Common carbon-containing salts in aqueous solutions and their treatment
Carbon-containing compounds are common in water, but their form depends on pH and the other dissolved ions. In water, soluble carbonate and bicarbonate salts dissociate into ions; sparingly soluble carbonates may occur as minerals or precipitated scale rather than as dissolved salt molecules.
Important distinction: “Carbon in water” can refer to inorganic carbon (CO2, bicarbonate and carbonate) or organic carbon (such as DOC and TOC). This page focuses on common carbonate and bicarbonate compounds; it does not represent the full range of organic carbon in water.
Common carbon-containing salts and compounds in aqueous systems
| Carbonate or bicarbonate compound | Formula | Typical application / industry / where used | Typical aqueous streams where encountered |
|---|---|---|---|
| Calcium carbonate (calcite, aragonite, limestone) | CaCO3 | Construction and mineral processing; lime softening and neutralisation; common scale mineral. | Groundwater and process water in contact with carbonate rock; softening sludge, mineral-processing water and carbonate scale deposits. |
| Calcium bicarbonate (aqueous calcium and bicarbonate) | Ca2+ + 2HCO3− (often written Ca(HCO3)2) | A convenient representation of carbonate hardness in natural-water chemistry, not usually an isolated stable salt handled as a dry product. | Groundwater and surface water where CO2-bearing water has dissolved limestone or other carbonate minerals. |
| Magnesium carbonate (magnesite and related phases) | MgCO3 | Mineral processing, refractory materials and specialty chemical production. | Carbonate-mineral waters and mineral-processing streams; dissolved magnesium is often accompanied by bicarbonate, while solid magnesium-carbonate phases depend on the conditions. |
| Sodium bicarbonate | NaHCO3 | Food and chemical production, buffering and selected water-treatment applications. | Natural waters with bicarbonate alkalinity, dosing and rinse streams, and concentrated process waters. |
| Sodium carbonate (soda ash) | Na2CO3 | Glass and detergent manufacture, pH adjustment and water softening. | Process solutions and wastewater where soda ash is used; concentrates and cleaning streams. |
| Potassium bicarbonate and potassium carbonate | KHCO3, K2CO3 | Food, fertilizer, specialty chemical and pH-control applications. | Relevant process and wash waters where these chemicals are used; concentrations depend on dosing and subsequent dilution or recovery. |
| Ammonium bicarbonate and ammonium carbonate | NH4HCO3, (NH4)2CO3 | Food, fertilizer and chemical processes. | Process waters and wastewater associated with use of ammonium salts; ammonia/ammonium and carbonate speciation changes with pH and temperature. |
| Iron(II) carbonate (siderite) | FeCO3 | Iron-bearing mineral deposits and geochemical systems; may form deposits in reducing environments. | Some reducing groundwater and mine or process waters where dissolved iron and carbonate chemistry permit precipitation. |
Solubility and speciation depend on water chemistry
Soluble salts such as sodium bicarbonate dissociate into ions in water. The expressions “calcium bicarbonate” and “magnesium bicarbonate” commonly describe aqueous combinations of metal ions and bicarbonate; they should not be mistaken for stable, undissociated molecules in a dilute solution. Carbonate minerals such as calcite and siderite have limited solubility, and their formation depends on pH, temperature, pressure, ionic strength and the concentrations of relevant ions.
As pH changes, the balance among dissolved CO2, bicarbonate and carbonate shifts. Calcium or magnesium carbonate may precipitate when the water becomes supersaturated. In concentrated brines, activities and interactions between ions differ from dilute pure-water conditions, so simple solubility tables cannot by themselves predict precipitation or scaling in an operating plant.
Potential treatment routes
The appropriate process depends on whether the objective is to remove dissolved ions, control alkalinity or scale, reduce organic carbon, or recover and manage a concentrate. The routes below are examples for evaluation, not universal recommendations.
| Water-treatment objective | Potential technologies | Important selection factors |
|---|---|---|
| Reduce hardness or control calcium-carbonate precipitation | Lime softening, pH/alkalinity adjustment, precipitation and solids separation | Hardness, alkalinity, pH, temperature, sludge handling and target residual concentration. |
| Reduce dissolved carbonate/bicarbonate ions and overall dissolved salts | Reverse osmosis (RO), nanofiltration (NF) or electrodialysis (ED/EDR) | Membrane separation depends on species, feed chemistry, pretreatment, recovery and concentrate management. NF separation differs by ion and membrane; test against the target. |
| Manage free dissolved carbon dioxide | Aeration or degassing, sometimes combined with pH adjustment or downstream treatment | Gas transfer, alkalinity, pH, temperature and the required final water quality. |
| Reduce selected organic compounds or biodegradable organic matter | Biological treatment or activated-carbon adsorption | Activated carbon targets adsorbable organic compounds, not bicarbonate or carbonate ions; performance depends on the target and competing matter. |
| Concentrate or manage a carbonate-bearing residual | Membrane concentration, precipitation/crystallisation and, where justified, zero liquid discharge (ZLD) | Scale control, energy use, salt purity, residual handling and the overall water balance. |
More background: alkalinity and water, carbonate and bicarbonate in irrigation water, and activated carbon adsorption.
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Last reviewed: by Lenntech.