Dyes are extensively used in paper, textile, dye-houses and printing to color
the final products (Aksu, 2005). They usually have a synthetic origin and complex
aromatic structures making them difficult to degraded (Aksu and Tezer, 2005).
Dyes are classified as anionic, cationic and nonionic dyes (Fu and Viraraghavan,
2001). Existing colored wastewater treatment methods involve in a combination
of physical and chemical processes. Adsorption is one of the effective methods
to remove colored textile contaminants from wastewater. Recently, attentions
have been focused on the development of low cost adsorbent for the application
concerning treatment of wastewater (Namasivayam et al., 2001; Forgacs
et al., 2004; Gurses et al., 2006; Hamdaoui, 2006; Grini et
al., 2007). In the last 3 years, our laboratory has developed fast and cost
effective methods for the removal of metals using waste materials like egg shells
(Yeddou and Bensmaili, 2007). Skin almonds, being a low-cost and easily available
adsorbent, could be an alternative for more costly wastewater treatment processes.
To the best of our knowledge, there is no information in literature on the use
of skin almonds as an adsorbent. In this study, the potentials for the use of
skin almonds as an adsorbent for crystal violet and methyl orange removal from
solution were investigated.
The objective of this study was to explore the feasibility of using Skin Almonds (SA), a new agriculture sorbent for removal of crystal violet and methyl orange.
MATERIALS AND METHODS
The study was conducted at the Laboratory of chemical reaction, Faculty of Mechanical Engineering and Process of Engineering, Houari Boumediene University of Science and Technology, Algiers, Algeria (USTHB) During 2008.
Adsorbate The basic dye crystal violet and the acid dye methyl orange
are widely used in the textile, pharmaceutical and paper manufacturing. Double
distilled water was employed for preparing all the solutions.
Before use, the skin almonds were washed in distilled water for several
times and dried at 100°C for 2 h. After drying, the skin almond was sieved
to obtain a particle size range of 0.1-0.25 mesh. Studies were carried out using
two different forms of skin almonds, natural (designated as NSA) and treated
(designated as TSA). In order to enhance the adsorption aptitudes of these adsorbent,
different types of chemical treatments: acidic treatment (H2SO4),
alkaline treatment (NaOH) and salt treatment (MgCl2) were investigated.
Activation will increase the surface area and the fraction of mesopore volume
simultaneously (Wua et al., 2005). One hundred grams of skin almonds
was mixed with solution of reagent. The mixture was stirred for different times
(Table 1) after which it was washed several times with distilled
water and filtrated; the optimization of chemical treatment operating conditions
such as: temperature, contact time and chemical concentration of solution reagent
||Operating conditions of the treatment of skin almonds
||Effect of chemical treatment on the adsorption of methyl orange
Figure 1 and 2 show the results for the removal of crystal violet and methyl orange by natural and treated skin almonds. Among the different chemical treatment used, treatment with H2SO4 is the most effective for skin almonds. It was observed from Fig. 1, that the percentage of removal of methyl orange increases from 37.5-78.12% for the treatment by H2SO4 (Temp. is 70°C and treatment time of 4 h). Little increase of the adsorption capacity of the tested skin almonds was obtained by NaOH (37.5 to 45%) and MgCl2 (37.5 to 52%). Figure 2 shows that the effect of treatment of skin almonds on the removal of crystal violet was not significant. Consequently, methyl orange removal studies were carried out using different skin almonds forms, natural and treated.
Adsorption experiments were carried out by adding a fixed amount of adsorbent
(0.3 g) to a series of conical flasks filled with 50 mL diluted dye solutions.
The conical flasks were placed in a thermostatic shaker for crystal violet and
methyl orange, respectively. The adsorption amount on natural and modified skin
almonds was calculated indirectly from the difference of dye concentration in
solution before and after the experiment. The dye concentration was determined
by measuring the absorbance of the solution using U-V visible (Jenway 6305UV/Vis
spectrophotometer) at maximum wavelength, 504 nm for crystal violet and 466
nm for methyl orange. Experiments were repeated for different initial dye concentration
(5, 10, 20, 30 and 40 mg g-1) and temperature (23, 30, 40 and 50°C)
The removal efficiency (E) of dye on skin almonds, the sorption capacity (q) and distribution ratio (Kd) were calculated by the following equations:
||Effect of chemical treatment on the adsorption of crystal
where, Ci and Cf are the initial and final concentrations of dye (mg L-1) in aqueous solution, respectively, V is the volume of the solution (L) and m represents the weight of the adsorbent (g).
