Adsorption of Methylene blue and Malachite Green in Aqueous Solution using Jack Fruit Leaf Ash as Low Cost Adsorbent

The adsorption of mixture of two basic dyes methylene blue and malachite green in aqueous solution onto jack fruit leaf ash in a batch system with respect to initial dye concentrations, pH, contact time, shaker speed and adsorbent doses was investigated.. The pseudo-first-order and pseudo-second- order kinetics model were used to describe the kinetic data. The rate constants at different pH values (3-9.1) were evaluated. The experimental data fitted well with the pseudo-second-order kinetic model. Equilibrium isotherms were analyzed by Langmuir, Freundlich and Temkin isotherm models. Maximum adsorption capacity was 20.41mg/g was achieved by Langmuir isotherm model. Error analysis was done to find the best model that described the experimental data well and it was the Langmuir model. The result indicated that jack fruit leaf ash could be fruitfully employed as low cost adsorbent for the removal of mixture of two basic dyes MB and MG from the wastewater.


I. INTRODUCTION
The textile and other dye using industries consume large amount of water for its different operational phases. The use of huge water for their production purposes, it is inhabitable that they discharge a huge volume of wastewater which is rich in color. Due to more and more increasingly stringent restrictions, it is essential to eliminate dye contamination from wastewater before discharging to the open environment. There are various techniques such as adsorption, chemical oxidation, filtration, coagulation etc. have been used to remove dye pollutants from the wastewater. Among these different methods for the removal of colors, it has been well established that adsorption is the most useful and effective technique to remove dyes from the wastewater. The advantages of adopting this technique in wastewater treatment is because of its simple operation, higher efficiency and ability to separate wide range of chemical compounds. The use of commercially available activated carbon is better choice but it involves huge cost, makes adsorption process infeasible. So in the recent years research is mainly focused on utilizing natural agricultural by products such as jack fruit leaf, neem leaf, rice husk, bagasse fly ash etc. available as abundant materials in rural Bengal. The use of such waste materials as low cost adsorbent is a typical attempt for this purpose. The basic dyes methylene blue and malachite green are one of the important and widely used dyes in textile and other dye using industries. In practice the wastewater from industries contain mixture of dyes. However, no literature is available since last 20 years to remove mixture of dyes from the wastewater. Both the dyes MB and MG are water soluble and very difficult to remove from the wastewater by simple removal technique. So the adsorptive removal of mixture of basic dyes using jack fruit leaf ash (JFLA) as low cost adsorbent is a typical attempt of the present work to solve the real life problem. The effects of adsorbent doses, contact time, pH, shaker speed and initial concentration of the solution dyes were investigated. The Langmuir, Freundlich and Temkin isotherms were used to fit the equilibrium data. Pseudofirst-order and Pseudo-second-order kinetic models were attempted.
Methylene Blue (MB) and Malachite Green (MG) are two cationic dyes. Two dyes of equal proportion was taken to prepare stock solution of 1000mg/L. The working solution was prepared by diluting the stock solution to give the appropriate concentrations.

Methylene Blue
Malachite Green Fig.1: Chemical structure of MB and MG

Batch adsorption experiments
All adsorption experiments were carried out by agitating the jack fruit leaf ash of required amount with 200 mL dye solution of different concentrations as required in a 250 mL bottle at constant room temperature in a shaker at 120 rpm except during varying shaker speed observation. The experiment was carried out for various JFLA dosage, initial concentrations, shaker speed and pH. At the end of the predetermined shaking time the samples were withdrawn and centrifuged at 5000 rpm for 10 minutes. The resulting supernatant was then analyzed using spectrophotometer.
The amount of adsorption at equilibrium, qe (mg/g), was computed as follows: where, C0 and Ce are the initial and equilibrium solution concentrations (mg/L) respectively, V is the volume of the solution (L), and ms is the weight of JFLA used (g).
In the experiments of batch kinetic adsorption, 200 mL of the chosen desired concentration of the stock solution of two dyes in mixture (1:1) were placed in the measuring bottle of 250 mL together with 5 gm JFLA and agitated by shaker at room temperature (30 0 C) and at normal pH. At predicted intervals of times, samples were taken, and their concentrations were determined by spectrophotometer.

Effect of adsorbent dosage
The effect of JFLA dosage on the amount of dyes adsorbed was investigated by containing 200mL of dye solution with initial concentration of each dyes 12.5mg/L, mixing equally (1:1) to make 25mg/L at room temperature 30 0 C at 120 rpm. Different amount of JFLA (0.1 to 12 gm) for methylene blue and malachite green were applied. The result showed that when the adsorbent dosage increased from 0.1 gm to 5gm the removal of two dyes increased from 15.83 to 90.49% and then reached a plateau. It is reported that the larger adsorption surface caused higher adsorption of dyes. Therefore, for convenience the adsorbent dosage for the present study was selected as 5 gm as equilibrium dosage. The plot of dye removal (%) versus adsorbent dosage (g/L) is shown in the fig.2. The adsorbent dose was taken as 5gm, initial concentrations of mixed dye solution as 25mg/L, shaker speed as 120 rpm. The experiment was done for 6 hrs. It can be seen that the rate of adsorption under various fixed other operating conditions, the percentage removal increased rapidly for initial 10 to 165 mins and beyond that contact time no noticeable change in the percentage removal was observed. As the equilibrium time is also function of initial dye concentration, 165 min is sufficient to reach equilibrium study. The percentage removal at 165 min was noticed as 95.72%, and considered to be the optimal contact time for the adsorption study. Natural pH Shaker speed = 120 rpm Adsorbent dose = 25mg Initial concentration od dyes = 25mg/l at initial concentration 25mg/L was 88.53% and that decreased to 58.25% at concentration of 150mg/L, under fixed other operating conditions. It can be concluded that the percentage removal is decreased by increasing the initial concentrations of dye mixture.

