Equilibrium and Thermodynamics Studies for Decolorization of Reactive Black 5 by Adsorption

14 Pages·2017·0.49 MB·English

Iranian Journal of Health Sciences 2015; 3(3): 15-28 http://jhs.mazums.ac.ir

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Equilibrium and Thermodynamics Studies for Decolorization of Reactive Black 5 by
Adsorption onto Acid Modified Banana Leaf Ash

Edris Bazrafshan1 Somaiieh Rahdar1 Davoud Balarak1 Ferdos Kord Mostafapour1 *Mohammad Ali Zazouli2

1- Health Promotion Research Center, Zahedan University of Medical Sciences, Zahedan, Iran
2- Department of Environmental Health, School of Public Health AND Health Sciences Research Center, Mazandaran University
of Medical Sciences, Sari, Iran

*[email protected]
(Received: 25 Feb 2015; Revised: 13 Jun 2015; Accepted: 1 Aug 2015)

Abstract
Background and purpose: At present study, acid modified banana leaf ash was used as an

adsorbent for the successful removal of Reactive Black 5 (RB5) dye from aqueous environments.
Materials and Methods: Th e effect of various operating parameters such as pH of solution, dye
concentration, contact time, adsorbent dosage, and the temperature was investigated.
8 ] Results: Maximum adsorption capacity of the banana leaf ash was 191.32 mg/g at pH 2, the
1-1 initial concentration of 200 mg/l and 323° K when 95.66% of the dye was removed. The process
0
3- followed pseudo-second-order kinetics. Furthermore, equilibrium data were better represented by
2
0 Freundlich isotherm among Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich
2
on equilibrium isotherm models. The negative values of free energy change confirmed the feasibility
c.ir of the process and the spontaneous nature of adsorption. Furthermore, from the magnitude of ∆H,
a
s. the process was found to be endothermic physisorption.
m
zu Conclusion: According to the results of this study, it was found that the acid modified banana
a
m
leaf ash is not only a low-cost adsorbent, but also has high performance in the removal of RB5
s.
h
m j from aqueous environments.
ed fro f[oBarz rDafeshcaonl oE,r iRzaahtdiaorn S , oMfo sRtaefaapcotuirv eFK B, Blaalcarka k5 D ,b *yZ aAzoduslio MrpA.t iEoqnu oilnibtor iuAmci da nMdo Tdihfeierdm Bodaynnaanma icLse aSft uAdsiehs.
d
nloa Iran J Health Sci 2015; 3(3): 15-28] http://jhs.mazums.ac.ir
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Key words: Decolorization, Reactive Black 5, Adsorption, Banana Leaf

3 ]
0
0
3.
0
5.
1
0
2
s.
h
8/ij
0
5
7
0.
1
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Decolorization of RB5 by acid modified banana leaf ash E. Bazrafshan et al.

