Isotherm models for the adsorption of Crystal violet dye ... issue/Paper 37.pdf · Isotherm models...

375Int J Nano Corr Sci and Engg 2(5) (2015) 375-391International Conference on Chemical and Environmental Research (ICCER 2015), 17th December 2015,

PG and Research Department of Chemistry JAMAL MOHAMED COLLEGE (Autonomous), Tiruchirapalli,Tamilnadu, India

Available Online http://www.ijncse.comISSN Online: 2395-7018

2(5) (2015) 375-391

Isotherm models for the adsorption of Crystal violet dye onto

Zinc chloride activated carbonV. Nandhakumar[a] *, A.Rajathi [a], K. Ramesh[b] and A. Elavarasan[c]

[a] Department of Chemistry, A.V.V.M Sri Pushpam College, Poondi.

[b] Department of Chemistry, Arasu Engineering College, Kumbakonam.

[c] Department of Chemistry, Sengunthar College of Engineering College, Thiruchengodu.

Corresponding Author :E- mail id: [email protected]

ABSTRACT

An effective adsorbent was prepared from Terminalia catappa Linn fruit shell by Zinc

chloride activation method and its adsorption characteristics was studied for the removal of a

cationic Crystal violet (CV) dye from aqueous solution. The adsorbent was characterized for

its surface area and pHzpc. Batch mode adsorption experiments were adopted. Maximum dye

removal capacity was observed at a pH of 9. Equilibrium data were obtained at 303, 313, 323,

333 and 343K for the initial concentrations of 16, 18, 20, 22 and 24 mg/L. Adsorption

isotherm models such as Langmuir, Freundlich, Temkin and Dubinin – Radus-Kevich

isotherms were used to correlate the equilibrium data. Parameters obtained from the isotherm

models were discussed in detail.

Keywords: Crystal violet dye, adsorption, Terminalia catappa Linn fruit shell, activated

carbon pHzpc, Langmuir, Freundlich, Temkin, and Dubinin–Radus-Kevich isotherms.

INTRODUCTION

Various types of synthetic dyestuffs appear in the effluents of industries such as

textiles, printing, plastics, leather and food. The removal of synthetic dyes is of great concern

Nandhakumar et al.,



because most of them and their degradation products cause serious environmental problems

due to their high stability and complex aromatic structures . Crystal violet (CV) dye belongs

to the tri phenyl methane class and is used largely as histological stain in veterinary medicine,

as bacteriostatic agent and skin disinfectant in the medical community. CV is harmful and

can cause life-threatening injury to the conjunctiva, skin irritation and permanent blindness.

Hence it is necessary to remove the dye from effluent prior to discharge into water sources.

There are several methods used for the treatment of dye containing wastewater. Some of

them involve reverse osmosis, chemical oxidation, photo degradation and adsorption [1].

Among these methods, adsorption is proved to be superior to other techniques. Adsorption

using activated carbon gave fruitful results for the removal of dyes from wastewater.

Preparation of activated carbon from waste plant bio masses and evaluating its adsorbing

potential is the recent trend of research. Fruit shell of Terminalia catappa Linn is waste plant

bio mass which is chosen as precursor for the present investigation [2] and the Zinc chloride

activation method is adopted as it has the advantage of producing excellent activated carbons

as reported in earlier literatures [3].

MATERIALS AND METHODS

Adsorbate

Crystal violet dye (Molecular formula: C25H30N3Cl, M.W: 407.979, C.I.no. 42555,

CAS: 548-62-9, mp: 2050C) of Analar grade purchased from Merck company was used as

such without further purification. Stock solution of 1000 mg/L was prepared by dissolving

1gm of dye in 1000 mL. Required initial concentrations of the solution say 16,18,20,22 and

24 mg/L were prepared from the stock solution by proper dilution [4,5].

Fig. 1 Structure of Crystal violet dye

Maximum wavelength (λmax) of this dye is 590nm

Nandhakumar et al.,



Preparation of Adsorbents

The Terminalia catappa fruit shell were collected from A.V.V. Sri pushpam college

campus, Thanjavur Dt., washed with distilled water to remove the surface adhered particles,

dried in sun light for 8 hours, chopped into small pieces and powdered in a pulveriser. 50g of

the powder was mixed with 100 ml of 60% ZnCl2 solution. The slurry was kept at room

temperature for 24 hours, to ensure the complete access of the ZnCl2 to the Terminalia

catappa shell powder. The slurry was heated in muffle furnace at 4500C for 30 minutes.

