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JOURNAL OF ENVIRONMENTAL PROTECTION SCIENCE (2009), Vol. 3, pp.11 – 22.
Utilization of Fly ash as Low-Cost Adsorbent for the Removal of
Methylene Blue, Malachite Green and Rhodamine B Dyes from Textile
Wastewater
Tabrez A. KHAN*, Imran ALI, Ved VATI SINGH and Sangeeta SHARMA
Department of Chemistry; Jamia Millia Islamia; Jamia Nagar, New Delhi-110025, India
Abstract
Fly ash was utilized as a potential low-cost adsorbent for the removal of methylene blue, malachite green and rhodamine B
from artificial textile wastewater. The adsorbent was characterized by its physico-chemical analyses, porosity, surface area,
ignition loss measurements and scanning electron micrograph. Adsorption studies were carried out in a batch process with
different concentrations of dyestuffs, pH, temperature and contact time. The removal of methylene blue, malachite green
and rhodamine B varied from 0.228 to 0.814, 0.219 to 0.644, and 0.184 to 0.618 mgg-1 respectively when the initial dye
concentration was raised from 5 to 20 mgL-1. The amount of dye adsorbed (mgg-1) was found to increase with increase in
the contact time; with 80 minutes for malachite green and rhodamine B and 100 minutes for methylene blue. The
equilibrium data closely followed both Langmuir and Freundlich isotherms, but the latter isotherm fitted the data better.
The changes in standard free energy (∆G°), standard entropy (∆S°) and standard enthalpy (∆H°) were calculated. The
adsorption of all the dyes onto fly ash was found to be physical and exothermic in nature.
Keywords: Methylene Blue; Malachite Green; Rhodamine B; Fly ash; Adsorption, Langmuir and Freundlich Isotherms.
JEPS (2009), Vol. 3, pp. 11 – 22.
__________________________________________________________________________________
1. Introduction
The discharge of colored wastewater from paper and pulp,
textile and dyeing, leather, printing industries and food
process industries is posing a serious environmental
concern due to their poor biodegradability, carcinogenicity
and toxicity [1-4]. Similarly, the disposal of fly ash, which
is a voluminous by-product of coal burning power plants,
is posing a major problem as per its storage space and the
cost involved. The fly ash is abundantly available in India
and according to one estimate about 125 million tons of fly
ash is generated per annum and it is expected to increase to
about 200 million tons in the near future [5] At present
about 2 % of the total fly ash is gainfully utilized [5].
Adsorption [6,7] has emerged as an effective method for
the removal of many aqueous contaminants. Activated
carbon, due to its large surface area and the presence of
many different types of surface functional groups, is a very
effective adsorbent. However, the high cost of activated
carbon has led to the development of new cost-effective
adsorbents with similar adsorption characteristics.
* To whom all correspondence should be addressed: Tel: + 011-
26985938; Fax: + 11-26985507; E-mail: takhan501@yahoo.com;
drimran_ali@yahoo.com
A number of low-cost adsorbents such as activated carbon
prepared from various wastes [8-15], diatomaceous earth
[16], industrial waste products [17-19], bagasse fly ash
[20], clay mineral [21], biodegradable waste [22],
hydrotalcite [23], coffee grounds [24], dusts [25-27],
kudzu [28], ‘waste’ metal hydroxide sludge [29],
agricultural waste [30], dolomitic sorbents [31], charcoal
from extracted residue of coffee beans [32], bentonite and
polyaluminum hydroxide [33] have been studied for
adsorption of different dyes from solutions. Fly ash,
containing about 2-5 % un-burn carbon, has been reported
to adsorb metals [34-37] and dyes [38-41] from aqueous
solutions. In the present paper, reports are presented on the
removal of the most commonly used coloring dyes (i.e.
three basic dyestuffs methylene blue, malachite green and
rhodamine B, Fig. 1) from artificial textile wastewater
using fly ash as low-cost adsorbent. These dyes are chosen
for the present study because they are the brightest class of
soluble dyes used in the textile industry [42].
Figure 1. Chemical structure of methylene blue, malachite
green and rhodamine B.
