L.-X. Zhao, S.-E. Song, N. Du and W.-G. Hou,A sorbent concentration-dependent Freundlich isotherm,Colloid Polym. Sci.,2013,291(3), 541-550.

During study of the adsorption isotherms at solid-liquid interfaces, a sorbent concentration effect (C (s)-effect) phenomenon was observed. In order to describe the C (s)-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 C (s)-effect. Based on the SCA model, the activity coefficient of adsorption sites should be a function of the sorbent concentration (C (s)), and a C (s)-dependent Freundlich equation (Freundlich-SCA equation) was derived. It was confirmed that the Freundlich-SCA equation can describe the C (s)-effect observed in adsorption experiments. Its parameters (n (S) and K (S)) can be simulated with experimental adsorption data and are independent of C (s). Thus, these parameters obtained at given C (s) values can be used to predict the adsorption behavior of the adsorbate at any C (s) value.