Adsorption of Rose Bengal Dye from Aqueous Solution onto Zinc Chloride Activated Carbon

Now a day’s synthetic dyes have been widely used in many industrial processes especially in the textile industry. The varieties of dyes include; acidic, reactive, basic, disperse, azo, diazo, anthraquinone based and metal complex. These dyes have a complex structure and low biodegradability [1]. These dyestuffs cause huge threat to the environment. Rose Bengal (RB) dye is harmful and can cause severe toxic effects on the human corneal epithelium. The dye is extremely hazardous when it comes in contact with skin and creates irritation, itching, scaling, reddening and even blistering. It also causes inflammation, redness, watering and itching in the eyes. On ingestion by inhalation it damages mucous membranes, which leads to respiratory irritation in humans. Numerous efforts and research have been made to remove these dangerous chemical compounds, there are many traditional techniques applied in the removal process such as coagulation, flocculation, activated carbon adsorption, membrane filtration, photo degradation and sedimentation. Among these methods, adsorption is proved to be superior to other techniques. Adsorption using activated carbon gave fruitful results for the removal of dye from wastewater. The extent of dye removal depends upon the kind of dye, nature of adsorbent and experimental conditions. Preparation of activated carbon from waste plant bio masses and evaluating its adsorbing potential is the recent trend of research. Fruit shell of Terminalia catappa Linn is a waste plant bio mass which is chosen as precursor for the preparation of activated carbon in this present investigation [2] and the Zinc chloride activation method is adopted as it has the advantage of producing excellent pores as reported in earlier literatures [3].


Introduction
Now a day's synthetic dyes have been widely used in many industrial processes especially in the textile industry.The varieties of dyes include; acidic, reactive, basic, disperse, azo, diazo, anthraquinone based and metal complex.These dyes have a complex structure and low biodegradability [1].These dyestuffs cause huge threat to the environment.Rose Bengal (RB) dye is harmful and can cause severe toxic effects on the human corneal epithelium.The dye is extremely hazardous when it comes in contact with skin and creates irritation, itching, scaling, reddening and even blistering.It also causes inflammation, redness, watering and itching in the eyes.On ingestion by inhalation it damages mucous membranes, which leads to respiratory irritation in humans.Numerous efforts and research have been made to remove these dangerous chemical compounds, there are many traditional techniques applied in the removal process such as coagulation, flocculation, activated carbon adsorption, membrane filtration, photo degradation and sedimentation.Among these methods, adsorption is proved to be superior to other techniques.Adsorption using activated carbon gave fruitful results for the removal of dye from wastewater.The extent of dye removal depends upon the kind of dye, nature of adsorbent and experimental conditions.Preparation of activated carbon from waste plant bio masses and evaluating its adsorbing potential is the recent trend of research.Fruit shell of Terminalia catappa Linn is a waste plant bio mass which is chosen as precursor for the preparation of activated carbon in this present investigation [2] and the Zinc chloride activation method is adopted as it has the advantage of producing excellent pores as reported in earlier literatures [3].for 3 hours.Thus the carbonized samples were washed with 0.5M HCl followed with distilled water until the pH of the washings attain 7.0.Then it was dried in a hot air oven at 383K for 1 hour.The dried material was ground and sieved to get particle size in between 73 µm and 150 µm.It was designated as T. catappa Zinc Chloride Activated Carbon (TCZAC).

Adsorption experiments
Batch experiments were conducted.The experiments were carried out in an orbital shaker at a constant speed of 130 rpm using 250 mL conical flasks containing predetermined dose of TCZAC with 50 mL of dye solution.After agitating, the samples were drawn from the flasks and the adsorbents were separated from the solution by centrifugation at 1000 rpm for 10 min.The absorbance of the of the supernatant solution was determined spectrophotometrically at λ max 530 nm using Systronics 2202 make UV-Visible spectrophotometer to estimate the residual dye concentration.Percentage of removal and the quantity of adsorbate adsorbed were calculated using the following equations.
( ) Where; C i and C t are the concentration of adsorbate (mg/ L) at initial stage and at time't' respectively V is the volume of solution (L) W is the mass of adsorbent (g).
Experimental results obtained from the effect of initial concentration and contact times were employed in testing the applicability of isotherm and kinetic models.

Effect of solution pH
Solution pH is an important factor which influences the transfer of adsorbate from aqueous solution to adsorbent by altering the speciation of the adsorbate as well as the surface charges of adsorbent.To study the effect of pH on adsorption, 50 mg of TCZAC was added to 50 mL of 20 mg/ L of the RB dye solution of the different pH.Initial pH of the solution was maintained by adding 0.1M HCl or 0.1NaOH.

