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Environmental fate & pathways

Phototransformation in water

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Reference
Endpoint:
phototransformation in water
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
Please refer to the justification for grouping of substances provided in IUCLID Section 13.
Reason / purpose for cross-reference:
read-across source
% Degr.:
44
Sampling time:
30 d
Test condition:
natural sunlight
Remarks on result:
other: mole% conversion of dimethylsilanediol to methylsilanetriol and silicic acid
Transformation products:
yes

PHOTOCHEMICAL REACTIVITY

Since dimethylsilanediol exhibits no UV absorption in the region of 220 – 400 nm, chromophoric impurities were postulated to explain the observed photochemical reactivity. Thus, a series of dimethylsilanediol solutions containing potential inorganic chromophores were photolyzed to test this postulate. The results indicated that only the oxides of nitrogen significantly mediated the demethylation process by showing an evident correlation between NOx concentrations and the extent of demethylation. Since the enhancement of demethylation rates was even observed at part per billion levels of NOx chromophore species, the miniscule amounts of demethylation observed with distilled water and with sulfate, phosphate and iodide salts may also be attributable to trace NOx impurities. Furthermore, there are sufficient levels of nitrates and nitrites in inland lakes and waterways to anticipate significant photolytic oxidative demethylation of methylsiloxanols in these media. The data obtained for natural waters showed good correlation between chromophore concentration and extent of demethylation.

DEMETHYLATION WITH NATURAL SUNLIGHT

Demethylation with natural sunlight was also demonstrated for 60 µg/mL tetramethyl­disiloxanediol (dimer diol) in distilled water with 10 µg/mL added sodium nitrate after 30 d exposure to central Michigan mid-summer sunlight. After 30 d exposure, a 44 mole % conversion of dimethylsilanediol to methylsilanetriol and silicic acid in a mole ratio of about 3:1 was observed.

DEMETHYLATION PROCESS

The term “demethylation” in the context of nitrate-mediated photolytic degradations does not refer to a direct hydrolytic cleavage of Si-CH3 bond to form methane. On the contrary, the process was shown to be oxidative rather than hydrolytic by simply purging the solutions with N2 to remove most of the oxygen prior to photolysis. Furthermore, CO2 was conclusively identified by GLC, IR-MS analysis as the only gaseous product of photolytic demethylations. It is well established that the aqueous photolysis of NOx species in the presence of oxygen produces the highly reactive hydroxyl radical (OH) and it was presumed to be the responsible species for the observed “demethylations”.

DIATOM STUDIES

The effects of dimethylsilanediol and photolysis on diatom growth were investigated.

No significant growth was noted in media with no added silicate or photolyzed dimethylsilanediol showing that diatoms are not capable of metabolizing even simple water-soluble dimethyl substituted silicon species. Excellent growth curves were obtained in diatom medium containing Na2SiO3 and in Si-free diatom medium with dimethylsilanediol exposed to UV (18 d) prior to diatom inoculation, which contained photolytic degradation products from dimethylsilanediol as the only source of silicon-containing nutrient. Analytical results confirm the ecological acceptability of demethylated silicon moieties, demonstrate that the transformation products have no significant toxicity to diatoms and show that even monomethyl silicon moieties are not utilized as silicic acid sources by diatoms.

 

Conclusions:
An experimental study on photolytic oxidative demethylation of aqueous Dimethylsilanediol (source substance) resulted into a 44 mole % conversion of dimethylsilanediol to methylsilanetriol and silicic acid in a mole ratio of about 3:1 after 30 d of exposure to central Michigan mid-summer sunlight. DMSD is the hydrolysis product of the target substance. Since the hydrolysis half life is < 1 h the results of the hydrolysis product describes the expected results for the target substance Dimethoxydimethylsilane.

Description of key information

Key value for chemical safety assessment

Additional information

Dimethoxydimethylsilane (CAS 1112-39-6) hydrolyses rapidly to dimethylsilanediol and methanol under environmental conditions (DT50 < 0.6 h at pH 7, pH 9 and 25 °C). Thus, the hydrolysis products rather than the parent substance are expected to be present in the environment after environmental release.

Direct photolysis is unlikely to be relevant for both hydrolysis products as they contain no chromophores that would absorb visible or UV radiation. However, experiments carried out for dimethylsilanediol indicate that aqueous photolytic oxidative demethylation with tropospheric ultraviolet radiation in the presence of suitable chromophores, such as nitrogen oxides, are possible (Buch et al. 1984).