INTRODUCTION
Colored compounds are the most easily recognizable pollutants in
the environment. Most industries use dyes and pigments to color their
color products. Discharge of dye-bearing wastewater into natural streams
and rivers from textile, paper, carpet and printing industries have a
severe problem since dyes impart toxicity to aquatic life and are damaging
the aesthetic nature of the environment. Generally, this discharge is
directed to the nearest water sources such as rivers, lakes and seas.
Textile dyeing process is an important source of contamination responsible
for the continuous pollution of the environment. The volume of wastewater
containing processed textile dyes is on steady increase. Over 7.0x105
tones and approximately 10000 different types of dyes and pigments are
produced world wide annually (Pearce et al., 2003; McMullan et
al., 2001). Many of the dyes used in industry are stable to light
and oxidation, as well as resistant to aerobic digestion. The colored
wastewater damages the aesthetic nature of water and reduces the light
penetration through the water`s surface and photosynthetic activity of
aquatic organisms due to the presence of metals, chlorides, etc., in them
(O`Neill et al., 1999; Robinson et al., 2001). Basic dyes
known as the brightest class of soluble dyes are used by the textile industries
such as acrylic, nylon, silk and wool dyeing. Their tinctorial value is
very high; less than 1 ppm of the dye produces obvious coloration. Basic
dyes can also cause allergic dermatitis, skin irritation, cancer and mutations.
Although biological treatment processes effectively remove BOD, COD and
suspended solids they are largely ineffective in removing color from wastewater.
Therefore it is necessary to remove the dye pollutions (Banat et al.,
1996; Robinson et al., 2001).
Phenolic compounds are also considered to be the major and undesirable
pollutions in wastewaters apart from basic dyes. Phenol derivatives are
formed in the course of many industrial processes worldwide, for instance
in the paper, petroleum and pesticide industries and are present in many
manufactured goods, such as plastics, drugs, antioxidants and dyes. Hence,
they frequently occur in the environment as a result of contamination
from a variety of sources. Nitrophenols belong to major phenolic pollutants
that have been analyzed in the environment. Nitrophenols, coming from
pesticide degradation products, car exhaust and industrial waters are
listed as priority pollutions by the US Environmental Protection Agency
(Guo et al., 2004). They have great potential toxicities of carcinogenesis,
teratogenesis and mutagenesis. Because of their detriment and vast scale
distribution in the ecological environment, their separation and determination
have been become one of the important studies of environmental analysis.
Phenol is considered to have toxic effects on human health even when present
in small concentrations. The ingestion of such contaminated water in the
human body causes protein degeneration and tissue erosion of the central
nervous system and also damages the kidney, liver and pancreas. Therefore
removal, destruction or modification to less noxious structures of phenolic
compounds is essential for purification of wastewaters as well as raw
water (Caturla et al., 1988; Derylo-Marczewska and Marczewski,
2000).
The various treatment methods for the removal of color and dye are coagulation
using alum, lime, ferric sulfate, ferric chloride, chemical oxidation
using chlorine and ozone, membrane separation processes, adsorption and
so on. The removal of dyes from effluent in an economic fashion remains
a major problem although a number of successful systems have been designed
recently. Among the treatments options, adsorption has become one of the
most effective and comparable low cost method for the decolourization
of textile wastewater. Many types of adsorbents including natural materials,
biosorbents and waste materials from industry and agriculture have been
proposed by several workers. Some of the reported sorbents include clay
materials (bentonite, kaolinite), zeolites, siliceous material (silica
beads, alunite, perlite), agricultural wastes (bagasse pith, maize cob,
rice husk, coconut shell) industrial waste products, biosorbents (chitosan,
peat) fly ash and others (starch, cyclodextrin) (Allen et al.,
1994; Annadurai and Krishnan, 1997; Annadurai et al., 1999; Bagane
and Guiza, 2000; Banks and Parinson, 1992; Chiou et al., 2004;
Chou et al., 2001; Crini and Morcellet, 2002).
