Adsorption kinetics of methyl violet onto perlite

Adsorption kinetics of methyl violet onto perlite

Mehmet Do�ggan, Mahir Alkan *

Department of Chemistry, Faculty of Science and Literature, Balıkesir University, 10100 Balıkesir, Turkey

Received 15 January 2002; received in revised form 16 September 2002; accepted 24 September 2002

Abstract

This study examines adsorption kinetics and activation parameters of methyl violet on perlite. The effect of process

parameters like contact time, concentration of dye, temperature and pH on the extent of methyl violet adsorption from

solution has been investigated. Results of the kinetic studies show that the adsorption reaction is first order with respect

to dye solution concentration with activation energy of 13.2 kJ mol�1. This low activation energy value indicates that

the adsorption reaction is diffusion controlled. The activation parameters using Arrhenius and Eyring equations have

been calculated. Adsorption increases with increase of variables such as contact time, initial dye concentration, tem-

perature and pH.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Perlite; Methyl violet; Activation parameters; Kinetics of adsorption; Dye; Intra-particle diffusion; Rate constant

1. Introduction

In the natural aquatic systems, sorption processes

play a dominant role in the transport and ultimate fate

of pollutants. Thus, the adsorption behaviour of organic

contaminants on natural particles has been of central

interest in recent years (Liu et al., 2001). Pignatello and

Xing (1996) as well as Luthy et al. (1997) have reviewed

the sorption/desorption of organic compounds on nat-

ural particles. Numerous studies have been devoted to

sorption kinetics, equilibrium modelling, and mecha-

nism as well as to the factors that affect adsorption. It

has been shown that the sorption of low-polarity, non-

polar, and nonionic organic compounds onto natural

particles occurs largely by the partition of these com-

pounds to organic matter involved in the solid (Xia and

Ball, 1999), and the interface behaviour of ionic organic

compounds is controlled mainly by the specific chemical

and electrostatic interactions (Undabeyti and Nir, 1999).

Dyes are widely used for textiles, printing, and dye-

ing, as well as in the food and paper-making industries.

Environmental research has paid special attention to

dye compounds because of the extensive environmental

contamination arising from dyeing operations. Most

studies have focused on the development of a treatment

technique and method for dye wastewater, such as a

physical–chemical technique destroying the color groups

(Meyer, 1992; Lin and Peng, 1994), a bio-degradation

process mineralizing the colorless organic intermediates

(Specchia and Gianetto, 1984), and chemical oxidation

including homogeneous (Kuo, 1992) and heterogeneous

photocatalytic oxidation (Choi et al., 1994). However,

because of its higher organic concentration, particular

toxicity, complex composition, and poor degradabil-

ity, dye wastewater cannot be efficiently purified and

treated. Thus, a large amount of wastewater containing

dye compounds in low concentrations may be leaked

and released into natural waters, leading to aquatic en-

vironmental contamination. Investigations have been

conducted on physico-chemical methods of removing

color from textile effluent. These studies include the use

of coagulants, oxidising agents, ultra-filtration, electro-

chemical and adsorption techniques. The advantages

Chemosphere 50 (2003) 517–528

www.elsevier.com/locate/chemosphere

* Corresponding author.

E-mail addresses: [email protected] (M. Do�ggan),

[email protected] (M. Alkan).

0045-6535/03/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0045-6535 (02 )00629-X

mail to: [email protected]

and disadvantages of each technique have been exten-

sively reviewed. Of these methods, adsorption has been

found to be an efficient and economic process to remove

dyes, pigments and other colorants and also to control

the bio-chemical oxygen demand. Activated carbon, in-

organic oxides, natural adsorbents (such as clays and

clay minerals, cellulosic materials, chitin, and chitosan)

have been extensively used as adsorbents.

Activated carbon adsorption has been found to be

an effective and widely employed means of water and

wastewater treatment. Despite its profilic use in water

and wastewater industries, commercial activated carbon

remains an expensive materials. This has led to a search

for low-cost materials as alternative adsorbents (Kan-

nan and Sundaram, 2001).

The purpose of this work is to study the kinetics and

mechanism of adsorption of methyl violet on perlite.

