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

TL;DR

This study examines how unsymmetrical dimethylhydrazine (UDMH) degrades in different soil types, identifying key factors and pathways that influence its breakdown.

Contribution

The study quantitatively links soil properties to UDMH degradation rates and pathways using controlled experiments, a novel approach in environmental chemistry.

Findings

UDMH degraded rapidly in all three soil types, with black soil showing the highest degradation rate.

Other degradation pathways, such as catalytic transformation, dominated UDMH breakdown, accounting for 68.75% of the process.

Soil physicochemical properties significantly influenced UDMH degradation dynamics across tested matrices.

Abstract

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.

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1. Introduction

Unsymmetrical dimethylhydrazine (UDMH), a high-energy liquid fuel, is a key propellant widely used in the global aerospace industry [1,2,3,4,5]. The launching and operation of aerospace vehicles inevitably leads to the leakage of UDMH into the environment, with soil ecosystems as the primary receptor, threatening ecological balance and human health. As a highly toxic organic compound, UDMH impairs soil microbial activity and biochemical processes [6,7,8,9,10,11]. Given its inherent carcinogenic, mutagenic, teratogenic and embryotoxic properties, the IARC (International Agency for Research on Cancer) has designated UDMH as a possible human carcinogen, falling into Group 2B. Additionally, UDMH’s strong chemical reactivity induces oxidative transformation and self-polymerization, generating toxic nitrogenous derivatives with comparable or higher toxicity [12,13,14,15,16]. With the increasing use of UDMH in aerospace activities, associated soil contamination has intensified, creating an urgent need for efficient and sustainable remediation technologies [17,18,19,20].

Prior to formulating efficient and eco-friendly remediation strategies, it is imperative to conduct a comprehensive characterization of the environmental behavior of UDMH in contaminated soils, encompassing its degradation, adsorption and retention dynamics, degradation pathways, environmental fates, and toxicological profiles. This process-specific mechanistic understanding lays a scientific foundation for the rational design of targeted remediation schemes, thereby effectively mitigating exposure risks posed by UDMH and its transformation products (TPs) into soil systems.

In recent years, numerous studies have investigated UDMH-induced soil contamination. Kenessov et al. [21] explored the distribution of UDMH TPs in sandy soils at the impact sites of the Baikonur Cosmodrome in Central Kazakhstan. Their results demonstrated that soil contamination was confined exclusively to the epicenters of these impact sites, with an affected range spanning 8–10 m in diameter. Semi-volatile TPs dominated the surface soil horizon, driven by the rapid volatilization and microbial breakdown of volatile UDMH derivatives. Rodin et al. [22] reported a higher initial UDMH concentration in moist soils compared to dry soils. Notably, regardless of soil moisture conditions, the total residual UDMH concentration in soil matrices decreased to less than 0.5% of the initial content 90 days after spillage. Koroleva et al. [23] investigated the environmental effects associated with “Proton” and “Soyuz” carrier rocket launches at the Baikonur Cosmodrome over the period 2014–2016. Their measurements revealed that the UDMH concentration in sporadic soil samples peaked at 1.5 mg·kg^−1^; additionally, at the leakage sites, the soil’s pH value increased significantly from the original 6.6 to 8.3–9.4. Previous reports [24,25] have explored the TPs of UDMH upon exposure to atmospheric oxygen. Their findings revealed that most UDMH undergoes rapid oxidative conversion, primarily via a radical-driven reaction mechanism, yielding a diverse array of related products. These include formaldehyde dimethylhydrazone (FDMH), N-nitrosodimethylamine (NDMA), formic acid dimethylhydrazine (FADMH), dimethylamine (DMA), N,N-dimethylformamide (DMF) and other harmful substituted heterocycles. Krechetov et al. [26] revealed that the environmental behavior of UDMH in soils was jointly determined by the intrinsic properties and concentration of UDMH, as well as the physicochemical characteristics of the soil, including its acid-base properties, organic matter content, particle size distribution and clay mineral composition. Ul’yanovskii et al. [18] explored the migration behavior of unsymmetrical dimethylhydrazine in histosols within the rocket stage impact zones in northern Russia (Plesetsk Cosmodrome). The maximum UDMH concentration, reaching up to 240 mg∙kg^−1^, was detected in the vicinity of the crater center. Notably, UDMH and its primary transformation products exhibit strong binding affinity to natural organic matter and humic substances, leading to the long-term persistence of such pollutants in the environment. Although extensive studies have investigated the spatial distribution of unsymmetrical dimethylhydrazine and its major TPs at multiple contaminated districts (primarily in Russia and Kazakhstan), critical research gaps still persist. Specifically, previous researchers have investigated the contamination characteristics of UDMH-contaminated sites, but research on temporal degradation behavior remains lacking; the effect of soil physicochemical characteristics on UDMH environmental fate in certain districts has been studied, but the comprehensive impacts of soil physicochemical properties on its degradation behavior in comparative soil matrices are not well understood. Thus, the temporal degradation dynamics of unsymmetrical dimethylhydrazine in distinct soil substrates, correlations between the physicochemical characteristics and UDMH transformation, and its main transformation pathways will be addressed in the present study. The paucity of fundamental data regarding UDMH degradation dynamics in diverse soil types and its transformation pathways severely constrains the assessment of environmental safety risks and the development of efficient remediation strategies.

