., 1916. The constitution and

fundamental properties of solids and liquids,

J. Am. Chem. Soc., 38, 2221-2295.

9) Low, M.J.D., 1960. Kinetic modeling of liquid

phase adsorption of reactive dyes and metal

ions on chitosan, Chem. Rev., 60, 267-312.

10) Machado, L.C.R., Torchia, C.B and Lago,

R.M., 2006. Floating photocatalysts based on

TiO2 supported on high surface area exfoliated

vermiculite for water contamination,

Catal. Commun., I, 538-541.

11) Mehmet, E.A., Sukru, D., Celalettin, O. and

Mustafa, K., 2006. Heavy metal adsorption by

modified oak sawdust: Thermodynamics and

kinetics. Journal of Hazardous Materials.

12) Namasivayam, C and Periasamy, K., 1993.

Bicarbonate treated peanut hull carbon for Hg

(II) removal from aqueous solution, Water

Res., 27, 1663–1668.



13) Ozacar, M., 2003. Equilibrium and kinetic

modeling of adsorption of phosphorous on

calcined alunite, Adsorption 9, 125-132.

14) Zili Liu., FengPeng and Xiaoguo Liu., 2012.

Adsorption of heavy metals by sodium

polyacrylate-humic acid-rectorite composite as

a novel adsorbent, Adv. Mater. Res., 550-553,

2428-2435.

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