Sequestration of HEDPA, NTA and phosphonic NTA derivatives towards Al 3+ in aqueous solution Paola Cardiano, Concetta De Stefano, Claudia Foti, Fausta Giacobello, Ottavia Giuffrè ⁎, Silvio Sammartano Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, Università di Messina, Viale F. Stagno d'Alcontres 31, 98166 Messina, Italy abstract article info Article history: Received 5 March 2018 Received in revised form 29 March 2018 Accepted 2 April 2018 Available online 05 April 2018 The sequestration of Al 3+ 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 Al 3+ -NTA system ML(OH) 2 and M 2 L 2 (OH) 2 , for NTAP system ML 2 , for NTA2P and NTA3P systems MLH 2 species, for HEDPA system ML 2 and ML 2 OH species were detected as well. The formation constant values for ML species show the trend NTA b NTAP b HEDP b NTA2P b NTA3P. Investigations using 1 H NMR spectroscopy were also performed for the study of Al 3+ -NTAP sys- tem. The 1 H 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 ti- tration calorimetry at T = 298.15 K and I = 0.15 mol L -1 in NaCl, for the main Al 3+ -NTA, -NTAP, -NTA2P and NTA3P species, are mainly endothermic, as typical for hard-hard interactions. The sequestering ability of the li- gands under study towards Al 3+ was also evaluated, under different pH conditions by the empirical parameter pL 0.5 . © 2018 Elsevier B.V. All rights reserved. Keywords: Sequestration Al 3+ complexes Etidronic acid Nitrilotriacetic acid and its phosphonic deriva- tives Aqueous solutions Thermodynamic parameters 1 H NMR spectroscopy 1. Introduction Aluminium is the third constituent of the earth's crust and the most abundant metallic element. It has a wide variety of uses, such as medi- cines, surgery materials, cosmetics, as well as in water purification, building, food packaging, beverage cans, food additives, cooking tools, etc. [1]. It occurs ubiquitously in the environment, but since it is not an essential element in bioprocesses, it results toxic for animals, plants and humans [1–3]. The aluminium hydrolysis significantly affects its solubility and its bioavailability [1]. Because of its hard nature, it prefer- entially interacts with various ligands having the same character, such as phosphate, carboxylate, phenolate and catecholate [4–7]. Among chelating agents, polyphosphonates are of great interest, since these ligands are used in medicine, biology, environmental field and in several industrial and technological applications as scale inhibi- tors [8–12]. Phosphonates are characterized by a low toxicity, by high chemical stability and resistance against enzymatic hydrolysis [9]. Di- phosphonic acids, containing the P-C-P backbone, are commonly named bisphosphonates and are employed for the treatment of several diseases related to the excessive bone resorption, such as osteoporosis, myeloma and bone metastases [9,13]. Polyphosphonate derivatives were also studied for applications as therapeutic radiopharmaceuticals. They are also present in several detergent formulations, since they bind Ca(II) ions, improving the cleaning action [10,14,15]. As a result these li- gands enter the aquatic ecosystem being released with domestic waste waters [14,15]. Flowing through plants of water potabilization, they un- dergo a process of coagulation/flocculation with aluminium or ferric hy- droxides [15]. Other applications of HEDPA include those for scale inhibition in water treatment, as dye-fixing agent, in therapeutic treat- ments [10,16]. Several biological uses of phosphonates are based on their ability to give electrostatic interactions, to form hydrogen-bonds and to interact with various metal ions. Di- and tri-phosphonates can give rise to a multi oxygen coordination to metal ions forming oligo- meric or polymeric structures [9,13]. The most common phosphonates are structural analogues of aminopolycarboxylates, such as ethyl- enediaminetetraacetate (EDTA) and nitrilotriacetate (NTA)[11]. Aminopolycarboxylates present a tertiary nitrogen atom in the cen- tre of the molecule and acidic groups linked to alkyl residues. Ac- cordingly, at least four donor groups, can potentially form 1:1 species with metal cations [17]. Phosphonates are effective in the complexation of strongly hydrolyzed cations, such as Al 3+ [12]. As Journal of Molecular Liquids 261 (2018) 96–106 ⁎ Corresponding author. E-mail address: ogiuffre@unime.it. (O. Giuffrè). https://doi.org/10.1016/j.molliq.2018.04.003 0167-7322/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq