a comparative study of 8-hydroxyquinoline-5-sulphonic … loaded on f-400 granular activated carbon...

INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 3, No 6, 2013

© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0

Research article ISSN 0976 – 4402

Received on May 2013 Published on July 2013 2048

A comparative study of 8-hydroxyquinoline-5-sulphonic acid and its 7-nitro

derivative loaded on F-400 granular activated carbon for removal of

copper ions from aqueous solutions Doss V.R, Kodolikar S.P

Department of Chemical Engineering, Sinhgad College of Engineering, Vadgaon (Bk),

Pune -411041, Maharashtra, India

[email protected]

doi: 10.6088/ijes.2013030600024

ABSTRACT

The purpose of this study was to compare 8-hydroxyquinoline -5-sulphonic acid (QSA) and

its 7- nitro derivative (NQSA) loaded on F-400 GAC as a suitable adsorbent material for

adsorption of Cu ions in aqueous solutions. Batch experiments were performed to evaluate

effect of various parameters like pH, contact time, temperature, agitation speed, etc. Kinetic

studies were done which showed 90.14% removal of copper for QSA and 94.19% removal

for NQSA. The adsorption isotherm data were validated to Langmuir, Freundlich and DR

isotherms. Kinetic data showed that it follows pseudo second order kinetics for both QSA and

NQSA. The thermodynamic parameters of the adsorption process like Gibb’s free energy,

entropy and enthalpy were also determined for both the ligands. The Langmuir constants

have been calculated at different temperatures and the adsorption is found to be exothermic

(∆ H = -36.08 kJ/mole for QSA and ∆ H = -48.11 kJ/mole for NQSA). Maximum removal is

observed at pH 5.5 for both the ligands. It has been observed in the qe values that even though

NQSA is adsorbed to a marginally lower extent than QSA, it is able to remove more copper

than QSA because of proper orientation to form a 1:2 complex with copper ions on the

surface of ligand loaded GAC. The results demonstrates that NQSA loaded GAC has an edge

over QSA as an effective adsorbent for removal of copper ions. The 7-nitro derivative owing

to its bulky nature shows better adsorption capacity.

Keywords: Adsorption, Isotherms, Kinetics, Thermodynamics, Heavy metals, Ligands, 8-

hydroxyquinoline -5-sulphonic acid and Granular Activated Carbon.

1. Introduction

Many industrial processes involve the use of materials that are potentially toxic if released

into the environment. The industrial effluents containing heavy metals pose severe threat to

ecology when allowed to mix with surface waters by altering the chemistry of the receiving

water. The treatment and disposal of heavy metal bearing waste presents a unique challenge.

Many physico-chemical processes are employed for the removal of heavy metals but the

conventional methods like precipitation, ion-exchange, electrolysis, are becoming non-

effective and economically non-feasible to achieve the required stringent standards1.

(Kurniawan, 2006)

Activated carbon adsorption appears to be a particularly competitive and effective process for

removal of heavy metals at trace levels. Some researchers have emphasized on the use of

natural adsorbents for metal removal like peat, wood, peanut shell, saw dust, rice husk, etc 2

(P.SenthilKumar, 2011) and also on modified surface of the adsorbent. Huang using 8-

A comparative study of 8-hydroxyquinoline-5-sulphonic acid and its 7-nitro derivative loaded on F-400

granular activated carbon for removal of copper ions from aqueous solutions

Doss V.R, Kodolikar S.P

International Journal of Environmental Sciences Volume 3 No.2, 2013 2049

hyrdroxyquinoline as an effective chelating agent improved Cu ion adsorption3. (Huang,

1978). Natarajan et al. separated Cu ions from aqueous medium either alone or in admixtures

with other divalent ions using GAC containing adsorbed 8-hydroxyquinoline-5-sulphonic

acid 4(Deshmukh, 1989). Similarly, Ruey-shin juang et al studied the adsorption ability of

Cu(II) using chitosan from simulated rinse solutions containing chelating agents including

ethylenediaminetetraacetic acid (EDTA), citric acid, tartaric acid and sodium gluconate 5.

(Ruey-shin juang, 1999) Activated carbon can, therefore be used to enhance the adsorption

capacity for metal ions in aqueous solutions by ligands adsorbed on the surface.

