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Environmental fate & pathways

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Hydrazine is mainly abiotically degraded through oxidative pathways when released in the aquatic, terrestrial and atmospheric compartment. Biotic degradation processes can be neglected as hydrazine is an inorganic compound and cannot serve as energy supply for microorganisms. Nevertheless, a test similar to a test on inherent biodegradability (OECD 302 B) showed a hydrazine elimination of 99 % after 24 hours when brought in contact with activated sludge. No distinction can be made between biotic, abiotic or physico-chemical elimination processes, but the results provide evidence that hydrazine will be removed to a significant extent subjecting hydrazine containing waste waters to a sewage treatment. A refined description of the environmental behaviour of hydrazine will thus be possible.

Dissipation of hydrazine in aqueous media is influenced by a multitude of factors, but there is evidence that hydrazine will be rapidly degraded in an aqueous media mainly due to autooxidative processes. In the aqueous compartment between 10 % up to 99 % of the applied hydrazine degraded within 4 days in the presence of oxygen, organic matter, carbonate and metal ions, whereas the degradation rate is determined by the sum of all these factors, rather than by one factor alone. Furthermore, hydrazine was shown to be not stable when introduced into culture media of two biological test systems. Effective concentrations decreased to about 3.13 % and 17 %, respectively after 48 h of incubation. No correlation between test conditions, medium composition and degradation rate can be found. An investigation on the stability of hydrazine in natural surface waters yielded DT50 values of approximately 2.67 h and 24 h, respectively, whereas the degradation was faster in water with the lower hardness and DOC values. The results from the accompanying analytical monitoring of two biological assays and the stability tests in natural surface waters provide evidence that the environmental half-life of hydrazine in aqueous media will be below 24 h. Nitrogen and water are the primary degradation products, whereas the presence of catalysts like phosphate or cupric ions is paralleled by the formation of ammonia. The half-life of 24hours in natural media is taken as a key value for the calculation of the environmental exposure.

Furthermore, hydrazine will undergo a rapid abiotic degradation in the soil compartment in the presence of oxygen and exchangeable metal cations held by the clay. Based on one older study in soil it be concluded that a half-life of 12 hours in soil is a realistic worst-case assumption.

In the atmosphere, hydrazine was shown to degrade in the presence of hydroxyl radicals. Assuming an OH-concentration of 500000 molecules/cm³ the atmospheric half life of hydrazine accounts for 6.3 h yielding only a limited potential for a long range atmospheric transport. In the absence of light, the degradation of hydrazine in air is characterized by half lives smaller than 6 h strongly depending on the prevailing humidity. In the presence of ozone, the atmospheric half life decreases below 2 h.

The potential to bioaccumulate in aquatic organisms is scored as very low due to low logKow of -0.16, the high water solubility and the rapid degradation of hydrazine in the aquatic environment.

A test on adsorption/ desorption cannot be conducted as it is technically not possible. Nevertheless, there are several studies available dealing with the behaviour of hydrazine in the presence of soil. Assessed individually, they all have several drawbacks. But combining the information of these studies, it can be concluded that hydrazine might sorb or degrade depending on the prevailing conditions. At low pH values, mainly sorption occurs, whereas at higher pHs, degradation is supported. Additionally, the soil composition might contribute to hydrazine degradation. No definite value for sorption on soil can be determined as a multitude of factors determine the behaviour of hydrazine in that environmental compartment.

Environment Canada (2011) supports these findings and concluded without knowledge of the latest studies performed at Currenta (2010): "There are several lines of evidence to suggest that the persistence of hydrazine in natural ecosystems is low to moderate: all four half-lives in air are less than 1 day; the 26 halflives in water range from 0.2 to 125 days; the three half-lives in soil are less than or equal to 3 days; the one estimated biodegradation half-life in aerobic sediment is 1.6 day. Ready biodegradability tests indicate that the degree of degradation depends on hydrazine loading. Tests on natural water samples indicate that biodegradability is also a function of bacterial abundance and species present. Decrease of water temperature can decrease degradation half-lives in this compartment but this effect is not established with certainty given the low level of detail provided by James (1989) and Jingqiu et al. (1994) for their experiments