Journal of Colloid and Interface Science 287 (2005) 6–13 www.elsevier.com/locate/jcis Thermodynamic behaviour and the effect of temperature on the removal of dyes from aqueous solution using modified diatomite: A kinetic study M. Al-Ghouti a , M.A.M. Khraisheh b,∗ , M.N.M. Ahmad a , S. Allen a a School of Chemical Engineering, Queen’s University Belfast, Stranmillis Road, Belfast, BT9 5AG, UK b Department of Civil and Environmental Engineering, University College London, Gower Street, London WC1E 6BT, UK Received 17 August 2004; accepted 1 February 2005 Available online 10 March 2005 Abstract The effect of solution temperature and the determination of the thermodynamic parameters of adsorption of methylene blue (MB), Cibacron Reactive black C-NN (RB) and Cibacron Reactive golden yellow MI-2RN (RY) onto manganese-oxides-modified diatomite (MOMD), such as activation energy, E, enthalpy of activation, H ∗ , entropy of activation, S ∗ , and free energy of activation, G ∗ , on the adsorption rates is important in understanding the adsorption mechanism. The rate and the transport/kinetic processes of dye adsorption onto the adsorbents were described by applying various kinetic adsorption models. This would lead to a better understanding of the mechanisms controlling the adsorption rate. The pseudo-second-order model was the best choice among all the kinetic models to describe the adsorption behaviour of RB onto MOMD, suggesting that the adsorption mechanism might be a chemisorption process. The activation energies, E, for RB, RY and MB were −6.74, 56.65 and 99.80 kJ/mol, respectively. The negative value of the activation energy suggested that the rise in the solution temperature did not favour RB adsorption onto MOMD. Moreover, the activation energy of the diffusion process, E ′ , for RB, RY and MB increased as MB > RY ≫ RB. It means that the RB molecules are much faster moving and a lower energy is needed to diffuse into MOMD than RY and MB molecules. E ′ , the activation energy for adsorption into pores, of RY is higher than E, indicating that the rate-limiting step of RY adsorption onto MOMD might be diffusion controlled, while the activation energy of the diffusion process, E ′ , of MB is slightly lower than E, suggesting that the rate-limiting step is a combination of chemical and diffusion adsorption.  2005 Elsevier Inc. All rights reserved. Keywords: Adsorption; Diatomite; Modified adsorbents; Thermodynamic parameters; Kinetic studies; Reactive dyes; Methylene blue 1. Introduction The removal of coloured and colourless organic pollu- tants from industrial wastewater is considered an important application of adsorption processes using a suitable adsor- bent [1]. There is growing interest in using low cost, com- mercially available materials for the adsorption of dyes. Di- atomite, a siliceous sedimentary rock available in abundance in various locations around the world, has received attention for its unique combination of physical and chemical proper- ties (such as high permeability, high porosity, small particle size, large surface area, low thermal conductivity and chem- * Corresponding author. Fax: +44-20-7380-0986. E-mail address: m.khraisheh@ucl.ac.uk (M.A.M. Khraisheh). ical inertness) and as low cost material for the removal of pollutants from wastewater [2]. Previous studies by the au- thors established that chemical modification of diatomite, especially with manganese oxides, enhanced its dye removal capacity and its feasibility for large-scale application to the treatment of textile effluents containing reactive dyes; a dif- ficult class of dyes to treat in traditional methods. Detailed information about the effect of this chemical modification can be found in Ref. [3]. For a successful scale-up of such a process, kinetic studies are essential since they describe the adsorbate uptake rate, which in turn controls the resi- dence time in the adsorbent–solution interface. A number of experimental parameters are usually considered in these studies, including the effect of agitation speed, particle size, adsorbent mass, initial adsorbate concentration, as well as 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.02.002