In view of the high toxicity, persistent and increasing use of chemicals, the most important

non-ferrous metal copper having varied uses in industries like metal cleaning and plating

baths, paper and pulp and fertilizer industry6 (Shell, 1981) and also household appliances was

chosen for the present work. Copper is highly toxic to humans and animals in high dosages.

Prolonged oral administration of copper in excess may cause damage to vital organs.

Consequently its removal from wastewater assumes importance. Aquatic animals also get

influenced by its toxicity depending on alkalinity, pH, hardness and organic compounds

present in water7.(Hodgen, 1997).The tolerance limit for inland surface water discharge is 3

ppm for copper while drinking water tolerance limit is 0.05 ppm.8 (ISI, Drinking water

specification, 1991).

In the present work the sorption of Cu ions from aqueous solutions by using ligand QSA and

NQSA loaded granular activated carbon was investigated. A comparative adsorption capacity

of adsorbent QSA and its derivative NQSA was conducted in batch experiments.

2. Material and methods

2.1 Adsorbent F-400 GAC: Preparation and its Characterization

F-400 GAC is a well documented adsorbent in literature9 (Tiwari 1989). The samples were

gifted by m/s Calgon Corporation Inc, Pittsburg, USA. The sample of GAC was first sieved

as per the ASTM method DDY74910

(American Society for Testing and Materials, 1994),

particles having mesh size 12 x18 was chosen. The samples were washed thoroughly and

dried in a dessicator. In a period of 10 days time the GAC is fairly of constant composition in

regard to residual moisture content.

The sample was characterized by SEM and FTIR in order to understand the basic

morphology and presence of surface active groups responsible for the process of adsorption.

The SEM was done on a Cambridge stereoscan S 250 MKIII model instrument and FTIR

recorded on a Nicolet Magna IR 550 Spectrometer. The SEM photograph and FTIR are

shown in figures (1) and (2).





Figure 1: SEM photograph of F-400 GAC

Figure 2: FTIR spectra of F-400 GAC

Table 1 enlists the properties of F-400 GAC used. For modification of its surface it was

loaded with ligands QSA and NQSA.

Table 1: Characteristic properties of GAC

Carbon

Type*

Surface Area[N-

BET]

Particle

Density

Apparent

Density True Density

Pore

Volume Porosity

sq.m./g g/cc g/cc g/cc cc/g

F-100 841 --- --- 2.679 0.549 0.26

F-200 825 0.858 0.53 2.267 0.724 0.53

F-300 970 0.7303 0.48 2.1 0.85 0.64

F-400 998 0.795 0.48 2.308 0.825 0.65

*Origin for all carbon types is Bituminous Coal

2.2 Copper solution





All chemicals used were of analytical reagent grade. A stock solution of 1000 ppm Cu (II)

was prepared and used for all adsorption and kinetic studies. Experimental solutions of the

desired copper concentration were obtained by successive dilutions. Using Na-DDC method

the concentration of copper was determined spectrophotometrically at 520 ηm11

( Vogel,

1979)

2.3 Ligand 8-hydroxyquinoline sulphonic acid and its 7-nitro derivative.

The properties of 8-Hydroxyquinoline and its derivatives as chelating agents are well

documented in literature12

(Moeller, 1954). These ligands behave as a bidentate (N, O)

univalent ligand and thus could chelate effectively with metal ions like copper. Variation in

the structure of ligand could help in understanding the role of substituent groups in the

adsorption process on the GAC. The sulphonic acid derivatives have a hydrophilic character

due to which the metal complexes have an unusually high water solubility13

(Bailer,

1956).Further the ligand has a crystalline nature and is easily soluble in hot water thus

suitable for chemical analysis.

QSA was of Riedel de Haen, West Germany make. Estimation was done by pH titration

using standard alkali solution. The sulphonic acid group undergoes neutralization at pH 4-5.

The molecular weight determination experimentally was found to be 243.8 while reported

literature value is 243.2. 7-nitro QSA was prepared in the laboratory using the method of

Fresco and Freiser14

(Fresco, 1963). The molecular weight determination was done by

sulphonic acid group pH titration which was found to be 270.82 while reported literature

value is 271.2.

