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www.wjpps.com │ Vol 10, Issue 8, 2021. │ ISO 9001:2015 Certified Journal │
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Mohammed et al. World Journal of Pharmacy and Pharmaceutical Sciences
THE RMODYNAMIC STUDY OF THE ADSORPTION OF (E110, 122,
102, 133, 124, 123, 127) DYES EMPLOYED IN THE FOOD INDUSTRY
USING ACTIVATED CARBON AS ADSORBENT SUBSTANCE
Ibrahim Y. Mohammed* and Khaleel Ibraheem A. Al-Niemi
College of Education for Pure Sciences - Dep. of Chemistry. Mosul University- Iraq.
ABSTRACT
This research included a thermodynamic study of the adsorption of
some food coloring dyes (E110, 122, 102, 133, 124, 123, 127) on the
surface of commercial activated carbon as an adsorbent. The analytical
method was used to opted the calibration curves for each dye and to
estimate the amount of adsorbed and remain of food coloring dyes
according to (Beer's –Lambert law) by using spectrometer
photometricmethod for UV- visible spectrophotometric . The effect of
temperature on the adsorption of food coloring dyes or food coloring
molecules was studied within an experimental temperature range (298-
318 kº) and calculate the thermodynamic functions of the adsorption
process (∆H, ∆G°, ∆S°, ∆S) from the adsorption equilibrium
constant(Kc) using the (Vant-Hoff) equation, as well as, calculating those functions from the
adsorption isotherms from the Langmuir and Freundlich constants, All results showed that
the adsorption process is endothermic and of a physical nature through positive ∆H values at
all concentrations with range of temperatures used in this study. It was also found that the
adsorption process occurs spontaneously, where the values of (∆G°) were negative and few,
as well as, it turns into more spontaneous with increasing temperature, but some food
coloring dyes have positive (∆Gº) values refers to that the adsorption process is a non-
spontaneous process, In general, the entropy values (∆S°) were slightly greater than the
values of ( ∆S) that is, the regularity of the adsorbed molecules at equilibrium is greater than
their regularity at any step of adsorption. The results obtained for the thermodynamic
functions calculated from the Langmuir and Freundlich constants showed great agreement
with those that were calculated from the equilibrium constants, which indicates the possibility
of using those Constants to calculate thermodynamic functions successfully.
WORLD JOURNAL OF PHARMACY AND PHARMACEUTICAL SCIENCES
SJIF Impact Factor 7.632
Volume 10, Issue 8, 2202-2227 Research Article ISSN 2278 – 4357
*Corresponding Author
Dr. Ibrahim Y.
Mohammed
College of Education for
Pure Sciences - Dep. of
Chemistry. Mosul
University- Iraq.
Article Received on
17 June 2021,
Revised on 07 July 2021,
Accepted on 27 July 2021
DOI: 10.20959/wjpps20218-19646
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Mohammed et al. World Journal of Pharmacy and Pharmaceutical Sciences
KEYWORDS: Adsorption, Thermodynamic functions, Activated carbon, Adsorption
isotherms.
INTRODUCTION
Food coloring dyes are organic chemicals containing aromatic rings that are widely used
globally in the food, pastry, soft drinks and various juice industries, and this has led to water
pollution mainly in the places of their manufacture or from their domestic or artisanal use for
their manufacture or in dry mixes and dairy products and others.[1-4]
These food colorings are
chemical compounds that did not exist in nature and are manufactured with high purity,
which constitutes an important and very wide sector and one of the important industries as
well.[5]
Chemically, food coloring dyes are involved In the presence of the chromophore azo
group(N N), sulfur (C S), aryl and ionic rings.[6-12]
To treatment the problem of water
pollution with food pollutants, the adsorption technique was used among other methods, for
its efficiency, ease and economically inexpensive, and the use of different effective adsorbent
materials such as clay and activated carbon, and other natural or synthetic materials.[13-20]
Adsorption process is usually accompanied by a change in free energy (∆G) of the surface on
which adsorption occurs and is accompanied by a decrease in entropy due to the adsorption
of molecules on the surface and their attachment to it, loss of some degrees of its freedom
compared to its state before the adsorption process and as a result of the decrease in free
energy and entropy simultaneously. One, this will cause a decrease in enthalpy (∆H).[21-24]
The values of thermodynamic functions are calculated based on certain equations, assuming
that the adsorption process occurs according to the following:
Food coloring dye + Activated carbon prod.
at t = 0 0
at t = teq ( Ce) Cads.
Cads. = Ci – Ce ………….(1)
Where
C0 = the initial concentration of the dye (mg/L).
Ce = the residual concentration of the dye after adsorption (mg/L).
Cads. = adsorbed concentration of dye (mg/L).
and the value of the equilibrium constant is represented as follows:
Kc = Cads. / Ce ……………….(2)
∆G° = -RT ln Kc ………………(3)
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Ln Kc = lnC - ∆H/ RT ………….(4) (Vant-Hoff equation)
Where
Kc = equilibrium constant.
R = gas constant.
T = temperature (k°).
It has been assumed that the values of (∆G) = 0 at equilibrium state.
