tungsten zirconium oxide

Adsorption / desorption

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

adsorption / desorption: screening

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

weight of evidence

Justification for type of information:

Read across from several studies reporting on adsorption of zirconium to particulate matter. The read across justification document is attached to IUCLID Section 13.

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Key result

Phase system:

soil-water

Type:

log Kp

Value:

4.13

Remarks on result:

other: Key log Kp soil-water for Zr based on the read across study from Ferrand (2005) (see also Ferrand et al., 2006).

Key result

Phase system:

sediment-water

Type:

log Kp

Value:

5.47

Remarks on result:

other: Key log Kp sediment-water for Zr based on the read across study from Roychoudhury and Starke (2006).

Key result

Phase system:

suspended matter-water

Type:

log Kp

Value:

5

Remarks on result:

other: Key log Kp suspended matter-water for Zr based on the read across studies from Veselý et al. (2001) and Gobeil et al. (2005).

Reference 2

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: In this study, Kp values were determined for Zr using a water soluble Zr compound (ZrOCl2) and two different soils.

Remarks:

The Kp values were determined using a batch equilibrium experimental setup similar to that described in the corresponding OECD guideline. The experiments seem to be well performed and a suitable method of analysis was used. A Klimisch 2 reliability score was assigned to this study because not all results were presented and because total recovery of the added Zr was not discussed.

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)

GLP compliance:

not specified

Type of method:

batch equilibrium method

Media:

soil

Radiolabelling:

no

Test temperature:

20°C

Analytical monitoring:

yes

Details on sampling:

- Concentrations: a single concentration of 9.12 µg/L Zr was used (50 mL of a 0.1 µM ZrOCl2 dilute suspension in a 0.1 M KNO3 ionic buffer)
- Sampling interval: contact time ranged from 5 min to 48 h

Details on matrix:

COLLECTION AND STORAGE
- Geographic location: two agricultural soils were sampled close to the underground research laboratory (Meuse/Haute Marne, France) of the National Agency for management of radioactive wastes (Andra)
- Sampling depth (cm): top soils 0-20 cm
- Soil preparation (e.g.: 2 mm sieved; air dried etc.): air-dry soils were crushed and sieved under 2 mm

PROPERTIES
Soil A (acidic sandy clayey loamy)
- % sand: 31.9
- % silt: 48.7
- % clay: 19.4
- pH: 5.45
- Organic carbon (%): 31.8
- CEC (meq/100 g): 9.0 cmol/kg
- Background Zr content: 417.4 mg/kg dw
Soil B (clayey calcareous soil)
- % sand: 10.7
- % silt: 50.7
- % clay: 38.6
- pH: 8.3
- Organic carbon (%): 33.6
- CEC (meq/100 g): 10.02 cmol/kg
- Background Zr content: 164 mg/kg dw

Details on test conditions:

TEST CONDITIONS
- Buffer: a 0.1 M KNO3 ionic buffer
- pH: at soil pH
- Suspended solids concentration: 200 mg soil (on a dry weight basis)

TEST SYSTEM
- Type, size and further details on reaction vessel: polypropylene tubes
- Amount of soil/sediment/sludge and water per treatment (if simulation test): 200 mg soil (on a dry weight basis)
- Soil/sediment/sludge-water ratio (if simulation test): 4:1 (50 mL solution)
- Number of reaction vessels/concentration: not reported

Computational methods:

[Zr]adsorbed by soil = V ([Zr]initial - [Zr]solution)/Msoil
Kp in L/kg = [Zr]adsorbed by soil / [Zr]solution
The calculated Zr adsorbed by soil could afterwards be compared to the cumulative concentration desorbed by extraction with CaCl2, DTPA and NaPP

Phase system:

soil-water

Type:

Kp

Value:

6 000 L/kg

Temp.:

20 °C

Matrix:

Soil A

% Org. carbon:

31.8

Phase system:

soil-water

Type:

Kp

Value:

30 000 L/kg

Temp.:

20 °C

Matrix:

Soil B

% Org. carbon:

33.6

Adsorption and desorption constants:

Kp = 6000 L/kg dw for soil A.
Kp = 30000 L/kg dw for soil B.