RESULTS AND DISCUSSION
Effect of Initial Dye Concentration
The effect of initial crystal violet and methyl orange concentration on
adsorption by skin almonds was investigated in the range of 5-40 mg L-1
of the initial dye concentration. From Fig. 3a-c,
it was observed that the uptake capacity of treated skin almonds was higher
than raw one. The amount of methyl orange adsorbed increases from 3.2 to 15
mg g-1 and 4.6 to 31.25 mg g-1 as the initial adsorbate
concentration increases from 5 to 40 mg L-1 for natural and treated
skin almonds, respectively.
The experimental results showed that the removal of the dye was highly concentration
dependent. The increase in uptake capacity of the sorbent with increasing dye
concentration may be due to the increase of sorbate quantity. At lower initial
dye concentrations, sufficient adsorption sites are available for the sorption
of dye ions.
||Effect of time and initial dye concentration on adsorption
capacity (a) CV-NSA (b) MO-NSA and (c) MO-TSA
Conversely, the numbers of dye ions at higher initial concentrations are relatively
more as compared to the available adsorption sites. As highlighted earlier,
the difference in the removal capacity is a result of the difference in their
chemical affinities and ion exchange capacity with respect to the chemical functional
group on the surface of the adsorbent. In this case, the availability of free
adsorption sites dominates (Ajay et al., 2005).
Effect of Temperature
Temperature is a highly significant parameter in adsorption process, this
dependence of these two dyes was studied with a constant initial concentration
of 20 mg L-1. The adsorption studies were carried out at four different
temperatures 23, 30, 40 and 50°C for these three systems and the results
are shown in Fig. 4a-c. On increasing the
temperature of the reaction from 23 to 50°C, the amount of crystal violet
on natural skin almonds, methyl orange on natural skin almonds and methyl orange
on treated skin almonds decreased from 46.08 to 21; 14.7 to 10.4 and 23 to 15
mg g-1, respectively similar result was also observed for methyl
orange removal from wastewater using De-oiled Soya and Bottom Ash (Mittal et
The retention capacity of the skin almonds is enhanced with decreasing temperature it indicates that the adsorption reaction is exothermic in nature.
The effect of temperature on the adsorption of crystal violet is more than the effect of temperature on the adsorption of methyl orange. These results indicate that these two dyes escape to the liquid phase from the solid phase with the rise in temperature.
||Dye adsorption at various temperatures (a) CV-NSA (b) MO-NSA
and (c) MO-TSA
The isotherms data were analyzed using two of the most commonly used equilibrium
models (Lin et al., 2008; Tan et al., 2008).
The Langmuir equation is given in the following equation:
where, qe the amount adsorbed at equilibrium (mg g-1), qmax the maximum amount of sorbate per unit weight of adsorbent (mg g-1), Ce the concentration of adsorbate at equilibrium (mg L-1) and b (L mg-1) is the constant related to the affinity of the binding sites. qmax and b can be determined from the linear plot of Ce/qe versus Ce.
The Freundlish equation is an empirical equation employed to describe heterogeneous systems, in which it is characterized by the heterogeneity factor 1/n. Hence the empirical equation can be written:
where, Kf is the Freundlich constants and 1/n is a measure of the adsorption intensity. Equation 5 can be linearized in logarithmic form and the Freundlich constants can then be determined.
The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL (Batzias abd Sidiras, 2004) as:
||Langmuir adsorption isotherm for CV-NSA at various temperatures
||Langmuir adsorption isotherm for MO-NSA at various temperatures
Figure 5-10 show the fitted equilibrium
data in Freundlich and Langmuir expressions. The isotherm parameters and the
correlation coefficients (R2) are shown in Table 2.