Therefore, the present study aims to (1) investigate the temporal degradation dynamics of unsymmetrical dimethylhydrazine across contrasting soil matrices; (2) clarify correlations between UDMH degradation behavior and soil physicochemical characteristics; and (3) explore degradation pathways of UDMH in a typical soil matrix. These findings are expected to deepen the understanding of UDMH degradation and transformation behavior and provide a theoretical basis for optimizing soil remediation approaches.

2. Materials and Methods

2.1. Soil Sampling and Determination of Physicochemical Properties

Yellow-brown soil (classified as Luvisols, abbreviated as YS), red soil (classified as Ferralsols, abbreviated as RS) and black soil (classified as Phaeozems, abbreviated as BS), classified according to the WRB international soil classification system, were sampled from Northwest China (Xi’an, Shaanxi), Southern China (Changsha, Hunan) and Northeast China (Heilongjiang, Suihua), respectively. Soil samples were collected from the 0–25 cm surface layer at the study site. Prior to soil sampling, surface plant residues and superficial floating soil were removed first. Soil samples collected from the same sampling area were homogenized thoroughly (3~6 composite samples). After manually picking out large sand grains, gravels and other exogenous impurities, the soil samples were subjected to shade air-drying, followed by sieving through a 40-mesh sieve, and then preserved for subsequent experimental analysis. Detection assays verified the absence of unsymmetrical dimethylhydrazone (UDMH) in all collected samples. Table 1 shows the physicochemical properties of the tested soils. The pH value was determined by a pH meter (1:2, w/v; Seven Compact, Mettler Toledo, Greifensee, Switzerland) [27]. Cation exchange capacity was measured according to the EDTA–ammonium acetate exchange approach. Electrical conductivity was determined via electrode method. Soil organic matter content was quantified with a total organic matter determinator (JX-S7066, Jingxin Instruments, Shanghai, China). Total and available contents of potassium and phosphorus were assayed in accordance with the Methods of Soil Analysis [28,29] using an ICP–OES (5110, Agilent Technologies, Santa Clara, CA, USA). The contents of nitrate nitrogen and ammonia nitrogen were detected by a UV–visible spectrophotometer (TU-1900, Puxi Instrument, Beijing, China). Total nitrogen content was measured by a Kjeldahl nitrogen determinator (K1160, Hanon Instruments, Jinan, Shandong, China) [29]. Soil particle size distribution was analyzed with a laser particle size analyzer, classifying on the basis of the USDA particle size classification method, which categorizes soil particles into sand (0.2–2.0 mm), silt (0.002–0.02 mm and 0.02–0.2 mm), and clay (<0.002 mm). The unsymmetrical dimethylhydrazine reagent used in the present experiment was of analytical grade. UDMH was determined via headspace solid–phase microextraction and a GC–MS analyzer (EXPEC 3500, PuYu, Hangzhou, China). Each measurement was performed in triplicate, and the data are presented as the mean ± standard deviation (SD).

2.2. Methods

2.2.1. Experiment of the Temporal Degradation Characteristics of UDMH

Unsymmetrical dimethylhydrazine (UDMH)-contaminated soil was prepared via exogenous addition of the contaminant, and the degradation characteristics of UDMH over time were investigated. The pretreated YS, RS and BS samples were prepared into samples with an initial UDMH concentration of 2000 mg∙kg^−1^ via the addition of UDMH solution referring to the previous reports (A high UDMH dose of 240,000 mg kg^−1^ was applied in the research of UDMH transformation in soils by Rodin [22], which simulated the initial propellant release moment from rocket debris. Ermakov [30] reported an initial UDMH concentration of 6763.6 mg∙kg^−1^ in rocket crash site soil as well). After all the samples were treated, the beakers with 5 g soil samples were placed in a constant-temperature incubator, with the temperature held at 25 ± 2 °C and the relative humidity controlled at 90% RH. During the experiment, deionized water was periodically replenished using the weighing method to keep the soil moisture content at 60% of the field capacity. UDMH and its metabolites in soil samples were determined at the incubation times of 1 h, 12 h, 1 d, 3 d, 7 d, 10 d, 14 d, 20 d, 25 d and 30 d. Each treatment was performed in triplicate.