Thus the carbonized samples were washed with 0.5 M HCl followed by distilled water until

the pH of the washings attain 7.0. Then it was dried in a hot air oven at 110 °C for 1 .The

dried material was ground and sieved to get particle size of 150 µm and stored in an air tight

container. It was designated as Terminalia catappa Zinc chloride Activated carbon (TCZAC)

[6,7].

Fig. 2 Terminalia catappa fruit shell

Batch equilibrium method

Experiments were carried out in various temperatures such as 303, 313,323,333 and

343K in an orbital shaker at a constant speed of 130 rpm using 250 mL conical flasks

containing 40 mg of TCZAC with 50mL of dye solution. Samples were agitated for pre-

determined time and the adsorbent was separated from the solution by centrifugation. The

absorbance of the centrifugate was estimated to determine the residual dye concentration. The

absorbance of the dye solution was measured at λ max = 590 nm using Systronics Double

Beam UV-visible Spectrophotometer: 2202 [8].

The percentage of removal dye was calculated using the following equation

(%) of removal = ((Ci-Ce)/Ci) X 100 (1)

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where, Ci = Initial dye concentration (mg/L)

Ce = Final dye concentration (mg/L)

The amount of adsorbate adsorbed at equilibrium condition, qe (mg/g) was calculated using

the following equation:

RESULTS AND DISCUSSION

Determination of Zero Point Charge

In solution, the presence of a net charge on a particle affects the distribution of ions

surrounding it, resulting in an increase in the concentration of counter ions. At pH zpc, the

total sum of positive charges and the negative charges on the adsorbent is zero that is the

adsorbent is in neutral charge. When the solution pH is below the pH zpc of the adsorbent,

surface of the adsorbent will possess positive charge. On the other hand the surface of the

adsorbent will possess negative charge when the solution pH is above the pH zpc of the

adsorbent.

The pH of the zero point charge (pH ZPC) was determined by the pH drift method [9],

by placing 0.2 g of adsorbent in glass stopper bottle containing 50 ml of 0.01M NaCl

solutions. The initial pH of these solutions was adjusted by either adding 0.1 M NaOH or 0.1

M HCl [10]. The bottles were placed in an incubator shaker at 298 K for 24 h, and the final

pH of supernatant has been measured. A graph was plotted between final pH and initial pH of

the solution. A straight line was drawn connecting the same pH values of horizontal axis and

vertical axis [11]. The point of intersection of the straight line and the graph was taken as the

pHzpc of the TCZAC which was found to be 7 as shown in the Fig. 3.

Nandhakumar et al.,



Fig. 3 pHzpc of the carbon

Effect of pH

pH is one of the most important factors controlling the adsorption of dye onto

adsorbent particles, which affects the surface charge of the adsorbents as well as speciation of

the solutes [12]. The hydrogen ion and hydroxyl ions are adsorbed quite strongly and

therefore the adsorption of other ions is affected by the pH of the solution. It is usually

expected that increase of cationic dye adsorption with the increase of pH due to the increase

of the negative surface charge on the adsorbents [13]. The effect of solution pH was studied

between initial pH range of 2 to 10, initial pH of the solution was maintained by the addition

of 0.1M HCl,0.01M HCl , 0.1M NaOH and 0.01M NaOH solutions and agitated with 50 mg

of adsorbent for 1 hour at 34°C. The results of effect of initial pH of dye solution on the

adsorption of CV for initial dye concentration of 16 mg/L was presented in Fig. 4

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Fig. 4 Effect of pH

The lower adsorption at acidic pH was probably due to the presence of excess H+ ions

in the solution which compete with the cationic dye for adsorption sites. As surface positive

charge density decreases with an increase in the solution pH, the electrostatic repulsion

between the positively charged dye and the surface of the adsorbent is lowered, which results

in an increase in the extent of dye adsorption. Higher percentage removal was occurred at pH

9.0 .But still at an alkaline medium; percentage of removal was not good. This might be due

to the interionic attraction between the OH‒ ions which present in the solution in excess and

dye cations. Hence the remaining experiments were conducted at pH 9 ± 0.5.