Removal of organic molecules including dyes and phenolic compounds from
solutions is widely achieved by adsorption onto activated carbon in powder
or granular form. Activated carbon which is currently the most widely
used adsorbent for wastewater treatment is a structurally homogenous material
of high surface area, has microporous structure and show radiation stability
and as a consequence, showing a high efficiency for the adsorption of
bigger size compounds such as dyes (Lin, 1993). It is therefore widely
used in various industrial processes as adsorbent, catalyst or catalyst
support. The adsorption capacity of activated carbon depends on various
factors such as surface area, pore size distribution and surface functional
groups of adsorbent; polarity, solubility and molecule size of adsorbate;
solution pH and the presence of other ions in solution etc. (Crini, 2006;
Shawabkeh and Abu-Nameh, 2007).
This study reports the results of the adsorption of Safranin-O, phenol and
three nitrophenols i.e., m-nitrophenol, p-nitrophenol and o-nitrophenol
from aqueous solutions on activated carbon. Chemical structures and characteristics
of Safranin-O, phenol, m-nitrophenol, p-nitrophenol and o-nitrophenol
are shown in Scheme 1, respectively (Vidic et al.,
1997).
The main goal of this study is to compare of adsorption of Safranin-O, phenol
and nitrophenols i.e., m-nitrophenol, p-nitrophenol and o-nitrophenol
on activated carbon in terms of their molecular differences such as size, shape,
polarity and solubility in water. The adsorption behaviour of Safranin-O and
phenols were performed as a function of initial dye and phenol concentrations
on activated carbon under kinetic and equilibrium conditions. The effect of
temperature were also discussed the efficiency in Safranin-O and phenolic compounds
removal from aqueous solutions.
MATERIALS AND METHODS
Reagent grade phenol, m-nitrophenol, p-nitrophenol
and o-nitrophnenol (all from Merck) were of the highest purity available
and used without further purification. Safranin-O was obtained from Fluka
and its purity is higher than 99% were provided by Sigma Activated carbon
was purchased from VWR PROLABO BDH. Chemical and physical characteristics
of activated carbon were done by Turkish Scientific and Technical Research
Center. The specific surface area, cross sectional area, bulk density
and the porosity of the adsorbent were 1014.44 m2 g-1,
16.2 Å2, 0.63 g cm-3 and 0.4, respectively
with the elemental analysis of the adsorbent quoted at C: 86.5%, H: 1.09%
and N < 1% (w/w).
The effects of important parameters such as initial concentrations of
Safranin-O and phenols and temperature on the adsorptive removal of Safranin-O
and phenols were studied by batch experiments. In each kinetic experiment,
a known quantity of adsorbent contacted with an adsorbate solution in
a 250 mL flask at desired temperature was shaken in a thermostat rotary
shaker at constant agitation speed (250 rpm) for a given time intervals.
At various time intervals the flasks were successfully removed, the liquid
was separated from the solid by centrifugation and the remaining concentration
of dye and phenols in solution were measured spectrophotometrically on
a UV-1700 Shimadzu at a wavelengths of 520 nm for SO and 270, 317, 274,
279 nm for phenol, p-nitrophenol, m-nitrophenol and o-nitrophenol,
respectively. Equilibrium adsorption isotherms for Safranin-O and phenols
were undertaken at 25±1°C. The adsorption behaviors of the
Safranin-O and phenols on the activated carbon were studied at four temperatures
(298, 303, 313 and 323 K). The adsorption equilibrium isotherm experiments
were repeated in duplicate and the average values are reported.
The concentration retained in the adsorbent phase (qe, mg g-1)
was calculated by using the following equation;
where, C0 is the initial dye concentration and Ct
is concentration of the dye and phenols (mg L-1) at any time
(t). V is the volume of solution (L) and Ws is the mass
of activated carbon (g).
RESULTS AND DISCUSSION
It is seen that in all the concentrations studied of Safranin-O and Phenol
while the concentration of Safranin-O decreased with time up to 150min
the concentration of Phenol decreased with time up to 100 min and then
the curves become flattened (Fig. 1a, b).
It is also seen that the equilibrium time is independent of solution concentration.
