Chapter 8
Interactions Between Atrazine and Smectite Surfaces David A. Laird
Downloaded by FUDAN UNIV on April 24, 2017 | http://pubs.acs.org Publication Date: June 21, 1996 | doi: 10.1021/bk-1996-0630.ch008
National Soil Tilth Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 2150 Pammel Drive, Ames, IA 50011
Smectites contribute much of the inorganic surface area of soils and therefore have a large potential for influencing the fate of atrazine in soil environments. The sorption capacity of smectites for atrazine varies widely depending on the surface charge density of the smectite, nature of the adsorbed cation, and pH of the equilibrating solution. Under neutral conditions, molecular atrazine is initially sorbed on smectites by a combination of water bridging between electronegative moieties on the atrazine molecule and adsorbed metal cations and hydrophobic bonding between the alkyl-side chains on the atrazine molecule and hydrophobic microsites on the smectite surface. Surface acidity, arising principally from enhanced hydrolysis of solvation water for adsorbed metal cations, catalyzes protonation and hydrolysis of atrazine sorbed on smectite surfaces. The hydrolysis product is protonated hydroxy/keto-atrazine, which may exist in any of 14 different tautomeric forms. Tautomerism and resonance allow protonated hydroxy/keto-atrazine to adapt to heterogeneous microsites on smectite surfaces which results in strong bonding.
Clay minerals are the dominant adsorbents of nonionic organic compounds in subsoils and sediments that contain less than 1% organic C . Clays may also contribute significantly to the sorption of nonionic organic compounds in surface soils (7-5). Dunigan and Mcintosh (
