Zinc oxide

Bioaccumulation: aquatic / sediment

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

bioaccumulation in aquatic species: fish

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Remarks:

Does not compare with bulk or element

Qualifier:

no guideline followed

Principles of method if other than guideline:

A series of exposure studies was undertaken with zebrafish (Danio rerio) and rainbow trout (Oncorhynchus mykiss), exposing them to various sonicated metal oxide NPs either via the water column under semistatic conditions, for between 24hand 14 days, or via an oral dose by incorporation into feed pellets over a 21-day period. Exposure via the water avoided the use of dispersants, to allow investigation of the coreNPalone without the possibility of mixture effects. Gill, liver, skin, brain, gut, blood, and kidney were analyzed for zinc, cerium, or titanium content with inductively coupled plasma mass spectrometry (ICP-MS) or optical emission spectroscopy (ICP-OES).

GLP compliance:

not specified

Remarks:

the publication does not specify GLP compliance

Radiolabelling:

no

Vehicle:

no

Test organisms (species):

Oncorhynchus mykiss (previous name: Salmo gairdneri)

Route of exposure:

aqueous

Test type:

semi-static

Water / sediment media type:

natural water: freshwater

Total exposure / uptake duration:

14 d

Nominal and measured concentrations:

Nominal:
50, 500, 5000 µg L−1.

Reference substance (positive control):

no

Type:

BCF

Value:

1 050 dimensionless

Basis:

organ d.w.

Remarks:

gill

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:500 µg/l

Type:

BCF

Value:

720 dimensionless

Basis:

organ d.w.

Remarks:

liver

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:500 µg/l

Type:

BCF

Value:

680 dimensionless

Basis:

organ d.w.

Remarks:

brain

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:500 µg/l

Type:

BCF

Value:

2 060 dimensionless

Basis:

organ d.w.

Remarks:

skin

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:500 µg/l

Type:

BCF

Value:

106 dimensionless

Basis:

organ d.w.

Remarks:

gill

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:5000 µg/L

Type:

BCF

Value:

80 dimensionless

Basis:

organ d.w.

Remarks:

liver

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:5000 µg/l

Type:

BCF

Value:

78 dimensionless

Basis:

organ d.w.

Remarks:

brain

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:5000 µg/l

Type:

BCF

Value:

182 dimensionless

Basis:

organ d.w.

Remarks:

skin

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:5000 µg/l

Elimination:

not specified

Validity criteria fulfilled:

yes

Conclusions:

No significant uptake of zinc in any of the four tissues (gill, liver, brain, and kidney) analyzed at either exposure concentration adopted in this study (500 or 5000 μg L-1

Reference 2

Endpoint:

bioaccumulation in aquatic species: invertebrate

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

ENP stock suspensions (Stock 1) containing either 1.0 gL−1 CeO2 or ZnO in NanoPure water were prepared by vortexing 5 min, followed by 30 min sonication. They were then diluted in natural filtered seawater (0.22 m) to achieve a final concentration of 100 mg L−1 (Stock 2). Stock 2 suspensions were then sonicated for
10 min and then diluted in natural filtered seawater to achieve concentrations of 1.0, 2.5, 5.0 and 10.0 mg L−1. The composition of the filtered seawater is provided in [7].
Mussels were obtained from a mariculture farm (Taylor Shellfish Farms Shelton, WA, USA) and were acclimated in running seawater for two weeks before the experiments. Commercial feed (Shellfish Diet 1800, Campbell, CA, USA) was added to each container, along with the ENPs, to achieve a cell density of 30 × 104 cells L−1, with a mix of four microalgae, Isochrysis sp, Pavlova sp, Thalossiosira weissflogii, and Tetraselmis sp. The clearance rates of the mussels, i.e., the amount of phytoplankton removed by the organisms from the water column over time were calculated by measuring the fluorescence (Turner Designs Model# 7200-043 with Chl-a in vivo module excitation wavelength 485 nm, emission wavelength 685 nm) before and after exposure of mussels to the ENP-containing media, and expressed as the number of phytoplankton cells present in the media per mL per minute (cells mL−1 min−1). There was minimal interference from the ENPs in the fluorescence signal from the phytoplankton. All mussels were in excellent condition and were very similar in size, ranging from 2.9 to 3.1 cm in length. Our experiments were carried out in 500 ml plastic cups at a constant 15 ◦C and 35 parts per thousand salinity, and were aerated for 4 days. Seawater media containing the feed and CeO2 or ZnO was changed and re-dosed every 24 h.

