Adsorption Study of Some Sulphanilic Azo Dyes on Charcoal

Abstract: Studies on the removal of two dyes (sulphanilic azo antipyrine and sulphanilic azo imidazole) from aqueous solution by adsorption on charcoal as an adsorbent were carried out. A series of experiments were under taken in a batch adsorption technique to access the effect of the process variables i.e. contact time, initial dye concentration, initial pH, adsorbent dose and temperature. Adsorbent dosage (0.1 g) higher value for both dyes. The equilibrium in the solution was observed within (35 min) of two sulphanilic dyes on charcoal. The equilibrium isotherms for both dyes were determined to describe the adsorption process. The results showed that the equilibrium data was fitted by of the Freundlich isotherms on charcoal surface. The result obtained shows that the adsorption isotherm for both dyes on charcoal was according to Giles classification. The thermodynamic factors such as ∆Η, ∆G and ∆S were calculated.


Introduction
Charcoal is often used as an adsorbent for the removal of a different of organic dyes because it is highly inert and thermally stable, it can also be used at different hydrogen function range 1 .Activated carbon is the most commonly used adsorbent, although other materials such as activated clay, wood and types of cellulose have also been studied 2,7 .Some of azo compounds are stable and do not suffer biodegradation and those including metals are toxic.Consequently it is important to remove these pollutants from the wastewaters before their final disposal 8,9 .Adsorption methods have been reported as an effective process to remove organic dyes from wastewaters.The widely used adsorbent for industrial residue is activated carbon 10,11 .Many methods including coagulation precipitation, filtration and oxidation have been used for the treatment of dye-containing effluents 12,13 .These methods are costly and can not be used effectively to treat the wide range of dye wastewaters when compared with adsorption processes.The aim of this study was to determine the adsorption capacity of charcoal in aqueous organic dyes varying with process factors including initial dye concentration, contact time, initial pH, adsorbent dose and temperature.

Experimental
All spectral and absorbance measurements were carried out on APEL -PD -303 UV visible recording spectrophotometer.Shaking inductor GCA/precision scientific Chicago, U.S.A and Maga fuge 1.0 Herouse centrifuge were used .A digital pH meter Hanna was used.The charcoal sample used as adsorbent was supplied from India where as the sulphanilic azo imidazol and sulphanilic azo antipyrine were prepared according to the equations by Hussain 14 (Scheme 1).From the difference between initial concentration and equilibrium, the amounts of dye A and B adsorbed were calculated by the following relation: Where, Q e amount of dye adsorbed (mg.g -1 ), C o and C e are the initial and equilibrium concentration (mg.L -1 ) respectively.M is the adsorbent dosage (in g), V sol is the solution volume (in mL).The adsorption capacity was determined with the effect of contact time, initial concentration of dye A and B, pH, temperature and agitation rate.The equilibrium concentration, adsorption capacity at equilibrium was determined to fit in the adsorption isotherms.

Calibration graphs of two dyes
In to a series of ten calibrated flask, different volumes of dye A and B working solution (100 mg.mL -1 ) were transferred to cover the range of the calibration curve.Linear calibration graphs of two dyes were obtained, that obeyed Beer's law over the concentration range of (10 -100 ppm) with correlation coefficient 0.9981, 0.9889 and an intercept of 0.2, 0.53 for dye A and B respectively.

Adsorption isotherms
The Freundlich models are the most frequently employed models.The Freundlich isotherms have been widely adopted to characterize the adsorption capacity of dyes pollutant using different adsorbent by fitting the adsorption data (Figure 3).(2) This equation can be modified as: Where, K f and 1/n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively, of the sorbent.The values of K f and 1/n can be obtained from the intercept and slope respectively of the linear plot of experimental data of log Q e versus log C e (Table 2 Figure 4&5).The values showed that the equilibrium data for two dyes fitted well to the Freundlich isotherms in the studied concentration ranges.Based on the correlation coefficients (r) the equilibrium data was fitted in the isotherms Freundlich equations (Table 2).
Many authors have used these isotherm to evaluate the adsorption capacity by different adsorbent with different dyes 15,16 .The results showed that the adsorption of dye A and B on charcoal was found to be effective at pH 12 and 2 respectively.The Freundlich equations were used to study data concerning the dependence of the adsorption of two dyes on charcoal at pH 12 and 2.
The adsorption constants evaluated from the isotherms with correlation coefficient are (Table 2) very high, regression coefficient (0.9601, 0.9669) was found for dye A and B respectively.The regression values showed that the equilibrium data for dyes fitted well to the Freundlich isotherm in the studied concentration ranges.

