Holmium salts in water: solubility, removal and concentration
Holmium chloride and nitrate are water-soluble sources of Ho(III), while carbonate, oxalate and fluoride are poorly soluble compounds. The chemical form present after mixing with water determines treatment behaviour; the name of the original salt is not enough.
For sources and environmental pathways, see holmium in water.
Common holmium salts and solubility
These qualitative descriptions are not saturation concentrations or dosing specifications. Check data for the exact hydrate, temperature and solution composition. Acidity, complexing substances and other ions can change how much holmium remains dissolved.
| Salt | Representative formula | Behaviour in water | Practical relevance |
|---|---|---|---|
| Holmium(III) chloride hexahydrate | HoCl3·6H2O | Water-soluble. | Supplies dissolved Ho(III); hydration changes the holmium mass fraction. |
| Holmium(III) nitrate pentahydrate | Ho(NO3)3·5H2O | Water-soluble. | Both holmium and nitrate enter solution. Verify hydrate and assay on the product certificate. |
| Holmium(III) sulfate | Ho2(SO4)3 | Moderately water-soluble according to supplier information. | Check hydrate, temperature and sulfate composition before preparing or concentrating a solution. |
| Holmium(III) carbonate hydrate | Ho2(CO3)3·xH2O | Poorly soluble in water. | A suspension is not a dissolved solution. Variable hydration affects mass calculations. |
| Holmium(III) oxalate hydrate | Ho2(C2O4)3·xH2O | Poorly soluble in water. | A possible precipitation product; measure residual dissolved holmium rather than assuming complete capture. |
| Holmium(III) fluoride | HoF3 | Poorly soluble in water. | Include fluoride as well as holmium in the assessment of water quality and residues. |
What happens after dissolution?
These compounds contain trivalent holmium, Ho(III). Changes in acidity or complexing substances can alter dissolved species and the tendency to form particles or bind to surfaces. A solution prepared from a soluble salt can therefore change after mixing with another stream.
Measure total holmium and, where relevant, the dissolved fraction defined by a specified sampling and filtration method. Fine particles and colloids can affect this distinction. Record preparation methods so that before-and-after treatment results are comparable.
Removing holmium from water
Solids separation and precipitation
Settling and filtration can remove holmium-bearing particles if those particles can be separated effectively. They do not by themselves establish removal of dissolved Ho(III). Precipitation can transfer dissolved holmium into a poorly soluble solid when the solution chemistry permits.
For any proposed precipitation step, check acidity, complexing substances, competing ions, residual dissolved holmium and the filterability of the solids. There is no universal dose or operating pH established by a qualitative solubility label. Account for added reagents and sludge as part of the treatment balance.
Adsorption and ion exchange
Primary research has examined holmium adsorption on amino-functionalized magnetic graphene oxide and modified alginate composites. The alginate study investigated pH, contact time, initial concentration, temperature, competing metals and regeneration. These results support testing possible capture mechanisms, rather than assuming the same performance in a different water.
Ion exchange is used in rare-earth separation, but practical selection requires testing the actual mixture. Evaluate capacity, contact time, breakthrough, competing-ion effects and regeneration. A simple holmium salt solution may behave differently from a mixed rare-earth liquor or a water containing organic ligands.
Concentration: mg/L as Ho versus mg/L of salt
A value expressed as elemental holmium excludes counterions and water of crystallisation. Using a holmium atomic mass of 164.930 g/mol:
mg/L as Ho = mg/L of salt × (number of Ho atoms per formula unit × 164.930) / molar mass of the salt.
| Pure salt | Formula | Approximate molar mass | Ho from 100 mg/L of salt |
|---|---|---|---|
| Anhydrous holmium chloride | HoCl3 | 271.29 g/mol | 60.8 mg/L as Ho |
| Holmium chloride hexahydrate | HoCl3·6H2O | 379.38 g/mol | 43.5 mg/L as Ho |
| Holmium nitrate pentahydrate | Ho(NO3)3·5H2O | 441.02 g/mol | 37.4 mg/L as Ho |
These examples assume the stated pure compound and complete dissolution; they are not solubility limits. Correct for product assay, moisture, the actual hydrate and undissolved material. A concentration of 1 mmol/L of holmium corresponds to 164.930 mg/L as Ho. Total elemental analysis alone cannot identify the original salt.
Concentrating and recovering holmium
Adsorption followed by desorption or ion exchange followed by elution can move captured holmium into another liquid stream. A smaller final volume can increase its concentration. A concentrate may also contain other rare earths, salts and impurities, so recovery and purity need separate measurements.
For an illustrative mass balance, 1 m3 of water at 10 mg/L as Ho contains 10 g of holmium. Recovering 90% of that mass into a final liquid volume of 10 L would give 900 mg/L as Ho, a concentration factor of 90. This is a calculation, not an expected treatment yield. Include all liquid outlets, residual solids and losses in an actual balance.
Useful process inputs include flow, temperature, pH, total and dissolved holmium, other rare-earth elements, major ions, suspended solids and known complexing substances. Define both the treated-water target and any recovery objective before establishing performance through laboratory or pilot testing.
Sources
- Thermo Scientific Chemicals: holmium chloride hexahydrate and holmium nitrate pentahydrate.
- American Elements: holmium sulfate.
- Inorganic Ventures: Periodic Table Guide, holmium solution chemistry.
- Recovery of heavy rare earth elements using amino-functionalized magnetic graphene oxide.
- Modified renewable-based alginate composites for adsorption of samarium and holmium.
- Royal Society of Chemistry: holmium atomic data and separation.