Hydrogen fluoride

Adsorption / desorption

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

adsorption / desorption, other

Remarks:

Published adsorption / desorption data

Type of information:

other: literature review

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Remarks:

Published literature review from the World Health Organisation

Qualifier:

no guideline followed

Principles of method if other than guideline:

Various studies are summarised in the review

GLP compliance:

not specified

Remarks:

literature review of published studies

Type of method:

other: various

Media:

soil

Specific details on test material used for the study:

Although sodium fluoride has been identified as the test material for the current study summary, it should be noted that the publication (WHO, 2002) is based on adsorption/desorption data for various fluoride species, not just sodium fluoride.

Radiolabelling:

not specified

Test temperature:

No information on temperature is provided in the review

Remarks on result:

other: Fluoride adsorption in soil is strongly dependent on pH. Fluoride is generally extremely immobile in soil.

Factors that influence the mobility of inorganic fluorides in soil are pH and the formation of aluminium and calcium complexes. In more acidic soils, concentrations of inorganic fluoride are 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. The fate of inorganic fluorides released to soil also depends on the chemical form, rate of deposition, soil chemistry and climate.

Fluoride in soil is mainly bound in complexes. The maximum adsorption of fluoride to soil was reported to occur at pH 5.5. In acidic soils with pH below 6, most of the fluoride is in complexes with either aluminium or iron. 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.

Fluoride binds to clay by displacing hydroxide from the surface of the clay. 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.

The changes in free fluoride ions and total fluoride levels following equilibrium of either poorly soluble fluoride species, such as calcium fluoride and aluminium fluoride or wastes from aluminium smelters, were determined. 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 also shown to be extremely immobile in soil as determined by lysimeter experiments: 75.8–99.6% of added fluoride was retained by loam soil for 4 years and was correlated with the soil aluminium content. Soil phosphate levels may also contribute to the mobility of inorganic fluoride. It is reported that approximately 0.5–6.0% of the annual addition of fluoride (from atmospheric pollution and artificial fertilizers) to a forest and agricultural areas was leached from the surface to lower soil horizons. It has also been 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 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. The activity of the fluoride ion in acid sandy soils that had been limed was studied. 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. Low amounts of fluoride are reported to be 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, are reported to eb 377, 0.38 and 21.7 mg/kg for total fluoride, water-soluble fluoride and resin-exchangeable fluoride, respectively.

The water-soluble fluoride in sodic surface soil treated with gypsum increased with increasing exchangeable sodium levels. The increase in exchangeable sodium also caused an increase in soil pH, which in turn caused an increase in water-soluble fluoride. Incubation studies reveal 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.

Insoluble calcium fluoride is reported to be 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. 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 it was 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. It was thought that the fluoride was present as the insoluble calcium fluoride.

The vertical distribution of fluoride in the soil profiles sampled near an industrial region was determined. 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. 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. In the uncontaminated soil, losses of aluminium from the acid soil were higher than those from the calcareous soil. 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. 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.

Validity criteria fulfilled:

not specified

Conclusions:

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.

Executive summary:

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.

Reference 2

Endpoint:

adsorption / desorption, other

Remarks:

Published adsorption / desorption data

Type of information:

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

Adequacy of study:

key study

Justification for type of information:

Hydrogen fluoride is known to rapidly react upon contact with water to form fluoride, therefore available data from studies with fluoride are given as indication of sorption properties of hydrogen fluoride.

Reason / purpose for cross-reference:

read-across source

Media:

soil

Remarks on result:

other: Fluoride adsorption in soil is strongly dependent on pH. Fluoride is generally extremely immobile in soil.

Factors that influence the mobility of inorganic fluorides in soil are pH and the formation of aluminium and calcium complexes. In more acidic soils, concentrations of inorganic fluoride are 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. The fate of inorganic fluorides released to soil also depends on the chemical form, rate of deposition, soil chemistry and climate.

Fluoride in soil is mainly bound in complexes. The maximum adsorption of fluoride to soil was reported to occur at pH 5.5. In acidic soils with pH below 6, most of the fluoride is in complexes with either aluminium or iron. 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.

