Grantee Research Project Results
Final Report: The Role of Natural Organic Matter in the Transport, Disposition and Binding of Atrazine
EPA Grant Number: R827589E02Title: The Role of Natural Organic Matter in the Transport, Disposition and Binding of Atrazine
Investigators: Larive, Cynthia K. , Carper, W. Robert , Bhandari, Alok , Xia, Kang
Institution: University of Kansas , Kansas State University , Wichita State University
EPA Project Officer: Chung, Serena
Project Period: June 1, 1999 through March 30, 2003
Project Amount: $169,613
RFA: EPSCoR (Experimental Program to Stimulate Competitive Research) (1998) RFA Text | Recipients Lists
Research Category: EPSCoR (The Experimental Program to Stimulate Competitive Research)
Objective:
The objective of this research project is to provide a molecular level understanding of the general biological, chemical, and physical processes that determine the fate of atrazine in agricultural soils and the nature of its association with soil, sediment, and aquatic organic matter. The results of these experiments will improve our general understanding of the fate and effects of anthropogenic organic compounds using the disposition of atrazine in the Hillsdale Lake Basin as a model system.
Summary/Accomplishments (Outputs/Outcomes):
Atrazine is one of the most widely used postemergent herbicides on both agricultural and nonagricultural land. The U.S. Geological Survey found elevated levels of triazine herbicides throughout the Hillsdale Lake Basin from 1994 to 1995, with the highest concentrations occurring during the month of May following typical herbicide application periods. To provide insight into the problem, Dr. Bhandari and his student, Heather Lesan, investigated atrazine interactions with two Hillsdale surface soils of varying soil organic matter (SOM) content: an agricultural soil (3.4 percent SOM) and a woodland soil (6.2 percent SOM). Specifically, this research compared the two soils in relation to: (1) atrazine adsorption and desorption; (2) atrazine distribution among humic acid, fulvic acid, and soil/humin fractions of the soils after adsorption; and (3) aging effects on atrazine adsorption, desorption, and distribution. The two soils were compared in adsorption and desorption experiments using 14C-U-ring-labeled atrazine in sterile batch reactors. Aqueous atrazine concentrations of 0.25, 1.0, 2.5, 10.0, and 25.0 µM were used to simulate residual atrazine in soil, normal application rates, and over-application or spill scenarios. Atrazine that sorbed to the soil was sequentially water-, solvent-, and alkali-extracted to quantify the water-extractable, solvent-extractable, humic acid-bound, fulvic acid-bound, and soil/humin-bound atrazine. A desorption equilibrium study also was conducted for both soils. Phase-distribution relationships (PDRs) were described for contact times of 1 hour, 1 day, and 1, 2, 4, 8, and 12 weeks using the Freundlich model. The sorption capacity (KF) of both soils was observed to increase with time, while the sorption linearity (n) generally decreased. As contact time increased, more atrazine remained bound to the soil and SOM. More than 90 percent of the atrazine was removed with water after 1 hour of adsorption contact time; however, as little as 30 percent was water extractable after 12 weeks of adsorption. Atrazine desorption linearity decreased with a decrease in the amount of sorbed herbicide.
Because water-soluble organic matter (WSOM) plays a major role in controlling the transport of organic and inorganic contaminants in the environment, experiments also were performed by Dr. Xia's research group to examine atrazine binding to WSOM. Each year, large quantities of NO3- and H2PO4-/HPO42- are introduced into the aquatic environment because of agricultural use of N and P fertilizers. Limited information is available on how N and P nutrients affect the sorption and desorption reactions of atrazine with WSOM. WSOM samples were collected from different locations in the Hillsdale watershed using a portable reverse osmosis system. The collected WSOM samples were purified using a 250 mL column of cation exchange resin (H+ form). Batch dialysis equilibrium cells were constructed and used to determine the effects of phosphate and nitrate on the sorption-desorption of atrazine by WSOM. A dialysis membrane with a molecular cut-off weight of 500 Daltons was placed between the two half cells to monitor the partition of atrazine between aqueous solution and WSOM solution. Our study suggests that nonspecific accumulation rather than specific functional group binding is most likely the sorption mechanism of atrazine on the WSOM. Thus, the formation of multiple weak interactions on WSOM may control atrazine sorption and desorption in the aquatic environments. Compared to the International Humic Substance Society Suwannee River humic acid, the WSOM had significantly higher sorption capacity and capability for atrazine. At high NaNO3 or KH2PO4 concentrations, the atrazine sorption on WSOM was decreased. However, further research is needed to investigate the effect of low levels of NaNO3 or KH2PO4 on atrazine interactions with WSOM. Dr. Xia left Kansas State University in December 2001, and she is now an assistant professor at the University of Georgia.
