Removal of Malachite Green from Aqueous Solution by Activated Carbon Developed from Cocoa ( Theobroma Cacao ) Shell-A Kinetic and Equilibrium Studies

The removal of malachite green (MG) by cocoa (Theobroma cacao) shell activated carbon (CSAC) was investigated in present study. Adsorption studies were performed by batch experiments as a function of process parameters such as initial pH, contact time, initial concentration and adsorbent dose. A comparison of kinetic models applied to the adsorption of MG on CSAC was evaluated for the pseudo-first order and pseudo-second order kinetic models. Results showed that the pseudo-second order kinetic model was found to correlate the experimental data well. The experimental equilibrium adsorption data was represented with Langmuir, Freundlich, Tempkin, DubininRadushkevich and Flory-Huggins isotherms. The experimental data obtained in the present study indicated that activated carbon developed from cocoa shell can be attractive options for dye removal from waste water.


INTRODUCTION
Dyeing industries are the largest sector of chemical industries in India.Over 7 × 10 5 tons and approximately 10000 different types of dyes and pigments are produced worldwide annually 1 .Dyes used as colouring materials are toxic to aquatic organisms when unspent substances are discharged into the environment.It is estimated that 10% of the dye is lost in the effluent during the dyeing process 2 .The treatment process for removing dyes from waste water comprises ozonation 3 , coagulation 4 , ultrafiltration 5 , oxidation 6 , photocatalytic degradation 7 and adsorption 8 .Adsorption has been found highly efficient for removal of color from wastewater in terms of initial cost, ease of operation and simplicity of design 9 .Adsorption using activated carbon is the most widely used technique because of excellent adsorption efficiency for organic compounds 10 .Activated carbon is a highly porous material; therefore it has an extremely high surface area for contaminant adsorption.The equivalent surface area of 1 pound of activated carbon ranges from 60 to 150acres 11 .In recent years due to high cost of activated carbon, attention has prompted interest in the production of carbon based adsorbents from agricultural waste products 12 .The production of activated carbon from wastes saves the non renewable natural resources and producing a valuable product with potential applications in pollution control 13 .
In the present study it was proposed to evaluate the capacity of cocoa shell, an agricultural waste as a precursor for the preparation of activated carbon material.This adsorbent was successfully used to remove Malachite Green (MG) dye from aqueous solution.Batch studies were carried out involving processes such as initial dye concentration, contact time, pH and adsorbent dosage.Equilibrium and kinetic data analysis were conducted to understand sorption process and optimization of various parameters in dye recovery was investigated.

Preparation of adsorbent
Cocoa shell collected from local agricultural field was air -dried and carbonized with concentrated sulphuric acid in the weight ratio of 1:1(w/v).The resulting black product was kept in muffle furnace maintained at 550°C for 7 hours.The carbon obtained was washed with distilled water until it became free from excess acid and dried at 110°C for overnight in a hot air oven.The carbonized material was taken out, grounded to fine powder and sieved to 150µm size and stored in a vacuum dessicator.

Preparation of dye solution
Stock solution of Malachite Green (Chemical formula: C 23 H 25 N 2 Cl, M.W.:364.92,C.I. 42000, CAS no.:569-64-2) was prepared by dissolving 1g of dye in 1L of distilled water to give concentration of 1000 mg/L.The serial dilutions say 20, 40, 60, 80 mgL -1 were made by diluting the dye stock solution in accurate proportions.The pH of dye solutions were adjusted with 0.1M NaOH or 0.1M HCl using a pH meter.

Batch Adsorption experiments
Adsorption experiments were carried out in temperature controlled orbital shaker at a constant speed of 125 rpm at 35°C using 250ml conical flasks containing 100mg of CSAC with 50ml of dye solutions.All the experiments (except the study of pH effect) were carried out at pH of 7.0.After agitating the flasks for predetermined time intervals samples were withdrawn from the flasks.The adsorbents were separated from the solution by centrifugation (REMI make) at 2000rpm for 10 minutes 14 .The absorbance of the supernatant solution was estimated to determine the residual dye concentration 15 , measured at λ max = 617nm spectrophotometrically using Elico make UV-Visible spectrophotometer.
The adsorption isotherms were specified at pH 7 for adsorption of MG onto CSAC at 35°C.Adsorption data obtained from the effect of initial concentration and contact time were employed in testing the applicability of isotherm and kinetic equations, respectively.