Effect of pH
In order to study the hydrogen ion concentration (pH) on sulphanilic azo imidazole and sulphanilic azo antipyrine by adsorbent charcoal, dyes solutions were prepared in the range of 10-100 ppm and adjusted to different pH values (2, 4, 7, 9 and 12) using 0.1 N of HCl and 0.1 N of NaOH .The results are shown in Figure 6 & 7.It is clear that the amount of dyes removed at various pH.pH 12 produces a large adsorbed quantity for dye A, whereas pH 2 produces a large adsorbed quantity for dye B. It is clear that, the lower pH value (Figure 6) explained on the basis of formation of a positively charged surface on charcoal.At solution pH acidic, the removal capacity was expected to decrease, as the adsorbent was positively charged and the dye molecules were neutral charged Figure 7.

Contact time
The influence of the contact time on the adsorption capacity of two dyes by adsorbent charcoal was conducted through batch experiments to achieve the equilibrium as shown in Figure 8.

Adsorbent dosage
In order to study the effect of adsorbent dosage on dyes removal as the adsorption capacity with fixed initial concentration of type of dye, pH, temperature ,agitation rate and charcoal as an adsorbent.Different weight of dosage was used (0.02, 0.03, 0.05, 0.07, 0.09, 0.10 and 0.12 g).The maximum removal of dyes was observed with the dosage more than 0.09 g and 0.1 g used for all subsequence experiments.And the entropy was calculated from the equation 20 ∆G = ∆ H -T ∆ S (7) The thermodynamic factors evaluated from isotherms are shown in Table 3.The up take of two dyes decreases with an increasing temperature up to 40 °C.The results indicated that the adsorption process is endothermic in nature 21,22 .Many variables such as the molecular volume of the dyes, its planarity and its ability to bind to the adsorbent, among others, can affect the degree of adsorption.The two temperatures was the favorite for adsorption of sulphanilic azo dyes on charcoal due to the thickness of the boundary layer decrease with the increase in solution temperature.Therefore, the increasing tendency of dyes to escape from the solid phase to the liquid phase, as a result of increase in kinetic energy of the adsorbent species at high temperature 23 .
More dye B is adsorbed on charcoal than dye A dye.This difference is most related to the higher affinity of the dye B on the charcoal surface.Also this may be due to a tendency for the dye A molecule to escape from the solid phase of charcoal to the liquid phase of dye with an increase in temperature of the solution 24  The apparent adsorption enthalpy is -4.49kcal /mol for the sulphanilic azo imidazole dye and -7.41 kcal /mol for sulphanilic azo antipyrine.The low value of adsorption enthalpy indicates a physical adsorption 17 .

Figure 1 & 2 )
which corresponds to the maximum absorption peak of dye A and B.

Figure 3 .
Figure 3. Adsorption isotherms of dye A and B at pH 12 and 2 respectively The Freundlich isotherms have the general from such as: Qe =K f Ce ½(2) This equation can be modified as:

Figure 4 .Figure 5 .
Figure 4. Freundlich isotherm of dye A with charcoal at pH 12

Figure 6 .
Figure 6.Removal of colour from aqueous solution of sulphanilic azo antipyrine at various pH

Figure 7 .
Figure 7. Removal of colour from aqueous solution of sulphanilic azo imidazole at various pH

Figure 8 .
Figure 8. Adsorption capacity against contact time of dyes A and B with charcoal The results indicated that the high affinity between the sorbent surface and the reactive dye molecules the dye molecule favorably adsorb on the carbon surface with low competition from water molecule Adsorption isotherms were taken for the dyes in the temperature range of 298 to 313 K.The results are shown in Figure9& 10.The thermodynamic parameters ln K, ∆G, ∆S and ∆H were computed.

Figure 9 . 12 Figure 10 .
Figure 9.Effect of temperature on the adsorption capacity of dye A with charcoal at pH 12

Figure 11 Figure 11 .Figure 12 .
Figure 11.Relationship between the log Xm and 1/T of dye A with charcoal as an adsorbent

Table 1 .
Structures, molecular weights and wavelengths of the dyes

Table 2 .
Frundlich constant for two dyes in aqueous solution using charcoal as an adsorbent

Table 3 .
Thermodynamic values of sulphanilic dyes in aqueous solution using charcoal as an adsorbent