To characterize the WSOM isolated by Dr. Xia, Dr. Larive's group measured ultraviolet and infrared spectra for several samples. The most definitive characterization experiments utilizing 13C nuclear magnetic resonance (NMR) spectroscopy failed to produce spectra of sufficient quality for these samples despite numerous attempts. Therefore, Dr. Larive and her students have focused on the development of new NMR methods for the analysis of humic substances using alternative materials, and they have applied these methods in the study of the aggregation properties and interactions of humic substances with organic compounds. A paper, which currently is in review for the Journal of Colloid and Interface Science, examines aggregation and hydrophobic interactions of a soil humic acid isolated by Kim and Bleam at the University of Wisconsin. Although we attempted to study the interactions of atrazine with this humic substance, the atrazine proton resonances were extensively broadened and could not be detected. In this paper, NMR diffusion measurements, along with measurements of viscosity, were used to probe the aggregation properties of this humic acid, which had been previously demonstrated to form micelles. Our results suggested that the behavior of this molecule was more consistent with that reported for associative polymers, and a proposed modification to the currently accepted micelle model for aggregation of humic substances was proposed. A second study involving William Otto, has been published in the Journal of Colloid and Interface Science and currently is in review. This study uses NMR diffusion measurements to probe the interactions of aquatic humic substances with surfactants. Both natural and manmade surfactants may affect the behavior of humic substances in surface waters, especially those impacted by waste water from cities or large-scale agriculture operations. Such humic substance-surfactant interactions are likely to dramatically affect the disposition of organic pollutants, such as atrazine.
To develop a molecular-level understanding of atrazine in the environment, a series of molecular simulations were performed by Dr. Carper and his student, Zhizhong Meng. A semiempirical (PM3) study of atrazine dimer hydration in which the supermolecule approach was used to simulate a solvent sheath of water molecules varying from 7 to 24 H2Os around the atrazine dimer. Enthalpies of complex formation and reaction are reported along with entropies of complex formation and free energies of reaction and formation. The hydration sphere around the atrazine dimer is reduced stepwise by removing the outermost water molecule followed by calculation of the new structure. In the case of the hydrated dimer with 21 water molecules (see Figure 1), the removal of one water molecule results in rearrangement of the majority of the solvent sheath (see Figure 2). This is accompanied by a change in the number of hydrogen bonds and the average hydrogen bond distance between water molecules, ring chlorines, a ring nitrogen, and other water molecules. There are no additional major changes in the solvent sheath as one continues the stepwise removal of water. In addition, reaction enthalpies and entropies show a systematic change at a point where the solvent sheath contains 17 water molecules. The atrazine rings and their attached nitrogens maintain their planarity during the solvation process, unlike the case where metal ion binding to the atrazine dimer occurs.
Figure 1. Molecular Structure (PM3) of Atrazine Dimer Solvated With 21 Water Molecules.
Figure 2. Molecular Structure (PM3) of Atrazine Dimer Solvated With 20 Water Molecules. The atrazine dimer occupies the same position as in Figure 1.
In addition, Drs. Carper and Meng recently have completed a series of ab initio calculations using high-level basis sets to accurately reproduce the 13C and 1H chemical shifts of both atrazine and atrazine dimers. These results provide additional evidence for the existence of atrazine dimers in the liquid state, and they provide a basis for determining equilibrium constants for these species. This work is in press in the Journal of Molecular Structure: THEOCHEM.
The calculated gauge including atomic orbitals (GIAO) and experimental 1H NMR chemical shifts are similar to those reported previously. The N-H H's undergo considerable changes in chemical shift as a function of concentration, solvent, and temperature. In CDCl3, one observes multiple N-H 1H peaks that include two intense peaks at 6.563 and 5.527 ppm. There also are two less intense N-H 1H peaks at 6.247 and 5.212 ppm. The peaks at 5.527 and 5.211 ppm are doublets, and they can be assigned to N-H's directly connected to i-propyl groups with JH-N-C-H = 6.8 Hz. The peaks at 6.563 and 6.247 ppm are singlets and are assigned to the N-H's connected to the ethyl groups.
The ratio of peak areas for the 6.563 and 6.247 ppm 1H peaks is 2.91 for a 0.2 M atrazine solution at 25°C. This ratio decreases to 2.83 for a 0.5 M atrazine solution and increases to 3.16 for a 0.02 M atrazine solution at 25°C, indicating some type of atrazine monomer-dimer equilibrium. Assuming that atrazine dimer formation is an exothermic process suggests that the more intense peaks (6.563 and 5.527 ppm) can be assigned to the monomer and the lesser peaks (6.247 and 5.211 ppm) to the atrazine dimer.
In CDCl3, multiple 1H chemical shifts for the CH and CH2 groups also are observed, consistent with the existence of more than one atrazine species in solution. The ethyl CH2 group chemical shift consists of two groups of overlapping multiplets (JH-C-C-H = 7.2 Hz) that separate into two pairs of quartets. The most intense pair of quartets have centers at 3.471 and 3.454 ppm, and the second pair of quartets have centers at 3.384 and 3.367 ppm. The more intense pair of quartets (3.471 and 3.454 ppm) has been assigned to the atrazine monomer, and the second pair of weaker quartets (3.384 and 3.367 ppm) to the dimer. In a similar manner, we also observe C-H multiplets that can be separated (not the only solution) as two pairs of overlapping septuplets. The most intense pair of overlapping septuplets have centers at 4.212 and 4.170 ppm (JH-C-C-H = 6.4 Hz), and the weaker pair of overlapping septuplets have centers at 4.073 and 4.035 ppm. As is the case throughout this report, we assign the more intense peaks to the atrazine monomer and the weaker peaks to the atrazine dimer. The ethyl CH3 H's appear as a strong triplet centered at 1.213 ppm (JH-C-C-H = 7.2 Hz) and a weaker triplet centered at 1.185 ppm. Similarly, the i-propyl CH3 H's exist as a strong doublet centered at 1.232 ppm (JH-C-C-H = 6.4 Hz) and a weaker doublet centered at 1.204 ppm. The stronger CH3 H peaks are assigned to the atrazine monomer (1.213 and 1.232 ppm) and the weaker peaks (1.185 and 1.204 ppm) to the atrazine dimer.
The calculated GIAO and experimental 1H NMR chemical shifts for Hartree-Frock (HF) and density function theory (DFT) versions of the atrazine dimer show some interesting differences. The N-H (i-propyl) H's undergo a considerable increase in chemical shift from the monomer to the dimer in all cases. This increase in chemical shift is considerably greater than the increase in chemical shift for the N-H (ethyl) H's, resulting in a reversal of the order indicated for the atrazine monomers. A possible explanation is that solvation is more extensive in atrazine monomers, and that the i-propyl-N-H's are somewhat protected from the bulk solvent.
Calculated chemical shifts of the remaining H's match reasonably well with the experimental results. The methyl H's are averaged out assuming a rapid rotation about the C-C axis in each case. There are at least two C-H (i-propyl) values that can be explained by the existence of atrazine monomers and atrazine dimers in solution. The root mean square (RMS) deviations of the results indicate that the HF method provides a somewhat better fit for 13C chemical shifts than the DFT (B3LYP) method. The exception is the B3PW91/6-311+G(d,p) calculation that is better than the HF/6-311+G(d,p) calculation of the atrazine dimer. The HF method also provides the best fit with the experimental 13C chemical shifts for the atrazine dimer. With the exception of the N-H H's, both the HF and DFT methods are in reasonable agreement with the 1H spectrum of atrazine.
The gas-phase ab initio models of atrazine and atrazine dimers calculated with the HF and DFT methods produce structures that support the concept of hydrophobic interactions between atrazine molecules. Application of the GIAO method yields 13C and 1H chemical shifts that are consistent with experimental spectra, with the exception of multiple N-H H's. Both previous and present NMR data support the formation of multiple atrazine species including monomers and dimers in solution. Finally, total energy calculations support the concept of weak atrazine dimer formation in the gas phase. In general, it appears likely that atrazine dimers exist in the liquid state as well as in the gas phase.
Journal Articles on this Report : 7 Displayed | Download in RIS Format
Other project views: | All 17 publications | 7 publications in selected types | All 7 journal articles |
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Bhandari A, Lesan HM. Isotherms for atrazine desorption from two surface soils. Environmental Engineering Science 2003;20(3):257-263. |
R827589E02 (Final) R827457E03 (Final) |
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Lesan HM, Bhandari A. Atrazine sorption on surface soils: time-dependent phase distribution and apparent desorption hysteresis. Water Research 2003;37(7):1644-1654. |
R827589E02 (Final) R827457E03 (Final) |
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Meng Z, Carper WR. Effects of hydration on the molecular structure of atrazine dimers: a MOPAC (PM3) study. Journal of Molecular Liquids 2002;96-97:397-407. |
R827589E02 (Final) R827457E03 (Final) |
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Meng Z, Carper WR. GIAO NMR calculations for atrazine and atrazine dimers: comparison of theoretical and experimental 1H and 13C chemical shifts. Journal of Molecular Structure: THEOCHEM 2002;588(1-3):45-53. |
R827589E02 (Final) R827457E03 (Final) |
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Meng Z, Carper WR. Effects of hydration on the molecular structure of metal ion-atrazine dimer complexes: a MOPAC (PM3) study. Journal of Molecular Structure: THEOCHEM 2000;531(1-3):89-98. |
R827589E02 (Final) R827457E03 (Final) |
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Otto WH, Larive CK. Improved spin-echo-edited NMR diffusion measurements. Journal of Magnetic Resonance 2001;153(2):273-276. |
R827589E02 (Final) R827457E03 (Final) |
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Otto WH, Britten DJ, Larive CK. NMR diffusion analysis of surfactant-humic substance interactions. Journal of Colloid and Interface Science. 2003;261(2):508-513. |
R827589E02 (Final) R827457E03 (Final) |
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Supplemental Keywords:
atrazine, nuclear magnetic resonance, NMR, sorption, nutrients, runoff, agricultural watershed, agriculture runoff, agrochemicals, computer modeling, contaminated sediment, fate and transport, herbicides, natural organic matter, NOM, soil organic matter, SOM, water-soluble organic matter, WSOM., Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, PESTICIDES, Environmental Chemistry, Contaminated Sediments, Fate & Transport, Ecology and Ecosystems, Pesticide Types, fate and transport, agricultural watershed, contaminated sediment, transport contaminants, agriculture runoff, computer modeling, natural organic matter, atrazine, agrochemicalsProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.