Niobium

Toxicity to aquatic algae and cyanobacteria

Link to relevant study record(s)

Reference

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

experimental study

Adequacy of study:

key study

Study period:

14 - 17 Feb 2022

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 201 (Alga, Growth Inhibition Test)

Deviations:

no

Principles of method if other than guideline:

Evidence of particles in the test medium, indicating that effects are linked to physical effects; Analytically measured niobium concentrations in the test medium were not reliable and are expected to i) not reflect the exposure concentrations during the test and ii) to not reflect the dissolved niobium test medium concentration. Thus effect values were based on nominal concentrations.

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

yes

Remarks:

ICP-MS

Details on sampling:

- Test start: all treatment and control solutions, prior to addition of algae (n = 2; 15 mL)
- Test termination: all treatment and control replicates (n = 2; 15 mL), algae were removed by filtration (0.45 µm PES filter)
- Sampling method: Before sampling the test solution was shaken and samples were taken from the middle of the respective flask using a pipette
- Sample storage conditions before analysis: one sample was stored (4 ± 3 °C) until further analysis

Vehicle:

no

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION
- Method: Test item solutions were prepared as highly saturated test solutions according to OECD GD No. 23:
- Test medium: OECD growth medium adjusted to pH 8.5
- > 1 mg/L: Individually prepared; Amounts of 1.00, 3.17, 10.01, 31.62 and 100.08 mg of the test item were transferred into sterile glass flasks and 1 L sterile modified growth medium was added. The test solutions were stirred vigorously for about 96 hours at room temperature. After stirring, the prepared test solutions were filtered through a 0.45 µm filter (PES) to remove undissolved test item.
- < 1 mg/L: Dilution: 0.32 mg test item/L, 320 mL of the 2nd lowest test concentration (1 mg test item/L) were mixed with 680 mL of modified growth medium. After dilution the solution was treated in the same way as the test solutions prepared by direct weighing.
- Controls: growth medium without test item; the growth medium was treated in the same way as the test solutions
- Test concentration separation factor: approx. 3.16
- Evidence of undissolved material: Sedimenting particles were observed by means of dynamic light scattering (DLS) in the test medium of the 10, 31.6 and 100 mg/L (nominal loading) treatment groups during the course of the experiment (0, 24, 48 and 72 h).

PRELIMINARY EXPERIMENTS - SEPARATION METHOD AND STIRRING TIME
a) Additional settling time of 24 h after 96 h of stirring: 10 and 100 mg/L (nominal, loading) prepared as described in 'Preparation and application of test solution' without filtration step; visual observation of undissolved test item after 0, 2, 4, 6 and 24 h
b) 0.45 µm filtration after 96 h of stirring: 10 and 100 mg/L (nominal, loading) prepared as described in 'Preparation and application of test solution'; After stirring, fresh samples (0h) were taken before filtration and again after filtration through 0.45 µm and 0.2 µm PES filter. The test solutions were then distributed to replicates and incubated under test conditions of an algal growth inhibition test (OECD 201). Samples for analytical measurements of Nb were taken at 24 h, 48 h and 72 h from individually prepared replicates per test concentration. 0.20 µm filtered samples were only measured after 72 h.

REFERENCES:
- OECD Series on testing and assessment, Number 23. Guidance Document on Aqueous-Phase Aquatic Toxicity Testing of Difficult Test Chemicals. ENV/JM/MONO(2000)6/REV1, 6-Jul-2018.

Test organisms (species):

Raphidocelis subcapitata (previous names: Pseudokirchneriella subcapitata, Selenastrum capricornutum)

Details on test organisms:

TEST ORGANISM
- Common name: green algae
- Source (laboratory, culture collection): SAG, Culture Collection of Algae at Pflanzenphysiologisches Institut of the University at Göttingen, Albrecht von Haller Institut, Untere Klarspüle 2, 37073 Göttingen, Catalog No 61.81 SAG
- Age of inoculum (at test initiation): 4 days
- Method of cultivation: according to OECD 201 in culture medium recommended by Bringmann and Kühn (1980); Precultures: OECD growth medium

REFERENCES
- Bringmann, G. and Kühn, R. (1980). Comparison of the toxicity thresholds of water pollutants to bacteria, algae, and protozoa in the cell multiplication inhibition test. Water Research 14(3), 231-241.