A comparison of the experimental isotherms with the adsorption isotherm models showed that the Langmuir equation represented the poorest fit of experimental data as compared to the isotherm Freundlich equation (R2>0.99) for systems MO-NSA and MO-TSA.
||Langmuir adsorption isotherm for MO-TSA at various temperatures
||Freundlish adsorption isotherm for CV-NSA at various temperatures
It is clear from Table 2 that the values of n were greater than 1, indicating favourable adsorption condition. Both Kf and n reached their corresponding maximum values, at 23°C. This implies that binding capacity reaches the highest value and the affinity between the sorbent and dye ions was also higher than other temperature values investigated. The constant b represents affinity between the sorbent and sorbate, b increases with decreasing temperature from 23 to 50°C.
It can be observed that the corresponding qmax values of the system MO/untreated skin almonds for 23-50°C temperature range were not affected with changes of temperature solution.
||Freundlish adsorption isotherm for MO-NSA at various temperatures
||Freundlich adsorption isotherm for MO-TSA at various temperatures
The value of RL indicates whether the isotherm is irreversible (RL
= 0), favourable (0<RL<1), unfavourable (RL>0).
The results are shown in Fig. 11 and 12.
It was found that the value of RL is between 0.205 and 0.728 for
all systems studied and confirmed that the treated and untreated skin almonds
is favourable for adsorption of methyl orange and crystal violet under conditions
used in this study.
Pseudo Second Order Model
Based on equilibrium adsorption, the pseudo second-order kinetic equation
(Kavitha and Namasivayam, 2007a, b) is expressed as:
||Freundlich and Langmuir isotherm model constants and correlation
||Separation factor at 23°C at various dye concentrations
||eparation factor at various temperatures
||Pseudo-second-order kinetics for the sorption of dye at 23°C
(a) CV-NSA, (b) MO-NSA and (c) MO-TSA
||Pseudo-second-order kinetics for the sorption of dye at various
temperatures (a) CV-NSA, (b) MO-NSA and (c) MO-TSA
where, K2 is the rate constant of pseudo-second order adsorption (g mg.min-1).
The initial adsorption rate, h, (mg gmin-1) is expressed as:
The application of the linear form of pseudo-second-order kinetic model on
our experimental results is shown in Fig. 13a-c
and 14a-c. Both constants K2
and h were calculated from the intercept and slope of the line obtained by plotting
t/qt versus t. It can be shown from Table 3 that
the kinetics of dye adsorption onto skin almonds (treated and untreated) follow
this model with correlation coefficients higher than 0.99. Further, the rate
constant, K2, decreased with increase in initial dye concentration.
Similar phenomena had been observed in the adsorption of methylene blue by hazelnut
shells and wood sawdust (Fererro, 2007), acid blue 193 onto BTMA-bentonite (Ozcan
et al., 2005) and adsorption of basic black dye using calcium alginate
beads (Aravindhan and Nishtar, 2007).
||Comparison of the pseudo-second-order and intraparticle diffusion
models of crystal violet on natural skin almonds
The variations of t/qt versus t at various temperatures of dye solution
under the initial concentration of 20 mg L-1 still confirmed to fit
the pseudo-second-order model. The values of model parameters (K2,
h and qe) for different temperatures have been calculated from Eq.
7 and 8 and the results are shown in Table
3, revealing that the fitted adsorption capacity at equilibrium, qe,
decreased with increasing temperature. The values of the initial sorption rate,
h, increases from 0.777 to 7.856 as the solution concentration increases from
5 to 40 mg L-1 and decreases with increasing temperature for these
three systems (CV-NSA, MO-NSA and MO-TSA).
These results imply that physic-sorption mechanism may play an important role for the adsorption of each dye on the raw and treated skin almonds. Based on the values of R2 shown in Table 3, it is clear that the pseudo-second-order equation is better in describing the adsorption kinetics.
Intra-Particle Diffusion Model
The pseudo-second-order kinetic model could not identify the diffusion mechanism
and the kinetic results were then analyzed by using the intraparticle diffusion
model. Although, the kinetic studies help in identifying the adsorption process,
the determination of the adsorption mechanism is important for design purposes.
