Studies on Synthesis and Characterization of Activated Carbon Prepared From Cocoa Theobroma Cacao Shell and Its Adsorption Modeling of Dissolved Organic Pollutants

Abstract

Adsorption is one of the most commonly used methods in watertreatment processes. It is attractive due to it easy operation and the availabilityof a wide variety of commercial adsorbents. Adsorbents prepared from wastematerials for the treatment of dyeing effluents have high significance inenvironmental sustainability. The objective of the present study is tocharacterize quality and evaluate the efficiency of using an agriculturalbyproduct, cocoa(Theobroma cacao) shell activated carbon as an adsorbentfor the removal of acid (AR18, AB92), basic (RhB, MG) ,direct (CR, DB GL)and reactive (RO16, RB2) dyes. The BET surface area and pore sizedistribution of CSAC were determined by adsorption-desorption nitrogenisotherms at the temperature of –196°C using surface area analyzer. Surfacemorphology, thermal stability and functional groups were examined usingScanning electron microscopy (SEM), Thermogravimetric analysis (TGA)and FTIR studies, respectively. The operation parameters investigated inbatch mode process include initial dye concentration and contact time, pH ofsolution, adsorbent dosage and temperature.The equilibrium adsorption isotherms have been examined byapplying Langmuir, Freundlich, Temkin, Harkins-Jura and Halsey isothermmodels. The adsorption of all dyes was found to follow Langmuir model. Thecalculated dimensionless separation factor, RL, indicated that the adsorptionof all dyes onto CSAC was favorable. Pseudo-first order, pseudo-second order ivkinetics and Elovich model were used to analyze the kinetic data. Adsorptionkinetics analyzed using the regression results showed that the adsorptionkinetics was more accurately described by a pseudo-second order model forwhole systems of dyes studied. To test the recoverability of the adsorbed dyemolecules, desorption experiments were done with neutral pH water,sulphuric acid (1M), sodium hydroxide (1M) and 50 % acetic acid (v/v) as thedesorbing agents. Desorption of all dyes studied except DB GL in acetic acid(organic medium) indicated the adsorption of dyes on CSAC was through byactivated chemisorption mechanism. Desorption of DB GL by alkalinemedium confirmed the physisorption mechanism.Mass transfer studies by intraparticle and liquid film diffusionmodels for the adsorption process was found to be controlled by both particleand film diffusion. Analysis of adsorption data using Boyd plot confirms thatfilm diffusion was the rate limiting step in the sorption process. Themagnitude of activation energy (Ea) was evaluated for confirming the type ofadsorption. Various thermodynamic parameters, such as Gibbs free energychange (ǻG0), enthalpy change (ǻH0) and entropy change (ǻS0), werecalculated, which indicated that the present system was spontaneous andendothermic process. A single stage batch adsorber design of the adsorptionof all dyes onto CSAC has been studied based on the Langmuir isothermequation. The results indicated that CSAC could be employed as analternative to commercial adsorbents in wastewater treatment for the removalof colour and dyes.