Adsorption of Cu(II) and Pb(II) onto a grafted silica: isotherms and kinetic models

The isotherms and kinetics of adsorption of lead(II) and copper(II) onto a grafted silica are studied at 20°C. A commercial silica is grafted with an ethylediamine derivative, N-[3-(trimethoxysilyl)propyl]-ethylenediamine. From the Langmuir isotherms, maximum adsorption capacities of the grafted silica towards Pb(II) and Cu(II) are determined (0.184 mmol Pb(II) g1 and 0.261 mmol Cu(II) g1) and compared to those of non-modified silica (respectively, 0.019 and 0.036 mmol g1). Four kinetic models, i.e., pseudo-first order, pseudo-second order, Langmuir and double-exponential are applied to fit the experimental kinetic data. The kinetic parameters are determined which allow to calculate the theoretical metal uptake as a function of time. The results are discussed and indicate the best fit is obtained with the double-exponential model. A discussion on the adsorption mechanism with respect to the double-exponential model leads to two possible interpretations: the metal uptake may follow a diffusion-controlled mechanism or a two-site adsorption process.