Adsorption of sulfometuron and other anions on pillared clay

The adsorption of several anions of agronomic and ecological importance, su lfometuron 2-[3-(4,6-dimethylpyrimidin-2-yl)ureidosulfo-nylbenzoic acid, su lfate, acetate, and chloride on pillared clay was studied. Pillared clay (P C) is a very effective adsorbent of anions from aqueous solutions. The frac tions 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 mel, 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 adsorpt ion of sulfate and acetate. Acetate at 1000-fold larger excess competes wit h sulfometuron for the surface sites, but sulfate does not. Acetate decreas ed dramatically the adsorption of sulfate when both anions were added simul taneously, but acetate did not adsorb on the clay with pre-adsorbed sulfate . The sequence of adsorption affinity of anions to the PC surface is: sulfo meturon >> sulfate > acetate >> chloride. The binding coefficient was 12 00 0 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 s trong electrostatic interactions. The proposed interactions of sulfate with PC are outer- and inner-sphere complexation. The pH is reduced with sulfat e adsorption and Al is released. The proposed mode of acetate interactions with PC is mainly via inner-sphere complexation. In this case the pH increa ses with acetate adsorption, indicating exchange of acetate with surface hy droxyls, and Al is not released.