Equilibrium studies

Adsorption of dye is considered to be a fast physical/chemical process; it is a

collective term for a number of passive accumulation processes which include ion exchange,

co-ordination, complexation, chelation, Vander Waal’s attraction and micro precipitation.

Proper analysis and design of adsorption separation processes require relevant adsorption

equilibria as one of the vital information.

In equilibrium, certain relationship prevails between solute concentration in solution

and in adsorbed state .Equilibrium concentrations are the function of temperature. Therefore,

the adsorption equilibrium relationship at a given temperature is referred to as adsorption

isotherm. The concentration of dye solution at equilibrium (Ce) and the quantity adsorbed at

equilibrium (qe ) at different temperatures are collected in Table 1.

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Table 1 Equilibrium parameters for adsorption of dye onto activated carbon

[pH :9, [Ci ] in mg/L:16,18,20,22,24, Dose = 40 mg/ 50 mL]

[CV]

(mg/L)

Ce (mg/L) qe (mg/g)

Temperatures Temperatures

303 313 323 333 343 303 313 323 333 34316 1.176 0.980 0.784 0.588 0.392 18.52 18.77 19.01 19.26 19.50

18 2.156 1.764 1.568 1.176 0.784 19.80 20.29 20.53 21.02 21.51

20 2.941 2.549 2.156 1.764 1.176 21.32 21.81 22.30 22.79 23.52

22 3.921 3.529 2.941 2.352 1.764 22.59 23.08 23.82 24.55 25.29

24 4.901 4.313 3.529 3.137 2.549 23.87 24.60 25.58 26.07 26.81

Isotherm studies

The presence of equilibrium between two phases (liquid and solid phase) is

rationalized by adsorption isotherm. The equilibrium data obtained from the experiments

were processed with the following isotherm equations such as Langmuir, Freundlich,

Temkin, and Dubinin-Raduskevich adsorption isotherm models. Inference obtained from

each isotherm was discussed in detail one by one

Langmuir isotherm

It is a widespread-used model for describing dye sorption onto adsorbent. Langmuir

equation relates to the coverage of molecules on a solid surface and the concentration of

contacting solution at a fixed temperature.

This isotherm is based on the following assumptions such as adsorption limited to

monolayer coverage, all surface sites being alike, one site accommodates one species of the

adsorbates and the ability of a molecule to be adsorbed on a given site independent of its

neighboring sites occupancy. Linear form of Langmuir equation is written in the following

form [14]

C e/q e = 1/qmb + Ce /qm (2)

where qe is the amount of solute adsorbed per unit weight of adsorbent (mg/g), Ce the

equilibrium concentration of solute in the bulk solution (mg/L), qm is the maximum

monolayer adsorption capacity or saturation capacity (mg/g) and b is the adsorption energy, b

is the reciprocal of the concentration at which half saturation of the adsorbent is reached. The

essential characteristics of Langmuir isotherm can be described by a separation factor, RL,

which is defined by the following equation

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RL = 1 / (1+ bC0) (3)

Where C0 is the initial concentration of the adsorbate solution. The separation factor

RL indicates the nature of the adsorption process as given below:

RL value Nature of the process

RL> 1 Unfavourable

RL = 1 Linear

0 < RL< 1 Favourable

RL = 0 Irreversible

The results obtained from Langmuir isotherm model for the adsorption of dye

presented in Table 2. Concerned isotherm plots are shown in Fig. 5

Table 2 Langmuir isotherm constants for the adsorption of dye

[pH for CV :9, Ci for CV (mg/L):16,18,20,22,and 24, Dose = 40 mg/ 50 mL]

Temperature

(K)

qm

(mg/g)

b

(L/mg)R2

303 27.0 1.608696 0.995

313 27.0 1.947368 0.995

323 28.5 2.058824 0.990

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333 29.4 2.615385 0.994

343 29.4 4.250000 0.997

The regression coefficient (R2) values are ranged from 0.990 to 0.997 for the five

studied temperatures viz. 303, 313,323,333 and 343 K. These results show the best fitting of

the equilibrium data in the Langmuir isotherms.

The adsorption capacity is the most important characteristic of an adsorbent. It is

defined as the amount of adsorbate taken up by adsorbent per unit mass of adsorbent. This

variable is governed by a series of properties such as pore size and its size distribution,

specific surface area, cation exchange capacity, pH, surface functional groups and also

temperature.

The mono layer adsorption capacity qm values (mg/g) for adsorption of CV dye

ranged from 27.0270 to 29.4117.