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

72 h

Test temperature:

22.0 - 22.5 °C

pH:

Control/ treatment group test initiation: 8.23 /8.11 - 8.49
Control/ treatment group test termination: 7.82 /7.84 - 7.90

Nominal and measured concentrations:

Nominal: 0, 0.32, 1.00, 3.17, 10.0, 31.6 and 100 mg test item/L

Details on test conditions:

TEST SYSTEM
- Test vessel: 250 mL conical glass flasks, acid-washed (10% HNO3) and rinsed six times with purified water
- Type: covered with air permeable silicone-sponge caps , sterile
- Material, size, headspace, fill volume: 100 mL
- Aeration: no
- Initial cells density: 10 000 cells/mL
- Control end cells density: 17.9 x 10^4 cells/mL
- No. of vessels per concentration (replicates): 4 (randomly placed in the incubator)
- No. of vessels per control (replicates): 8 (randomly placed in the incubator)

GROWTH MEDIUM
- Standard medium used: no, OECD medium according to OECD 201, prepared with pruified water, but the pH was modified to a pH of 8.5

TEST MEDIUM / WATER PARAMETERS
- Source/preparation of dilution water: purified water (ELGA „PURELAB Ultra“)
- Culture medium different from test medium: yes, pH culture medium: about 8.1

OTHER TEST CONDITIONS
- Sterile test conditions: yes
- Adjustment of pH: yes, 8.5
- Photoperiod: continuously illuminated (OSRAM 'day light')
- Light intensity and quality: 93.41 to 95.84 µE m-2 s-1 (direct light above the tray of the incubator was measured)
- Temperature: measured continuously during the test in an additional test vessel
- pH measurement: 0 h: one replicate; 72 h: all replicates

EFFECT PARAMETERS MEASURED :
- Determination of cell concentrations: in the inoculum culture prior to the addition to the test vessels at test start and after 24, 48 and 72 hours in the test cultures during the growth test; electronic particle counter (CASY TT, OMNI Life Science GmbH & Co. KG, Bremen, Germany)
- Microscopic observation: 72 h

RANGE FINDING TEST
- Spacing factor for test concentrations: 10
- Range finding study : yes
- Test concentrations: 1, 10, 100 mg/L (nominal loading)
- Results used to determine the conditions for the definitive study: yes

FIRST REPETITION OF MAIN TEST:
- Prior to the definitive test (results reported in this robust study summary) a first repetition of the main test was performed in the same way as the definitive test (except that no particles size measurements were conducted)
- Test concentrations: 0, 1.00, 3.16, 10.00, 31.60, 100,00 mg/L (nominal)

CULTURING APPARATUS
- Details on culturing apparatus used: test vessels were placed on an orbital laboratory shaker (150 rpm) (INFORS, Switzerland) in an incubator

Reference substance (positive control):

yes

Remarks:

3,5-dichlorophenol

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Remarks on result:

other:

Remarks:

Effect values are based on nominal instead of measured concentrations since the measured concentrations were not reliable.

Duration:

72 h

Dose descriptor:

EC10

Effect conc.:

16.1 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Remarks on result:

other: 95% CI: 2.78 - 34.4 mg/L

Remarks:

Effect values are based on nominal instead of measured concentrations since the measured concentrations were not reliable.

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Remarks on result:

other:

Remarks:

Effect values are based on nominal instead of measured concentrations since the measured concentrations were not reliable.

Details on results:

- Exponential growth in the control: yes
- Observation of abnormalities: no
- Unusual cell shape: no
- Colour differences: no
- Flocculation: no
- Adherence to test vessels: no
- Aggregation of algal cells: no
- Any stimulation of growth found in any treatment: In the treatment groups between 1.00 and 10.0 mg/L (nominal loading) a non-significant induction of growth rate (+0.9 to +24.3%) was observed.
- Any observations that might cause a difference between measured and nominal values: Sedimenting particles were observed by means of dynamic light scattering (DLS) in the test medium of the 10, 31.6 and 100 mg/L (nominal loading) treatment groups during the course of the experiment (0, 24, 48 and 72 h).
- Effect concentrations exceeding solubility of substance in test medium: Measured Nb concentrations in the test medium were found to exceed the Nb concentrations released in transformation dissolution tests according to OECD GD No. 29 with different Nb materials (≥100 mg/L loading rate, 7 d, pH 8, dissolved Nb: 2.77 - 3.5 µg/L; 1 - 10 mg/L loading rate, 7 or 28 d, pH 8: dissolved Nb concentration < LOQ of 0.01 or 0.4 µg/L; for details please refer to IUCLID section 4.8).

