Trinitrotoluene and Metabolites Binding to Humic Acid

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. Equi librium 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.