Sequestration of HEDPA, NTA and phosphonic NTA derivatives towards Al3+in aqueous solution

The sequestration of Al3+ by etidronic acid (1‑Hydroxyethane‑1,1‑diyil)bis(phosphonic acid) (HEDPA), nitrilotriacetic acid and its phosphonic derivatives, namely N‑(phosphonomethyl)iminodiacetic acid (PMIDA, NTAP), N,N‑bis‑(phosphonomethyl)glycine (NTA2P) and [bis(phosphonomethyl)amino]methylphosphonic acid (NTA3P) was studied in aqueous solution at T = 298.15 K and I = 0.15 mol L−1 in NaCl. Formation constants and speciation models are discussed on the basis of potentiometric results. The speciation models found for all the systems include MLH, ML and MLOH species, in addition for Al3+-NTA system ML(OH)2 and M2L2(OH)2, for NTAP system ML2, for NTA2P and NTA3P systems MLH2 species, for HEDPA system ML2 and ML2OH species were detected as well. The formation constant values for ML species show the trend NTA < NTAP < HEDP < NTA2P < NTA3P. Investigations using 1H NMR spectroscopy were also performed for the study of Al3+-NTAP system. The 1H NMR findings are in agreement with the speciation model obtained by potentiometry, confirming the stabilities of the main species. The dependence of formation constants on ionic strength over the range I = 0.15 to 1 mol L−1 in NaCl is also reported for NTA, NTAP, NTA2P systems. Enthalpy change values obtained by titration calorimetry at T = 298.15 K and I = 0.15 mol L−1 in NaCl, for the main Al3+-NTA, -NTAP, -NTA2P and NTA3P species, are mainly endothermic, as typical for hard-hard interactions. The sequestering ability of the ligands under study towards Al3+ was also evaluated, under different pH conditions by the empirical parameter pL0.5.