Magnesium fluoride

Endpoint summary

Description of key information

Magnesium fluoride (MgF2) is inorganic white crystalline salt composed of one magnesium ion and two fluoride ions, and is used for optical elements. The biodegradation and hydrolysis is not a relevant fate process for an inorganic substance and the bioavailable constituents of the substance. The substance profile (low partition coefficient and vapour pressure) suggests that aquatic media is the target compartment of concern.

In aqueous solution, it is dissociated into the magnesium ion (Mg2+) and the fluoride anion (F -). MgF2 is a stable structure that in contact with water will form complexes of fluoride and magnesium (Buckner et al.). A complete reaction can occur high temperature. It has been investigated by Donald R. Messier (1965):

                            1. MgF2+H20 --> MgF2.H2O

                            2. MgF2.H2O --> Mg(OH)F+ HF

                            3. Mg(OH)F --> MgO + HF

There is a primary formation of a hydroxyfluoride complex stable (1 and 2) and decomposition of the hydroxyfluoride complex to form products (3). In general, dilution and dispersion in the environment will favour the formation of hydrolytic species. Therefore, the hydrolysis is not a relevant fate process for the inorganic substance.

MgF2 is a poorly soluble substance composed of 2 components (Mg and F) with different solubility. Therefore, the environmental fate and pathways of the degradation products in the aquatic compartment is assessed.

Aquatic compartment

Magnesium is found in seawater and plant and animal tissues and is an essential element for human survival. In water, the transport and transformation of inorganic fluorides are influenced by pH, water hardness and the presence of ion-exchange materials such as clays (Environment Canada, 1994). Fluoride is usually transported through the water cycle complexed with aluminium (Ares, 1990). In areas of extreme acidity and alkalinity, inorganic fluorides may leach from fluoride-containing minerals into surface water or groundwater Cuker & Shilts, 1979).The proportion of fluoride in the form of magnesium fluoride (MgF2) ranged from 0.4 to 33.7%. Fluoride concentrations were reported to increase with salinity (within a salinity range of 0.1–16%). In seawater fluoride is divided among the following fractions (Stumm & Morgan, 1981):

Fraction	Proportion (%)	Concentration (mol/litre)
F–	51	4.1 × 10–5
MgF+	47	3.7 × 10–5
CaF+	2	1.6 × 10–6

In seawater, fluorides are removed by the formation of complexes with calcium compounds, principally carbonate and phosphate (Carpenter, 1969). Undissolved fluoride is generally removed from the aquatic phase by sedimentation (US EPA, 1980). Carpenter (1969) calculated a residence time for fluoride in ocean sediment to be 2–3 million years. Fluor silicic acid and hydrofluoric acid in high aquatic concentrations such as may be found in industrial waste ponds may volatilize, releasing silicon tetrafluoride and hydrogen fluoride into the atmosphere (US NAS, 1971).

Therefore, magnesium and fluorides sources are natural and/or anthropogenic. The transportation, distribution and transformation are influenced by factors like pH, water hardness and the presence of ion-exchange materials such as clays.