Research Article Open Access
Adsorption of Rose Bengal Dye from Aqueous Solution onto Zinc Chloride Activated Carbon
V Nandhakumar1*, A Rajathi1, R Venkatachalam1, K Ramesh2 and S Savithiri3
1Department of Chemistry, AVVM Sri Pushpam College, Poondi, India
2Department of Chemistry, Arasu Engineering College, Kumbakonam, India
3Department of Chemistry, Manakula Vinayagar Engineering College, Puducherry, India
*Corresponding author: V Nandhakumar, Associate professor of Chemistry, A.V.V.M Sri Pushpam College, Poondi, Tamil Nadu, S. India – 613006, Tel:+91 9894439942; E-mail: vnchem14@gmail.com, vnandhakumar12@yahoo.com
Received: July 10, 2015; Accepted: July 29, 2015; Published: August 10, 2015;
Citation: Nandhakumar V, Rajathi A, Venkatachalam R, Ramesh K, Savithiri S (2015) Adsorption of Rose Bengal Dye from Aqueous Solution onto Zinc Chloride Activated Carbon. SOJ Mater Sci Eng 3(2): 1-9. DOI: http://dx.doi.org/10.15226/sojmse.2015.00126
Abstract
A low cost activated carbon was prepared from fruit shell of Terminalia catappa Linn (Indian almond), waste plant residues and evaluated its adsorbing behaviour of Rose Bengal (RB) dye from aqueous solution by batch mode adsorption experiments. The activated carbon was prepared using zinc chloride as activating agent. This study examined the effect of contact time, adsorbent dosage, initial dye concentration, pH, other ions and temperature. Equilibrium data were obtained at 303, 313, 323, 333 and 343 K temperatures for the initial dye concentrations of 10, 20, 30, 40 and 50 mg/ L. Maximum dye removal capacity was observed at pH 6. Experimental data obtained were fitted into Lagergren, Ho and Weber Morris kinetic models for pseudo first-order, pseudo secondorder and intra-particle diffusion kinetic study. The adsorption kinetic was found to follow pseudo second-order kinetic model as per the statistical tool 'Sum of Squared Error' (SSE) result. The equilibrium adsorption data were analysed by Langmuir, Freundlich, Temkin and Dubinin–Radushkevich and isotherms. Thermodynamic parameters; Gibb's free energy (ΔG0), Enthalpy (ΔH0) and Entropy (ΔS0) of the adsorption process were also evaluated. Analysis of these values inferred that the adsorption was endothermic, spontaneous and proceeded with increased randomness.

Keywords: Adsorption; Rose Bengal dye; ZnCl2 Activated Carbon; Kinetics; Isotherms; Thermodynamic Parameters
Introduction
Materials and Methods
Rose Bengal, all so known as Acid Red 94 [Molecular formula C20H4Cl4I4O5] [IUPAC Name: 4,5,6,7–Tetra chloro – 3',6'– dihydroxy-2',4',5',7'-tetraiodo – 3H- Spiro [isobenzofuran-1,9'- xanthen]-3-one]. Maximum wavelength (λmax) for the absorption of this dye is 530 nm. Stock solution of 1000 mg/ L was prepared by dissolving 1 g of dye in 1000 mL of double distilled water. Required experimental solutions say 10, 20, 30, 40 and 50 mg/ L were prepared from the stock solution by proper dilution.
The Terminalia catappa Linn fruit shells were collected from A.V.V.M. Sri Pushpam College campus, Thanjavur District., washed with distilled water to remove the surface adhered particles, dried in sun light for 4 days, chopped into small pieces and powdered in a pulveriser. 50 g of the powder was mixed with 100 mL of 60% ZnCl2 solution. The slurry was kept at room temperature for 24 hours, to ensure the complete access of the ZnClS2 to the T. Catappa shell powder. Excess of solution was decanted and the slurry was heated in muffle furnaces at 723K 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).
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 λmax530 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;
Ci and Ct 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]

