Less so the weathering, but the Terbium is extreme low on the...

Less so the weathering, but the Terbium is extreme low on the weathered samples.

https://www.researchgate.net/publication/339359335_Characteristics_of_Precipitation_of_Rare_Earth_Elements_with_Various_PrecipitantsGrok, summarises the Terbium and Europium spikes as follows:

Here's a summary of the carbonate system's participation in relation to europium (Eu) and terbium (Tb):

Europium (Eu):

Precipitation Behavior: Europium generally forms insoluble carbonates at higher pH levels. This is due to the low solubility of
Eu²⁺ and Eu³⁺ carbonates, making carbonate precipitation an effective method for europium recovery or separation from other REEs.

Redox Sensitivity: Since europium can exist in both divalent (Eu²⁺) and trivalent (Eu³⁺) oxidation states, the carbonate system's interaction can be influenced by the redox environment. In an oxidizing environment, Eu³⁺ forms more stable carbonate complexes, whereas in reducing conditions, Eu²⁺ might be more prevalent, altering the precipitation dynamics.

Selectivity: The carbonate system can be used to selectively precipitate europium from a mixture of REEs by fine-tuning the pH, as different REEs have slightly different solubility products for their carbonates.

Terbium (Tb):

Precipitation Characteristics: Terbium, being trivalent (Tb³⁺), also forms insoluble carbonates, but its behavior can differ slightly from europium due to its ionic radius and electronic configuration. The precipitation of terbium carbonate typically requires a higher pH than for europium due to its lower solubility product.

Complexation: Terbium can form stable carbonate complexes in solution, which can influence its precipitation behavior. These complexes might require higher pH or different conditions to precipitate out of solution compared to free Tb³⁺ ions.

Separation Challenges: The carbonate system's participation in terbium recovery or separation often aims at exploiting differences in solubility and complexation behavior compared to other REEs. However, due to the chemical similarity among REEs, achieving high selectivity can be challenging, often requiring additional steps or combined methods for effective separation.

General Considerations:

pH Sensitivity: Both Eu and Tb carbonate precipitation is highly pH-sensitive. Adjusting pH is a key strategy in controlling the precipitation process to either selectively precipitate one over the other or to precipitate them together for bulk separation from other elements.

Environmental Impact: The use of carbonates is often seen as more environmentally friendly compared to other precipitants like oxalates or phosphates due to the benign nature of carbonate residues.

Process Integration: Carbonate precipitation can be integrated into broader separation schemes where it serves as an initial or intermediate step, followed by other methods like solvent extraction or ion exchange for further purification.

In summary, the carbonate system plays a crucial role in the precipitation and separation of europium and terbium, leveraging pH adjustments and the different solubility properties of their carbonate forms to achieve separation from other REEs or from each other under specific conditions.


This is visualised in the infographic used in presentation slide and 2023 FY annual report.