Perlite, a glassy volcanic rock, has been chosen as a

adsorbent, which is essentially a metastable amorphous

alumina silicate (Do�ggan, 1997, 2001; Do�ggan et al., 1997,

2000; Alkan and Do�ggan, 1998, 2001, 2002). In our pre-

vious works, we investigated the electrokinetic proper-

ties (Do�ggan et al., 1997) and surface titrations of perlite

suspensions (Alkan and Do�ggan, 1998), and also the

adsorption of copper (II) (Alkan and Do�ggan, 2001) and

methylene blue (Do�ggan et al., 2000) onto perlite samples.

Thus, this investigation aims at to study kinetics and

thermodynamic activation parameters, to describe the

rate and mechanism of adsorption, to determine the

factors controlling the rate of adsorption and to find out

the possibility of using this material as low-cost adsor-

bent for the removal of dye. The effects of solution pH,

concentration and temperature on methyl violet ad-

sorption rate have been evaluated.

2. Materials and methods

2.1. Materials

The perlite sample was obtained from Cumaovası

Perlite Processing Plants of Etibank (_IIzmir, Turkey).

The chemical composition of the perlite found in Turkey

is given in Table 1 (Uluatam, 1991). Before using in the

experiments, the perlite sample was treated as follows

(Do�ggan, 1997, 2001; Do�ggan et al., 1997, 2000; Alkan and

Do�ggan, 1998, 2001, 2002): a suspension containing 10

g l�1 perlite was mechanically stirred for 24 h. After

waiting for about two minutes the supernatant sus-

pension was filtered through a white-band filter paper

(U ¼ 12:5 cm) and the solid sample was dried at 110 �C

Nomenclature

K adsorption constant, g mol�1

Qe equilibrium dye concentration on adsorbent,

mol g�1

R linear regrassion coefficient

t time, s

T temperature, K

m mass of adsorbent, g

Qt the amount of dye adsorbed per unit mass of

the adsorbent at time t, mol g�1

C0 initial dye concentration in aqueous solu-

tion, mol l�1

k adsorption rate constant, s�1

t1=2 the half-adsorption time of dye, s

kdif intra-particle diffusion rate constant,

mol min�1=2 g�1

bL mass transfer coefficient, m s�1

SS the surface area of adsorbent, m2 g�1

Ct dye concentration in solution at time t,mol l�1

r0 the radius of the adsorbent particle, cm

D diffusion coefficient, cm2 s�1

Ea activation energy, kJ mol�1

Rg gas constant

DH � enthalpy of activation, kJ mol�1

DS� entropy of activation, J mol�1 K�1

DG� free energy of activation, kJ mol�1

kb Boltzmann�s constant, J K�1

h Planck�s constant, J s

Table 1

Chemical composition of perlite

Constituent Percentage present

SiO2 71–75

Al2O3 12.5–18

Na2O 2.9–4.0

K2O 4.0–5.0

CaO 0.5–2.0

Fe2O3 0.1–1.5

MgO 0.03–0.5

TiO2 0.03–0.2

MnO2 0.0–0.1

SO3 0.0–0.1

FeO 0.0–0.1

Ba 0.0–0.1

PbO 0.0–0.5

Cr 0.0–0.1

518 M. Do�ggan, M. Alkan / Chemosphere 50 (2003) 517–528

for 24 h, then sieved by 100-mesh sieve. The particles

under 100-mesh are used in further experiments.

The cation exchange capacity (CEC) of perlite was

determined by the ammonium acetate method, while the

density was determined by the picnometer method. The

specific surface area of perlite was measured by BET N2

adsorption. Zeta potential of perlite was measured using

a Zeta Meter 3.0 equipped with a microprocesser unit.

The results are summarized in Table 2. All chemicals

were A.R. grade.

2.2. Methyl violet

Methyl violet contains a secondary amine group. The

color of methyl violet changes from yellow to green in

the pH range 0.13–0.5, from green to blue in the pH

range 2.0–3.0. When pH is higher than 3, the solubility

of methyl violet are essentially unchanged. Methyl violet

has a molecular weight of 393.96 g mol�1. The structure

of this dye is shown in Fig. 1 (Dai, 1994).

2.3. Method

Methyl violet was A.R. grade from Carlo Erba (C.I.