Where;
qe and qt are the adsorption capacity at equilibrium and at time t respectively (mg/ g). k1 is the rate constant of pseudo first-order adsorption. From the plot drawn between log (qe-qt) and t, values of k1 and qe 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].
$\text{t}}{{\text{q}}_{\text{t}}}=1}{{\text{k}}_{2}{\text{q}}_{\text{e}}^{2}}+{1}{\text{q}}}_{\text{e}}×\text{t}$
Where; qe and qt are the adsorption capacity at equilibrium and at time t respectively (mg/ g).
The initial adsorption rate, h (mg/ (g min)),
The Plot drawn between of t/ qt and t gave a straight line, from which 'calculated adsorption capacity qe (cal) mg/ g, and the second-order rate constants kS2 (g/ (mg min)) can be determined from the slopes and intercepts of plots respectively.
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 [7-9] by various researchers showed that the plot of qt versus t1/2 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 kp is defined by the equation:
${\text{q}}_{\text{t}}={\text{k}}_{\text{p}}{\text{t}}^{1/2}+\text{C}$
Weber and Morris plot was drawn between qt and t1/2 gave a straight line. Where kp (mg/ g/ min0.5) is the intra particle diffusion rate constant and C is the thickness of the boundary film [11]. The kp 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]
${\text{C}}_{\text{e}}}{{\text{q}}_{\text{e}}}=1}{{\text{q}}_{\text{m}}}\text{b}+{\text{C}}_{\text{e}}}{{\text{q}}_{\text{m}}}$
where qe is the amount of solute adsorbed per unit mass of adsorbent (mg/ g), Ce the equilibrium concentration of solute in the bulk solution (mg/ L), qm 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 'qm' and 'b' can be calculated from the slopes and intercepts of the graph is drawn between Ce/ qe and Ce.

The essential characteristics of Langmuir isotherm can be described by a separation factor, RSL, which is defined by the following equation
Where;
C0 is the initial concentration of the adsorbate solution. The separation factor RL indicates the shape of the isotherm and the nature of the adsorption process as given below:
 RL value Nature of the process RL > 1 Unfavourable RL = 1 Linear 0 < RL < 1 Favourable RL = 0 Irreversible
Freundlich isotherm
The liner form of Freundlich isotherm model is represented by [13]
Where; qe is the amount of adsorbate adsorbed (mg/ g), Ce is the equilibrium concentration of dye solution (mg/ L) and kf and n are the constants incorporating all factors affecting the adsorption capacity and intensity of adsorption, respectively. Values of 'n' and 'kf' were determined from slopes and intercepts of the graph drawn between the lnCe and lnqe.
Temkin isotherm
The Temkin isotherm assumes that the heat of sorption in the layer would decrease linearly with coverage due to sorbate - sorbent 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,
bT is the Temkin constant related to the heat of sorption (J/ mg) and aT the equilibrium binding constant corresponding to the maximum binding energy (L/ g) The Temkin constants aT and bT were calculated from the slopes and intercepts of graph drawn between qe versus lnCe.
Dubinin - Radushkevich isotherm
The Linear form of Dubinin-Radushkevich isotherm is [15].
Where,
qD is the theoretical saturation capacity (mg/ g) B is a constant related to the mean free energy of adsorption per mole of the adsorbate (mol2/ J2) and ε is Polanyi potential which is related to the equilibrium concentration as follows.
A plot of lnqe vs. ε2 gives a linear line and the constants qD 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.
Thermodynamic parameters such as change in free energy ΔG°(kJ/ mol), enthalpy ΔH°(kJ/ mol) and entropy ΔS° (J/ K/ mol) were determined using the following equations.
Figure 1: Structure of Rose Bengal dye.
Figure 2: Structure of Rose Bengal dye.
Figure 3: Effect of pH on the adsorption of RB dye. [RB]: 20 mg/ L; Dose: 50 mg/ 50 ml; contact time: 160 min
Figure 4: [RB]: 20 mg/ L; Contact time: 160min; pH: 6.
$\Delta {\text{G}}^{°}=-\text{RT}\mathrm{ln}{\text{k}}_{\text{d}}$

${\text{lnk}}_{\text{d}}=\Delta {\text{S}}^{°}}{\text{R}}-\Delta {\text{H}}^{°}}{\text{RT}}$
Where;
kd is the equilibrium constant, Csolid is the solid phase concentration at equilibrium (mg/ L), Cliquid 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 lnkd 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.
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. 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 RL 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.

The results obtained from the Temkin isotherm model are recorded in Table 3 and concerned isotherm plots are shown in figure 12.
Table 1: Percentage of removal of dye and amount of dye adsorbed at 303K.
 Initial concentration (mg/ L) Percentage of removal of dye at equilibrium Amount of dye adsorbed at equilibrium (mg/ g) 10 77.5 07.75 20 75.0 15.00 30 71.7 21.50 40 68.8 27.50 50 65.5 32.75
Figure 5: [RB]: 10-50 mg/ L; Dose: 50 mg/ L; pH: 6.
Table 2: Kinetic parameters for the removal of RB dye onto TCZAC.
 Initial dye concentration (mg L-1) Kinetic parameters 10 20 30 40 50 Pseudo – first order constant k1 (min-1) 0.032 0.025 0.020 0.018 0.014 qe, exp (mg g-1) 7.750 15.00 21.50 27.5 32.75 qe, cal (mg g-1) 5.26 10.18 13.87 17.82 19.41 R2 0.996 0.994 0.987 0.994 0.997 SSE 7.58 Pseudo-second-order constants k2 (g mg-1 min-1) 0.0130 0.0037 0.0020 0.0013 0.0012 qe, cal (mg g-1) 8.197 16.667 24.390 32.258 37.037 h (mg/ g min) 0.876 1.037 1.220 1.307 1.597 R2 0.996 0.996 0.996 0.995 o.994 SSE 2.83 Intra-particle diffusion constants kp (mg g-1 min-1) 0.63 0.94 1.10 2.04 2.19 C (mg g-1) 2.308 5.337 8.747 6.085 8.703 R2 0.998 0.998 0.997 0.999 0.998
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.
Figure 6: [RB]: 10-50 mg/ L; Dose: 50 mg/ 50 mL; pH: 6.
Figure 7: [RB]: 10-50 mg/ L; Dose: 50 mg/ 50 mL; pH: 6.
Figure 8: [RB]: 10-50 mg/ L; Dose: 50 mg/ 50 mL; pH: 6.
Figure 9: Adsorption isotherm of RB onto TCZAC at different temperature. [RB]: 10-50 mg/ L; Dose: 50 mg/ 50 mL; Contact time: 160 min
Discussion
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.