The same behaviours were also seen for p-nitrophenol, m-nitrophenol
and o-nitrophenol. Based on these findings, the shaking time was
used in all further experiments. The amount adsorbed, qe (mgg-1)
is plotted against the equilibrium concentration, Cs (mg L-1)
to obtain the adsorption isotherms. Adsorption isotherm data of Safranin-O
and phenols are shown in Fig. 2a and b.
The shape of the isotherms indicates L behavior according to Giles and
| |
| Fig. 1a: |
Variations in the initial concentrations
of Safranin-O with time |
| |
| Fig. 1b: |
Variations in the initial concentrations of phenol with time |
| |
| Fig. 2a: |
The adsorption isotherms of Safranin-O onto activated carbon
at 298 K |
Smith classification (Giles et al., 1974). Type L suggests that
the aromatic rings adsorbs parallel to the surface and no strong competition
exists between the adsorbate and the solvent to occupy the adsorption
sites.
The correlation of the experimental adsorption data with a number of
adsorption models was undertaken to gain an understanding of the adsorption
behavior and the heterogeneity of the adsorbent surface. The linearized
forms of Langmuir (1918), Freundlich (1906) and Temkin (1941) isotherm
equations can be represented Eq. 2-4
as follows:
| |
| Fig. 2b: |
The adsorption isotherms of Phenol and Nitrophenols onto activated
carbon at 298 K |
where, qe is the amount of adsorbate adsorbed per unit mass
of adsorbent at equilibrium in mg g-1; Ce the final
concentration at equilibrium in mg L-1; qmax the
maximum adsorption at monolayer coverage in mg g-1; b the adsorption
equilibrium constant related to the energy of adsorption in L mg-1;
KF a Freundlich constant representing the adsorption capacity
in (mg g-1) (L mg-1)1/n; n a constant
depicting the adsorption intensity; k1 the Temkin isotherm
energy constant in L mg-1 and k2 the Temkin isotherm
constant. The main difference between these three isotherm models is in
the variation of heat of adsorption with the surface coverage. Langmuir
model assumes uniformity, Freundlich model assumes logarithmic decrease
and Temkin model assumes linear decrease in heat of adsorption its surface
coverage.
The experimental isotherm data were fitted to these equations by applying
linear regression analysis. One way to assess the goodness of fit of experimental
isotherm data to these equations is to check the regression coefficients
obtained during the regression analysis. As shown in Table
1, adsorption data for all compounds obeyed mostly Langmuir isotherm.
The efficiency of adsorption process can be predicted by the dimensionless
equilibrium parameter RL, which is defined by the following
equation:
| Table 1: |
Parameters of Langmuir, Freundlich and Tempkin adsorption
isotherm equations at 298 K |
 |
where, b is the Langmuir constant (L mg-1), C0
the initial concentration of SO and phenols in mg L-1. Isotherm
is considered to be unfavourable when RL >1, linear when
RL = 1, favorable when 0<RL<1 or irreversible
when RL = 0. The RL values calculated for the adsorption
of SO and phenols studied are given in Table 1 for the
highest initial concentrations. Since all the RL values are
between 0 and 1, the adsorption processes in all cases are favourable.
It is known that the Langmuir and Freundlich isotherm constants do not
give any idea about the adsorption mechanism. In order to understand the
adsorption type, equilibrium data was tested with D-R isotherm. D-R equation
can be written as:
where, ε, RT 1n (1+1/Ce), K' is the constant related
to the adsorption energy, R the gas constant (8.314 J mol-1
K-1) and T is the temperature in Kelvin. The quantities of
q, qmax and Ce have the same meaning as above. Straight
lines are obtained upon plotting lnq versus ε2 indicating
that adsorption of both Safranin-O and phenols on activated carbon also
obey the D-R isothermal equation in the entire concentration range studied.
Values of qmax and K' calculated from the intercepts and slopes
of the plots were given in Table 2. From the value of
K' it is possible to calculate the mean free energy of adsorption (Ea),
defined as the free energy change when 1 mol of ion is transferred to
the surface of the solid from infinity in solution using the following
Eq.:
It is known that magnitude of Ea is useful for estimating
the type of adsorption and if this value is between 8 and 16 kJ mol-1
the adsorption type can be explained by ion exchange. But the value of
Ea found in this study is within the energy range of physical
adsorption (Ea< 8). The calculated values of Ea
are between 2 and 4 kJ mol-1 for Safranin-O and phenols (Table
2). This indicates that adsorption of Safranin-O and phenols onto
activated carbon are physical in nature.