42535) and was used without further purification. Me-

thyl violet was first dried at 70–80 �C for 4 h to remove

moisture befor use. All of the methyl violet solution was

prepared with distilled water. Since the dye is difficult to

dissolve in water, the dye solution was allowed to stand

for 1–2 days until the absorbance of the solutions re-

mained unchanged. Studies of the kinetics of methyl

violet adsorption onto perlite were carried out from

its solution. In systems, the dye concentration was

2:0 � 10�4 mol l�1, except those in which the effect of

concentration was investigated. For the experiments of

adsorption kinetics, 10-g-perlite sample was added into

a litre of methyl violet solution at the desired concen-

tration, temperature and pH. The pH of the solution

was adjusted with NaOH or HNO3 solution by using a

Orion 920A pH-meter with a combined pH electrode.

The pH-meter was standardized with NBS buffers before

every measurement. A preliminary experiment revealed

that about 30 min time is required for the adsorption

process to reach the equilibrium concentration. The

mixture was continuously agitated by a magnetic stirrer

at 30 �C and 500 rpm for 30 min. A constant tempera-

ture bath was used to keep the temperature constant. At

the end of the adsorption period, the solution was cen-

trifuged for 15 min at 5000 rpm. The samples at ap-

propriate time intervals were pipetted from the reactor

by the aid of the very thin point micropipette, which

prevents the transition to solution of perlite samples.

Preliminary experiments had shown that the effect of the

separation time on the amount of adsorbed dye were

negligible. The amount of adsorbed dye on perlite at any

time, t, was determined from absorbency as measured

with a Cary j1Ej UV–visible spectrophotometer (varian)

at a 584 nm wavelength. The amounts of dye adsorbed

were calculated from the concentrations in solutions

before and after adsorption. Each experimental point

was an average of two independent adsorption tests

(Yang and Al-Duri, 2001).

3. Results and discussion

3.1. Equilibrium time

The optimum period for the adsorption of methyl

violet on perlite can be observed by looking at the final

absorbance of methyle violet solution after adding per-

lite. As shown in Fig. 2, preliminary investigations on

the rate of adsorption by the perlite under study have

indicated that adsorption occurs rapidly. In Fig. 2, Qe

represents the ratio of adsorbed amount of methyl violet

to the gram amount of adsorbent (mol g�1). The amount

of adsorbed methyl violet after adding perlite as a

function of time is shown in Fig. 2. The rate of ad-

sorption of methyl violet onto perlite was followed by

looking at the increase in the adsorption of the methyl

violet as a function of time until the adsorption of

the methyl violet remained constant; implying equilib-

rium was reached. The gradual increase in adsorption

was observed, and, after 30 min for perlite, a gradualFig. 1. The structure of methyl violet.

Table 2

Some physico-chemical properties of perlite used in the study

Cation exchange capacity (CEC meg/100 g) 25.97

Density (g ml�1) 2.30

Specific surface area (m2 g�1) 1.22

Zeta potential (mV) )23.50

M. Do�ggan, M. Alkan / Chemosphere 50 (2003) 517–528 519

approach to the limiting adsorption was observed.

Therefore, the optimum agitation period of methyl vi-

olet is about 30 min for perlite. Similar results were

obtained by other workers for other adsorbate–adsor-

bent systems (Ngah and Musa, 1998).

3.2. Adsorption rate

The application of the adsorption technique to larger

scale studies is facilitated by clear elucidation of the

kinetic parameters and sorption characteristics of the

adsorbent material. Four issues of prime importance in

this respect are the influence of initial adsorbate con-

centration, contact time, temperature and pH on uptake.

3.2.1. The effect of contact time on adsorption process

Contact time experiments investigate the influence of

system parameters on the rate and extent of adsorption

of methyl violet onto perlite, and also the kinetic data

essential for the identification of the rate determining

step(s) in the adsorption process. The influence of dif-

ferent initial methyl violet concentrations, 1:0 � 10�4;

1:5 � 10�4; 2:0 � 10�4 and 2:5 � 10�4 mol l�1 at pH ¼ 7

and 30 �C, on the equilibrium time is shown in Fig. 2,

which indicates that the initial concentration did not

have a significant effect on the time to reach equilibrium.

