Common oxygen-containing salts in water: occurrence, solubility and treatment
“Oxygen salts” is an informal way to describe salts whose anions contain oxygen. In water, these compounds occur as dissolved ions or related aqueous species—not as intact salt molecules—and they are distinct from dissolved oxygen gas (O2). Common examples include bicarbonates, carbonates, sulfates, nitrates, phosphates and silicates.
These species are found in natural waters through mineral weathering, soil processes and biological activity, and in industrial waters through raw-water intake, chemical dosing, product processing, mining, cooling, desalination and wastewater treatment. Their concentrations and treatment needs depend on the complete water analysis, pH, temperature and other dissolved ions.
Common oxygen-containing salts and ions in water
| Representative salt or species | Where it may occur | Solubility in water | Water-treatment relevance |
|---|---|---|---|
| Sodium bicarbonate (NaHCO3) | Groundwater, surface water, boiler and process-water streams | About 9.6 g per 100 g water at 20 °C for the solid salt; bicarbonate in real waters is governed by carbonate equilibrium. | Alkalinity and pH buffering; can contribute to carbonate scale when water is concentrated, heated or degassed. |
| Sodium carbonate (Na2CO3) | Industrial cleaning, chemical processing and treatment-dosed waters | About 21.5 g per 100 g water at 20 °C for anhydrous sodium carbonate; hydrate formation affects reported data. | Raises alkalinity and pH; may cause calcium carbonate precipitation in hard water. |
| Calcium carbonate (CaCO3) | Hard groundwater, karst waters, scale and mineral-processing streams | Sparingly soluble in CO2-free pure water; apparent dissolved calcium rises in water containing CO2 because bicarbonate species form. | A common scale and precipitate. pH, alkalinity, temperature and dissolved CO2 determine saturation. |
| Sodium sulfate (Na2SO4) | Natural saline waters, textile and chemical process waters, and RO concentrates | About 19.5 g per 100 g water at 20 °C; the curve is not linear with temperature and hydrate phases matter. | Contributes to salinity; sulfate can form sparingly soluble scale with calcium or barium. |
| Magnesium sulfate (MgSO4) | Seawater, mineralized groundwater, and industrial brines | Soluble; published values depend on temperature and whether the value is reported for anhydrous salt or a hydrate. | Contributes to hardness and salinity. Concentration can affect scale and crystallization behavior. |
| Calcium sulfate (CaSO4, gypsum) | Groundwater, mining waters, desalination brines and evaporator circuits | Only moderately soluble: gypsum dissolves at roughly 2 g/L near room temperature, with temperature and ionic composition affecting the limit. | Gypsum scale may limit RO recovery and foul heat-transfer surfaces during evaporation. |
| Sodium nitrate (NaNO3) and potassium nitrate (KNO3) | Agricultural drainage, groundwater, fertilizer manufacture and process effluents | Both are highly soluble; at 20 °C, representative values are about 88 and 32 g per 100 g water, respectively. | Nitrate is a drinking-water contaminant of concern. Removal may use ion exchange, RO, electrodialysis or biological denitrification, depending on the stream. |
| Orthophosphate salts (for example, sodium or calcium phosphates) | Natural waters, fertilizer and food processing, and municipal or industrial wastewater | There is no single value: solubility depends strongly on the cation, phosphate form, pH and solid phase. | Phosphorus is commonly removed by biological uptake or chemical precipitation, followed by solids separation. |
| Silicate species (reported as dissolved silica, often SiO2) | Groundwater, geothermal waters, industrial process water and RO concentrate | Not represented by one “salt solubility”: dissolved silica speciation, polymerization and precipitation depend on pH, temperature and concentration. | Silica can deposit on membranes and equipment; pretreatment and recovery limits should be based on water chemistry. |
Interpreting the values: The figures above are approximate pure-water reference values for specified solid forms, not guaranteed limits for natural or industrial water. In mixed waters, ion activities, pH, temperature, complexation and competing salts change solubility and mineral saturation. A water analysis and suitable equilibrium model are needed for design.
Removal, concentration and recovery options
There is no universal process for every oxygen-containing anion. The appropriate treatment depends on which species must be removed, the required product-water quality, the feed composition and how residual brine or solids will be managed.
| Water challenge | Potential treatment approaches | Key design point |
|---|---|---|
| High salinity or mixed dissolved salts | Reverse osmosis, nanofiltration, or electrodialysis and electrodialysis reversal | Membranes separate ions but generate a concentrate stream; scaling limits and concentrate management matter. |
| Hardness and carbonate scale | Ion exchange, water softening, pH adjustment, precipitation and solids separation | Choice depends on hardness, alkalinity, sodium limits and whether salt or sludge residuals can be handled. |
| Nitrate in drinking or process water | Selective ion exchange, RO, electrodialysis, or biological denitrification | Account for competing ions, nitrate residuals and the intended use of treated water. |
| Phosphate in wastewater | Biological or chemical phosphorus removal, followed by clarification or filtration | Precipitation transfers phosphorus to solids; it does not destroy it. |
| Concentrated brine or salt recovery | Evaporation and crystallisation, sometimes after membrane concentration | Mixed salts may co-precipitate; achievable recovery and product purity require stream-specific assessment. |
Discuss an oxygen-containing salt or water-treatment challenge
Lenntech can assess treatment options for a specific natural or industrial water composition. To discuss a project, contact info@lenntech.com.
Last reviewed: 7 November 2026 by Lenntech
Sources and further reading
- USGS: Ground-water quality — major dissolved constituents and water chemistry.
- USGS: Major ions in natural waters.
- PubChem: Sodium bicarbonate; Sodium carbonate; Sodium sulfate.
- PubChem: Calcium sulfate; Calcium carbonate.
- PubChem: Sodium nitrate; Potassium nitrate.
- USGS: Nitrogen and water.
- US EPA: Addressing nitrate in drinking water.
- US EPA: Nutrient Control Design Manual.
- ILO/WHO International Chemical Safety Card: Magnesium sulfate heptahydrate.