Effect of adsorbent dosage
The effect of dose was studied by taking adsorbents from 10 to 100 mg/ 50 mL for 20 mg/ L of the RB dye solution.

Effect of contact time and initial concentration
Effect of initial concentration was studied by taking 10, 20, 30, 40 and 50 mg/ L of RB dye solutions.The % of removal was determined at regular time intervals up to 160 minutes of agitation time.

Kinetic Studies
Kinetic studies are necessary to determine the different operation conditions for the sorption of dye.The kinetics of RB onto TCZAC was analysed using pseudo-first order, pseudosecond order and intra particle diffusion kinetic models as below.

Pseudo -first-order kinetic model
The linearised form of the pseudo-first order equation of Lagergren is [4] ( ) e t e 1 log q q log q k / 2.303 t − = − × Where; q e and q t are the adsorption capacity at equilibrium and at time t respectively (mg/ g).k 1 is the rate constant of pseudo first-order adsorption.From the plot drawn between log (q e -q t ) and t, values of k 1 and q e can be calculated from the slopes and intercepts of the plot respectively.

The second-order kinetic model
The pseudo-second order kinetic model (Ho equation) is represented by the following linear equation [5].
Where; q e and q t are the adsorption capacity at equilibrium and at time t respectively (mg/ g).

Intra-particle diffusion study
The mechanism of adsorption of a sorbate on a sorbent follows a series of steps.The slowest of these steps control the overall rate of the process.Generally, pore and intra particle diffusion are often rate limiting in a batch reactor while for a continuous flow system, film diffusion is the rate limiting step [6].Previous studies [7][8][9] by various researchers showed that the plot of q t versus t 1/2 represents multi linearity which characterizes two or more steps involved in the sorption of sorbate by a sorbent.These involve transport of the solute molecules from the aqueous phase to the surface of the solid particulates and diffusion of the solute molecules into the interior of the pores, which is usually a slow process.
The effect of contact time experimental results can be used to study the rate limiting step in the adsorption process, as shown by Weber and Morris [10].Since the particles are vigorously agitated during the adsorption period, it is probably reasonable to assume that the rate is not limited by mass transfer from the bulk solution to the particle external surface; one might then postulate that the rate limiting step may be either film diffusion or intraparticle diffusion.As they act in series, the slower of the two will be the rate determining step.According to Weber and Morris, an intra-particle diffusion Co-efficient k p is defined by the equation: Solution onto Zinc Chloride Activated Carbon.SOJ Mater Sci Eng 3(2): 1-9.

Adsorption of Rose Bengal Dye from Aqueous Solution onto Zinc Chloride Activated Carbon
Copyright: © 2015 Nandhakumar et al.
Weber and Morris plot was drawn between q t and t 1/2 gave a straight line.Where k p (mg/ g/ min 0.5 ) is the intra particle diffusion rate constant and C is the thickness of the boundary film [11].The k p and C values obtained from the slopes and intercepts.

Effect of temperature studies
The adsorption study was carried out at different temperatures (303, 313, 323, 333, and 343K) for the RB solutions initial concentration of between 10 mg/ L, 20 mg/ L, 30 mg/ L, 40 mg/ L and 50 mg/ L, with the stirring for a period of 160 min.

Isotherm studies
It is important to fit the equilibrium data in different isotherm equations to give useful information of the adsorption process for the conception of the four isotherm models of adsorption were used in this study namely Langmuir, Freundlich, Temkin and Dubinin-Radushkevich.

Langmuir isotherm
The linear form of Langmuir isotherm equation is often written in as [12] e e e m m where q e is the amount of solute adsorbed per unit mass of adsorbent (mg/ g), C e the equilibrium concentration of solute in the bulk solution (mg/ L), q m is the maximum monolayer adsorption capacity or saturation capacity (mg/ g) and b is the adsorption energy, b is the reciprocal of the concentration at which half saturation of the adsorbent is reached.Values of 'q m ' and 'b' can be calculated from the slopes and intercepts of the graph is drawn between C e / q e and C e .
The essential characteristics of Langmuir isotherm can be described by a separation factor, R L , which is defined by the following equation ( ) Where; C 0 is the initial concentration of the adsorbate solution.The separation factor R L indicates the shape of the isotherm and the nature of the adsorption process as given below:

R L value
Nature of the process

Freundlich isotherm
The liner form of Freundlich isotherm model is represented by [13] ( ) e f e 1 lnq lnk lnC n = + Where; q e is the amount of adsorbate adsorbed (mg/ g), C e is the equilibrium concentration of dye solution (mg/ L) and k f and n are the constants incorporating all factors affecting the adsorption capacity and intensity of adsorption, respectively.Values of 'n' and 'k f ' were determined from slopes and intercepts of the graph drawn between the lnC e and lnq e .