There is a possibility that the transport of the crystal violet or methyl orange
from the solution into the pores of adsorbent is rate controlling. Hence, the
data was further processed for testing the role of diffusion in the adsorption
In the model developed by Indra et al. (2006), the initial rate of intra-particle diffusion is calculated by linearization of the Eq. 9:
where, C (mg g-1) is the intercept and Ki is intra-particle diffusion rate constant (mg g min-½). According to this model, the plot of uptake, qt versus the square root of time (t1/2) should be linear if intraparticle diffusion is involved in the adsorption process and if these lines pass through the origin then intraparticle diffusion is the rate controlling step.
The deviation of straight lines from the origin (Fig. 15a-c,
16a-c) indicates that the pore diffusion
is not the sole rate controlling step. The values of rate constants (Ki)
are shown in Table 3. The intraparticle diffusion, Ki,
values were obtained from the slope of the straight line portions of plot of
qt versus t1/2 for various dye concentrations and solutions
temperature (Fig. 15, 16). It was observed
that intraparticle rate constant values (Ki) increased with initial
dye concentration. The increase in Ki values with increasing initial
dye concentration can be explained by the growing effect of driving force resulted
in reducing the diffusion of dye species in the boundary layer and enhancing
the diffusion in the solid. Also, as shown in Table 3, increasing
the temperature decreased the pore diffusion in sorbent particles.
Test of Kinetic Models
In order to quantitatively compare the applicability of each model in fitting
to data a normalized standard deviation, Δq was calculated:
where, the qt,exp and qt, cal are the experimental and calculated values and N is the number of data points.
||Plots of intraparticle diffusion modelling for the sorption
of dye at 23°C (a) CV-NSA, (b) MO-NSA and (c) MO-TSA
||Plots of intraparticle diffusion modelling for the sorption
of dye at various temperatures (a) CV-NSA, (b) MO-NSA and (c) MO-TSA
||Arrhenius plots for adsorption of crystal violet and methyl
orange at various temperatures
Theoretical calculated values of the remaining sorption capacities of each dye were found to have a close agreement with the experimentally-measured values for the pseudo second as compared to the intraparticle model (Table 3). Based on the values of R2 and Δq given in Table 3, it is clear that the pseudo-second-order equation is better in describing the adsorption kinetics.
The thermodynamic parameters of the adsorption process were determined from
the experimental data obtained using the following equations (Kavitha and Namasivayam,
where, Kd is the distribution coefficient for the adsorption, ΔS°, ΔH° and ΔG° are the change of entropy, enthalpy and the Gibbs energy, T is the absolute temperature, R is the gas constant.
The second-order rate constant is expressed as a function of temperature by the Arrhenius equation (Eq. 11). K0 is the temperature independent factor (g mg min-1); Ea is the activation energy of sorption (kJ mol-1). The activation energy values were calculated to be -9.688, -28.189 and -20.951 kJ mol-1 for CV-NSA, MO-NSA and MO-TSA, respectively (Fig. 17).
These results (Table 4) show that these dyes adsorption process
by skin almonds natural or treated were exothermic. This low value of activation
energy suggested that the adsorption process was governed by the physisorption
(Hameed et al., 2007). The values of ΔH° and ΔS° were
determined from the slope and intercept of the plot of LnKd vs. 1/T
||Van t Hoff plots for the adsorption of crystal violet
and methyl orange
ΔH° and ΔS° values are all negative for adsorption of these
three systems. The negative value of ΔG° indicated the feasibility
of the process and the spontaneous nature of the adsorption; while the negative
value of ΔS° suggested that this adsorption would lead to decreasing
The skin almonds, an agriculture waste were successfully employed in natural form for removal of basic dye and with H2SO4 treatment for removal of acid dye. The removal of crystal and methyl orange were systematically investigated under various conditions. From the results, it could be concluded that the adsorption was dependent on initial concentration and temperature solution. The maximum adsorption capacities of crystal violet onto skin almond and methyl orange onto natural and treated skin almond were 85.47, 21.10 and 31.94 mg g-1, respectively and for the temperature of 23°C. Adsorption equilibrium for these three systems on untreated and treated skin almonds was best represented by the Freundlish isotherm. Two kinetic models, pseudo-second-order and intraparticle diffusion were tested to investigate the adsorption mechanism. The experimental data fitted very well the pseudo second order kinetic model and also followed by intraparticle diffusion model for all initial concentrations and temperatures. The thermodynamics data shows the process is exothermic and spontaneous nature.