This is in agreement with the experimental results of

McKay et al. (2001). On changing the initial concen-

tration of methyl violet solution from 1:0 � 10�4 mol l�1

to 2:5 � 10�4 mol l�1, the amount adsorbed increases

from 9:8 � 10�6 to 1:5 � 10�5 mol g�1 at pH ¼ 7 and 30

�C. It is evident from Fig. 2 that with a lower initial

concentration of adsorbate the amount of adsorbate

attained on the solid phase is smaller than the amount

attained with higher initial concentrations are used.

However, the percent removal of methyl violet was

greater with lower initial concentrations and smaller

with higher initial concentrations.

3.2.2. The effect of temperature on the adsorption process

The results of the studies on the influence of tem-

perature on methyl violet adsorption is presented in Fig.

3. A study of the temperature dependence of adsorption

reactions gives valuable information about the enthalpy

change during adsorption. The effect of temperature on

the adsorption rate was studied by carrying out a series

of experiments at 30, 40, 50 and 60 �C for perlite. The

adsorption of methyl violet on perlite increased from

1:4 � 10�5 to 1:9 � 10�5 mol g�1, indicating that the

adsorption process is endothermic in nature when tem-

perature was increased from 30 to 60 �C (Fig. 3) at

pH ¼ 7 and C0 ¼ 2:0 � 10�4 mol l�1. The increase in the

equilibrium sorption of dye with temperature indicates

that dye removal by adsorption on perlite favours a high

temperature (Do�ggan et al., 2000). This may be a result of

increase in the mobility of the large dye ion with tem-

perature. An increasing number of molecules may also

acquire sufficient energy to undergo an interaction with

active sites at the surface. Furthermore, increasing tem-

perature may produce a swelling effect within the in-

ternal structure of the perlite enabling large dyes to

penetrate further (Do�ggan et al., 2000).

3.2.3. The effect of pH on adsorption process

The magnitude of electrostatic charges imparted by

the ionized dye molecules is primarily controlled by the

Fig. 2. The effect of contact time and concentration on the removal of methyl violet from aqueous solutions with perlite.


pH of medium. The amount of dye adsorbed or rate of

adsorption tends to vary with pH (3–9) of aqueous

medium. The amount of methyl violet adsorbed on the

perlite as a function of pH at an initial dye concen-

tration of 2:0 � 10�4 mol l�1 and temperature 30 �C is

shown in Fig. 4. The amount of methyl violet adsorbed

increased with increase in pH (i.e., as the solution be-

came more basic). The adsorption of methyl violet on

perlite increased from 1:3 � 10�5 to 1:5 � 10�5 mol g�1,

when pH was increased from 3 to 9 (Fig. 4) at 30 �Cand C0 ¼ 2:0 � 10�4 mol l�1. This indicates that the

adsorption capacity of the perlite is pH dependent. Thus,

it is clear that pH determined the speciation of the

methyl violet as well as providing a favourable adsor-

bent surface charge for the adsorption to occur. As

the pH increased from 3 to 9, the number of ionizable

sites on the perlite increased. These observations are

in line with earlier findings of Lafferty and Hobday

(1990).

SOH þ OH� ¼ SO� þ H2O ð1Þ

It had been shown that perlite samples had no point

of zero charge and exhibited negative zeta potential

values in the pH range 3–11 (Do�ggan et al., 1997; Alkan

and Do�ggan, 1998). As the pH of the dye solution became

Fig. 3. The effect of contact time and temperatures on the removal of methyl violet from aqueous solutions with perlite.

Fig. 4. The effect of contact time and pH on the removal of methyl violet from aqueous solutions with perlite.


higher, the association of dye cations with more nega-

tively charged perlite could more easily take place;

SO� þ Dyeþ ¼ S–O–Dye ð2Þ

3.3. Adsorption kinetics and mechanism

In order to examine the controlling mechanism of

the adsorption process such as mass transfer, diffusion

control and chemical reaction, several kinetic models are

used to test experimental data. From a system design

viewpoint, a lumped analysis of adsorption rates is thus

sufficient to practical operation (Wu et al., 2001).

