The effect of three chemical oxidants on subsequent biodegradation of 2,4-dinitrotoluene (DNT) in batch slurry reactors

BACKGROUND: Recent studies indicate that chemical oxidation may be compatible with subsequent biodegradation in contaminated soils. To test this, soil contaminated with 2,4-dinintrotoluene (2,4-DNT) was treated in batch slurry reactors with (1) ozone, (2) modified Fenton chemistry (MFC), and (3) iron-activated sodium persulfate (SPS). Chemical and subsequent biological oxidation were monitored, and compared with biodegradation alone. Release of nitrite and nitrate distinguished biological from chemical oxidation of 2,4-DNT, respectively. DNT-degrading microorganisms were enumerated. The disappearance of volatile fatty acids (VFAs) accumulated during chemical oxidation was also monitored. RESULTS: In the biological reactor 66% of the 2,4-DNT was degraded, but further biodegradation was inhibited by nitrite concentrations approaching 18 mmol L-1. At the doses tested, all oxidants achieved chemical oxidation followed by biodegradation, resulting in 98% DNT removal overall. Ozone achieved the greatest DNT removal (70%), but also caused the greatest reduction in DNT degraders and the longest rebound time (60 days) before biodegradation of the remaining DNT and VFAs. SIPS resulted in the least DNT removal by chemical oxidation (37%), but showed no obvious rebound period for DNT degraders, and even signs of co-existing chemical and biological oxidation. By releasing nitrate, which is less toxic than nitrite, the oxidants kept nitrite levels below 18 mmol L-1, enabling the follow-on biodegradation step to attain lower concentrations of 2,4-DNT than biodegradation alone. CONCLUSIONS: All three chemical oxidants were compatible with biodegradation of residual 2,4-DNT. Post-oxidation bioremediation should be included in remedial designs. (C) 2009 Society of Chemical Industry