# Uranyl(VI) Interaction with 2-Phosphonobutane-1,2,4-Tricarboxylic Acid (PBTC): A Spectroscopic and Computational Study over a Wide pH Range

## TL;DR

This study explores how a chemical called PBTC interacts with uranium in different pH conditions, using various spectroscopic and computational methods.

## Contribution

The first investigation of PBTC's complexation behavior with U(VI) across a wide pH range using multiple spectroscopic and computational techniques.

## Key findings

- PBTC forms a chelate complex with U(VI) via phosphonate and geminal carboxylate groups across most of the pH range.
- UV-Vis and factor analysis revealed the distribution and stability of protonated chelate species.
- TRLFS confirmed the spectroscopic findings and provided complementary insights into the complexation.

## Abstract

Organophosphonates have manifold applications in the chemical industry, of which one of the most commonly used is 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC). It is widely used as a cement additive and may pose a potential risk of complexing radionuclides such as uranium in nuclear waste repositories. PBTC, in its fully deprotonated form, has four negatively charged groups, one phosphonate and three carboxylate groups, which makes it a superior ligand for metal ion complexation. In this study, for the first time, its complexation behavior towards hexavalent uranium, U(VI), in the pH range from 2 to 11, has been investigated using various spectroscopic methods. The structure-sensitive methods NMR, IR, and Raman spectroscopy were used to characterize the complex structure. The interpretation of the results was supported by density functional calculations. Over almost the entire pH range studied, U(VI) and PBTC form a chelate complex via the phosphonate and the geminal carboxylate group, highlighting the strong chelating ability of the ligand. UV-Vis spectroscopy combined with factor analysis was applied to determine the distribution of differently protonated chelate species and their stability constants. Time-resolved laser-induced luminescence spectroscopy (TRLFS) was additionally used as a complementary method.