Molybdic acid

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Well documented experimental investigations.

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

2009

Materials and methods

Objective of study:

other: in vitro bioavailability

Test guideline

Qualifier:

no guideline followed

Principles of method if other than guideline:

Solubility of test items in artificial biological fluids.

GLP compliance:

no

Test material

Test material information

Constituent 1

Reference substance name:

molybdenum metal, molybdenum dioxide, molybdenum trioxide, roasted molybdenite concentrate, sodium molybdate dihydrate, ammonium octamolybdate, ferromolybdenum, molybdenum disulfide

IUPAC Name:

molybdenum metal, molybdenum dioxide, molybdenum trioxide, roasted molybdenite concentrate, sodium molybdate dihydrate, ammonium octamolybdate, ferromolybdenum, molybdenum disulfide

Details on test material:

Several different molybdenum substances, which are within the scope of REACH registrations prepared by the Molybdenum Consortium were tested, as follows: molybdenum metal, molybdenum dioxide, molybdenum trioxide, sodium molybdate dihydrate, ammonium octamolybdate and roasted molybdenite concentrate.

Radiolabelling:

no

Test animals

Species:

other: in vitro (simulated human body fluids)

Details on test animals or test system and environmental conditions:

The different substances were exposed to five different test media covering relevant pHs ranging from about 1.5 to 7.4. The test media were:
• Phosphate-buffered saline (PBS, pH 7.4), a standard physiological solution that mimics the ion strength of human blood serum. It is generally used within research and medical health care and normally serves as reference test solution for comparison of data under simulated physiological conditions.
• Gamble’s solution (GMB, pH 7.4) mimics interstitial fluid within the deep lung at normal health conditions.
• Artificial sweat (ASW, pH 6.5) which simulates the hypoosmolar fluid, linked to hyponatraemia (loss of Na+ from blood), which is excreted from the body upon sweating.
• Artificial lysosomal fluid (ALF, pH 4.5) simulates interstitial conditions in the lung occurring in conjunction to phagocytosis by cells, which involves relatively aggressive conditions similar to an immunologic reaction of the body.
• Artificial gastric fluid (GST, pH 1.5) mimics the very aggressive digestion milieu of high acidity in the stomach.

The test media where chosen to simulate a relevant inhalation or ingestion scenarios. It should be stressed that the different test media only simulate physiological conditions to a limited extent. The complexity and function of the real body fluids are difficult to simulate, however, in vitro results in synthetic biological media can, in a simple way, provide information that could be relevant for a real situation.

The test solutions were prepared using ultra-pure water and chemicals of analytical grades. The chemical composition (g/L) of ALF and GMB is presented below. The pH of ALF and GMB was adjusted using 50% NaOH and 25% HCl respectively. The pH of PBS was adjusted with 50% NaOH.

Artificial gastric fluid was prepared according to the ASTM standard (ASTM D5517).
Reference: ASTM D5517-03 (2003), ”Standard Test Method for Determining Extractability of Metals from Art Materials”

Administration / exposure

Details on study design:

Experimental Procedure
A particle loading of 0.1 g/L was selected since it is experimentally feasible even when low concentrations of released metal are expected. It is also used for testing according to the OECD transformation/dissolution protocol for sparingly soluble metals and metal compounds, which facilitates comparison with other data of metal release/dissolution (OECD, 2001).

The time periods for the exposure were 2 and 24 hours, which were selected to be relevant for a fictitious inhalation and ingestion scenario and to enable comparison with other metal ion release/dissolution data often generated after this time period. The approximate time for digestion is about 2 hours. The 24 hour exposure was selected as a standard time duration that is relatively easy to compare with metal ion release/dissolution data as well as toxicity data for further evaluation of the bioaccessibility of released metals.

Triplicate samples were prepared for exposure to each of the five different test media for two time periods, respectively. In addition, one blank sample containing only the test solution was incubated together with the triplicates for each time period. In each sample vial, 5 ± 0.5 mg of the samples were weighed into a TPX Nalgene® jar. Then, 50 mL of the test solution were added to the powder sample before incubation under gentle bi-lineary shaking (25 cycles per minute) at 37 ± 2 °C.

Details on dosing and sampling:

After the testing period, the samples were allowed to cool to ambient room temperature before the final pH of the test solution was measured. The test medium was then separated from the powder particles by centrifugation at 3000 rpm for 10 minutes, resulting in a visually clear supernatant. The supernatant solution was decanted into a polypropylene storage flask and acidified to a pH less than 2 (not needed in the case of artificial gastric fluid) with 65% pure HNO3 prior to solution analysis.

Results and discussion

Main ADME results

Main ADME results 1

Type:

other: Bioavailability based on dissolution in artificial biological fluids

Results:

For the purpose of human health risk assessment, the substances sodium molybdate, ammonium octamolybdate, molybdenum trioxide and roasted molybdenum concentrate are considered to be of high bioavailability.