The Vant - Hoff equation is usually used to calculate the enthalpy values (∆H) of the
adsorption process, and it represents one of the important measures in determining the type of
bonding forces between the adsorbed food dye molecules and the activated carbon surface,
and at the same time represents the amount of energy required to recover the adsorbent from
the solid surface, as well as, the type of adsorption physical or chemical. Also, the negative or
positive sign of (∆H) values indicates that the adsorption process is exothermic or
endothermic respectively.[25]
The value of (∆H) less than (40 kJ / mole) confirms that the
forces controlling the adsorption process are of a physical nature and more than (80 kJ /
mole) It indicates that the adsorption is chemical with real bonds. Thus, each quantity has a
thermodynamic function that gives an indication and an accurate description of the
thermodynamic adsorption process in terms of whether it occurs spontaneously or non-
spontaneously through (∆Gº) values, as well as, the randomness of the adsorption system of
the adsorbed molecules before and after adsorption. The isotherms constant for Langmuir
isotherm (kL) and Freundlich isotherm (kf) of adsorption process used to calculated
thermodynamic functions on the surface of activated carbon for adsorption of food coloring
dyes on surface activated carbon to calculate thermodynamic functions. The Langmuir
constant (kL) which is related to the maximum adsorption capacity, and Freundlich constant
(kf) which is related to the adsorption capacity,were calculated at the same temperature range
(25-45c°) that is used to calculate the equilibrium constant (Kc) for the adsorption process
and for a range of concentrations of the food coloring dyes under study, When The values of
thermodynamic functions obtained from the isotherms constants are consistent with those
calculated from the equilibrium constant, then it can be described as a correct and successful
study.[24,26,27]
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Experimental
Scientific research depends widely on the practical results, which the methods of preparing
stander solution of food coloring dyes chemicals and laboratory devices in accomplishing this
research.
1- The chemicals: All chemicals from used these materials from BDH and Fluka a
company:
a) Activated charcoal (Adsorbent)
b) Hydrochloric acid
c) Sodium hydroxide
d) The food coloring dyes that were used in this study were produce from market of our city
in bottles from well-known companies.
Table 1: The names and molecular formulas of the food coloring Dyes and Some of their
physical Properties and Optimum conditions.
coloring
dyes
Dye name Molecular formula The
color
Meltin
g point
cº
λmax.
nm.
M.wt.
g/ mol
ɛ max.
L/mol.Cm.
optimum conditions.
C0
T c° gm/L PH
E110 Sunset C16H10N2Na2O7S2 Yellow 300 480 452.37 19707 45 45.237 8.11
E122 Azorubine C20H12N2Na2O7S2 Food
Red
300 515 502.431 17256 45 50.243 7.71
E102 Tartrazine C10H11N2Na3O10S3 Yellow 870 426 534.3 8592.7 45 53.430 6.50
E124 Ponceau C20H11N2Na3O10S3 Red - 503 604.46 19361 45 60.446 7.12
E123 Amaranth C20H11N2Na3O10S3 Red 120 523 604.473 13122 45 60.447 6.93
E133 Billiant C37H34N2Na3O10S3 Blue - 630 792.85 81630 45 7.929 7.88
E127 Erythrosine C20H6N4Na2O5 Red - 530 879.86 12680 45 87.986 7.42
2- Instruments
a) Electrothermol melting point of the type (9300 meter).
b) The pH measurement device of the type (JENWAY 3510).
c) An absorbance measuring device of the type (T92+ UV spectrophotometric PG lin) which
used to find the values of (λ max) for all food coloring dyes and measure the absorbance
spectrum for them using a device
d) (CECIL spectrophotometer 1000S) measure the adsorption for food coloring dyes
solution before and after adsorption process using distilled water as a solvent and (blank)
with glass cells with a thickness of (1 cm ).
e) Water bath with a program vibrator (Julabo Sw 23) to shake solutions at a speed of (100
rpm).
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3- Preparation of standard solutions
The standard solutions of the studied food coloring dyes were prepared by dissolving (1 gm)
of each dye in a liter of distilled water, then solutions were prepared with concentrations
(2,4,6,8,10)*10-5
M. for all the dyes under study except for the dye E133, which was prepared
in concentrations (2,4,6,8,10)*10-6
M., was tracked by diluting the calculated volume from
the original standard solution.
4- Analytical method
a) A UV-VIS spectrophotometric device of the type (T92+ spectrophotometric) was used to
estimate the concentration of food coloring pigments in the solutions under study. The
test was done based on the ability of these food colorings to absorb electromagnetic rays
in the UV-visible range. To complete this work, at first, the value of the maximum
absorption wavelength (λ max ) was determined for each dye separately, then the amount
of adsorbent material was tracked after a period of time according to the nature of each
dye. Draw the relationship between absorbance and concentration to choose the best
concentration for the study
b) The best five concentrations were selected from the calibration curve to prepare (5)
solutions containing different concentrations of solutions of each food coloring dye, and
to each of them the necessary amount of activated carbon was added.
c) These solutions were then shaken separately for (60) minutes and at a range of
temperatures (25, 30, 35, 40 and 45 °C) respectively using the programmed vibrator after
setting the temperature.
d) The five solutions were filtered and the absorbance value was recorded before and after
adsorption using equations (1,5 to find the adsorption capacity and efficiency.
The change of quantity adsorbed material from food coloring dye with time using Lambert
Beer, s law to draw the calibration curve at (λ max) for each dye between absorbance and
concentration, as in the following equation:
A= ɛ L C ………………5
Where
A = absorbance (nm) before adsorption.
ɛ = molar absorption coefficient (L/ mol .cm).
C = food coloring dye concentration (M.).
L = length of the absorber cell (1 cm.).
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5- Determination of the concentration of solutions of food coloring dyes
The concentration of dyes before and after adsorption was estimated using a UV - visible
spectrometer, and the results were obtained in (mg/L) using the standard curve that was
prepared previously for each dye. Express the amount of the adsorbent in terms of adsorption
capacity (qe) and adsorption efficiency (%) by estimating the amount of residual and
adsorbent from the food coloring dye solution as in the following equations:
Cads.= C0 –Ce ……………………….6
qe = ((C0-Ce) /m ) * VL……………7
%= ( (C0 – Ce ) / C0 ) * 100 ………..8
Where
Ce = residual dye concentration (mg/L).
C0 = the initial concentration of the dye (mg/L).