Details on results (Batch equilibrium method):

Kinetic experiments showed that Zr is retained on soil components in few minutes.
The curves can be described by a first order kinetic model:
[Zr]adsorbed = [Zr]sat (1-e^-kt)
Saturated Zr adsorbed concentrations were 0.002 mg Zr/g dry soil for soil A.
Saturated Zr adsorbed concentrations were 0.00215 mg Zr/g dry soil for soil B.
At equilibrium, Zr concentrations measured in solution are lower for soil B (10^-9 M) than for soil A (4x10^-9 M).
Soil B has a higher affinity for Zr than soil A.

Conclusions:

In this study, batch equilibrium experiments were conducted with ZrOCl2 solutions and two different soils (an acidic sandy clayey loamy soil and a clayey calcareous soil). The Kp values resulting from this study are 6000 L/kg (dw) for the acidic soil and 30000 L/kg (dw) for the calcareous soil.

Reference 3

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: In this study, Kp values were determined for Zr using a water soluble Zr compound (ZrOCl2) and two different soils.

Remarks:

The Kp values were determined using a batch equilibrium experimental setup similar to that described in the corresponding OECD guideline. Additionally, desorption experiments were conducted with the soils used in the adsorption experiments. The experiments seem to be well performed and a suitable method of analysis was used. A reliability score of Klimisch 2 was assigned to this study because total recovery of the added Zr was not discussed and because the results may have been affected by the very low soil:solution ratio.

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)

GLP compliance:

not specified

Type of method:

batch equilibrium method

Media:

soil

Radiolabelling:

no

Test temperature:

20°C

Analytical monitoring:

yes

Details on sampling:

Adsorption experiments
- Concentrations: a single concentration of 9.12 µg/L Zr was used (50 mL of a 0.1 µM ZrOCl2 dilute suspension in a 0.1 M KNO3 ionic buffer)
- Sampling interval: contact time ranged from 5 min to 48 h
Desorption experiments:
- Sampling interval: contact time ranged from 5 min to 48 h

Details on matrix:

COLLECTION AND STORAGE
- Geographic location: two agricultural soils were sampled close to the underground research laboratory (Meuse/Haute Marne, France) of the National Agency for management of radioactive wastes (Andra)
- Sampling depth (cm): top soils 0-20 cm
- Soil preparation (e.g.: 2 mm sieved; air dried etc.): air-dry soils were crushed and sieved under 2 mm

PROPERTIES
Soil A (acidic sandy clayey loamy)
- % sand: 31.9
- % silt: 48.7
- % clay: 19.4
- pH: 5.45
- Organic carbon (%): 31.8
- CEC (meq/100 g): 9.0 cmol/kg
- Background Zr content: 417.4 mg/kg dw
Soil B (clayey calcareous soil)
- % sand: 10.7
- % silt: 50.7
- % clay: 38.6
- pH: 8.3
- Organic carbon (%): 33.6
- CEC (meq/100 g): 10.02 cmol/kg
- Background Zr content: 164 mg/kg dw

Details on test conditions:

TEST CONDITIONS
- Buffer: a 0.1 M KNO3 ionic buffer
- pH: at soil pH
- Suspended solids concentration: 200 mg soil (on a dry weight basis) in 50 mL solution

TEST SYSTEM
- Type, size and further details on reaction vessel: polypropylene tubes
- Amount of soil/sediment/sludge and water per treatment (if simulation test): 200 mg soil (on a dry weight basis)
- Soil/sediment/sludge-water ratio (if simulation test): 1:250 (50 mL solution)
- Number of reaction vessels/concentration: two + one control without soil
- In the desorption experiments, dried soil used in the adsorption experiments were mixed with milliQ water. All other test conditions as mentioned above.

Computational methods:

Adsorbed Zr in soil was determined by mass balance calculations using the initial concentration in solution and those measured in solution after certain contact periods.
Kp in L/kg = [Zr]adsorbed by soil / [Zr]solution (taking into account soil:solution ratio).

