Evaluating the bioavailability of explosive metabolites, hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX) and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX), in soils using passive sampling devices

The uptake kinetics of two major RDX (hexahydro-1,3,5-trinitro-1,3,5-triazacyclohexane) metabolites, hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX) and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX), into passive sampling devices (PSDs), and the ability of PSDs to serve as surrogates for evaluating bioavailability of MNX and TNX were investigated in laboratory sand and two soil types. The results indicate that MNX and TNX absorption into PSDs was best fitted with a polynomial curve model: y = ax2 + bx + c (y: amount of MNX or TNX absorbed into PSD; x: incubation time of PSDs in soil), with an excellent correlation coefficient (>0.95) for each type of soil amended with 10 mg/kg MNX or TNX. TNX was more readily absorbed by PSDs than MNX. Soil conditions, especially organic matter content, affected MNX and TNX uptake into PSDs. A relatively good correlation between MNX and TNX uptake into PSDs and uptake into earthworms was obtained in two types of natural soils (a silt loam soil from Nebraska and a sandy loam soil from Texas) and laboratory sand. A linear relationship between PSD uptake and earthworm uptake was observed. The correlation coefficients (r2) were ≥0.82 for all test soils spiked with MNX or TNX. Organic matter content is one soil factor that affected the ratio of MNX or TNX uptake into earthworms versus uptake into PSDs. These data indicate that C18 PSDs may be used as a surrogate for soil organisms such as earthworms and provide a simple and easy chemical test for assessing the bioavailability of contaminants in soils.