Influence of Ubiquitous Electron Acceptors on In Situ Anaerobic Biotransformation of RDX in Groundwater

摘要

A series of column studies, with aquifer material from the former Nebraska Ordnance Plant, were performed to explore the phenomenon of electron competition from ubiquitous inorganic electron acceptors (nitrate and sulfate) present in contaminated groundwater. Acetate was used as a source of readily biodegradable carbon in all of the treatment column systems. Influent hexahydro- 1,3,5-trinitro-1,3,5-triazine (RDX) concentrations (1 to 1.8 mg L(exp -1)) were completely removed to below detection levels of 20 micrograms L(exp -1) in all treatment column systems without any nitroso-metabolites. In the control column system (with no carbon amendment), significant levels ((caret)30 percent of the inlet molar RDX of nitroso-substituted RDX derivates were observed in the effluent stream. The estimated first-order biodegradation rate coefficient for RDX was highest (0.79 hr(exp -1)) in the treatment column system where acetate was the only amendment, about 52 times higher than the rate coefficient (0.015 hr(exp - 1) obtained in the control column system. The presence of sulfate (100 mg L(exp -1) in influent groundwater temporarily delayed the onset of RDX biotransformation without any adverse effects on overall RDX biotransformation. Coexistence of low (100 mg L(exp -1) nitrate levels in the influent feed water reduced the first-order biodegradation rate coefficient obtained in the absence of nitrate by about 80 percent to 0.16 hr(exp -1). These nitrate levels, however, were low to halt the RDX biodegradation, probably because the available carbon levels were high enough to exceed the demands for nitrate reduction. High levels of nitrate (500 mg L(exp -1)) initially halted RDX removal and significantly reduced the rate of RDX biotransformation by about 98 percent to 0.02 hr(exp -1), thereby increasing the half-life from 0.9 hr in the absence of nitrate to about 32 hr, with noticeable levels of untreated RDX in the effluent stream.