2.4 Sorption experiments

Batch experiments were carried out in order to determine the adsorption isotherm of a

particular ligand on F-400 GAC. 0.1g-1.5 g of GAC was transferred into 6 clean dry shaking

bottles. 200 ml of 0.002M ligand solution, QSA added to each of the bottles. The bottles were

fixed on to a mechanical shaker and subjected to shaking for 8 hrs. There was practically no

visible attrition of the GAC sample. The contents were filtered and dried in oven at 1100C for

an hour and then placed in a dessicator for further use. Same procedure was followed for

ligand NQSA.

The value of Ce, the equilibrium concentration was obtained by determining the residual

ligand concentration by UV-visible spectrophotometer of make systronics digital

spectrophotometer type 166 at 350 nm for both the ligands. Using this value of Ce, the

equilibrium ligand concentration on GAC in moles/g was calculated using the expression15

(Eloussaief, 2010)

dVCCq ee /)( 0

where qe = equilibrium ligand concentration on GAC in moles/g

C0 and Ce = concentration of ligand in solution at initial and equilibrium conditions in moles/l,

V= volume of solution in liters, d= weight of GAC in g

Due to formation of a monolayer of adsorbate on the surface of the adsorbent the value of qe

reaches a saturation value which represents the maximum amount of ligand that can be





adsorbed for the given amount of adsorbent under given set of experimental conditions. To

verify adherence to various adsorption models like Dubinin-Radushkevich, Langmuir and

Freundlich isotherms, some representative plots of ln qe vs e2, 1/qe vs 1/ Ce and ln qe vs ln Ce

are shown in figures 3,4 , and 5 for QSA and figures 6, 7 and 8 for NQSA respectively. These

figures clearly indicate that these isotherms are all validated as referred to in literature16

(Chereminisoff, 1978)

Adsorption isotherm studies were carried out with different initial concentrations of Cu (II)

and a fixed concentration of carbon. In order to study adsorption of Cu ions on these ligand

loaded GAC, the GAC was first saturated with the ligand to hinder direct adsorption of the

copper by the GAC. 0.5 g of GAC was shaken with 100 ml of 0.002M sulphonated oxine for

8 hrs. The solution was filtered and washed thoroughly with distilled water to remove all

adhering ligand solution. The residual ligand solution after filtering was checked for its

absorbance and only those GAC systems were used for later work which fixed the same

amount of ligand on them. To this ligand loaded GAC was then added 100 ml of copper

solution 25-120 mg/l and contents of the flask were shaken for 8 hrs in a mechanical shaker.

The initial concentration of copper i.e. C0 was determined spectrophotometrically at 520 nm

using Na-DDC method. Similar procedure was adopted for NQSA. Once equilibrium is

reached then Ce, the equilibrium metal ion concentration in mg/l was determined. Using both

these quantities the value of qe, the amount of copper on the ligand loaded GAC was

determined using a similar relation

dVCCq ee /)( 0

where qe = concentration of copper ion on ligand loaded GAC in mg/mmole of ligand C0

and Ce = initial and equilibrium concentration of copper ion in solution in mg/l, V= volume

of solution in litres, d= millimoles of ligand on GAC

Plot of qe vs Ce representing adsorption isotherm of copper on the ligand loaded GAC is

shown in figure 3 and 7 for both QSA and NQSA.

To check the adherence of the adsorption process to Freundlich, Langmuir and DR adsorption

isotherm plot of log qe vs log Ce, Ce/qe vs Ce and ln qe vs e2 are given in figures 4, 5 and 6 for

the ligand QSA and figures 8, 9 and 10 for ligand NQSA.

The % removal of copper is calculated by the following equation

100/)(Re% 00 xCCCmoval e

It was found to be 90.14 % using QSA and 94.19% using NQSA.





