Effect of the modification of natural clay minerals with hexadecylpyridinium cation on the adsorption-desorption of fungicides

Clay minerals, montmorillonite (M), illite (I), kaolinite (K), muscovite (Mu), sepiolite (S) and atapulgite (A), modified with the organic cation hexadecylpyridinium (HDPY) were studied as sorbents of two fungicides of different hydrophobicity, penconazole and metalaxyl, using adsorption-desorption isotherms. The Freundlich constant values were low for the adsorption of the fungicides by natural clays and increased when the minerals were saturated with the organic cation. In HDPY-M, the K f constant increased 190-fold for the adsorption of penconazole and 67-fold for the adsorption of metalaxyl. Adsorption of the fungicides by the organo-clays at an initial concentration of 25 mug/mL was related to their organic matter content, the highest adsorption values being found in HDPY-M, HDPY-I and HDPY-A, and the increase in adsorption was greatest for the more hydrophobic fungicide penconazole. Desorption of fungicides from modified clays pointed to a high degree of stability for penconazole adsorbed by HDPY-M. In the remaining samples, the stability of this pesticide was low and the stability of metalaxyl adsorbed by all the organo-clays was also low. The results obtained point to the potential value of clay minerals modified with HDPY for immobilising pesticides with different degrees of hydrophobicity. Of special interest is the HDPY-M sample, which could be considered for use in clay barriers aimed at protection of soil and water pollution by hydrophobic pesticides.