Common samarium salts in water: solubility, removal and concentration
Common samarium salts include chloride, nitrate and sulfate, while carbonate, oxalate, phosphate and fluoride are relevant as sparingly soluble solids. This guide compares their behaviour in water and explains how to report samarium concentration, remove it from water and concentrate it for recovery.
For occurrence and environmental context, see samarium in water. “Common” here refers to laboratory and process chemistry, not a ranking of salts detected in natural waters. Once a salt dissolves, its original formula does not describe all the species present: Sm(III) can form dissolved complexes or transfer to solids.
Common salts and water solubility
| Salt | Formula and hydrate examples | Water solubility | Practical significance |
|---|---|---|---|
| Samarium(III) chloride | SmCl3; SmCl3·6H2O | Water-soluble; check the specified hydrate. | A soluble Sm feed salt. Removing Sm does not necessarily remove chloride. |
| Samarium(III) nitrate | Sm(NO3)3; commonly hydrated | Water-soluble. | Used to prepare Sm solutions; nitrate remains a separate water-quality consideration. |
| Samarium(III) sulfate | Sm2(SO4)3; octahydrate | Limited solubility; strongly dependent on temperature and solid phase. A study reports 0.033 mol/kg water for the octahydrate at 25 °C [7]. | Sulfate also forms dissolved complexes; sulfate-bearing water does not automatically precipitate all Sm. |
| Samarium(III) carbonate | Sm2(CO3)3·xH2O | Poorly soluble in water; acid promotes dissolution. | Potential precipitation and recovery product. |
| Samarium(III) oxalate | Sm2(C2O4)3·xH2O | Poorly soluble; pH and excess ligand affect dissolved Sm. | Relevant to rare-earth precipitation and concentration. |
| Samarium(III) phosphate | SmPO4 | Sparingly soluble; acidity and complexation affect dissolution. | Can immobilize Sm in a solid phase. |
| Samarium(III) fluoride | SmF3 | Poorly soluble in water. | A possible precipitate; adding fluoride introduces a further contaminant and is not a general water-treatment recommendation. |
These are qualitative comparisons, not saturation concentrations. A numerical solubility must identify temperature, pH, hydrate or solid phase, solution composition, and units. Values in g per 100 g water cannot be used directly as g/L solution. Sm hydroxides and oxides are also relevant solids, but are not included as salts in this table. [1–4]
What determines dissolved samarium?
Solubility is an equilibrium property; measured concentration also depends on the amount introduced, residence time and solids separation. Acidity, carbonate, sulfate, phosphate, fluoride and organic ligands can change the balance between free Sm3+, dissolved complexes and precipitates. A low free-ion concentration does not necessarily mean low total dissolved Sm. Precipitation predictions should account for complexation and the actual solid phases. [3,4]
Concentration: mg/L as Sm versus mg/L of salt
Report results as elemental samarium, for example µg/L as Sm or mg/L as Sm, and state whether the sample represents dissolved or total material. Do not equate a salt dose with dissolved Sm remaining after equilibration. There is no single representative concentration for all natural or industrial waters.
Calculated Sm concentration (mg/L) = salt dose (mg/L) × n × 150.36 / molar mass of the salt (g/mol).
Here n is the number of Sm atoms per formula unit. Include water of crystallization in the molar mass and correct for the product assay where appropriate.
| Defined salt | Approximate molar mass (g/mol) | Sm from 100 mg/L salt, calculated |
|---|---|---|
| SmCl3, anhydrous | 256.71 | 58.6 mg/L as Sm |
| SmCl3·6H2O | 364.80 | 41.2 mg/L as Sm |
| Sm(NO3)3·6H2O | 444.47 | 33.8 mg/L as Sm |
These stoichiometric examples assume pure material, complete dissolution and no subsequent loss; they are neither solubility measurements nor environmental concentrations. One mmol/L of Sm corresponds to approximately 150.36 mg/L as Sm. Conversely, 1 mg/L as Sm is approximately 6.65 µmol/L.
Removal from water
- Precipitation and solids separation: evaluate pH adjustment and suitable precipitants with the actual water. Carbonate, oxalate and phosphate chemistry can transfer rare earths into solids. Follow precipitation with settling or filtration; producing a precipitate alone does not remove it from the water stream. Co-precipitation can produce a mixed-metal sludge. [4]
- Ion exchange: capture dissolved Sm on a suitable resin and regenerate it into a smaller liquid volume. Sm(III) uptake has been studied with acidic cation-exchange resin; acidity, competing ions and loading influence performance. Laboratory capacity is not a guaranteed operating capacity for a plant. [5]
- Adsorption and speciation: sorbent uptake depends on pH, competing ions and ligands. A samarium study using a strong-acid resin found that resin accumulation was related to free Sm in solution in the tested systems. Total dissolved Sm and free Sm ions are therefore not interchangeable measurements. [6]
Filtration alone targets particles, not freely dissolved Sm ions. Treatment selection should be based on the measured influent, the required outlet concentration, competing metals and the resulting waste or recovery streams. Removing Sm does not by itself establish that chloride, nitrate, sulfate or other dissolved constituents meet the required water quality.
Concentration and recovery
Removal lowers Sm in the treated water; recovery collects it in a useful product. Ion-exchange elution can concentrate Sm in a smaller liquid volume, while precipitation concentrates it in a solid. Neither step alone establishes separation from neighbouring rare earths or a saleable product purity. [4,5]
Liquid concentration factor = Sm concentration in the collected eluate / Sm concentration in the feed. Determine recovery separately as collected Sm mass divided by feed Sm mass. For example, 1 m³ containing 10 mg/L as Sm contains 10 g Sm. Recovering 90% into 10 L would give 900 mg/L as Sm and a concentration factor of 90. This is an illustrative mass balance, not a treatment-performance prediction.
Information for treatment assessment
Useful inputs include flow, dissolved and total Sm, pH, temperature, major anions, competing metals, suspended solids and the target outlet concentration. Define sampling and analytical preparation with the laboratory so that dissolved and total results remain comparable. Bench testing should measure residual Sm, solids separation, reagent consumption and the composition of any concentrate.
Sources and further reading
- Santa Cruz Biotechnology: samarium(III) chloride hexahydrate.
- Stanford Materials: samarium nitrate.
- Stanford Materials: samarium compounds.
- Han (2020): Characteristics of Precipitation of Rare Earth Elements with Various Precipitants.
- Samarium(III) Removal by Weak Acid Exchanger Amberlite IRC-50 in (H+) and (Na+) Forms.
- Speciation of a lanthanide (Sm) using an ion exchange resin (2016).
- Solubility and thermodynamic functions for samarium sulfate in water and sulfuric acid solutions (1975).