Removal of di- and tri-alkyltin(IV) compounds by polyphosphonate ligand: A speciation perspective Paola Cardiano, Claudia Foti ⁎, Ottavia Giuffrè 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 7 April 2017 Received in revised form 11 May 2017 Accepted 15 May 2017 Available online 17 May 2017 The potential of employing diethylenetriamine-N,N,N′,N″,N″-pentakis(methylenephosphonic acid), DTPMP, to remove (CH 3 ) 3 Sn + , (C 2 H 5 ) 3 Sn + , (C 3 H 7 ) 3 Sn + , (CH 3 ) 2 Sn 2+ , (C 2 H 5 ) 2 Sn 2+ cations from aqueous solution was evaluated on the basis of speciation modeling. For this purpose, the interactions of DTPMP towards alkyltin(IV) cations in NaCl at I = 0.1 mol L -1 (for (CH 3 ) 2 Sn 2+ , also in the ionic strength range 0.1 ≤ I ≤ 1 mol L -1 ) was stud- ied. Potentiometric measurements evidenced the formation of MLH j species, with 0 ≤ j ≤ 5, and stability constant values that follow the trend (CH 3 ) 3 Sn + N (C 2 H 5 ) 3 Sn + N (C 3 H 7 ) 3 Sn + , for trialkyltin(IV) cations and (CH 3 ) 2 Sn 2+ N (C 2 H 5 ) 2 Sn 2+ , for dialkyl ones. 1 H NMR investigations in solution allowed to confirm all the models proposed on the basis of potentiometric findings and, in some cases, to gain information about the coordination arrangement around the tin. Once assessed the most reliable speciation models, the sequestering ability of the polyphosphonic ligand was evaluated by pL 0.5 empiric parameter (ligand concentration required to sequester 50% of the metal cation present in traces). © 2017 Elsevier B.V. All rights reserved. Keywords: Polyphosphonic ligand Alkyltin(IV) cations Speciation models Coordination mode Sequestering ability 1. Introduction In the last years, the use of classical chelating agents like EDTA has been reduced in favour of new classes of compounds which should be, possibly, more effective, selective, non-toxic, cheap and “eco friendly”. In particular, the class of (poly)phosphonates looks very promising for the replacement of old “complexones”. Organophosphates are com- pounds bearing the R-C-P(O)-(O) 2 2- group, with a stable carbon- phosphorus bond. The presence of the carbon atom between the phosphate unit and an organic backbone imparts a high stability to the molecule which is, as a result, less prone to enzymatic hydrolysis [1]. The high stability is often associated to a low toxicity and to a strong complexing ability towards a variety of metal cations. This has made or- ganophosphates very appealing in a variety of applications in industry, technology, biology and biomedicine, and nanotechnology as well [2– 5]. Organophosphates are, in fact, employed in chelation and prevention of scale formation, as complexing ligands potentially active in living systems, as antimetabolites, as molecular tools in biologically relevant molecules sensing and recognition, in the drug delivery and treatment, or, if anchored to a polymer matrix, for cation exchange separations of several metals. From the second half of the twentieth century, these compounds have been widely used as chelating agents since they provided very interesting performances as suitable alternatives to clas- sic complexones [2,6,7]. All these applications are (directly or not) related to their coordina- tion chemistry, so that the comprehension of their binding ability (and, therefore, their speciation) and coordination mode towards cations is crucial, especially considering that, in real systems, many other compet- ing ligands and interfering cations may be present, thus reducing the ef- ficacy of the chelation process. Moreover, over the pH range of natural waters, phosphonates show very strong interactions with mineral sur- faces, also when coordinated to metal cations. This strong adsorption hinders the release of metal cations [4]. Speciation studies play there- fore a key role for the assessment of the optimal conditions for their use and to define the role of chelating agents in the distribution of metal cations in natural waters. In particular, the knowledge of the ther- modynamic parameters is of great importance for the application to real systems, making possible the speciation prediction in systems featured by very variable composition, pH, ionic strength, temperature. Here we report on the coordination ability of an EDTA phosphonate derivative, the diethylenetriamine-N,N,N′,N″,N″-pentakis(methylene phosphonic acid) (DTPMP), towards alkyltin(IV) cations. DTPMP mo- lecular structure is shown in Chart 1, together with the atom labeling employed in the discussion of 1 H NMR spectra. Alkyltin(IV) cations under study are: (CH 3 ) 3 Sn + , (C 2 H 5 ) 3 Sn + , (C 3 H 7 ) 3 Sn + , (CH 3 ) 2 Sn 2+ , (C 2 H 5 ) 2 Sn 2+ . The presence of these compounds in the environment and in the biological systems is known, as well as their toxicity and their strong biological activity. Fraction of these compounds may be Journal of Molecular Liquids 240 (2017) 128–137 ⁎ Corresponding author. E-mail address: cfoti@unime.it (C. Foti). http://dx.doi.org/10.1016/j.molliq.2017.05.067 0167-7322/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq