Environ. Sci. Technol. 1997, 31, 584-589
Trinitrotoluene and Metabolites Binding to Humic Acid A N Z H I Z . L I , † K E N N E T H A . M A R X , * ,† J O H N W A L K E R , ‡ A N D D A V I D L . K A P L A N * ,‡ Department of Chemistry, University of MassachusettssLowell, Lowell, Massachusetts 01854, and Biotechnology Division, U.S. Army Natick Research, Development & Engineering Center, Natick, Massachusetts 01760-5020
Systematic studies were conducted to observe the binding interactions between the class of compounds including nitroaromatic munitions pollutants trinitrotoluene (TNT) and certain of its breakdown products and dissolved Aldrich humic acid (HA), which is used as a model soil matrix. Equilibrium dialysis followed by HPLC quantitation was used to determine the effect of ligand concentration, HA concentration, pH, and ionic strength on the formation kinetics and ligand binding level of the ligand-HA complex. It was found that TNT and its byproducts 2,6-diamino-4-nitrotoluene (2,6DAmNT) and 2-amino-4,6-dinitrotoluene (2AmDNT) are all able to bind to HA at different binding levels in a slow kinetic process. The HA concentration was observed to have the same inverse effect on the binding of both TNT and 2,6DAmNT, while pH had opposite effects on binding for the two compounds. Nearly a 2-fold increase in binding of TNT to HA was observed for a 5-fold increase in ionic strength of phosphate buffer. A linear binding model represented the best fit for the 2,6DAmNT isotherm data while the Langmuir model best fit the TNT isotherms. The maximum binding density of TNT for HA calculated from the Langmuir model ranged from 6 to 30 µM TNT/µM HA of average size 5000 for all conditions studied. These facts suggest that the binding mechanisms are different for the above two ligands due to their different chemical structures.
Introduction Explosives residues in soil and most of their transformation products are all environmental concerns (1, 2). The energetic nitroaromatic compounds were used as munitions for many decades. Since they are long-lived in the soil, they represent a significant contamination problem at many military sites in the United States and worldwide as well. The major munitions contaminant species 2,4,6-trinitrotoluene (TNT) along with its transformation products (1-3), such as 2,6diamino-4-nitrotoluene (2,6DAmNT), represent the class of compounds that is the focus of this study. The system to be studied is a complicated one. Nitroaromatic compounds TNT, 2,6DAmNT, and 2-amino-4,6-dinitrotoluene (2AmDNT) are all difficult to work with, soil is a complex matrix, and humic acid (HA) is itself a complex soil component. Moreover, the interactions between these nitroaromatic compounds and the complex soil matrix have not been extensively studied, and very few references could be found (4, 5). The steadystate adsorption to and desorption of TNT from test soil was * Corresponding authors. † University of MassachusettssLowell. ‡ U.S. Army Natick Research.
584
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 2, 1997
investigated. It was found that batch TNT adsorption isotherms were best fit by the Langmuir model, and binding was found to be most highly correlated with extractable Fe, cation exchange capacity, and clay content for all soils (4). The humic substances, of which humic acid is a component, constitute up to 80% of the organic matter in most inorganic soils. They are formed from the chemical and microbial degradation of both animal and plant material. Humic substances interact with a number of small ligands, including metal ions, and organics such as toxic pollutants (6). HA is considered to be a flexible, linear polyelectrolyte at low concentrations ( 3.5), and at moderate ionic strength ( 2AmDNT > TNT, is consistent with the conclusion from a previous study (5).
Discussion We present new observations on the binding between hydrophobic pollutants TNT, 2,6DAmNT, and 2AmDNT and hydrophilic HA, a polymeric component of soil. TNT, 2AmDNT, and 2,6DAmNT bind to HA at different levels that are influenced by many factors, including HA concentration, ligand concentration, pH, ionic strength, ligand structure, temperature, and incubation time. A common observation for the nitroaromatic compounds TNT and 2,6DAmNT binding to HA was slow binding kinetics. This is consistent with that observed previously for TNT binding to whole soil (4). These results do not distinguish the possibility that some portion of the binding we observed for these nitroaromatic compounds could be due to irreversible, covalent interaction with HA. Role of Polyelectrolyte HA. HA exhibits flexible, linear, polyelectrolyte behavior under the conditions of low HA concentration ( 3.5, and moderate ionic strength (