pHzpc of the adsorbent is 7.When the solution pH is below pHzpc 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
Figure 10: [RB]: 10-50 mg/ L; Dose: 50 mg/ 50 mL; Time: 160min.
Figure 11: [RB]: 10-50 mg/ L; Dose: 50 mg/ 50 mL; Time: 160 min.
Figure 12: [RB]: 10-50 mg/ L; Dose: 50 mg/ 50 mL; Time: 160 min.
Table 3: Isotherm parameters for removal of RB dye onto TCZAC.
 Isotherm model Temperature (K) 303 313 323 333 343 Langmuir constants qm ( mg g-1) 66.67 71.43 76.92 83.33 90.91 b (L mg-1) 0.059 0.062 0.067 0.075 0.086 R2 0.999 0.995 0.996 0.995 0.997 Freundlich constants kf (mg g-1) 32.73 41.59 54.83 74.30 110.9 n 1.410 1.399 1.389 1.372 1.348 R2 0.994 0.996 0.996 0.996 0.995 Temkin constants bT(J/ mg) 20.41 25.92 30.86 35.08 38.34 aT (L/ mg) 0.758 0.833 0.9345 1.065 1.267 R2 0.983 0.977 0.976 0.975 0.976 Dubinin-Radushkevich constants qD(mg g-1) 26.91 28.06 29.38 30.80 33.03 E (kJ mol-1) 0.500 0.707 0.745 0.845 1.000 R2 0.891 0.885 0.882 0.879 0.898
Figure 13: [RB]: 10-50 mg/ L; Dose: 50 mg/ 50 mL; Time: 160 min.
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
Table 4: Dimensionless separation factor (RL) for adsorption of RB dye onto TCZAC.
 Ci for RB dye Temperature (K) 303 313 323 333 343 10 0.146 0.617 0.596 0.573 0.538 20 0.079 0.446 0.425 0.402 0.368 30 0.054 0.349 0.330 0.309 0.280 40 0.041 0.287 0.270 0.251 0.225 50 0.033 0.243 0.228 0.212 0.189
Table 5: Thermodynamic parameters of rose Bengal by Terminalia catappa Linn.
 Ci for RB dye ΔG (kJ mol -1) ΔH (kJ mol-1) ΔS (Jk-1 mol-1) 303 K 313 K 323 K 333 K 343 K 10 -3.116 -3.608 -4.165 -4.803 -5.550 15.215 60.260 20 -2.768 -3.219 -3.724 -4.294 -5.237 15.364 59.487 30 -2.338 -2.745 -3.322 -3.985 -4.765 16.079 60.418 40 -1.987 -2.442 -2.951 -3.627 -4.423 16.304 60.044 50 -1.615 -2.083 -2.604 -3.192 -4.044 16.395 59.154
Figure 14: Plot of lnkd vs. 1/T for adsorption of RB onto TCZAC.
Figure 15: [RB]: 50 mg/ L; Dose: 50 mg/ 50 mL; Contact time: 60 min.
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 (R2) 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' (qe(cal)) values from the respective kinetic models with the experimentally determined adsorption capacity (qe(exp)) at equilibrium. The kinetic model which gives the closer 'qe(cal)' values with the experimental 'qe(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, ((qe)exp) is the experimental qe and ((qe)cal) is the calculated qe.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.

The kp 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 (R2) 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 RL 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 (R2) 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 kf (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 (R2) values ranged from 0.975to 0.983 and these results show the best fitting of the equilibrium data with Temkin isotherm. Equilibrium binding constant 'aT' values (L/ g) and the Temkin constant related to heat of sorption, bT 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 (R2) values are ranged from 0.879 to 0.898 for the five studied temperatures. The theoretical saturation capacity qD 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.
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 RL 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.
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