Fluridone adsorption–desorption on organo-clays

Abstract

The adsorption–desorption of the herbicide fluridone on Na-montmorillonite and several organo-montmorillonite complexes was studied at a variety of loadings of the organic cation and pH levels. The aim was to find the organo-clay complex, which would be an optimal adsorbent for the hydrophobic fluridone. The organic cations studied were hexadecyltrimethylammonium (HDTMA), benzyltriethylammonium (BTEA), benzyltrimethylammonium (BTMA) and methylene blue (MB) at loadings equal to 25%, 50% and 100% of the cation exchange capacity (CEC) of the clay-mineral. The adsorbed amount of fluridone increased several-fold when montmorillonite was preadsorbed by the organic cation HDTMA up to its CEC and with BTMA at a loading of 5/8 of the CEC. BTEA and MB did not improve the adsorption capacity of the clay for fluridone. The results suggest that interactions between the phenyl rings of the herbicide and that of a small organic cation are geometrically easier to establish than with a larger organic cation. A reduced interaction between the phenyl rings of MB and those of fluridone may account for the low affinity of fluridone adsorption on montmorillonite-MB. In all cases, fluridone adsorption increased with decreasing pH and reached 100% for pH 2.7. Protonation of fluridone molecules with decreasing pH would result in increased adsorption through cation exchange. Thus, by regulating the pH, complete fluridone adsorption can be achieved. Desorption isotherms demonstrate high degree of irreversibility of the adsorption–desorption process and suggest that strong binding mechanisms dominate the fluridone-clay and organo-clay interactions. The results for fluridone adsorption–desorption demonstrate that, for similar molecules, a clay-based slow release formulation can be designed by first lowering the pH.