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Environmental fate & pathways

Hydrolysis

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Hydrolysis

The hydrolysis of DOTTG is highly differentiated depending on the circumstances

1 Exposure to HCl, low pH

the investigations on alky tin compounds in general and on DOTTG in particular aim to simulate the gastric hydrolysis as an important step in metabolism.

1.1 Pure substance

The pure substance reacts with HCl in in a ring opening reaction to DOTC-TA, followed by a reaction with a second molecule HCl to DOTC (Naßhan 2019)

DOTTG + HCl --> DOTCE-TA | DOTCE-TA +HCl --> DOTC

1.2 DOTTG in DOTE solution

DOTTG is manufactured and marketed only in a max 10 % DOTE (DOTI) solution.

The degradation product DOTC formed by reaction with HCl reacts with the excess DOTE rapidly to DOTEC, the electronically stabilized monochloro-thioester with a cyclic structure close to the DOTTG structure. (Naßhan 2019)

DOTTG +2 HCl --> DOTC | DOTC + DOTE --> 2 DOTEC

1.3 DOTTG while acting as a PVC stabilizer in a mixture with DOTE and MOTE

During the stabilization process of PVC, DOTTG is exposed to HCl from the degradation of PVC. DOTTG is completely transformed to DOTEC during this process. (Frenkel 2016).

DOTTG + PVC --> DOTC | DOTC + DOTE --> DOTEC

2 Exposure to water, pH 4-9

The studies done on organotins at higher pH pursue the goal to demonstrate the stability during manufacturing process, which includes contact to water and to simulate the exposure and behavior in the environment.

2.1 Dioctyltins in ACN/water solution

Dioctyltins hydrolyse in acetonitrile/water solutions to form dioctyltin oxide. It was determined that contact of these dioctyltins with water alone produces the oxide

The formation of the ligand EHTG could be observed unequivocally, whereas the formation of DOTO was postulated. The DOTO signal intensity remained constant in the experiment, which was attributed to the very low solubility of DOTO. (Yoder 2003)

2.1.2 DOTTG in Acetone-d6 solution hydrolysis with D2O:

After addition of 100µMol D2O the NMR signal for DOTTG completely disappeared. The chemically only meaningful hydrolysis product here is DOTO together with the ligand thioglycolic acid. Since DOTO is very unsoluble in all common solvents the 119Sn-spectrum of the hydrolysis did not show any signal (Nasshan 2019)

The two studies (Yoder 2003, Nasshan 2019) show, that dioctyltin compounds in solution react rapidly with water to form Dioctyltin oxide and the ligand. The scenario is relevant for any dissolved dialkyltin compounds which are dissolved in water, e.g. during sewage treatment.

2.2 Alkyltin substances as neat substances

Hydrolysis studies at pH 4-7 with neat substances have been conducted with Dimethyl, Dibutyl and Dioctyltin substituted organotin substances with thiobonds, namely:

DMTE (Dimethyltin bis(2-ethylhexylthioglycolate) CAS-No.: 57583-35-4)

DBT-MPTD (Diisotridecyl 3,3'-[(dibutylstannylene)bis(thio)]dipropionate- / CAS 84896-44-6)

DOTE (2-ethylhexyl 10-ethyl-4,4-dioctyl-7-oxo-8-oxa-3,5-dithia-4-stannatetradecanoate), CAS 15571-58-1

All substances remained stable when the neat substances were exposed to buffer solutions for 5 days / 50 °C. 119Sn-NMR spectroscopy was used to determine the (unchanged) organotin species.

The scenario of neat organotin substances is relevant for the manufacturing of this compouns since they are manufactured in contact with water.

3 Ligands:

The ligand which is released during hydrolysis of dialkyltin thioglycolates (Yoder 2003) is Ethylhexyl thioglycolate (EHTG)

It is is self-classified as Aquatic Acute 1 and Aquatic Chronic 1.

The ligand of DOTTG, Thioglycolic acid is not classified for the environment and thus does not pose any additional risk for the environment when released from DOTTG by hydrolysis

It degrades in water by fast oxidation to dithiodiglycolate.

Based on the physico-chemical properties of thioglycolic acid and salts (high solubility and low Log P), it is considered that they are not expected to adsorb to suspended solids, sediments and soils and are mobile in soil

Thioglycolic acid and its main oxidation product, the diammonium dithiodiglycolate, it can be considered that thioglycolic acid and its salts are ready biodegradable and do not raise concern in terms of persistency.

Thioglycolic acid and its salts are highly soluble in water (> 1000 g/L at 20°C) (Sablowski, 2007b; reliability 2) and have a partition coefficient octanol-water equal to -2.99 at 22°C and pH 7. Therefore thioglycolic acid is not expected to bioaccumulate according to technical guidance documents

 

 

 

 

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