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Sudan is applied to color products food and are synthetic industrial, used in plastics, oil and waxes. However, the dye should be within the safety limit set by the food regulations. In this work, a simple method was proposed for study of adsorption of the sudan (III) by using activated carbon. Activated carbon was found effective for up taking the dye with maximum capacity of 7.8 mg/g at 40 oC where the following experimental conditions maintained at mass of carbon 0.2 g, volume of solution 50 mL, dye concentration range 10-50 mg/L with participle diameter 300-500 μm. The adsorption data of sudan (III) were modeled using Langmuir and Ferundlich isotherms. The model parameters were found to be 1.7 (KL), 7.8 mg/g (qm), 0.8557 (R2) and 1.37 (n), and 0.8620 (R2) which were obtained at 40 oC. The activated carbon was proved to be a perfect absorbent for sudan (III) as indicated from the distribution coefficient value (9.9 L/g) and this value was reported at the following conditions: 10 mg/L dye, 0.2 g carbon mass, 3 days agitation, and pH 8.5.
Thermodynamic studies indicated that sudan (III) adsorption onto activated carbon was an exothermic process and spontaneously at 293, 303 and 313 K. The values of free Gibbs energy (ΔG) were -4.8, -4.9 and -5.0 KJ/mol respectively. The enthalpy value of adsorption was -2.4 KJ indicate a physiosorption process. The process occurred with positive entropy.
Sudan dye is classified of azo dye was used for variant field in industrial and scientific applications (coloring of fuel and staining for microscopy) [1,2]. Sudan dyes in figure (1.1) are synthetic industrial, used in plastics, oil and waxes . Because of this color low cost and available . Synthetic organic colorants, have a group (-N=N-). The international Agency for Research on cancer (IARC) classified sudan dyes as class 3 carcinogens [5,6]. In some European countries sudan dyes have been found in food products such as chili powder to mimic. sudan dyes have been found in six hundred products in UK such as fish sauce, noodle soup, Worchester sauce and pizza. The detection limit of sudan dyes is 0.5-1 mg/kg .
Figure1.1. Chemical structure of sudan III
Sudan dyes have different techniques to be used for their separation and identification. The traditionally analytical techniques for the determination of the sudan dyes include solid-phase spectrophotometry [8,9,10], HPLC-Fluorimetry , thin layer chromatography , capillary electrophoresis , HPLC coupled with photo-diode array , UV-visible , chemiluminescence  and MS [17,18,19,20,21].
The determination of sudan dyes by using conventional methods is difficult because of high-cost instruments and time-consuming pretreatment technique separations, such as equipment such as liquid and gas chromatography are not available for small laboratories due to their high cost.
The application of activated carbon is investigated in this work. The studied dye is sudan (III). This dye has a wide application. Effect of experimental conditions such as initial dye concentration, carbon mass, agitation time, pH, salt and temperature were studied.
The effect of treatment time, initial dye concentration, carbon mass, pH, salt and temperature was determined by estimating the distribution coefficient (Kd) of dye between solid phase and aqueous solution. The value of (Kd) was simply estimated using Equation (1), .
Kd = qe/Ce ……………………………… (1)
Where kd, qe, and Ce, and are distribution coefficient (L/g), surface concentration of dye (mg/g), and concentration of dye remaning in solution after time (t) or at equilibrium (mg/ L) respectively. The value of qe can be calculated from Equation (2):
qe= (C0-Ce)V/ m ………………………(2)
Where C0, V, and m are initial dye concentration (mg/ L), volume of solution (L) and mass of extractant (g) respectively. A large Kd (>> 1.0 mg/g) signifies a high affinity between dye and the extractant.
The aim of work study adsorption of the Sudan dye (III) in methanol by using activated carbon Effect of experimental conditions such as initial dye concentration, carbon mass, agitation time, pH, salt and temperature were studied.
All materials used in this research were analytical grade reagents and used as received without any further purification: The dye sample of Sudan (III) was obtained from Lobal Chemie (India). Methanol from Sigma- Aldrich Chemie (Germany). Activated carbon was purchased from Nen Tech Ltd (UK). Sodium Hydroxide and sodium chloride from Lobal Chemie (India). Hydrochloric acid from Riedel-de Haen.
UV-Visible spectrophotometer (SP-300, Optima, Japan) for calculation of dye concentration, Samples were shaken and thermostated using Wise Bath (Daihan scientific, Korea) shaker. pH was measured using (Lovibond pH-meter, Germany). Masses were accurately measured (±0.0001 g) using Wiggen Hanser. Samples were stirrer using Magnetic stirrer Wigen Hauser.
