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>PYR>GA>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.