VL = volume of the model (ml) .
m = weight of the adsorbent (activated carbon)( g/L).
6- Determining the amount of adsorbent material
Five different weights of the adsorbent material (0.01-0.09 g) were tested at the natural
acidity function and the initial concentration was stable to choose the best weight for it to
adsorb these food coloring dyes until reaching the equilibrium state.
7- Determination of the adsorption isotherm
To find the adsorption isotherm, concentration solutions were used for each dye described
previously, where (50 ml) of each solution were taken and placed in contact with a known
weight of activated carbon in a standard flask of (250 ml) capacity equipped with a tight seal
and placed in a water bath equipped with a vibrator at a temperature of (298-318k0).) for a
specified period of time. After filtration, the absorption was measured with a UV-visible
spectrometer and the residual concentration after adsorption was calculated, including
calculating the amount of adsorbent per unit weight of the adsorbent based on equation (7)
and the following equation:
Ce= Ai / ɛ ............................................ 9
Where:
Ai = absorbance after adsorption.
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8- The effect of temperature
To estimate the effect of temperature on the adsorption rate, the adsorption of each food
coloring dye solution was studied at temperatures (25,30,35,40,45 C0) and by the same steps
mentioned previously. These results were used for application in the Vant - Hoff equation by
drawing the relationship between (ln Kc) versus (1/T) The thermodynamic values were found
according to equations.[2,3,4,5]
RESULTS AND DISCUSSION
In the adsorption process of food coloring dyes on the surface of activated carbon we can
expected that bond formation include this process consequently, the molecules of food
coloring dyes will be associated with a change in the thermodynamic parameters of the
adsorption system. The parameters were used to explain the variation food coloring removal
efficiency at different temperatures and to confirm the adsorption nature of food coloring
dyes on the surface of activated carbon.
The calculated of thermodynamic parameters give feasibility of using different temperatures
in adsorption process and the spontaneous nature of this process.
Temperature is a factor affecting the adsorption process,[28]
as the increase in temperature
leads to an increase in the kinetic energy of the non adsorbed molecules in the solution, and
thus the ease of their transfer to the solid surface for adsorption and an increase in the
efficiency of adsorption.
The increase of temperature useful to over come on the intermolecular interactions[29,30]
and
electrical attractions, as well as, solvent effect at a high concentration of dyes solutions, on
other hand, the effect of increasing at temperature on the adsorbing molecules on the surface
of adsorbent material in the case of physical adsorption leads to decrease the efficiency of
adsorption system, because the increase of temperature cause increase in the kinetic energy of
adsorbed molecules on the surface leads to their disengagement from the adsorbent surface
and their return to the solution , for this reason the adsorption efficiency will decrease,[31,32]
this situation is prevalent in exothermic adsorption system.
The results of effecting of temperature in adsorption process tabulated in table (2) at optimum
conditions.
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Table 2: The effect of temperature on the percentage of adsorption of food coloring dyes
under study at the best dye Concentration and The best weight of activated carbon at
optimum conditions.
% qe mg/g Ce mg/L T k0
C0 mg/L Activated
Carbon (gm)
Dye
81.032 5.236 1.716 298 9.047 0.07 E110
84.039 5.431 1.444 303
87.278 5.640 1.151 308
90.052 5.819 0.900 313
92.141 5.954 0.711 318
86.128 6.182 1.394 298 10.049 0.07 E122
86.864 6.235 1.320 303
87.611 6.289 1.245 308
89.342 6.413 1.071 313
91.820 6.591 0.822 318
84.611 6.459 1.644 298 10.686 0.07 E102
87.422 6.673 1.344 303
89.000 6.794 1.175 308
91.701 7.000 0.886 313
95.313 7.275 0.501 318
76.166 2.013 0.378 298 1.586 0.03 E133
81.463 2.153 0.294 303
86.255 2.280 0.218 308
87.831 2.322 0.193 313
90.984 2.405 0.143 318
80.045 6.912 2.412 298 12.089 0.07 E124
82.080 7.088 2.166 303
84.580 7.303 1.864 308
88.208 7.617 1.425 313
93.424 8.067 0.795 318
39.518 3.412 7.312 298 12.089 0.07 E123
46.849 4.045 6.425 303
56.746 4.900 5.229 308
71.408 6.166 3.456 313
89.369 7.717 1.285 318
40.430 5.082 10.482 298 17.597 0.07 E127
44.604 5.606 9.748 303
52.572 6.608 8.346 308
63.955 8.039 6.343 313
79.132 9.947 3.672 318
When checking the results in table (2) about the effect of temperature on the adsorption of
food coloring dyes on the surface of activated carbon, we clearly noticed an increase in the
values of the adsorption capacity, and efficiency with the increase in temperature. Therefore,
the amount of adsorbed dye is proportional to the amount of heat absorbed by the adsorption
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Mohammed et al. World Journal of Pharmacy and Pharmaceutical Sciences
system, because the largest possible number of molecules possessing sufficient activation
energy to bind to the surface.
The conclusions that were reacted to it from the practical data of adsorption were within the
temperature range used (25-45cº), which is expected to be the dominant forces on the process
of binding food coloring molecules on the surface of activated carbon of a physical nature,
and include an sorption process for diffusion with its various particles and process. In any
case, the results of the values and sign of the enthalpy(∆H) of adsorption pores will confirm
these conclusion through later thermodynamic study, in other words, a decrease in the energy
absorbed by the adsorption system with an increase in concentration within temperature rang
used, and this leads to a decrease in the amount of food coloring dyes adsorbed on the surface
of activated carbon, causing a decrease in its adsorption efficiency at constant concentration
and a certain temperature range, the high heat absorbed by the adsorption system it is
accompanied by a high adsorption efficiency.
