A COMPARISON OF CHEMISORPTION KINETIC MODELS APPLIED TO POLLUTANT REMOVAL ON VARIOUS SORBENTS Y. S. HO and G. McKAY (FELLOW) Department of Chemical Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong A comparison of kinetic models describing the sorption of pollutants has been reviewed. The rate models evaluated include the Elovich equation, the pseudo-®rst order equation and the pseudo-second order equation. Results show that chemisorp- tion processes could be rate limiting in the sorption step. The pseudo-second order equation may be applied for chemisorption processes with a high degree of correlation in several literature cases where a pseudo-®rst order rate mechanism has been arbitrarily assumed. Keywords: sorption; kinetics; Elovich equation; pseudo-®rst order and pseudo-second order. INTRODUCTION Many attempts have been made to formulate a general expression describing the kinetics of sorption on solid surfaces for liquid-solid phase sorption systems. The pseudo-®rst order equation was ®rst represented by Lagergren 1 for the sorption of oxalic acid and malonic acid onto charcoal. Several applications of the Lagergren equation have been widely applied throughout the years. An early application of the pseudo-®rst order rate equation of Lagergren was the sorption of cellulose triacetate from chloroform onto calcium silicate by Trivedi et al. 2 . Numerous studies report ®rst order Lagergren kinetics for the sorption of metals such as the sorption of As(III) from aqueous solutions by haematite 3 , the sorption of nickel(II) from aqueous solutions by Woolastonite and china clay 4,5 , the sorption of chromium(VI) by bismuth trioxide 6 , the sorption of cadmium(II) onto hydrous ceric oxide 7 , the sorption of chromium(III) by natural moss and chromium(VI) by copper-coated moss 8 , the sorption of mercury(II) onto hydrous zirconium oxide 9 , the sorption of lead(II) onto kaolinitic clay 10 , the sorption of arsenite(III) and arsenate(V) using basic yttrium carbonate 11 and the sorption of arsenic(V) on haematite and feldspar 12 ; of dyes such as the sorption of Omega Chrome Red ME (OCRME) using a 1:1 ratio of ¯y ash and coal 13 , the sorption of Methylene Blue on water hyacinth roots 14 , the sorption of Orlamar Red BG (ORBG) by Fomitopsis carnea 15 , the sorption of Congo Red, Procin Orange and Rhodamine-B by waste orange peel 16 , the sorption of Congo Red on red mud 17 and the sorption of Acid Blue 29 and Reactive Blue 3 on chrome sludge 18 . Others studies utilizing the pseudo- ®rst order model include the sorption of Acid Violet dye onto waste banana pith 19 and the sorption of ¯uoride, phosphate and arsenate(V) using lanthanum-impregnated silica gel 20 . This present paper presents a literature review of over 250 systems analysed by the Lagergren pseudo-®rst order kinetic model. Three of these results have been analysed and compared by the pseudo-second order rate mechanism and the Elovich equation, for the sorption of phosphate onto tamarind nut shell activated carbon 21 and the sorption of Pb(II) and Cu(II) onto bottom ash 22 . DISCUSSION In order to investigate the mechanism of sorption and potential rate controlling steps such as mass transport and chemical reaction processes, kinetic models have been used to test experimental data. These kinetic models included the pseudo-®rst order equation, the pseudo- second order equation and the Elovich equation. The Pseudo-First Order Equation The pseudo-®rst order equation of Lagergren 1 is generally expressed as follows: d q t d t k 1 q e q t 1 where q e and q t are the sorption capacity at equilibrium and at time t , respectively (mg g 1 ) and k 1 is the rate constant of pseudo-®rst order sorption (l min 1 ). After integration and applying boundary conditions t 0 to t t and q t 0 to q t q t , the integrated form of equation (1) becomes: log q e q t log q e k 1 2.303 t 2 The equation applicable to experimental results generally differs from a true ®rst order equation in two ways 23 : · The parameter k 1 ( q e q t ) does not represent the number of available sites. · The parameter log( q e ) is an adjustable parameter and often it is found not equal to the intercept of a plot of log( q e q t ) against t , whereas in a true ®rst order log( q e ) 332 0957±5820/98/$10.00+0.00  Institution of Chemical Engineers Trans IChemE, Vol 76, Part B, November 1998