Figure 3: Adsorption data [F-400: QSA] Figure 4: Freundlich Isotherm [F-400:QSA]

Figure 5: Langmuir Isotherm [F-400:QSA] Figure 6: D R Isotherm [F-400: QSA]

Figure 7: Adsorption data [F-400:NQSA] Figure 8: Freundlich Isotherm [F-400:NQSA]





Figure 9: Langmuir Isotherm [F-400:NQSA] Figure 10: D R Isotherm [F-400: NQSA]

2.5 Kinetic studies

Experiments were carried out to study the rate of adsorption. A study of the rates of diffusion

could verify on the type of diffusion operative in the adsorption process of ligands used in

this work by the GAC. 500 ml of ligand solution was taken in a flask placed in a thermostat at

25±1°C. The concentration of the ligand approximated to a value of Ce in the adsorption

isotherm. This was necessary to allow rapid rise of qe during the adsorption process. 0.5 g of

GAC sample was added and 5 ml sample of ligand was withdrawn after every 15 minutes in

the first hour and then after every half an hour for the remaining 3 hrs by which time the fall

in concentration of ligand solution almost stabilized. The agitation speed was maintained by a

remi stirrer (model RQ 122) at 800 RPM. This was necessary in order to eliminate film

diffusion from operating in the process. The absorbance values were measured at 350 ηm

using digital spectrophotometer model 166. A plot of Ct vs time gives an idea of fall in ligand

concentration with time17

(Chatwal, 2007). At definite time intervals the value of qt the

amount of ligand absorbed at these time intervals are calculated as

txVCCq et /)( 0

qt and Ct represent the values at the intervals of time and not equilibrium values18

(Yenkie,

1985). However it was possible to find out the equilibrium concentration of ligand on GAC

using this value of Ct, and in conjunction with the adsorption isotherm curve, value of qe was

read at the value of Ce=Ct. this represented the value of q*. A plot of q*and q vs time then

indicated the process approach to equilibrium. The difference between values of q* and q at

any time is the driving force operative in the process leading to adsorption on GAC. At

definite time intervals values of dq/dt and (q*2-q

2)/2q were also computed from q* and q

values. These represented the QDF plots as shown in figures 11[a] and 11[b].19

(Vermeulen,

1953). The expression proposed for QDF plot is

q

qqx

a

D

dt

dq

2

*. 22

2

2

The QDF equation was used to obtain the values of D; the diffusion coefficient for the system.





Figure 11[a]: QDF plot [F-400: QSA] Figure 11[b]: QDF plot [F-400: NQSA]

To study the kinetics of copper adsorption on ligands loaded GAC, 0.5 g of GAC F-400 was

shaken for 4 hrs with 0.002M 100 ml of sulphonated oxine solution. The solution was filtered

and filtrate discarded. To the ligand loaded GAC 500 ml of copper solution at a concentration

obtained from the adsorption isotherm was added. 2 ml samples were withdrawn

intermittently over a period of 4 hours. The value of q, the concentration of copper ions on

GAC containing adsorbed ligand was estimated using the relation

dxVCCq t /)( 0

Where q is concentration of copper on ligand loaded GAC in mg/mmole of ligand, C0 and Ct

are concentration of copper in mg/l at t=0 and t=t, V is volume of copper solution in liters, d

is mmoles of ligand on 0.5g of GAC. The results are depicted in figures 12 and 13 for QSA

and NQSA ligands respectively.

Kinetic Data [F-400:QSA-Cu]

0

1

2

3

4

5

6

0 1 2 3 4 5

time hrs

Ct

mg

/l

Kinetic Data [F-400:NQSA-Cu]

0

1

2

3

4

5

6

0 1 2 3 4 5

time hrs

Ct

mg

/l

Figure 12: Kinetic Data [F-400: QSA] Figure 13: Kinetic Data [F-400: NQSA]

3. Result and conclusion

3.1 Characterization of F-400 GAC

The SEM image at 1000x magnification reveals the nature of the surface of GAC in figure 1.

It shows a layered loosely packed porous irregular structure with cavities, cracks and pores

thus makes possible the adsorption of copper ions on different parts of the adsorbent.The

FTIR shown in figure 2 shows presence of surface active functional groups. At 3500cm -1

the





peak observed is due to presence of free OH stretch vibration of phenolic OH group and a

peak around 1760-1690 cm-1

was due to asymmetric O=C=O stretch vibration of the free

COOH group. These surface active groups contribute significantly to adsorption of ligands on

GAC and also on adsorption of copper on ligand loaded GAC. Presence of free COOH group

favors adsorption of ligand on the GAC20

. (Mattson, 1969)

3.2 Adsorption isotherm studies

The sorption data for copper ions on ligand loaded GAC were tested against the standard

isotherm models, the Langmuir, Freundlich and D-R isotherms. The representation plots

justify the adherence to these models as shown in figures 14, 15, 16 and 17 for QSA and 18,

19, 20 and 21 for NQSA.

