Methenamine

Endpoint summary

Description of key information

The degradation of methenamineis strongly dependent on the pH where abiotic and biotic pathways are possible. With decreasing pH abiotic degradation (mainly hydrolysis) increases. In a new state of the art study (Muckle, 2009)Methenaminewas shown to be not ready biodegradable at neutralpH.The following data were determined for the test item: 10-day-window: day 8 - 18 degradation at the end of 10-day-window: 22% degradation at the end of the test: 35% However, other studies clearly indicate a good biodegradation on the chosen test conditions. Taken all together, hydrolysis seems to be the major degradation pathway formethenaminein the environment. At acidic pH-levels,methenamineis quickly degraded hydrolytically. At neutral and basic pH, the rate of hydrolysis decreases, and degradation is supported by microbial activity. Consequentlymethenaminecan be regarded as degradable but not as “ready biodegradable”.

Additional information

In aqueous systems, methenamine is susceptible to hydrolysis. The process is strongly pH dependent. At acidic pH-levels the substance is degraded in a few hours, at neutral and basic pH-levels the half life increases to several days. For environmental relevant conditions, hydrolytical degradation can last several days – weeks, e.g. in surface water with pH > 7.

The available tests on biodegradation revealed different results. In a new state of the art ready biodegradabability study methenamine did not meet the criteria for ready biodegradable although some degradation was noted. In other standard screening tests on ready biodegradation, degradation levels between 28 % and > 100 % were determined depending on the selected experimental conditions e.g. pH. In sewage treatment plant simulation studies the elimination rate was between 12 % and 53 %, depending on the test conditions e.g. the aging of the sludge. These differences might also be explained by the susceptibility to hydrolysis at different pH-levels, because hydrolysis is expected to be the decisive step also in the available biodegradation tests. In addition, the rate biological degradation might depend on the amount of protonated methenamine. At acidic pH-levels, most of the substance is protonated while in a more basic environment the amount of protonated methenamine is decreasing. The neutral molecule is considered to be more bioavailable and therefore subject to microbial degradation.

Taking these findings together, hydrolysis seems to be the major degradation pathway for methenamine in the environment. At acidic pH-levels, methenamine is quickly degraded hydrolytically. At neutral and basic pH, the rate of hydrolysis decreases, and degradation is supported by microbial activity.

No information about degradation of methenamine in soil is available. Since releases of methenamine into the soil compartment can be excluded due to fast hydrolysis and controlled conditions during manufacturing and processing of the substance. Further information is not required.

Consequently, methenamine can be regarded as degradable but not as “ready biodegradable”.