GLP compliance:

not specified

Remarks:

the publication does not specify GLP compliance

Radiolabelling:

no

Vehicle:

no

Test organisms (species):

other aquatic mollusc: mussels

Details on test organisms:

Mussels were obtained from a mariculture farm (Taylor Shellfish Farms Shelton, WA, USA) and were acclimated in running seawater for two weeks before the experiments

Route of exposure:

aqueous

Test type:

static

Water / sediment media type:

natural water: marine

Total exposure / uptake duration:

4 d

Total depuration duration:

24 h

Test temperature:

15°C

Salinity:

35 parts per thousand

Details on test conditions:

Water samples were collected immediately after addition of feed/ENP suspension and every hour for 4 h to determine clearance rates of the mussels, i.e., the amount of phytoplankton removed by the organisms from the water column over time. A water sample was also collected at 24 h to determine the amount of metal remaining in the water column. Pseudofeces samples were collected at 24, 48, 72, and 96 h from each container and rinsed with NanoPure water followed by centrifugation at 5000 rpm for 10 min thrice. After the fourth day of the experiment, mussels were allowed to clear their digestive track for 24 h in filtered seawater with no added ENPs or feed, followed by dissection. During mussel dissections, all of the soft tissue inside of the shells were removed and soaked for 5 s in 5% HNO3 followed by a 5 s rinse with NanoPure water. The acid-NanoPure water rinse was done three times to avoid carryover of metal ions or ENPs that could have been present in the media. All samples were oven-dried for 72 h at 60 ◦C before recording the dry weight for uptake calculations.

Nominal and measured concentrations:

Nominal:
1.0, 2.5, 5.0 and 10.0 mg L−1.

Reference substance (positive control):

no

Type:

BCF

Value:

ca. 250 dimensionless

Basis:

whole body d.w.

Time of plateau:

24 h

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:1 mg/l

Type:

BCF

Value:

ca. 132 dimensionless

Basis:

whole body d.w.

Time of plateau:

24 h

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:2.5 mg/l

Type:

BCF

Value:

ca. 80 dimensionless

Basis:

whole body d.w.

Time of plateau:

24 h

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:5 mg/l

Type:

BCF

Value:

ca. 88 dimensionless

Basis:

whole body d.w.

Time of plateau:

24 h

Calculation basis:

steady state

Remarks on result:

other: Conc.in environment / dose:10 mg/l

Elimination:

not specified

Reported statistics:

A randomized design with five replicates per treatment was followed during the experiments. Quantitative data was reported as a mean ± standard error (SE) and significant differences between means were identified using one-way ANOVA, with ENP concentration as the main factor and either clearance rate or concentration in tissue/pseudofeces as the dependent variable. When significant differences were found, Tukey’s HSD was used to determine which groups differed. All statistical analyses were performed with the statistical program R (version 2.10.1, The R Foundation for Statistical Computing).

Validity criteria fulfilled:

yes

Conclusions:

Tested concentrations had no influence on clearance rates nor any noticeable effect on feeding habits. Zn accumulation is dependent on its concentration. Zn concentration increased in pseudofaeces with increasing ENP concetrations in the media.

Executive summary:

To understand how biotransformation influences the fate and transport of ENPs in marine ecosystems, suspension-feeding mussels were exposed to ZnO, over a range of concentrations from 1 mg L−1 to 10 mg L−1, in a laboratory experiment. Mussels

exposed to 10 mg L−1 accumulated 880 g g−1 of Zn on a dry tissue basis but rejected 63,000 g g−1 for Zn in pseudofeces. Scanning electron microscope evidence indicates CeO2 remained as ENPs but ZnO did not after being rejected by the mussels. A significant fraction of Zn remained in solution.