A simple kinetic analysis of adsorption is the Lag-

ergren equation. In this equation the average value of

the rate constant, k, for adsorption of methyl violet on

perlite can be calculated in the form

dQt

dt¼ kðQe � QtÞ ð3Þ

where k is the rate constant of the first-order adsorption

(s�1), and Qe and Qt are the amount of dye adsorbed per

unit mass of the adsorbent (mol g�1) at equilibrium and

time t, respectively. The first-order kinetics only de-

scribes the sorption sites and not the adsorption process

as a whole. After definite integration by applying the

initial conditions Qt ¼ 0 at t ¼ 0 and Qt ¼ Qt at t ¼ t,Eq. (3) becomes

lnðQe � QtÞ ¼ lnQe � kt ð4Þ

The plot of lnðQe � QtÞ versus t gives staight lines.

The straight lines in Figs. 5–7 agree with the linearity of

the Lagergren equation. The fact that all the linear re-

gression coefficients were higher than 0.99 indicates the

applicability of this kinetic equation and the first-order

nature of the adsorption process of methyl violet on

perlite. These lines suggest that the adsorption process

is a first-order process. The k values of the various ad-

sorption system were calculated from these plots and are

listed in Table 3. Moreover, it is found that the variation

of rate constant with temperature is higher than that

with pH. The rate constants of adsorption generally

increased with increasing temperature and pH at con-

stant concentration. More specifically, k values were

found to increase from 2:40 � 10�3 to 3:04 � 10�3 s�1

with increase in the solution pH from 3 to 9, and from

2:86 � 10�3 to 4:55 � 10�3 s�1 with increase in the so-

lution temperature from 30 to 60 �C.

The half-adsorption time of the dye, t1=2, is the time

required for the perlite to take up half as much dye as

it would at equilibrium. This time is often used as a

measure of the rate of adsorption and is given by

t1=2 ¼ln 2

kð5Þ

The values of k and t1=2 are given in Table 3 at the dif-

ferent initial dye concentrations, temperatures and pHs

of solution.

The mechanism of adsorption from solution consists

of three stages: (a) diffusion across the boundary layer

surrounding the particle (a process measured by an ex-

ternal mass transfer coefficient); (b) diffusion within

the particle assuming pore diffusion or solid diffusion

mechanism and (c) adsorption at site. Steps (a) and (b)

are considered to be the major rate controlling steps;

(c) is assumed to be rapid. It is suggested that diffusion-

controlled systems can be differentiated from chemically

controlled systems by two factors, mainly fluid velocity

Fig. 5. Lagergren plots for adsorption of methyl violet at different initial dye concentrations from its aqueous solutions with perlite.


and temperature. Since the uptake of dye at the active

sites of perlite is a rapid process, the rate of adsorption is

mainly governed either by external mass transfer rate or

intra-particle diffusion rate. The adsorption mechanism

can be simply determined using mass transfer and intra-

particle diffusion models (Allen et al., 1997).

The external mass transfer coefficient, bL (m s�1) of

methyl violet at the perlite–solution interface, was de-

termined by using Eq. (6)

lnCt

C0

�� 1

1 þ mK

�¼ ln

mK1 þ mK

� �� 1 þ mK

mK

� �bLSSt

ð6Þ

where K is the Langmuir constant (L g�1), m is the mass

of adsorbent (g), and SS is the surface area of adsorbent

(m2 g�1).

A linear graphical relation between ln½ðCt=C0Þ � 1=ð1 þ mKÞ versus t was not obtained. In plotting the

curves, K values for the adsorption of methyl violet onto

perlite were taken from our previous work (Do�ggan,

2001). This result indicates that the model mentioned

above for the system is not valid. The values of regres-

sion coefficient (r-values) calculated from equation men-

tioned above are given in Table 3.

It has been stated that the specific surface areas of

non-porous adsorbents change more frequently in the

Fig. 7. Lagergren plots for adsorption of methyl violet at different pH from its aqueous solutions with perlite.

Fig. 6. Lagergren plots for adsorption of methyl violet at different temperatures from its aqueous solutions with perlite.


range of 0.1–1.0 m2 g�1 (Oscik, 1982). The specific sur-

face areas of perlite samples have changed in the range

of 1.23–2.30 m2 g�1 (Do�ggan, 2001). Despite the result

above, it can be said that perlite is more or less a porous

adsorbents.

