Common rubidium salts in water: solubility, recovery and treatment technologies
Rubidium from dissolved salts occurs in water primarily as the hydrated ion Rb+. In ordinary water and brines, it is not present as separate RbCl, Rb2SO4 or Rb2CO3 molecules. Those formulas describe solid source or product salts; after dissolution, rubidium and the anions are distributed in the solution according to its chemistry.
Rubidium is a trace constituent in most natural waters. In chloride-rich seawater, salt-lake brines and process liquors it occurs alongside much larger concentrations of sodium and potassium. Sulfate or carbonate salts may be used as rubidium products, or arise in particular processing routes, but it is misleading to assign one dominant rubidium salt to a natural water solely from its anion composition.
Representative rubidium salts and solubility
The values below describe dissolution of pure crystalline salts in water. They are reference data, not typical rubidium concentrations in environmental water or a prediction of saturation in a mixed brine.
| Salt | Reference solubility in water | Where it is relevant |
|---|---|---|
| Rubidium chloride, RbCl | 90.32 g per 100 g water at 18.7 °C | A common reference and product salt; chloride-rich waters contain dissolved Rb+ with Cl−, rather than discrete RbCl molecules. |
| Rubidium sulfate, Rb2SO4 | 48.2 g per 100 g water at 20 °C | Relevant to sulfate-based leaching and product streams; in solution it dissociates into Rb+ and SO42−. |
| Rubidium carbonate, Rb2CO3 | 4,500 g/L at 20 °C (manufacturer specification) | A soluble specialty product. It is not automatically the form of rubidium in lithium-processing mother liquor. |
| Rubidium nitrate, RbNO3 | Soluble in water; a single comparable value is not given here | Used mainly as a specialty chemical and laboratory or process reagent, rather than a typical natural-water salt. |
Solubility varies with temperature and the measurement basis. In concentrated brines, high ionic strength and competing ions can shift activities and phase equilibria, so pure-water values should not be used directly for design. Rubidium-bearing mineral liquors may be produced by leaching or roasting of minerals such as lepidolite, zinnwaldite or pollucite. In lithium-processing circuits, rubidium may become enriched in a residual liquor, but its actual counterions and concentration depend on the process; it should not be assumed to be present mainly as rubidium carbonate.
Rubidium removal, recovery and concentration
Rubidium is difficult to recover selectively because it is chemically similar to potassium and usually present at much lower concentrations than sodium, potassium, magnesium and calcium. The best route depends on whether the objective is to remove trace Rb+ from water, recover rubidium as a product, or reduce the volume of a saline stream.
- Selective ion exchange and adsorption: Specially selected inorganic sorbents, including copper hexacyanoferrate materials, have been investigated for capturing Rb+ from salt-lake brines. Ordinary cation-exchange resins may also retain rubidium, but selectivity and capacity can be limited by potassium and other competing ions. Regeneration or elution produces a smaller, rubidium-enriched stream that may need further purification.
- Solvent extraction: The extractant t-BAMBP has been studied for separating rubidium from potassium in alkaline rubidium-ore liquor. This is a feed-specific hydrometallurgical method, not a universal treatment for ordinary water.
- Precipitation and crystallization: Fractional crystallization or precipitation of selected rubidium compounds can produce an intermediate or final salt after the liquor has been concentrated and purified. The sequence must be designed around the other salts present; indiscriminate precipitation does not selectively remove rubidium.
- Reverse osmosis and evaporation: Reverse osmosis can separate water from dissolved salts and concentrate Rb+ in the reject stream, but it does not normally separate rubidium selectively from other monovalent ions. Evaporation can further concentrate a brine before crystallization, with energy use, scaling and mixed-salt purity considered in the process design.
For specific rubidium recovery, the feed analysis should include Rb, K, Na, Li, Cs, Mg, Ca, chloride, sulfate, carbonate/alkalinity, pH and suspended solids. Bench testing is important before selecting a sorbent, extractant or crystallization sequence.
See also Rubidium and water and Lenntech’s ion-exchange systems.
Last reviewed: 8 November 2026 by Lenntech
Discuss a water-treatment challenge
Rubidium recovery or removal depends on the complete water composition and the required product or discharge quality.
Discuss a water-treatment challenge
Sources and further reading
- IUPAC–NIST Solubility Data Series: Rubidium chloride and water
- Rubidium sulfate solubility data (citing reference handbooks)
- Thermo Scientific: Rubidium carbonate product data
- Xing et al., “Rubidium extraction from mineral and brine resources: A review”
- Xing et al., “Separation of rubidium from potassium in rubidium ore liquor by solvent extraction with t-BAMBP”
- Zhang et al., “An electrochemistry-ion exchange approach for targeted extraction of rubidium from different type raw salt lakes without evaporation”