Temkin isotherm
The Temkin isotherm assumes that the heat of sorption in the layer would decrease linearly with coverage due to sorbatesorbent interactions.Further the fall in the heat of adsorption is not logarithmic as stated in Freundlich expression.Linear form of Temkin isotherm equation is [14].Where, b T is the Temkin constant related to the heat of sorption (J/ mg) and a T the equilibrium binding constant corresponding to the maximum binding energy (L/ g) The Temkin constants a T and b T were calculated from the slopes and intercepts of graph drawn between q e versus lnC e.

Dubinin -Radushkevich isotherm
The Linear form of Dubinin-Radushkevich isotherm is [15].Where, q D is the theoretical saturation capacity (mg/ g) B is a constant related to the mean free energy of adsorption per mole of the adsorbate (mol 2 / J 2 ) and ε is Polanyi potential which is related to the equilibrium concentration as follows.
A plot of lnq e vs. ε 2 gives a linear line and the constants q D and B calculated from the slopes and intercepts respectively.The mean free energy of adsorption E was calculated from B using the following equation ( ) From this mean free energy of activation we can predict whether an adsorption is physisorption or chemisorption.If this energy is lesser than 8 kJ/ mol, the adsorption is physisorption and if the energy is more than 8 kJ/ mol, the adsorption is chemisorption.

Adsorption thermodynamics
Thermodynamic parameters such as change in free energy ∆G°(kJ/ mol), enthalpy ∆H°(kJ/ mol) and entropy ∆S° (J/ K/ mol) were determined using the following equations.

Results
Results of effect of pH study were depicted as graph in figure 3. Color of the RB dye solution disappeared below the solution pH 4 even in the absence of adsorbent.Hence the pH dependence of RB dye adsorption onto TCZAC was studied in the pH range of 4 to 11.The percentage removal increased as the pH of the solution increased in acidic medium up to 6. Afterwards the percentage removal found to decrease up to pH 11.Maximum percentage removal was observed at pH 6.
Percentage of removal of RB dye from aqueous solution with respect to different doses was shown in figure 4. The percentage adsorption increased with the increase of the carbon dosage.
Result of the percentage of removal of RB dye from aqueous solution with respect to different contact time and with different initial concentration (10, 20, 30, 40 and 50 mg/ L) was given Table 1 and shown in figure 5, and it is observed that the amount of solute adsorbed by the adsorbent, increased with the increase of initial concentrations of dye.
The results obtained from pseudo-first order and pseudosecond order kinetic models are presented in table 2 and concerned plots are shown in figure 6 and figure 7 correspondingly.
The results obtained from Intra particle diffusion model are presented in table 2 and concerned plots are shown in figure 8.The results obtained from the Temkin isotherm model are recorded in Table 3 and concerned isotherm plots are shown in figure 12.

Adsorption of Rose Bengal Dye from Aqueous Solution onto Zinc Chloride Activated Carbon
Copyright: © 2015 Nandhakumar et al.The constants obtained from Dubinin-Radushkevich isotherm are collected in table 3 and the concerned isotherm plots are shown in figure 13.
The results obtained from thermodynamics study are presented in table 5 and representative plots are shown in figure 14.
The effect of ionic strength is shown in the figure 15.

Adsorption of Rose Bengal Dye from Aqueous Solution onto Zinc Chloride Activated Carbon
Copyright: © 2015 Nandhakumar et al.
the increase of percentage removal of the dye up to pH 6.When the solution pH is above 6, the surface of the adsorbent becomes slightly positively charged and the dye might have acquired negative charge which would be attracted by the adsorbent.This positive charge on the adsorbent would have decreased with the increase of pH and hence the attraction between the dye anion and the adsorbent would have decreased.The decrease of percentage removal of the dye in alkaline medium that is pH of the solution is above 7 is attributed to electrostatic repulsion between the dye anion and the negative charge on the surface of the adsorbent.

Effect of adsorbent dosage
The increase of percentage adsorption with the increase of carbon dosage is due to increased carbon surface area and the availability of more adsorption sites [17].