2. Experiments
2.1 Materials and Methods
Fly ash, obtained from Indraprastha Thermal Power
Station, New Delhi, India. 10.0 g of fly ash was washed
with distilled water (50 mL) five times, dried in an oven at
110 °C for 24 h, sieved to desired particle sizes (75, 150
and 300 µm) and finally stored in vacuum desiccators.
Stock solutions of the dyes were prepared in double
distilled water. The chemical analysis of fly ash was done
following standard methods [43,44]. Surface area was
determined by N2-BET method [45] employing a
Quantasorb surface analyzer, QS/7. Porosity was measured
by mercury intrusion porosimeter, Micrometric model
9310 [46] while the mean particle diameter was
determined [46] with the Laser particle size analyzer,
Malvern 36000. Absorbance measurements were made
with a Jasco-7800 UV-VIS spectrophotometer at
corresponding wavelength for maximum absorbance (λmax)
665 nm for methylene blue, 615 nm for malachite green
and 555 nm for rhodamine B. The pH measurements were
made using a pH-meter. The scanning electron micrograph
was obtained with a scanning electron microscope (Model
JEOL JSM 840).
2.2 Adsorption Studies
Batch experiments were carried out using a series of
Erlenmeyer flask of 50 mL capacity covered with
aluminum foil to prevent the introduction of any foreign
particle contamination. The effect of pH, concentration,
dose, temperature and shaking time was studied. Isotherms
were run by taking selected different concentrations of
methylene blue, malachite green and rhodamine B at
desired temperatures (20, 30 and 40 ºC) and pH. After the
required experimentation, the solutions were filtered and
the concentrations of methylene blue, malachite green and
rhodamine B were determined in filtrate using a UV-
visible spectrometer. For kinetic studies the batch
technique was used due to its simplicity. A series of
Erlenmeyer flasks of 50 mL capacity containing a defined
volume of solutions of methylene blue, malachite green
and rhodamine B of known concentrations were kept in a
thermostatic shaking water bath. After attaining the desired
temperature (30 ºC), a known amount of the adsorbent was
added to each flask and the flasks were allowed to agitate
mechanically. At given time intervals, the solutions were
filtered and the supernatants were analyzed for methylene
blue, malachite green and rhodamine B as mentioned
above.
One gram of fly ash was maintained in contact with 50 mL
dye solution (initial concentration; 5, 10, 15, 20, 25 mg L-
1) in an Erlenmeyer flask and was shaken in a thermostatic
water bath (120 cycle/min). After the different contact
times, the solution was filtered on a Whatman filter paper
No. 42. The residual dye concentration in each solution
was measured spectrophotometrically at the corresponding
λmax (665, 615 and 555 nm for methylene blue, malachite
green and rhodamine B, respectively). The original pH of
the dye solution was adjusted to the desired value by
adding required quantities of decimolar solutions of
sulphuric acid or sodium hydroxide. Effects of initial dye
concentration, particle size, and pH at different agitation
time were also studied following the same experimental
procedures. In order to eliminate error due to adsorption of
dyes on filter paper; a parallel control set (without fly ash)
was run in an identical manner. The percentage removal of
dye was calculated on the basis of color of wastewater in
the control set.
Figure 2. Scanning Electron Micrograph of the fly ash
sample
2.2.1 Adsorption Isotherms
The adsorption equilibrium models often provide insight
into the sorption mechanism, surface properties and
affinity of adsorbent. The most commonly used
equilibrium models are Langmuir and Freundlich
isotherms [47]. The fractional coverage, θ, on an adsorbent
surface at constant temperature is given by Langmuir
isotherm. Langmuir isotherm is based on the assumption of
12
uniform energy of adsorption on the surface of the
adsorbent. The total monolayer capacity of the adsorbent is
equal to Q°, a Langmuir constant. The rearranged
Langmuir isotherm is represented by following equation:
Ce/qe = 1/ Q° b + Ce/Q° (1)
where, Ce is the equilibrium concentration of dye (mg L-1),
qe is the amount of dye adsorbed at equilibrium (mgg-1), Q°
is the monolayer adsorption capacity (mgg-1), and b is the
constant related to the free energy of adsorption. Hence, a
plot of Ce/qe versus Ce yields a straight line with Q°
calculated from the slope and the value of b as its
intercept.