1- Thermodynamic study
In order to understand in detail the effect of temperature on the adsorption process at
equilibrium, It was necessary to calculate the thermodynamic parameters of the adsorption
system. In order to know the path way of adsorption process and the type of forces affecting
it, and the nature of the molecular interactions [33,34]
that it can be known through the value of
the enthalpy (heat of adsorption) which gives an indication of the nature of these bonding
forces between the adsorbed molecules and the adsorpted surface, which can be calculated
from Vant- Hoff equation (equation 4) after calculation the values of the equilibrium constant
for the adsorpting process:
The value of (∆H) can be calculated from drawing the relationship between (ln Kc) with (1/
T), which is supposed to give a straight line with an slope equal (- ∆H / R) and high
coloration confection (R2) (Figures 1 A,B,C).
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A B
C
Figures 1: The relationship between (ln Kc) with (1/ T) for the adsorption of food
coloring dyes at different temperatures.
After calculated (∆H) value, the other thermodynamic parameters (∆Gº, ∆Sº and ∆S ) values
calculated from it, the (∆Gº) value represent the change in the free energy at any step of
adsorption process, while the (∆G) value which represent the free energy at equilibrium state
of adsorption for this reason the (∆S) and (∆Sº) values was calculated which represent the
randomness of the adsorption system :
∆Gº = ∆H - T∆Sº ……………. 10
∆Sº = (∆H - ∆Gº) / T ……………11
∆S = ∆H / T ………………………..12
The thermodynamic parameters values of (∆H, ∆Gº, ∆Sº and ∆S) were listed in table (3):
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Table 3: Thermodynamic Parameters and Equilibrium constants for the adsorption of
food coloring dyes at different temperatures.
∆S
J/mole.k0
∆S0
J/mole.k0
∆G0
kJ/mole
R2
∆H
kJ/mole
kc T k0
C0 mg/ L Dye
42.607 39.829 +0.828 0.9753 12.697 0.716 298 45.237 E110
42.904 39.752 +0.652 0.772 303
41.224 40.068 +0.356 0.870 308
40.565 40.125 +0.138 0.948 313
39.928 39.786 +0.045 0.983 318
25.396 27.601 -0.657 0.9115 7.568 1.304 298 50.243 E122
24.977 28.063 -0.935 1.449 303
24.571 27.932 -1.035 1.498 308
24.179 28.096 -1.226 1.601 313
23.799 28.057 -1.354 1.669 318
62.785 63.349 -0.168 0.9879 18.710 1.070 298 53.430 E102
61.749 63.244 -0.453 1.197 303
60.747 63.381 -0.812 1.373 308
59.776 62.064 -0.716 1.316 313
58.836 65.953 -2.263 2.354 318
35.218 37.678 -0.733 0.9889 10.495 1.345 298 7.929 E133
34.637 41.455 -2.066 2.270 303
34.075 41.565 -2.307 2.462 308
33.530 41.629 -2.535 2.649 313
33.003 41.509 -2.705 2.781 318
16.24295 15.55633 +0.204614 0.9784 4.8404 0.920732 298 60.446 E124
15.97492 15.46951 0.153138+ 0.941021 303
15.71558 15.45163 0.081299+ 0.96875 308
15.46454 15.48716 -0.00708 1.002725 313
15.22138 15.60605 -0.12232 1.047354 318
75.48725 68.58355 +2.057304 0.9935 22.4952 0.435888 298 60.447 E123
74.24158 68.81647 1.643808+ 0.520728 303
73.03636 68.49045 1.400142+ 0.578812 308
71.86965 68.58628 1.027693+ 0.673734 313
70.73962 68.95422 0.567758+ 0.806746 318
176.1339 159.4284 +4.978247 1.000 52.4879 0.134079 298 87.986 E127
173.2274 159.6712 4.107536+ 0.195826 303
170.4153 159.925 3.231008+ 0.283155 308
167.693 160.3599 2.295248+ 0.413948 313
165.0563 161.1474 1.243011+ 0.624908 318
To confirm the adsorption nature of the food coloring dyes and to explain the variation in it
removal efficiency by changing of temperature and the spontaneous nature of adsorption
process, It is observed that (∆H) have a positive value, which thermodynamically
substantiates the assumption that the adsorption of food coloring dyes on the surface of
activated carbon is endothermic . Its values ranged (12.697 – 52.4879 kJ/ mole) and this
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Mohammed et al. World Journal of Pharmacy and Pharmaceutical Sciences
indicates that the physical adsorption predominate in the adsorption system. When studying
the effect of increasing the initial concentration of food coloring dyes in the solution, we
noticed that the enthalpy values (∆H) decrease with their positive with increase in the initial
concentration of the food coloring dyes within the temperature range used. In addition,
increasing, the concentration of food coloring dyes works to resist the mass transfer of the
adsorbed substance between the liquid and solid phase and the enthalpy values appears from
the strength of the bonding of the adsorbed molecules to the surface, the amount of energy
required to recover the adsorbed substance from the surface and the type adsorption.[35, 36]
The enthalpy values(∆H) decrease with increase of dyes concentration, in other words, a
decrease in the value of energy absorbed by the adsorption system, this mean decrease in the
number of food coloring dyes molecules allowed to transition from the solution to the
surface. At it is known that increasing the food coloring dye in the solution leads to the
closeness of the molecules to each other, which in itself is a large molecule that contains
aromatic rings and different active groups depending on the type of dye, as will as, the effect
of the solvent. In this research the system of adsorption, it is basically an endothermic and
accordingly, the number of food dye molecules that have the ability to overcome those
interaction or have more freedom of movement than other and the temperature used are the
ones that have opportunity to move from the solution and adsorption on the surface of
activated carbon. As the concentration increases, the number of molecules absorbing energy
decrease for adsorption system, because of this, the absorbed energy will decrease steadily .
