Thermodynamic Solution Properties of a Biodegradable Chelant (L-glutamic-N,N-diacetic Acid, L-GLDA) and Its Sequestering Ability toward Cd(2+)

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, (CH(3))(4)N(+). Formation constants of GLDA with Cd(2+) were determined in NaCl((aq)) at different ionic strength values. Five complex species were found, namely CdL(2−), CdHL(−), CdH(2)L(0)((aq)), Cd(2)L(0)((aq)), and Cd(OH)L(3−), 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 Cd(2)L(0)((aq)) was observed only for C(Cd) ≥ C(GLDA) and concentrations of >0.1 mmol dm(−3). The sequestering ability of GLDA toward Cd(2+), 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 Cd(2+) sequestration.