Determining degradation kinetics, byproducts and toxicity for the reductive treatment of Nitroguanidine (NQ) by magnesium-based bimetal Mg/Cu

Abstract

Energetic-laden process water from industrial munition facilities can be treated by zero-valent metals (ZVMs) or zero-valent iron (ZVI) to remove residual energetics. This reduction-based treatment is significantly enhanced with the addition of a secondary catalytic metal (i.e. forming a bimetal reagent). The reagent is further enhanced by using a more reductive base metal, such as Mg. In this work, the reductive degradation of nitroguanidine (NQ) in aqueous solutions by Mg/Cu bimetal is investigated. Two initial pH conditions (unadjusted and pH 2.7) were studied. Under unadjusted initial pH conditions, 90% of NQ degraded within 30 min reaction time. After 150 min, NQ degradation generated a suite of products including guanidine (44%), cyanamide (31%), formamide (15%), aminoguanidine (AQ) (6%), urea (2%) and cyanoguanidine (0.03%), leading to 100.0% carbon closure when accounting for residual NQ. The experimentally-derived degradation reaction pathway consisted of two parallel reactions: nitroreduction led to formation of AQ with further degradation to urea, cyanamide and formamide, or reductive cleavage of the N-N bond led to guanidine formation. Toxicological assessments indicated only cyanamide and AQ were toxic to S. obliquus at certain concentrations. A lowered initial pH promoted AQ transformation to benign formamide, thus reducing toxicity and complexity of products.