Freundlich isotherm is an exponential equation and can
be written as:
qe = KF Ce1/n
log qe = log KF + 1/n log Ce (2)
where, KF is the constant indicative of the relative
adsorption capacity of the adsorbent (mg g-1), and 1/n is
the constant indicative of the intensity of the adsorption.
The Freundlich equation possesses two constant, KF and
1/n. High and low values of KF and 1/n indicate high
adsorption throughout the concentration range studied
whereas high values of 1/n and low values of KF show low
adsorption. When 1/n = 1, the adsorption is favorable.
2.2.2 Adsorption Thermodynamics
The adsorption at any interface between two phases can be
regarded as an equilibrium process, the point of
equilibrium being dictated by the relative energies of the
adsorbate in the two phases. These energy values can be
defined in terms of thermodynamic parameters such as
change in free energy (∆G°), enthalpy (∆H°), and entropy
(∆S°). The feasibility of the removal process is often
evaluated by determining these thermodynamic parameters
using the following equations:
∆G° = -RT ln b (3)
∆H° = R(T2 T1) / (T2 – T1) ln (b1 / b2), (4)
∆S° = (∆H° - ∆G°) / T (5)
where b, b1, and b2 are the Langmuir constants at
temperature T, T1 and T2, respectively.
2.2.3 Adsorption Kinetics
The study of kinetics of adsorption describes the solute
uptake rate at the solid-solution interface. The rate constant
of adsorption of dyes on to fly ash, Kad, has been studied
using the Lagergren first order rate equation:
log (qe – qt) = log qe - Kad t / 2.303 (6)
where, qe is the amount of dye adsorbed at equilibrium,
and qt is the amount of dye adsorbed at time t (both in
mgg-1).
3. Results and Discussion
3.1 Characterization of the Adsorbent
The chemical analyses indicated that the major constituent
of the fly ash was SiO2, (60.10 %) followed by Al2O3
(18.60 %) and Fe2O3 (6.40 %). Other constituents included
CaO (6.30 %) and MgO (3.60 %). The adsorbent had
surface area of 40.16 m2 g-1; porosity, 0.43 cm3 g-1; bulk
density, 3.51 g cm-3; and showed an ignition loss of 4.90
%. The scanning electron micrograph at 1000 x
magnification (Fig. 2) of fly ash (75 µm) shows typical fly
ash morphology and surface texture. The adsorbent
consisted mainly of solid spheres of a wide range of sizes
with the zero charge of the fly ash being 5.8. Some
adsorbents such as fly ash and bottom ash contain 2-15 %
un-burnt carbon, which may attach organic functional
groups containing oxygen. On contact with the aqueous
medium, these oxides form surface hydroxyl compounds
and are amphoteric in nature. The overall interaction of
metal oxides with water may be described according to
Ahmed [48]. The charged interface thus formed interacts
with the charged aqueous pollutant species of the
wastewater. Electrical charge on the interface is also
determined by zero point charge (pHzpc), of the adsorbent
species. It is understood that below pHzpc, the adsorbent
acquires positive charge and, above it, the surface of the
adsorbent remains negatively charged. Weber and Morris
[49] correlated molecular weight of substances with
capacity of adsorption and concluded that by increasing
the molecular weight the capacity was significantly
increased. It is noteworthy that the rate dependence on
molecular size can be generalized only within a particular
chemical class of molecules. For example, large
molecules of one chemical class of compounds may adsorb
more rapidly than smaller ones of another class. Further,
this rate dependence on size is applicable for rapidly
agitated batch reactors.
3.2. Effect of Initial Concentrations
To study the effect of different concentrations of dyes on
adsorption behavior three concentrations (5, 10 and 20 mg
L-1) were used and the amounts adsorbed were calculated
and given in Table 1 and plotted in Fig. 3. Table 1 and Fig.
