Adsorption of zearalenone by organophilic montmorillonite clay

Acidic montmorillonite clay was chemically modified via exchange with various ammonium- and pyridinium-based organic cations. These organo clays were then shown to adsorb zearalenone (ZEN) with an efficacy related to the exchanged cation hydrophobicity, as gauged by estimated critical micelle concentration values (cmc) and length of the alkyl chain. ZEN adsorption was greatest for clay samples that had been modified by interlayer exchange with cations possessing 16-carbon alkyl chains: hexadecyltrimethylammonium (HDTMA) and cetylpyridinium (CP). Isothermal analysis of ZEN adsorption (at 37 degrees C, pH 6.5) indicated that the clay exchanged with CP at a level of less than or equivalent to the cation exchange capacity (CEC) binds ZEN primarily by a partition process. In contrast, clay exchanged at the 1.5 CEC level gave an S-shaped curve with at least two plateaus, suggesting additional mechanisms of adsorption. A capacity (Q(max)) and distribution coefficient (K-d) for the first site were estimated. At acid pH, the adsorption capacity for 1.5 CEC CP clay decreased and the isotherm shape predicted a simple partition process. Conversely, the shape remained curved at alkaline pH, suggesting an adsorption site. These data imply an adsorption mechanism at neutral pH that involves hydrophobic attraction of ZEN into the interlayer in addition to multisite binding of the anionic form to hydroxyl groups on the edge of the clay via ion-dipole interactions and electrostatic attraction to excess exchanged surfactant cations.