2.2.2. Experiment of UDMH Degradation Pathways

The orthogonal experiment method was adopted. Yellow-brown soil contaminated with UDMH at a concentration of 2000 mg∙kg^−1^ was divided into two groups: sterilized and unsterilized. Each group was further set up with three controls, namely, non-film mulching, transparent film mulching and black film mulching (light-impermeable), resulting in the following combined treatments, denoted as T1, T2, T3, T4, T5 and T6, respectively: T1 represents sterilization + non-film mulching, T2 represents sterilization + transparent film mulching, T3 represents sterilization + black film mulching, T4 represents non-sterilization + non-film mulching, T5 represents non-sterilization + transparent film mulching, T6 represents non-sterilization + black film mulching and T0 represents the initial contamination concentration of UDMH. The following relationships exist: Light degradation fraction = T3 − T2; Volatilization fraction = T2 − T1; Microbial degradation fraction = T6 − T3; Other loss fractions = T0 − Light degradation fraction − Volatilization fraction − Microbial degradation fraction − Soil residual fraction. (It was assumed that the individual degradation pathways (photodegradation, volatilization, microbial degradation) were considered to have independent action processes during the experimental period). Potential interaction bias among pathways (e.g., the synergistic promotion of UDMH volatilization by light-induced soil temperature rise or the inhibition of microbial activity by photodegradation intermediates may cause deviation in the calculated percentage contribution of individual pathways). A 30-day soil incubation experiment was planned for all the above treatments. Soil incubation experiments were conducted in a light-controlled constant temperature incubator (Jingcheng JC-600C, Shandong, China). The incubation conditions were set as follows: light intensity of 7500 lux (constant light), constant temperature of 25 ± 2 °C, relative humidity (RH) of 90% and forced air exchange was set to 2 air changes per hour. Prior to the experiment with the sterilized groups, the glassware used for soil culture was sterilized by autoclaving at 121 °C for 30 min under high-pressure steam. Each treatment was performed in triplicate.

2.2.3. Determination of UDMH and Its Metabolites

Headspace solid–phase microextraction (HS–SPME) was adopted as the pretreatment method, and gas chromatography–mass spectrometry (GC–MS) was used for the qualitative analysis of samples to identify UDMH and its intermediate products. Gas chromatography–mass spectrometry (GC–MS) was applied for the quantitative analysis of UDMH and its main degradation product, formaldehyde dimethylhydrazone (FDMH). A soil sample of 1 g was put into a headspace vial of 30 mL, and 4 mL extracts (water) with sodium hydroxide and sodium chloride were injected into the headspace vial. Then, the headspace vial was placed in a solid-phase microextraction (SPME) pretreatment unit and heated at 50 °C for 30 min to achieve sample equilibration. Subsequently, fiber (75 μm PDMS/DVB) extraction was performed under the following conditions: 50 °C, 1000 rpm and an extraction duration of 10 min. A polar capillary chromatographic column was selected for the gas chromatography–mass spectrometry analyzer (GC–MS). Initial column temperature was fixed at 40 °C and isothermally held for two minutes. Subsequently, the temperature was elevated to 150 °C at 10 °C∙min^−1^, with an isothermal hold for 1 min. Finally, the temperature was ramped to 250 °C at 20 °C∙min^−1^, with a 3 min isothermal hold. Ultra-high-purity helium (≥99.999%) was employed as a carrier gas at a flow rate of 1.0 mL min^−1^. Full-scan mode was adopted for the analysis of UDMH and its transformation products in the samples. The quantitative standard curves and regression equations of UDMH and FDMH are presented in Figure S1a,b (Supplementary Materials). The limit of detection (LOD) of UDMH and FDMH for GC–MS analysis was determined via the spiking method, where a signal-to-noise ratio (S/N) of 3:1 was defined as the LOD and an S/N of 10:1 as the limit of quantification (LOQ). The LOD of UDMH was 2.0 mg∙kg^−1^, with the corresponding LOQ being 6.0 mg∙kg^−1^. The LOD of FDMH was 0.2 mg∙kg^−1^, and the LOQ was 0.6 mg∙kg^−1^. The average spiked recovery of UDMH and FDMH was 70.6–75.9% and 82.1–87.3%, respectively.

2.3. Statistical Analysis

All soil sample experiments were performed in triplicate, with mean values presented together with standard errors. The validity of experimental results was confirmed via statistical analysis using SPSS V13.0.

3. Results and Discussion

3.1. Temporal Degradation Dynamics of UDMH

When unsymmetrical dimethylhydrazine (UDMH) enters the soil environment, it is adsorbed partially by soil organic matter, while the rest undergoes degradation, oxidation, decomposition and other processes through various pathways over time. The temporal degradation behavior of UDMH in YS, RS and BS was investigated, which could serve as a basis for evaluating the applicability of monitored natural attenuation. As for low-concentration contaminated sites, it is feasible to determine whether the safety threshold can be achieved without manual intervention by monitoring the degradation rate.

