ENHANCED HUMIFICATION AS AN IN-SITU BIOREMEDIATION TECHNIQUE FOR 2,4,6-TRINITROTOLUENE (TNT) CONTAMINATED SOILS

The microbial remediation of 2,4,6-trinitrotoluene (TNT) contaminated soils is difficult because it is not possible to achieve mineralizatio n of this substance. TNT can only be transformed via the sequence TNT --> 4-aminodinitrotoluene (4-ADNT) --> 2,4-diaminonitrotoluene (2,4-DA NT) --> 2,4,6-triaminotoluene (TAT). TAT is formed when redox potentia ls are below - 250 mV, otherwise 2,4-DANT is accumulated. If under in- situ conditions TNT or its metabolites are irreversibly incorporated i nto the humic matrix (humification) instead of being sorbed, the humif ication might be used as a remediation technology. Equilibrium dialysi s showed that TNT, 4-ADNT and 2,4-DANT did not adsorb on humic substan ces. However, in TNT-degradation studies under nitrogen limitation, th e more humic substances were added the less metabolites were recovered proposing an efficient microorganism-mediated incorporation (humifica tion), probably based on stress-induced radical enzyme reactions. This assumption was supported by the observation that 2,4-DANT but not TNT or 4-ADNT is attacked by the radicalic working enzyme peroxidase. Mol ar mass chromatography showed that 2,4-DANT is incorporated into the h umic matrix by the peroxidase. The microbiologically formed ''bound re sidues'' could not be released by drastic pH shifts to pH 2 or 12. Onl y methanolic saponification resulted in the release of about 10% of th e bound residues. An in-situ remediation technology may include at fir st a stepwise microbial reduction of TNT to 2,4-DANT. Although under i n-situ conditions TAT cannot be formed in a further step rue to the re dox potential higher than required, this is not necessary because the metabolite 2,4-DANT can be efficiently incorporated into humic substan ces.