Kinetics and Equilibrium Studies of Adsorption of Anionic Dyes Using Acid-Treated Palm Shell G. Sreelatha, † S. Kushwaha, ‡ V. J. Rao, § and P. Padmaja* ,‡ Department of Applied Chemistry, Faculty of Technology and Engineering, The Maharaja Sayajirao UniVersity of Baroda, Vadodara, India, Department of Chemistry, Faculty of Science, The Maharaja Sayajirao UniVersity of Baroda, Vadodara, India, and Department of Metallurgy and Material Science, Faculty of Technology and Engineering, The Maharaja Sayajirao UniVersity of Baroda, Vadodara, India This study investigates the potential uses of palm shell, pretreated with sulfuric acid (APSP) for the adsorption of AOII, DSB, and AV7. The effects of different system variables like adsorbent dosage, pH, contact time, and temperature were studied. Optimum pH values for all the three dyes were determined as 1.0. Equilibrium was achieved within 30 min. Langmuir I, II, III, IV and Freundlich isotherm models were applied to describe the equilibrium isotherms at different temperatures, and the langmuir model was found to agree very well with the experimental data. The maximum adsorption capacity was found to be 2180.05 mg/g, 1199.99 mg/g, and 243.9 mg/g for AOII, DSB, and AV7, respectively. Thermodynamic parameters such as change in free energy (∆G 0 ), enthalpy (∆H 0 ), and entropy (∆S 0 ) were also determined. Pseudo-first-order, pseudo-second- order, and intraparticle diffusion models were used to fit the experimental data. The pseudo-second-order equation was able to fit well and provide a realistic description of the adsorption kinetics. Introduction Waste waters from textile industries, dye manufacturing industries, paper and pulp mills, tanneries, electroplating industries, distilleries, food industries, etc. contain dyes, thus polluting water resources. The conventional methods for treating dye containing waste waters are activated sludge, chemical evaporation, activated carbon adsorption, electrochemical treat- ment, reverse osmosis, hydrogen peroxide catalysis, etc. Most of the above methods have some disadvantage or other and are not economically effective. Low-cost treatment methods have therefore been used as an alternative for dye removal. These include peat, chitin, wood, china clay, apple pomace, biogas waste slurry, wheat straw, organo-montmorillinite, coir pith, slag from manufacture of steel, fly ash, and activated slag from fertilizer plants. 1-14 Recently, Esther Forgacs et al., 15 V. K. Gupta et al. 16 and Gregorio Crini 17 have reviewed the use of low cost adsorbents for the removal of dyes. Palm shell or Borassus Flabellifer is abundant in coastal areas throughout India. Preliminary inves- tigations done to study the removal of anionic dyes, acid orange II, acid violet-7, and disulphine blue, using untreated palm shell powder revealed that removal was only 41.961, 29.351, and 22.866%, respectively (results not shown). In this work, we have investigated further the ability of acid treated palm shell powder (APSP) to remove acidic anionic dyes like acid orange II, acid violet-7, and disulphine blue. The kinetic and equilibrium data of adsorption studies were processed to understand the adsorp- tion mechanism of the dye molecules onto the APSP. Materials and Methods Preparation of Adsorbent. Palm shells obtained from the coastal areas of Andhra Pradesh were washed, sundried for 24 h, and ground using a jaw crusher. They were then dried at 110 °C, and the cleaned powder was mixed with conc. H 2 SO 4 (sp. gr. 164) in 1:1.5 weight ratio and allowed to stand in an oven maintained at 140-160 °C for 24 h. The resulting char was thoroughly washed with water followed by 2% solution of NaHCO 3 until effervescence ceased and then left to soak in a 2% solution of NaHCO 3 overnight. The APSP was then separated, washed with water until free of bicarbonate, and dried at 105 °C. Preparation of Dye Solutions. Stock solutions of dyes (1 g/L) were prepared by dissolving an accurately weighed amount of AOII, DSB, and AV7 in double distilled water and subsequently diluting to the required concentration. Adsorption Experiments. A series of dye sorption experi- ments were conducted to study the effect of pH, dose, and temperature. In the adsorption experiment, 25 mL of dye solution of known initial concentration was kept in contact with a required dose of PSP at room temperature. Adsorption studies were carried out with 100 mL Durasil Stoppered flasks in a thermo-regulated water bath shaker at 180 rpm. The pH of solutions was adjusted using 0.1 N HCl or 0.1 N NaOH solution. After a specific time period the reaction mixture was filtered. The dye concentration in the filtrate was determined by measuring absorbance at the wavelength of maximum absorption (490, 640, and 530 nm for AOII, DSB, and AV7, respectively) using a SYSTRONICS digital 166 model visible spectropho- tometer. The percentage removal of the dye and the amount adsorbed (mg/g) were calculated by the following relationship: where C i is the initial concentration of dye in mg/L, C e is the equilibrium concentration of dye in mg/L, m is the mass of adsorbent in g/L, and q e is the amount of dye adsorbed per gram of adsorbent. The experiments done without adsorbent were treated as blanks, and they showed that no precipitation of dye occurred under the conditions selected. Adsorption Isotherms. The results of the adsorption experi- ments were analyzed using Freundlich and Langmuir I, II, III, * To whom correspondence should be addressed. E-mail: p_padmaja2001@yahoo.com. † Department of Applied Chemistry. ‡ Department of Chemistry. § Department of Metallurgy and Material Science. q e ) (C i - C e )/ m Ind. Eng. Chem. Res. 2010, 49, 8106–8113 8106 10.1021/ie101004q  2010 American Chemical Society Published on Web 07/23/2010