Common europium salts in water: solubility, removal and concentration
Common europium salts include chloride, nitrate and sulfate. Carbonate, phosphate and fluoride are relevant poorly soluble forms. Europium also has a useful distinction between Eu(III) and Eu(II): the oxidation state can change its solubility and separation behaviour.
For occurrence and environmental context, see europium in water. “Common” here refers to laboratory and process chemistry, not a ranking of salts detected in natural waters. Once a salt dissolves, the original formula does not describe every species in solution.
Common salts and water solubility
| Salt | Formula and hydrate examples | Water solubility | Practical significance |
|---|---|---|---|
| Europium(III) chloride | EuCl3; EuCl3·6H2O | Water-soluble; check the specified hydrate [1]. | A soluble Eu feed salt. Removing Eu does not necessarily remove chloride. |
| Europium(III) nitrate | Eu(NO3)3·6H2O | Water-soluble [2]. | Used to prepare Eu solutions; nitrate remains a separate water-quality consideration. |
| Europium(III) sulfate | Eu2(SO4)3·8H2O | Listed as soluble in water; saturation depends on temperature, hydrate and solution composition [3]. | Do not confuse this trivalent salt with Eu(II) sulfate. |
| Europium(II) sulfate | EuSO4 | Sparingly soluble; used in selective precipitation after reduction [4]. | A specialist recovery route requiring control of europium oxidation state. |
| Europium(III) carbonate | Eu2(CO3)3·xH2O | Poorly soluble in water [5]. | Solid form relevant to processing; acidic or complexing solutions require separate assessment. |
| Europium(III) phosphate | EuPO4 | Listed as water-insoluble [6]. | A solid phase that can retain Eu; insoluble does not mean zero dissolved concentration. |
| Europium(III) fluoride | EuF3 | Water-insoluble [7]. | Adding fluoride introduces another contaminant and is not a general water-treatment recommendation. |
These are qualitative comparisons, not numerical saturation concentrations. A numerical value should identify temperature, pH, oxidation state, hydrate or solid phase, solution composition and units. Values in g per 100 g water cannot be substituted directly for g/L solution. Hydroxides and oxides are relevant solids but are not included as salts in this table.
Eu(III), Eu(II) and dissolved species
Eu(III) can form dissolved complexes or transfer to particles and precipitates. Hydrolysis, carbonate chemistry and sorption affect aqueous behaviour [8]. Free-ion concentration, total dissolved Eu and total Eu are different measurements. Organic ligands and competing ions can alter treatment performance.
Reduction to Eu(II) has been used before selective sulfate precipitation [4]. This does not mean all Eu salts become insoluble after reduction. EuCl2 is soluble in water: recovery results based on chloride precipitation from low-water alcohol mixtures cannot be transferred directly to an aqueous water-treatment process [9]. Assess redox conditions and oxygen exposure alongside pH.
Concentration: mg/L as Eu versus mg/L of salt
Report elemental europium, for example µg/L as Eu or mg/L as Eu, and define the sample fraction. The mass of an added salt includes its anions and water of crystallization. A dose calculation does not establish dissolved Eu remaining after equilibration.
Calculated Eu concentration (mg/L) = salt dose (mg/L) × n × 151.964 / molar mass of the salt (g/mol).
Here n is the number of Eu atoms per formula unit. Include water of crystallization and correct for product assay when appropriate.
| Defined salt | Approximate molar mass (g/mol) | Eu from 100 mg/L salt, calculated |
|---|---|---|
| EuCl3, anhydrous | 258.31 | 58.8 mg/L as Eu |
| EuCl3·6H2O | 366.40 | 41.5 mg/L as Eu |
| Eu(NO3)3·6H2O | 446.06 | 34.1 mg/L as Eu |
These calculated examples assume pure material, complete dissolution and no subsequent loss. They are not solubility measurements or environmental concentrations. A concentration of 1 mmol/L Eu corresponds to approximately 151.964 mg/L as Eu; 1 mg/L as Eu corresponds to approximately 6.58 µmol/L. The atomic mass used is 151.964 [10].
Removal of europium from water
- Precipitation and solids separation: test suitable pH adjustment and precipitants with representative water. Eu hydroxide precipitation has been investigated in process solutions [11]. Follow precipitation with settling or filtration; forming a solid does not itself remove it from the water stream.
- Adsorption and ion exchange: assess a suitable sorbent or resin for dissolved Eu, including competing metals, ligands, acidity and regeneration. Eu(III) uptake has been investigated with experimental alginate-based sorbents [8]. Laboratory capacity does not guarantee plant performance or selective separation from other rare earths.
- Redox-based separation: reduction and EuSO4 precipitation can support selective recovery in suitable process liquors [4]. Reagents, oxidation-state stability and residual streams require process-specific evaluation.
Filtration alone targets particles rather than freely dissolved Eu ions. Removing Eu does not establish that chloride, nitrate, sulfate or other constituents meet the required water quality.
Concentration and recovery
Removal lowers Eu in treated water; recovery collects Eu in another stream or product. Precipitation concentrates it in a solid. Elution from a loaded resin may collect Eu in a smaller liquid volume, subject to testing. Further purification can be necessary before a useful product is obtained.
Liquid concentration factor = Eu concentration in the collected liquid / Eu concentration in the feed. Determine recovery separately from the collected Eu mass divided by the feed Eu mass. For example, 1 m³ containing 10 mg/L as Eu contains 10 g Eu. Recovering 90% into 10 L gives 900 mg/L as Eu and a concentration factor of 90. This is an illustrative mass balance, not a performance prediction.
Information for treatment assessment
Useful inputs include flow, dissolved and total Eu, pH, temperature, redox conditions, major anions, organic ligands, competing metals, suspended solids and the desired outlet concentration. Agree sampling and analytical preparation with the laboratory. Test residual Eu, solids separation, reagent consumption and the composition of concentrates.
Sources and further reading
- ProChem: europium chloride hexahydrate.
- ProChem: europium nitrate hexahydrate.
- Thermo Scientific: europium(III) sulfate octahydrate.
- Europium separation from a middle rare earths concentrate derived from Egyptian black sand monazite.
- American Elements: europium carbonate hydrate.
- ProChem: europium(III) phosphate.
- ProChem: europium(III) fluoride.
- Eu(III) removal using polyurea-crosslinked alginate aerogels.
- Photochemical recovery of europium from non-aqueous solutions.
- Royal Society of Chemistry: europium.
- Yttrium and europium separation by solvent extraction with undiluted thiocyanate ionic liquids.