Results with reference substance (positive control):

- Results with reference substance valid: yes (February 2022)
- EC50: ErC50 value of 3.40 mg/L (nominal; 95% confidence limits: 2.80 – 4.13 mg/L)

Reported statistics and error estimates:

Biological data were statistically analysed to determine EC50 and EC10 values together with 95 % confidence intervals, where possible.
For growth rate and yield the data was analysed using non- linear regression procedures (3 – parametric normal), however, the data showed a significant lack of fit for the tested 3-parametric non-linear regression procedures (3-parametric normal, logistic and Weibull) and therefore, the ECx values were calculated based on linear regression procedures (Probit).

The NOEC for growth rate was determined using the Williams Multiple Sequential t-test Procedure, while the NOEC for yield was determined using the Multiple Sequentially-rejective Welsh-t-test After Bonferroni-Holm (significance level of 0.05, one-sided smaller).

The computer program ToxRat® was used for statistical evaluations.

RANGE FINDING TEST

Concentration dependent effect were observed in the treatment groups 1 to 100 mg/L (nominal) after 72 h of exposure, ranging from 1.3 to 42.9% inhibition of growth rate compared to control.

 

Table: Measured Nb concentrations in the non-GLP range finding test.

Sample	Nominal concentration [µg Nb/L]	Filtration	Measured concentration [µg Nb/L]	Mean Nb conc. [µg/L]	Deviation from 0h samples [%]	Recovery of filtered 3d samples compared to 0h sample [%]
Test initiation „direct measurement“ (August 23, 2021)	100,000	filtered 0.45 µm	141	-	-	-
Test initiation “stability; measured after 3 days” (August 26, 2021)	100,000	filtered 0.45 µm	150	145	3.2
			156
			136
			138
Test termination (August 26, 2021)	100,000	filtered 0.45 µm	156	-	-	111
		not filtered	163			-

August 23, 2021: LOQ = 0.0013; August 26, 2021: LOQ = 0.0047 µg/L

DEFINITIVE TEST

After exposure of algae to the test solutions for 72 h the growth rate of algae was inhibited in a concentration dependent manner up to 28.3% compared to control, except for the lowest test concentration. Here a strong, not significant, inhibition of growth (22.6%) was observed. Measured niobium concentrations (104 µg/L) were higher in the lowest nominal test concentration (0.32 mg/L (nominal)) than in the highest nominal test concentration of 100 mg/L (86.7 µg/L), indicating that this higher effect was due to an application error (see chemical results for a detailed discussion). Dynamic light scattering analysis, revealed that non-dissolved niobium particles were available in the test medium, demonstrating exposure of algae to non-dissolved niobium. This means that the measured niobium concentrations do not reflect the dissolved fraction of niobium in the test medium only and that the observed effects are caused by direct physical effects of the non-dissolved niobium.

The test item is a powder consisting of a broad range of particle sizes. Due to this inhomogeneity, it was not possible to apply homogenous and reproducible aliquots of this powder, in terms of particle size distribution, to each treatment group. It is assumed that due to this property, different amounts of particles e.g. of < 0.45 µm are available at each test item application, resulting in a huge variation of the measured test medium concentrations. Furthermore, due to the dynamic behaviour of the particles in the test medium (sedimentation/agglomeration) it is assumed that the measured concentrations do not reflect the real exposure situation in the test vessels. This was confirmed by the measured niobium concentration in the test solutions at test initiation and test termination which varied significantly between various analytical pre-tests, the range finder and a 1st main test. For example, measured niobium concentrations in the 100 mg/L

(nominal, stirred for 96 h, 0.45 µm filtered)

treatment group ranged from 86.7 – 870 µg/L in the preliminary, range finder and main tests.

 

Thus, based on following facts, it is concluded that the measured concentrations were not reliable and all endpoints were evaluated based on the nominal concentrations:

- Evidence of non-dissolved niobium material in the test medium demonstrates that the measured niobium concentrations do not only reflect the dissolved niobium fraction in the test medium.