Generally, a process is diffusion controlled if its rate

is dependent upon the rate at which components dif-

fuse toward one another. A diffusion-controlled reaction

would have a comparatively small activation energy, for

if Ea is large (Ea=RT � 1), then the rate of the reaction is

not controlled by the diffusion rate but by the number of

molecules with energy greater than the activation energy

(Woodbury, 1997). The possibility of intra-particle dif-

fusion was explored by using the intra-particle diffusion

model:

Qt ¼ kdif

ffiffit

pþ C ð7Þ

where Qt is the amount of dye adsorbed at time t(mol g�1); C is the intercept and kdif is the intra-particle

diffusion rate constant (mol s�1=2 g�1). According to Eq.

(7), a plot of Qt versus t1=2 should be a straight line with

a slope kdif and intercept C when adsorption mechanism

follows the intra-particle diffusion process. Values of

intercept (Table 3) give an idea about the thickness of

boundary layer, i.e., the larger intercept the greater is the

boundary layer effect (Kannan and Sundaram, 2001).

The kdif values were calculated by using correlation

analysis (Table 3). The high regression coefficient values

suggest that the uptake of methyl violet varies almost

linearly with the half power of time (t1=2). This func-

tional relationship corresponds to the characteristic of

intra-particle diffusion. It is found that diffusion rate was

increased with increase of dye concentration, tempera-

ture and pH (Figs. 8–10).

3.4. Diffusion coefficient

The diffusion coefficients for the intra-particle trans-

port of methyl violet within the pores of perlite particles

have been calculated at different temperatures, initial

dye concentrations and pH by employing Eq. (8)

t1=2 ¼0:030r2

0

Dð8Þ

where D is the diffusion coefficient with the unit cm2 s�1;

t1=2 is the time (s) for half-adsorption of methyl violet

and r0 is the radius of the adsorbent particle in cm. The

value of r0 was calculated as 3:75 � 10�3 cm for per-

lite sample. In these calculations, it has been assumed

that the solid phase consists of spherical particles with

an average radius between the radii corresponding to

upper- and lower-size fractions (for <150 lm mesh-size).

For the present study, the pore diffusion coefficient

values obtained from Eq. (8) are given in Table 3. The

values of diffusion coefficient for adsorption of methylTab

le3

Kin

etic

svalu

esca

lcu

late

dfo

rm

eth

yl

vio

let

ad

sorp

tio

no

np

erlite

Tem

per

atu

re

(�C

)

Co

nce

ntr

ati

on

�10

4(m

oll

�1)

pH

Lager

gre

neq

uati

on

Mass

tran

sfer

Intr

a-p

art

icle

diff

usi

on

D�

10

10

(cm

2s�

1)

t 1=2

(s)

rk�

10

3(s

�1)

rk d

if�

10

7

(mo

ls�

1=2

g�

1)

rC

30

1.0

70.9

992

2.5

60.9

979

2.1

50.9

928

0.3

015.6

271

30

1.5

70.9

996

2.6

40.9

860

2.1

70.9

955

0.5

416.1

262

30

2.0

70.9

998

2.8

60.9

807

2.5

60.9

916

0.6

817.4

243

30

2.5

70.9

995

3.2

50.9

735

2.6

20.9

963

0.8

219.8

213

30

2.0

30.9

998

2.4

00.9

779

2.3

10.9

934

0.5

814.6

289

30

2.0

50.9

994

2.6

50.9

770

2.3

90.9

944

0.6

316.1

262

30

2.0

90.9

997

3.0

40.9

876

2.6

20.9

973

0.7

618.5

228

40

2.0

70.9

995

3.2

40.9

705

2.6

10.9

884

0.7

919.7

214

50

2.0

70.9

996

3.8

80.9

602

2.9

60.9

742

0.9

323.6

179

60

2.0

70.9

999

4.5

50.9

794

3.0

00.9

940

1.2

527.7

152


violet increase from 17:4 � 10�10 to 27:7 � 10�10 cm2 s�1

with change in temperature from 30 to 60 �C. These

results agree with those of Chibowski et al. (2001a,b)

and McKay and McConvey (1985). Compared to as-

trozane blue and telon blue on wood (McKay and Po-

ots, 1980), the D values for methyl violet on perlite

are higher. The diffusion coefficients vary from 6:0�10�13 to 1:8 � 10�13 cm2 s�1 for astrazone blue and from

3:0 � 10�13 to 8:0 � 10�13 cm2 s�1 for telon blue. It can

be result of crystalline defects in the structure of perlite

(Vieth, 1991).

