Studies on the Preparation and Characterization of Activated Carbon from Elephant Elephas Maximus Dung and its Adsorption of Dyes from Aqueous Solution

Abstract

Adsorption is a process by which an adsorbent can attract acomponent to its surface in aqueous solution and form an attachment viaphysical or chemical bond, thus removing the component from the liquidphase. Adsorption process is an excellent choice because it does not requireany additional pretreatment steps before its application. The advantages ofadsorption over other treatment methods are: high efficiency, minimization ofchemical or biological sludge, no additional nutrient requirement, andregeneration of the adsorbent. Adsorption by activated carbon has been foundto be an effective and widely employed means of water and wastewatertreatment. The main focus of this study is to prepare and characterizeactivated carbon from elephant dung (EDAC), an animal waste, and to utilizeit as an effective low cost adsorbent for the removal of acid [Acid Blue92(AB92) and Acid yellow 17 (AY17)], basic [Rhodamine B (RhB), CrystalViolet (CV) and Malachite Green (MG)] and reactive [Reactive Orange 16(RO16) and Reactive Yellow 15 (RY15)] dyes from aqueous solution.The adsorbent (EDAC) was characterized by spectral, elementalanalysis and physico chemical methods. The presence of functional groupswas analyzed by Fourier Transform Infra red Spectroscopy (FTIR). Thesurface morphology was examined by Scanning Electron Microscope (SEM).The amorphous nature of the adsorbent was confirmed by X-Ray Diffractionmethod (XRD). The thermal stability of EDAC was tested by Thermo ivGravimetric Analysis (TGA). Estimation of Carbon, Hydrogen, Nitrogen andSulphur was done by Elemental analysis. The surface area of EDAC wasdetermined by Braunneur–Emmet–Teller (BET) surface area analyzer. Theacidic (carboxylic, phenolic and lactonic) and basic groups covering thesurface of EDAC were determined by Boehm titration method. The otherphysico-chemical parameters were characterized by standard procedures.The adsorption performance of EDAC was studied by conductingbatch adsorption experiments. The operation parameters investigated includeinitial dye concentration and contact time, pH of the solution, adsorbentdosage and temperature. Three different kinetic models such as Pseudo firstorder, Pseudo second order and Elovich model were employed to analyze thekinetics of adsorption. The results indicated that the kinetics followed Pseudosecond order model. Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models were adopted to describe the existence ofadsorbate between the liquid and solid phase. The adsorption of all dyes wasbest described by Langmuir isotherm. The effect of pH on the removal of alldyes was studied over a pH range of 2-11. Highly acidic solution pH favoredthe removal of acid and reactive dyes whereas neutral and alkaline mediumfavored the removal of basic dyes, except RhB which showed maximumremoval at pH 3. The mechanistic aspects of adsorption and possibility ofrecovery of adsorbent were investigated by conducting desorption studies.Acid and reactive dyes were desorbed by alkaline medium indicating thephysisorption mechanism. Basic dyes were desorbed by organic mediumindicating chemisorption mechanism. vMass transfer studies were examined by carrying out intraparticlediffusion and film diffusion models to elucidate the mechanism of adsorption.The results suggested that the adsorption process was controlled by bothparticle diffusion and film diffusion. The magnitude of activation energy (Ea)was evaluated for confirming the nature of adsorption. The thermodynamicparameters such as Gibb’s free energy of adsorption (G), enthalpy ofadsorption (H) and entropy of adsorption (S) were determined byconducting a series of experiments at different temperatures (301, 308, 318,328 and 338 K). The results indicated that the adsorption was endothermicand spontaneous.The amounts of EDAC required for the 90% removal of volume of20 L of AB92, AY17, RhB, CV, MG, RO16 and RY15 solution ofconcentration of 60 mg/L were calculated based on the Langmuir isothermequation. The applicability of EDAC was tested by treating with an effluentcollected from a dyeing unit. Competitive adsorption of dyes was investigatedat the same time by taking AB92, RhB, CV, MG and RY15 mixture. From theresults it was found that more than 90% of acid and basic dyes were removedwhereas reactive dye was not removed considerably. The adsorptionperformance of EDAC for the removal of RY15 was compared withCommercial Activated Carbon (CAC) by carrying out kinetics and isothermalstudies. The results revealed that EDAC could be employed as an alternativeto commercial adsorbents in wastewater treatment for the removal of dyes.