Strontium

Toxicity to microorganisms

Link to relevant study record(s)

Reference

Endpoint:

activated sludge respiration inhibition testing

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: Guideline study, conducted under GLP

Justification for type of information:

Strontium metal is highly reactive and instantly oxidizes upon contact with water. It decomposes completely. During the redox-reaction with water, a strong evolution of hydrogen gas and an immediate precipitation of a white, crystalline solid (i.e. Sr(OH)2) is observed (Sr2+ + 2OH- + H2 (g). The amount of dissolved Sr cations is determined by the solubility of the Sr(OH)2 precipitate. According to OECD guideline 105 (1995) and EU method A.6 (2006), the water solubility of strontium was determined to be 6.74 ± 0.14 g/L under the conditions of the test (flask method under protective gas atmosphere; loading of 41 g Sr/L, at 20.0 ± 1.0 °C, pH >13).
Due to the buffering capacity of most environmental systems, it may reasonable be assumed that the formed hydroxide ions are neutralised in the environment by different processes including precipitation.

The solubility of strontium is not greatly affected by the presence of most inorganic anions as there is little tendency for strontium to form complexes with inorganic ligands (Krupka et al. 1999. EPA 402-R-99-004B and references therein). Free Sr2+ cations are mobile under most environmental conditions, despite the relatively low solubility of strontium carbonate and strontium sulfate at neutral to high pHs. In solutions with a pH below 4.5, the Sr2+ ion is dominant. Under more neutral conditions (pH 5 to 7.5), SrSO4 forms. Strontium carbonate controls strontium concentrations in solutions only under highly alkaline conditions. Further, dissolved strontium forms only weak aqueous complexes with chloride and nitrate (Salminen et al. 2015 and references therein, Krupka et al. 1999. EPA 402-R-99-004B). Regarding monodentate and bidentate binding to negatively-charged oxygen donor atoms, including natural organic matter, alkaline earth metals, such as strontium, tend to form complexes with ionic character as a result of their low electronegativity. Ionic bonding is usually described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions (Carbonaro and Di Toro. 2007. Geochim Cosmochim Acta 71 3958–3968; Carbonaro et al. 2011. Geochim Cosmochim Acta 75: 2499-2511 and references therein). Thus, strontium does not form strong complexes with fulvic or humic acids based on the assumption that strontium would exhibit a similar (low) stability with organic ligands as calcium and that strontium could not effectively compete with calcium for exchange sites because calcium would be present at much greater concentrations (Krupka et al. 1999. EPA 402-R-99-004B). In sum, strontium ions are highly mobile, occur only in one valence state (2+), i.e. are not oxidized or reduced, and do not form strong complexes with most inorganic and organic ligands (Krupka et al. 1999. EPA 402-R-99-004B; Salminen et al. 2015). Thus, it may further be assumed that the behaviour of the dissociated strontium ions in the environment determine the fate of strontium upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning as well as the distribution in environmental compartments (water, air, sediment and soil) and subsequently the ecotoxicological potential.Therefore, the assessment of the ecotoxicity of strontium is based on elemental strontium concentrations. Read-across of ecotoxicity data available for soluble strontium substances is applied since the strontium ions determine the ecotoxicological potential of strontium.

Qualifier:

according to guideline

Guideline:

OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test

Principles of method if other than guideline:

Procedures were designed to meet the test methods of the Commission Regulation (EC) No. 440/2008 of 30 May 2008, Publication No. L142, Part C11 and ISO Standard 8192 (2007)

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

no

Details on sampling:

Analytical monitoring not required according to Guideline

Details on test solutions:

A stock solution of 0.5 g/L was prepared by adding 0.1254 g test substance to 250 mL of Milli-reverse osmosis water. Volumes of the clear and colourless stock solution corresponding to the test concentration were then added to the test media

Test organisms (species):

activated sludge of a predominantly domestic sewage

Details on inoculum:

- micro-organisms in activated sludge were obtained from the municipal sewage treatment plant "Waterschap de Maaskant, 's-Hertogenbosch, The Netherlands", receiving predominantly domestic sewage- the sludge was coarsely sieved, washed and diluted with ISO-medium. A small amount of the sludge was weighed and dried overnight at ca. 105°C to determine the amound of suspended solids- 50 mL of synthetic sewage feed was added per litre of activated sludge at the end of the collection day- the sludge was kept aerated at test temperature until use