The temporal degradation behavior of UDMH in YS, RS and BS is shown in Figure 1, Figure 2 and Figure 3. Figure 1 presents the qualitative analysis results of UDMH and its TPs in soil at the initial and final incubation time via solid-phase microextraction/gas chromatography–mass spectrometry (SPME/GC–MS), and the compounds are listed in Table S1 (Supplementary Materials). Figure 2 shows the temporal degradation behavior of UDMH in YS, RS and BS over a 30-day incubation period, and its main transformation product formaldehyde dimethylhydrazone (FDMH) is presented in Figure 3. As shown in Figure 1, the GC–MS chromatogram of UDMH and its TPs in soil at the initial and final incubation time displayed that UDMH was first isolated at a retention time of approximately 1.20 min, followed by its TPs, formaldehyde dimethylhydrazone (FDMH), acetaldehyde dimethylhydrazone (ADMH) and 1,1,4,4-tetramethyltetrazene (TMT), with a retention time of 1.46 min, 2.22 min and 4.64 min, respectively (mass spectra of TPs are presented in Figure S2). The types of UDMH transformation products were consistent in YS, RS or BS samples during this incubation period. Table 2 shows the peak area in the GC–MS chromatogram of UDMH and its TPs. Comparing the behavior of UDMH and its TPs at the initial and final incubation time (taking 12 h and 21 days, for example, the amount of UDMH in soil was relatively low after 21 days of incubation), the contents of UDMH in YS, RS and BS matrices significantly decreased from the initial to the final incubation period, as shown in Figure 3 and Table 2. Nevertheless, the amount of FDMH increased during the soil incubation, transforming from UDMH in all soil matrices; a slight variation was observed in RS, and substantial changes were observed in both YS and BS matrices, as shown in Table 2. The amount of ADMH was relatively low, decreasing slightly in YS and BS, but increasing in RS. TMT accounted for a large proportion of the transformation products in all three soil matrices. The amount of TMT showed a minor reduction in YS and RS, but increased in BS.

The quantitative analysis of UDMH variation in YS, RS and BS during the 30-day incubation is presented in Figure 2. The content of UDMH continuously decreased with increasing duration: from an initial content of 1559 mg kg^−1^ (0 h) to 151.4 mg kg^−1^ (24 d), and then to an undetectable level, in YS; from an initial content of 1777 mg kg^−1^ (0 h) to 81.91 mg kg^−1^ (28 d), and then to undetectable level, in RS; and from an initial content of 1644 mg kg^−1^ (0 h) to 92.35 mg kg^−1^ (21 d), and then to undetectable level, in BS. Rapid degradation of UDMH was observed within 7 days of soil incubation; the degradation rates of UDMH in YS, RS and BS were 66.03%, 67.51% and 73.13%, respectively, and the degradation rate slowed down significantly in the subsequent period. UDMH completely degraded within 24 days, 28 days and 21 days in YS, RS and BS matrices, respectively, during this soil incubation experiment. UDMH transformed more rapidly in the BS matrix, followed by YS and RS matrices, due to the different characteristics of the three soil matrices. The variation in FDMH in YS, RS and BS with increasing incubation time is displayed in Figure 3. A distinct upward trend in FDMH can be observed in YS and BS matrices, and a relatively gentle trend was observed in the RS matrix, indicating that the conditions of YS and BS were beneficial for the transformation of UDMH into FDMH. It had been reported by Rodin [22] that UDMH decreased to 65.4% on the third day and decreased to 1.04% after 30 days of incubation in soddy-podzolic soil.

Owing to its pronounced reactivity, UDMH is susceptible to a variety of chemical transformations, such as isomerizing to FDMH and dimerizing to TMT (Figure 4). The C–N bond is susceptible to hydrolytic, oxidative and reductive cleavage. The formation of FDMH is attributed to the interaction of UDMH with formaldehyde solution, and formaldehyde is generally derived from the initial oxidation of a methyl group of UDMH [31,32]. Additionally, UDMH usually undergoes catalytic oxidation via O_2_, metal ions or microorganisms, generating intermediate products such as dimethylamine and dimethyldiazene ((CH_3_)2_N=N–). A fraction of the methyl groups (–CH_3) can be oxidized to formaldehyde, which is further converted to acetaldehyde either via direct oxidation or a two-step pathway: C–N bond cleavage and methyl coupling of UDMH, followed by oxidation. Subsequently, UDMH reacts with acetaldehyde present in the system via a condensation–dehydration reaction to form ADMH [12]. Under the action of environmental oxidants (O_2_, ClO^−^, NO_3_^−^, etc.) or photocatalysis, UDMH loses one electron to form a dimethylhydrazine radical ((CH_3_)2_N–NH·) or undergoes deprotonation to generate a dimethyldiazenide anion ((CH_3)2_N=N^−^). These two intermediates would undergo radical–radical or radical–anion coupling at the N-terminus, yielding (CH_3)2_N−N=N−N(CH_3)2—the thermodynamically stable major product, which corresponds to the E-configuration of TMT. Subsequent proton transfer in neutral or weakly acidic environments could convert it into neutral TMT [12,21]. Briefly, UDMH is an unstable and highly active molecule, which could easily transform in the soil matrix and transform diversely due to the distinct properties of the soil matrices.

