A Unified Sorption Variable for Environmental Applications of the Freundlich Equation
- Z. Chen,
- B. Xing and
- W. B. McGill *
The Freundlich equation (x/m = Kf C1/ne) has been used widely in sorption studies in chemistry and environmental research. Two problems arise in comparing sorbate-sorbent systems if Ce ≠ 1 or if 1/n values differ. First, it is often not possible to compare Kf values from the literature if they were derived from different units of x/m and Ce. Second, the exponent 1/n, determines the interpretation of Kf. Consequently, comparing values of Kf or Koc—f is problematic. We propose a way to characterize sorption dynamics in systems that follow the Freundlich isotherm that avoids problems caused by comparing Kf values. We show that: (i) The units of Kf are: mmol1-1/n kg−1 L1/n, or mg1-1/n kg−1 L1/n, etc. depending on the units of x/m and Cc. Therefore, Kf parameters cannot be compared among samples whenever the 1/n parameters are different; (ii) For Kf values that are associated with different units, the units-conversion factor for Kf is also a function of 1/n. It has the form of: A1-1/n, where A is the units-conversion factor if 1/n = 1; (iii) We propose a new variable, Ku, to unify the units of Kf and to conduct sensitivity analyses of Kf and 1/n. Numerically Ku is the slope, (x/m)/Ce, of the sorption isotherm at any value of Ce or x/m and has units of L kg−1.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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