Thermodynamic Solution Properties of a Biodegradable Chelant (L-glutamic-N,N-diacetic Acid,L-GLDA) and Its Sequestering Ability toward Cd2+

The thermodynamics of the interaction of L-glutamic-N,N-diacetic acid (GLDA) with protons was studied potentiometrically at different temperatures, ionic strengths and ionic media. Four protonation constants and corresponding enthalpy changes occurred at infinite dilution together with temperature and ionic strength coefficients. The medium effect was also interpreted in terms of the formation of weak complexes between the ligand and the cations of supporting electrolytes, resulting in a greater tendency of GLDA to chemically interact with Na⁺ rather than K⁺ and, in turn, (CH₃)₄N⁺. Formation constants of GLDA with Cd²⁺ were determined in NaCl_(aq) at different ionic strength values. Five complex species were found, namely CdL²⁻, CdHL⁻, CdH₂L⁰_(aq), Cd₂L⁰_(aq), and Cd(OH)L³⁻, whose formation constant values at infinite dilution were log β = 12.68, 17.61, 20.76, 17.52, and 1.77, respectively. All the species results were relevant in the pH range of natural waters, although the Cd₂L⁰_(aq) was observed only for C_Cd ≥ C_GLDA and concentrations of >0.1 mmol dm⁻³. The sequestering ability of GLDA toward Cd²⁺, evaluated by means of pL_0.5, was maximum at pH~10, whereas the presence of a chloride containing a supporting electrolyte exerted a negative effect. Among new generation biodegradable ligands, GLDA was the most efficient in Cd²⁺ sequestration.