Common Selenium Salts and Compounds in Water
Selenium in water is usually discussed as dissolved selenate and selenite oxyanions, rather than as intact salt molecules. The salts below are useful reference compounds because they can dissolve and release these species. Their solubility and the best way to remove or recover selenium depend on temperature, pH, redox conditions, sulfate and other competing ions.
Key distinction: sodium and potassium selenates and selenites are soluble compounds. In natural waters, selenium speciation is controlled by water chemistry; poorly soluble metal selenides and elemental selenium are generally solid phases, not common dissolved salts.
For background on selenium sources and chemistry, see Selenium in water.
Common selenium salts and compounds
| Salt or compound | Formula | Approximate water solubility | Typical applications or occurrence | Water-treatment relevance |
|---|---|---|---|---|
| Sodium selenate | Na2SeO4 | Very soluble; reported about 41.5 g per 100 g water at 20 °C (83 g per 200 g water). | Reference and process compound; selenate can occur in oxygen-rich, alkaline waters affected by selenium-bearing geology or industrial activity. | Produces mobile Se(VI) oxyanions. Often harder to remove by iron-based adsorption or conventional clarification than selenite; anion exchange, membranes, or reduction-based processes may be considered. |
| Sodium selenite | Na2SeO3 | Very soluble; reported about 85 g per 100 g water at 20 °C (reported values use different water-volume or mass bases). | Reference, reagent and process compound; selenite may occur in mildly oxidizing waters and selenium-bearing process streams. | Produces Se(IV) oxyanions, which generally adsorb more strongly to iron oxides than selenate and can be more amenable to iron coagulation or adsorption. |
| Potassium selenate | K2SeO4 | Soluble in water; exact reported values vary with temperature and data source. | Specialty chemical and reference compound; not usually assumed to be the specific salt present in natural water. | Dissolves to potassium and selenate ions; treatment is governed mainly by the selenate species and water matrix, not by the original counter-ion. |
| Potassium selenite | K2SeO3 | Soluble in water; quantitative values depend on temperature and reference conditions. | Specialty chemical and reference compound; may represent a Se(IV) source in process or laboratory waters. | Dissolves to potassium and selenite species. Iron-based adsorption/coagulation, anion exchange, membranes or reduction may be evaluated for the actual stream. |
| Metal selenides (examples: CuSe, CdSe) | CuSe, CdSe | Low solubility; behavior depends on solid phase and conditions. | Mineral, metallurgical and materials-processing solids; may occur in reduced residues or particulates. | Usually addressed as solids or residuals. Analyze dissolved and particulate selenium separately before choosing treatment. |
| Elemental selenium | Se0 | Insoluble in water. | May form in reducing environments or as a product of biological or chemical selenium reduction. | Must be separated from water after formation, for example by solids separation appropriate to particle size and process design. |
Solubility figures are approximate reference values for pure compounds, not predictions of selenium concentration or speciation in a natural or industrial water. Hydration state, temperature, solution composition and the measurement basis can change reported values. In dilute environmental waters, selenium is present at trace levels as species governed by pH and redox conditions, not as a saturated solution of a named salt.
Where selenium species occur
Selenate (SeO42−) is generally more mobile under oxidizing conditions, particularly in alkaline waters, because it is highly soluble and often adsorbs weakly to mineral surfaces. Selenite (SeO32−) is commonly retained more strongly by iron oxyhydroxides. Under reducing conditions, selenium may be converted to elemental selenium, selenides or organic forms. The dominant form varies with pH, redox potential, mineralogy, microbial activity and competing ions.
Removal, separation and recovery options
Process selection depends on whether selenium is mainly selenate or selenite, the required effluent quality, concentration, sulfate and other competing ions, salinity, pH and the handling route for residuals. The options below are general approaches; treatability testing is needed for a specific water.
- Iron-based coagulation and adsorption: Ferric salts or iron-rich media can capture selenium, especially selenite, through adsorption or co-precipitation. Selenate is often more difficult to remove this way. Iron-bearing solids must be separated and managed. See coagulation and flocculation and adsorption.
- Anion exchange: Strong-base anion exchange resins can remove dissolved selenium oxyanions. Sulfate and other anions compete for resin capacity, and regeneration produces a concentrated selenium-bearing brine requiring further treatment or recovery. See ion exchange.
- Reverse osmosis or nanofiltration: Membranes can separate dissolved selenium from permeate and concentrate it in a smaller reject stream. Performance depends on species, membrane selection and feed chemistry; the concentrate still requires management or recovery. See reverse osmosis.
- Biological or chemical reduction: Reduction can convert soluble oxyanions to less-soluble elemental selenium. The resulting particles or biomass then need to be separated, and process conditions must be controlled to avoid transferring the problem to an unmanaged residual.
- Combined treatment: Pretreatment, reduction, clarification, filtration, ion exchange or membranes may be combined when a single step does not meet the treatment objective. The sequence should be based on water analysis and testing.
Discuss a water-treatment challenge
For selenium-bearing water, treatment design starts with representative data on total and dissolved selenium, relevant species, pH, redox conditions, sulfate, salinity and suspended solids.
Discuss a water-treatment challenge
Last reviewed: 8 November 2026 by Lenntech.
Sources and further reading
- U.S. Geological Survey: Geochemistry and occurrence of selenium—an overview.
- ATSDR/International Programme on Chemical Safety: Selenium chemistry and speciation.
- National Toxicology Program: Sodium selenate and sodium selenite, toxicity report (physical and chemical properties).
- ILO/WHO International Chemical Safety Card: Sodium selenite.
- Treatment technologies for selenium-contaminated water: a critical review (2022).
- Removal of selenium oxyanions from aqueous solutions by ion exchange: equilibrium, kinetics and mechanistic modeling.
- Australian National Health and Medical Research Council: Selenium technical report (drinking-water treatment evidence).