Calcium fluoride

Adsorption / desorption

Administrative data

Endpoint:

adsorption / desorption, other

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Data source

Reference

Reference Type:

publication

Title:

Environmental Health Criteria 227: Fluorides

Author:

World Health Organization (WHO)

Year:

2002

Bibliographic source:

ISBN 92 4 157227 2; ISSN 0250-863X

Materials and methods

Test guideline

Qualifier:

no guideline followed

Principles of method if other than guideline:

Review publication, no information on method avaialble.

GLP compliance:

not specified

Test material

Test material information

Constituent 1

Reference substance name:

Fluoride (cation unspecified)

Cas Number:

16984-48-8

Molecular formula:

F

IUPAC Name:

Fluoride (cation unspecified)

Radiolabelling:

no

Results and discussion

Adsorption coefficient

Key result

Type:

log Koc

Remarks on result:

not measured/tested

Any other information on results incl. tables

Environmental transport, distribution and transformation of fluorides

The transport and transformation of fluoride in water are influenced by pH, water hardness and the presence of ion-exchange materials such as clays. Fluoride is usually transported through the water cycle complexed with aluminium. 

The transport and transformation of fluoride in soil are influenced by pH and the formation of predominantly aluminium and calcium complexes. Adsorption to the soil solid phase is stronger at slightly acidic pH values (5.5–6.5). Fluoride is not readily leached from soils. 

In more acidic soils, concentrations of inorganic fluoride were considerably higher in the deeper horizons. The low affinity of fluorides for organic material results in leaching from the more acidic surface horizon and increased retention by clay minerals and silts in the more alkaline, deeper horizons (Davison, 1983; Kabata-Pendias & Pendias, 1984). This distribution profile is not observed in either alkaline or saline soils (Gilpin & Johnson, 1980; Davison, 1983). The fate of inorganic fluorides released to soil also depends on the chemical form, rate of deposition, soil chemistry and climate (Davison, 1983).

Fluoride in soil is mainly bound in complexes. The maximum adsorption of fluoride to soil was reported to occur at pH 5.5 (Barrow & Ellis, 1986). In acidic soils with pH below 6, most of the fluoride is in complexes with either aluminium or iron (e.g., AlF²⁺, AlF₂⁺, AlF₃⁰, AlF₄^–, FeF²⁺, FeF₂⁺, FeF₃⁰) (Perrott et al., 1976; Murray, 1984b; Elrashidi & Lindsay, 1986). Fluoride in alkaline soils at pH 6.5 and above is almost completely fixed in soils as calcium fluoride, if sufficient calcium carbonate is available (Brewer, 1966).

Fluoride binds to clay by displacing hydroxide from the surface of the clay (Huang & Jackson, 1965; Bower & Hatcher, 1967; Meeussen et al., 1996). The adsorption follows Langmuir adsorption equations and is strongly dependent upon pH and fluoride concentration. It is most significant at pH 3–4, and it decreases above pH 6.5.

Pickering et al. (1988) determined changes in free fluoride ions and total fluoride levels following equilibration of either poorly soluble fluoride species, such as calcium fluoride and aluminium fluoride, or wastes from aluminium smelters. The experiments were carried out on materials that had different cation-exchange capacities, such as synthetic resins, clay minerals, manganese oxide and a humic acid. Increased amounts of fluoride were released from fluoride salts and fluoride-rich wastes when solids capable of exchanging cations were present. The effect was greatest when there were more exchange sites available and when the fluoride compound cation had greater affinity for the exchange material. In a few cases, soluble complex ions were formed when the released fluoride attacked the substrate, such as illite or alumina wastes.

Fluoride is extremely immobile in soil, as determined by lysimeter experiments. MacIntire et al. (1955) reported that 75.8–99.6% of added fluoride was retained by loam soil for 4 years. Fluoride retention was correlated with the soil aluminium content. The leaching of fluoride occurred simultaneously with the leaching of aluminium, iron and organic material from soil (Polomski et al., 1982). Soil phosphate may contribute to the mobility of inorganic fluoride (Kabata-Pendias & Pendias, 1984). Oelschläger (1971) reported that approximately 0.5–6.0% of the annual addition of fluoride (atmospheric pollution and artificial fertilizers) to a forest and agricultural areas was leached from the surface to lower soil horizons. Arnesen & Krogstad (1998) found that fluoride (added as sodium fluoride) accumulation was high in the upper 0–10 cm of soil columns, where 50–90% of the accumulated fluoride was found. The B-horizons sorbed considerably more fluoride than the Ah-horizons, due to higher content of aluminium oxides/hydroxides. A study by McLaughlin et al. (2001) involving long-term application of phosphate fertilizers has shown a large portion of fluoride applied as impurities in the fertilizer to remain in the 0- to 10-cm depth of the soil profile.