Phase system:

soil-water

Type:

Kp

Value:

6 000 L/kg

Temp.:

20 °C

Matrix:

Soil A

% Org. carbon:

31.8

Phase system:

soil-water

Type:

Kp

Value:

30 000 L/kg

Temp.:

20 °C

Matrix:

Soil B

% Org. carbon:

33.6

Adsorption and desorption constants:

Kp = 6000 L/kg dw for soil A.
Kp = 30000 L/kg dw for soil B.

Recovery of test material:

not reported

Details on results (Batch equilibrium method):

ADSORPTION
Kinetic experiments showed that Zr is retained on soil components in few minutes.
The curves can be described by a first order kinetic model:
[Zr]adsorbed = [Zr]sat (1-e^-kt)
With
- [Zr]adsorbed = mg Zr/kg dry soil
- [Zr]sat = concentration of Zr sorbed at saturation (mg Zr/kg dry soil)
- k = adsorption constant (min^-1)
- t = contact time (min)
The time constants (T = 1/k) were 3 min for soil A and 2.5 min for soil B.
Saturated Zr adsorbed concentrations were 2.09 mg Zr/kg dry soil for soil A.
Saturated Zr adsorbed concentrations were 2.2 mg Zr/kg dry soil for soil B.
At equilibrium, Zr concentrations measured in solution are lower for soil B (10^-9 M; 0.091 µg/L) than for soil A (4x10^-9 M; 0.365 µg/L).
Soil B has a higher affinity for Zr than soil A.

DESORPTION
At desorption equilibrium, the Zr concentrations in solution were 0.365 µg/L in soil A and 0.219 µg/L in soil B.

Statistics:

not reported

- Soil B (the calcareous soil) had more affinity for Zr than soil A (the acidic soil) and sorption also occurred faster in soil B. This may be explained by the fact that the H+ ions present in the acidic soil enter in competition with Zr ions for adsorption to available sites on the solid phase.

- The very low soil:solution ratio used in this study was necessary because at higher ratios the concentrations of Zr in solution would be below the detection limit of the available method of analysis. However, such low soil:solution ratios favor adsorption and therefore the Kp values may have been affected by these experimental conditions.

- The method of Kp determination does not allow to distinguish between the different solid forms of Zr in the experiment (adsorbed to iron oxides, adsorbed to organic matter, precipitated as hydroxydes or carbonates). According to formerly obtained results, the authors mention that adsorption to iron oxides may be the predominant process in soil.

- The desorption experiments indicate that the concentrations of Zr in soil remain largely unaffected, suggesting that non-reversible processes are involved such as inner sphere complexation or surface precipitation.

Conclusions:

In this study, batch equilibrium experiments were conducted with ZrOCl2 solutions and two different soils (an acidic sandy clayey loamy soil and a clayey calcareous soil). These experiments indicate very fast adsorption of Zr to soil (time constants (1/k) = 2.5-3 min). The Kp values resulting from this study are 6000 L/kg (dw) for the acidic soil and 30000 L/kg (dw) for the calcareous soil. Very low soil to solution ratios were used because otherwise concentrations in solution would become unquantifiable. Such low soil to solution ratios however favor adsorption. This should be kept in mind when using the Kp values resulting from this study. Desorption experiments indicated very limited desorption suggesting that non-reversible adsorption processes such as inner sphere complexation or surface precipitation are involved.

Reference 4

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Well performed field study. The results can be considered reliable with restrictions, keeping in mind that log Kp values were calculated based on average concentrations of Zr in water and suspended particulate matter.

Qualifier:

no guideline available

Principles of method if other than guideline:

Paired samples of (filtered) water and suspended particulate matter were taken monthly during one year at two locations along the St. Lawrence River: 1) at a municipal water filtration plant located near the mouth, and 2) at the Montreal City purification plant (treated urban effluents). Zr concentrations were determined in both filtered water and suspended particulate matter.