The study of adsorption kinetics of Safranin-O, phenol, m-nitrophenol,
p-nitrophenol and o-nitrophenol describe the solute uptake
rate and evidently this rate controls the residence time of adsorbate
uptake at the solid-solution interface. In case of m-, p-
and o-nitrophenols trends of their kinetics curves are similar
to those of phenol and therefore, similar reasoning can be given. Adsorption
rate constants for the Safranin-O and phenol were calculated by using
pseudo-first-order, second-order and intraparticle diffusion kinetic models
and which were used to describe the mechanism of the adsorption. The conformity
between the experimental data and the model-predicted values was expressed
by the correlation coefficients (R2). A relatively high R2
values indicate that the model successfully described the kinetics of
the Safranin-O and Phenol adsorption.
A pseudo-first-order equation can be expressed in a linear form as:
| Table 2: |
Parameters of D-R adsorption isotherm at 298 K |
 |
| Table 3: |
Comparison of the pseudo-first and second-order kinetic models
of Safranin-O and phenol adsorption for the different initial concentrations
on activated carbon at 298 K |
 |
where, qe and q are the amount of Safranin-O and phenol adsorbed
(mg g-1) on the adsorbents at the equilibrium and at time t,
respectively and k1 is the rate constant of adsorption (min-1)
(Lagergren, 1898). Values of k1 were calculated from the plots
of ln (qe-q) versus t for different concentrations of Safranin-O
and Phenol, respectively and shown in Table 3.
The pseudo-second-order adsorption kinetic rate equation is expressed
as:
where, k2 is the rate constant of pseudo-second-order adsorption
(g mg min). Integrating and applying the initial conditions, we have a
linear form as:
where, qe is the amount of dye adsorbed at equilibrium (mg
g-1). Values of k2 and qe were calculated
from intercept and the slope of the linear plots of t/qt versus
t (Ho and McKay, 1999).
It may be observed from Table 3 that the adsorption
of Safranin-O and Phenol on activated carbon fits pseudo-second order
kinetics better than the first order. The same observation can be said
phenol adsorption. It can be seen from the better fit of qe
values by using pseudo-second order kinetics when experimental qe
values are compared.
In adsorption studies, it is necessary to determine the rate-limiting
step. Therefore, the results obtained from the experiments were used to
study the rate-limiting step in the adsorption process. The rate constant
for intraparticle diffusion (kid) is calculated by the following
equation:
where, q is the amount dye adsorbed (mg g-1) at time (t) and
kid (mg g-1 min0.5) is the rate constant
for intraparticle diffusion. Values of kid were calculated
from the slope of the linear plots of q versus t0.5 (Namasivayam
and Yamuna, 1995). The kid values were obtained from the slope
of the linear portions of the curves and were found to be as 10.517 and
12.33mg g-1min0.5 for the initial concentrations
of 125 and 200mg L-1, for Safranin-O and 10.824 and 21.409
mg g-1min0.5 for the initial concentrations of 20
and 40 mg L-1 for Phenol, respectively. It is seen from Fig.
3a and b, that all the plots have the same general
features (the initial curved portion and a linear portion). The initial
curved portion is attributed to the boundary layer diffusion, while the
linear portion is responsible for the intraparticle
| |
| Fig. 3a: |
Plots of amount of Safranin-O adsorbed vs t0.5
for intraparticle diffusion of SO |
| |
| Fig. 3b: |
Plots of amount of phenol adsorbed vs t0.5 for
intraparticle diffusion of phenol |
diffusion. The linear portions of the curves do not pass the origin in
Fig. 3a and b. This indicates that mechanism
of Safranin-O and Phenol removal on Activated Carbon is complex and both,
the surface adsorption as well as intraparticle diffusion contribute to
the rate determining step. Similar trends were also observed for nitrophenols.