Adsorption of sudan (III) dye
The adsorption properties for dye were studied using batch-Equilibrium technique in the following manner: A certain amount of activated carbon at specific diameter (300-500 µm) was contacted with 50 mL dye solution and the mixture was agitated to reach equilibrium. The equilibrium dye concentration was detected using the calibration graph Figure (3.1). Effect of carbon mass, dye concentration, solution acidity, ionic strength, and temperature on sudan (III) dye adsorption was investigated. As shown in Figure (3.2-3.7).
2.3.1 The effect of time on dye
The equilibrium time for dye adsorption was determined as following: Different dye solutions were agitated for different periods (1-8days). The remaining dye concentration in each solution was determined. The following variables were maintained: mass of carbon = 0.2g, temperature = 25 oC, volume of solution = 50 mL, initial concentration 35 mg/L, pH = 8.5 and particle diameter = 300 - 500µm. As shown in Figure (3.2).
2.3.2 Effect of adsorbent mass on dye removal
Different masses of activated carbon (0.10-0.35 g) were mixed with 50-mL of 35 mg/L dye solution. The mixtures were agitated for 3 day. The following variables were maintained: pH = 8.5, temperature = 25 oC, volume of solution =50 mL, and particle diameter =300-500 µm. As shown in Figure (3.3).
2.3.3 Adsorption isotherm determination
Adsorption isotherm of sudan (III) by activated carbon was recorded at different temperatures. A fixed amount of activated carbon was added to set of dye solutions of variable levels (10-50 mg/L). The mixtures were agitated until equilibrium. The following variables were maintained: agitation time = 3 days, mass of carbon = 0.2 g, temperature = 25 oC, volume of solution = 50 mL, pH=8.5 and particle diameter =300 - 500µm. As shown in Figure (3.4).
2.3.4 Effect of ionic strength on dye
Different solutions of sudan (III) of variable ionic strengths were prepared and agitated with activated carbon for 3 days. The remaining dye concentration in each solution was determined. The remaining dye concentration in each solution was determined. The amount of dye removed was calculated in each case. The following variables were maintained: mass of carbon = 0.2 g, temperature = 25 oC, volume of solution = 50 mL, initial concentration 35 mg/L, pH=8.5 and particle diameter = 300 - 500µm. As shown in Figure (3.5).
2.3.5 Effect of solution pH on dye
Different solutions of sudan (III) dye were prepared at different pH conditions: 1, 3, 10, and 12. The mixtures were carefully agitated for 3 days. The remaining dye concentration in each solution was determined. The following variables were maintained: mass of carbon = 0.2 g, temperature = 25 oC, volume of solution = 50 mL, contact time = 3 days, initial concentration 35 mg/L, and particle diameter = 300-500 µm. As shown in Figure (3.6).
2.3.6 Effect temperature on dye
Adsorption isotherm of sudan (III) dye was recorded at different temperatures (20, 30, and 40 oC). The following variables were maintained: agitation time = 3 days, mass of carbon = 0.2 g, initial concentration=35mg/L, temperature = 25 oC, volume of solution = 50 mL, pH=8.5 and particle diameter = 300 - 500µm. As shown in Figure (3.7).
Result and discussion
Adsorption properties of the activated carbon
The adsorption properties of the activated carbon toward sudan (III) were studied by determination of dye concentrations remaining in solution using spectrophotometer after building up analytical calibration curve for sudan (III).
Linear calibration curve was obtained for dye using a blank sample and a series of standard samples in the range of 1.0-8.0 mg L-1. The calibration graph Figure (3.1) was obtained by plotting absorbance values against dye concentration. A linear calibration graph was obtained with R2 = 0.9887 and this graph were repeated every week for more accuracy. The calibration plot was carried out at 510 nm, the wavelength at the maximum absorption of the dye (λmax).
Figure (3.1). Calibration curve of sudan (III) in methanol (λmax. = 510 nm)
3.2 The effect of time on dye
Adsorption of dye by the activated carbon 0.2g was determined as mentioned earlier over long period of time (1-8) days, pH = 8.5, T = 25 oC and initial concentration of 35 mg/L. The results of these experiments are shown in Figure (3.2).
Figure (3.2) Dye uptake by the activated carbon as a function of contact time
Effect of agitation time on dye extraction at 35mg/L is shown in figure (3.2). As the contact time increase, the adsorption of sudan (III) increases to become nearly steady after 3 days. The distribution value was slightly changed after that time. Accordingly, the optimum agitation time was selected to be three day. As can be noted, the uptake process is favorable after the day number four where Kd is higher than unity.
