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Toxicity to microorganisms

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Description of key information

No effects were observed up to 511 mg/L precipitated tin (IV) hydroxides which is equivalent to 1000 mg/L tin (IV) chloride pentahydrate. OECD 209, Clark (2010)

Key value for chemical safety assessment

EC10 or NOEC for microorganisms:
511 mg/L

Additional information

A respiration inhibition test completed by VITO in 2010 used tin (IV) chloride solutions and encountered interfering effects from pH change. Two preliminary range finding tests were performed: the first (RIT09001) without any adaptation of pH and the second (RIT09002) with adjusted back to circumneutral values. Adjusting the pH from acid to neutral values caused opaqueness in the freshly prepared dilutions at the highest concentrations. The pH in the test vessels for RIT09001 ranged from 2.42 to 7.03.

In the first range finding test (RIT09001), toxic effects were seen with a calculated EC50 of 201 mg/L SnCl4.5H2O. However, a steep pH shift was seen in the EC50 range with EC50 pH being 4.5. In the pH adapted second range finding test no toxicity was observed (EC50 > 1000 mg/L SnCl4.5H2O).

These two experiments showed that the pH effect is the cause of toxicity either by increased bioavailability and/or by direct pH effects on sludge respiration. The optimal pH range for sludge activity is 6 -8. Below and above these figures respiration is inhibited.

In the third test (RIT09003), no pH adaptation was used and the concentration range tested narrowed to 50 to 200 mg/L SnCl4.5H2O. Unexpectedly, no effects on pH or toxicity were seen. In test RIT09001 the pH was 4.5 at 200 mg/L loading but was 7.14 in RIT09003.This illustrates how different batches of activated sludge can have varying buffering characteristics. The test was repeated again (RIT09003bis) using a range of 100 to 500 mg/L with no pH adaptation and toxic effects were seen with an EC50 of 232 mg/L SnCl4.5H2O.

SnCl4.5H2O has a negative effect on the respiration rate of activated sludge and the effect is related to the acidifying properties of the test material. This study may be useful for providing data on the environmental hazards of SnCl4.5H2O. However, the purpose of the study was to assess the effect of tin ions arising from the dissolution of tin metal on microrganisms and this study provides little useful data to inform that assessment. The maximum dissolved tin that can be achieved with a 100 mg/L loading of the finest tin powder is 31 µg/L (CIMM 2010). Therefore, no effects on microrganisms will be seen directly from that concentration of dissolved tin. 31 µg/L of dissolved tin will also have no material effect on the pH of activated sludge.

To overcome these methodological difficulties and to investigate the potential for precipitated tin hydroxides arising from dissolved tin to cause adverse affects on micro-organisms a different sample introduction process was adopted in the Harlan 2010 study. Amounts of tin (IV) chloride pentahydrate were dispersed into dechlorinated tap water and allowed to precipitate. The precipitated tin material ( assumed to be tin hydroxides) was washed to minimise any pH effects from residual tin (IV) chloride, dried and weighed. The filter paper (GF/A grade) together with the weighed mass of precipitated tin hydroxide was then added to the test vessel.

No effects were observed up to 511 mg/L precipitated tin (IV) hydroxides which is equivalent to 1000 mg/L tin (IV) chloride pentahydrate. A separate study was undertaken (ITRI 2010) to characterise the dried precipitated material. The precipitated material was found to contain 72% Sn.

The overall objective of the sludge respiration testing is to provide data on the hazard to micro-organisms at sewage treatment works or related scenarios arising from the dissolution of soluble ions from tin metal followed by their subsequent precipitation. Tin solutions are not stable at concentrations above a few tens of micrograms per litre and will form precipitates at higher concentrations. In practice, the risk of mg/L quantities of precipitated tin compounds developing from solubilised tin is small as the maximum soluble tin formed from 100 mg/L finely powdered tin is 31 µg/L (CIMM 2010).