Description of key information

Zinc is an essential element which is actively regulated by organisms, so bioconcentration/bioaccumulation is not considered relevant for all inorganic zinc substances. As a rule, the ranges of BCF values observed have no relation to toxicity. They are the result of these active regulation mechanisms that keep the internal zinc concentration of the organisms within an optimal range. 

 

However, three studies investigated the bioaccumulation of nano zinc oxide particles in the aquatic environment: one in fish (Johnston et al, 2010), one in mussels (Montes et al, 2012) and one in algae (Merdzan et al, 2014).

* In the first study by Johnston et al (2010), no significant uptake of zinc in any of the four tissues (gill, liver, brain, and kidney) analyzed at either exposure concentration adopted in this study (500 or 5000 μg L-1).

* In the second study by Montes et al (2012), tested concentrations had no influence on clearance rates nor any noticeable effect on feeding habits. Zn accumulation is dependent on its concentration. Zn concentration increased in pseudofaeces with increasing ENP concentrations in the media.

* In the third study by Merdzan et al (2014), no significant differences in algal uptake and bioaccumulation were oberved for Zn salt vs. Zn nano. However, the bioaccumulation of nZnO-HMP was greater than could be attributed to the release of free Zn from the particles. The increased Zn bioaccumulation was hypothesized to have resulted from the biological stimulation of C. reinhardtii due to phosphate from the particle coating.

Additional information

Bioaccumulation is not considered relevant for essential elements because of the general presence of homeostatic control mechanisms.

 

However, three studies investigated the bioaccumulation of nano zinc oxide particles in the aquatic environment: one in fish (Johnston et al, 2010), one in mussels (Montes et al, 2012) and one in algae (Merdzan et al, 2014).

In the first study by Johnston et al (2010), a series of exposure studies was done with zebrafish (Danio rerio) and rainbow trout (Oncorhynchus mykiss), exposing them to various sonicated metal oxide NPs (including zinc oxide) either via the water column under semi-static conditions, between 24h and 14 days, or via an oral dose by incorporation into feed pellets over a 21-day period. The nominal concentrations of the nanoparticles were of 50, 500 and 5000 μg L-1. Gill, liver, skin, brain, gut, blood, and kidney were analyzed for zinc, cerium, or titanium content with inductively coupled plasma mass spectrometry (ICP-MS) or optical emission spectroscopy (ICP-OES).

No significant uptake of zinc in any of the four tissues (gill, liver, brain, and kidney) analyzed at either exposure concentration adopted in this study (500 or 5000 μg L-1).

In the second study by Montes et al (2012), suspension-feeding mussels were exposed to ZnO in order to understand how biotransformation influences the fate and transport of ENPs in marine ecosystems. The nominal concentrations of the nanoparticles were of 1.0, 2.5, 5.0 and 10.0 mg L−1. Mussels exposed to 10 mg L−1 accumulated 880 g g−1 of Zn on a dry tissue basis but rejected 63,000 g g−1 for Zn in pseudofeces. Scanning electron microscope evidence indicates CeO2 remained as ENPs but ZnO did not after being rejected by the mussels. A significant fraction of Zn remained in solution.

Tested concentrations had no influence on clearance rates nor any noticeable effect on feeding habits. Zn accumulation is dependent on its concentration. Zn concentration increased in pseudofaeces with increasing ENP concentrations in the media.

In the third study by Merdzan et al (2014), Chlamydomonas reinhardtii was exposed to either a soluble Zn salt or nZnO with different stabilizers: (i) bare nZnO, (ii) polyacrylic acid-stabilized, nZnO-PAA, or a (iii) sodiumhexametaphosphate-stabilized, nZnOHMP. Multiple techniques were used to quantify particle agglomeration and dissolution. Bioaccumulation experiments were performed by adding either the soluble Zn salt or the nZnO to the exposuremedium in order to obtain a total Zn concentration of 42–1350 μg L−1 (6.4 × 10−7 M–2.0 × 10−5M).

Measurements of total, dissolved and free ion were made. No significant differences in algal uptake and bioaccumulation were oberved for Zn salt vs. Zn nano. However, the bioaccumulation of nZnO-HMP was greater than could be attributed to the release of free Zn from the particles. The increased Zn bioaccumulation was hypothesized to have resulted from the biological stimulation of C. reinhardtii due to phosphate from the particle coating.