Main ADME results 2

Type:

other: Bioavailability based on dissolution in artificial biological fluids

Results:

For the purpose of human health risk assessment, the substances molybdenum (metal), ferromolybdenum, molybdenum dioxide and molybdenum disulfide are considered to be of negligible bioavailability.

Any other information on results incl. tables

Results and discussion:

 

Specific surface area (m²/g)

 

Test items	Mo (1)	FeMo (2)	MoO2(3)	MoO3(4)	Na2MoO4∙2H2O(5)	(NH4)4Mo8O26(6)	MoS2(7)	MoO3‐a (8)	RMC (9)
BET area	0.23	0.11	1.18	2.97	1.15	1.82	4.15	4.59	0.32

 

SEM images

 

See attached graphs.

 

 

Surface composition (XPS analysis)

 

For the substances molybdenum (1) and ferromolybdenum (2) approximately 1/3 of the surface atoms were oxygen, indicating the expected presence of an oxide layer on these metallic materials. All other test item showed the expected elemental surface composition, in accordance with their chemical nature.

 

 

Bioaccessibility data / dissolution in synthetic biological fluids

 

The measured molybdenum concentrations in the various media and after two time points were used to calculate the fraction of the sample that has dissolved. The results are presented in the table below (variability omitted here for reasons of clarity). All samples were introduced into the media at a loading of 0.1 g/L, with the exception of sample (4), MoO₃, which was tested at a loading of 1 g/L.

 

Table: Fraction of material dissolved (%)

(Test item) Material	Exposure period	GMB pH 7.4	PBS pH 7.2	ASW pH 6.5	ALF pH 4.5	GST pH 1.6
(1) Mo	2h 24h	0.17 0.17	0.32 6.1	0.32 2.4	0.21 0.58	0.44 0.50
(2) FeMo	2h 24h	0.44 0.64	0.58 2.2	1.8 12.7	0.27 2.5	0.16 0.85
(3) MoO2	2h 24h	0.16 0.25	0.18 0.48	0.14 0.93	0.17 0.25	0.28 0.08
(4) MoO3	2h 24h	26.7 95.3	42.5 95.1	42.0 106	7.8 77.2	2.1 10.4
(5)Na2MoO4.H2O	2h 24h	78.7 89.4	85.2 80.2	80.8 82.0	47.1 58.2	84.8 83.2
(6)(NH4)4Mo8O26	2h 24h	78.9 67.8	81.5 86.5	56.0 88.0	36.6 66.7	64.1 94.0
(7) MoS2	2h 24h	0.007 0.04	0.001 0.007	0.008 0.02	< DL  0.002	0.006 0.004
(8) MoO3‐a	2h 24h	100.6 98.6	91.9 94.2	n/a	48.2 51.1	41.9 71.0
(9) RMC	2h 24h	36.5 114.5	50.3 101.6	n/a	28.2 55.6	6.6 24.8

 

 

Discussion:

 

The tested substances can be divided into two groups:

 

A) The molybdates sodium molybdate (5) and ammonium octamolybdate (5), as well as molybdenum trioxide (samples 4 and 8) dissolved completely or almost completely already after 2 hours in solution or after 24 hours at the latest. Into the same group falls roasted molybdenite concentrate, which - from a chemical standpoint - is essentially molybdenum trioxide. The dissolution of the oxides in artificial gastric juice is less than in other media, as a result of the low pH of the medium. The dissolution of MoO₃in aqueous media is in fact a chemical reaction which liberates protons. This reaction is therefore considerably restricted in acidic solutions such as artificial gastric juice (which is essentially 0.1% hydrochloric acid).For the purpose of human health risk assessment, the substances sodium molybdate, ammonium octamolybdate, molybdenum trioxide and roeasted molybdenum concentrate are considered to be of high bioavailability.

 

B) Dissolution of the other substances is considerably less.

Molybdenum disulfide (7) hardly dissolved at all. Less than 0.04% of the added MoS₂material dissolved, independently of medium or exposure time. Less than 1% of molybdenum dioxide (3) dissolved after 24h in all media. Molybdenum metal and ferromolybdenum, in which the metals are in their elemental form (oxidations state zero), show similar dissolution behaviour. For these two substances the dissolved molybdenum and iron is derived from the surface layer of oxides on these materials (existence of this oxide layer shown by XPS surface analysis). The dissolved concentrations will depend on the extent of the layer and how strongly it adheres to the substrate (the underlying compound). Once the oxide layer is dissolved, molybdenum metal and ferromolybdenum are considered to be inert. Even the exposure to artificial gastric juice (which is essentially 0.1% hydrochloric acid) does not lead to a significant dissolution of these metallic materials.

For the purpose of human health risk assessment, the substances molybdenum (metal), ferromolybdenum, molybdenum dioxide and molybdenum disulfide are considered to be of negligible bioavailability.

 

 

 

 

 

Applicant's summary and conclusion

Conclusions:

For the purpose of human health risk assessment and regarding systemic bioavailability, molybdenum substances can be grouped into high or negligible bioavailability.