The insensitive high explosive (IHE) constituent, 3-nitro-1,2,4-triazol-5-one (NTO), has a very high aqueous solubility (16,640 mg L-1) and low hydrophobicity. These properties may provide conditions for elevated concentrations of NTO in munitions manufacturing wastewater. Likewise, NTO is very mobile in soil and aquifers, possibly creating a concern for surface water and groundwater. Thus, developing a cost-effective treatment method that can target NTO as the predominant explosive in munitions wastewater or impacted groundwater is of interest. The objective of this project was to demonstrate that reactive minerals in a sequence of reducing and oxidizing packed bed reactors can rapidly degrade NTO to safe end-products.

The basis of the project was to utilize redox-active minerals with reactive surfaces to sequentially (i) reduce and (ii) oxidize the NTO molecule. In the first step of the process, NTO was reduced by zero-valent iron (ZVI) to its daughter product, 3-amino-1,2,4-triazol-5-one (ATO), which is susceptible to surface-catalyzed oxidation. In the next step, ATO was oxidized by birnessite (MnO₂) to environmentally safe end-products (urea, N_2(g) and CO_2(g)). In order for this technology to be a significant game changer, the project aimed to accomplish each step in the process with very low hydraulic retention times. The tasks were arranged around:

1. Optimizing each of the sequential reactions.
2. Assessing the longevity of the reactive minerals and developing methods to prevent surface passivation of the minerals.
3. Evaluating and controlling the solid phase mineralogy over time in the packed bed reactors.
4. Expanding the technology by testing the reduction of NTO and co-occurring IHE compounds, 2,4-dinitroansisole (DNAN) and nitroguanidine (NQ), by ZVI and by naturally occurring or commercially available iron sulfide (FeS) minerals.

The investigations demonstrated that ZVI and FeS minerals rapidly reduce NTO to ATO. This project established that depassivating pretreatments could improve the performance of NTO reduction by ZVI. The approach required for effective depassivation varied depending on the thickness of the passivated layer. For a ZVI containing a thick passivated layer (ca. 880 nm), the best pretreatment was with 1 M HCl. In contrast, for a ZVI with a thinner passivated layer (ca. 300 nm), a milder treatment with 60 mM bicarbonate solution was sufficient. ZVI-packed columns were also operated treating NTO-containing effluent at pH 3 or 6 for six months. Both columns effectively reduced NTO to ATO. The column treating the pH-3.0 influent exhibited prolonged longevity in reducing NTO, treating 11-fold more pore volumes (PV) than the column treating the pH-6.0 influent until the breakthrough point (defined as when 85% of NTO was removed). The ZVI in the columns was oxidized to iron (oxyhydr)oxide minerals such as magnetite, lepidocrocite, and goethite, as confirmed by synchrotron X-ray absorption spectroscopy and X-ray diffraction measurements. The spent ZVI in the columns regained full NTO-reducing capacity by reactivation using 1 M HCl.

This project has also demonstrated that FeS, including commercial FeS and mackinawite, a naturally-occurring mineral, can reduce NTO and DNAN to their respective aromatic amines. These results indicate that this reaction may be driving the fate of insensitive munitions compounds (IMC) in aquatic sediments and anoxic subsurface environments rich in FeS minerals. Solid phase analysis showed concurrent oxidation of mackinawite to goethite, an iron (oxyhydr)oxide mineral [α-FeO(OH)],  and elemental sulfur.

To address the second objective of this project, the complete oxidation of ATO to final inorganic products (urea, NH₄⁺, CO_2(g), and N_2(g)), was tested using Mn^IV oxide minerals. Two commercial granular MnO₂ materials, i.e. Pro-OX^TM and Greensand Plus^TM, were assessed in packed-bed columns under a simulated wastewater treatment regime (1 mM ATO, empty bed contact time [EBCT] = 1 h). The column packed with Pro-OX reached breakthrough (C/C₀ ≥ 0.05) at 2400 PV, as compared to only 55 PV for the column packed with Greensand Plus. Under an accelerated groundwater flow regime (0.1 mM ATO, EBCT= 24 h), both materials removed ATO effectively for the duration of operation (660 PV for Pro-OX or 380 PV for Greensand Plus). Characterization of the solid phase mineralogy indicated that, after reacting with ATO, the Mn oxide materials that initially were dominated by Mn^IV (e.g., pyrolusite, ramsdellite, todorokite) were converted to Mn^III minerals (e.g., groutite, manganite) and soluble Mn^II (recovered in effluent) indicating a general reduction of Mn oxides.