1. Introduction are mutagenic and carcinogenic to living
Dyes and pigments are considered as one of organisms (29,30). These dyes also cause
the most important group of water serious ecological problems; for example,
contaminants and also these compounds are they significantly affect the photosynthetic
one of the most hazardous chemical activity of aquatic plants by reducing light
compound classes found in industrial effluents penetration, and they may be toxic to some
and need to be treated since their presence in aquatic organisms (31).
water sources reduces light penetration, The adsorption process is the most efficient
precluding the photosynthesis of aqueous flora procedure for removal of synthetic dyes from
(1-4). They are also aesthetically industrial effluents because the dye species are
objectionable for drinking and other purposes transferred from the water effluent to a solid
(3) and can cause allergy, dermatitis, skin phase, diminishing the effluent volume to a
irritation (5), and also provoke cancer (6) and minimum. Furthermore, adsorption has proven
mutation in humans (7). to be a reliable treatment method due to its low
Inappropriate treatment and disposal of capital investment cost, simplicity of design,
wastewaters from textile, dyeing, printing, ease of operation and insensitivity to toxic
ink, and related industries have provoked substances, but its application is limited by the
severe environmental concerns all over the high price of some adsorbents and the large
world (8-11). Removal of dye in wastewater amounts of wastewater normally involved.
has been made by physical, physicochemical, Activated carbon is the most popular and widely
biological and/or chemical processes (12-15). used dye adsorbent, but it suffers from several
Conventional treatment involves a process of drawbacks such as its high cost of both
coagulation or flocculation. This is a versatile manufacturing and regeneration and it is
8 ] process, which can be used alone or combined ineffective against disperse and vat dyes (32).
1
1- with biological treatments, as a way of Agricultural waste-based carbon has the
0
23- removing suspended solids and organic advantage of exhibiting low ash content,
0
n 2 material, as well as promoting the extensive reasonable hardness and high surface area
o
c.ir removal of dyes from textile industry effluents and/or adequate porous structures (28,33). The
s.a (16,17). However, this approach presents the choice of activated carbon precursor typically
m
u disadvantage of generating a large volume of depends on its availability, cost, and purity, but
z
a
m sludge. Furthermore, biological and enzymatic the manufacturing process and intended
s.
m jh treatment (10,18-21), ozone treatment applications of the product are also important
ed fro (o1x0id,2a2ti,o2n3 ), anndan oppahrtoictolecsa tal(y2ti4c) , pcrhoecmesisceasl cboionmsiadsesr aisti ognestt in(3g4 i)n.c rTeahseerdef aotrtee,n teiovna liuna atilol no voefr
d
a
nlo (11,25,26), photochemical and sonochemical the world as it is renewable, widely available,
w
o processes (27) and membrane processes cheap, and environmental friendly (35).
D
[ (8,10,28), were used for removal of dye from At present study, adsorption of Reactive
textile effluents. However, some of these Black 5 (RB5) from aqueous solutions on
methods are limited due to their high activated carbon prepared from banana leaf
operational costs and problems. ash was studied. The effects of various
3 ] Azo dyes, widely used in the textile parameters, including initial pH of the
00 industry, are considered recalcitrant solution, adsorbent dosage, RB5
3.
5.0 xenobiotic compounds due to the presence of concentration, and contact time were studied.
1
20 a nitrogen double bond (-N = N-) and other In additional, equilibrium isotherms and
s.
08/ijh gearosiulpy sb i(oid.ee.g, rasduelpdh. oInni ca ddgirtoiounp,) mtohsatt aazroe dyneost tdheesrcmriobde ytnhaem exicp epraimraemnetatel rdsa twa.e re explored to
5
7
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Decolorization of RB5 by acid modified banana leaf ash E. Bazrafshan et al.

2. Materials and Methods 2.3. Dye removal experiments
2.1. Chemicals and reagents Dye removal experiments with the banana leaf
RB5 is an anionic dye with a molecular ash were carried out as batch tests in 250 ml
weight of 991.82 g/m and maximum flasks under magnetic stirring. Each test
absorption (λ ) 597 nm. The RB5 consisted of preparing a 100 ml of dye
max
(C H N Na O S ) used in this work was the solution with a desired initial concentration
26 21 5 4 19 6
analytical grade (Merck, Germany). The and pH by diluting the stock dye solution with
chemical formula of RB5 is shown in figure 1. distilled water, and transferring it to the
This dye is characterized as a diazo beaker on the magnetic stirrer. The pH of the
compound, which bears two sulfonate and two solution was adjusted using 1 N HCl or NaOH
sulphatoethylsulphon groups that have solutions. A known mass of banana leaf ash
negative charges in an aqueous solution. For (adsorbent dosage) was then added to the
treatment experiments, the dye solutions with solution, and the obtained suspension was
concentrations in the range of 10-200 mg/l immediately stirred for a predefined time.
were prepared by successive dilution of the After the desired contact time, the samples
stock solution (1000 mg/l) with distilled were withdrawn from mixture by using a
water. All other chemicals used in this study micropipette and centrifuged for 5 minutes at
were of analytical grade. 5000 rpm. RB5 concentration was determined
spectrophotometrically at λ = 597 nm
max
according to the Lambert–Beer law using an
UV-VIS spectrophotometer (T80 PG
Instruments Ltd). Then the amount of RB5
adsorbed, q (mg/g), was obtained as follows:
e
8 ]
1 (C -C )V
1- q = 0 e
3-0 e M (1)
2
0
2
n Where, C and C are the initial and
o 0 e
c.ir equilibrium liquid phase concentration of RB5
a Figure 1. Structure of Reactive Black 5
s. (mg/g), respectively. V is the volume of the
m