Effect of contact time and initial concentration
The adsorption process is characterized by a rapid up take

Effect of solution pH
Disappearance of color of the RB dye below the solution pH 4 in the absence of adsorbent may be due to change in the structure of the RB dye as described in indicator theories.Trend in percentage of adsorption with respect to pH of the solution can be interpreted by; the point of zero charge (pH ZPC ) of TCZAC and the speciation of the RB dye at different pH of the solution as below.pH zpc of the adsorbent is 7.When the solution pH is below pH zpc that is 7,the surface of the adsorbent is positively charged [16].The lower percentage removal at lower pH infers that the dye might have acquired positive charge as reduced molecular ions (CH + ) and surface of the adsorbent would have rendered electrostatic repulsion towards dye cations.Increase of solution pH would decrease the positive charge on the adsorbent which explains

Adsorption of Rose Bengal Dye from Aqueous Solution onto Zinc Chloride Activated Carbon
Copyright: © 2015 Nandhakumar et al.   at initial stages.The percentage of removal increased with the increase of contact time.However, the time to attain equilibrium decreased with the increase of initial concentration of the dye solution.This is due to the decrease in the ratio of available adsorption sites to the concentration of solute in the solution with the increase of the initial concentration [18,19].Increase of quantity adsorbed with the increase of initial concentration is due to higher driving force obtained because of concentration effect in the solution phase.

Pseudo-first order and Pseudo-second order kinetic models
From the result of kinetics study it is observed the initial sorption rate, 'h', increased from 0.8757 to 1.5974 with an increase of initial concentration of adsorbate from 10 mg/ L to 50 mg/ L.
The regression coefficient (R 2 ) values were ranged from 0.987 to 0.997 and from 0.994 to 0.996 for the pseudo-first order and pseudo-second order kinetic models respectively.The best fitting kinetic model was determined by comparing the 'calculated adsorption capacity' (q e(cal) ) values from the respective kinetic models with the experimentally determined adsorption capacity (q e(exp) ) at equilibrium.The kinetic model which gives the closer 'q e(cal) ' values with the experimental 'q e(exp) ' values can be considered as the best fitting kinetic model.This can be known from the statistical tool 'Sum of Squared Error' (SSE).

( ) ( )
Where; N is the number of data points, ((q e ) exp ) is the experimental q e and ((q e ) cal ) is the calculated q e .The kinetic model which gives lowest SSE value is the best describing kinetic model.
Between the pseudo-first order and pseudo-second order, the pseudo second order kinetic model seems to best describe the adsorption of RB dye as the SSE value of pseudo-second order model is smaller than the pseudo-first order kinetic model.

Intra-particle diffusion study
The linear plots are attributed to the macro pore diffusion which is the accessible sites of adsorption.This is attributed to the instantaneous utilization of the most readily available adsorbing sites on the adsorbent surface.

Adsorption of Rose Bengal Dye from Aqueous Solution onto Zinc Chloride Activated Carbon
Copyright: © 2015 Nandhakumar et al.
The k p values were found to increase from 0.63 to 2.19 with an increase of RB dye concentration which revealed that the rate of adsorption is governed by the diffusion of RB dye within the pores of the adsorbent.

Effect of temperature
The increase of percentage of removal with the increase of temperature is due to increased surface activity and increased in kinetic energy of the dye molecule.Since the % of removal increased with the increase of temperature, the adsorption was endothermic nature.

Langmuir isotherm
The regression coefficient (R 2 ) values are ranged from 0.995 to 0.997 for all the studied temperatures.These results show the best fitting of the equilibrium data with Langmuir isotherm.Further it is noticed that adsorption capacities were slightly increased with an increase of temperature.
These the R L values were lie in between 0 and 1 which indicates the favourable adsorption of RB dye onto TCZAC.
In general Langmuir constant values infer a better performance of TCZAC.

Freundlich isotherm
The regression coefficient (R 2 ) for Freundlich isotherms are ranged from 0.994 to 0.969 for all the studied temperatures viz.303, 313,323,333 and 343K.It indicates that the experimental data fit well into the Freundlich isotherm models.
Further it is noticed that Freundlich constant the adsorption capacity k f (mg/ g) increased from32.734 to 110.917 with the increase of temperature.
The adsorption intensity 'n' values are ranged from 1.347 to 1.4104 i.e., between 1 and 10, which indicate the favourable physical adsorption [20].

