Optimum sorption isotherm by linear and non-linear methods for malachite green onto lemon peel

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Abstract

Equilibrium studies were carried out at 305 K for the sorption of malachite green onto lemon peel. The equilibrium data were fitted to the Freundlich, Langmuir and Redlich–Peterson isotherms by linear and non-linear methods. Non-linear method is a better way to obtain the isotherm parameters. The best fitting isotherm was found to be the Langmuir and Redlich–Peterson isotherm. Redlich Peterson is a special case of Langmuir when the constant g equals unity.

Introduction

Adsorption process is proved to be an effective process for the removal of colour from dye wastewaters [1]. Activated carbon is the most commonly used adsorbent for the removal of various pollutants from wastewaters. However, due to the high cost and the loss involved in regeneration, there is a continuous search for low cost adsorbents as an alternate to activated carbon. In the present research, lemon peel, a waste generated from juice manufacturing industry was used as an adsorbent. The analysis and design of adsorption process require information on the equilibrium adsorption isotherms [1]. The three widely used isotherms were the Freundlich [2], Langmuir [3] and Redlich–Peterson [4] expressions. Linear regression by the method of least squares was the most commonly used method to estimate the isotherm parameters [1], [5]. However, linearization of non-linear isotherm expressions may alter the error distributions and violate the normality assumptions of the least square method [1], [5]. In the present study both linear non-linear methods were used to estimate the isotherm parameters of malachite green onto lemon peel. The dye malachite green is selected as a model compound in order to evaluate the capability of lemon peel to remove dye from wastewaters.

The lemon peel used in the present study was obtained from the university canteen. The obtained lemon peel was cut into small pieces using scissors. Then the lemon peels were dried at 100 °C for 24 h using hot air oven. The dried materials were then ground using a domestic Sumeet mixer. The ground materials were then sieved through −90 + 105 mesh to get uniform geometrical size for use.

The dye used in all the experiments was malachite green, a basic (cationic) dye. Synthetic dye stock solutions were prepared by dissolving 2 g of malachite green in 2 L of double distilled water. All working solutions were prepared by diluting the stock solution with distilled water.

Batch sorption experiments were carried out at 305 K. Dye solution (30 mL) of dye concentration ranging from 200 mg/L to 25 mg/L was taken in 100 mL capped conical flasks. Accurately weighed 0.05 g of lemon peel was added to the solution. Then the flasks were agitated using a water bath shaker at a constant agitation speed of 180 rpm. The contact was made for 24 h, which is more than sufficient time required to reach equilibrium. After shaking, the samples were then centrifuged to separate the lemon peel from the solution. The left out concentration in the supernatant dye solution was analyzed using UV spectrophotometer.

Section snippets

Results and discussions

The non-linear and the linearized expressions of the Freundlich [2], Langmuir [3] and Redlich–Peterson [4] isotherms are shown in Table 1. From Table 1 it was observed that Langmuir isotherm can be linearized to at least four different types. The Freundlich isotherm parameters can be obtained from the plot between Log qe and Log Ce. Similarly, the Langmuir isotherm parameters for type 1, type 2, type 3 and type 4 Langmuir can be obtained from the plots between Ce/qe and Ce, 1/qe and 1/Ce, qe and q

Conclusions

The present study shows that the waste material, lemon peel, can be effectively used as an adsorbent for the removal of the malachite green from its aqueous solution. The equilibrium data were found to be well represented by the Freundlich, Langmuir and Redlich–Peterson isotherms. Non-linear is the more appropriate method to obtain the isotherm parameters. Redlich–Peterson is a special case of Langmuir when the constant g equals unity.

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