u
z solution (L) and M is the amount of adsorbent
ma 2.2. Adsorbent preparation
s. used (g).
h Banana leaf used in the batch experiments
m j To express the percent of dye removal, the
o were collected from lands near to Chabahar
d fr city (25° 17' 44" N , 60° 38' 2" E) of Sistan and following equation was used:
de
oa Baluchestan province in the southeastern part (C -C )
nl %= 0 f .100
w of Iran. This natural wastes were firstly washed C
Do 0 (2)
[ with distilled water to remove impurity such as
sand and leaves and soluble and colored Where, C and C represent the initial and
0 f
components, dried at 110° C for 12 hours, final (after adsorption) dye concentrations,
burned at 700° C for 2 hours, crushed in a respectively. All tests were performed in
3 ] domestic grinder and sieved to obtain particle duplicate to insure the reproducibility of the
0 size in the range of 60-200 mesh. The results; the mean of the two measurements is
0
3.
0 powdered adsorbent was stored in an airtight reported. The plot of equilibrium adsorption
5.
01 container until use. No other chemical or capacity against equilibrium concentration in
2
hs. physical treatments were used prior to the liquid phase graphically depicts the
08/ij adsorption experiments. equilibrium isotherm.
5
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Decolorization of RB5 by acid modified banana leaf ash E. Bazrafshan et al.

The investigated ranges of the experimental 60 minutes. As it can be seen from figure 3,
variables were as follows: RB5 dye the uptake of the dye increased rapidly with
concentration (10, 20, 40, 50, 60, 80, 100, increased amount of adsorbent from 0.1 to
150, 200 mg/g), pH of solution (2-12), banana 0.8 g and slowed down from 0.8 to 1.2 g.
leaf ash dosage (0.1-1.2 g/l) and mixing time
(5-210 minutes). 240 100
210 90
qe, (mg/g) 80 %)
3. Results 180 Efficiency, % 70 y (
150 60 nc
3T.h1e. Eeffffeecctt ooff iinniittiiaall ppHH on adsorption of RB5 e (mg/g) 19200 4500 al efficie
was studied over a wide pH range of 2-12 at q 30 ov
60 m
room temperature, constant initial dye 20 e
r
concentration of 50 mg/g, adsorbent dose of 30 10 Dye
0 0
3 g/l and contact time of 60 minutes. Figure 2
0 0.2 0.4 0.6 0.8 1 1.2 1.4
depicts that the pH significantly affects the
Adsorbent dose (g)

extent of adsorption of dye over the adsorbent
Figure 3. Effect of adsorbent dosage on Reactive
and a reduction in the amount adsorbed with
Black 5 (RB5) adsorption onto banana leaf ash
increasing pH was observed. Figure 2 also
specifies that maximum uptake of the RB5 is 3.3. Effect of contact time and initial dye
observed at pH 2. The percentage of the concentration
amount of the dye adsorbed then decreases up The effect of contact time on the RB5
to pH 7.0. After this pH, it remains almost adsorption by the banana leaf ash was
8 ] constant. Thus, all further studies were carried investigated for 210 minutes at different initial
1
1- out at pH 2.0 in each case. dye concentrations. It is clear from figure 4 that
0
23- the extent of adsorption is rapid in the initial
0
2
n 80 120 stages and becomes slow in later stages till
o
c.ir (%) 70 Efficiency, % 100 saturation is attained after 120 minutes. On the
ms.a ncy qe, (mg/g) other hand, according to figure 4a. The RB5
u e 60 80 was rapidly adsorbed in the first 20 minutes
az ci
m fi g (55-85%) for various initial concentrations, and
d from jhs. emoval ef 4500 4600 qe, mg/ tfhroemn t2h0e toa d1s0o0rp mtioinnu treast ea ndde cfrineaaslleyd regarcahdeuda ltloy
e r equilibrium in about 120 minutes.
nload Dye 30 20 The effect of initial concentration of RB5
ow 20 0 on the extent of adsorption by banana leaf ash
D
0 2 4 6 8 10 12 14
[ Initial pH was studied, and the relevant data are given in