3 indicate that the amount of dye adsorbed is increased
from 0.228 to 0.814 mgg-1, 0.219 to 0.644 mgg-1 and
0.184 to 0.618 mgg-1 when the initial concentration was
increased from 5 to 20 mg L-1, for methylene blue,
malachite green and rhodamine B, respectively. The
observed increase in the adsorption of dyestuffs with
increasing concentration may be due to sufficient
adsorption sites at adsorbent [50].
13
Table 1. Amount (mg g-1) and percentage of dyes
adsorbed at 30±1 oC
3.3 Effect of Contact Time
The adsorption experiments were also carried out at
different time intervals (from 15-200 minutes) and the
results of these findings are plotted in Fig. 4. The effect of
contact time was also studied at different values of pH (pH
3, 5, 7 and 9). The uptake of adsorbate species was rapid in
the initial stages of the contact period and became slow
near the equilibrium. In between these two stages of the
uptake, the rate of adsorption was found to be nearly
constant. This result is expected because a large number of
surface sites are available for adsorption at the initial
stages and after a lapse of time, the remaining surface sites
are difficult to occupy because of repulsion between the
solute molecules of the solid and bulk phases [50]. The
structure of dyes indicates the presence of secondary and
tertiary amines, carboxylic group, and oxygen and sulphur
atoms and adsorbents contains silica, iron and calcium
oxides. Therefore, adsorption may be due to hydrogen
bonding, van der Waal forces, and others. Examination of
dye structure indicates that malachite green should absorb
strongly but methylene blue adsorbed the most. This
behavior may be explained on the basis of steric effect in
malachite green and rhodamine B dyes versus methylene
blue that has no steric effect
3.4 Effect of pH
The effect of pH and contact time on removal of dyestuffs
is shown in Fig. 5 and it is evident from this figure that
maximum adsorption of malachite green is at pH value of
7.0 while the adsorption of methylene blue and rhodamine
B was maximum at high pHs (7-9). But due to pH of
natural water being in the range of 7 to 8, 7.5 was
considered as the optimum pH for maximum adsorption of
these dyes. The effect of variation of pH can be seen in
Fig. 4 and it is clear from this figure that adsorption of
dyes increases from 0.426 to 0.467, 0.232 to 0.394 and
0.286 to 0.367 mg g-1 for methylene blue, malachite green
and rhodamine B, respectively as the pH is increased from
3 to 9. It appears that silica and alumina, which are chief
constituents of fly ash, form metal-hydroxide complexes in
solution and the subsequent acidic or basic dissociation of
these complexes at the solid-solution interface leads to
either positive or negative surface charge [51]. At acidic
pH, the dissociation of the metal-hydroxide complexes
causes the surface to become positively charged. However,
with increasing pH, the surface becomes negatively
charged as in the alkaline medium the silica and alumina
get converted into SiO2- and Al2O3- type of functional sites
and, therefore, the binding of positively charged dyes onto
these surfaces become much favorable resulting in
enhanced adsorption of dyes [38,52].The variation of
adsorption with pH can be explained by considering the
difference in the structure of the dyes, as well as the point
of zero charge of the fly ash (which is 5.8). The main
constituents of fly ash are silica and alumina. The ZPC (a
concept; related to the adsorption process; describes the
condition when the electrical charge density on a surface is
zero) of silica is 2.3, while that of alumina is 8.2, and as
such the surface of fly ash would have high positive charge
density below pH value of 5.8, i.e. ZPC of the fly ash
[48]. Under these conditions the uptake of positively
charged dyes would be low; with increasing pH, the
negative-charge density on the surface increases resulting
in enhanced removal.