Figure 14: Adsorption data[F-400:QSA:Cu] Figure 15: Langmuir Isotherm[F 400:QSA:Cu]

Figure 16: Freundlich Isotherm[F-400:QSA:Cu] Figure 17: D R Isotherm [F-400:QSA:Cu]





Figure18: Adsorption data Figure 19: Langmuir Isotherm

[F-400: NQSA: Cu] [F-400: NQSA: Cu]

Figure 20: Freundlich Isotherm Figure 21: D R Isotherm

[F-400: NQSA: Cu] [F-400: NQSA: Cu]

3.3 Effect of Parameters

3.3.1 Effect of pH

The pH of solution is an important parameter which relates to the surface chemistry and

binding sites of the adsorbent. On the basis of the electrostatic interaction model21

(Bhattacharya, 1984) the influence of pH on Cu (II) removal on ligand loaded GAC can be

explained. The pH was maintained at 5.5 for all the experiments. At lower pH the surface of

the carbon exhibits an increasing positive charge due to increasing H3 O+ since the species to

be adsorbed i.e., Cu (II) is also positive it experiences a repulsive force and thus adsorption is

not favored and to add to this the H3 O+

would also compete with Cu (II) for the adsorption

sites available thus reducing the uptake of Cu (II) by the ligand loaded GAC. On the contrary,

as pH increases till pH 5.5, the adsorbent surface becomes negatively charged and favors the

adsorption of positively charged Cu (II) 22

(Periasamy, 1996). However, on further

increasing the pH it resulted in precipitation of copper as its hydroxide derivative.23

(Reddy,

1997).Irrespective of the ligand used the maximum removal of Cu ions was observed at pH

5.5.





3.3.2 Effect of adsorbent dosage

The dependence of adsorption of copper ions on the dosage of carbon is shown in figure

22.The adsorption experiments were carried out in the copper concentration range of 25-

120mg/lit for an adsorbent dose of 0.5g of GAC loaded with 100ml 0.002M for both QSA

and NQSA at pH 5.5. However, the amount of Cu adsorbed in mg/g of GAC was found to

decrease with increasing adsorbent dosage due to high number of unsaturated adsorption

sites24

. (Shukla, 2002)The higher concentrations of heavy metal resulted in depletion of

surface active sites. As the surface coverage of active sites increase, adsorption to these sites

becomes limited thereby leading to a decrease in the adsorption intensity. The effect of

weight of adsorbent for QSA and NQSA showed a similar trend.

Figure 22: Effect of mass of adsorbent [F-400: QSA, NQSA:Cu]

3.3.3 Effect of contact time

Figure 23 shows the effect of contact time on removal of Cu (II) by GAC. The adsorption of

Cu (II) was found to increase with increase in contact time for a fixed adsorbate dosage and

fixed concentration of copper ions and the removal in mg of Cu/g of carbon increases with

time and attains equilibrium in 4 hrs for GAC for an initial Cu (II) concentration of 5.68 mg/l.

Initially adsorption is rapid due to external surface adsorption of copper which occurs

instantaneously and later on becomes slower once equilibrium uptake was achieved. A

comparative account does not show any significant difference in the effect of contact time.

Figure 23: Effect of Contact Time [F-400: QSA, NQSA:Cu]

3.3.4 Effect of shaking speed





For the experimental runs an optimum speed of 800 rpm was maintained for both the ligands.

At higher speed of 1200 rpm, the diffusion coefficient of metal ions was significantly

affected as the energy due to high speed probably assisted in breaking bonds formed between

metal ion and their adsorbent. At lower speed of 500 rpm the degree of mixing reduces. The

maximum removal efficiency was 90.14% for QSA and 94.19% for NQSA loaded GAC-Cu

system at 800 rpm as shown in figure 24.

Figure 24: Effect of shaking speed.