- Sedimentation of particles demonstrates a dynamic exposure system, therefore it is expected that e.g. sedimented material was not recorded during sampling.

- Wide particle size distribution of the test item and the presence of particles < 0.45 µm (filter pore size) did not allow a reproducible and homogeneous application of the test item, resulting in a high variation of measured niobium concentrations in the preliminary, range finder and the main tests.

TEST ITEM CONCENTRATION IN THE TEST MEDIUM - DEFINITIVE TEST

Table: Results of the chemical analysis of Nb concentration in the test media at test initiation and test termination after 72 hours exposure (All values are considered as reliable; potential error due to carry over during the analytical measurement of < 25%)

Sampling	Nominal concentration [µg test item/L]	Measured Nb concentration [µg/L]	% of nominal [%]	% of initial [%]
0d (Analytical sample)	Control	1.65	-	-
	320	104	32.3	-
	1000	40.7	4.1	-
	3170	8.14	0.3	-
	10000	35.1	0.4	-
	31600	91.3	0.3	-
	100000	86.7	0.1	-
0d (Retain sample)a	Control	1.64	-	-
	320	102	31.7	-
	1000	40.5	4.1	-
	3170	8.07	0.3	-
	10000	34.6	0.3	-
	31600	100	0.3	-
	100000	87.9	0.1	-
0d (Mean)	Control	1.65	-	-
	320	103	32.0	-
	1000	40.6	4.1	-
	3170	8.11	0.3	-
	10000	34.9	0.3	-
	31600	95.4	0.3	-
	100000	87.3	0.1	-
3d	Control	1.33	-	-
	320	74.8	23.4	73.0
	1000	32.5	3.2	80.0
	3170	6.23	0.2	76.9
	10000	25.7	0.3	73.6
	31600	71.2	0.2	74.6
	100000	63.6	0.1	72.9

a:        Since the data did not reflect the results of previous pre-tests, the retain samples were measured as well but confirmed the results of the analytical samples with a recovery of 0.1% to 32.3% of the nominal concentration. Therefore, the mean value of the determined concentrations was calculated and used to determine the stability of the Nb concentrations in terms recovery of the initially measured concentrations.

b:        The lowest test concentration was prepared by serial dilution from the 2nd lowest test concentration. Due to the indicated presence of inhomogeneously distributed particles (< 0.45 µm) it was assumed that at the preparation of the lowest test concentration a larger part of particles was part of the aliquot used for the dilution. By this, a considerably higher concentration as envisaged was achieved in the lowest test concentration.

PARTICLE SIZE DISTRIBUTION OF THE TEST ITEM IN TEST MEDIUM - DEFINITIVE TEST

Although the particle size distribution results are not reliable, due to a high PDI (0.306 - 1.000), the results indicate that there were particles in the test medium which had a size below around 200 nm (Table 1).

Table: Particle size distribution of the test item in test medium in the three highest test concentration throughout the test.

