KINETIC MODELS FOR THE SORPTION OF DYE FROM AQUEOUS SOLUTION BY WOOD Y. S. HO and G. MCKAY (FELLOW) Department of Chemical Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong T he sorption of Basic Blue 69 and Acid Blue 25 onto wood has been studied in terms of pseudo-second order, intraparticle diffusion and pseudo-®rst order chemical sorption processes. The batch sorption model, based on the assumption of a pseudo-second order mechanism, has been developed to predict the rate constant of sorption, the equilibrium capacity and initial sorption rate with the effect of initial dye concentration, wood particle size and temperature. An equilibrium capacity of sorption has been evaluated with the pseudo- second order rate equation as well as by the Langmuir isotherm and operating line method. In addition, an activation energy of sorption has also been determined based on the pseudo-second order rate constants. Key words: wood; dye; kinetics; sorption. INTRODUCTION Adsorption of dyes from waste water has been studied. The rate at which sorption takes place is of the utmost importance when designing batch sorption systems; conse- quently it is important to establish the time dependence of such systems under various process conditions. A number of sorption processes for dyes has been studied in an attempt to ®nd a suitable explanation of the mechanism and the kinetics. Alexander et al. 1 evaluated an overall diffusion coef®cient for the kinetics of the adsorption of Astrazone Blue on silica. Khare et al. 2 reported that removal of Victoria Blue from aqueous solution by ¯y ash follows a ®rst order adsorption rate expression. In 1984, McKay 3,4 developed a model to predict the rate of adsorption of Astrazone Blue dye onto Sorbsil silica which is controlled by the internal mass transport processes based on a pore diffusion mechanism. The author presented a two resistance mass transfer model which is based on ®lm resistance and homogeneous solid phase diffusion for the adsorption of Basic Blue 69 by silica. In addition, McKay et al. 5 presented a three-step model for the adsorption of Astrazone Blue dye on peat. In the case of the adsorption of dyes on chitin, McKay et al. 6 found that there is a branched pore mechanism and that the rate of adsorption of dyes onto chitin in the initial stages of the adsorption process depends on the ®lm mass transfer coef®cient 7 . In the present research the mechanism of basic and acid dye sorption on wood has been studied. The factors studied include the in¯uence of wood particle size, temperature and initial dye concentration on the diffusion processes and the sorption kinetics of the system. A method is discussed for the determination of the activation energy of sorption of basic dye and acid dye onto wood based on a pseudo-second order sorption process. A rate parameter, k, has been de®ned and used to describe the sorption of acidic and basic dyes onto wood and a method for determining the diffusion rate coef®cients, using data plotting techniques, has been developed. A comparison of the intraparticle diffu- sion parameter, k i , a pseudo-second order sorption para- meter, k, and a pseudo-®rst order rate parameter, k 1 , is also discussed. MATERIALS AND METHODS Sorbent The wood sawdust particles were of the spruce wood type, and no form of pretreatment was applied except for sieving into various particle size ranges. Sorbates The sorbates and their structures used in the experiments are listed below. The dyestuffs were used as the commercial salts. Acid Blue 25 (Telon Blue ANL) CI 62055 was supplied by Bayer. Basic Blue 69 (Astrazone Blue FRR) was supplied by Bayer. No structure is available for this dye. It belongs to the methine class, of which the chromophore is a conjugate chain of carbon atoms terminated by an equivalent 183 0957±5820/98/$10.00+0.00 q Institution of Chemical Engineers Trans IChemE, Vol 76, Part B, May 1998