Figure 2. Effect of initial pH on Reactive Black 5 figure 4a. As can be seen, when the initial dye
(RB5) adsorption onto banana leaf ash concentration is increased, the percent of dye
removal decreased. In contrast when the initial
3 ] 3.2. Effect of amount of adsorbent dye concentration is increased, the amounts of
0
0 In order to determine the effect of adsorbent adsorbed dye also increase (Figure 4b), so the
3.
5.0 dosage on adsorption, 0.1-1.2 g/l adsorbent removal of dye depends on the concentration of
1
20 were used for adsorption experiments at fixed the dye. For example, when the initial RB5
s.
8/ijh initial pH (pH 2), initial dye concentration concentration increases from 10 to 200 mg/l
0 (50 mg/g), and temperature 20° C for (at contact time 5 minutes), the equilibrium
5
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Decolorization of RB5 by acid modified banana leaf ash E. Bazrafshan et al.

sorption capacities of banana leaf ash increase efficiency of RB5 for all initial dye
from 5.99 to 93.92 mg/g. This increase in the concentrations was increased, when the
proportion of removed dye may be probably temperature was increased from 293 to 323° K.
due to equilibrium shift during sorption process.
3.5. Kinetics of the adsorption process
3.4. Effect of temperature on RB5 dye Figure 6 illustrates the adsorption kinetics of
adsorption and thermodynamic studies RB5. The removal rate of RB5 was fast during
The effect of temperature on RB5 dye the initial stages of the adsorption processes,
adsorption was investigated at (293-323° K). especially for an initial dye concentration of
As it can be seen from figure 5, the removal 150 and 200 mg/l. However, the adsorption

100 a
95 10, mg/L
) 90 20, mg/L
%
( 85
y 40, mg/L
c 80
n
e 50, mg/L
ci 75
ffi 70 60, mg/L
e
al 65 80, mg/L
v
o 60
m 100, mg/L
e 55
r
ye 50 150, mg/L
D
45 200, mg/L
8 ] 40
1-1 0 20 40 60 80 100 120 140 160 180 200 220 240
0
3- Time (min)
2
0
2
on 250 b
c.ir 10, mg/L 20, mg/L 40, mg/L 50, mg/L 60, mg/L
a
s. 80, mg/L 100, mg/L 150, mg/L 200, mg/L
m 200
u
z
a
m
s.
h
m j 150
)
o g
d fr mg/
e
ad e ( 100
o q
nl
w
o
D
[ 50
0
0 20 40 60 80 100 120 140 160 180 200 220
3 ] Time (min)
0
0
3. Figure 4. Effect of contact time for the adsorption of Reactive Black 5 (RB5) onto banana
0
5. leaf ash, a: trend of dye removal, b: trend of adsorbed dye
1
0
2
s.
h
8/ij
0
5
7
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Decolorization of RB5 by acid modified banana leaf ash E. Bazrafshan et al.

100
95
%)
(
y
c 90
n
e
ci
al effi 85 10, mg/L 50, mg/L 100, mg/L
v
o
m
Re 80
150, mg/L 200, mg/L
75
290 295 300 305 310 315 320 325
Temperature (K)

Figure 5. Effect of temperature on Reactive Black 5 (RB5) dye adsorption onto banana ash

