Wat. Res. Vol. 35, No. 14, pp. 3357–3366, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$-see front matter PII: S0043-1354(01)00056-2 ADSORPTION OF BASIC DYES ON GRANULAR ACIVATED CARBON AND NATURAL ZEOLITE V. MESHKO 1 *, L. MARKOVSKA 1 , M. MINCHEVA 1 and A. E. RODRIGUES 2 1 Faculty of Technology and Metallurgy, University ‘‘St.Cyril & Methodius’’ Ruger Boskovic 16, 91000 Skopje, Republic of Macedonia and 2 Laboratory of Separation and Reaction Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal (First received 10 May 2000; accepted in revised form 12 January 2001) Abstract}The adsorption of basic dyes from aqueous solution onto granular activated carbon and natural zeolite has been studied using an agitated batch adsorber. The influence of agitation, initial dye concentration and adsorbent mass has been studied. The parameters of Langmuir and Freundlich adsorption isotherms have been determined using the adsorption data. Homogeneous diffusion model (solid diffusion) combined with external mass transfer resistance is proposed for the kinetic investigation. The dependence of solid diffusion coefficient on initial concentration and mass adsorbent is represented by the simple empirical equations. # 2001 Elsevier Science Ltd. All rights reserved Key words}basic dye, activated carbon, natural zeolite, homogeneous diffusion model, external mass transfer resistance NOMENCLATURE a 1 constant in equation (15) A parameter in the Freundlich equation, mgg 1 (mgdm 3 ) n b 1 constant in equation (15) b L parameter in the Langmuir equation, dm 3 mg 1 c e equilibrium liquid-phase concentration, mgdm 3 c l liquid-phase concentration, mgdm 3 c 0 initial value of c,mgdm 3 c s liquid-phase concentration at liquid–pellet inter- face, mgdm 3 d p particle diameter, m D stirrer diameter, m D M bulk phase diffusivity, m 2 s 1 D 0 constant factor in equations (15) and (16), m 2 s 1 D s solid diffusivity, m 2 s 1 f degrees of freedom H height of liquid, m k constant in equation (16) k f external mass-transfer coefficient, ms 1 m constant in equation (11) M mass of adsorbent, g n parameter in the Freundlich equation N rotation speed, s 1 q solid-phase concentration, mgg 1  q average value of q,mgg 1 q e equilibrium solid-phase concentration, mgg 1 q m parameter in the Langmuir equation, mgg 1 q s value of q at liquid–pellet interface, mgg 1 r radial distance measured from the center of a pellet, m R pellet radius, m t time, s T tank diameter, m V volume of solution, m 3 Greek symbols e p porosity of pellet n kinematic viscosity of fluid, m 2 s 1 r p particle density, kgm 3 r s solid density of adsorbent, kgm 3 y equilibrium surface coverage INTRODUCTION The removal of dyes and organics in an economic way remains an important problem although a number of systems have been developed with adsorption techniques. Adsorption has been found to be superior to other techniques for water re-use in terms of initial cost, simplicity of design, ease of operation and insensitivity to toxic substances. Granular activated carbon is the most popular adsorbent and has been used with great success (McKay, 1982), but is expensive. Consequently, new materials as chitin (McKay, 1982), silica gel (McKay et al., 1980), natural clay (El-Geundi, 1991, 1993a, b), bagasse pith (McKay, 1998) are being studied. A very limited amount of information is available on the use of natural zeolites as a method for dye removal (Meshko et al., 1999). A full description of low cost adsorbents for waste and wastewater treatment: a review has been presented by Pollard et al. (1992). *Author to whom all correspondence should be addressed. Tel.: +389-91-364588; fax: +389-91-365389; e-mail: ameshko@ukim.edu.mk 3357