A Unified Sorption Variable for Environmental Applications of the Freundlich Equation

Abstract The Freundlich equation ( x/m = K f C 1/n e ) has been used widely in sorption studies in chemistry and environmental research. Two problems arise in comparing sorbate‐sorbent systems if C e ≠ 1 or if 1/n values differ. First, it is often not possible to compare K f values from the literature if they were derived from different units of x/m and C e . Second, the exponent 1/n, determines the interpretation of K f . Consequently, comparing values of K f or K oc—f is problematic. We propose a way to characterize sorption dynamics in systems that follow the Freundlich isotherm that avoids problems caused by comparing K f values. We show that: (i) The units of K f are: mmol 1‐1/n kg −1 L 1/n , or mg 1‐1/n kg −1 L 1/n , etc. depending on the units of x/m and C c . Therefore, K f parameters cannot be compared among samples whenever the 1/n parameters are different; (ii) For K f values that are associated with different units, the units‐conversion factor for K f is also a function of 1/n. It has the form of: A 1‐1/n , where A is the units‐conversion factor if 1/n = 1; (iii) We propose a new variable, K u , to unify the units of K f and to conduct sensitivity analyses of K f and 1/n. Numerically K u is the slope, ( x/m )/ C e , of the sorption isotherm at any value of C e or x/m and has units of L kg −1 .