MODIFIED LANGMUIR EQUATION FOR S-SHAPED AND MULTISITE ISOTHERM

Standard isotherm equations do not estimate capacity (Q(max)) and dist ribution coefficient (K-d) for complex or non-Langmuir-shaped isotherm plots. In this study, two mycotoxins, that is, aflatoxin B-1 (AfB(1)) and cyclopiazonic acid (CPA), were mixed with kaolinite and a natural ly acidic montmorillonite clay (LPHM) at 25 degrees C, respectively. I sotherm data gave S-type plots. The data were fitted to the models of Langmuir (LM) and multi-langmuir (MLM); however, these models did not provide a good fit for data that displayed multisite adsorption or mul tiple plateaus. While a published modification of the Langmuir equatio n (QKLM), which defines an effective partition coefficient as a functi on of the surface coverage, was able to fit simple isotherm plots from all categories (H, L, S, C), it did not fit complex or multisite isot herm plots. Importantly, an equation that enables the estimation of Q( max) and K-d for both S-shaped and multisite isotherm plots has not ye t been reported. Since the LM, MLM, and QKLM did not provide adequate fitting of the data, several modifications of the LM were developed: s hifted Langmuir, shifted squared Langmuir, shifted cubed Langmuir, shi fted exponential Langmuir, exponential Langmuir, and shifted modified Langmuir. These equations were used to derive information about the ad sorption of mycotoxins to clay and to gain insight into the molecular mechanism(s) and site(s) of adsorption. The objectives of this study w ere to present a series of modified Langmuir equations that can be use d to estimate the Q(max) and K-d of a specific adsorption site and to relate Q(max) to available adsorption area.