3.2. Effect of carbon mass on dye uptake
The effect of varying mass of activated carbon on dye uptake was studied using batch procedure in the range: 0.10 - 0.35 g. the sorption capacity profile is illustrated in Figure (3.3).
Figure (3.3) Dye uptake as a function of carbon weight
The distribution value has been decreased by increasing carbon mass from 0.1 to 0.35 g and slightly changed after that mass. Accordingly, the optimum mass of activated carbon was maintained at 0.2 g.
3.3. Effect of dye concentration
Effect of dye adsorption was at different initial concentrations of sudan (III) (10-50 mg/L) while keeping the other variables at: T = 25 oC, contact time 3 days, pH = 8.5, carbon mass = 0.2 g, and solution volume = 50 mL. The distribution values were calculated at each concentration and presented in Figure (3.4).
Figure (3.4) Effect concentration of dye on Kd.
The extent of sudan (III) adsorption was studied over a wide concentration range (10-50 mg/L). As can be noted Figure (3.4), the Kd value is significantly decreased as concentration increased. For example, the Kd value was highly decreased from 9.98 to2.47 when dye level increased from 10 to 50 mg/L. Accordingly, the values of Kd will be much larger than 9.98 at lower initial dye concentration because the remaining dye concentration (Ce) will be very small. .
3.4. Effect of ionic strength on dye adsorption
The effect of sudan (III) adsorption by activated carbon at different ionic strength values was studied using batch procedure and ionic strength range: 0.5-3.0 mol/L NaCl. Figure (3.4) presents the data.
Figure (3.4) Effect of solution ionic strength on sudan (III) uptake
The adsorption of activated carbon has been considerably decreased when the salt is introduced to the mixture. The distribution values were decreased when the salt level become more than 0.5 M. A number of intermolecular forces have been suggested to explain the aggregation between dye molecules in solutions and these forces include van der Waals forces, ion-dipole forces, and dipole-dipole forces which occur between dye molecules in the solution. These forces found to be more favorable when salt is added to the dye solution. The salt ions force dye molecules to aggregate and migrate toward carbon surface .
3.5. pH-dependence of dye sorption by activated carbon
The effect of dye uptake at variable pH values (1-12) was studied and the data were given in Figure (3.5).
Figure (3.5): Effect of solution pH on extent of dye distribution value
The adsorption of sudan (III) by activated carbon was affected when pH was changed. The change in sudan (III) uptake with solution pH is explained according to following mechanisms:
1) Electrostatic interaction between the protonated surface at pH 3 of activated carbon and negatively charged dye molecule.
2) Hydrophobic interactions between the activated carbon and the hydrophobic part of dye molecule. The large reduction in carbon adsorption at basic medium can be attributed to the electrostatic repulsion between negatively charged activated carbon (due to adsorption of OH- ions on the surface) and the deprotonated dye molecule .
3.6. Effect of Temperature on dye adsorption
The adsorption isotherms were determined for dye at different temperatures (20 oC, 30 oC, 40 oC) in the concentration range 10 - 50 mg/L. Adsorption results are presented in figure (3.6).
Figure (3.6) Adsorption isotherms of dye on activated carbon at different temperatures.
Adsorption of sudan (III) by activated carbon was studied at different temperatures (20, 30 and 40 °C) and adsorption isotherms were presented in Figure (3.7). The Kd of the dye was increased by increasing temperature; however, this increase is dependent on the initial concentration. This indicates that temperature has a higher effect on Kd but at lower concentration and small effect at higher concentration. The high adsorption at higher temperature may be attributed to increased penetration of dye inside micropores at higher temperatures or the creation of new active sites.
3.6.1. Analysis of adsorption isotherms by Langmuir and Ferundlich models
The adsorption data were modeled by using Langmuir and Ferundlich isotherms. The Langmuir model (homogeneous sorption) was chosen for the estimation of the maximum adsorption capacity corresponding to the saturation of the polymer surface using the linearized forms of Langmuir isotherms:
Kd = qe/Ce = qmK - Kqe ………………….. (Linear form I)……………….… (3)
1/qe = (1/ (qmKL)) 1/Ce + 1/qm ………..… (Linear form II)…..………….… (4)
Ce/qe = 1/ (qmK) + (1/qm) Ce …………..…… (Linear form III)…............…… (5)
Therefore, a plot of (qe / Ce) against (qe) should be a straight line with slope = - K and an intercept = qm K for Langmuir form (I), a plot of 1/qe versus 1/Ce gives a straight line of slope (1/qmK) and intercept 1/(qm) for Langmuir form (II), a plot of Ce/qe against Ce should be a straight line with slope = 1/qm and an intercept = 1/(qmK) for Langmuir form (III) where Kd is the distribution coefficient, KL is a constant related to the adsorption/desorption energy, and qm is a maximum adsorption capacity upon complete saturation of the clay surface. On the other hand, The Freundlich model (heterogeneous sorption) is an empirical equation used to estimate the adsorption intensity of the dye towards the clay using the linearized form of the Freundlich isotherm:
log qe = log KF + 1/n log Ce.................................................................... (6).
