Department of Civil, Environmental, and Geo- Engineering, University of Minnesota, 500 Pillsbury Drive SE, Minneapolis, Minnesota 55455-0116, United States.
Department of Environmental Chemistry, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland.
Environ Sci Technol. 2021 May 18;55(10):6752-6763. doi: 10.1021/acs.est.0c08420. Epub 2021 Apr 26.
Subsurface contamination with the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) at ordnance production and testing sites is a problem because of the persistence, mobility, and toxicity of RDX and the formation of toxic products under anoxic conditions. While the utility of compound-specific isotope analysis for inferring natural attenuation pathways from stable isotope ratios has been demonstrated, the stable isotope fractionation for RDX reduction by iron-bearing minerals remains unknown. Here, we evaluated N and C isotope fractionation of RDX during reduction by Fe(II) associated with Fe minerals and natural sediments and applied N isotope ratios to the assessment of mineral-catalyzed RDX reduction in a contaminant plume and in sediment columns treated by chemical reduction. Laboratory studies revealed that RDX was reduced to nitroso compounds without denitration and the concomitant ring cleavage. Fe(II)/iron oxide mineral-catalyzed reactions exhibited N isotope enrichment factors, ε, between -6.3±0.3‰ and -8.2±0.2‰, corresponding to an apparent N kinetic isotope effect of 1.04-1.05. The observed variations of the δN of ∼15‰ in RDX from groundwater samples suggested an extent of reductive transformation of 85% at an ammunition plant. Conversely, we observed masking of N isotope fractionation after RDX reduction in laboratory flow-through systems, which was presumably due to limited accessibility to reactive Fe(II).
在弹药生产和测试场地，由于 RDX 的持久性、迁移性和毒性以及在缺氧条件下形成有毒产物，爆炸物六氢-1,3,5-三硝基-1,3,5-三嗪（RDX）的地下污染是一个问题。虽然已经证明了利用特定化合物同位素分析从稳定同位素比值推断自然衰减途径的有效性，但含铁矿物还原 RDX 的稳定同位素分馏仍然未知。在这里，我们评估了与铁矿物和天然沉积物相关的 Fe(II)还原 RDX 过程中的 N 和 C 同位素分馏，并将 N 同位素比值应用于评估污染物羽流和通过化学还原处理的沉积物柱中矿物催化的 RDX 还原。实验室研究表明，RDX 被还原为亚硝基化合物，而没有脱氮和伴随的环裂解。Fe(II)/铁氧化物矿物催化反应表现出 N 同位素富集因子，ε 值在-6.3±0.3‰和-8.2±0.2‰之间，对应于明显的 N 动力学同位素效应为 1.04-1.05。在弹药厂，从地下水样本中观察到 RDX 的 δN 变化约为 15‰，这表明其还原转化程度为 85%。相反，我们在实验室流动系统中观察到 RDX 还原后 N 同位素分馏的掩盖，这可能是由于反应性 Fe(II 的可及性有限。
