Adsorption of dicamba (3,6-dichloro-2-methoxy benzoic acid) in aqueous solution by calcined-layered double hydroxide

Layered double hydroxides (LDHs) are anionic clays with high anion exchange capacities. Calcination of LDHs increases their anion-exchange capacities significantly, resulting in calcined-LDHs that are better adsorbents for removal of anion pollutants than uncalcined-LDHs. In this study, layered double hydroxides, LDH-CO32- and calcined-LDH were prepared and characterized by FT-IR and X-ray diffraction analysis, with the products evaluated for their ability to adsorb dicamba (3,6-dichloro-2-methoxy benzoic acid), an ionizable organic pesticide. Results indicated that dicamba could be adsorbed on calcined-LDH, but no adsorption occurred with LDH-CO32-. The adsorption isotherms of dicamba on calcined-LDH are typical S-type curves, suggesting a hydrophobic adsorption mechanism was involved. Dicamba adsorption on calcined-LDH was a rapid process that reached a quasi-equilibrium within 30 min. Competing anions strongly affected the adsorption process, with dicamba adsorption, in the presence of different anionic species, increasing in the order: SO42- < HPO42- < CO32- < NO3- approximate to F- approximate to Cl- approximate to Br- approximate to I-. Adsorbed dicamba on calcined-LDH could be desorbed completely, with the release rate dependent upon the type of competing anion in the aqueous solution. FT-IR and X-ray diffraction analysis verified that adsorbed dicamba formed a monolayer that was oriented perpendicularly from the interlayer mineral surfaces. (C) 2002 Elsevier Science B.V. All rights reserved.