Predicting Abiotic Reduction Rate Constants of Munition Compounds in Soils

We report an empirical poly-parameter linear free energy relationship (LFER) for estimating the mass-normalized rate constants for the abiotic reduction of munition compounds (MC) in soil. A total of 131 kinetic experiments were performed, using three classes of MC (nitroaromatic [TNT, DNAN], nitramine [RDX], and azole [NTO]) and 11 soils having highly varied organic carbon and iron contents and reduced with dithionite to different electron contents. The LFER has the same form as that for MC reduction by Fe^III (oxyhydr)oxide-Fe_aq^II redox couples and predicts MC reduction rate constants to within an order of magnitude, using only the aqueous-phase one electron reduction potential (E_H¹) of the MC and the pe and pH of the soil. As previously shown for azoles, which exhibited markedly higher reactivity toward iron than toward carbon reductants relative to all neutral MC, NTO reduction rate depended on soil composition and hence a correction to model prediction was necessary at soil iron-to-carbon mass ratios ≲1. This is the first successful attempt to predict the reduction kinetics of structurally diverse nitro compounds in compositionally complex soils based on their thermodynamic properties. The LFER would be useful in the management/restoration (e.g., natural or enhanced attenuation) of soils impacted by MC or other nitro pollutants.