3.4 Adsorption Isotherm Models

In the present study the distribution of metal ions between liquid and solid phases has been

described by using various adsorption isotherm models. The Langmuir isotherm model 25

(Langmuir, 1916) is valid for monolayer adsorption onto surface containing finite number

of identical sorption sites The Langmuir equation, was applied for adsorption equilibrium for

both QSA and NQSA

00 //1/ QCbQqC eee

Where Ce is the equilibrium copper concentration (mg /l), qe the amount of copper adsorbed

at equilibrium (mg/g), Q0 and b are Langmuir constants related to maximum adsorption

capacity (mg/g) and energy of adsorption (L /mg) respectively.

The linear plots of Ce/qe vs qe show that the adsorption obeys Langmuir isotherm model for

both QSA and NQSA. (figures 15 and 19).The correlation coefficients are found to be 0.999

and 0.996 respectively and the values of Q0 and b were determined from the slopes and

intercepts of the Langmuir plot and are found to be 27.03 mg of Cu/g for ligand and 0.027

lit/mg of Cu for QSA and 32.258 mg of Cu/g for ligand and 0.5484 lit/mg of Cu for NQSA.

The maximum adsorption capacity of copper by ligand loaded GAC was 27.03 mg/ g for

QSA and 32.258 mg/g for NQSA (Table 2).On comparing the value of maximum adsorption

capacity obtained from this study with values from other reported adsorbents, ligand loaded

GAC is suggested as a potential scavenger for removal of copper in wastewater. It was

observed that the adsorption capacity for copper using ligand loaded GAC is comparable with

other reported adsorbents as shown in Table 2. 26

(Ahmaruzzaman, 2011)

Table 2: Langmuir, Freundlich and Dubinin–Radushkevich isotherm constants and

correlation coefficients





Isother

m

Model

Langmuir Freundlich

Dubinin–Radushkevich

Type of

ligand

qmax

(mg/

g)

b

(L/mg

)

R2 KF

(mg/

g)

RL KF

(mg/g)

n R2 qm

(mg/

g)

E

(kJ/m

ol)

R2

QSA 27.03 0.027 0.99

9

0.46

1

4.22 5.55 0.93 23.01 50 0.792

NQSA 32.25

8

0.548

4

0.99

6

0.03

2

3.804 7.63 0.97 28.73 9.129 0.661

The Langmuir model can be expressed in terms of RL, a dimensionless constant separation

factor given by the following equation to predict the affinity between the sorbate and

sorbent27

(Hall, 1966):

01

1

bCRL

where b is the Langmuir constant (l/ mg) and C0 is the initial copper concentration (mg /l). It

has been established that for favorable adsorption, 0 < RL < 1. All the values of RL was found

to be between 0.1 and 0.5 for the initial copper concentration range from 25 to 120 mg/l

indicating favorable adsorption of copper onto ligand loaded GAC .

The Freundlich isotherm model is derived to model the multilayer adsorption and applicable

to highly heterogeneous surface, and is given as:

eFe CnKq log/1loglog

where KF is maximum adsorption capacity (mg/ g) and n is related to adsorption intensity.

If the value of n is greater than 1, it indicates favorable adsorption of metal ions on the

surface of adsorbent28

(Mohanty, 2006). The value of n determined from Freundlich

isotherm was 5.55 for QSA and 7.63 for NQSA as shown in Table 2, indicating that Cu (II)

ions are favorably adsorbed by ligand loaded GAC.

The Dubinin-Radushkevich (D-R) equation can be expressed as:

2lnln Keqq me

where e (Polanyi potential) is equal to RT ln (1 + 1/Ce), qm is the maximum adsorption

capacity (mg/ g),

K is related to mean adsorption energy E in kJ mol−1

as:

KE 2/1

The mean adsorption energy (E) gives information about chemical and physical adsorption29

(Erdem, 2004).

3.5 Thermodynamics of copper adsorption





To determine the spontaneity of a process thermodynamic parameters such as enthalpy

change (∆H°), entropy change (∆S°) and free energy change (∆G°) have to be taken into

consideration. For the present study the experiments at temperatures of 295,300, and 303 K

were conducted. ∆G°, ∆H° and ∆S° are calculated using the following equations:

e

Adc

C

CK

cKRTG ln0

RTHRSKc //ln 00

Where Kc is the equilibrium constant, CAd is the concentration of copper adsorbed on solid at

equilibrium (mg /L), Ce is the equilibrium concentration of copper in the solution (mg /L),R is

the gas constant (8.314 J/K. mol) and T is the temperature in Kelvin.

