Adsorption of sulfometuron and other anions on pillared clay

The adsorption of several anions of agronomic and ecological importance, sulfometuron 2-[3-(4,6-dimethylpyrimidin-2-yl)ureidosulfonylbenzoic acid, sulfate, acetate, and chloride on pillared clay was studied. Pillared clay (PC) is a very effective adsorbent of anions from aqueous solutions. The fractions of sulfometuron adsorbed from aqueous solutions were more than 90% of added, but low solubility limited the added amounts. The maximal adsorbed amount of sulfate was 0.72 mol_c kg^-1 and of acetate was 0.41 mol_c kg^-1. Addition of 100 and 500 mM of NaCl as the background electrolyte did not affect the adsorption of sulfometuron, but drastically reduced the adsorption of sulfate and acetate. Acetate at 1000-fold larger excess competes with sulfometuron for the surface sites, but sulfate does not. Acetate decreased dramatically the adsorption of sulfate when both anions were added simultaneously, but acetate did not adsorb on the clay with pre-adsorbed sulfate. The sequence of adsorption affinity of anions to the PC surface is: sulfometuron ≫ sulfate > acetate ≫ chloride. The binding coefficient was 12 000 M^-1 for sulfometuron, 650 M^-1 for sulfate, 350 M^-1 for acetate, and 15 M^-1 for chloride. The dominant mode of sulfometuron adsorption on PC is via strong electrostatic interactions. The proposed interactions of sulfate with PC are outer- and inner-sphere complexation. The pH is reduced with sulfate adsorption and Al is released. The proposed mode of acetate interactions with PC is mainly via inner-sphere complexation. In this case the pH increases with acetate adsorption, indicating exchange of acetate with surface hydroxyls, and Al is not released.