Effect of agitation time and initial concentration
To study the effect of dyes initial concentration and contact time on adsorption uptake, MG solution with initial concentrations 20-80 mgL -1 were agitated with 100mg of CSAC.The experimental results of sorption of MG on to CSAC at various initial concentrations are shown in Figure1.The adsorption at different dye concentrations was rapid at the initial stages and then gradually decreases with the progress of adsorption until the equilibrium was reached.The rapid adsorption at the initial contact time can be attributed to the availability of the positively charged surface of activated carbon.As shown in Fig. 1, the contact time needed for MG solution to reach equilibrium was 120min.The results indicated that there was no change in the sorption capacity after 120mins, therefore 180mins was fixed as the agitation time for isotherm studies.The adsorption capacity at equilibrium (q e ) increased from 9.09 to 50.00 mgg -1 with an increase in the initial concentrations from 20 -80 mgL -1 .

Effect of adsorbent dosage
In order to investigate the effect of adsorbent mass on the adsorption, a series of adsorption experiments was carried out with different adsorbent dosages at initial concentration of 40mg/lt without changing the volume of dye  2).Similarly the pH (7.0) and temperature (35°C) was kept constant.The results follow the expected pattern, in which the percentage sorption increased with increased adsorbent dosage.The increase in carbon dosage is due to adsorbent dosage or due to conglomeration of carbons at higher doses 16 .

Effect of pH
The pH is one of the most important factor for controlling the adsorption of dye on to the adsorbent.The pH of the system exerts influence on the adsorptive uptake of adsorbate molecules.The effect of solution pH was studied between 2 to 10, initial pH controlled by the addition of 0.1M HCl or 0.1M NaOH and agitated with 100mg of adsorbent for 3 hours.The effect of initial pH of dye solution on the adsorption of MG for initial dye concentration of 40 mg/L was illustrated in Fig. 3.As the pH increases from 2 to 7, the percent adsorption increases from 33.33 to 86.67% and from pH 7 to 10 the percent adsorption decreases from 86.67 to 40.00%.The maximum uptake of MG by CSAC was obtained at pH 7.0.So, pH 7.0 was chosen for the study on adsorption isotherm.

Adsorption Isotherms
The five most common adsorption isotherm models were used to fit the equilibrium adsorption data.These are Langmuir, Freundlich, Tempkin, Dubinin-Radushkevich and Flory-Huggins model.

Langmuir isotherm
Langmuir adsorption isotherm model was usually adopted for homogenous adsorption and it is used successfully in monomolecular adsorption processes 17 .Linear form of Langmuir model was expressed by ... (1)   where, C e is equilibrium constant of dye (mg/L), q e is amount of dye adsorbed at equilibrium (mg/g), Q 0 is Langmuir constant related to adsorption capacity (mg/g), b is Langmuir constant related to energy of adsorption capacity (L/mg).The linear plot of C e /q e versus C e is shown in Fig. 4(a).The constants Q 0 and b can be calculated from slope and intercept of the plot and the values are tabulated in Table 1.The shape of the Langmuir isotherm was investigated by the dimensionless constant separation term (R L ) to determine high affinity adsorption.R L is calculated as follows ... (2)   where, C o is the initial dye concentration(mg/L).R L indicates the type of isotherm to be irreversible (R L = 0), favorable (0 < R L < 1), linear (R L = 1) (or) unfavorable (R L > 1) 18 .In the present investigation, the R L values were less than one which shows the adsorption process was favorable.

Freundlich isotherm
The Freundlich isotherm model was chosen to estimate the adsorption intensity of the adsorbate on the adsorbent surface 19 .Linear form of Freundlich model is expressed by ... (3)   where, q e is dye concentration in solid at equilibrium (mg/g), C e is dye concentration in solution at equilibrium (mg/L), K f (L/mg) and 1/n are adsorption capacity at unit concentration and adsorption intensity, respectively.1/n values indicate the type of isotherm to be irreversible (1/n = 0), favorable (0 < 1/n < 1), unfavorable (1/n > 1).The linear plot of logq e versus logC e is shown in Fig. 4(b).The values of 1/n and k f can be calculated from the slope and intercept respectively and the results are given in Table 1.The value of 1/n was less than one indicating the favorable adsorption 20 .

