Zinc oxide

Toxicity to aquatic plants other than algae

Link to relevant study record(s)

Reference

Endpoint:

toxicity to aquatic plants other than algae

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Study not according to standardised protocol but good quality. Culture medium, test water conditions well described and relevant for EU freshwater.

Qualifier:

no guideline followed

Principles of method if other than guideline:

lab-designed dose-response test

GLP compliance:

not specified

Specific details on test material used for the study:

ZnO NPs were purchased from Aipurui Co., Ltd., Nanjing, China. The surface area of the NPs was further determined using the multipoint Brunauer–Emmett–Teller method (Brunauer et al. 1938). The morphology of the NPs was examined using transmission electron microscopy (H-7500; Hitachi, Japan), and the crystalline phase was examined by x-ray diffraction (XRD, X’TRA; ARL, Switzerland).

Analytical monitoring:

no

Vehicle:

no

Details on test solutions:

The nutrient solution used for growth was prepared according to Hoagland and Arnon (1938) and contained the following (mg/L): Ca(NO3)24H2O 118; KNO3 5.055; MgSO47H2O 4.932; KH2PO4 0.68; FeSO47H2O 0.307; K2SO4 0.348; H3BO3 0.286; MnSO47H2O 0.155; ZnSO4 0.022; CuSO4 0.0079; NiSO47H2O 0.00478; NaWO42H2O 0.00179; (NH4)6Mo7O244H2O 0.0128; and Co(NO3)26H2O 0.0049. The pH was adjusted to 6.5 by the addition of 0.1 N NaOH.

Test organisms (species):

other: Spirodela polyrhiza

Details on test organisms:

S. polyrhiza (L.) Schleid was obtained from Linyi Fengheyuan Garden Co., Ltd, a garden company for aquatic plants in Linyi City, China (E118o350, N35o050).

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

96 h

Test temperature:

24+/-2°C

pH:

6.5

Nominal and measured concentrations:

The NP exposures were at 0 (negative control, CK), 1, 10, and 50 mg/L. At the 50 mg/L concentration, two exposure methods were adopted: (1) exposure for the full 96 h (50a treatment) and (2) replacement of the ZnO spiked medium with culture medium without NPs after 12 h (50b treatment).

Reference substance (positive control):

no

Duration:

96 h

Dose descriptor:

NOEC

Effect conc.:

> 10 - < 50 mg/L

Nominal / measured:

nominal

Conc. based on:

dissolved

Remarks:

Zinc

Basis for effect:

other: growth rate

Validity criteria fulfilled:

yes

Conclusions:

Test adequacyconsidered sufficient

Executive summary:

In the present study, the results of the growth and biochemical responses indicate that the ZnO NPs could generate adverse effects to S. polyrhiza when the concentration reached 50 mg/L in the culture medium. The Zn released from NPs might be the main source of toxicity to S. polyrhiza because the 50a group and the positive control (ZnSO4 group) exhibited similar changes in enzymatic activity.

Description of key information

The general observation of similar toxicity by the Zn-ion and the ZnO-NP is most relevant for the discussion on the aquatic PNEC developed for zinc substances; it can be concluded that the available data show that the general Zn-ion based PNEC for freshwater is relevant for the nano-ZnO, too.

 

In the Hu et al study (2013), results of the growth and biochemical responses indicate that the ZnO NPs could generate adverse effects to S. polyrhiza when the concentration reached 50 mg/L in the culture medium. The Zn released from NPs might be the main source of toxicity to S. polyrhiza because the 50a group and the positive control (ZnSO4 group) exhibited similar changes in enzymatic activity.

In the Chen et al study (2016), no significant differences in biomass growth rate (%) bewteen nano ZnO and dissolved Zn at 0, 1 or 10 mg/L after 7 days exposure. At high concentrations (10 mg/L) biomass growth rates for both ZnO nano and dissolved Zn were lower when compared to 0 and 1 mg/L. IC50 values reported for biomass showed no differences in biomass growth rate or chlorophyll a content when comparing dissolved zinc to nano ZnO. The authors concluded that nano-ZnO toxicity is primarily caused by dissolved Zn.

Additional information

Nano ZnO acute

The present short-term acute aquatic toxicity database for nano-ZnO, covers 1 acute study on 1 sediment freshwater species. For reasons of comparison, in the attached tables, the data are presented as normalized values with the ecotoxicity value observed for the soluble zinc salts (Zn2+) as reference (ECx = 100%). The ratio ECx Zn2+/ECx nano ZnO is provided. The detail of the studies is in the relevant IUCLID sections. The full set of EC50 values are presented in the background information tables.

Nano ZnO chronic toxicity

The present review focused notably on these chronic data, which are considered to be of major relevancy for the risk assessment and the related PNECs. Like for the acute toxicity, the data are presented as normalized values with the ecotoxicity value observed for the soluble zinc salts (Zn2+) as reference (ECx = 100%). The ratio ECx Zn2+/ECx nano ZnO is provided. The detail of the studies is in the relevant IUCLID sections.

The freshwater chronic dataset covers 1 aquatic plant species. The full set of EC10/NOEC values are presented in the tables attached as background documents.

For freshwater, the comparison of the chronic ecotoxicity data obtained within the same studies for the same species/endpoints after exposing the organisms to either the soluble Zn2+ion or the ZnO-NP form show that there is generally no difference in toxicity between the two Zn-forms.

The general observation of similar toxicity by the Zn-ion and the ZnO-NP is most relevant for the discussion on the aquatic PNEC developed for zinc substances; it can be concluded that the available data show that the general Zn-ion based PNEC for freshwater is relevant for the nano-ZnO, too.

 

In the Hu et al study (2013), Spirodela polyrhiza was exposed to NP at 0 (negative control, CK), 1, 10, and 50 mg/L in a laboratory designed dose-response test. At the 50 mg/L concentration, two exposure methods were adopted: (1) exposure for 96 h (50a treatment) and (2) replacement of the ZnO spiked medium with culture medium without NPs after 12 h (50b treatment).

The results of the growth and biochemical responses indicate that the ZnO NPs could generate adverse effects to S. polyrhiza when the concentration reached 50 mg/L in the culture medium. The Zn released from NPs might be the main source of toxicity to S. polyrhiza because the 50a group and the positive control (ZnSO4 group) exhibited similar changes in enzymatic activity.

 

In the Chen et al study (2016), four L.minor colonies, each with 3 fronds were exposed to nano-ZnO in concentrations ranging from 0 to 10 mg/L (0, 1, 10 mg/l; i.e. 0,0.0123 mM, 0.123 mM (measured)) for 7days. In parallel, L. minor was cultured in medium supplemented with the “dissolved Zn equivalent of dissolved nano-ZnO”. In this case, Zinc ions (as ZnSO4, Sigma–Aldrich) were gradually added to the medium, such as to mimic the dissolved Zn from nano-ZnO measured during the same period of 7 days.

No significant differences in biomass growth rate (%) bewteen nano ZnO and dissolved Zn at 0, 1 or 10 mg/L after 7 days exposure. At high concentrations (10 mg/L) biomass growth rates for both ZnO nano and dissolved Zn were lower when compared to 0 and 1 mg/L. IC50 values reported for biomass showed no differences in biomass growth rate or chlorophyll a content when comparing dissolved zinc to nano ZnO. The authors concluded that nano-ZnO toxicity is primarily caused by dissolved Zn.