Common magnesium salts in aqueous solutions and their removal/purification technologies
Magnesium salts contribute to water hardness and occur naturally in groundwater and seawater. In water, they are present mainly as dissolved magnesium ions (Mg2+) balanced by ions such as chloride, sulfate and bicarbonate; the water’s composition and conditions determine which minerals may precipitate.
Managing magnesium hardness, a magnesium-rich brine or deposits in process water? The right treatment depends on the complete water analysis, the desired water quality or recovered product, and how residuals and concentrate will be managed.
From water analysis to a treatment concept
Characterise the stream (magnesium, calcium, alkalinity, sulfate, chloride, silica, TDS, pH, temperature and flow) → define the objective (softening, desalination, reuse or magnesium recovery) → compare treatment routes → evaluate residuals and concentrate. This is a decision framework; the process train depends on the actual water chemistry.
Common magnesium salts and where they are encountered
| Magnesium salt or form | Formula | Typical application / industry / where used | Typical aqueous streams where encountered |
|---|---|---|---|
| Magnesium chloride | MgCl2 | Magnesium production, chemical manufacture, de-icing and dust control. | Seawater, saline groundwater, potash-processing bitterns, chemical-process water and some de-icing runoff. Magnesium and chloride occur as dissolved ions, not intact salt molecules. |
| Magnesium sulfate | MgSO4 | Fertilizer and chemical production; mineral processing. | Seawater, sulfate-rich groundwater, mining and process brines, fertilizer-related streams and desalination concentrates. |
| Magnesium bicarbonate in hard water | Mg2+ with HCO3− | Natural carbonate-water chemistry; not normally isolated as a stable dry salt. | Groundwater in contact with dolomite or other magnesium-bearing carbonate rocks. Heating or pH changes can alter carbonate equilibria and promote deposits. |
| Magnesium nitrate-bearing water | Mg2+ with NO3− | Fertilizer and specialty chemical production. | Fertilizer-related runoff and process waters. Nitrate and magnesium are dissolved ions; the water should not be assumed to contain a discrete magnesium-nitrate compound. |
| Magnesium carbonate minerals / precipitates | MgCO3 and hydrated carbonate phases | Magnesite and related minerals; possible recovered solids in some treatment or mineral-processing operations. | May occur as suspended mineral solids or form when water chemistry favours precipitation; it is not generally a highly soluble source of dissolved magnesium. |
| Magnesium hydroxide precipitate | Mg(OH)2 | Formed during high-pH precipitation, softening or magnesium recovery. | Produced as a solid in some treatment processes after pH adjustment; it is not typically present as a dissolved salt in neutral water. |
For scale, average seawater contains about 1,350 mg/L magnesium. Freshwater concentrations vary with local geology; water interacting with magnesium-rich carbonate, sulfate or chloride deposits can contain substantially more than water from silicate-rich formations.
Solubility, hardness and deposits
Magnesium chloride and magnesium sulfate are very soluble, so they can remain in solution as water is concentrated. By contrast, magnesium carbonate and magnesium hydroxide are sparingly soluble solids under relevant water-treatment conditions. A magnesium result by itself does not predict scaling: pH, alkalinity, temperature, ionic strength and other dissolved ions affect mineral saturation and precipitation.
In natural and industrial waters, dissolved magnesium is part of a mixed-ion system. Evaluate the full analysis and operating conditions before drawing conclusions about deposition, recovery or the performance of a treatment process.
Separation, softening and concentration solutions
Potential routes for magnesium-bearing water serve different purposes. Process selection should account for competing ions, water recovery, residuals and any target magnesium product.
| Water-treatment challenge | Potential process route | Selection notes |
|---|---|---|
| Reduce hardness caused by magnesium and calcium | Water softening or cation exchange | Softening can remove hardness by precipitation or ion exchange. Ion exchange replaces hardness ions with other cations and produces a regeneration stream that needs management. |
| Reduce dissolved salts in brackish or saline water | Reverse osmosis (RO) | Retains dissolved ions in a concentrate. Pretreatment and recovery depend on hardness, sulfate, alkalinity, silica, salinity and the concentrate-management route. |
| Preferentially separate many divalent ions from monovalent salts | Nanofiltration (NF) | NF can reject magnesium and other multivalent ions to differing degrees; performance varies with membrane, pressure and feed composition. It does not create a pure magnesium product by itself. |
| Concentrate or separate dissolved ionic streams | Electrodialysis (ED) or electrodialysis reversal (EDR) | Ion transport depends on membrane selection and mixture composition. Confirm magnesium transport, selectivity and scaling control for the specific application. |
| Recover magnesium or manage high-strength brine | pH-controlled precipitation, followed where appropriate by solid-liquid separation; or evaporation and crystallisation in a ZLD train | Precipitation can produce magnesium hydroxide under suitable conditions. Mixed salts, product purity, reagent demand and residual handling need to be assessed for each stream. |
Assessing a magnesium-bearing stream
A representative analysis should include magnesium, calcium, sodium, potassium, chloride, sulfate, alkalinity, silica, TDS, pH and temperature, alongside flow and the required treated-water quality. For a recovery project, define product specifications and residual-management requirements as well. Laboratory testing or pilot operation may help assess performance where feed variability, precipitation or fouling is important. Lenntech’s process feasibility assessment describes its evaluation approach.
Discuss a water-treatment challenge
To discuss a specific water or brine composition, contact Lenntech at info@lenntech.com.
Sources and further reading
- USGS: Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast — major constituents of average seawater, including magnesium.
- USGS PHREEQC: Seawater composition and aqueous speciation example.
- U.S. EPA WaterSense at Work: Water purification and softening.
- U.S. EPA: Reference Guide to Treatment Technologies for Mining-Influenced Water — reverse osmosis and pretreatment.
- USGS: Collection and Analysis of Water Samples for Dissolved Minerals and Gases — magnesium in natural waters.