Adsorptive Removal of Methylene Blue onto ZnCl 2 Activated Carbon from Wood Apple Outer Shell : Kinetics and Equilibrium Studies

Methylene blue dye removal from aqueous solution was investigated using ZnCl2 activated carbon prepared from wood apple outer shell (Limonia acidissima, biomass waste). Influence of agitation time, adsorbent dose, dye concentration, pH and temperature were explored. Two theoretical adsorption isotherms namely Langmuir and Freundlich were used to describe the experimental results. The Langmuir adsorption capacity (Qo) was found to be 35.1 mg/g and the equilibrium parameter (RL) values indicate favourable adsorption. The experimental data were well fitted with Langmuir isotherm model and pseudo second order kinetic model. Desorption studies showed that ion exchange mechanism might be involved in the adsorption process.


Introduction
Industrial development is directly related to pollution in all countries.Pollution caused by dyes is a common problem faced by countries which is likely to cause health hazards, harm to ecology, damage to structure or amenities and interferences with legitimate use of water 1 .Methylene blue (MB) dye causes eye burns and may cause nausea, vomiting, profuse sweating, mental confusion, painful micturition and methemoglobinemia or blue baby syndrome 2,3 .Therefore, the treatment of effluent containing such dye is of vital interest due to its esthetic impacts of receiving waters.
Adsorption process using activated carbons is widely used to remove pollutants from wastewaters.However, commercially available activated carbon is expensive.This has led to search for low cost material as alternative adsorbent material.In the last years, many researchers have studied the production of various type of activated carbons from different sources viz., palm-tree cobs 3 , palm kernels 4 , cassava outer shell 5 , bagasse 6 , jute fibre 7 , rice husks 8 , olive stones 9 , date pits 10 , fruit stones and nut shells 11 and wood apple outer shell 12 .Manufacturing advantage lies in utilizing rejected waste as raw material for making new effective carbon will renew the waste and cost effective.
In this work, we used ZnCl 2 activated wood apple carbon (ZAWAC) as adsorbent to remove the methylene blue dye from waste water.Wood apple is rich in oxalic, malic, citric acids and concentrated tannic acid.A mixture of the ripe pulp of the fruit and cardamom, honey and cumin seeds taken in the morning tones up sagging breast.It is also useful in preventing cancer of breast and uterus and helps to cure sterility.The carbon made from this biomass waste could yield a carbon with high porous structure and high surface area.

Experimental
The adsorbate, methylene blue (MB) was received from qualigens fine chemicals, India.Double distilled water was used for preparing all the solutions and reagents.

Preparation and characterization of activated carbon
Wood apple outer shell raw material (biomass waste) collected from industries was washed with hot distilled water.It was dried in sun light for 10 h and then the resultant material was grounded and stirred into a boiling solution containing ZnCl 2 in the ratio of 2:1.The filtered material after drying was carbonized by using Brick Kiln at elevated temperature for 3 days and then material was leached out using dilute HCl.Then the carbon was repeatedly washed to ensure the removal of excess of ZnCl 2 and dried.The carbonized material was sieved to 200 µm size and used for experiment.

Analysis of methylene blue
The concentration of methylene blue in the supernatant solution before and after adsorption was determined using a double beam spectrophotometer (Elico, Biospectrophotometer, India) at 661 nm.It was found that the supernatant from the activated carbon did not exhibit any absorbance at this wavelength and also that the calibration curve was very reproductive and linear.

Batch equilibrium studies
Adsorption isotherms were performed in Erlenmeyer flasks (250 mL) where solutions of dye (100 mL) with different initial concentration (10-60 mg/L) were placed in these flasks.Equal mass of 100 mg of activated carbon was added to dye solution and kept in a mechanical shaker for 7 h.to reach equilibrium of solid-solution mixture.Similar procedure was followed for another set of erlenmeyer flask containing the same dye concentrations without activated carbon to be used as a blank, the pH was adjusted to 7 by adding either few drops of diluted HCl (or) NaOH (0.1 M).The flasks were then removed from the shaker and the final concentration of dye in the solution was analyzed.Each experiment was duplicated under identical conditions.The amount of adsorption at equilibrium (q e ) was calculated by Where, C o and C e (mg/L) are the liquid-phase concentrations of the dye at initial and equilibrium, respectively.V is the volume of the solution (I) and W is the mass of dry adsorbent used (g).