3.5. Activation parameters

The rates of most chemical reactions increase mark-

edly as the temperature is increased, typically doubling

Fig. 8. Intra-particle diffusion plots for adsorption of methyl violet at different initial dye concentrations from its aqueous solutions

with perlite.

Fig. 9. Intra-particle diffusion plots for adsorption of methyl violet at different temperatures from its aqueous solutions with perlite.


with a temperature rise of a few degrees kelvin. In the

rate law, the temperature dependence appears in the

rate constant. Quite remarkably, the temperature de-

pendence of virtually all reactions can be fit successfully

to the Arrhenius equation (Laidler and Meiser, 1999):

ln k ¼ lnA� Ea

RgTð9Þ

where Ea is the Arrhenius activation energy, and A is the

Arrhenius factor. To extract A and Ea from kinetic data,

we plot the series of rate constants measured at different

temperatures in a graph of ln k versus 1=T . As shown in

Fig. 11, the corresponding activation energy was deter-

mined from the slope of linear plot. The result obtained

is 13.2 kJ mol�1 for adsorption from its solution. As

known when the rate is controlled by intra-particle dif-

fusion mechanism, the activation energy is very low and

hence it can be concluded that process is controlled by

intra-particle diffusion which is a physical step in the

adsorption process (Sismanoglu and Pura, 2001).

Fig. 10. Intra-particle diffusion plots for adsorption of methyl violet at different pHs from its aqueous solutions with perlite.

Fig. 11. Arrhenius plots for adsorption of methyl violet.


Free energy (DG�), enthalpy (DH �) and entropy (DS�)

of activation can be calculated by Eyring equation

(Laidler and Meiser, 1999);

lnkT

� �¼ ln

kb

h

� �þ DS�

Rg

� DH �

RgTð10Þ

where kb and h are Boltzmann�s and Planck�s constants,

respectively. According to Eq. (10), a plot of lnðk=T Þversus 1=T should be a straight line with a slope DH �=Rg

and intercept ½lnðkb=hÞ þ DS�=Rg. DH � and DS� were

calculated from slope and intercept of line, respectively

(Fig. 12). Gibbs energy of activation may be written in

terms of entropy and enthalpy of activation:

DG� ¼ DH � � T DS� ð11Þ

DG� was calculated at 333 K from Eq. (11). It is found

that the values of the free energy (DG�), enthalpy (DH �)

and entropy (S�) of activation is 83.8 kJ mol�1, 15.8

kJ mol�1 and )204.2 J mol�1 K�1, respectively. The free

energy of activation, DG� have also been computed at

333 K. The fact that the activation entropy is negative

is a result of interactions between dye and perlite as ex-

pected.

4. Conclusions

The use of perlite for the adsorption of methyl violet

from aqueous solutions has been examined. The fol-

lowing results are obtained:

• Initially, the rate of adsorption of methyl violet

onto perlite was very high. This was then fol-

lowed by a slower rate, and gradually approached a

plateau.

• The kinetics of the adsorption of methyl violet by

perlite can be approximated to first-order kinetics.

• The effect of the initial concentration of the solute on

the rate of reaction was found to be of considerable

significant. The uptake rate of methyl violet increased

with the increasing initial concentration.

• The rate of adsorption of methyl violet increased

with increasing temperature and pH.

• Perlite is capable of removing methyl violet from an

aqueous solution.

• Because of crystalline defects rather than changes in

amorphous phase, the results of this research demon-

strate that an intra-particle diffusion mechanism

plays a significant role in the adsorption of methyl vio-

let by perlite, and it is apparent that the adsorption

rate is controlled by diffusion process.

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Adsorption kinetics of methyl violet onto perlite

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