3.2. Correlation Analysis of Soil Properties and UDMH Degradation Behavior

Variations in the physicochemical properties of soil matrices directly dictate the degradation kinetics, transformation pathways, migration behavior and biological effects of contaminants in soils [13,23,33]. Table 1 presents the soil physicochemical properties of YS, RS and BS. The divergent degradation behaviors of UDMH across different soil types were predominantly governed by their unique physicochemical properties. Accordingly, Spearman correlation analysis (a statistical method typically employed for non-normally distributed data) was performed to elucidate the relationships between the physicochemical properties and UDMH degradation rate of YS, RS and BS, as depicted in Figure 5a–c. A conceptual correlation diagram is shown in Figure 6. The physicochemical properties of pH, CEC, EC, OM, TN, NO_3_-N, NH_4_-N, P, AP, K, AK and soil particle size distribution were analyzed. As shown in Table 1, YS samples were alkaline (pH = 7.92), BS samples were weakly alkaline (pH = 7.45), and RS samples were acidic (pH = 5.20). Such pH divergence was attributed to the distinctive geological settings and regional environmental conditions corresponding to the soil matrix. The soil’s pH value exerted a notable influence on the speciation and transformation of pollutants in the soil environment (p < 0.001, r > 0.933). As demonstrated in Section 3.1, a comparative analysis of UDMH degradation behavior across the three soil matrices revealed that acidic soil showed a significantly elevated capacity for UDMH retention with lower degradation kinetics. In addition, EC values reflect the combined synergistic effects of ionic concentration and mobility within the soil matrix, a factor that further modulates the migration kinetics of pollutants in soil systems. A significant positive correlation between EC and UDMH degradation rate (%C_1d_, %C_7d_, %C_21d_) can be observed, and there was a highly significant positive correlation (p < 0.01, r = 0.833) for RS at any incubation time, as shown in Figure 5b. From Figure 5a, for YS, it can be seen there was a significant positive correlation between EC and %C_1d_ (p < 0.05, r = 0.733), and a highly significant positive correlation between EC and %C_7d_ and %C_21d_ (p < 0.01, r = 0.817). Likewise, there was a significant positive correlation between EC and UDMH degradation rates (%C_1d_, %C_7d_, %C_21d_) (p < 0.05, r = 0.717) for BS, as shown in Figure 5c. EC exhibited a decreasing trend in the order of BS > YS > RS, suggesting that UDMH tended to stabilize in soils with lower ion concentrations and slower ion migration rates. CEC could quantify the soil’s ability to adsorb and immobilize cationic species (e.g., K^+^, Ca^2+^, Mg^2+^, NH_4_^+^). Consequently, soils with higher CEC values exhibit superior capacity to absorb cationic nutrients, leading to more effective preservation of soil fertility. Table 1 shows that the CEC of BS reached 233.6 mmol kg^−1^, which was obviously higher than that of YS (164.0 mmol kg^−1^) and RS (186.4 mmol kg^−1^). Correspondingly, BS exhibited the highest nutrient content, including TN, NO_3_^−^-N, NH_4_^+^-N, P, AP, K, and AK; YS and RS followed. It is worth noting that soil nutrient composition is not only closely correlated with its CEC but also subject to substantial impacts from variations in ecological settings, geological backgrounds and anthropogenic agricultural activities. Figure 5a reveals a highly significant positive correlation between CEC and UDMH degradation rate at the 1-day or 7-day incubation time (%C_1d_, %C_7d_) (p < 0.01, r = 0.800 and r = 0.833), and a significant positive correlation between the TN, P, AK contents and UDMH degradation rate at the 21-day incubation time (%C_21d_) (p < 0.05, r = 0.767) for YS. Figure 5b shows a highly significant positive correlation between CEC and UDMH degradation rates (%C_1d_, %C_7d_, %C_21d_) for RS (p < 0.01, r = 0.867). Figure 5c depicts a significant positive correlation between CEC and UDMH degradation rates (%C_1d_, %C_7d_, %C_21d_) (p < 0.05, r = 0.750) for BS. SOM harbors a suite of essential elements (i.e., C, N, P and S) that are indispensable for plant growth and development. In addition to its nutritional functions, SOM possesses pronounced hydrophilicity, which allows it to adsorb and retain considerable amounts of soil moisture, ultimately contributing to a marked improvement in the water-holding capacity of soil. As shown in Table 1, OM across the three soil types displayed the tendency of BS > YS > RS. Specifically, OM levels in BS and YS samples were comparable, whereas RS featured a markedly lower OM. Figure 5a shows an extremely significant positive correlation between OM and %C_21d_ (p < 0.001, r = 0.912), a highly significant positive correlation between OM and %C_7d_ (p < 0.01, r = 0.828) and a significant positive correlation between OM and %C_1d_ (p < 0.05, r = 0.0.778) for YS. Figure 5b presents significant positive correlations between OM and UDMH degradation rates (%C_1d_, %C_7d_, %C_21d_) (p < 0.05, r = 0.707) for RS. Similarly, Figure 5c reveals a significant positive correlation between OM and UDMH degradation rates (%C_1d_, %C_7d_, %C_21d_) (p < 0.05, r = 0.724) for BS. Given the high water solubility of UDMH and the strong hydrophilicity of OM, these physicochemical properties promoted the adsorption of UDMH by OM. In essence, the regulatory effect of OM on UDMH adsorption was primarily ascribed to the deprotonated functional groups of it (e.g., –COO^−^), serving as available UDMH adsorption sites. Additionally, OM was able to adsorb UDMH via hydrogen bonding with its polar functional groups, or through the combination of its electron-rich moieties with the electron-deficient sites on the OM matrix [33,34]. The strong adsorption capacity of OM enabled it to adsorb UDMH molecules onto its surface, resulting in the local enrichment of this contaminant. This enrichment effect elevated the contact probability between UDMH and oxidizing substances, reduced mass transfer resistance and thereby expedited the degradation reaction [35,36,37]. In comparison with low-OM soils, high-OM soils possessed a stronger UDMH adsorption capability, which increased the substrate concentration for the degradation reaction and consequently led to a higher UDMH degradation rate. Additionally, soil organic matter facilitated the formation of soil aggregate structure, increased soil porosity and consequently enhanced oxygen diffusion efficiency in the soil matrix. Sufficient oxygen availability can then significantly accelerate the mineralization process of UDMH.

