Common fluoride salts and compounds in aqueous solutions and their removal/purification technologies
In water, fluorine is normally discussed as dissolved fluoride (F−), not elemental fluorine. Fluoride may come from natural minerals or from industrial and treatment chemicals. The compounds below are representative sources and process materials; the original salt or reagent may dissolve, dissociate or react, so routine water analysis generally reports fluoride rather than identifying the original salt.
Managing elevated fluoride or fluoride-bearing process water? Treatment depends on fluoride concentration, pH, calcium and other ions, solids, target water quality and residual-stream handling. A representative analysis is needed to compare removal and recovery options.
From water analysis to a treatment concept
Characterise the stream (fluoride, pH, alkalinity, calcium, aluminium, sulfate, silica, TDS and suspended solids) → define the target (drinking water, process reuse, discharge or recovery) → compare treatment routes → evaluate residuals and verify performance. This is an indicative decision path, not a fixed plant design.
Common fluoride salts and related compounds
| Salt or compound | Formula | Typical application / industry / where used | Typical aqueous streams where encountered |
|---|---|---|---|
| Calcium fluoride (fluorite) | CaF2 | Naturally occurring fluoride mineral and an important fluorine-bearing mineral; also forms as a precipitate during some fluoride-removal processes. | Groundwater influenced by fluoride-bearing rocks; mineral-processing water; lime-treatment sludge or solids. |
| Sodium fluoride | NaF | Fluoride reagent used in selected water-treatment programmes and in glass, metal-surface and chemical processes. | Fluoride dosing solutions, equipment rinse water and relevant industrial process wastewater. |
| Sodium hexafluorosilicate (sodium fluorosilicate) | Na2SiF6 | Solid fluoride reagent used for drinking-water fluoridation in some jurisdictions and as an industrial chemical. | Reagent make-up and dosing streams, spills or rinse water; after dissolution and reaction, fluoride is measured in the water. |
| Potassium fluoride | KF | Fluoride reagent used in specialty chemical manufacture, glass processing and surface-treatment operations. | Industrial process solutions, etching or wash waters and equipment rinses where the reagent is used. |
| Ammonium fluoride | NH4F | Fluoride reagent used in glass, metal and chemical surface treatment. | Process baths, rinses and industrial wastewater containing fluoride and ammonium. |
| Ammonium bifluoride (ammonium hydrogen difluoride) | NH4HF2 | Acidic fluoride reagent used for glass etching and some metal-cleaning or surface-treatment processes. | Etching wastewater and acidic rinse streams; fluoride speciation depends on pH. |
| Fluorapatite | Ca5(PO4)3F | Fluoride-bearing phosphate mineral found in phosphate rock and some igneous rocks. | Phosphate mining and beneficiation water, tailings seepage and drainage affected by mineral processing. |
| Cryolite | Na3AlF6 | Fluoride-bearing mineral and industrial compound associated with aluminium production. | Selected aluminium-process waters, wash water and residues; the presence and dissolved fluoride depend on process chemistry. |
| Fluorosilicic acid (related fluoride reagent; not a salt) | H2SiF6 | Aqueous reagent used for drinking-water fluoridation in some jurisdictions and in industrial processes. | Reagent storage, dilution and dosing systems, plus related rinse or process water. |
These names describe solid salts, minerals or reagents. Once mixed into water, many dissolve or react; the resulting fluoride may be present as F−, with speciation affected by pH and other dissolved ions. Sodium hexafluorosilicate has comparatively low water solubility, while sodium fluoride is more soluble. Fluorite and fluorapatite are sparingly soluble minerals, but geochemical conditions can still allow fluoride to accumulate in groundwater over time. Pure-compound solubility alone does not predict the fluoride concentration in a real water stream.
Where fluoride enters water
Natural fluoride is associated with minerals such as fluorite and fluorapatite. Groundwater concentrations can vary with the geology and water chemistry; some aquifers develop elevated fluoride where calcium is removed from solution or where water-rock interactions favour fluoride release. Industrial sources can include glass etching, metal finishing, phosphate processing, aluminium production and fluoride chemical manufacture. Drinking-water fluoridation is used only in some jurisdictions and is governed by local policy and requirements.
Fluoride removal and treatment options
The appropriate process depends on whether the stream is a drinking-water source, a high-strength industrial wastewater or a concentrated brine. The routes below are options for evaluation, not a guarantee of a particular finished-water concentration.
| Water-treatment challenge | Potential process route | Selection notes |
|---|---|---|
| Elevated fluoride in groundwater or drinking-water feed | Activated alumina adsorption; reverse osmosis (RO) or nanofiltration (NF); ion exchange in suitable cases. | Performance depends on pH, alkalinity, competing ions, media or membrane selection, and regeneration or concentrate management. Confirm results by analysis of treated water. |
| High fluoride loading in industrial wastewater | Calcium-based chemical precipitation to form CaF2, followed by clarification or filtration; polishing may be needed. | Residual fluoride depends on feed chemistry and operating conditions. Sludge and solids require appropriate characterization and handling. |
| Acidic fluoride-bearing etching or rinse water | Equalisation and controlled pH adjustment, followed by precipitation and solids separation; additional treatment may be evaluated. | Acidity, ammonium, metals and complexing agents can affect reagent demand, treatment and residuals. |
| Fluoride in a saline or membrane-concentrate stream | Evaluate selective treatment, further concentration or a zero-liquid-discharge (ZLD) train where appropriate. | Assess calcium-fluoride scaling, co-ions, recovery objectives, energy and concentrate or solids management before selecting a process. |
For drinking-water guidance, the World Health Organization guideline value for fluoride is 1.5 mg/L. The EU Drinking Water Directive also sets a parametric value of 1.5 mg/L. These values apply within their respective guidance or regulatory contexts; requirements for a specific supply or industrial discharge must be checked with the responsible local authority.
Read more about reverse osmosis, nanofiltration, ion exchange, precipitation and zero liquid discharge. For fluoride occurrence and health context, see fluoride in water.
Developing a treatment solution
A representative water analysis helps identify the treatment objective, process options and residual-management requirements. Where appropriate, laboratory testing and pilot-scale validation can help evaluate performance under representative conditions before a full-scale design is selected.
Discuss a water-treatment challenge
To discuss a specific water composition or fluoride-removal objective, contact Lenntech at info@lenntech.com.
Last reviewed: 7 November 2026 by Lenntech.
Sources and further reading
- World Health Organization: Chemical fact sheet on fluoride.
- European Union: Drinking Water Directive (EU) 2020/2184, Annex I.
- U.S. Geological Survey: Fluoride in thermal and non-thermal groundwater.
- Australian Drinking Water Guidelines: Sodium fluorosilicate.
- PubChem: Sodium fluoride.
- PubChem: Sodium hexafluorosilicate.
- U.S. EPA: Design manual for fluoride removal by activated alumina.
- U.S. EPA: Effluent guidelines document describing fluoride wastewater treatment by precipitation.