Consequently, a constant lumped Deff is usually selected as the mass transfer controlling diffusion coefficient as in Model 1. The variable Deff may be expressed as a function of initial solute concentration (McKay and Al-Duri, 1990) or in terms of fractional surface coverage (Ho and McKay, 2000). Another approach widely adopted by many researchers is to assign a constant Deff to ‘‘best fit’’ a range of experimental data. The suitability of the experimental system, conditions and data must be careful appraised before this method is adopted. The results of Model 1 indicated that this approach is not particularly suitable for the three metal ion–peat systems.