The results of effect the concentration on the enthalpy values listed in table (4,5,6)for some
food coloring dyes. It is also notice from the results in table (4,5,6), that increasing the
concentration of food coloring dyes leads to a degrease in the values of the equilibrium
constant when the temperature is constant, while the values of (∆G°) decreases it is negative
values, with in creasing concentration to convert in to a positive values at the highest
concentration due to a degrease in spontaneous adsorption.
So the adsorbed energy decreases within the temperature range used and this leads to a
decrease in the amount of food coloring dyes adsorbed on the surface and that’s a decrease in
adsorption efficiency. At a constant concentration and a certain temperature range, the high
energy absorbed is accompanied by a high adsorption efficiency. The positive enthalpy
indicate the occurrence of an sorption within the adsorption process to the inner micro- pores
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of activated carbon , and this requires additional external energy to be received by adsorption
system.
The negative values of (∆Gº) which listed in Table (3) indicated the spontaneity and
feasibility of adsorption of food coloring dyes on the surface of activated carbon.
Increasing the temperature of the food coloring dyes solution at constant concentration leads
to an increase in the negative value of the Gibbs free energy, meaning an increase in its
spontaneous adsorption from solution to the surface with an increase in temperature, because
the process of its adsorption is endothermic. In this type of adsorption system, the thermal
energy added to this system is also useful in removing molecular interactions between the
food coloring molecules and the surface, because the predominate values of (∆Gº) are less
than (-2.705 kJ/mole).[37]
and inductive that the physical adsorption is prenominal in the
sorption process, as well as, this values refers to that the energy of adsorption process for
food coloring dyes are less than liberation of the previously adsorbed water.
In the sorption system, the (∆Gº) is the driving force, as will as, the basic criterion for the
spontaneous realization of the sorption. In any case, some food coloring dyes showed a
difference in the values of (∆Gº) from the other food coloring dyes under study, such as
(E123) and (E127) dyes, where the values of (∆Gº) had less than (1 kJ/mole) and the positive
value decreased with increase in temperature, meaning it is shift towards spontaneity. Also
(E110) dye had positive values of (∆Gº) at the highest concentration, with the temperature
range used. This indication thermodynamically unfavorable and non spontaneous adsorption
process as is not self-evident, as well as, the driving force is important to manning adsorption
possible. The small positive value of (∆Gº) for (E110) dye difficult to tell us if the sorption
process occurred spontaneously, and we can say the adsorption of (E110) dye take place and
close to spontaneous process. This conclusion indicated by the value of (∆H) at (45 cº) at the
same concentration, while the (∆Gº) values for (E123) and (E127) dyes coloring are very
small even under normal condition it seems that at low concentration the values for it
converted from non spontaneous to spontaneous process with increasing of temperature, but
at other high concentration the (∆Gº) obtained positively at all temperatures, but decreased
with increasing of temperatures by means close to spontaneous adsorption process.
Thermodynamically the entropy (∆S) represent the degree of random (non-regularity) in
adsorption process for adsorbed molecules and refers to the adsorption system condition
resulting from the binding of food coloring dye molecules to the surface of activated carbon .
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The results obtained for entropy values (∆Sº, ∆S) are presented in table (3), the positive
values for entropy parameters of the adsorption of food coloring dyes on activated carbon
give evidence of irregularity during adsorption process between the solid-solution interface.
The (∆Sº) values refers to random for adsorption process at any step of adsorption, it has
positive values that are close to each other and depend on the nature of the dye. The
convergence in the values of (∆Sº) and its stability with increasing temperature and within the
studied range, supports the fact that the adsorption system is of physical nature, it refers to the
increase in randomness on the surface of activated carbon and the interactions in the food
coloring dye solution during the adsorption process, and this significant of increasing
freedom degree for adsorbed dye molecules, which dose not suffer obstruction on the surface
of activated carbon, and all this confirms the predominate of physical adsorption for food
coloring dyes. This statement supports by the lower values of (∆Gº). Finally, the change of
entropy has an important role in the process of adsorption, the (∆Sº) value had been estimated
to be so large which indicated an increase of entropy as a result of adsorption .Before
adsorption, the molecules of dyes near the adsorbent surface were in ordered from than in
subsequent adsorption state and the ratio of free dye molecular to the interacting dye
molecular with the adsorbent will be higher than in the adsorbed state. The distribution and
translational energy will increase with increasing adsorption by giving a positive entropy
value, at high temperature the adsorption is likely to occurs spontaneously because the (∆H)
and (∆Sº) value more than zero value.
Table 4: Effect of concentration on the thermodynamic parameters at optimum
Condition and Different temperatures for the adsorption of (E110) dye.
∆S
J/mole.k0
∆S0
J/mole.k0
∆G0
kJ/mole
R2
∆H
kJ/mole
kc T k0
C0 mg/
L
129.752 141.822 -3.597 0.9956 38.666 4.272 298 9.047
127.611 141.419 -4.184 5.265 303
125.539 141.552 -4.932 6.860 308
123.534 141.850 -5.733 9.052 313
121.591 142.060 -6.509 11.724 318
110.138 117.903 -2.314 0.9527 32.821 2.545 298 18.095
108.320 117.366 -2.741 2.967 303
106.562 118.893 -3.798 4.405 308
104.859 117.706 -4.021 4.690 313
103.211 118.151 -4.751 6.030 318
63.819 65.738 -0.572 0.9107 19.018 1.260 298 27.142
62.766 67.132 -1.323 1.691 303
61.747 66.445 -1.447 1.760 308
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60.760 66.738 -1.871 2.052 313
66.107 66.107 -2.004 2.133 318
52.047 51.490 +0.166 0.9717 15.510 0.935 298 36.190
51.188 52.429 -0.259 1.109 303
50.357 51.370 -0.312 1.130 308
49.553 51.780 -0.697 1.306 313
48.774 51.783 -0.957 1.436 318
42.607 39.829 +0.828 0.9753 12.697 0.716 298 45.237
42.904 39.752 +0.652 0.772 303
41.224 40.068 +0.356 0.870 308
40.565 40.125 +0.138 0.948 313
39.928 39.786 +0.045 0.983 318
Table 5: Effect of concentration on the thermodynamic parameters at optimum
condition and different temperatures for the adsorption of (E133) dye.