Batch kinetic studies
The procedures of kinetics experiments were basically identical to those of equilibrium tests.The aqueous samples were taken at present time intervals, and the concentrations of the dye were similarly measured.The amount of adsorption at time (t), q t (mg/g), was calculated by Where, C t (mg/g) is the liquid-phase concentrations of the dye at any time (t), respectively.

Textural characteristics of prepared activated carbon
Textural of the carbon was characterized by before and after adsorption using FT-IR, SEM, XRD, surface area through BET studies.Surface functional groups and other parameters were determined using standard methods 13,14 .The characteristics of an activated carbon were presented in the Table 1.The FT-IR spectrums of the ZAWAC before and after adsorption were given in Figures 1 and 2 and the spectral interpretations were given in Table 2. Scanning Electron Microscope of the surface morphology of ZAWAC was given in Figure 3. From the SEM picture we find well aligned uniform pore net work is developed upon activation and pore shape is spherical layer structure.

Effect of agitation time and concentration of dye on adsorption
A series of contact time experiments for MB dye have been carried out at different initial concentration (10-60 mg/L) and at temperature of 28 °C.The amount of adsorption (q e ) at equilibrium increases from 1.74 to 22.8 mg/g with an increase in the initial dye concentration from 10 to 60 mg/L.It is evident that the activated carbon prepared from wood apple is efficient to adsorb MB dye from aqueous solution, the process attaining equilibrium gradually.This is due to the fact that activated carbon is composed of porous structure with large surface area (794 m 2 /g).Three consecutive mass transport steps are associated with the adsorption of solute from solution by porous adsorbent 15 .First, adsorbate migrates through the solution, i.e. film diffusion, followed by pore diffusion and finally the adsorbate is adsorbed into the active sites at the interior of the adsorbent particle.The similar phenomena were observed for the adsorption of MB dye from aqueous solution on jute fiber 7 and bamboo-based activated carbon and the equilibrium time was 24 h 16 .

Adsorption isotherms
The adsorption isotherm indicates how the adsorption molecules distribute between the liquid-Phase when the adsorption process reaches an equilibrium state.

Langmuir isotherm
Langmuir isotherm is based on the assumption that a) Maximum adsorption corresponds to a saturated monolayer of adsorbate molecules on the adsorbate surface.b) The energy of the adsorption is constant.c) There is no transmigration of adsorbate molecules in the plane of adsorbent surface 17

Freundlich isotherm
A well -known logarithmic form of Freundlich model is given by the following Equation, log q e = log k f + 1/nlog c e (5) Where q e is the amount of adsorption, k f is the Freundlich constant related to sorption capacity and 1/n is a constant related to energy or intensity of adsorption.This gives an expression encompassing the surface heterogeneity and the exponential distribution of activated sites and their energies.This isotherm dose not predicts any saturation of the adsorbent surface.The Freundlich exponents k f and 1/n can be determined from the linear plot of log q e vs. log c e is shown in Figure 8.The values of the Freundlich constants K F and 1/n are 4.5708 and 0.706 respectively shown in Table 4.The slope 1/n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneous, becoming more heterogeneous as its value gets closer to zero 18 .The results show that the adsorption also follows Freundlich isotherm.

Adsorption kinetics
The adsorption kinetics data of MB dye is analyzed using the lagergren 19 first order rate Equation, ln (q e ─ q t ) = ln q e ─ k 1 t  Where q e and q t are the amount of MB dye adsorbed (mg/g) at equilibrium and at time' t' (h -1 ) respectively and k 1 is the lagergren rate constant of first order adsorption (h -1 ).The values of k 1 calculated from the slope of the plot of ln (q e─ q t ) vs. t are shown in Figure 9.It is found that the calculated q e values do not agree with the experimental q e Values.This suggests that the adsorption of MB dye does not follow first order kinetics 20 .The second order kinetic model 21 can be represented as t/q t = 1/k 2 q e 2 + t/q e (7) Where k 2 is the equilibrium rate constant of second order adsorption [g(mg h) -1 ].The values of k 2 and q e are calculated from the plot of t/q vs. t is shown in Figure 10.The calculated q e values agree with experimental q e values and also the correlation coefficients for the second order kinetic plots at all studied concentrations above 0.9917 which is shown in Table 5.These results indicate that the adsorption system studied belongs to the second order kinetic model.Similar phenomenon has been observed in the adsorption of Cr(VІ) by used tires and saw dust 22 and Congo red on coir pith carbon 23 .

