Thermodynamic insights into Trans-Aconitate interactions with H+, Cd2+, Mn2+, and Pb2+: Equilibrium constants, enthalpy changes and sequestering ability

The thermodynamics of interaction of trans-aconitate (L3-) with proton, sodium and potassium cations was studied by means of potentiometric titration performed at different temperatures, ionic strengths, and aqueous ionic media (NaCl, KCl and (C2H5)4NI). Three protonation constants and corresponding enthalpy changes are reported at infinite dilution together with their dependence parameters on temperature and ionic strength according to van’t Hoff and Extended Debye Hückel equations, respectively. Enthalpy changes resulted slightly endothermic at infinite dilution and values decrease with increasing ionic strength. Proton binding resulted always entropic in nature. Weak association constants of Na+/L3- and K+/L3- species were determined using the pure water model approach. Formation constants of trans-aconitate with Cd2+, Pb2+and Mn2+were determined in KCl(aq) at different ionic strength values and at 298.15 K. Three complex species were found with all metal cations (ML-, MHL0(aq), MH2L+), whose formation constant values at infinite dilution and at T =298.15 K were log β =4.54, 9.94, 13.77 for cadmium, 5.00, 10.57, 14.81 for manganese and 4.97, 10.44, 14.13 for lead. The as found values resulted unexpectedly high. The sequestering ability of trans-aconitate towards M2+was evaluated by determining pL0.5 (the ligand total concentration required to bind 50% of the metal cation), and the results show that, throughout the investigated pH range, trans-aconitate shows the highest sequestering ability towards Mn2+. Data reported in this paper were critically compared to other tricarboxylic acid, namely citric and tricarballylic acid.