Fig. 5 Langmuir isotherm constants for the adsorption of dye

Further it is noticed that adsorption capacities were slightly increased with an increase

of temperature.

The dimensionless separation factor RL values calculated for various initial

concentrations at different temperatures are given in Table 3 for the adsorption of dye. These

values were lie between 0 and 1 which indicate the favourable adsorption of dyes onto

TCZAC.

Table 3 RL values

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[CV]mg/L

Temperature 0K

303 313 323 333 343

16 0.037 0.031 0.029 0.023 0.014

18 0.033 0.027 0.026 0.020 0.012

20 0.031 0.025 0.023 0.018 0.011

22 0.027 0.022 0.021 0.017 0.011

24 0.025 0.020 0.019 0.015 0.009

Freundlich Isotherm

Freundlich isotherm is an empirical equation. It is the most popular model for a single

solute system based on the distribution of solute between the solid phase and aqueous phase

at equilibrium. It suggests that sorption energy exponentially decreases on completion of the

sorptional centres of an adsorbent. The Freundlich model describes the adsorption with in a

restricted range only. It is capable of describing the adsorption of organic and inorganic

compounds on a wide variety of adsorbents [15].

The linear form of the equation has the following form:

ln qe = ln Kf + 1/n lnCe (4)

where qe is the amount of adsorbate adsorbed (mg/g) at equilibrium, Ce is the

equilibrium concentration of adsorbate in solution (mg/L) and Kf and n are the constants

incorporating all factors affecting the adsorption capacity and intensity of adsorption

respectively

As a robust equation, Freundlich isotherm has the ability to fit into nearly all

experimental adsorption–desorption data and is especially excellent for fitting data from

highly heterogeneous sorbent systems. 1/n is the heterogeneity factor and it is a measure of

deviation from linearity of adsorption. A favourable adsorption tends to have n value between

1 and 10. The larger value implies a stronger interaction between the adsorbent and

adsorbate while 1/n equal to 1 indicates linear adsorption leading to identical adsorption

energies for all sites.

Sorption of solute on any sorbent can occur either by physical bonding, ion exchange,

complexation, chelation or through a combination of these interactions. In the first case of

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physical bonding, as the solute is loosely bound, it can easily be desorbed using distilled

water. Different mechanisms as mentioned can be involved as the interaction between sorbent

and solute molecules depending upon the functional groups such as hydroxyl, carbonyl and

carboxyl can present within the structure of adsorbent. The parameter ‘n’ value of Freundlich

equation expresses these phenomena.

The results obtained from Freundlich isotherm model are given in Table 4. The

concerned isotherm plots are shown in Fig. 6.

Table 4 Freundlich isotherm results


Temperature

(K)n

kf

(mg/g)R2

303 5.6 18.6 0.980

313 5.6 17.7 0.971

323 5.2 19.5 0.950

333 5.5 20.9 0.978

343 5.7 22.8 0.994

The regression coefficient (R2) for Freundlich isotherms are ranged from 0.950 to 994

for all the studied temperatures viz. 303, 313, 323,333 and 343 K. It indicates that the

experimental data fit well into Freundlich model.

Freundlich constant asdsorption capacity Kf (mg/g) values for adsorption of CV dye

ranged from 17.68 to 22.76 respectively. Further it is noticed that the adsorption capacity

increased with

the increase of

temperature .

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Fig. 6 Freundlich isotherm results

The adsorption intensity constant ‘n’ values are ranged from 5.2 to 5.7 for all the

studied temperatures, i.e., between 1 and 10, which indicate the favourable physical

adsorption. In general Freundlich constant values infer a better performance of TCZAC.

Temkin isotherm

The Temkin isotherm assumes that the heat of sorption in the layer would decrease

linearly with coverage due to sorbate - sorbent interactions. Further the fall in the heat of

adsorption is not logarithmic as stated in Freundlich expression [16].

The linear form of Temkin equation is.

qe = RT/bT ln aT + RT/bT ln Ce (5)

Where, bT is the Temkin constant related to the heat of sorption (J/mg) and aT the

equilibrium binding constant corresponding to the maximum binding energy (L/g). The

Temkin constants aT and bT were calculated from the slopes and intercepts of qe versus ln Ce.

