Plutonium is an important element for nuclear energy production and radioactive waste disposal, and evaluating its species distribution in natural water systems is essential for investigating its migration behavior. Recently, with the advancements in related research, some new plutonium species have been discovered and confirmed, hence previous related research is now inadequate for accurately describing the species distribution of plutonium in solutions.

This study aims to understand the speciation distribution of plutonium in different water composition systems.

The geochemical calculation software PHEEQC was employed to systematically evaluate the effects of the pH value and coexisting ion concentration on the species distribution of plutonium. The plutonium species proportion in different natural water systems was estimated in according with the latest thermodynamic data embedded into PHEEQC software.

The results reveal that hexavalent plutonium in oxidized groundwater with low hardness mainly exists as PuO₂²⁺ or PuO₂CO₃ under acidic conditions, whereas PuO₂(CO₃)₂^2- or PuO₂(CO₃)₃^4- dominate under neutral or alkaline conditions. Although PuO₂²⁺ and PuO₂CO₃ are the main species of plutonium in acidic environments, CaPuO₂(CO₃)₃^2- is the dominant species under neutral or basic conditions whereas the calcium concentration is high in the solution.

Therefore, the hardness of water (particularly, the calcium concentration) can be concluded to be among the important factors affecting the species of plutonium and must be carefully considered in the geological disposal of plutonium.