Binding of 2,4,6-trinitrotoluene and its degradation products in a soil organic matter two-phase system

The widely used explosive 2,4,6-trinitrotoluene (TNT) and its degradation products are of large environmental concern because of their toxic properties and high concentrations encountered in contaminated soils. Batch experiments were used to study TNT* (the sum of TNT and its degradation products) bonding to dissolved (DOM) and particulate (POM) soil organic matter. Reversed-phase high performance liquid chromatography (RP-HPLC) was used as a separation technique in combination with 14C-labeled TNT to determine free TNT and TNT* bound to DOM. By use of dialysis we showed that DOM did not interfere with the HPLC analysis of free TNT. Depending on pH and total TNT concentration, the relative distribution of TNT* among water, POM, and DOM varied between 60 to 90, 10 to 30, and 0.5 to 6%, respectively, after 22 h of equilibration. The association of TNT* to DOM was strongly pH dependent and followed a nonlinear Langmuir isotherm. The association of TNT* to POM was less pH dependent and data were equally well fitted by linear and nonlinear isotherms. Particulate organic matter had 6.4 (pH 6.2) to 22 (pH 5.2) times greater capacity to bind TNT* than DOM, but the binding strength (the slope of the isotherm) was greater for DOM. The TNT degradation was enhanced with increasing concentration of soil organic matter, resulting in a stronger bonding of TNT* to DOM and POM. Based on our results, combined with other recent findings, we suggest that it is mainly the degradation products of TNT that associate with DOM and POM, and that the association with DOM is mainly of ionic character involving specific DOM sites. The greater binding capacity and a weaker, linear type of isotherm suggests a nonspecific type of partitioning in POM, possibly of hydrophobic character.