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The distribution in a sewage treatment plant (STP) has been estimated using the SimpleTreat model (implemented in EUSES 2.1.2), see table below. Due to the rapid hydrolysis of the parent substance to trimethylsilanol and ammonia, the distribution calculation is done for the hydrolysis products.

Table. Distribution modelling for STP

 

Trimethylsilanol

Ammonia

 

Fraction of emission directed to air by STP

6.5

 23.2

[%]

Fraction of emission directed to water by STP

93

 76.7

[%]

Fraction of emission directed to sludge by STP

0.534

 0.163

[%]

Fraction of the emission degraded in STP

0

 0

[%]

Hexamethyldisilazane hydrolyses very rapidly in contact with water to trimethysilanol and ammonia. Trimethylsilanol is not expected to undergo any significant biodegradation, has moderate Henry’s Law constant and low log Kow/log Koc. Water and air are the main compartments to which it is expected to partition in a sewage treatment plant.

The physicochemical properties of ammonia (presented in IUCLID Section 4) are used in EUSES 2.1.2; its conversion to nitrite and nitrate is not taken into account as a worst case.

The distribution of ammonia/ammonium is discussed further below.

Ammonia (unionised) is lost from water by volatilisation (Environment Agency, 2007). Based on its solubility, ammonia (un-ionised) is not expected to adsorb to soil particulate matter, suspended solids or sediment. (Environment Agency, 2007).

The ammonium ion is bound in soil by the attraction of the positive charge on the ammonium ion to the negatively charged soil micelles. In soil, ammonium is adsorbed primarily by four mechanisms: chemical (exchangeable), fixation (non-exchangeable), reaction with organic matter and physical attractive forces.

Ammonia or ammonium ion is rapidly converted to nitrate by nitrification under aerobic conditions in the aquatic environment. Ammonia is part of the nitrogen cycle. As the ammonium salts are inorganic salts, standard biodegradation tests cannot be performed. However, it is known that ammonia (NH3or NH4+) is easily mineralised to nitrite ion (NO2-) by numerous species of bacteria such as Nitrosomonas europea, Nitrosococcus, Nitrosospira(OECD, 2007).

Ammonium is an important intermediate in the assimilation of nitrogen from the soil by plants. Nitrogen is present in the soil largely in the organic form and is unavailable to plants. Microbial processes must mineralise it. As nitrification is an energy-yielding process, the rates of conversion are rapid, so that ammonium rarely accumulates in soil while bacteria are active. Organic nitrogen compounds are reduced to ammonium, which is converted to nitrite (NO2) by Nitrosomonas and then to nitrate by Nitrobacter. Most plants can assimilate the ammonium ion, but it is usually oxidised to the nitrate ion, the most common form of mineralised nitrogen in soil, which may be assimilated by plants as well (Environment Canada, Health Canada 2001).

In general, natural ammonia levels in soil are very low (<1 mg/kg) due to the rapid conversion of ammonium to nitrite b yNitrosomonas species and then to nitrate by Nitrobacter species in the temperature range 0–35ºC (Henry 1995, cited in Environment Canada, Health Canada, 2001).

Environment Agency, 2007. Proposed EQS for Water Framework Directive Annex VIII substances: ammonia (un-ionised).

Science Report: SC040038/SR2. SNIFFER Report: WFD52(ii)

OECD (2007). SIDS Initial Assessment Report for SIAM 24, Paris, France, 17-20 April 2007, Ammonia Category.