Date	Nominal conc. [mg test item/L]	Z-Average	PdI	Peak 1 Mean Int	Peak 1 Area Int	Peak 2 Mean Int	Peak 2 Area Int	Mean Count Rate
		d.nm		d.nm	%	d.nm	%	kcps
0 hours	100	2481	1.000	100.6	100	0	0	93.9
		603.3	0.625	194.7	70.9	66.92	29.1	85.6
		307.8	0.415	96.04	51.5	281.3	48.5	75.3
	31.6	1829	1.000	82.31	100	0	0	126.7
		940.2	0.801	103.5	100	0	0	124.6
		791.3	0.785	164.2	82.5	51.8	17.5	117.7
	10.0	991.1	0.846	77.05	100	0	0	33.9
		181.3	0.263	170.3	93.6	5191	6.4	29.4
		140.3	0.341	141.5	81.7	2076	18.3	29.7
24 hours	100	678.8	0.588	147.4	100	0	0	143.7
		955.7	0.786	169.9	100	0	0	149.7
		288.7	0.383	179.5	100	0	0	122.5
	31.6	1162	0.973	102.2	100	0	0	193.0
		897.6	0.757	129.0	100	0	0	174.9
		497.3	0.513	199.7	81.4	75.36	18.6	167.2
	10.0	1555	1.000	106.6	100	0	0	82.8
		607.9	0.618	203.9	82.7	66.44	17.3	72.5
		280.0	0.594	160.3	61.7	751.5	34.4	62.5
48 hours	100	3030	1.000	57.3	100	0	0	105.1
		583.4	0.611	143.4	100	0	0	89.9
		260.9	0.477	122.7	69.7	489	30.3	71.3
	31.6	1334	1.000	115.7	100	0	0	103.3
		635.0	0.562	121.5	100	0	0	92.7
		382.2	0.415	164.5	87.1	58.07	12.9	93.7
	10.0	1247	1.000	84.5	100	0	0	37.9
		274.2	0.306	245.7	72.5	73.94	26.2	36.1
		171.6	0.389	359.2	100	0	0	32.4
72 hours	100	589.2	0.636	90.1	100	0	0	44.5
		165.0	0.357	232.8	94.0	4149	6	44.9
		175.8	0.409	137.1	54.2	595.2	45.8	46.3
	31.6	946.9	0.802	99.0	100	0	0	72.7
		395.7	0.435	137.3	100	0	0	68.7
		198.3	0.424	189.5	93	5197	7	66.2
	10.0	1127	0.949	82.7	100	0	0	41.7
		239.0	0.586	112.0	56.8	327.1	39.7	35.4
		150.8	0.367	175.6	89.3	3303	10.7	30.1

PDI: Polydispersity index; Int: Intensity; kcps: kilo counts per second

GROWTH RATES - DEFINITIVE TEST

Table: Inhibition of growth rate and yield compared to controls after 72 hours.

Nominal concentration [mg test item/L]	Growth rate		Yield
	Growth rate [1/d]	% Inhibition	Yield [cells x 104/mL]	% Inhibition
Control	0.960	-	16.9	-
0.32	0.744a	22.6 (-)[FS1] a	8.3a	50.8 (+)a
1.00	0.969	-1.00 (-)	17.3	-2.7 (-)
3.17	1.194	-24.3 (-)	35.0	-107.2 (-)
10.0	0.969	-0.90 (-)	17.3	-2.4 (-)
31.6	0.834	13.1 (+)	11.2	33.5 (+)
100	0.689	28.3 (+)	6.9	59.2 (+)

A negative value indicates an increase of the parameter compared to the control.

(+) significantly/ (-) not significantly different from controls, Williams Multiple Sequential t-test Procedure (growth rate) and Multiple Sequentially-rejective Welsh-t-test after Bonferroni-Holm (yield), significance level 0.05, one-sided smaller.

a:        The lowest test concentration was prepared by serial dilution from the 2nd lowest test concentration. Due to the indicated presence of inhomogeneously distributed particles (< 0.45 µm) it was assumed that at the preparation of the lowest test concentration a larger part of particles was part of the inoculum used for the dilution. By this, a considerably higher concentration as envisaged was achieved in the lowest test concentration leading to a higher toxicity at a lower nominal test concentration.

 

CELL NUMBERS - DEFINITIVE TEST

Table: Cell numbers (x 104) per mL dependent on nominal concentrations and time.

Nominal concentration [mg test item/L]	Control	0.32	1.00	3.17	10.0	31.6	100
0 h	1	1	1	1	1	1	1
25 h	3.875	2.059	3.133	3.740	2.585	2.125	2.050
	3.777	2.021	2.977	3.514	2.840	2.119	1.589
	3.926	1.943	2.610	3.905	2.924	2.271	1.902
	3.811	2.109	2.707	3.995	2.870	2.059	2.197
	4.189
	4.021
	3.765
	3.677
Mean:	3.880	2.033	2.857	3.788	2.805	2.143	1.934
Std.Dev.:	0.164	0.070	0.241	0.211	0.151	0.090	0.260
n:	8	4	4	4	4	4	4
CV:	4.2	3.4	8.4	5.6	5.4	4.2	13.4
48 h	10.570	4.672	6.450	11.920	7.568	5.233	4.279
	9.354	4.562	6.185	10.660	6.650	4.933	4.956
	9.401	4.520	6.372	11.720	6.307	5.646	4.260
	8.921	4.511	6.139	12.940	6.515	4.883	4.334
	8.990
	9.301
	9.555
	8.578
Mean:	9.334	4.566	6.286	11.810	6.760	5.174	4.457
Std.Dev.:	0.591	0.074	0.148	0.935	0.557	0.351	0.334
n:	8	4	4	4	4	4	4
CV:	6.3	1.6	2.4	7.9	8.2	6.8	7.5
72 h	19.680	8.686	18.910	38.280	18.760	13.000	8.001
	20.850	9.841	17.140	34.010	17.820	12.380	8.193
	17.880	9.537	18.970	33.960	17.770	11.920	7.720
	16.160	9.200	18.340	37.700	18.820	11.630	7.662
	17.830
	17.170
	17.310
	16.220
Mean:	17.887	9.316	18.340	35.987	18.293	12.233	7.894
Std.Dev.:	1.630	0.495	0.849	2.325	0.575	0.598	0.248
n:	8	4	4	4	4	4	4
CV:	9.1	5.3	4.6	6.5	3.1	4.9	3.1