GLP compliance:

no

Type of method:

other: field study

Media:

other: suspended particulate matter

Radiolabelling:

no

Analytical monitoring:

yes

Details on sampling:

- All sample material was acid washed before use.
- 0.45 µm polycarbonate membrane filters were used for filtering water samples.
- Monthly sampling between July 2000 and July 2001 at the Lévis municipal water filtration plant located near the mouth of the St. Lawrence River and at the Montreal City purification plant (treated urban effluent, samples taken during 24 h proportionally to flow rate).
- Filtrate was acidified with ultrapure nitric acid to pH 2.
- Filters with residu were frozen until further treatment.
- Filters and their contents were oven-dried at 65°C for 12 h, digested with 2 mL of concentrated nitric acid and 1 mL of concentrated hydrofluoric acid first for 12 h at ambient air temperature and then for 1 h in an oven at 125°C, acids were then evaporated and residues diluted with 15 mL of 0.1 N nitric acid.

Details on test conditions:

Suspended matter concentrations varied between 16 and 27 mg/L during the sampling period.

Computational methods:

Concentration Zr in SPM (mg/kg) / concentration Zr in filtered river water (mg/L) = Kp.

Phase system:

suspended matter-water

Type:

log Kp

Value:

6.26

Matrix:

St. Lawrence River

Phase system:

suspended matter-water

Type:

log Kp

Value:

5.51

Matrix:

Effluent from Montreal City purification

Adsorption and desorption constants:

Average log Kp in St. Lawrence River water = 6.26 (mean Zr concentration in filtered water and SPM = 7.2 ng/L and 13.1 mg/kg, respectively).
Average log Kp in effluent from Montreal City purification plant = 5.51 (mean Zr concentration in filtered water and SPM = 22 ng/L and 7.13 mg/kg, respectively).

Statistics:

Average Zr concentrations were reported and used for Kp calculation.

Conclusions:

In this study, paired samples of (filtered) water and suspended particulate matter were taken monthly during one year at two locations along St. Lawrence River. At one location river water was sampled and at the other location effluent of the Montreal waste water treatment plant was sampled. Based on average concentrations of Zr in filtered water and suspended particulate matter, log Kp values of 6.26 and 5.51 were calculated for these locations, respectively.

Reference 5

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Well performed field study reporting paired measurements of Zr concentrations in filtered water and sediment samples from 20 sites along the Blesbokspruit, as well as the range of Kp values obtained from these measurements.

Qualifier:

no guideline available

Principles of method if other than guideline:

In this study, Zr concentrations were determined in paired samples of filtered water and sediment from 20 sites along the Blesbokspruit, South Africa. Based on the reported Zr concentrations, Kp values can be calculated for the individual sites.

GLP compliance:

no

Type of method:

other: field study

Media:

sediment

Radiolabelling:

no

Test temperature:

Between 5.7 and 24.2°C.

Analytical monitoring:

yes

Details on sampling:

- Paired samples of water and sediment were taken at 20 randomly chosen sites along the Blesbokspruit (both upstream and downstream of a liming plant and the site where mine water from Grootvlei Gold Mine is discharged).
- Surface water samples were filtered through a 0.45 µm nylon membrane filter using a hand-held vacuum pump.
- One of two filtered samples was acidified with 3 M HNO3 to a pH < 2 to prevent metal precipitation.
- Water samples were stored in plastic bottles pre-rinsed with HNO3 and deionized water. Bottles were sealed and stored at 4°C.
- Sediment samples were taken close to the sediment-water interface by inserting 50-mL polypropylene centrifuge vials.
- Vials were capped without leaving headspace and stored under anoxic conditions on ice.
- Digestion of sediment samples: 50 mg dried sediment + 4 mL of a 4:1 mixture of 28M HF and 14M HNO3, digested for 48 h at 50-60°C. After evaporation to dryness, 2 mL of 14M HNO3 was added repeatedly for digestion at 50-60°C until complete dissolution, followed by evaporation to dryness at 75°C. After cooling, samples were diluted 1000 times with an internal standard.