Fluoride binds to clay by displacing hydroxide from the surface of the clay. 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.

The changes in free fluoride ions and total fluoride levels following equilibrium of either poorly soluble fluoride species, such as calcium fluoride and aluminium fluoride or wastes from aluminium smelters, were determined. 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 also shown to be extremely immobile in soil as determined by lysimeter experiments: 75.8–99.6% of added fluoride was retained by loam soil for 4 years and was correlated with the soil aluminium content. Soil phosphate levels may also contribute to the mobility of inorganic fluoride. It is reported that approximately 0.5–6.0% of the annual addition of fluoride (from atmospheric pollution and artificial fertilizers) to a forest and agricultural areas was leached from the surface to lower soil horizons. It has also been 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 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. The activity of the fluoride ion in acid sandy soils that had been limed was studied. 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. Low amounts of fluoride are reported to be 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, are reported to eb 377, 0.38 and 21.7 mg/kg for total fluoride, water-soluble fluoride and resin-exchangeable fluoride, respectively.

The water-soluble fluoride in sodic surface soil treated with gypsum increased with increasing exchangeable sodium levels. The increase in exchangeable sodium also caused an increase in soil pH, which in turn caused an increase in water-soluble fluoride. Incubation studies reveal 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.

Insoluble calcium fluoride is reported to be 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. 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 it was 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. It was thought that the fluoride was present as the insoluble calcium fluoride.

The vertical distribution of fluoride in the soil profiles sampled near an industrial region was determined. 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. 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. In the uncontaminated soil, losses of aluminium from the acid soil were higher than those from the calcareous soil. 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. 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.

Validity criteria fulfilled:

not specified

Conclusions:

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.

Executive summary:

Hydrogen fluoride is known to rapidly react upon contact with water to form fluoride, therefore available data from studies with fluoride are given as indication of sorption properties of hydrogen fluoride.

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.

Description of key information

Hydrogen fluoride is known to rapidly react upon contact with water to form fluoride, therefore available data from studies with fluoride are given as indication of sorption properties of hydrogen fluoride.

Only qualitative data are available regarding the sorption characteristics of fluorides. In soil, fluoride is mainly bound in complexes with aluminium, iron or calcium dependent upon the pH and availability of these counter ions. Fluoride binds to clay by displacing hydroxide from the surface of the clay. Adsorption of fluorides is shown to be strongly dependent upon pH and fluoride concentration, with adsorption to the soil solid phase shown to be stronger at slightly acidic pH values (5.5–6.5). Low affinity of fluorides for organic material results in leaching from the more surface horizon and increased retention by clay minerals and silts in the more alkaline, deeper horizons. Increased amounts of fluoride are released from fluoride salts and fluoride-rich wastes in soil with high cation exchange capacity. This effect is greatest when there were more exchange sites available and when the fluoride compound cation had greater affinity for the exchange material. Fluoride is shown to be extremely immobile in soil as determined by lysimeter experiments, which showed that 75.8-99.6% of added fluoride was retained by loam soil for 4 years and was correlated with the soil aluminium oxides/hydroxides content. Adsorption data in sediment is not available.

From the data available, no actual Koc values can be determined. At neutral pH the major part of fluoride retention in soil appears to be a result of formation of complexes. True adsorption of fluoride and consequential formation of equilibrium between soil/sediment and porewater is not expected based on the anionic character of fluoride. Therefore, fluoride is assumed to have low solids-water partitioning coefficients in the different environmental compartments. For pragmatic reasons, for environmental exposure assessment a calculated Koc of 1.959 L/kg was calculated using the QSAR for non-hydrophobics in EUSES 2.2.0, based on a log Kow of -1.4 as suggested in the EU-RAR for hydrogen fluoride.

Key value for chemical safety assessment

Koc at 20 °C:

1.959

Additional information

Relevant information are reported in the EHC (2002). 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 essentially immobile, and is not readily leached from soil.