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MOLECULAR DYNAMICS MODELING OF SORPTION OF PESTICIDES ONTO THE SURFACES OF KAOLINITE
Citation:
Yan, L. AND G W. Bailey. MOLECULAR DYNAMICS MODELING OF SORPTION OF PESTICIDES ONTO THE SURFACES OF KAOLINITE. Presented at 220th American Chemical Society National Meeting, Washington, DC, August 20-24, 2000.
Impact/Purpose:
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Description:
To accurately predict the fate of contaminants in the environment and to make sound decisions about environmental remediation, we must accurately understand sorption mechanisms and surface reactivity of environmental particles. Sorption of selected pesticides on kaolinite surfaces was simulated with molecular dynamics. The kaolinite model was built based on crystallographic data, the model humic acid, and four types of lignin oligomers were constructed to study the effect of various functional groups. Electrostatic potential-derived charge was determined by ab initio quantum mechanical calculations to assign the atomic charges for both the kaolinite crystal structure and for the pesticide molecules. The Gasteiger-HUckel method was used to calculate the atomic charges for the humic acid and the oligomers. Simulated annealing and energy The association energy for 2,4-D [(2,4-dichloro)phenoxyacetic acid], atrazine [(2-chloro-4- ethylamino-6-isopropylamino-1,3,5-triazine)], hydroxyatrazine [(2-hydroxy-4-ethylamino-6- isopropylamino-1,3,5-triazine)] and metolalachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2- methoxy-l-methylethyl)acetamide] for the Al-OH octahedral surfaces of kaolinite was higher than for the Si-O tetrahedral surfaces. Alachlor [(2-chloro-N-(2,6-diethylphenyl)-N- (methoxymethyl)acetamide)] has a lower association energy on the Al-OH surface. Humic acid and negatively charged organic oligomers were more strongly bonded to the Al-OH surfaces than to Si-O surfaces. Sorption energies for humics varied from 37 to -84 kcals mole-'; for the
various oligorners, it varied from -16 to -269 kcals mole- . The electrostatic energy component dominates the adsorption energy, therefore, the charge properties of both the mineral and humic acid/oligomers play a critical role in determining the environmental fate of pesticides.