24 equilibrium was reached at 120 minutes for all
10 mg/L
21 50 mg/L the five concentrations tested. As it can be seen
100 mg/L from figure 6 the data fitted well with the
18
150 mg/L second order kinetics model (R2 > 0.99).
g) 15 200 mg/L
g/
m 12
min/ 9 3T.h6e. Eisqouthileirbmrisu mba saedds oornp ttihoen eixsoptehreimrmen tal data
8 ] t/qt ( 36 and the parameters obtained from nonlinear
1-1 regression by four models are presented in
0 0
3- table 1. According to results of this table 1,
2 0 25 50 75 100 125 150 175 200 225
0
n 2 Time (min) the correlation coefficient of the Freundlich
ac.ir o ReaFcitgivuer eB l6a.c Pks 5eu (dRoB-s5e)c aodnsdo roprtdieorn k oinn ebtiacn apnloat sle faofr ash minodidcealt ingw ast haht igthheer Ftrheaunn dloicthh er momdoedl eliss,
ms. at different initial concentration of dye (dose = 1 g, pH
suitable for describing the adsorption
u
z = 2, time = 5-210 minutes, concentration = 10-200
ma equilibrium of RB5 dye onto banana leaf ash.
mg/l in 100 ml of synthetic water)
s.
h
m j
o Table 1. Isotherm parameters for adsorption of Reactive Black 5 (RB5) onto activated carbon
d fr obtained from banana leaf ash at various temperatures
e
d
a Langmuir isotherm
o
nl 293° K 298° K 303° K 308° K 313° K 323° K
w
o q (mg/g) 94.04 94.90 92.29 96.11 99.6 97.82
D m
[ kRL2 (l/mg) 0.09.84636 0.19.8065 4 04.9.0832 9 05.9.7884 05.9.5864 5 01.19.83262
Freundlich isotherm
k 18.69 27.57 39.49 48.35 54.73 65.60
f
n 1.89 2.11 2.47 2.46 2.29 2.39
R2 0.9941 0.9942 0.9910 0.9960 0.9915 0.9843
Temkin isotherm
3 ] K 1.39782E-07 1.9856E-17 4.93545E-27 2.49034E-33 2.95647E-37 7.47417E-43
0 T
0 B 0.0323 0.0346 0.0385 0.0382 0.0345 0.0345
03. R2 0.8123 0.8190 0.7952 0.8308 0.8301 0.7903
5. Dubinin-Radushkevich isotherm
1
0 q 4.62 4.68 4.72 4.81 4.88 4.91
2 m
s. β -0.00063 -0.00043 -0.00028 -0.00026 -0.00026 -0.00022
h
8/ij R2 0.8732 0.8883 0.8875 0.9139 0.9231 0.9153
50 RB5: Reactive Black 5
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Decolorization of RB5 by acid modified banana leaf ash E. Bazrafshan et al.