Concn. of
dyes
Methylene blue Malachite green Rhodamine B
(mgL-1) (mg g-1) (%) (mg g-1) (%) (mgg-1) (%)
5.0 0.228 91.20 0.219 87.60 0.184 73.64
10.0 0.434 86.80 0.387 77.38 0.334 66.80
20.0 0.814 81.38 0.644 64.42 0.618 6178
3.5 Effect of Particle Size
The relationship between the amounts of dye adsorbed at
75, 150 and 300 µm particle sizes is shown in Fig. 6,
which shows that the adsorption capacity increases with
decreasing particle size of the adsorbent. This could be due
to substantial increase in the surface area for small particle
[53,54]. Adsorption capacity at 300 µm is very low and
for particle sizes between 75 and 150 µm, adsorption
capacity is 12.4, 15.8 and 12.6 percent higher than that at
75 µm for methylene blue, malachite green and rhodamine
B respectively. Therefore, 75 µm is considered as optimum
particle size.
3.6 Adsorption Dynamics
The adsorption of dyes onto the fly ash at different time
intervals is depicted in Fig. 4. The adsorption of dye
increased with lapse of time and gradually attained
equilibrium at 80 minutes. The adsorption rate constant,
Kad at 20, 30 and 40 oC, is calculated from the slope of the
linear plots of log (qe - q) vs. t (Fig. 7), based on Lagergren
first order rate equation 6 [55] The adsorption rate
constants were determined from the slopes of the plots and
were found to be 6.45, 5.01 and 4.51 ×102 min-1 for
methylene blue, 4.70, 4.42 and 4.23 ×102min-1 for
malachite green and 4.79, 4.65 and 4.61 × 102 min-1 for
rhodamine B at 20, 30 and 40 °C respectively (Table 2).
The plots were found to be linear with significant
regression coefficients in the range 0.923-0.975, indicating
that Lagergren’s equation is applicable to the dye
adsorption process with first order process.
3.6.1 Adsorption Isotherms
Both Langmuir and Freundlich isotherm models have been
employed to evaluate the adsorption data for methylene
14
Figure 3. Effect of concentration on the adsorption of dyes,
(Temp. 30ºC, Particle size 75 µm)
Table 2. The values of Langmuir constants
Adsorbate
(Dyes) Temp.
(oC) QoB r2RL
Kad x
102/min-
1
20 4.2793 0.0123 0.9758 0.0242 6.45
30 2.9434 0.0183 0.9230 0.0770 5.01
Methylene
blue 40 2.6868 0.0197 0.9251 0.0749 4.51
20 1.5805 0.0358 0.9661 0.0339 4.70
30 1.3717 0.0407 0.9694 0.0306 4.42
Malachite
green 40 1.2290 0.0455 0.9669 0.0334 4.23
20 2.3257 0.0190 0.9230 0.0770 4.79
30 1.8706 0.0231 0.9492 0.0508 4.65
Rhodamine
B 40 1.3045 0.0317 0.9461 0.0539 4.61
blue, malachite green and rhodamine B. Langmuir
isotherm is based on the assumption of uniform adsorption
energy throughout the surface of the adsorbents. When
Figure 4. Effect of contact time at different pHs
(Temp. 30ºC, Particle size 75 µm, Conc. 10 mgL-1)
adsorption is in accordance with the Langmuir equation,
the total monolayer capacity of the adsorbent is equal to
Qo, a Langmuir constant. When the amount adsorbed at
equilibrium is quite small, the equilibrium concentration
shows a linear relationship with the amount of adsorption
at equilibrium. The experimental data on the uptake of
dyestuffs at 20, 30 and 40º C have been fitted in the
rearranged Langmuir equation (equation no. 1).
Adsorption is in accordance with the Langmuir equation,
the total monolayer capacity of the adsorbent is equal to
the plot of Ce/q versus Ce at different temperature is linear
(Fig. 8). This suggests the applicability of the Langmuir
adsorption model, and is indicative of monolayer coverage
of the adsorbate at the outer surface of the adsorbent. The
values of Qo and b, at different temperatures, determined
15
from the slopes and intercepts of the respective plots, and
the
Table 3. The values of Freundlich constants
Adsorbate
(Dyes) Temperature,
(oC) KF1/n r2
20 0.0619 0.8703 0.9969
30 0.0655 0.8277 0.9894
Methylene
blue 40 0.0650 0.8183 0.9873
20 0.0777 0.7103 0.9812
30 0.0797 0.6777 0.9824
Malachite
green 40 0.0796 0.6626 0.9720
20 0.0552 0.8150 0.9906
30 0.0556 0.7856 0.9892
Rhodamine
B 40 0.0575 0.7214 0.9855
regression coefficients, r2 are summarized in Table 2. The
values of Qo (i.e. maximum uptake) decrease with increase
in temperature, thereby confirming that the process is
exothermic. The dimensionless separation factor or
equilibrium parameter [38], RL (RL = 1/1+bCi) at different
temperature is calculated from the Langmuir isotherms.
