Influence of clay minerals on adsorption and degradation of a sulfonylurea herbicide (Cinosulfuron)

A study was conducted to examine the interactions between montmorillonite saturated with Na, Ca, Al and Cu(II) and a sulfonylurea herbicide. Adsorption isotherms were calculated from aqueous solution at 7 and 22 ﾛC. Cu-montmorillonite adsorbed a larger amount of herbicide than other clays; only the Cu-montmorillonite showed a significant penetration in the interlayer space as revealed by XRD. Moreover, a significant difference in adsorption at the two temperatures was found only for Cu-montmorillonite. Herein, for this adsorbent, the isosteric heat of adsorption (delta Hi) was calculated. Desorption experiments showed that cinosulfuron adsorbed on Na-, Ca- and Al-montmorillonite is almost completely removed by water washings, whereas on Cu-montmorillonite about half of the adsorbed amount remained bound. These results indicated the formation of a complex between cinosulfuron and Cu-montmorillonite. Fourier transform IR analyses showed that the C=O group of cinosulfuron is involved in the adsorption from the organic solvent on each adsorbent; furthermore, it is proposed a protonation of the triazinyl moiety. In adsorption from the organic solvent on Al-montmorillonite, cinosulfuron is partially decomposed to several products; one of these was identified as 4,6-metoxy,2-ol,1,3,5 s-triazine. This product was also observed in lesser amounts on the other adsorbents kept in a desiccator over P2O5 for several months. This product is absent on 1 N HCI acidic hydrolysis