Figure 25: Van’t Hoff plot of adsorption of copper onto ligand loaded GAC.

The values of ∆H° and ∆S° can be obtained from the slope and intercept of Van’t Hoff plot as

shown in figure 25. The negative values of ∆H° (−36.08 kJ /mol for QSA and -48.1116 kJ

/mol

for NQSA) as shown in Table 3 indicates exothermic nature of adsorption. The low value of

heat of adsorption obtained in this study indicates that adsorption is likely due to

physisorption through weak intermolecular interactions30

(Alkan, 2004).

The results of these thermodynamic calculations are shown in Table 3 below. The negative

value of free energy change (∆G°) shows that it is a spontaneous process. However, the

values of ∆G° decreased with increasing temperature, indicating that adsorption of Cu (II)

ions on ligand loaded GAC became less favorable at higher temperature. The negative value

of entropy change (∆S°) shows a decreased disorderliness at the solid/liquid interface during

copper adsorption. As the temperature increases, the mobility of copper ions increases





causing the ions to escape from the solid phase to the liquid phase. Therefore, the amount of

copper that can be adsorbed will decrease.

Table 3: Thermodynamic parameters of copper adsorption by ligand loaded GAC

Type of ligand Temperature (K)

∆G° (kJ/ mol) ∆H° (kJ/mol) ∆S° (1/kJ mol)

QSA 295 -8731.362999

-36.0837 -73.3702

299 -8972.228552

303 -9262.883023

NQSA 294 -8701.765158

-48.1116 -85.3981

300 -9002.236005

305 -9324.024165

3.6 Adsorption Kinetics

Kinetic models, namely first-order and pseudo-second order models have been validated

with the experimental adsorption data for copper (II) onto ligand loaded GAC. The study of

adsorption kinetics describes the rate of solute uptake which further controls the residence

time of adsorbate uptake at the solid-solution interface including the diffusion process.

For the kinetic studies 0.5 g of F-400 GAC was adsorbed on 100ml of 0.002 M QSA and

NQSA separately by shaking for 8 hours on a mechanical shaker. 500 ml of Cu solution at a

concentration equivalent to a value from the descending portion of the adsorption isotherm of

Cu F-400 NQSA system was chosen. The run lasted for 4 hrs by which the fall in

concentration of the Cu ion in solution stabilized. The data for both QSA and NQSA were

then regressed against the Lagergren equation, which represents a first-order kinetic equation, 31

(Namasivayam, 1995)

where qt is the metal uptake per unit weight of GAC (mg/g) at time t, qe is the metal uptake

per unit weight of GAC (mg/g) at equilibrium, and k1 (min−1

) and k2 (g mg−1

min−1

) are the

rate constants of the first-order and pseudo-second-order kinetics equations, respectively. The

slopes and intercepts of these curves were used to determine the values of k1 and k2, as well

as the equilibrium capacity (qe). The first-order kinetics model was considered initially which

gave R2 value (0.965 for QSA and 0.908 for NQSA) (Figure 26[a] and 26[b]); however, the

linearized second-order kinetics model (Figure 27[a] and 27[b]), provided R2

values (0.998

for QSA and NQSA). As a result, the sorption system appears to follow pseudo-second-order

reaction kinetics for both the ligands.





Figure 26[a]: First-order kinetics model Figure 26[b]: First-order kinetics model

[F-400: QSA: Cu] [F-400: NQSA: Cu]

Figure 27[a]: Pseudo-2nd

order kinetics Figure 27[b]: Pseudo-2nd

order kinetics

model [F-400: QSA: Cu] model [F-400: NQSA: Cu]

3.7 Adsorption capacities

The comparative adsorption capacities (saturation values of qe) of the two ligands on F-400

GAC used in the present investigation could be estimated from the figures 3 and 7. The

saturation values of qe for the ligands QSA and NQSA were found to be 12.90 x 10-4

and 7.11

x 10-4

moles/g as shown in table 4.

