Theme: 01. Remediation Technology Innovations

Topic Area: 01b. Bioremediation: Advances in Amendment Formulations

Formatted Title:

Biological Degradation of High Concentrations of 2,4- and 2,6-DNT on Laboratory and Field Scale

Background/Objectives:

On a former toluene-di-isocyanate production site in Brazil, a variety of contaminants are present, including nitrotoluenes, chlorobenzenes, BTEX and some chlorinated compounds. The main components of concern are 2,4 and 2,6-dinitrotoluene (DNT). The highest maximum concentrations for DNT that were found were 940 mg/L 2,4-DNT and 540 mg/L 2,6-DNT. Greensoil was given the opportunity to investigate the feasibility of both aerobic and anaerobic biodegradation of the different contaminants with the focus of DNT by means of laboratory degradation tests and a subsequent field test.

Approach/Activities:

Based on scientific literature DNT can be biodegraded both aerobically via removal of the nitro groups, leading to the production of nitrite as a degradation product and anaerobically, where the nitro groups are being reduced to amino groups. Since aminotoluenes are considered more recalcitrant under anaerobic conditions, the anaerobic conditions are typically followed by aerobic conditions, to further biodegrade the aminotoluenes. Even though switching from anaerobic to aerobic conditions takes extra effort, costs, etc. from a practical point of view, this approach was considered since dichlorobenzene (DCB) was being used as a solvent for DNT. In case of aerobic degradation, it is expected that DCB is preferentially degraded over DNT with the potential risk of crystallization of DNT, which has been observed at the site. The feasibility of biological degradation of DNT was investigated both on laboratory and field scale. For the field test, two different layers are investigated in order to be able to potentially test the reduction at different pHs if necessary.

Results/Lessons Learned:

During the laboratory degradation test > 99% DNT degradation was observed under anaerobic conditions. Since the natural pH was ± 6.0, it was decided to also test a buffered condition with neutral pH. At natural pH, the main degradation products in the presence of an electron donor were aminonitrotoluene (ANT) isomers, while at neutral pH they were 2,4-and 2,6-diaminotoluene (DAT). Only at neutral pH, the degradation products were completely further degraded under aerobic conditions.

Under aerobic conditions complete degradation of 2,4-DNT (> 99%) was observed in all biological conditions, including the biological control, however no degradation of 2,6-DNT could be observed under aerobic conditions.

Anaerobic reduction was confirmed by CSIA by demonstrating an isotopic shift on the nitrogen atom. Next Generation Sequencing showed an enrichment in Enterococcus and Dysgonomonas, two groups that are not previous known to be able to reduce DNT.

Based on the degradation tests, a field test was started to further investigate the feasibility of stimulated anaerobic reductive biodegradation under field conditions. So far complete reduction to DAT has been observed and increased in time. After 4 months in the shallow layer with an average pH of 6.6, the average degree of DNT reduction was 82% in a ratio 43:57 ANT/DAT. In the deeper layer with higher concentrations of DNT and an average pH of 6.1, the degree of DNT reduction was on average 3.6% in a ratio 96:4 ANT/DAT so far.

Based on laboratory tests, anaerobic reduction of DNT to ANT and DAT and subsequent degradation of ANT and DAT under aerobic conditions seems to be the most feasible approach for the treatment of high concentrations of both 2,4-DNT and ,2,6-DNT. The approach is being tested in a field study and so far a significant part of the DNT is being reduced to mainly DAT during the anaerobic phase in the more shallow layer with a slightly higher pH and lower DNT concentrations.

This study proved the value of laboratory testing prior to a field test and showed also high concentrations of DNT can be treated by bioremediation.

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Fernanda Martins