291 Research Article Received: 11 March 2008 Revised: 5 June 2008 Accepted: 21 July 2008 Published online in Wiley Interscience: 9 October 2008 (www.interscience.wiley.com) DOI 10.1002/jctb.2038 Optimization and analysis of nickel adsorption on microwave irradiated rice husk using response surface methodology (RSM) Magesh Ganesa Pillai, a I. Regupathi, b M. Helen Kalavathy, a T. Murugesan c,a and Lima Rose Miranda a∗ Abstract BACKGROUND: The removal of heavy metals using adsorption techniques with low cost biosorbents is being extensively investigated. The improved adsorption is essentially due to the pores present in the adsorbent. One way of improving the porosity of the material is by irradiation of the precursor using microwaves. In the present study, the adsorption characteristics of nickel onto microwave-irradiated rice husks were studied and the process variables were optimized through response surface methodology (RSM). RESULT: The adsorption of nickel onto microwave-irradiated rice husk (MIRH) was found to be better than that of the raw rice husk (RRH). The kinetics of the adsorption of Ni(II) from aqueous solution onto MIRH was found to follow a pseudo-second-order model. Thermodynamic parameters such as standard Gibbs free energy (G ◦ ), standard enthalpy (H ◦ ), and standard entropy (S ◦ ) were also evaluated. The thermodynamics of Ni(II) adsorption onto MIRH indicates that it is spontaneous and endothermic in nature. The response surface methodology (RSM) was employed to optimize the design parameters for the present process. CONCLUSION: Microwave-irradiated rice husk was found to be a suitable adsorbent for the removal of nickel(II) ions from aqueous solutions. The adsorption capacity of the rice husk was found to be 1.17 mg g −1 . The optimized parameters for the current process were found as follows: adsorbent loading 2.8 g (100 mL) −1 ; Initial adsorbate concentration 6 mg L −1 ; adsorption time 210 min.; and adsorption temperature 35 ◦ C. c  2008 Society of Chemical Industry Keywords: adsorption; response surface methodology; microwave irradiation; central composite rotary design; nickel NOTATION a f Multilayer adsorption capacity and intensity of adsorption (mg g −1 ) a L Langmuir constants (L mg −1 ) b Langmuir adsorption intensity constant (L mg −1 ) b f Freundlich isotherm exponent k c Equilibrium constant k, Number of factors in cube portion of design q e , Equilibrium adsorption capacity (mg g −1 ) q t Amount of Nickel ion adsorbed per unit mass of adsorbent at time (mg g −1 ) t Contact time (min) x i x j , Dimensionless coded value of ith variable A Adsorbent concentration (g (100 mL) −1 ) B Initial adsorbate concentration of Solution (mg L −1 ), C Adsorption time (min) D Adsorption temperature ( ◦ C) C Ae Equilibrium concentration of Ni (II) on the solution (mg L −1 ) C Be Equilibrium concentration of Ni (II) on the adsorbent (mg L −1 ) C o Initial concentration of adsorbate in solution (mg L −1 ) F Number of points in cube portion of design K 1 First-order rate constant (min −1 ) K 2 Second-order rate constant (g mg −1 min −1 ) K L Langmuir constants (L g −1 ) R Universal gas constant (kJ kg −1 mol −1 K −1 ) R 2 Coefficient of determination T Temperature (K) V Volume of adsorbent (L) V o Initial volume of adsorbent (L) X Independent variable X i , The natural value of the ith variable X max Highest limits of the ith variable ∗ Correspondence to: Lima Rose Miranda, Department of Chemical Engineering, A.C. College of Technology, Anna University, Chennai 600025, India. E-mail: limamiranda2007@gmail.com a Department of Chemical Engineering, A.C. College of Technology, Anna University, Chennai 600025, India b Department of Chemical Engineering, National Institute of Technology Karnataka, Mangalore, India c Chemical Engineering Programme, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750, Tronoh, Perak, Malaysia J Chem Technol Biotechnol 2009; 84: 291–301 www.soci.org c  2008 Society of Chemical Industry