Hydration Structure and Hydrolysis of U(IV) and Np(IV) Ions: A Comparative Density Functional Study Using a Modified...

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Physical Chemistry

Background:

• Uranium(IV) and neptunium(IV) ions are critical in nuclear fuel cycles and environmental remediation.
• Accurate prediction of their solution behavior, including hydration and hydrolysis, is essential for safety and management.
• Existing solvation models often struggle with highly charged actinide species.

Purpose of the Study:

• To investigate the hydration and first hydrolysis reaction of U(IV) and Np(IV) ions in aqueous solution.
• To develop and validate an improved continuum solvation model for charged species.
• To accurately calculate hydration energies and hydrolysis constants for U(IV) and Np(IV).

Main Methods:

• Application of a relativistic density functional method.
• Utilization of a modified continuum solvation model with charge-dependent effective atomic radii for solute cavities.
• Calculation of solvation energies, geometries, hydration energies, and hydrolysis constants.

Main Results:

• The modified solvation model provides improved solvation energies for charged species.
• Calculated hydrolysis constants show acceptable accuracy.
• Free hydration energies of U(IV) and Np(IV) are estimated in good agreement with empirical data.
• U(IV) coordination number is predicted as 8-9 water molecules; Np(IV) coordination number is predicted as 8.

Conclusions:

• The refined solvation model accurately predicts the behavior of U(IV) and Np(IV) in aqueous solutions.
• The model offers a reliable method for calculating hydration energies and hydrolysis constants of highly charged actinide ions.
• This work advances computational chemistry for nuclear actinide speciation.