Common praseodymium salts in water: solubility, removal and concentration
Common praseodymium 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 praseodymium concentration, remove it from water and concentrate it for recovery.
For occurrence and environmental context, see praseodymium 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: Pr(III) can form dissolved complexes or transfer to solids.
Common salts and water solubility
| Salt | Formula and hydrate examples | Water solubility | Practical significance |
|---|---|---|---|
| Praseodymium(III) chloride | PrCl3; PrCl3·6H2O or ·7H2O | Water-soluble; check the specified hydrate. | A soluble Pr feed salt. Removing Pr does not necessarily remove chloride. |
| Praseodymium(III) nitrate | Pr(NO3)3; commonly hydrated | Water-soluble. | Used to prepare Pr solutions; nitrate remains a separate water-quality consideration. |
| Praseodymium(III) sulfate | Pr2(SO4)3; octahydrate | Limited solubility; strongly dependent on conditions and solid phase. | Sulfate also forms dissolved complexes; sulfate-bearing water does not automatically precipitate all Pr. |
| Praseodymium(III) carbonate | Pr2(CO3)3·xH2O | Poorly soluble in water; acid promotes dissolution. | Potential precipitation and recovery product. |
| Praseodymium(III) oxalate | Pr2(C2O4)3·xH2O | Poorly soluble; pH and excess ligand affect dissolved Pr. | Relevant to rare-earth precipitation and concentration. |
| Praseodymium(III) phosphate | PrPO4 | Sparingly soluble; acidity and complexation affect dissolution. | Can immobilize Pr in a solid phase. |
| Praseodymium(III) fluoride | PrF3 | 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. Pr hydroxides and oxides are also relevant solids, but are not included as salts in this table. [1–4]
What determines dissolved praseodymium?
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 Pr3+, dissolved complexes and precipitates. A low free-ion concentration does not necessarily mean low total dissolved Pr. Precipitation predictions should account for complexation and the actual solid phases. [3,4]
Concentration: mg/L as Pr versus mg/L of salt
Report results as elemental praseodymium, for example µg/L as Pr or mg/L as Pr, and state whether the sample represents dissolved or total material. Do not equate a salt dose with dissolved Pr remaining after equilibration. There is no single representative concentration for all natural or industrial waters.
Calculated Pr concentration (mg/L) = salt dose (mg/L) × n × 140.90766 / molar mass of the salt (g/mol).
Here n is the number of Pr 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) | Pr from 100 mg/L salt, calculated |
|---|---|---|
| PrCl3, anhydrous | 247.26 | 57.0 mg/L as Pr |
| PrCl3·6H2O | 355.35 | 39.7 mg/L as Pr |
| Pr(NO3)3·6H2O | 435.01 | 32.4 mg/L as Pr |
These stoichiometric examples assume pure material, complete dissolution and no subsequent loss; they are neither solubility measurements nor environmental concentrations. One mmol/L of Pr corresponds to approximately 140.91 mg/L as Pr. Conversely, 1 mg/L as Pr is approximately 7.10 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 Pr on a suitable resin and regenerate it into a smaller liquid volume. Pr(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: sorbents can bind Pr, but pH, contact time and water composition matter. Pr-specific sediment adsorption research supports testing rather than assuming universal removal. Assess regeneration or disposal of the spent sorbent. [6]
Filtration alone targets particles, not freely dissolved Pr ions. Treatment selection should be based on the measured influent, the required outlet concentration, competing metals and the resulting waste or recovery streams. Removing Pr does not by itself establish that chloride, nitrate, sulfate or other dissolved constituents meet the required water quality.
Concentration and recovery
Removal lowers Pr in the treated water; recovery collects it in a useful product. Ion-exchange elution can concentrate Pr 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 = Pr concentration in the collected eluate / Pr concentration in the feed. Determine recovery separately as collected Pr mass divided by feed Pr mass. For example, 1 m³ containing 10 mg/L as Pr contains 10 g Pr. Recovering 90% into 10 L would give 900 mg/L as Pr 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 Pr, 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 Pr, solids separation, reagent consumption and the composition of any concentrate.
Sources and further reading
- Stanford Materials: praseodymium chloride product information.
- CDH: praseodymium nitrate specification.
- Inorganic Ventures: periodic table guide, praseodymium compatibility and solubility.
- Han (2020): Characteristics of Precipitation of Rare Earth Elements with Various Precipitants.
- Modeling and equilibrium studies on the recovery of praseodymium (III), dysprosium (III) and yttrium (III) using acidic cation exchange resin (2022).
- Natural sediment at a dam and its inorganic materials as adsorbents of praseodymium (Pr(III)) (2023).