Thermodynamic Modeling and Speciation of Uranium(VI) Transuranic Complexation with Humic Substances in Saline Aquifers

International Journal of Chemistry | Vol. 17, No. 8, August 2026 | pp. 73–80

DOI: 10.46882/2026/IJC/000216

Article Type: Original Research Paper

Title: Thermodynamic Modeling and Speciation of Uranium(VI) Transuranic Complexation with Humic Substances in Saline Aquifers

Names of Authors: M. F. de Oliveira¹, A. L. Green²*

Authors’ Affiliations:
¹Department of Chemical Engineering, Instituto Superior Técnico, Lisbon, Portugal.
²Department of Earth and Environmental Sciences, University of California, Berkeley, California, United States of America.

Abstract: Assessing the geochemical mobility of radionuclides in deep geological repositories requires accurate thermodynamic data on metal-organic interactions under variable ionic strengths. This study evaluates the chemical speciation and complexation parameters of hexavalent uranium [U(VI)] with purified aquatic humic acids (HA) across a range of NaCl concentrations (0.1 M to 3.0 M). Batch potentiometric titrations and time-resolved laser-induced fluorescence spectroscopy (TRLIFS) were used to track the complexation kinetics and monitor shifts in coordination environments. Spectroscopic data revealed the formation of a dominant 1:1 complex, represented structurally as UO₂HA. The conditional stability constants (log beta) were calculated using the non-linear structural modeling framework of the Pitzer specific ion interaction equations to account for high background electrolyte activities. The calculated log beta values showed a systematic decrease from 6.24 to 4.85 as the saline molarity rose from 0.1 M to 3.0 M, demonstrating a significant screening effect of sodium ions on the carboxylate binding sites of the humic framework. Thermodynamic activation constants computed from temperature-dependence datasets using the van 't Hoff relation yielded a negative enthalpy change (-12.4 kJ/mol) and a positive entropy change (45.8 J/mol K), proving that the coordination mechanism is electrostatically driven and spontaneous.

Keywords: Uranium speciation; Humic acid; Potentiometric titration; Laser-induced fluorescence; Pitzer equations; Radioactive waste management

Manuscript Timeline: Received: February 18, 2025; Revised: April 22, 2025; Accepted: May 28, 2025; Published: August 08, 2026.

Citation: de Oliveira, M. F., & Green, A. L. (2026). Thermodynamic Modeling and Speciation of Uranium(VI) Transuranic Complexation with Humic Substances in Saline Aquifers. International Journal of Chemistry, 17(8), 73–80.

Subscribe to read the full article: https://www.internationalscholarsjournals.org/subscribe-to-read