Temkin isotherm
The regression coefficient (R 2 ) values ranged from 0.975to 0.983 and these results show the best fitting of the equilibrium data with Temkin isotherm.Equilibrium binding constant 'a T ' values (L/ g) and the Temkin constant related to heat of sorption, b T values (kJ/ mg) have low values and B value ranged from 0.002 to 0.001.Hence the adsorption belongs to physisorption.

Dubinin -Radushkevich isotherm
The regression coefficient (R 2 ) values are ranged from 0.879 to 0.898 for the five studied temperatures.The theoretical saturation capacity q D values (mg/ g) found to increase from 26.907 to 33.029 with the increase of temperature.
Values of the mean free energy E (kJ/ mol) ranged from 0.5 to 1.These very low values of 'E' support the physisorption interaction.

Adsorption thermodynamics
The negative values of ΔG° show that the adsorption is spontaneous.The positive values of ΔH° show the endothermic nature of adsorption process.The ΔH° values were within the range of 1-93 kJ/ mol which confirms the physisorption [21,22].The positive values of ΔS° show the increased disorder and randomness at the solid solution interface.During adsorption some structural changes occur at the surface of the adsorbent.The adsorbed water molecules which are displaced by the adsorbate species, might have gained more translational entropy than is lost by the adsorbate molecules, thus allowing the prevalence of randomness in the system [23].The enhancement of adsorption capacity of the activated carbon at higher temperatures was attributed to the enlargement of the size of the pore and the activation of the adsorbent surface.

Effect of ionic strength
Adsorption of Rose Bengal found to increase with an increase of NaCl concentration as shown in figure 15.This may be due to compression of the electrical double layer by the Cl ‾ anion and also due to common ion effect of Na + ion which drive dye to the sorbent.

Conclusion
Zinc Chloride Activated (TCZAC) was prepared from fruit shells of Terminalia catappa Linn, found to have good capacity to absorb RB dye.Equilibrium of adsorption was achieved around 50 to 160 minutes for the dosage of 50 mg/ 50 mL of solution at room temperature of 303 K for the initial concentrations of RB dye solutions ranging from 10 to 50 mg/ L. Maximum removal (77.5 % for the dye concentration of 10 mg/ L at 303 K) was recorded at pH 6.The % of removal found to increase with the increase of temperature (86.25 % for the dye concentration of 10 mg/ L at 303 K).Kinetic of the adsorption process was found to follow the pseudo second order kinetic model.Adsorption capacity data obtained from the Langmuir, Freundlich, and Dubinin-Radushkevich isotherms indicated that TCZAC was effective in removing RB dye from aqueous solution.Further the adsorption energy values obtained from Dubinin-Radushkevich and Temkin isotherms supported physisorption mechanism.The separation factor R L values of Langmuir isotherm indicated that the adsorption was favourable.The thermodynamics parameters indicated that the adsorption process is endothermic and spontaneous and physisorption in nature.

Declarations
It is purely individual work and not having any association with academic bodies for financial support.Therefore conflict of interest is not applicable.No human subject was used for this study.
Adsorption of Rose Bengal Dye from Aqueous Solution onto Zinc Chloride Activated CarbonCopyright: © 2015 Nandhakumar et al.

Figure 3 :
Figure 3: Effect of pH on the adsorption of RB dye.[RB]: 20 mg/ L; Dose: 50 mg/ 50 ml; contact time: 160 min the equilibrium constant, C solid is the solid phase concentration at equilibrium (mg/ L), C liquid is the liquid phase concentration at equilibrium (mg/ L), T is the temperature in Kelvin and R is the gas constant.The Van't Hoff plot is drawn between lnk d versus 1/ T.The values of change in free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were calculated from the slopes and intercepts of the plot respectively.

Figure 9
Figure 9 demonstrates the effect of temperature.That the percentage of removal of RB dye increased with, the increase of temperature from 303to 343K.The results obtained from Langmuir model are presented in tables 3 and concerned isotherm plots are shown in figure 10.The dimensionless separation factor R L values calculated for various initial concentrations at different temperatures are given in table 4. The results obtained from the Freundlich isotherm model are given in table 3. Representative isotherm plots are shown in figure 11.

Table 1 :
Percentage of removal of dye and amount of dye adsorbed at 303K.

Table 2 :
Kinetic parameters for the removal of RB dye onto TCZAC.Initial dye concentration (mg L -1 )

Table 3 :
Isotherm parameters for removal of RB dye onto TCZAC.

Table 4 :
Dimensionless separation factor (R L ) for adsorption of RB dye onto TCZAC.

Table 5 :
Thermodynamic parameters of rose Bengal by Terminalia catappa Linn.