Table 4: Values of qe for the ligands and for copper on the ligand loaded GAC

Sr. No System qe

1. F-400-QSA 12.90 X 10-4 moles/g

2. F-400-NQSA 7.11 x 10-4 moles/g

3. F-400-QSA-Cu 26 mg/millimole

4. F-400-NQSA-Cu 37 mg/millimole





The lower value of qe in case of F-400-NQSA could be correlated to the Q0 values obtained

from Langmuir equation which represents the area on the surface of the carbon consisting of

a monomolecular layer which is formed at the saturation point. From value of Q0, S is

calculated as:

AQNS 0

0

Where N0= Avogadro's number, S= surface area in m2/g, Q

0= moles/g

The values of Q0, A and S for GAC F-400 are shown in table 5. The surface area values of

the GAC is likely to differ based on the area of each ligand molecule.

Table 5: Values of Q0, A and S for GAC F-400

Ligands Q

0 [moles/g] x 10

3

A [cm2 ]x 10

16 S [m

2/ g]

QSA 1.2 55.9 401

NQSA 0.8 60.1 301

The amount of copper held per mmole of ligand QSA was found to be 26 mg/mmole while

for NQSA it was higher 37 mg/mmole. The striking change in the qe values in QSA and

NQSA is probably not something to do with stability of the complex formed but probably due

to the hindrance on the surface of GAC. QSA ligand being smaller in cross sectional area as

compared to NQSA is able to penetrate deep into the pore of GAC and with narrowing of the

pores sufficient QSA ligand molecules are unavailable for proper orientation to form 1:2

complex with copper ions on the surface of GAC, thus making the value of qe, the amount of

copper per mmole of ligand in the case of QSA appear smaller. On the other hand the nitro

derivatives of oxine is not able to penetrate deep into the pores owing to the bulky nature of

the groups and further the already diffused material, which are in the wider macro portion of

the pores form a chelate readily with the copper. Thus the amount of copper per mmole is

higher in case of NQSA compared to QSA. This is further supported by the amount of ligand

adsorbed at the saturated level found to be lower in case of NQSA. Thus, pore structure of

GAC and chemistry of the ligand molecule probably contributes to removal of certain metal

species through ligand adsorbed on the surface from aqueous solutions.

A critical analysis of the values of diffusion coefficient (D) in Table 6 clearly speaks of

surface diffusion phenomena operating in the process in the present investigation. These

values were obtained from QDF plots of GAC -ligand and GAC -ligand Copper system used

in the present work as shown in figure 28[a] and 28[b] respectively. The values of D in QSA

and NQSA parallels the trends observed earlier regarding the qe values at the saturation level.

The value of 19.92 x 10-8

for QSA is unusually high compared to NQSA. This could be

because QSA being highly soluble in aqueous medium and having a smaller cross -sectional

area helps in diffusing readily into the pores and once adsorbed would readily undergo

surface diffusion on the GAC. However NQSA once adsorbed would find steric factors

affecting surface diffusion owing to substituent and hence has value lower than that of QSA.

Table 6: Diffusion Coefficient data

Sr. No. System K D x 108

1. F-400-QSA 0.3143 19.9234

2. F-400-NQSA 0.0735 4.6592





3. F-400-QSA-Cu 0.0929 5.8889

4. F-400-NQSA-Cu 0.1704 10.8016

Where K= Slope from QDF Plot, D= Diffusion coefficient in cm2/sec

Figure 28[a]: Quadratic driving force plot Figure 28[b]: Quadratic driving force plot

for system : F-400-QSA: Cu for system : F-400-NQSA: Cu

4. Conclusion

The results obtained from equilibrium studies show adherence to adsorption models

Langmuir, Freundlich and D-R isotherms while the kinetics fitted well to pseudo-second

order Lagergren kinetic equation. Further it can be concluded that the pore structure of GAC

and chemistry of the ligand molecule probably contribute significantly towards uptake of

copper ions by ligand loaded F-400 GAC. Based on the results of this analysis, external mass

transfer appears to control the rate of adsorption. The adsorption process was also

thermodynamically spontaneous under natural conditions.

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a comparative study of 8-hydroxyquinoline-5-sulphonic … loaded on f-400 granular activated carbon...

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