Dubinin-Raduskevich (D-R) isotherm
The D-R isotherm was used to determine the characteristic porosity and the apparent free energy of adsorption 21 .The linear form can be represented as ... (4)   where, K DR is a constant related to the mean free energy of adsorption (mol 2 J -2 ), q m is the theoretical saturation capacity(mol g -1 ), ε is the polyani potential (Jmol -1 ), which is related to the equilibrium concentration(C e , gL -1 ) as follows ... (5)   The slope of the plot of lnq e versus ε² gives K (mol 2 J -2 ) and the intercept yields the adsorption capacity, q m (mgg -1 ).Fig. 4(c) shows D-R plot and the results are given in Table 1.The mean free energy of adsorption (E) (KJmol -1 ) is calculated from the equation 22 ...( 6)

Tempkin isotherm
The Tempkin isotherm model predicts a uniform distribution of binding energies over the population of surface binding adsorption 23  T is the absolute temperature in Kelvin, R is the universal gas constant, 8.314 J (mol K) -1 , b is the Tempkin constant related to heat of sorption(J/ mg).The Tempkin constants α and b are calculated from the slope and intercept of q e versus lnC e (Fig. 4(d)) and parameters are given in the Table 1.

Flory-Huggins model
The Flory-Huggins model was used to determine for the degree of surface coverage characteristics of the adsorbate on the adsorbent 24 .The linear form of the Flory-Huggins equation is expressed as ... (10)   where, θ = (1-C e /C o ) is the degree of surface coverage, n is the number of dye occupying adsorption sites, K FH is the equilibrium constant (L mol -1 ).K FH and n can be determined by plotting ln (θ/C o ) versus ln (1-θ) (figure not shown) and the parameters are given in Table1.K FH and ΔG o are related as follows ... (11)   where, ΔG o is the standard free energy change , R is the universal gas constant and equal to 8.314 Jmol -1 K -1 and T is the absolute temperature.The values of ΔG o calculated was negative which shows that the adsorption process is spontaneous in nature and supports an exothermic nature.

Adsorption kinetics
Kinetic studies are necessary to optimize different operation condition for the sorption of dyes.The kinetics of MG onto CSAC was analyzed using pseudo-first order and pseudo-second order kinetic models.

Pseudo -first order kinetic model
It is based on the fact that the change in dye concentration with respect to time is proportional to the power one 25 .The differential equation is described as follows: ... (12)   where, q e and q t are the adsorption capacity(mg/g) at equilibrium and time t, respectively and k 1 is the rate constant (min -1 ) of pseudo first order kinetic model.
The integrated linear form of the model is as follows: ... (13)   A plot of log (q e -q t ) versus t gives a linear line(Fig.5(a)) from which the values of k 1 and q e can be determined from the slope and intercept respectively and presented in Table 2.This value showed that the q e calculated was not equal to q e experimental, although the values of R 2 are satisfactory.This shows that the kinetics for the entire process did not follow the pseudo-first order model.

Pseudo -second order kinetic model
The adsorption mechanism over a complete range of the contact time is explained by the pseudo -second order kinetic model 26  ... (14)   where, q e and q t are the adsorption capacity(mg/g) at equilibrium and time t, respectively and k 2 is the rate constant of pseudo second order adsorption (g/mg min).
The linearised form of the above model is ... (15)   The initial adsorption rate, h (mg/g min), as t → 0 can be defined as ...( 16) A plot of t/q t versus t gives a linear relationship (Fig. 5(b)), from which q e and k 2 can be determined from the slope and intercept of the plot respectively and presented in Table 2.The values of q e calculated show good agreement with experimental data.The correlation coefficient R 2 was close to unity.Thus the sorption of MG by CSAC could be approximated more appropriately by the pseudo -second order model 27 , supporting the assumption of chemisorptions as the rate-limiting mechanism through sharing or exchange of electrons between sorbent and sorbate 28 .

CONCLUSION
The present study shows that the agricultural waste material, cocoa shell, can be effectively used as an adsorbent for the removable of malachite green from aqueous solution.The adsorption characteristics have been examined by initial pH, contact time, initial concentration and adsorbent dose.The equilibrium data were evaluated by Langmuir, Freundlich, Tempkin, Dubinin-Radushkevich and Flory-Huggins isotherms.Kinetic studies showed that the adsorption process followed the pseudo-second order model and pseudo-second order rate constant, k 2 , decreases form 2.5 x 10 -3 to 1.03 x 10 -3 g/mg min with increase in initial dye concentration for CSAC.Based on the data of present study, it could be employed that CSAC is an effective adsorbent for the removal of MG from aqueous solution.