In addition, soil particle size analysis (Table 1) indicated the maximum proportion of fine particles in YS, with 15.68% in the <0.002 mm fraction and 39.14% in the 0.02–0.002 mm fraction, thus classifying it as a fine-textured clay soil. In contrast, RS was dominated by coarse particles, with relatively large proportions of 15.92% and 46.37% in the 2.0–0.2 mm and 0.2–0.02 mm fractions, respectively, corresponding to a sandy soil with a coarse texture. BS showed a particle size distribution that was intermediate to those of YS and RS. It is noteworthy that soil particle size distribution exerts direct control over contaminant speciation and adsorption capacity at soil surfaces and further modulates their migratory fate in soil by modifying interfacial interactions patterns and pore-scale accessibility. It can be seen from Figure 5a–c that coarse soil particles (2.0–0.2 mm) presented a highly significant negative correlation with UDMH degradation rates (%C_1d_, %C_7d_, %C_21d_) (p < 0.01, r(RS) = −0.883, r(BS) = −0.833) for RS and BS, and an extremely significant negative correlation with UDMH degradation rates (%C_1d_, %C_7d_, %C_21d_) (p < 0.001, r = −0.917, −0.967, −0.967) for YS. However, fine soil particles of <0.002 mm showed an extremely significant positive correlation with UDMH degradation rates (%C_1d_, %C_7d_, %C_21d_) (p < 0.001, r(YS) > 0.917, r(RS) = 0.933) for YS and RS, and a highly significant positive correlation with UDMH degradation rates (%C_1d_, %C_7d_, %C_21d_) (p < 0.01, r = 0.817) for BS. Coarse soil particles typically exhibit a smaller specific surface area, tending to result in a weak adsorption capacity. In contrast, small soil particles possess a larger specific surface area, a property that not only favors the adsorption of contaminants but also promotes subsequent transformation reactions. By comparing the particle size distributions of YS, RS and BS, it was found that YS and BS contained a higher proportion of fine particles, thus facilitating the adsorption and degradation of UDMH. Conversely, RS was dominated by coarse particles, leading to a relatively low UDMH adsorption capacity and poor reactivity. Consistent with these findings, numerous previous studies have demonstrated that soil properties such as high porosity, fine particle distribution, large specific surface area and abundant adsorption sites, collectively contribute to enhancing the reactivity of contaminants in soil environments [38]. This additionally constituted an important cause for the elevated UDMH degradation activity of YS and BS.

