Toxicity and bioavailability of explosive metabolites to invertebrates

RDX and its N-nitroso metabolites were rapidly absorbed into earthworms. The BCFs were 1.86, 0.39, and 0.05 for RDX, MNX and TNX, respectively. It is unlikely that earthworms reduce RDX to MNX and MNX to TNX. Other biotransformation pathways may be involved in earthworm biodegradation of RDX. MNX and TNX were also absorbed by PSDs. Organic matter content is one soil factor that affected the ratio of MNX or TNX uptake into earthworms vs. uptake into PSDs. A linear relationship between PSD uptake and earthworm uptake was observed. 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.

MNX and TNX inhibited earthworm growth, caused death, and inhibited reproduction. Earthworms were more sensitive to TNX in sandy loam soil than other conditions. The LOLC for MNX and TNX was 100 mg/kg in sandy loam soil, and 200 mg/kg in silt loam soil. At 7 days of exposure, the LOEC for earthworm growth was 50 mg/kg for TNX and 100 mg/kg for MNX. After 35 days of exposure, earthworm growth was reduced 8-39% by TNX in sandy loam soil, whereas TNX only inhibited earthworm growth 5-18% in silt loam soil.

MNX affected glutathione synthesis and the activity of GPX and ChE although no significant effects were observed on protein content, CAT, and GCS activity. Surprisingly, MNX enhanced ChE activity, especially the activity of AChE. AChE activity was enhanced more than 3-fold in earthworms exposed to 50 mg/kg MNX for 30 days. RDX N-nitroso metabolites, especially MNX, may cause oxidation stress in earthworms.

MNX and TNX affected cricket egg hatching. TNX was more toxic to eggs than MNX. After 30 days exposure, the EC20, EC50, and EC95 were 47, 128, and 247 ìg/g for TNX, and 65, 140, and 253 ìg/g for MNX in topical tests and 21, 52, and 99 ìg/g for MNX, and 12, 48, and 97 ìg/g for TNX in sand.

These data can be used for defining criteria for environmental management of RDX and performing specific risk assessment of RDX and its N-nitroso metabolites.