∆S
J/mole.k0
∆S0
J/mole.k0
∆G0
kJ/mole
R2
∆H
kJ/mole
kc T k0
C0 mg/
L
141.507 151.168 -2.879 0.9817 42.169 3.196 298 1.586
139.172 151.475 -3.728 4.395 303
136.912 152.185 -4.704 6.275 308
134.725 151.163 -5.145 7.218 313
132.607 151.830 -6.113 10.091 318
104.631 113.020 -2.500 0.9413 31.180 2.744 298 3.171
102.904 114.370 -3.474 3.970 303
101.234 114.146 -3.977 4.724 308
99.617 112.984 -4.184 4.994 313
98.050 114.072 -5.095 6.868 318
87.403 93.698 -1.876 0.9486 26.046 2.132 298 4.757
85.960 96.894 -3.313 3.724 303
84.565 95.834 -4.477 3.889 308
83.214 94.240 -3.451 3.766 313
81.906 96.796 -4.735 5.996 318
58.926 63.342 -1.316 0.9542 17.560 1.700 298 6.343
57.954 65.195 -2.194 2.390 303
57.013 65.510 -2.617 2.780 308
56.102 65.297 -2.878 3.022 313
55.220 66.969 -3.736 4.107 318
35.218 37.678 -0.733 0.9889 10.495 1.345 298 7.929
34.637 41.455 -2.066 2.270 303
34.075 41.565 -2.307 2.462 308
33.530 41.629 -2.535 2.649 313
33.003 41.509 -2.705 2.781 318
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Table 6: Effect of concentration on the thermodynamic parameters at optimum
condition and different temperatures for the adsorption of (E172) dye.
∆S
J/mole.k0
∆S0
J/mole.k0
∆G0
kJ/mole
R2
∆H
kJ/mole
kc T k0
C0 mg/
L
213.4634 210.2412 0.960222 0.9240 63.6121 0.678707 298 17.597
209.9409 208.1394 0.545856 0.805185 303
206.5328 207.3889 -0.26367 1.108456 308
203.2335 208.0008 -1.49214 1.774284 313
200.0381 211.1196 -3.52394 3.791946 318
200.797 193.9007 2.055086 0.9902 59.8375 0.436278 298 35.197
197.4835 192.7241 1.442105 0.564137 303
194.2776 192.1562 0.653384 0.774795 308
191.1741 192.2201 -0.32739 1.134065 313
188.1682 193.1414 -1.58148 1.818791 318
186.7302 176.5548 3.032266 0.9925 55.6456 0.294085 298 52.792
183.6488 175.7074 2.406259 0.384739 303
180.6675 175.3328 1.643092 0.526421 308
177.7815 175.339 0.764487 0.745444 313
174.9862 175.8004 -0.25892 1.102888 318
179.3228 165.4518 4.133574 0.9992 53.4382 0.188549 298 70.389
176.3637 165.5771 3.268344 0.27324 303
173.5006 165.6114 2.429892 0.387162 308
170.7291 165.7296 1.564837 0.548081 313
168.0447 165.9876 0.654131 0.780816 318
176.1339 159.4284 4.978247 1.000 52.4879 0.134079 298 87.986
173.2274 159.6712 4.107536 0.195826 303
170.4153 159.925 3.231008 0.283155 308
167.693 160.3599 2.295248 0.413948 313
165.0563 161.1474 1.243011 0.624908 318
The all values of (∆S) are positive at all concentration in a range of temperature used , which
calculated at equilibrium state, and its values less than (∆Sº) values, this a clear confirmed
indication that the adsorption system of food coloring dyes on activated carbon is less regular
when the adsorption process reaches at equilibrium state compare to the other steps that this
process goes through. We note that the values of (∆S) gradually decrease with increasing
temperature in all concentration, This means that increasing the temperature at constant
concentration reduces the random of the adsorption system and that the food coloring dyes
molecules are more regular in the solution compare to their regularity on the surface of
activated carbon.
The (∆Sº, ∆S) values decreased with increasing the concentration of dyes in solution with the
range of temperatures used, this means decrease the random in the adsorption system,
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because of decreasing in the number of adsorbed molecules on the activated carbon surface,
this is clear from the efficiency of adsorption.
The same applies to the rest of the food coloring dyes (E122,102,124,1232).
2. Calculation of thermodynamic parameters using Langmuir and Freundlich constants
Langmuir and Freundlich constants can be used to calculate the thermodynamic parameters
for adsorption process of food coloring dyes on activated carbon surface as are suitable,
Therefore, Langmuir constant is considered as an equilibrium constant calculated at a range
of concentration, and the same range of temperature to calculate the equilibrium constant.
The apparent thermodynamic function calculated from Langmuir and Freundlich constants
were estimates in the some range of concentration and temperature used to calculate the
equilibrium constants from which the thermodynamic function were calculated where the
equilibrium constant represents the ratio between the amount of the dye adsorbed and remain
in the solution at different concentration, as well as, all results are tabulated in table (7) for
food coloring dyes. Also the Langmuir (kL) and Freundlich (kf) is calculated from the
following relationship:
kL = Qmax x b …………….13
Where :
Qmax = maximum capacity of adsorption for adsorbent substance
(mg of adsorbent substance/gm of the solid adsorbent substance)
b = Langmuir constant (L / mg), which have relation with adsorption energy.
