A sorbent concentration-dependent Freundlich isotherm

During study of the adsorption isotherms at solid–liquid interfaces, a sorbent concentration effect (Cs-effect) phenomenon was observed. In order to describe the Cs-effect, we proposed a new adsorption model, i.e., surface component activity (SCA) model. It supposes that the interaction between the sorbent particles exists in the real adsorption system, which induces the deviation of a real adsorption system from an ideal one. It is the deviation that induces the emerging of the Cs-effect. Based on the SCA model, the activity coefficient of adsorption sites should be a function of the sorbent concentration (Cs), and a Cs-dependent Freundlich equation (Freundlich-SCA equation) was derived. It was confirmed that the Freundlich-SCA equation can describe the Cs-effect observed in adsorption experiments. Its parameters (nS and KS) can be simulated with experimental adsorption data and are independent of Cs. Thus, these parameters obtained at given Cs values can be used to predict the adsorption behavior of the adsorbate at any Cs value.