The adsorption of Safranin-O and phenols on activated carbon were studied
at temperatures of 293, 303, 313 and 323 K. The free energy of adsorption
(ΔG) was calculated from the following equation;
| |
| Fig. 4: |
Plot of lnK vs. 1/T for estimation of the thermodynamic parameters
for the adsorption of Safranin-O, Phenol and Nitrophenols onto activated
carbon |
where, K is the equilibrium constant and T is the solution temperature
(K); R is the gas constant (8.314 J mol-1 K). The apparent
enthalpy of adsorption, (ΔH) and entropy of adsorption, (ΔS),
were calculated from adsorption data at different temperatures using the
Van`t Hoff equation (Shawabkeh and Tutunji, 2003):
Values of ΔH and ΔS were computed from the slopes and intercepts
of linear variations of ln K with the reciprocal of temperature (Fig.
4). As seen in Table 4 ΔG° values were
negative in the studied temperature range of 293-323K for both Safranin-O
and phenols indicating that the adsorption process led to a decrease in
Gibbs free energy. Negative ΔG° indicates the feasibility and
spontaneity of the adsorption process. The negative values of ΔH°
indicate that the process is exothermic. Physical adsorption and chemisorption
can be classified, to a certain extent, by the magnitude of the enthalpy
change. It is accepted that bonding strengths of <84 kJ mol-1
are those of physical adsorption type bonds. Chemisorption bond strengths
can range from 84 to 420 kJ mol-1. Based on this the adsorption
of Safranin-O, phenol and nitrophenols on activated carbon appears to
be a physical adsorption process. The enthalpy of adsorption of organic
molecules from aqueous solution on activated carbon is usually within
the range 8-65 kJ mol-1. The negative values of ΔS°
suggest that the decreased randomness at the solid-solution interface
during the adsorption of the Safranin-O and phenols in aqueous solution
on the activated carbon. The less negative values of ΔS° found
for phenols reveal that a more ordered arrangement of phenol molecules
is shaped on the adsorbent surface.
On the basis of the data, it is concluded that active carbon can be used
to remove Safranin-O and phenols from aqueous solutions. The experimental
adsorption data showed good correlation with
| Table 4: |
Thermodynamic parameters for Safranin-O, phenol, p-nitrophenol,
m-nitrophenol and o-nitrophenol adsorption on activated carbon
under different temperatures |
 |
the Langmuir isotherm model. Regardless of temperature, the capacity of the
activated carbon used to adsorb these compounds presented following order:
The lowest adsorption affinity of Safranin-O onto activated carbon comparing
with nitrophenols including Phenol can be explained by molecular size
of Safranin-O. Phenols, having the smaller molecular size, are quickly
adsorbed onto activated carbon surface when comparing with Safranin-O.
This was also confirmed by the equilibrium time. Solubility which is the
main physicochemical features of phenols playing an important role in
adsorption mechanisms. The solubility of phenol was higher than all studied
nitrophenols, thus, phenol could be dissociated easily in the water. The
higher adsorption affinity of m-nitrophenol onto activated carbon
among the studied all nitrophenols can also be explained lower solubility
in water. The difference between phenol and nitrophenols come from having
-NO2 group of nitrophenols since -NO2 group acts
as a strong electron-withdrawing group. As a conclusion the presence of
nitro group on reduction of the electron density of the aromatic ring
and the interaction of phenols and carbon is enhanced which leads to an
increase in adsorption capacity. It can be concluded that this is also
possibly the major mechanism for the adsorption of nitrophenols onto activated
carbon. Thermodynamical parameters were also evaluated for the basic dye
Safranin-O and phenols and revealed that the adsorption is exothermic
in nature. Although activated carbons are relatively expensive and difficult
to regenerate, adsorption over activated carbon is one of the most effective
methods employed in the treatment of wastewaters containing both different
classes of dyes and low-molecular weight organic compounds such as phenols.
Our experimental results also showed that the activated carbon with a
large surface area (1014.44 m2 g-1) could potentially
be used in the removal of safranin-O, phenol and nitrophenols in aqueous
solutions and industrial wastewater managements.
ACKNOWLEDGMENT
This research was supported by the Research Fund of Marmara University.
Subscribe Today