The results obtained from Temkin model for the removal of CV dye were represented in

Table 5. Concerned isotherm plots were shown in Fig. 7. The regression coefficient (R2)

values ranged from 0.931 to 0.990 for the five studied temperatures viz. 303,313, 323, 333

and 343 K. These results show the best fitting of the equilibrium data with Temkin isotherm.

Table 5 Temkin isotherm results

[pH for CV :9, Ci (mg/L):16,18,20,22,and 24, Dose = 40 mg/ 50 mL]

Temperature

(K)

bT

(kJ/mg)

aT

(L/g)R2

303 6.721 1.238 0.959

313 6.807 1.229 0.968

323 6.357 1.242 0.931

333 6.776 1.215 0.965

343 7.154 1.189 0.990

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Equilibrium binding constant ‘aT’ values (L/g) for adsorption of CV dye are ranged

from 1.1897 to 1.2425.The Temkin constant related to heat of sorption, bT values (kJ/mg)for

adsorption of CV dye are ranged from 6.3579 to 7.1547. Low values of heat of adsorption,

supports he physisorption mechanism. Both the binding constant ‘aT’ values and heat of

sorption, bT values found to increase with the increase of temperatures.

Fig. 7 Temkin isotherm for CV dye

Dubinin – Radus-Kevich isotherm

The Linear form of Dubinin-Radushkevich isotherm is.

ln qe = ln qD - Bε2 (6)

where, qD is the theoretical saturation capacity (mg/g) B is a constant related to the

mean free energy of adsorption per mole of the adsorbate (mol2/J2) and ε is Polanyi potential

which is related to the equilibrium as given below [17]

ε = RT ln (1+1/Ce) (7)

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The constants qD and B were calculated from the slope and intercept of straight line

obtained from the plot of ln qe versus ε2. The mean free energy of adsorption E calculated

from B using the following equation.

E = 1/ (2B)1/2 (8)

E is a parameter used in predicting the type of adsorption. An E value less than ‘8’

kJ/mol is an indication of physisorption. Concerned isotherm plots were shown in Fig. 8

Table 6 Dubinin – Radus-Kevich isotherm results


Temperature

(K)

qD

(mg/g)

E

(kJ/mol)R2

303 23.6 0.2357 0.819

313 23.6 0.2673 0.835

323 24.3 0.2887 0.774

333 25.0 0.3536 0.823

343 26.0 0.4082 0.883

The regression coefficient (R2) values are ranged from 0.774 to 0.883 for the five studied

temperatures viz. 303, 313,323,333 and 343 K. These values reveal that fitting of equilibrium

data with D-R isotherm are not as good as other isotherms studied earlier.

The mono layer adsorption capacity qD values (mg/g) for adsorption of CV dye are

ranged from 25.0136 and 26.0342 respectively. Further it is noticed that adsorption capacity

increased with an increase in temperature. Values of the mean free energy E(kJ/mol) for the

adsorption of CV dye are ranged from 0.2357 and 0.4082. The very low values of E infer the

physisorption interaction.

Nandhakumar et al.,



Fig. 8 D-R isotherm for CV dye

CONCLUSION

Activated carbon prepared from Terminalia catappa Linn fruit shell by Zinc chloride

activation method (TCZAC) found to have pH zpc 7.But the maximum adsorption of Crystal

Violet dye was observed at the initial solution pH 9.Equlibrium data were well fitted into

Langmuir, Freundlich, Temkin isotherms having regression coefficient values (R2) around

0.9.The regression coefficient values (R2) for Dubinin – Radus-Kevich isotherm ranged from

0.774 to 0.883 only. ‘RL’ values obtained from Langmuir isotherm and ‘n’ values obtained

from Freundlich isotherm reveals the favourability adsorption of Crystal Violet dye onto

TCZAC. Equilibrium binding constant ‘aT’ values and the heat of sorption, bT values obtained

from the Temkin isotherm supports the physisorption mechanism and endothermic nature of

adsorption. The very low mean free energy values ‘E’ obtained from the Dubinin – Radus-

Kevich isotherm infer the physisorption interaction. The adsorption capacities obtained from

the isotherms show the feasibility of TCZAC as an effective adsorbent for the removal of

Crystal violet dyes from aqueous solution.

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Received: 1-12-2015Accepted: 7-12-2015

Isotherm models for the adsorption of Crystal violet dye ... issue/Paper 37.pdf · Isotherm models...

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