4. Discussion the maximum adsorption capacity of the
4.1. Effect of initial pH adsorbent, the effect of amount of adsorbent
The removal of pollutants from aqueous was monitored. As it can be seen from figure 3,
environments by adsorption is greatly the uptake of the dye increased rapidly with
influenced by the pH of solution which affects increased amount of adsorbent from 0.1 to 0.8 g
the nature of the surface charge of the and slowed down from 0.8 to 1.2 g. This result
adsorbent, as well as the extent of ionization can be explained by the fact that the sorption
and speciation of the aqueous adsorbate sites remain unsaturated during the sorption
species and consequently the rate of whereas the number of sites available for
adsorption. On the other hand, the solution pH sorption site increases by increasing the
would affect both aqueous chemistry and adsorbent dose. The maximum adsorption
surface binding sites of the adsorbent. So, the efficiency of RB5 onto banana leaf ash was
solution pH is an important parameter during found to be 98.8% (49.4 mg/g) at adsorbent
the dye adsorption process. concentration of 1.0 g/l. There was a non-
As presented in figure 2, the pH significantly significant increase in the percentage removal of
affects the extent of adsorption of dye over the RB5 when the adsorbent concentration
adsorbent and a reduction in the amount increases beyond the 1.0 g/L. This suggests that
adsorbed with increasing pH was observed. The after a certain dose of biosorbent, the maximum
percentage of the amount of the dye adsorbed adsorption is attained and hence the amount of
then decreases up to pH 7.0. After this pH, it pollutants remains constant even with further
remains almost constant. In addition, as can be addition of dose of adsorbent (46).
seen from figure 2, the maximum adsorption Obviously, the RB5 adsorbed per gram of
capacity of the adsorbent was 115.3 mg/g at pH adsorbent decreased rapidly with an increase
8 ] 2 and initial concentration of 50 mg/g, when in the amount of adsorbent. It can be related to
1
1- 69.18% of the dye was removed. Removal the fact that fixed dye concentration (50 mg/l)
0
23- efficiency at pH 7.0 was only 30.43% and led to unsaturated active site on adsorbent
0
n 2 adsorption capacity was 50.71 mg/g. Similar surface and increase in the adsorbent
o
c.ir results were reported by other researchers concentrations caused particle aggregation
s.a (12,36-39). At acidic conditions, binding sites (47). Similar results were reported by other
m
u of the adsorbent would be closely associated researchers (36,48,49).
z
a
m with the hydrogen ions which act as bridging
s.
m jh ligands between the adsorbent surface and the 4.3. Effect of contact time and initial dye
ed fro dthyee ammooleucnutl ed (y4e0 )a. dIsno arbdeddit iowni,t ha cinocnrsetaansitn fga llp iHn cTohnec eadnstroarbtiaotne concentration and contact time
d
a
nlo may be due to deprotonation, which hinders the between adsorbent and adsorbate species play
w
o diffusion (41). The lower pH values can be a significant role in the process of removal of
D
[ suitable for the adsorption of reactive dye pollutants from aqueous solutions by
(42-45). According to the above results pH 2 adsorption at a particular temperature and pH.
was selected for performing the subsequent The effect of contact time on the RB5
experiments. adsorption by the banana leaf ash was
3 ] investigated for 210 minutes at different initial
00 4.2. Effect of amount of adsorbent dye concentrations. It is clear from figure 4 that
3.
5.0 The adsorbent concentration is an important the extent of adsorption is rapid in the initial
1
20 parameter because this determines the capacity stages and becomes slow in later stages till
s.
08/ijh oinfi ttihael RadBs5o rbceonntc e(nbtarnaatinoan .l eTahf earsehfo) rfeo, r toa gatitvaeinn soabtvuiroautiso nfr iosm a tttahien efda cat fttehra t1 2a0 lamrgineu tneusm. Tbheirs oisf
5
7
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Decolorization of RB5 by acid modified banana leaf ash E. Bazrafshan et al.

surface sites are available for adsorption at the 4.4. Effect of temperature on RB5 dye
initial stages and after a lapse of time, the adsorption and thermodynamic studies
remaining surface sites are difficult to be The removal efficiency of RB5 for all initial
occupied because of repulsion between the dye concentrations were increased, when the
solute molecules of the solid and bulk phases. temperature was increased from 293 to 323°
A similar finding was reported by Cengiz and K. Increasing the temperature is known to
Cavas (50) and Gulnaz et al. (36). increase the rate of diffusion of the adsorbate
The effect of initial concentration of RB5 molecules across the external boundary layer
on the extent of adsorption by banana leaf ash and in the internal pores of the adsorbent
was studied and the relevant data are given in particle, owing to the decrease in the viscosity
figure 4a. As can be seen, when the initial dye of the solution. In addition, changing
concentration is increased, the percent of dye temperature will change the equilibrium
removal decreased. In contrast when the initial capacity of the adsorbent for a particular
dye concentration is increased, the amounts of adsorbate (52). Similar results were reported
adsorbed dye also increase (Figure 4b), so the by Bazrafshan et al. (53).
removal of dye depends on the concentration of Thermodynamic considerations of an
the dye. This increase in the proportion of adsorption process are necessary to conclude
removed dye may be probably due to whether the process is spontaneous or not.
equilibrium shift during sorption process. In Gibb’s free energy change, ∆G°, is the
fact, the initial dye concentrations provide an fundamental criterion of spontaneity.
important driving force to overcome the mass Reactions are spontaneously at a given
transfer resistance of the dye between the temperature if ∆G° is a negative value. The
aqueous phases and the solid phases, so thermodynamic parameters of Gibb’s free
8 ] increasing initial concentrations would enhance energy change, ∆G°, enthalpy change, ∆H°,
1
1- the adsorption capacity of dye. Similar results and entropy change, ∆S°, for the adsorption
0
3- have also been recorded for adsorption of processes are calculated using the following
2
0
n 2 Congo red from aqueous solution onto calcium- equations:
o
c.ir rich fly ash (51) and RR198 removal from ∆G0= - RT ln K
s.a aqueous solutions by potamogeton crispus (36). a (3)
m
u Furthermore, the time taken to reach ∆G0= ∆H0 - T∆S0
z (4)
a
m equilibrium was equal for all the initial dye
s.
m jh concentrations used, which was 120 minutes. Where, R is universal gas constant
o This finding is supported by the study carried (8.314 J/mol/K) and T is the absolute
ed fr out by Osma et al. (39), who reported that the temperature in K.
d
a
o initial concentration of dyes had only a small The thermodynamic parameter, Gibb’s free
nl
ow influence on the time of contact necessary to energy change, ∆G°, is calculated using Ka
D
[ reach equilibrium in the adsorption study of obtained from Freundlich equation 8 and
RB5 by sunflower seed shells. shown in table 2.