The values of RL lie between zero and one, suggesting that
the adsorption process is favorable [56,57]. Freundlich,
isotherm has been widely used for determining the
adsorption capacity of different unconventional adsorbents
The equilibrium adsorption data at different dye
concentrations are fitted in the linear form of Freundlich
isotherm model (equation no. 2). The plots of log qe
against log Ce, shown in Fig. 9 are linear and the values of
KF and 1/n, calculated from intercept and slope of the plot
(Figure 9), respectively, are given in Table 3. The
calculated values of 1/n are less than 1, which suggest the
favorable adsorption of dyestuffs onto the fly ash [58]. The
values of the regression coefficients at different
temperatures indicate that the data satisfactorily follow
both Langmuir and Freundlich models but the Freundlich
isotherm fits the experimental data better. The values of Qo
and KF are observed to be higher for methylene blue and
malachite green dyes, respectively.
3.6.2 Thermodynamic Parameters
The amount of dye adsorbed decreased from 0.234 to
0.221 mg g-1 (methylene blue), 0.223 to 0.209 mg g-1
(malachite green) and 0.193 to 0.173 mg g-1 (rhodamine
B) with rise in temperature from 20 to 40 oC and at 5.0 mg
L-1 concentration, suggesting the exothermic nature of the
adsorption process. It has been reported by several authors
[38, 59, 60] that the chemical potential of the adsorbates
are the main controlling factor in the adsorption process. If
the solubility of adsorbates increases with an increase in
temperature, the chemical potential is decreased, thereby
causing a decrease in adsorption. The steady decrease in
Qo values with increase in temperature indicates that the
adsorption is governed by the same factor.
Thermodynamic data for the adsorption of methylene blue,
malachite green and rhodamine B onto fly ash are
summarized in Table 4. The change in standard free
energy (∆G°), standard enthalpy (∆Ho) and standard
entropy (∆S°) are calculated using equations 3, 4 and 5.
The negative values of ∆G° show the spontaneous nature
of the adsorption process while the small negative values
of ∆Ho indicate adsorption process physical in nature. The
positive values of ∆S° suggest favorable affinity of the
adsorbent for the dyes.
Table 4. The values of thermodynamic parameters
Adsorbate
(Dyes) Temperature,
(oC) -∆Go
(kJmol-1) -∆Ho
(kJmol-1) ∆So
(kJmol-1)
20 10.719 29.354 63.57
30 10.083 5.818 14.07
Methylene
blue 40 10.224 59.778 18.76
20 8.115 9.478 4.65
30 8.069 8.799 2.41
Malachite
green 40 8.045 5.273 16.58
20 9.65 14.43 16.30
30 9.49 24.97 51.05
Rhodamine
B 40 8.98 55.775 17.52
3.7 Mechanism of Adsorption
The variation in the adsorption of cations may be
explained on the basis of the surface hydroxylation of
oxides at the solid-solution interface. It is established that
hydroxylated oxides surfaces are influenced by minute
concentrations of potential determination ions, H+ and OH-
and develops positive and negative charges at the surface.
The acid-base dissociation of solid surface may be
represented in the following manner [59,61].
M2+ H+
OH- M+
OH
H+
OH-MOH
OH OH-
H+ M O-
OH
OH-
H+MO-
O-
(Basic Dissociation) (Acidic Dissociation)
where M denotes Si, Ca, Fe etc. It is evident from the
above equilibrium that with a decrease in the pH of the
solution, the positive charge density on the surface
increases and, hence, the adsorption of cations decreases.