This project also investigated the treatment of NQ, an IMC and co-occurring component of IMC-in impacted (waste)water, by ZVI, mackinawite, and commercial FeS under anoxic conditions. NQ transformation followed pseudo first-order kinetics. The reaction rate constants decreased as follows: commercial FeS > mackinawite > ZVI. The primary mechanism of NQ transformation was nitro-reduction, which generated nitrosoguanidine (NsoQ) as an intermediate. Further degradation of NsoQ by ZVI yielded aminoguanidine as a significant product, whereas guanidine was preferably formed in the presence of mackinawite and commercial FeS. Other degradation products detected included cyanamide, cyanoguanidine, and biguanidine. The project team assessed the treatment of NQ in ZVI- and FeS-packed reactors operating at pH 3.0 and 5.5, the optimum pH conditions established for each mineral, respectively. NQ removal efficiency data obtained during the 45 days of operation confirmed that ZVI outperformed FeS. NQ was effectively removed until the end of the operation (45 d, 490 pore volumes or PV) in the ZVI-packed column. In contrast, NQ breakthrough (NQ removal efficiency < 85%) was observed after only 10 days (100 PV) in the FeS-packed column.  

Batch assays conducted to investigate the impact of co-occurring chemicals on the transformation of IMC revealed a marked decrease in the rate of NQ reduction by ZVI and FeS when NTO was also present at equimolar concentrations, compared to assays lacking NTO. The rate of NTO reduction also decreased when NQ was present, albeit to a lower extent. These results indicate that competition between reactive co-occurring chemicals (e.g. different IMC) can lead to a decrease in the transformation rate of the chemicals. Moreover, they signify the importance of performing studies with real (waste)water to determine design parameters for full-scale design.

The innovative use of reactive minerals presents an effective solution for both in situ groundwater remediation and high-rate treatment of munitions manufacturing wastewater impacted with NTO, NQ and other IMC. The robustness and longevity of the minerals used, coupled with the potential for ZVI rejuvenation through acid treatment, make this technology a promising option for the treatment of water streams containing IMC. Furthermore, the use of naturally occurring minerals such as mackinawite provides valuable insights into the potential for natural attenuation processes in impacted aquatic sediments and subsurface environments. Further research is recommended to test the feasibility of the proposed technology in pilot-scale studies with groundwater from impacted sites and/or industrial wastewater to establish design parameters for full-scale application. (Project Completion - 2023)

Menezes, O., Y. Yu, R.A. Root, S. Gavazza, J. Chorover, R. Sierra-Alvarez, and J.A. Field. 2021. Iron (II) Monosulfide (FeS) Minerals Reductively Transform the Insensitive Munitions Compounds 2, 4-Dinitroanisole (DNAN) and 3-Nitro-1, 4-Triazol-5-One (NTO). Chemosphere, 285:131409. doi.org/10.1016/j.chemosphere.2021.131409.

Rios-Valenciana, E.E., O. Menezes, X.Z. Niu, J. Romero, R.A. Root, J. Chorover, R. Sierra-Alvarez, and J.A. Field. 2022. Reductive Transformation of the Insensitive Munitions Compound Nitroguanidine by Different Iron-Based Reactive Minerals. Environmental Pollution, 309:119788. doi.org/10.1016/j.envpol.2022.119788.

Theses and Dissertations

Miller, M.P. 2022. Degradation of 3-nitro-1,2,4-triazol-5-one (NTO), An Insensitive High Explosive, and its Reduced Daughter Product by Reactive Minerals (Master’s Thesis). University of Arizona.

Yu, Y. 2022. Reductive Transformation of Insensitive Munitions Compounds by Reactive Iron-Based Minerals (Ph.D. Dissertation). University of Arizona.