A plot of log qe versus log Ce gives a straight line with a slope of 1/n and intercept of log KF. Were KF and n are the Freundlich constants, the value of n indicates the affinity of the metal towards the clay .
The models were presented in Figure (3.7 and 3.8).
Figure (3.7): Langmuir modeling for adsorption data at different temperatures
Figure (3.8): Modeling of adsorption data by Freundlich model at different temperatures
As can be noted form the Figures above, Ferundlich model was more representative for sudan (III) adsorption as can be noted from R2 values. For Langmuir model and at all temperatures, R2 values were in the range 0.6982 - 0.8900 while for other model better R2 values were obtained: 0.8620-0.9995.
The estimated parameters; qm, Kl, n, Kf and qmax of adsorption isotherms were calculated from the intercepts and the slopes of the corresponding linear plots for dye adsorption onto the activated carbon at different temperatures. The value of these parameters with their correlation coefficients (R2) is given in Tables (3.1) at optimum experimental conditions.
Table (3.1): Langmuir and Ferundlich parameters.
The correlation coefficients (R2) were determined for each isotherm using simple linear regression method. The Langmuir and Freundlich have different correlation coefficients for presenting the data as shown in the previous Tables (3.1). So we can conclude that there is type of sorption on the adsorbent, heterogeneous sorption. The n values for adsorption of sudan (III) were all more than unity, which reflects the favorable adsorption of the dye over the studied temperatures (20-40 oC). Furthermore, the surface of activated carbon is known to be highly heterogeneous and the energies of active sites are highly variable, which would also tend to make the values of n more than unity .
3.7. Thermodynamic parameters of adsorption
Thermodynamic parameters were determined using the distribution coefficient, Kd (qe/Ce) which depends on temperature. The change in free energy (ΔG), enthalpy (ΔH°) and entropy (ΔS°) associated to the adsorption of sudan (III) were all calculated using equations (7-8):
ΔG° = ΔH° - T ΔS° ………………………………..(7)
Where R is the universal gas constant (8.314 J/mol K) and T is temperature (K).
ln Kd = ΔS°/R - ΔH°/RT …………………………..(8)
According to the above equations, ΔH° and ΔS° functions can be calculated from the slope and intercept of the plot of ln K versus 1/T; respectively. This is represented in Table (3.2) and in Figure (3.9).
Table (3.2): Thermodynamic functions for the adsorption of 35 mg/L sudan (III) onto activated carbon.
Figure (3.9): Plots ln Kd versus 1/T for sudan (III).
The adsorption process was exothermic for sudan (III) where ΔH° has negative value for adsorption process (-2.4 kJ/mol), which means that the dehydration energy is lower than the adsorption energy. The positive values of entropy for dye may be due to some structural changes in the adsorbate and adsorbent during the adsorption process from aqueous solution onto the adsorbent. In addition, positive value of ΔS° indicates the increasing randomness at the solid-liquid interface during the adsorption of dye on the adsorbent. The free Gibbs energy change calculated for adsorption of dye decreases as the temperatures increase, indicates that the interaction are thermodynamically favorable. We can conclude the adsorption of dye is spontaneous.
Activated carbon was found an effective adsorbent for sudan (III) from solution. The maximum adsorption value of sudan (III) was 7.8 mg/g. Adsorption of sudan (III) decrease as mass of carbon increases and number decrease with temperature. The process of sudan (III) adsorption was found spontaneous at 20, 30, and 40 0C.
Adsorption of sudan (III) on activated carbon was more favorable at acidic conditions. Kd value was 15.42 at pH 1. The adsorption of sudan (III) was improved at lower content of NaCl.
Adsorption isotherms of sudan (III) on activated carbon were more fitted to Freundlich equation than to Langmuir equation. Accordingly, heterogeneous active site was available for sudan (III) adsorption.