Freundlich and Langmuir isotherms as models for the adsorption of toxicants on activated charcoal.

The Langmuir isotherm has been widely used to characterize the adsorption of solutes from aqueous solutions. Activated charcoal adsorption data obtained experimentally, using a wide range of adsorbate concentrations, fit the Langmuir isotherm poorly but evidence a good fit to the Freundlich isotherm. Statistical analysis reveals this to be also true of published data that was previously considered to adhere to the Langmuir isotherm. Over the range of possible adsorbate concentrations, the two isotherms predict rather different adsorption behavior. Of the two, the Freundlich isotherm is able to more fully account for observed antidotal effectiveness of activated charcoal in vivo. A method of graphical analysis is advanced that more readily distinguishes the relative goodness-of-fit of the two isotherms. This and the statistical paradigm employed to decide between the two competing hypotheses should allow the adsorption phenomena involving other adsorbents to be re-examined.