The values of both Q and b were calculated from plotting the linear relationship between Ce
versus Ce/ qe
Q = 1 / slope, intercepts = 1/ (Q* b)
When using Vant- Hoff equation to express the Langmuir constant (kL) as an equilibrium
constant, we get the following relationship :
Ln kL = (-∆H / RT) + X ………….14
Where
X= constant.
When drawing the relation between ln kL versus 1 / T , we get a liner relationship through
which the enthalpy (∆H) value can be calculated from the slope of the straight line from
drawing the (ln kL) verse (1/T) as the following equation(14), then (∆Gº, ∆Sº∆S) values are
calculated after that(equation 3,10,11,12), and in the same way the thermodynamic
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parameters are calculated from Freundlich constants (kf), when it is considered as an
equilibrium constant and then the thermodynamic parameters are obtained for adsorption
process.
The value of enthalpy (∆H) is calculated from drawing the ln kf verse 1 / T from following
equation
Ln kf = (-∆H / RT) + X ………………15
The value of kf was calculated from the graph of the relationship between log qe versus log
Ce:
Intercepts = log kf
The apparent thermodynamic function calculated from Langmiar and Frendlich constant ,
were estaimeted in the same rang of concentration and temperature used to calculated the
equilibrium constants from which the thermodynamic function were calculated where the
equilibrium constant represents the ratio between the amount of the dye adsorbed and remain
in the solution at different concentration. All results are tabulated in table (7) for food
coloring dyes.
Table 7: Isotherm constants of the Langmuir and Freundlich models for adsorption of
food coloring (dyes at different temperatures and concentration.
Dye T kº kL R2 kf R
2
E110 298 4.174 0.9946 4.721 0.9553
303 5.665 0.9999 5.237 0.9474
308 7.078 0.9959 6.112 0.9117
313 8.354 0.9979 6.584 0.9500
318 10.474 0.9971 7.263 0.9343
E122 298 4.135 0.9316 5.030 0.9596
303 5.379 0.9864 5.554 0.9994
308 5.955 0.9866 5.583 0.9855
313 6.614 0.9903 6.193 0.9904
318 8.078 0.9855 6.775 0.9610
E102 298 4.792 0.9980 6.578 0.8029
303 6.337 0.9989 8.498 0.9124
308 7.005 0.9840 9.296 0.9594
313 8.329 0.9933 10.486 0.9970
318 14.627 0.9883 11.912 0.9614
E133 298 6.663 0.9987 3.929 0.9786
303 9.539 0.9368 5.332 0.9650
308 11.623 0.9891 5.936 0.9897
313 12.728 0.9789 6.179 0.9983
318 18.112 0.9860 7.327 0.9714
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E123 298 4.3459 0.9838 5.0720 0.9302
303 4.8662 0.9868 5.509 0.9359
308 5.6306 0.9893 6.0954 0.9428
313 7.1276 0.9914 7.0632 0.520
318 10.2145 0.9916 8.8288 0.9670
E124 298 0.5388 0.9828 0.7718 0.9907
303 0.6859 0.9728 1.0914 0.9997
308 1.0019 0.9521 1.8344 0.9984
313 1.5019 0.8786 3.2931 0.9427
318 7.4239 0.9984 7.6243 0.9709
E127 298 0.9160 0.9981 2.0526 0.9718
303 1.1122 0.9935 2.4592 0.9263
308 1.5659 0.9940 3.3643 0.9081
313 2.0864 0.9996 4.3782 0.9533
318 3.9420 0.9912 6.7251 0.9886
The results show that the values of (kL) increase with increase in temperature and this
confirms the increase in the transfer of food coloring molecules from the solution and their
overcoming of the effective forces of adsorption on the surface of activated carbon leading an
increase in the values of the adsorption efficiency. Looking at table(8):
Table 8: Langmuir thermodynamic parameters for the adsorption of food coloring dyes
at different temperatures.
∆S J/mole.k0
∆S0 J/mole.k
0 ∆G
0
kJ/mole
R2
∆H
kJ/mole
T k0
Dye
113.047 124.926 -3.540 0.9898 33.688 298 E110
111.182 125.597 -4.368 303
109.377 125.646 -5.011 308
107.629 125.281 -5.525 313
105.937 125.465 -6.210 318
68.084 80.218 -3.516 0.9621 20.289 298 E122
66.960 80.954 -4.240 303
65.873 80.705 -4.568 308
64.821 80.527 -4.916 313
63.802 81.170 -5.523 318
140.443 153.470 -3.882 0.9678 41.852 298 E102
138.125 153.472 -4.650 303
135.883 152.071 -4.986 308
133.712 151.339 -5.517 313
131.610 153.915 -7.093 318
115.909 131.681 -4.700 0.9604 34.541 298 E133
114.000 132.746 -5.681 303
112.146 132.539 -6.281 308
110.355 131.504 -6.620 313
108.619 132.704 -7.659 318
105.5820 117.7974 -3.6401 0.9380 31.4635 298 E124
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103.8400 116.9953 -3.9861 303
102.1542 116.5226 -4.4255 308
100.5224 116.8508 -5.1108 313
98.9418 118.2620 -6.1438 318
171.6896 166.5481 1.5322 0.9844 51.1635 298 E123
168.8564 165.7219 0.9498 303
166.3453 166.1310 -0.0049 308
163.4617 166.8438 -1.0586 313
160.8915 177.5586 -5.3001 318
178.9295 178.2001 0.2174 0.9455 53.3210 298 E127
175.9769 176.8610 -0.2679 303
173.1201 176.8486 -1.1484 308
170.3546 176.4691 -1.9138 313
167.6761 179.0803 -3.6265 318
It is observed that the enthalpy have a positive value which refers to endothermic adsorption
of food coloring dyes on surface of activated carbon including sorption process is receiving
energy from adsorption system and the physical adsorption predominate, the range of (∆H)
values for food coloring dyes are between (20-53) kJ/mole .Figures (2A,B) represent the
drawing of (ln kL) constant values verses (1/ T) from Vant-Hoff equation in terms of
Langmuir constants, which Give a good relationship with a high correlation coefficient we
also noticed a convergence in the values of (∆Gº) calculated from the equilibrium constant
for adsorption process and the Langmuir constants, and they are similar to each other in terms
of increasing its negative values with an increase in temperature when the concentration is
constant , when checking the positive (∆Sº) and (∆S) values ,we noticed that the (∆S) values
are less than (∆Sº) meaning that the adsorption process at the equilibrium state is less random
than the other adsorption steps .The values of (∆S) decrease with increase of the temperature
and this confirms the decrease in the randomness of the adsorption system , because the
thermal energy that the adsorption process receives reduces the associations and interactions
of the food coloring dye molecules on the solution and increase its degree of freedom and it is
more regular at equilibrium state.