Mean: arithmetic mean; Std. Dev.: standard deviation; n: number of replicates; CV: coefficient of variation

Cell number at test start: 10 000 cells/mL

Table: Validity criteria for OECD 201 - Definitive test

Criterion from the guideline	Outcome	Validity criterion fulfilled
The biomass in the control cultures should have increased exponentially by a factor of at least 16 within the 72-hour test period.	17.9	yes
The mean coefficient of variation for section-by-section specific growth rates (days 0-1, 1-2 and 2-3, for 72-hour tests) in the control cultures must not exceed 35%	34.8%	yes
The coefficient of variation of average specific growth rates during the whole test period in replicate control cultures must not exceed 7% in tests with Pseudokirchneriella subcapitata and Desmodesmus subspicatus. For other less frequently tested species, the value should not exceed 10%.	3.1%	yes

Validity criteria fulfilled:

yes

Remarks:

For details please refer to field "any other information on results incl. tables"

Description of key information

ErC50 (72h) > 100 mg/L (nominal) for Raphidocelis subcapitata (OECD 201, static)

ErC10 (72 h) = 16. 1 mg/L (nominal) for Raphidocelis subcapitata (OECD 201, static; effects due to mechanical effects)

Additional information

One key study investigating the toxicity of niobium (CAS 7440-03-1) to freshwater algae is available.

 

Effects on the growth rate of the freshwater algae Raphidocelis subcapitata were investigated after exposure to saturated solutions of niobium for 72 h in a test according to OECD 201 (GLP). Six saturated solutions of niobium (adjusted to pH 8.5) were prepared in the assumption to test the dissolved fraction of niobium (0, 0.32, 1.00, 3.17, 10.0, 31.6 and 100 mg/L (nominal)). For this, an appropriate amount of niobium was weighed into test vessels and filled up with the specific volume of test medium, which was previously adjusted to pH 8.5 (as requested in the ECHA decision CCH-D-2114517391-56-01/F). After stirring for 96 h the saturated solutions were filtered (0.45 µm) and used in the algae test.

The test item concentration in the test medium was analytically monitored at test initiation and termination by means of inductively coupled plasma mass spectrometry (ICP-MS) in control and all treatment groups. Hydrodynamic diameters of niobium particles were analysed in additional replicates of the test medium (without algae) in the three highest test concentrations at test initiation, after 24, 48 and 72 h by means of dynamic light scattering (DLS).

An excessive effort was done to optimize the application method (for details see the robust study summary), by performing preliminary tests. Nevertheless, due to the particulate properties (sedimentation of particles, wide particle size distribution including particles sized < 0.45 µm) of the test item it was not possible to apply a homogenous and reproducible test item concentrations and to reliably measure the real exposure concentration in the test medium. For example, measured niobium concentrations in the 100 mg/L (nominal) treatment group ranged from 86.7 – 870 µg/L in the preliminary, range finder and main tests. Therefore, effect concentrations were based on nominal instead of measured concentrations.

Dynamic light scattering analysis, revealed that non-dissolved niobium particles were available in the test medium, demonstrating exposure of algae to non-dissolved niobium.

The 72 hour EC10 and EC50 values for growth rate were determined as 16.1 and > 100 mg/L (nominal), respectively. However, the observed effects are linked to mechanical effects and not to the intrinsic toxic properties of the test item. Therefore, it is concluded that niobium is neither acute nor chronic toxic to aquatic algae.