Table 2. Thermodynamics parameters for Reactive Black 5 (RB5) adsorption on
03 ] Temperature, °K ∆G0 (kbJa/mnaonl)a leaf ash∆ H0 (kJ/mol) ∆S0 (kJ/mol K)
0
3. 293 -7.138
5.0 298 -8.220
1 303 -9.260
0 32.01 0.14
2 308 -9.936
hs. 313 -10.420
8/ij 323 -11.240
0
5 RB5: Reactive Black 5
7
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A plot of Gibb’s free energy change, ∆G°, 4.5. Kinetics of the adsorption process
against temperature, T, was found to be linear As presented in figure 6, the removal rate of
(Figure 7). The enthalpy change, ∆H°, and the RB5 was fast during the initial stages of the
entropy change, ∆S°, for the adsorption adsorption processes, especially for an initial
process were obtained from the intercept and dye concentration of 150 and 200 mg/L.
slope of equation 4 and found to be 32.01 However, the adsorption equilibrium was
kJ/mol and 0.14 kJ/mol/K, respectively. The reached at 120 minutes for all the five
negative values of ∆G° confirm the feasibility concentrations tested. The kinetic data in
of the process and also the spontaneous nature figure 6 were treated with a pseudo-second-
of adsorption with a high preference of RB5 order rate equation. The second-order kinetic
by banana leaf ash. Furthermore, the decrease model (57,58) is expressed as:
in the negative value of ∆G° with an increase
t 1 t
in temperature indicates that the adsorption = +
q K q 2 q
process of RB5 on banana leaf ash becomes t 2 e e (5)
more favorable at higher temperatures (54).
Where, k is the pseudo-second-order rate
2
constant (g/mg/minutes); q the quantity of
e
T (K)
dye adsorbed at equilibrium (mg/g); q the
t
290 300 310 320 330
quantity of dye adsorbed at time t (mg/g), and
0
t is the time (minutes).
-2
-4 As it can be seen from figure 6 the data
)
mol -6 fitted well with the second order kinetics
J/ -8 model (R2 > 0.99). Furthermore, the calculated
K
((cid:12)-10 q values agree very well with the
18 ] ∆-12 exeperimental data (Table 3). Similar kinetic
1- -14
0 results were reported in the biosorption of
3-
02 Figure 7. Plot of Gibbs free energy change, ∆G°, RB5 by powdered active carbon and fly ash
2
n versus temperature, T (57) and RB5 biosorption by sunflower seed
o
c.ir shells (39).
ms.a Adsorption process can be classified as 4.6. Equilibrium adsorption isotherm
u physical adsorption and chemisorption by the
z Isotherms study can describe how an
a
m magnitude of the enthalpy change. It is
s. adsorbate interacts with adsorbent. The
h
m j accepted that if magnitude of enthalpy change isotherm provides a relationship between the
d fro is less 84 kJ/mol, adsorption is physical. concentration of dye in solution and the
e However, chemisorption takes place range
d amount of dye adsorbed on the solid phase
a
nlo from 84 to 420 kJ/mol (55). when both phases are in equilibrium. In order
w
o At present study, the magnitude of enthalpy to investigate the adsorption isotherm, four
D
[ change. Table 2 indicates that the adsorption equilibrium isotherms were analyzed: the
is physical in nature. Furthermore, the positive Langmuir, Freundlich, Temkin and Dubinin-
value of ∆H° indicates that the adsorption Radushkevich (D-R) isotherm.
reaction is endothermic. Entropy has been 4.6.1. Langmuir isotherm
3 ] defined as the degree of chaos of a system. The Langmuir isotherm model is valid for
0
3.0 The positive value of ∆S° suggests that some monolayer adsorption onto surface containing
5.0 structural changes occur on the adsorbent and finite number of identical sorption sites which
1
20 the randomness at the solid/liquid interface in is presented by the following equation:
s.
08/ijh tahdes orapdtsioonrp ptiroonc esssy s(5te6m). increase during the q = (cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:9) (6)
75 (cid:2) (cid:10)(cid:11)(cid:6)(cid:7)(cid:8)(cid:9)
0.
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Decolorization of RB5 by acid modified banana leaf ash E. Bazrafshan et al.

