Thermodynamics and dissociation constants of acetohydroxamic acid (AHA), pyrocatechol (PYR), acetic acid (AA) and glycolic acid (GA) at variable ion strength and temperature in NaCl solution and the corresponding activity correction to improve speciation scheme for U and V in cement leachates
Creators: name: Chen, Congying
Publisher: name: Imperial College London
Year: 2023

description: The uptake of uranium and vanadium in the leakage of geological disposal facilities (GDF) via ligands sourced from vegetation, bacterial and fungi are widely discussed. It probably results in the remobilisation of these two pollutants and lead to the radioactive and heavy-metal contamination in the environment. The quantification of their complexes is the important section to provide environment risk management with scientific evidences. In this study, acetohydroxamic acid (HAHA), pyrocatechol (PYR), glycolic acid (GA) and acetic acid (AA) as typical representatives of common ligands under natural conditions were investigated via potentiometric titration (PT). It aims to determine their dissociation constants (pKa) and thermodynamic parameters (Gibbs energy ∆𝐺 , enthalpy ∆H and entropy ∆S values) for improving U(VI)-L and V(IV)-L complexing models in sodium chloride (NaCl) solution. To increase the understanding of the effect of ion strength and temperature on ligands as well as their coordination with UO2 2+ and VO2+ , their pKa values at ion strength of 0.1M, 0.5M, 1.0M, 2.0M and 3.0M and at temperature of 298.15K, 315.15K and 335.15K were measured and showed the differences compared with the results in previous studies. Based on Van’t Hoff equation, their thermodynamic parameters are also calculated. As a result, the effect of ion strength on the pKa values among these ligands showed a decrease from ion strength from 0.1 to 1.0 M, while an increase was found as ion strength increased to 3.0 M. The pKa values of HAHA and PYR displayed a high sensitivity to the temperature in this work, while the ones of GA and AA were inert to the variation of the temperature. In the calibration of intrinsic values, specific Ion Interaction Theory (SIT) showed the high consistency of pKa values obtained from 1.0 M to 3.0 M, while the Davies equation (DE) was suitable for ion strength 0.1 M to 0.5M. Meanwhile, it was found that the intrinsic pKa values of HAHA and PYR respective to the calculation of SIT and DE existed large differences referred to the literature values. In Hyss models, the new pKa values showed differences combing with the established models. Additionally, according to the speciation curves of M-L complexes with the intrinsic pKa values, the curves derived from the SIT and DE was basically showed lower contents of targeted M-L complexes than the experimental ones. Based on the highest formative percentages in their complexing species, the affinity to the metal ions followed the order: HAHA&gt;PYR&gt;GA&gt;AA in terms of their largest formation percentages. The study showed an appropriate method to determine the ligand protonation under old cement leachate (OCL) and provide geological models with the data regarding the coordination of the targeted ligands with metal ions like uranyl and vanadyl ions as well.