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A B
Figures 2: The relationship between (ln kL) with (1/ T) for the adsorption of food
coloring dyes onto activated carbon at different temperatures.
While the positive values of (∆Sº) fluctuate with increase or decrease with close values very
stable and within the temperature range used at any step of adsorption process and its little
change indicates the physical nature of adsorption and also indicates that the adsorption dyes
molecules are less regular on the surface of activated carbon because they are not in specific
sites and pores.
The application of (Vant- Hoff) equation on Freundlich constant(kf) gave a linear relationship
with a high correlation coefficient to get the value of the enthalpy of the adsorption process
and then calculation other thermodynamic function (∆Gº, ∆Sº, ∆S), figures (3A,B,C):
A B
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C
Figures 3: The relationship between (ln kf) with (1/ T) for the adsorption of food
coloring dyes onto activated carbon at different temperatures.
This relationship show where the results included in the table (7) indicate that the values of
(kf) increase with increasing of temperature , so the adsorption efficiency increases .Positive
(∆H) value within the range of temperature used confirms that the adsorption process is
endothermic and physical in nature(table 9) :
Table 9: Freundlich thermodynamic parameters for the adsorption of food coloring
dyes at different temperatures.
∆S J/mole.k0
∆S0 J/mole.k
0 ∆G
0
kJ/mole
R2
∆H
kJ/mole
T k0
Dye
55.225 68.128 -3.845 0.9850 16.457 298 E110
54.313 68.083 -4.172 303
53.432 68.481 -4.635 308
52.578 68.249 -4.905 313
51.752 68.239 -5.243 318
35.785 49.211 -4.001 0.9582 10.664 298 E122
35.195 49.452 -4.320 303
34.623 48.922 -4.404 308
34.407 49.227 -4.744 313
33.534 49.440 -5.058 318
71.107 86.772 -4.668 0.9700 21.190 298 E102
69.934 87.726 -5.391 303
68.799 87.338 -5.710 308
67.700 87.236 -6.115 313
66.635 87.236 -6.551 318
70.883 82.255 -3.389 0.9147 21.123 298 E133
69.713 83.630 -4.217 303
68.581 83.390 -4.561 308
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67.486 83.968 -4.739 313
66.425 82.987 -5.267 318
68.554 82.0541 -4.0229 0.9515 20.4292 298 E124
67.423 81.610 -4.2986 303
66.329 81.356 -4.6286 308
65.269 81.522 -5.0872 313
64.243 82.351 -5.7584 318
242.2164 240.0629 0.6418 0.9849 72.1805 298 E123
238.2195 238.9466 -0.2203 303
234.3523 239.3965 -1.554 308
230.6086 240.5175 -3.1015 313
226.9830 243.8713 -5.3706 318
149.0047 154.9834 -1.7816 0.9737 44.4034 298 E127
146.5459 154.0271 -2.2668 303
144.1669 154.2536 -3.1067 308
141.8639 154.1407 -3.8426 313
139.6333 155.4785 -5.0388 318
The values of (∆Gº) are few and negative, and its negative value increases with increasing
temperature and this confirms an increase in the spontaneous adsorption of food coloring
dyes on the surface of activated carbon (table 9) .The values of (∆Sº) are positive and close to
other ,this confirms that the food coloring dyes molecules in the solution are move regular
than they are on the surface and at any step of the adsorption process, because of the increase
in the number of adsorbed molecules on the surface and a decrease in their number in the
solution, these results are consistent with the values of thermodynamic parameters calculated
from the values of the equilibrium constant (table 9). When comparing the values of the
apparent thermodynamic function obtained from the Langmuir and Freundlich constants
calculated at different concentrations and temperature with those calculated from the
equilibrium constant, we arrived to the following conclusion:
1) The (∆H) value calculated from Langmuir and Freundlich constants are positive too, the
(∆H) value calculated from Langmuir constant close to values of (∆H) calculated from
equilibrium constant at The lower concentration (dilute solution), this refers to agreement
between two methods, while the (∆H) calculated from Freundlich constant was less than
that. In away, it can be said that those is confirming that the adsorption process is
endothermic and physical in nature.
2) When comparing the (∆Gº, ∆Sº, ∆S) values of all food coloring dyes with those
calculated from the Langmuir and Freundlich constants, we notice in general a clear
agreement between them, despite, the presence some slight discrepancy in the values with
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Mohammed et al. World Journal of Pharmacy and Pharmaceutical Sciences
increase or decrease in some dyes, but it a achieves the desired purpose of calculating it,
as well as, being realistic to significant and reliable in describing the adsorption system.
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