Table 3. Pseudo-second-order adsorption rate constants and q values for different initial
e
Reactive Black 5 (RB5) concentrations at pH 2
RB5 concentration, mg/L K q, mg/g R2
2 e
10 0.317 0.098 0.9999
50 0.151 0.019 0.9988
100 0.127 0.009 0.9969
150 0.089 0.006 0.9962
200 0.075 0.005 0.9955
RB5: Reactive Black 5

Where, q is the amount of metal adsorbed 4.6.3. Temkin isotherm
e
per specific amount of adsorbent (mg/g), C is The Temkin isotherm assumes that the fall in
e
equilibrium concentration of the solution the heat of sorption is linear and the
(mg/L), and q is the maximum amount of distribution of binding energies as uniform
m
RB5 dye required to form a monolayer (up to some maximum binding energy). This
(mg/g). The Langmuir equation can be model takes into account the presence of
rearranged to linear form for the convenience indirect adsorbate/adsorbent interactions and
of plotting and determining the Langmuir suggests that because of these interactions the
constants (K ) and maximum monolayer heat of adsorption of all molecules in the layer
L
adsorption capacity of banana leaf ash (q ). would decrease linearly with coverage
m
(59,60). The Temkin isotherm has generally
The values of q and K can be determined
m L
been applied in the following form:
from the linear plot of 1/q versus 1/C :
e e
8 ] (cid:10) (cid:10) (cid:10) (cid:10) q = BlnK +BlnC (10)
1 = + (7) (cid:2) (cid:23) (cid:2)
01- (cid:4)(cid:9) (cid:4)(cid:5) (cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:9)
23- The constant KT and B1 can be calculated
20 4.6.2. Freundlich isotherm using a linear plot of q versus lnC . K is the
n e e T
ac.ir o Tbahsee dF roenu nsdolripchti oenq uoant iohne teirso gpeunreeloyu se msuprifraiccael, ecqourrielisbproiunmdi ng tbol inmdianxgi mumc onbsitnadnitn g e(nl/emrggy)
s.
m which is commonly described by the and constant B is related to heat of adsorption.
u 1
z
ma following equation: The values are presented in table 1.
s.
h 4.6.4. D-R isotherm
m j 1
o q =K C n The D-R model is often used to estimate the
d fr e f e (8) characteristic porosity and the apparent free
e
d
oa Where, Kf and 1/n are the Freundlich energy of adsorption. The linear form of D-R
nl
w constants related to adsorption capacity and isotherm model is:
o
D
[ adsorption intensity, respectively. The logq =ln q -βε2
Freundlich equilibrium constants evaluated e m

from the intercept and the slope, respectively
Where β is a constant connected with the
of the linear plot of log q versus log C based
e e mean free energy of adsorption per mole of
3 ] on experimental data. The Freundlich the adsorbate (mol2/KJ2), qm is the theoretical
0 equation can be linearized in logarithmic form
0 saturation capacity (mg/g), and ε is the
3.
0 for the determination of the Freundlich
5. Polanyi potential (61).
1
0 constants as shown:
2 According to results of this study, the
hs.
8/ij logq = logK + (cid:10)logC (9) correlation coefficient of the Freundlich
50 (cid:2) (cid:18) (cid:19) (cid:2) model was higher than other models,
7
0.
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