Evaluation of Thermodynamic Parameters of 2 , 4-Dichlorophenoxyacetic Acid ( 2 , 4-D ) Adsorption

ermodynamic parameters of 2, 4-Dichlorophenoxyacetic acid (2, 4-D) adsorption were evaluated by studying the adsorption equilibrium and kinetics of 2, 4-D at different temperatures. Uptake capacity of activated carbon increases with temperature. Langmuir isotherm models were applied to experimental data of 2, 4-D adsorption. Equilibrium data �tted very well to the Langmuir equilibrium model. Adsorbent monolayer capacity Q, Langmuir constant aa and adsorption rate constant kkaa were evaluated at different temperatures for activated carbon adsorption.e activation energy of adsorption (EEaa) was determined using the Arrhenius equation. Using the thermodynamic equilibrium coefficients obtained at different temperatures, the thermodynamic constants of adsorption (ΔGG, ΔHH, and ΔSS) were evaluated. e obtained values of thermodynamic parameters show that the adsorption of 2, 4-D is an endothermic process.


Introduction
Along with industrial activities, agricultural activities also contribute in water pollution to the greater extent.Hence wastewater without an efficient treatment is becoming a serious problem.Among the numerous agrochemicals in use today, the herbicide 2, 4-D has been widely applied to control broad-leaved weeds in gardens and farming.Consequently, it has been frequently detected in water bodies in various regions of the world.e adsorption process is one of the efficient methods to remove organics from effluent [1,2].Activated carbons have also been employed to remove organic contaminants from wastewater and gaseous wastes [3][4][5].Electrostatic and van der Waals forces, H-binding, dipole-dipole interactions, ion exchange, covalent bonding, cation bridging, and water bridging can be responsible for the adsorption of organic compounds on activated carbon [6][7][8][9][10][11][12][13].It is reported that the pore size of the activated carbons signi�cantly in�uence the adsorption capacity of natural organic materials subject to their molecular sizes [14].Several researchers studying dye adsorption on activated carbon [15,16] have pointed out that the presence of mesopores together with micropores in the activated carbon enhances their adsorption capacities, especially, for large adsorbates [17,18].ese reports suggest that the pore size distributions of activated carbons decide their proper applications.
Generally an elevated temperature provides faster rate of diffusion of adsorbate molecules through the solution to the adsorbent surface and into adsorbent.Zogoroski [4,5] obtained the adsorption isotherm for phenol at different temperatures and concluded that adsorption decreases with the increase in temperature and concluded that adsorption is an exothermic process.On the contrary Tütem et.al. [19] concluded that the adsorption of chlorophenols from aqueous phase on bituminous shale was endothermic and basically of physical character.e equilibrium uptake of 2, 4-D by GAC was also affected by temperature and increases with increasing temperature [20].
In the present investigation, equilibrium and kinetics studies of adsorption of 2, 4-Dichlorophenoxyacetic acid (2, 4-D) from aqueous solution on granular activated carbon has been carried out at �ve different temperatures to evaluate thermodynamic parameters.e adsorption equilibrium data for the adsorbate-adsorbent systems studied were expressed by Langmuir isotherm model and a simpli�ed rate expression based on Langmuir adsorption theory was used to evaluate the adsorption "  " and desorption "  " rate constants.is data was then used to calculate the energy of activation of adsorption and desorption also the thermodynamic parameters, namely, the free energy of adsorption "Δ 0 ", enthalpy of adsorption "Δ 0 ", and the entropy of adsorption "Δ 0 ".

Adsorption Equilibrium Studies.
A 500 mL round bottom �ask containing 250 mL of distilled water was immersed in the thermostat bath.e content were constantly stirred at 800 ± 50 RPM and allowed to attain the temperature of the bath.Aer the temperature was reached, a calculated quantity of the stock solution was introduced into the distilled water with the help of graduated pipette.e solution was allowed to mix thoroughly and the same quantity of the resulting solution was pipetted out to maintain the �nal volume 250 mL, which was then used for the determination of initial concentration.0.25±0.001g of the adsorbate sample was then introduced into the solution with the stirring speed at 800 ± 50 RPM.e time of addition of GAC was noted.Stirring was continued till the concentration of the aqueous phase showed no detectable change in UV absorbance.Preliminary experiments show that the adsorbate uptake pro�le was independent of stirring speed above 600 rpm [21] and the equilibrium was attained in about 4 hours.As a precautionary measure, experiments were continued for �ve hours.Some of the experiments were carried out over a prolonged period of time where no signi�cant difference was observed in the adsorbate concentration.