Details on matrix:

Sediment characteristics were reported in Table 3. Generally:
- % gravel: 0-44
- % sand: 11-85
- % silt: 8-73
- % clay: 4-43
- % organic carbon: 0.5-9.0
Some trends are reported in the results section.

Details on test conditions:

Chemistry of surface water samples:
- pH: 7-9.2
- dissolved oxygen: 2.5-10.8 mg/L
- conductivity: 521-2400 µS/cm
- Cl: 28-161 mg/L
- SO4: 73-702 mg/L (outlier 4194 mg/L)
- Na: 69-855 mg/L
- Ca: 21-195 mg/L
- Mg: 9-56 mg/L
- K: 9-32 mg/L

Computational methods:

Zr concentration in sediment (mg/kg) / Zr concentration in filtered water (mg/L) = Kp.

Phase system:

sediment-water

Type:

log Kp

Value:

5.47

Matrix:

Sediments from sites along the Blesbokspruit.

Remarks on result:

other: average

Phase system:

sediment-water

Type:

log Kp

Value:

5.12 - 5.92

Matrix:

Sediments from sites along the Blesbokspruit.

Remarks on result:

other: range

Adsorption and desorption constants:

Log Kp between 5.12 and 5.92, average value 5.47.
Zr concentrations in filtered water were between 0.1 and 0.5 µg/L.
Zr concentrations in sediment were between 26.5 and 175.7 mg/kg.

Statistics:

Separate values can be calculated, average Kp and Kp range reported.

Conclusions:

In this study, paired samples of filtered water and sediment were taken from 20 sites along the Blesbokspruit. Based on Zr concentrations in these filtered water and sediment samples, an average log Kp of 5.47 could be calculated, the range being 5.12-5.92.

Reference 6

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Well performed field study, reporting mean Kp values for elements determined in paired samples of (filtered) water and suspended particulate matter.

Qualifier:

no guideline available

Principles of method if other than guideline:

Field study in which paired samples were taken from water and suspended particulate matter (through 0.4 µm filtration) from 54 Czech rivers at 119 localities. Analysis of the metal under consideration was done in the laboratory in both filtered water and suspended particulate matter.

GLP compliance:

no

Type of method:

other: field study

Media:

other: suspended particulate matter

Radiolabelling:

no

Test temperature:

not reported (samples taken during summers of 1997 and 1998)

Analytical monitoring:

yes

Details on sampling:

- 54 Czech rivers, 119 sites
- During summers of 1997 and 1998
- Paired samples were taken at each location of water and suspended matter.
- Suspended particulate matter was sampled by filtration (volume of water filtered: 200 mL, filters used: perforated polycarbonate filters 0.4 µm).
- Treatment of (filtered) water samples: acidification using HNO3, cooled storage in vessels cleaned with 10% HNO3 for at least 2 days.
- Treatment of filters before use: boiled with distilled water acidified by HNO3 for 10 min before use, dried at 105°C, weighed.
- Treatment of used filters: dried at 105°C and weighed.
- Filtration system: Antlia hand pump filter system (Schleicher & Schnell), air pressure of 5.3 kg/cm2 (regularly washed with 0.01 M HNO3 and distilled water before filtration in the field).

Details on matrix:

Mean % particulate organic matter in suspended particulate matter = 47%.
Using a factor of 2.0 for conversion of POM to particulate organic carbon results in 23.5% (range = 5.9-43%).

Details on test conditions:

Field study - paired samples of water and SPM
- Mean contents of SPM = 9.9 mg/L (range = 1.0-124 mg/L)
- Mean pH river waters = 7.74 (range = 6.9-8.8)
- Mean ionic strength river waters = 7.8 mM
- Mean specific conductance river waters = 538 µS/cm at 25°C
- Mean alkalinity river waters = 1.9 mM

Computational methods:

Concentration Zr in SPM / concentration Zr in filtered river water = Kp.

