Description of sorption data with isotherm equations

Analysis of sorption data is important for characterizing retention of chemicals by soil. In this paper I review the most common isotherm equations used in soil science. The mathematical descriptions of these equations are classified into rational, power, and transcendental functions which are related to the isotherm classification. I use most well-known isotherm equation as special cases of a general equation. The meaning of the parameters of this equation is discussed in terms of the Giles classification. Guidelines are presented for choosing the correct type of isotherm equations to describe a set of sorption data. In particular, I show that plotting the distribution coefficient vs. the amount of solute sorbed to the solid phase on a log–log scale is the best way to identify the class and subgroup of isotherms based on the Giles classification. Examples of how to apply the guidelines to determine and modify isotherm equations are presented.