Test type:

static

Water media type:

freshwater

Limit test:

yes

Total exposure duration:

3 h

Test temperature:

19.0 - 20.2 degrees Celcius

pH:

pH of activated sludge: 8.2 at the day of testingpH of control at start: 8.4pH of control at end: 8.5pH of test vessels: 8.4

Nominal and measured concentrations:

100 mg test substance/L

Details on test conditions:

- Number of micro-organisms was determined as the amount of mixed Liquor Suspended Solids (MLSS) per litre test medium- Batch of sludge was used one day after collection; 50 mL of synthetic sewage feed was added per litre of activated sludge at the end of the collection day- test medium was adjusted ISO-medium, formulated using tap-water purified by reverse osmosis (RO-water) (CaCl2.2H2O: 211.5mg/L ; MgSO4.7H2O: 88.8mg/L ; NaHCO3: 46.7mg/L ; KCl: 4.2mg/L)- test vessels: 300 mL glass oxygen bottles and 1L glass test bottles- following an overnight stirring period, the synthetic sewage feed (details provided in the report) and the activated sludge (200 mL) were added to the test substance mixture. Milli-RO water was added to provide a final volume of 500 mL with a final test substance loading of 100 mg/L. - the mixture was aerated during the contact time, using a pipette as an aeration device.- after the 3-hour contact time, a well mixed sample of te contents was poured into a 300 mL oxygen bottle, and the flast was sealed with an oxygen electrode connected to a recorder, forcing the air out of the vessel. - oxygen consumption was measured and recorded for approximately 10 minutes. - during measurement, the sample was not aerated but continuously stirred on a magnetic stirrer.- test was conducted in duplicate- respiration rate from each vessel (in mg O2/L/hr) was calculated fro mthe linear part of the respiration curve

Reference substance (positive control):

yes

Remarks:

3,5-dichlorophenol

Duration:

3 h

Dose descriptor:

EC50

Effect conc.:

> 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Duration:

3 h

Dose descriptor:

EC10

Effect conc.:

> 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Duration:

3 h

Dose descriptor:

NOEC

Effect conc.:

>= 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Duration:

3 h

Dose descriptor:

EC50

Effect conc.:

> 41.4 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Duration:

3 h

Dose descriptor:

EC10

Effect conc.:

> 41.4 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Duration:

3 h

Dose descriptor:

NOEC

Effect conc.:

>= 41.4 mg/L

Nominal / measured:

nominal

Conc. based on:

element

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Details on results:

Difference between the controls was within 15% of each otherNo inhibition of respiration rate (compared to the control) was noted in the test concentration of 100 mg test substance/L:- % inhibition in replicate 1: 0%- % inhibition in replicate 2: -2%

Results with reference substance (positive control):

the EC50 of 3,5-dichlorophenol was in the accepted range of 5-30 mg/L (i.e., 7.4 mg/L)

Reported statistics and error estimates:

a figure of more thzt 10% inhibition of respiration rate is considered significant. However, the test substance proved to be non-toxic at the tested concentration.

Validity criteria fulfilled:

yes

Conclusions:

An OECD-209-guideline test under GLP has been conducted, using strontium nitrate as test substance. A nominal test concentration of 100 mg/L (test substance) caused no effect; hence, the EC50 and EC10 are >100 mg/L test substance, and the NOEC is >= 100 mg/L test substance. These unbounded effect levels can be used for the estimation of a (worst-case) PNEC for micro-organisms.

Description of key information

One reliable toxicity study (Klimisch 1, GLP) has been identified for micro-organisms; a respiration inhibition test with activated sludge. Based on total amount of Sr added to the test solution, an unbound 3h-IC50 and a 3h-NOEC of >41.4 and >=41.4 mg Sr/L, respectively, were reported by Desmares-Koopman (2010), using  Sr(NO3)2 as test substance.

Additional information

One reliable toxicity data point (Klimisch 1, GLP) with activated sludge from a (predominantly) domestic sewage plant has been reported by Desmares-Koopman (2010). Inhibition of respiration was the endpoint under consideration.

At a nominal concentration of 100 mg Sr(NO₃)₂/L, no inhibition of the respiration rate was noted. Therefore - expressed as mg Sr_total/L, the 3h-IC₅₀ and 3h-NOEC were >41.4 and >=41.4 mg/L, respectively.