Adsorption of dyes from aqueous solutions on activated charcoal

Abstract

Adsorption of industrially important dyes namely bromophenol blue, alizarine red-S, methyl blue, methylene blue, eriochrome black-T, malachite green, phenol red and methyl violet from aqueous media on activated charcoal has been investigated. The effect of shaking time, pH and temperature on the adsorption behaviour of these dyes has been studied. It was noted that adsorption of all the dyes on activated charcoal decreases with an increase in the pH and the temperature. The adsorption isotherms at different temperatures were found to be of L-type. Adsorption data was fitted to Freundlich, BET and Langmuir isotherms and various adsorption parameters have been calculated. The thermodynamic parameters such as ΔG, ΔH and ΔS were calculated from the slopes and intercepts of the linear variation of ln K against 1/T, where K is the adsorption coefficient obtained from Langmuir equation, was used. The calculated values for the heat of adsorption and the free energy indicate that adsorption of dyes is favored at low temperatures and the dyes are chemisorbed on activated charcoal.

Introduction

Industrial, agricultural and domestic wastes, due to the rapid development in the technology, are discharged in the several receivers. Generally, this discharge is directed to the nearest water sources such as rivers, lakes and seas. Textile dyeing process is an important source of contamination responsible for the continuous pollution of the environment. The volume of wastewater containing processed textile dyes is on steady increase. Over 7 × 10⁵ tonnes and approximately 10,000 different types of dyes and pigments are produced world wide annually [1]. It is estimated that 10–15% of the dye is lost in the effluents during the dyeing process. Colour is a characteristic of wastewater, which is easily detected. Control of water pollution has importance for both organisms, which live in water and those who benefit from water. Many dyes reaching the water source are difficult to decompose and cause many problems due to their carcinogenicity [2], [3], [4]. Consequently, it is important to remove these pollutants from wastewater before their final disposal.

The methods of color removal from industrial effluents include biological treatment, coagulation, flotation, adsorption, oxidation and hyperfiltration. Among the treatment options, adsorption has become one of the most effective and comparable low cost method for the decolourization of textile wastewater [5], [6]. Different adsorbents have been used for the removal from aqueous solutions of various materials, such as dyes, metal ions and other organic materials includes perlite [7], [8], [9], [10], [11], [12], bentonite [13], silica gels [14], fly ash [15], [16], lignite [17], peat [18], silica [19], etc. Activated carbon is a structurally homogeneous material of high surface area, has microporous structure and show radiation stability. It is therefore widely used in various industrial process as adsorbent [20], [21], [22], catalyst or catalyst support [23]. The adsorption properties of activated carbon depend mainly on its particle size, porosity, ash contents, degree of carbonization and method of activation.

This paper reports the results of the adsorption of alizarine red-S, bromophenol blue, malachite green, methyl violet, methylene blue, phenol red, methyl blue and erichrome black-T from aqueous solutions on activated charcoal. The adsorption behavior of the dyes as a function of temperature, pH and shaking time were also studied. Adsorption data was fitted to Freundlich (Eq. (1)), Langmuir (Eq. (2)) and BET (Eq. (3)) isotherms and their corresponding adsorption parameters such as K_F, n and K and V_m, respectively, have been calculated.

$$\text{log}\frac{x}{m}=\text{log}{K}_{\text{F}}+\frac{1}{n}\text{log}{C}_{\text{s}}$$

where x/m is the amount adsorbed per unit mass of the adsorbate, C_s the equilibrium concentration, and 1/n and K_F are constants. The constant K_F is related to the degree of adsorption, n provides the rough estimation of the intensity of the adsorption:

$$\frac{C_{\text{s}}}{x/m}=\frac{1}{K{V}_{\text{m}}}+\frac{C_{\text{s}}}{V_{\text{m}}}$$

$$\frac{C_{\text{s}}/C_0}{x/m(1-C_{\text{s}}/C_0)}=\frac{1}{x_{\text{m}}C}+\frac{C-1}{x_{\text{m}}C}\frac{C_{\text{s}}}{C_0}$$

where K is the adsorption coefficient, V_m the monolayer capacity, x_m the amount of dye required to form the monolayer over the surface of adsorbent and C is the constant.

In order to fully understand the nature of adsorption, the thermodynamic studies play an important role. This paper also presents the thermodynamics parameters related to the adsorption of dyes such as free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS), which have been calculated using following equations [24], [25], [26], [27]:

$$\Delta G=-RT\text{ln}K$$

$$\text{ln}K=\frac{\Delta S}{R}-\frac{\Delta H}{RT}$$

$$\Delta G=\Delta H-T\Delta S$$