Common rhenium salts in water: solubility, removal and concentration
Rhenium salts in water are mainly relevant to specialised reagents and metal-recovery processes. In oxygenated solutions, dissolved rhenium commonly occurs as perrhenate, ReO4−. A dissolved salt name describes the starting material or product; it does not mean intact salt molecules remain in solution.
See rhenium in water for occurrence and aqueous chemistry, and rhenium in mining and processing waters for industrial sources and recovery circuits.
Representative rhenium salts and water solubility
| Compound | Formula | Water behaviour | Practical relevance |
|---|---|---|---|
| Ammonium perrhenate (APR) | NH4ReO4 | A supplier reports 62.3 g/L in water at 20 °C. Temperature and other dissolved ammonium salts affect saturation. | Crystalline intermediate recovered from purified rhenium-rich solutions. |
| Sodium perrhenate | NaReO4 | Highly water-soluble. Published crystallisation research reports more than 1,130 g/L at 298 K; this is not a limit for mixed process liquors. | Reagent and sodium-bearing recovery solutions. |
| Potassium perrhenate | KReO4 | Limited water solubility compared with sodium perrhenate; use measured temperature-specific data for crystallisation design. | Reagent and selected precipitation or purification routes. |
These are representative process salts, not a ranking of substances in natural water. Perrhenic acid, HReO4, is an acid rather than a salt; acidic recovery liquors are often described in these terms. A solubility expressed in grams of salt per litre must not be read as grams of elemental rhenium per litre. Values per litre of solution and per kilogram of water are also different reporting bases.
Pure-water data provide an initial reference. Actual saturation depends on temperature, acidity, counter-ions and the other dissolved salts. Studies of ammonium perrhenate with ammonium chloride, nitrate and sulfate demonstrate why a mixed liquor needs its own equilibrium assessment.
Removing rhenium from water
- Separate suspended solids: clarification and filtration remove rhenium-bearing particles. Ordinary particle filtration does not remove dissolved perrhenate.
- Capture dissolved perrhenate: anion exchange and selected adsorbents can transfer it from water to a solid phase. Test capacity, selectivity and breakthrough with the actual water, including sulfate, molybdate and other competing ions.
- Separate and purify process solutions: solvent extraction is used in rhenium hydrometallurgy. It needs a suitable feed and a managed extraction and stripping circuit, rather than direct extrapolation to drinking-water treatment.
- Recover a crystalline product: precipitation or crystallisation can recover perrhenate salts from sufficiently concentrated, conditioned solutions. Removing a trace concentration is a different objective from producing a high-purity salt.
Choose a treatment route using dissolved and total rhenium, flow, pH, redox conditions, salinity, competing metals and the required discharge, reuse or recovery endpoint. Bench testing and, where appropriate, pilot trials establish performance. Account for spent media, regeneration solutions, raffinate and solids; transferring rhenium to a residual stream is not destruction.
Concentration: mg/L as Re versus mg/L of salt
For a pure salt of known formula:
mg/L as Re = mg/L of salt × number of Re atoms per formula unit × 186.207 ÷ molar mass of the salt.
Ammonium perrhenate has a molar mass of approximately 268.24 g/mol and one rhenium atom per formula unit. Its theoretical rhenium mass fraction is 69.42%. Thus, 100 mg/L of pure ammonium perrhenate corresponds to approximately 69.42 mg/L as Re when all its rhenium is in the measured fraction. One mmol/L of rhenium corresponds to 186.207 mg/L as Re.
These are composition calculations, not solubility limits. Correct for reagent assay, hydration state and the exact chemical formula. Specify whether an analytical result represents dissolved or total rhenium. A result reported as perrhenate includes oxygen mass and cannot be used directly as a result reported as Re.
Concentrating rhenium and calculating recovery
Elution from an ion-exchange bed or stripping a solvent-extraction phase can transfer captured rhenium into a smaller liquid volume. Subsequent purification and crystallisation may produce ammonium perrhenate. Concentration depends on recovered mass and the final volume:
Recovered concentration = feed concentration × feed volume × recovered mass fraction ÷ recovered volume.
For example, 1,000 L containing 10 mg/L as Re holds 10 g of rhenium. Recovering 90% into 10 L gives 900 mg/L as Re, a concentration factor of 90. This illustrates a mass balance, not guaranteed treatment performance.
Report removal efficiency, mass recovery and product purity separately. Include wash and regeneration liquids in the volume balance, and verify impurities before treating a concentrated stream as a saleable product.
Discuss a rhenium treatment or recovery application
Discuss a water-treatment challenge
Sources and further reading
- Todini Chemicals: ammonium perrhenate solubility.
- Carl Roth: ammonium perrhenate safety data, water solubility at 20 °C.
- Casas and colleagues: crystallisation of sodium perrhenate at 298 K.
- Experimental study: solubility and density of ammonium and potassium perrhenate solutions.
- Experimental study: ammonium perrhenate solubility in ammonium salt mixtures.
- Experimental study: rhenium recovery from industrial leach solutions by solvent extraction.
- Review of rhenium extraction and recycling technologies.
- Thermo Scientific: ammonium perrhenate formula and molar mass.
- CIAAW: standard atomic weight of rhenium.