| Triazine herbicides such as atrazine and simazine have been applied for more than 40 years in the United States. Atrazine is one of the most common herbicide residues found in groundwaters of Midwest. Sorption to soil components can control the bioavailability, persistence, and transport of xenobiotics. We investigated the sorption of three triazine herbicides (atrazine, simazine, and metribuzin) by saponite and beidellite clay minerals saturated with K +, Cs+, Na+, and Ca2+. Saponite clay sorbed a larger fraction of each pesticide from aqueous solution than did beidellite clay. The lower cation exchange capacity in saponite (vs. beidellite) presumably results in a less crowded interlayer with more siloxane surface available for adsorption. Generally, Cs-saturated clays sorbed more triazines than clays saturated by K+, Na+ or Ca2+. We attribute this to the smaller hydrated radius of Cs +, which again increases the siloxane surface available for sorption. Furthermore, the relatively weak hydration of Cs+ reduces the swelling of clay interlayers, thus making sorption domains less hydrated and more receptive to hydrophobic molecules. Cs-saponite manifested a sorption of more than 1% atrazine by weight above equilibrium concentrations of 6 mg/L. This is the largest sorption of neutral atrazine from water yet reported for an inorganic sorbent. Molecular dynamics simulations indicate that atrazine interacts with both clay basal planes and with multiple cations in the clay interlayer forming both inner- and outer-sphere complexes. Similar effects of exchangeable cation on trichloroethene sorption by saponite clay were also observed, and our general concept of the factors controlling sorption of nonpolar organics by clay minerals is becoming more clear.; Molecular simulations were also used to estimate thermodynamics of polar organic sorption to montmorillonites. Our method shows promise in that simulated adsorption enthalpies for nitroaromatic compounds have the correct sign and are significantly different from that of p-xylene. Further work is still needed so that enthalpy predictions can be more precise. |
