A Sorbent Concentration-Dependent Langmuir Isotherm

Batch adsorption experiments of Pb(II) and Cu(II) on kaolinite as a function of kaolinite concentration were conducted. An obvious sorbent concentration effect (Cs-effect) was observed; namely, the adsorption isotherm declines as sorbent concentration (Cs) increases. The experimental data were fitted to the classic Langmuir model. The results showed that the classic Langmuir model could adequately describe the adsorption equilibria of Pb(II) and Cu(II) on kaolinite for a given Cs value, but could not adequately predict the Cs-effect observed in the adsorption systems. We proposed a surface component activity (SCA) model. It suggests that the interactions between the sorbent particles exist in the real adsorption system, and the activity coefficient of the adsorption sites of the sorbent surface is a function of Cs. Based on the SCA model, a Cs-dependent Langmuir (Langmuir-SCA) isotherm was derived. The applicability of the Langmuir-SCA isotherm was examined with the experimental adsorption data of Pb(II) and Cu(II) on kaolinite, as well as Cd(II) and Zn(II) on vermiculite and Pb(II) on coffee reported in the literature. The results showed that the Langmuir-SCA equation could describe the Cs-effect observed in adsorption experiments. The two intrinsic parameters of the Langmuir-SCA isotherm, the thermodynamic (Article)