3.3. Degradation Pathways of UDMH

UDMH and its main TPs exhibited notable environmental toxicity, with UDMH presenting considerable chronic environmental toxicity. FDMH and ADMH displayed high acute and chronic toxicity, and TMT showed baseline toxicity; these findings were obtained via the Quantitative Structure Activity Relationship (QSAR) Method, a computational toxicology tool for evaluating UDMH and its TPs in terms of environmental persistence and toxicity [33]. Thus, clarifying the degradation pathways of UDMH enables the identification of key intermediates, rate-limiting steps and favorable reaction directions during the degradation process, thereby providing a theoretical basis for the targeted development of efficient remediation technologies (e.g., advanced oxidation, microbial degradation, adsorption and immobilization). UDMH entering the soil undergoes a series of biotic (microbial degradation) and abiotic (photolysis, volatilization, etc.) migrations and transformation processes over time, leading to a gradual decrease in the soil matrix. Taking yellow-brown soil, for example, an experiment was designed with two groups and six different treatments to investigate the migration characteristics of UDMH in the soil. T1 represents sterilization + non-film mulching, T2 represents sterilization + transparent film mulching, T3 represents sterilization + black film mulching, T4 represents non-sterilization + non-film mulching, T5 represents non-sterilization + transparent film mulching and T6 represents non-sterilization + black film mulching. All the treatments specified above were subjected to a 30-day soil incubation trial. The GC–MS chromatograms of UDMH and its TPs in the YS incubation associated with the T1T6 treatments are depicted in Figure 7. The identified peaks in the chromatograms of all treatments were identical, which were UDMH and its TPs, formaldehyde dimethylhydrazone (FDMH), acetaldehyde dimethylhydrazine (ADMH) and 1,1,4,4-tetramethyltetrazene (TMT), with retention times of 1.20 min, 1.46 min, 2.22 min and 4.64 min, respectively. Table 3 shows the peak area in the GC–MS chromatograms of UDMH and its TPs in the T1T6 treatments. Under diverse degradation conditions, the comparative analysis of UDMH and its TPs demonstrated that the UDMH amount in the group of non-sterilization treatments was obviously lower than that of the sterilization group, indicating the effect of biodegradation. There was a slight fluctuation of FDMH across the treatments, which suggested that the production or transformation of FDMH was subject to minimal impact from the biodegradation conditions in the present experiment. Comparing the treatments in the sterilization group, the residual amount of UDMH in the treatments was ranked as follows: T3 > T2 > T1. Similarly, in non-sterilization groups, the residual amount of UDMH in the treatments was ranked as follows: T6 > T5 > T4. The results indicated that volatilization and photolysis both exert a certain degree of influence on the residual UDMH in soil. Additionally, an ascending trend in FDMH, ADMH and TMT was observed in the sterilization group, demonstrating that their production has little correlation with photolysis and volatilization. It was inferred that other pathways, including minor radical reactions, extracellular enzymatic transformation, co-adsorption catalyzed transformation, drying–wetting cycle-induced transformation and unidentified biotic–abiotic coupled processes, might also be the probable transformation routes. Based on calculations, the approximate proportion of UDMH degradation in soil through various pathways can be obtained. T0 represents the initial contamination concentration of UDMH. Light degradation fraction = T3 − T2; Volatilization fraction = T2 − T1; Microbial degradation fraction = T6 − T3; and Other loss fractions = T0 − Light degradation fraction − Volatilization fraction − Microbial degradation fraction − Soil residual fraction (based on T4). The pathways of photocatalytic degradation, volatilization, microbial degradation and others (such as oxidation, self-degradation, etc.) are displayed in Figure 8. The main results were as follows: (1) For photodegradation, the soil was placed in a dark incubator to shield from light, and the photodegradation contribution was calculated via comparison with the light group (2.91%). (2) For volatilization, the soil system was sealed with a gas-tight membrane to eliminate UDMH volatilization, and the residual UDMH content was determined to calculate the volatilization contribution (7.00%). (3) For microbiological degradation, soil microorganisms were inactivated by high-temperature sterilization (121 °C, 30 min), and the microbiological degradation contribution was determined via the difference method (10.30%). (4) The residues were obtained on the basis of the T4 treatment due to the adequate transformation conditions (11.04%). (5) The remaining degradation ratio (100% − 2.91% − 7.00% − 10.30% − 11.04% = 68.75%) was attributed to the combined effects of oxidation and self-degradation. Previous research [22,39] had reported that UDMH not only evaporated but was also oxidized with the formation of transformation products via air or microbiological transformation. As a typical hydrazine-based nitrogenous organic pollutant, UDMH undergoes transformation and degradation through multiple pathways, including photodegradation, volatilization, microbial degradation, air oxidation and chemical oxidation after entering the soil environment [12,21,40]. The photodegradation of UDMH in the soil matrix mainly relies on the absorption and transmission of sunlight via surface soil. The UDMH molecule itself could absorb ultraviolet light (200–300 nm) and part of visible light, with the N–N and C–N bonds in its molecular structure serving as photosensitive sites. Under sunlight irradiation, these chemical bonds undergo homolytic cleavage, generating reactive intermediates such as methyl radicals (·CH_3_) and hydrazino radicals (·NHNHCH_3_). Subsequent radical recombination or cleavage reactions further transform UDMH into products including formaldehyde, methylhydrazine (MMH), nitrogen gas and ammonia. In addition, natural photosensitizers present in soil (e.g., humic acid, fulvic acid and iron oxides) can produce endogenous reactive oxygen species (ROS) such as hydroxyl radicals(·OH) under illumination, which attack UDMH molecules and trigger oxidative degradation. UDMH is a polar organic amine with strong volatility. Its volatilization in soil is essentially a process where dissolved UDMH diffuses from the soil aqueous phase to the gas phase, which belongs to a physical migration and transformation behavior rather than complete degradation. Nevertheless, this process alters the spatial distribution of the pollutant and affects subsequent degradation processes. UDMH exists in two forms in soil: the ionic form (UDMH_2_^+^) and the molecular form (UDMH). The molecular form of UDMH has much higher volatility than the ionic form, and its proportion is determined by the soil’s pH value. When the pH value is higher than the pKa (negative logarithm of the acid dissociation constant, used for acid dissociation ability measurement) of UDMH (approximately 7.9), the molecular form dominates, leading to a significant enhancement in volatilization. In contrast, UDMH primarily exists in the ionic form in acidic soil matrices, resulting in a substantial reduction in volatilization rate. The essence of microbial degradation lies in the metabolic utilization of UDMH as a carbon or nitrogen source by microorganisms through enzyme-catalyzed reactions. Dehydrogenase catalyzes the oxidation of UDMH to form N-nitrosodimethylhydrazine, which then undergoes N–N bond cleavage under the action of hydrolase to produce dimethylamine and nitrous acid. Dimethylamine is further oxidized to formaldehyde and ammonia; formaldehyde enters the tricarboxylic acid cycle (TCA) and is completely decomposed into CO_2_ and HO, while ammonia is either assimilated by microorganisms or converted into nitrogen gas via nitrification/denitrification. However, according to our previous work [33], UDMH and its TPs are characterized by poor biodegradability associated with environmental persistence, which was assessed using computational toxicology modeling software, and may exert a prominent adverse impact on the ecological environment. In addition, oxidation is also an important transformation pathway for UDMH. The non-enzymatic oxidation reaction between oxygen in soil pores and UDMH mainly occurs in the aerobic zones of the soil surface layer. Under neutral or alkaline conditions, O_2_ can directly oxidize UDMH molecules; the key step of this reaction is the formation of a peroxo intermediate (UDMH–OOH), which then decomposes to produce nitrogen gas, dinitrogen monoxide, dimethylamine, formaldehyde and other products. The reaction rate is extremely low under acidic conditions because UDMH mainly exists in an ionic form, which is difficult for O_2_ to oxidize. In contrast, the proportion of molecular UDMH increases under alkaline conditions, leading to a significant enhancement in oxidation rate. Metal oxides in soil (e.g., MnO_2_ and Fe_2_O_3_) can act as catalysts to activate O_2_ and generate peroxide free radicals, thereby reducing the reaction activation energy and accelerating the aerial oxidation reaction [41]. The degradation of UDMH in soil does not proceed independently through a single pathway but rather results from the synergy or competition of multiple processes. For instance, volatilization migrates UDMH to the soil surface layer, increasing its contact probability with light and air and thereby facilitating photodegradation and aerial oxidation. Meanwhile, microbial degradation consumes UDMH in the soil, reduces the UDMH concentration in the aqueous phase and indirectly promotes the volatilization process [32,42].