Adsorption Kinetics.
For adsorption kinetics studies a 5-liter Borosil glass beaker �tted with six ba�es was used.ree liters of experimental solution was prepared by adding appropriate amount of stock solution into boiled and cooled distilled water.3.00 ± 0.001 g of given GAC sample was introduced into the solution at a given instant of time.5 mL of experimental solution was withdrawn at desired interval of time with the help of syringe and the concentration of adsorbate in the aqueous phase was estimated by UV analysis.Properties of adsorbent GAC F-400 and adsorbate used in the present study are given in Tables 1 and 2 respectively.

Result and Discussion
e adsorption isotherms obeyed the Langmuir equation with a very high regression coefficient above 0.98 indicating a very good linear �t in all the cases.e Langmuir adsorption isotherm is represented by the following equation [21]: Langmuir isotherm plot at different temperatures is depicted in Figure 1.e plot also shows the Langmuir equation obtained by linear regression of the data.e Langmuir isotherm characteristics can be expressed in terms of dimensionless constant separation factor   [22] represented as where  is the Langmuir constant and  0 initial adsorbate concentration;   values are given in  where, e adsorption "  " and desorption "  " rate constants were thus evaluated by plotting ln[(  −  )/(  +) against .Figures 2 and 3 depict these plots for the adsorbate-adsorbent for all the studied temperatures in the present work.e plots also show the rate expressions obtained by linear regression analysis of the kinetic data.
e experimentally determined values of adsorbent monolayer capacity " 0 ", Langmuir constant "" for the adsorbate-adsorbent systems at all the �ve temperatures are summarized in Table 3. e adsorption and desorption rate constants obtained are given in Table 4.

Effect of
Temperature.e results obtained in the present investigation are very informative.e equilibrium uptake of 2, 4-D by GAC is affected by temperature and increases with the rise in temperature.e adsorption desorption rate were found to decrease with the rise in temperature.e increase in the adsorption capacity with rise in temperature illustrates that 2, 4-D adsorption on to GAC F-400 is an endothermic process.e increase in the adsorption capacity of activated carbon at higher temperature may be attributed to the enlargement of pore size or activation of the adsorbent surface or creation of some new active sites on the surface of adsorbent due to bond rupture.is could also be due to the enhanced mobility of 2, 4-D ions from the bulk solution towards the adsorbent surface and extent of penetration within GAC structure overcoming the activation energy barrier and enhancing the rate of intraparticle diffusion [6,23,24].

ermodynamic Parameters.
From the variation of b (thermodynamic distribution coefficient or Langmuir constant) with temperature, thermodynamic parameters Δ 0 , Δ 0 , and Δ 0 namely, the standard free energy change, standard free enthalpy change, and standard free entropy change, respectively, were calculated using the following equations, and Vant' Hoff equation, e Vant' Hoff plot of ln  against 1/ was plotted (Figure 4) which shows an excellent linearity.Δ 0 and Δ 0 are obtained from slope and intercept of the plot and are given in Table 4. e values of Δ 0 for all the studied temperatures are found to negative.e negative values of free energy are the indicative of spontaneous process with a high affinity of the adsorbate to the surface of adsorbent.e positive value of enthalpy change points the endothermic nature of adsorption process.

Energy of Activation of Adsorption. e activation energy of adsorption was determined using Arrhenius equation
e plot of ln   versus 1/ was plotted (Figure 5), and the linear regression analysis gave a linear regression coefficient above 0.980.  value for the 2, 4-D-GAC system is given in Table 5.

Conclusion
(i) 2, 4-D adsorption using GAC is very rapid in the �rst hour of contact where 70% of the adsorbate is removed by GAC followed by a slow approach to equilibrium.(ii) e Langmuir constant (), related to the energy of adsorption, increased with temperature for all the (iii) Increase in the value of "ln " indicate that the rate of adsorption is much higher than the rate of desorption.
(iv) e 2, 4-D adsorption by GAC is a complex and is probably a combination of external mass transfer, intraparticle diffusion, and sorption process.ermodynamic constants were also evaluated using equilibrium constants changing with temperature.
(v) e negative value of Δ 0 indicated the spontaneity and the positive values of Δ 0 and Δ 0 showed the endothermic nature and increase in disorder of 2, 4-D adsorption, respectively.
(vi) Energy of activation of adsorption indicates the physical nature of the process.
(vii) e kinetics of adsorption is an activation process that is as the temperature of system increases the rate of the removal of adsorbate increases whereas adsorption capacity decreases.
(viii) us as the temperature of the system increases, the rate of adsorption increases.

F 5 :
Plot of energy of activation of 2, 4-D adsorption.

Table 3
o t (min) F 2: Adsorption kinetics of 2, 4-D at different temperatures.type of isotherm.  values between zero and one indicate favorable adsorption for 2, 4-D in the studied concentration range.
T 3: Effect of temperature on adsorption capacity and Langmuir constant.
T 5: Effect of temperature on adsorption rate constants.