Effect of pH of the initial solution of dyes
The effect of initial pH on the adsorption of mixed dyes MB and MG onto JFLA is shown in the figure 6.As these two are cationic dyes, adsorbed onto the JFLA effectively at higher pH. The removal efficiency increased from 94.87 to 99.18 % as the pH of the solution increased from 3.0 to 9.1. Fig.6: Effect of initial pH on adsorption onto JFLA

Adsorption kinetics
The mechanism of solute sorption onto a sorbent can be expressed by different kinetic models. To explore the fast and effective model the different parameters of adsorption mechanism such as chemical reaction, diffusion control and mass transfer to be evaluated by these kinetic model equations.
3.6.1 Pseudo-first-order equation Pseudo-first-order equation is generally represented as where qe is the amount of dye adsorbed at equilibrium, qt the amount of dye adsorbed at time t and k1 is the equilibrium rate constant of pseudo-first-order kinetics. Integrating the equation (1)

Pseudo-second-order equation
The Ho and Makay's pseudo-second-order chemisorption kinetic rate equation is expressed as where, qe and qt are the amount of dye adsorbed at equilibrium and any time t respectively. K2 is the rate constant of pseudo-second-order model.  From the above table, adsorption kinetics of dye mixture was studied and the rates of sorption were found to be confirmed to pseudo-second-order kinetics with good correlation coefficient value.

Adsorption isotherm
Design optimization of an adsorption system for the adsorption of adsorbate appropriate correlation for the equilibrium curve is very important. Langmuir, Freundlich and Temkin isotherm were tested in the present investigation.

3.7.1
Langmuir isotherm In this theory the basic assumption is that the sorption takes place at specific homogeneous sites within the adsorbent. The equation can be written as where, qe is the amount of dye adsorbed onto JFLA at equilibrium concentration of solution dyes, KL the equilibrium constant and Qm is the maximum adsorption capacity.
The linear form of the equation (5) is The essential characteristic of the Langmuir isotherm can be expressed by the dimensionless constant called equilibrium parameter, RL, defined by Where, kL is the Langmuir constant and C0 is the initial solution of the mixture of two dyes, RL value indicates the type of isotherm to be irreversible (RL=0), favourable (0<RL<1), linear (RL=1) or unfavourable (RL>1). The isotherm constants were determined from the linear plot of 1/qe versus 1/Ce (Figure- Where, KF is the adsorption capacity at unit concentration and 1/n is adsorption intensity. 1/n values indicate the type of isotherm to be irreversible (1/n=0), favourable (0 <1/n <1) or unfavourable (1/n> 1). To examine the acceptance of the model with respect to the experimental data, the plot of logCe versus log qe plotted. From the Figure 8 Freundlich constants KF and 1/n were evaluated and are given in the Table-3. where, r= gas constant (8.314 J/mol/K), T= Temperature in 'K' and B1= RT/b. From the plot of qe versus log Ce (Fig. 9) the isotherm constants were evaluated and are given in the Table-4.

Statistical analysis
The statistical analysis is employed in the present work due to inherent bias resulting from linearization. Five different error functions (Table-4) for statistical analysis were carried out to find the optimum isotherm model which best fited with the data obtained from the experimental run. The another type of statistical analysis named chi-square (X 2 ) test was also carried out to explore the significance of the effects of the parameters on present investigation of mixed dye removal and the residual concentrations of dye mixture at equilibrium.
Selection of best isotherm model Since each of the error function produces a different set of isotherm parameters, it is difficult to identify directly an optimum set. Thus to find the best model a normalization of each parameter was employed. In the normalization processes first each error function was selected in turn and the results for each parameter set were determined.
The normalized values of each isotherm against error analysis are given in the Table-5. The values from the Table clearly showed that Langmuir equation was best followed the equilibrium data, same as predicted by coefficient of regression (R 2 ) value. In order to avoid such uncertainty in claiming a specific isotherm model, it is thus imperative to carry out a more normalized error analysis as elucidated earlier in the form of chi-square test. The calculated values of chi-square test under various isotherm models are given in the Table -6. It can be seen that the adsorption of the two dyes in mixed solution follows Langmuir isotherm very well. IV. CONCLUSION Equilibrium and kinetic studies were done for the adsorption of mixture of two basic dyes methylene blue and malachite green onto jack fruit leaf ash. The biosorbent exhibited high sorption capacities towards the mixture of two dyes. The kinetic studies of dye mixture on JFLA were performed based on pseudo-first-order and pseudo-second-order rate mechanism. The data indicate that the adsorption kinetics followed pseudo-second-order rate expression at the pH values of 3.0 to 9.1. The equilibrium data using Langmuir, Freundlich and Temkin isotherms and the characteristic parameters for each isotherm have been determined. The results showed that the experimental results were correlated reasonably well by Langmuir adsorption model. Error analysis was conducted and established Langmuir model more appropriate over the other two models. Results of the adsorption showed that jack fruit leaf ash can effectively used as a biosorbents for the removal of model cationic dyes methylene blue and malachite green from the mixture of aqueous solution.