4. Conclusions

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 YS, RS and BS matrices. UDMH degraded quickly within the first 7 days of soil incubation, with degradation rates of 66.03%, 67.51% and 73.13% in YS, RS and BS, respectively. UDMH transformed more rapidly in the BS matrix than in the YS and RS matrices. The results revealed significant correlations between the physicochemical properties and UDMH degradation dynamics for contrasting soil matrices, indicating a substantial impact of soil physicochemical properties on UDMH degradation behaviors in soil matrices. UDMH degradation is promoted by alkaline soil; high EC, CEC and SOM; and fine particle size. The analysis of UDMH degradation pathways, including volatilization, photodegradation, microbiological degradation and others (catalytic transformation, induced transformation or unidentified biotic–abiotic coupled processes, etc.) demonstrated that other pathways acted as the dominant pathway governing its degradation. Research into degradation dynamics and pathways is expected to offer directional guidance and establish a technical basis for soil pollution remediation.

The findings on the soil-dependent degradation kinetics and transformation pathways of UDMH provide direct practical implications for its environmental risk assessment, as they enable a more accurate evaluation of UDMH persistence and ecological exposure risks in different soil matrices. For remediation design, the key soil physicochemical factors promoting UDMH degradation identified in this study provide a scientific basis for the targeted regulation of soil properties to enhance the in situ remediation efficiency of UDMH-contaminated soils with different matrix characteristics. The identified key factors governing UDMH degradation (e.g., soil organic matter content) also offer targeted references for optimizing site-specific in situ remediation measures (e.g., soil property regulation) for UDMH-contaminated soils. This research into the degradation dynamics and pathways is expected to offer directional guidance and establish a technical basis for soil pollution remediation. A major limitation of this study is that all incubation experiments were conducted under controlled laboratory conditions (restricted gas exchange, simplified microbial communities, constant temperature/moisture, accumulation of volatile degradation intermediates and feedback inhibition, etc.), which cannot fully simulate the dynamic and complex environmental factors in natural field soil systems. Future research should focus on field-scale experiments combined with long-term in situ monitoring to verify the UDMH degradation patterns observed in the laboratory and further explore the synergistic effects of multiple environmental factors on UDMH degradation in natural soil environments.