Gadolinium salts in water: solubility, removal and concentration
Gadolinium chloride and nitrate can introduce dissolved gadolinium into water, while carbonate, oxalate, phosphate and fluoride are poorly soluble solids. Selecting a removal or concentration process requires more than a total gadolinium measurement: the chemical form, pH and other dissolved substances matter.
This guide covers common commercially available gadolinium salts and practical water-treatment calculations. For occurrence and environmental pathways, see gadolinium in water.
Common gadolinium salts and water solubility
The table describes qualitative behaviour, not a universal saturation concentration. Temperature, hydrate form, acidity and complexing agents can change the amount that dissolves. Use data for the exact product and operating conditions when preparing solutions or designing a process.
| Salt | Representative formula | Behaviour in water | Practical relevance |
|---|---|---|---|
| Gadolinium(III) chloride hexahydrate | GdCl3·6H2O | Water-soluble. | A soluble source of Gd(III); the hydrate contains less gadolinium per gram than anhydrous chloride. |
| Gadolinium(III) nitrate hexahydrate | Gd(NO3)3·6H2O | Highly soluble. | Solution preparation must account for both gadolinium and nitrate loading. |
| Gadolinium(III) sulfate octahydrate | Gd2(SO4)3·8H2O | Soluble in cold water. | Check temperature-dependent solubility and sulfate chemistry before concentrating a solution. |
| Gadolinium(III) carbonate hydrate | Gd2(CO3)3·xH2O | Poorly soluble in water. | A suspended solid is not equivalent to dissolved gadolinium; variable hydration affects mass calculations. |
| Gadolinium(III) oxalate hydrate | Gd2(C2O4)3·xH2O | Poorly soluble in water; solubility depends on acidity and oxalate concentration. | Relevant to precipitation and recovery, with residual dissolved gadolinium requiring measurement. |
| Gadolinium(III) phosphate | GdPO4, also hydrated forms | Very low water solubility. | Formation of a solid can transfer gadolinium to sludge without destroying the element. |
| Gadolinium(III) fluoride | GdF3 | Poorly soluble in water. | Fluoride and gadolinium both need consideration in treatment and residue management. |
Dissolved salts are different from gadolinium chelates
Gadolinium in these salts is predominantly trivalent, Gd(III). After dissolution, its distribution among hydrated ions, complexes and solids depends on the water composition. The name of the salt originally added does not describe every species present later.
Gadolinium-based MRI contrast agents bind gadolinium within organic ligands. These chelates can remain dissolved and behave differently from uncomplexed Gd(III). A process tested with gadolinium chloride is therefore not evidence of equivalent removal of an intact contrast agent. Wastewater research has demonstrated incomplete removal of contrast-agent-derived gadolinium and the importance of compound-specific analysis.
Removing gadolinium from water
Solids separation and precipitation
Settling and filtration can separate particles containing gadolinium; they do not by themselves establish removal of dissolved species. Where the chemistry permits formation of a poorly soluble salt, precipitation followed by solids separation can transfer gadolinium into a recoverable or disposable solid. Gadolinium oxalate and phosphate studies illustrate this chemistry, but neither provides a universal operating pH or dose for wastewater.
Test the actual water for residual dissolved and total gadolinium, reagent consumption, solids separation and possible redissolution. Strong complexation can prevent the behaviour expected from a simple salt solution. Adding a precipitant also changes the treated water and creates a residue that requires management.
Adsorption and ion exchange
Adsorption and ion exchange may be investigated for dissolved gadolinium. Select media for the species present: a positively charged Gd(III) species and a neutral or negatively charged chelate need not bind to the same medium. Competing ions, organic matter, capacity, breakthrough and regeneration determine practical performance. Recent research on capturing medical gadolinium highlights the need to test specific chelates and realistic competing-ion conditions.
For a process assessment, useful information includes flow, pH, temperature, total and dissolved gadolinium, major ions, organic matter and any known contrast agents or other ligands. Laboratory and pilot results should establish the achievable effluent concentration and the composition of spent media, regenerant or sludge.
Concentration: mg/L as Gd versus mg/L of salt
Elemental gadolinium concentration and salt concentration are different quantities. Using a gadolinium atomic mass of 157.25 g/mol:
mg/L as Gd = mg/L of salt × (number of Gd atoms per formula unit × 157.25) / molar mass of the salt.
| Pure salt weighed | Approximate molar mass | Gadolinium from 100 mg/L of salt |
|---|---|---|
| Anhydrous GdCl3 | 263.60 g/mol | 59.7 mg/L as Gd |
| GdCl3·6H2O | 371.69 g/mol | 42.3 mg/L as Gd |
| Gd(NO3)3·6H2O | 451.35 g/mol | 34.8 mg/L as Gd |
These examples assume the stated pure hydrate and complete dissolution. Correct for product assay, moisture and any undissolved fraction. A concentration of 1 mmol/L of gadolinium corresponds to 157.25 mg/L as Gd. Total gadolinium analysis does not identify which salt or chelate supplied it.
Concentrating and recovering gadolinium
Removal from the main water stream and recovery as a useful product are separate objectives. A captured solid or regenerant can contain other rare-earth elements and contaminants. Concentration alone does not establish product purity or commercial value.
For an illustrative mass balance, 1 m3 of water at 10 mg/L as Gd contains 10 g of gadolinium. Capturing 90% of that mass in a final liquid volume of 10 L would give 900 mg/L as Gd, a concentration factor of 90. This is a calculation, not an expected treatment yield. Include all outlets, residual solids and losses when assessing a real process.
Sources
- ProChem: gadolinium chloride hexahydrate, gadolinium nitrate and gadolinium fluoride.
- Thermo Scientific Chemicals: gadolinium sulfate octahydrate.
- American Elements: gadolinium compounds and carbonate hydrate.
- Savannah River National Laboratory: Gadolinium Solubility as Functions of HNO3 and Oxalate.
- Synthesis and characterization of gadolinium phosphate neutron absorber.
- Speciation and isotope dilution analysis of gadolinium-based contrast agents in wastewater.
- Sustainable strategy for preventing medical gadolinium from entering surface water.
- Royal Society of Chemistry: gadolinium atomic data.