Biodegradation of 2,4‐ and 2,6‐dinitrotoluene by freshwater microorganisms

Abstract

The microbial degradation of 2,4‐ and 2,6‐dinitrotoluene was complete or nearly complete in surface water from two locations downstream from the Radford Army Ammunition Plant. No degradation was detected in surface water from four local (Frederick, MD area) sites. Either isomer could serve as a sole carbon and energy source, with about 60 percent of substrate carbon appearing as CO₂, and with an increase in the population of degrading organisms. In both the rate of mineralization in percent degraded per day increased with increasing substrate concentration. At 10 mg/L, degradation rates of 32 and 14.5 percent/day were observed for the 2,4 and 2,6 isomers, respectively. At very low concentrations of the 2,6 isomer a degrading population did not develop, and significant degradation did not occur. The rate of substrate utilization was far greater, and the lag time shorter, for the 2,4 isomer, consistent with a far greater density of 2,4‐DNT degraders.

Mixed enrichment cultures were developed for each DNT isomer separately, by sequential transfer to increasing substrate concentrations. Maximum substrate concentrations utilized were about 130 mg/L, and cell yields of 6.8 to 7.3 X 10⁵ CFU/μg input DNT were calculated. Disappearance of 2,4‐DNT in the presence of high concentrations of 2,4‐DNT mixed enrichment culture approximated first‐order kinetics; pseudo‐first order rate constants varied from 0.04 3 to 0.190 min⁻¹. The mean second‐order constant was 3.9 X 10⁻¹⁰ ml cell⁻¹ min⁻¹. If one assumes a concentration of 10⁶ cells/ml, at 25°C, a half‐life of 29.7 hours can be estimated for this isomer. Similarly, for 2,6‐DNT, the second‐order constant was 9.9 X 10⁻¹⁰ ml cell⁻¹ min⁻¹. The corresponding estimated half‐life was 11.6 hours. This rate is not likely to be realized in natural surface waters, due at least in part to the very low densities of 2,6‐DNT utilizers. From mixed enrichment cultures, pure cultures using 2,4‐DNT were isolated.

Key words:

• 2,4‐Dinitrotoluene
• 2,6‐Dinitrotoluene
• Microbial degradation
• Mineralization