Modelling of equilibrium heavy metal biosorption data at different pH: a possible methodological approach

In this paper a methodological approach to study and model equilibrium of heavy metals during biosorption has been proposed and discussed. Two cases of copper biosorption are reported as examples: biosorption of copper onto Sphaerotilus natans and copper adsorption onto calcium alginate. Different empirical and semi-empirical models have been proposed and summarised, to consider the pH effect on heavy metal up-take. The proposed models, originated from Langmuir isotherm, may be useful to fit experimental data by just monitoring the pH avoiding thus its control. The adsorption isotherms were built considering experimental procedures at constant pH and pH edge conditions. Both empirical and semiempirical models were able to fit these experimental results. The empirical approach can be applied in single cases considering the selected experimental results, whereas the proposed semi-empirical models seem to be more generally applicable. Although the validity of these last models should be checked also in other cases, this approach may be useful to describe and fit biosorption data in a single metal system during the building of an adsorption isotherm: the simple equilibrium pH monitoring during the tests (also carried out in pH-edge conditions) and the introduction of this factor as independent variable in the equilibrium modelling is suggested as methodological approach to study biosorption in a single metal system. The use of these models may be useful also in successive studies in which the experimental results are described by mechanistic models (like Surface Complex Model – SCM) permitting a right interpolation of the data. The results are not conclusive because the effect of some important factors as biomass concentration and presence of other ions in solution are not considered in these models. © 2003 SDU. All rights reserved.