A decrease in pH lowers the dissociation of acid and a high
pH produces negatively charged surfaces which would
favor the removal of cations from the bulk. The increase in
adsorption with pH is attributed due to the development of
negative charges on the surface of different adsorbents.
Thus, it is obvious that the adsorption of the cationic dyes
16
would be maximal at high (alkaline) pH ranges; due to
coulomb attraction [38]. The surface charge of the major
constituents of the adsorbents used plays an important role
in the removal of methylene blue, malachite green and
rhodamine B. The polar functional groups of the
adsorbents are involved in the formation of bonds with
cationic dyes. The observed decrease in the removal of
dyes at lower pH is apparently due to the higher
concentration of H+ ions present in the reaction mixture,
which compete with dye cations for the adsorption sites
[62,63]. The determining ions are H+ and OH-. Generally,
the solid surfaces adsorb anions favorably at low pH due to
association of H+ ions, whereas cations are adsorbed at
high pH due to deposition of OH- ions on the adsorbent
surface. The mechanism of adsorption may be described
as:
S H
A
S + H+ + A-
and
S OH
A
S + OH- + A+
where S and A denote the surface of the adsorbent and
adsorbate, respectively [64,65]. Oxides of metal and non-
metals are the main constituents of many unconventional
adsorbents like clays, minerals, soils, fly ash, furnace slag,
etc. Such oxides are first hydroxylated in contact with
water and then develop either positive or negative charges
on the interface, according to pH of the solution as
follows:
MO-
OH
OH-
-H2OMOH
OH M+
OH
H+
-H2O
In such systems, charged species of the aqueous phase are
withheld on the surface of the adsorbent by columbic
forces.
3.8 Validation of the Method
The developed adsorption method was validated by
carrying out seven sets (n = 7) of the experiments. The
regression analysis was carried out using Microsoft Excel
program. The values of standard deviation (SD) and
correlation coefficients (R2) ranged from ±0.10 to ±0.50
and 0.9997 to 0.9999 respectively. In addition, the
confidence levels ranged from 99.5 to 99.8 for all the
experiments.
Figure 5. Adsorption versus pH (Temp. 30 ºC, Particle
size 75 µm, Conc. 10 mgL-1).
17
Figure 6. Effect of particle size on adsorption of dyes
(Temp. 30ºC, Particle size 75 µm, Conc. 10 mgL-1)
Figure 7. Lagergren plots for the adsorption of dyes at different
temperatures (Temp. 30ºC, Particle size 75 µm, Conc. 10 mgL-1)
18
Rhodamine B
0.01
0.02
0.03
0.04
0.05
0102030
Ce
Ce/qex 10-3
40
Figure 8. Langmuir plots for the adsorption of dyes at different
temperatures (Temp. 30º C, Particle size 75 µm)
4. Conclusions
The results of the present sorption studies indicate that the
maximum adsorption of malachite green and rhodamine B
was obtained at 5.0 mg L-1, 80 minutes, 1.0 g L-1, 7.5, 75
µm and 30 ºC initial concentration, contact time, dose, pH,
particle size and temperature respectively. On the other
hand, the maximum adsorption of methylene blue was
Figure 9. Freundlich plots for the adsorption of dyes at different
temperatures (Temp. 30º C, Particle size 75 µm)
observed at 100 minutes contact times with other similar
conditions as in case of malachite green and rhodamine B
dyes. The adsorption process was of first order; physical
and exothermic in nature. The adsorption data was
analyzed by Langmuir and Freundlich models and fitted
well; slightly better fitted with Freundlich adsorption
isotherms; indicating the appropriateness of the
experiments. The fitness of Langmuir’s model indicated
the formation of monolayer coverage of the adsorbate on
19
the outer surface of the adsorbent. The fly ash adsorbent
was capable of adsorbing basic dyes with high affinity and
capacity indicating its potential as a low cost alternative
adsorbent. The negative values of ∆Go and ∆Ho indicated
that adsorption was a spontaneous and exothermic process.
The developed adsorption system is useful and can be used
for the removal of the reported dyes from contaminated
water.
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