Phase system:

suspended matter-water

Type:

log Kp

Value:

3.23

Remarks on result:

other: median value

Adsorption and desorption constants:

Median log Kp = 3.23 (median Zr concentration in filtered water = 0.07 mg/L, median Zr concentration in SPM = 109 mg/kg).

Statistics:

Medians were reported (for log Kp) because this allows taking into account sites where element concentration was below detection limits in filtered water and to eliminate the effect of several strongly polluted rivers.

Conclusions:

In this study, paired samples of filtered water and suspended particulate matter were taken from 119 sites along 54 Czech rivers and element concentrations were determined in both filtered water and suspended particulate matter. The suspended matter-water partition coefficient Kp was calculated by taking the ratio of the element concentration in suspended particulate matter to the element concentration in filtered water. The median log Kp for Zr was reported to be 3.23

Description of key information

Assessment of this endpoint and derivation of adsorption coefficients are element-based (i.e., not substance-based). Five studies reporting on adsorption of zirconium to particulate matter were used in a weight of evidence approach to cover the endpoint. 

Additional information

1. Information on zirconium (dioxide)

Adsorption of zirconium compounds (as such) to particles of suspended matter, sediment, or soil, is not expected to occur. It is rather the zirconium cation (or potentially other cationic zirconium species) that will adsorb to particulate matter. Therefore, the assessment of adsorption capacity and the derivation of adsorption coefficients is element-based (and not substance-based).

In total, five studies were considered sufficiently reliable to be included in the weight of evidence approach. Data were available for soil, sediment, and suspended matter and will be further discussed below.

For suspended matter, two studies were identified as useful and reliable (Klimisch score of 2). Veselý et al. (2001) reported a median log Kp of 3.23 for a series of samplings along Czech rivers. Gobeil et al. (2005) analysed samples from several locations along the St. Lawrence river, at one location river water was sampled and at the other location effluent of the Montreal waste water treatment plant was sampled. Based on average concentrations of zirconium in filtered water and suspended particulate matter, log Kp values of 6.26 and 5.51 were calculated for these locations. Because there is a limited amount of values available, the average log Kp (arithmetic mean) of 5.00 for these two studies is selected as key value for characterising distribution between suspended matter and water.

For sediment, only one reliable study is available (Klimisch score of 2). In this study, zirconium concentrations were determined in paired samples of filtered water and sediment from 20 sites along the Blesbokspruit, South Africa. Based on data from this study (Roychoudhury and Starke, 2006) an average log Kp value (arithmetic mean) of 5.47 was calculated, the range being 5.12-5.92.

For soil, two reliable studies (Klimisch score of 2) were retained for the determination of the key value for zirconium. Ferrand (2005) (see also Ferrand et al., 2006) conducted batch equilibrium experiments with ZrOCl2 solutions and two different soils (acidic sandy clayey loamy soil and a clayey calcareous soil). The Kp values resulting from this study were 6,000 L/kg (dw) (or log Kp of 3.78) for the acidic soil and 30,000 L/kg (dw) for the calcareous soil (or log Kp of 4.48). The average log Kp value (arithmetic mean) of 4.13 was taken as key log Kp for soil.

Overall, strong adsorption of zirconium to particulate matter is observed, whether soil, sediment, or suspended matter.

For adsorption to occur however, free zirconium has to end up in the aqueous phase of the environmental compartment under consideration (water column, or pore water in sediment/soil).

2. Information on tungsten (oxide)

No data on the adsorption of tungsten to particulate matter in the environment was included in the dossier.

3. Conclusion on tungsten zirconium oxide

Tungsten zirconium oxide is a substance of extremely low water solubility. The limited amounts of tungsten and zirconium that may be released from the substance upon its release to the environment will be subject to speciation and distribution. Any released zirconium can be expected to be strongly adsorbed to particulate matter in the environmental compartment under consideration. No data were included on the potential adsorption of tungsten to particulate matter in the environment, however, since the release of tungsten from tungsten zirconium oxide is expected to be limited, and since zirconium is the most dominant element in tungsten zirconium oxide on a weight by weight basis, it was considered justified to cover the endpoint using data on zirconium alone.