EQUILIBRIUM ISOTHERM STUDIES FOR THE SORPTION OF DIVALENT METAL IONS ONTO PEAT: COPPER, NICKEL AND LEAD SINGLE COMPONENT SYSTEMS Y. S. HO, J. F. PORTER and G. MCKAY ∗ Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, SAR China ( ∗ author for correspondence, e-mail: kemckayg@ust.hk) (Received 5 July 2000; accepted 6 September 2001) Abstract. The sorption of three divalent metal ions – copper, nickel and lead – from aqueous solution onto peat in single component systems has been studied and the equilibrium isotherms determined. The experimental data have been analysed using the Langmuir, Freundlich, Redlich-Peterson, Toth, Temkin, Dubinin-Radushkevich and Sips isotherm models. In order to determine the best fit iso- therm for each system, six error analysis methods were used to evaluate the data: the coefficient of determination, the sum of the errors squared, a hybrid error function, Marquardt’s percent standard deviation, the average relative error and the sum of absolute errors. The error values demonstrated that the Sips equation provided the best model for the three sets of experimental data overall. Keywords: copper, isotherm, lead, nickel, peat, sorption Nomenclature a F Freundlich isotherm constant; A t Temkin isotherm constant, dm 3 mmol -1 ; a L Langmuir isotherm constant, dm 3 mmol -1 ; a LF Langmuir-Freundlich isotherm constant ; a R Redlich-Peterson isotherm constant, (dm 3 mmol -1 ) b R ; a S Sips isotherm constant; a t Toth isotherm constant; B D Dubinin-Radushkevich isotherm constant; b T Temkin isotherm constant; b R Redlich-Peterson isotherm exponent; C e Solution phase metal ion concentration at equilibrium, mmol dm - 3; E Mean free energy of sorption, kJ mol -1 ; K L Langmuir isotherm constant, dm 3 g -1 ; K LF Langmuir-Freundlich isotherm constant; K R Redlich-Peterson isotherm constant, dm 3 g -1 ; Water, Air, and Soil Pollution 141: 1–33, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.