Ecotoxicological response of Scenedesmus obliquus to pure energetic compounds and metal ions found in wastewater streams from munitions manufacturing

The energetic compounds 1-methoxy-2,4-dinitrobenzene (DNAN), nitroguanidine (NQ), 3-nitro-1,2,4-triazole-5-one (NTO) and 1,3,5-trinitro-1,3,5-triazinane (Research Department Explosive, RDX) are widely used for explosive formulations production. Wastewater streams from industrial munition facilities are rich in nitrogen and contain considerable amounts of residual energetic compounds and metals such as zinc, nickel and copper. As such, these wastewater streams require physicochemical and/or biological treatment to meet regulatory discharge limits. The high nitrogen content of these streams makes them an attractive nutrient source for growing biomass such as microalgae. In previous studies we have identified some wastewater streams suitable for microalgal growth with minimal or no prior treatment, while others have shown moderate to high growth inhibition. The current study aims to elucidate the toxicity response of microalgae by performing growth-inhibition assays using solutions of single compounds and mixtures of energetic compounds and heavy metals. Using the freshwater green microalga Scenedesmus obliquus (S. obliquus) (ATCC (R) 11477) as a model organism, a microwell-plate growth-inhibition test procedure was performed to assess potential growth inhibition effects by DNAN, NQ, NTO, RDX, Zn, Ni and Cu. For the three metals tested, the order of growth rate inhibition effect on S. obliquus was Ni > Cu > Zn. The EC50s of Ni, Cu and Zn for S. obliquus were 0.08 mg/L, 0.26 mg/L and 10.13 mg/L, respectively. The effective concentrations for 50% growth inhibition (EC50) were 4.86 mg/L for DNAN and 8,700 mg/L for NTO. Conversely, RDX exhibited no toxicity below its solubility limit, and NQ had a mild toxic effect on S. obliquus with an EC20 of 303.3 mg/L.