In sandy acidic soils, fluoride tends to be present in water-soluble forms (Shacklette et al., 1974). Street & Elwali (1983) determined the activity of the fluoride ion in acid sandy soils that had been limed. Fluorite was shown to be the solid phase controlling fluoride ion activity in soils between pH 5.5 and 7.0. At pH values below 5.0, the fluoride ion activity indicated supersaturation with respect to fluorite. These data indicate that liming of acid soils may precipitate fluorite, with a subsequent reduction in the concentration of fluoride ion in solution.

Murray (1984b) reported that low amounts of fluoride were leached from a highly disturbed sandy podzol soil of no distinct structure. Even at high fluoride application rates (3.2–80 g per soil column of diameter 0.1 m with a depth of 2 m), only 2.6–4.6% of the fluoride applied was leached in the water-soluble form. The pH of the eluate increased with increasing fluoride application, and this was probably due to adsorption of fluoride, releasing hydroxide ions from the soil metal hydroxides. Over time, the concentration of water-soluble fluoride decreased due to increased adsorption on soil particles.

Mean soil concentrations in Pennsylvania, USA, were 377, 0.38 and 21.7 mg/kg for total fluoride, water-soluble fluoride and resin-exchangeable fluoride, respectively. The authors suggested that fluoride is relatively immobile in soil, since most of the fluoride was not readily soluble or exchangeable (Gilpin & Johnson, 1980).

The water-soluble fluoride in sodic surface soil treated with gypsum increased with increasing exchangeable sodium per cent (Chhabra et al., 1979). The increase in exchangeable sodium per cent also caused an increase in soil pH, which in turn caused an increase in water-soluble fluoride. Incubation studies revealed that a major portion of the added fluoride was adsorbed to soil within the first 8 days. Adsorption to soil followed Langmuir isotherms up to an equilibrium soluble fluoride concentration (11.4 mg/litre), with precipitation at higher concentrations.

Calcium fluoride was formed in soils irrigated with fluoride solutions. Calcium fluoride is formed when the fluoride adsorption capacity is exceeded and the fluoride and calcium ion activities exceed the ion activity product of calcium fluoride (Tracy et al., 1984). Less than 2% of applied fluoride was measured in the leachate, and between 15 and 20% of added fluoride was precipitated as calcium fluoride. Fluoride was precipitated in the upper profile, although the authors expected that once the adsorption mechanisms were exceeded, soluble fluoride would leach deeper into the soil with continued irrigation.

A large fraction of the fluoride in topsoil sampled at a distance of 0.5–1.0 km from an aluminium smelter was reported to be in water-soluble form (Polomski et al., 1982). The authors concluded that the fluoride was present as calcium fluoride.

Breimer et al. (1989) determined the vertical distribution of fluoride in the soil profiles sampled near an industrial region. In calcareous soils, fluoride (as extractable with hydrochloric acid) was restricted to the top 40–50 cm, probably due to the precipitation of calcium fluoride in the presence of lime. A slight leaching of fluoride into the Bt and C horizons was reported in non-calcareous soils. Water-extractable fluoride showed an increase with depth in the A horizons and subsequently decreased to base levels in the lower subsoil.

The adsorption of fluoride from the water phase may be an important transport characteristic in calcareous soils at low flow rates, but this exchange may be rate-limited at high flow rates (Flühler et al., 1982). Dissolved fluoride concentrations may be high around the root zone in soils with a high fluoride input such as from atmospheric deposition. The high concentrations exist only for a limited time until the fluoride is withdrawn from the solution. The adsorption isotherm was reported to be non-linear between initial concentrations of 10 and 50 mg fluoride/litre. Retention of fluoride in uncontaminated calcareous soil was higher than retention in calcareous soil from areas with fluoride contamination. The adsorption and desorption of fluoride in acidic soil were not related to previous fluoride contamination.

Fluoride-containing solutions increased the mobilization and leaching of aluminium from soils. Leaching of aluminium was reported to be greater from soil contaminated from an aluminium smelter than from uncontaminated soil (Haidouti, 1995). In the uncontaminated soil, losses of aluminium from the acid soil were higher than those from the calcareous soil. Arnesen (1998) also found that fluoride can solubilize aluminium, iron and organic material and can increase soil pH through exchange with hydroxide ions.

Unlike other soluble salts, fluoride was not leached from naturally salinized salt-affected soil. It was redistributed within the soil profile (Lavado et al., 1983). The adsorption of fluoride to soils increased with decreasing pH within the pH range 8.5–6. Retention of fluoride in the soil was positively correlated with ammonium acetate extractable iron.