Effect of temperature
The effect of temperature, a major factor influencing the sorption, was monitored in the range of 35-60 o C. The change in standard free energy, enthalpy and entropy of adsorption were calculated using the following Equations,

Desorption studies
The adsorbent (10 mg/100 mL) that was used for the adsorption of 10 mg/L of MB dye, loaded adsorbent was filtered using Whatman filter paper and washed gently with distilled water to remove any unadsorbed MB dye.Several such samples were prepared.Then the spent adsorbent was mixed with 100 mL of distilled water, adjusted to a pH value in the range of 2.0 to 11.0 and agitated at time intervals longer than the equilibrium time.The desorbed MB dye was estimated as before.The present desorption of MB increases from 70.2 to 34.6.This indicates that the dye adsorption is mainly due to ion exchange and physical adsorption.

Conclusion
The present investigation showed that wood apple can be effectively used as a raw material for the preparation of ZnCl 2 activated carbon for the removal of MB dye from aqueous solution over a wide range of concentration.MB dye is found to adsorb strongly on the surface of activated carbon.Adsorption behavior is described by a monolayer Langmuir type isotherm.Kinetic data follow second order kinetic model.The value of the maximum adsorption capacity, Q o (35.1 mg/g) is comparable with the values for commercial activated carbon reported in earlier studies.

Figure 3 .
Figure 3. SEM picture of ZAWAC Figure 4 and 5, show the percentage of removal and amount of dye adsorbed onto activated carbon increases with time and, at some point in time, reaches a constant value beyond which no more is removed from solution.At this point, the amount of dye desorbing from the adsorbent is in a state of Time, min % Removal Removal Time, min qe dynamic equilibrium with the amount of the dye being adsorbed onto the activated carbon.The amount of dye adsorbed at the equilibrium time reflects the maximum adsorption capacity of the adsorbent under those operating conditions.

Figure 4 .Figure 5 .
Figure 4.The removal of MB dye with adsorption time at various initial time

Figure 6
Figure 6 shows the removal of MB dye by ZnCl 2 activated wood apple carbon at different adsorbent dosages (25-200 mg) for 100ml of MB dye solution concentration of 10-60 mg/L.The increase in adsorbent dosage increased the percent removal for MB dye.The maximum removals of MB dye are 98.9, 95.6, 91.4,88.4, 83.4 and 80.1 at 200 mg of adsorbent dosage.Increase in the percentage of removal with increase in adsorbent dosage is due to the increase in adsorbent surface area.

Figure 6 .
Figure 6.The effect of adsorbent doses on MB dye onto ZAWAC . Langmuir isotherm is expressed as, C e /q e = 1/Q o b + C e /Q o (3) Where Q o and b are Langmuir constant related to adsorption capacity and energy of adsorption respectively.Plot of C e /q e vs. c e is linear shown in Figure 7.The values of Q o and b are 35.1 mg/g and 0.2217 L/mg, respectively shown in Table 3.The essential characteristics of the Langmuir isotherm can be expressed by a dimensionless constant called equilibrium Parameter R L .R L = 1/1 + bC o (4) R L values indicate the type of isotherm.The R L value between 0 and 1 indicates favourable adsorption.

Figure 9 .
Figure 9. Pseudo-first order kinetics for adsorption of MB dye onto ZAWAC.

Figure 10 .
Figure 10.Pseudo-second order kinetics for adsorption of MB dye onto ZAWAC ) Where R is the gas constant, K c is the equilibrium constant and T is the temperature According to van't Hoff equation, ln k c = ∆H * /RT + ∆S * /R (9) Where ∆S * and ∆H * are change in entropy and enthalpy of adsorption respectively.A plot of ln K c versus 1/T is linear as shown in Figure 11.Values of ∆S * and ∆H * were 1/T10 -3 , 1/K In K evaluated from the slope and intercept of Van't Hoff plots.The positive values of ∆H * confirm the endothermic nature of adsorption.The negative values of ∆G * 35 o C, 40 o C, 50 o C and 60 o C indicate spontaneous nature of adsorption of MB dye as shown in Table6.The positive values of ∆S * suggest the increased randomness at the solid/solution interface during the adsorption of MB dye on ZnCl 2 activated wood apple outer shell carbon.

Figure 11 . 6 .
Figure 11.Van't Hoff Plot for adsorption of MB dye onto ZAWAC Table 6.Thermodynamics parameters for the adsorptive removal of MB dye onto ZAWAC.

Table 5 .
First and second orders kinetic Result