Degradation Dynamics and Pathways of Unsymmetrical Dimethylhydrazine (UDMH) Across Contrasting Soil Matrices: Insights from Controlled Incubation Experiments

Unsymmetrical dimethylhydrazine (UDMH) serves as a high-performance liquid rocket propellant extensively utilized in the global aerospace industry, and its environmental release and leakage (particularly into soil systems) pose severe risks to ecological integrity and human health. As one of the few studies to quantitatively correlate soil physicochemical properties with UDMH degradation kinetics and pathway partitioning using controlled incubation experiments, this work aims to reveal the environmental hazards of UDMH in soil and provide a theoretical basis for subsequent remediation. The temporal degradation dynamics of UDMH in three comparative soil matrices (yellow-brown soil, red soil and black soil) were explored, correlations between soil physicochemical characteristics and UDMH degradation behavior were clarified, and UDMH degradation pathways were quantified. Headspace solid–phase microextraction (HS–SPME) was adopted as the pretreatment method, and gas chromatography–mass spectrometry (GC–MS) was used to identify UDMH and its transformation products (TPs) in soil incubation. From the GC–MS chromatogram, UDMH and its TPs—formaldehyde dimethylhydrazone (FDMH), acetaldehyde dimethylhydrazone (ADMH) and 1,1,4,4-tetramethyltetrazene (TMT)—were identified in the three soil matrices. UDMH underwent rapid degradation within the first 7 days of incubation, with degradation rates reaching 66.03%, 67.51% and 73.13% in yellow-brown soil (YS), red soil (RS) and black soil (BS), respectively. Degradation was most rapid in BS, followed by YS and RS. UDMH degraded completely and was undetectable within 30 days of soil incubation in the present study. Correlation analysis of soil physicochemical properties and UDMH degradation behavior revealed a significant influence of these edaphic properties on UDMH degradation dynamics across the tested soil matrices. The analysis of UDMH degradation pathways, including volatilization, photodegradation, microbiological degradation, and others (oxidation and self-degradation, etc.) demonstrated that other pathways (including catalytic transformation, induced transformation or unidentified biotic–abiotic coupled processes) acted as the dominant pathway governing its degradation (accounting for 68.75%). This study provides important insights and theoretical basis for unraveling the environmental fate of UDMH and remediating UDMH-contaminated soils.