Silvio Sammartano

8-hydroxyquinoline-2-carboxylic acid (8-HQA) has been found in high concentrations (0.5–5.0 mmol·dm⁻³) in the gut of Noctuid larvae (and in a few other lepidopterans), in which it is proposed to act as a siderophore. Since it is known that many natural siderophores are also involved in the uptake and metabolism of other essential elements than iron, this study reports some results on the investigation of 8-HQA interactions with molybdate (MoO₄²⁻, i.e., the main molybdenum form in aqueous environments), in order to understand the possible role of this ligand as molybdophore. A multi-technique approach has been adopted, in order to derive a comprehensive set of information necessary to assess the chemical speciation of the 8-HQA/MoO₄²⁻ system, as well as the coordination behavior and the sequestering ability of 8-HQA towards molybdate. Chemical speciation studies have been performed in KCl_(aq) at I = 0.2 mol·dm⁻³ and T = 298.15 K by ISE-H⁺ (glass electrode) potentiometric and UV/Vis spectrophotometric titrations. CV (Cyclic Voltammetry), DP-ASV (Differential Pulse-Anodic Stripping Voltammetry), ESI-MS experiments and quantum mechanical calculations have been also performed to derive information about the nature and possible structure of species formed. These results are also compared with those reported for the 8-HQA/Fe³⁺ system in terms of chemical speciation and sequestering ability of 8-HQA.

The interactions of epinephrine ((R)-(−)-3,4-dihydroxy-α-(methylaminomethyl)benzyl alcohol; Eph⁻) with different toxic cations (methylmercury(II): CH₃Hg⁺; dimethyltin(IV): (CH₃)₂Sn²⁺; dioxouranium(VI): UO₂²⁺) were studied in NaCl_aq at different ionic strengths and at T = 298.15 K (T = 310.15 K for (CH₃)₂Sn²⁺). The enthalpy changes for the protonation of epinephrine and its complex formation with UO₂²⁺ were also determined using isoperibolic titration calorimetry: ΔH_HL = −39 ± 1 kJ mol⁻¹, ΔH_H2L = −67 ± 1 kJ mol⁻¹ (overall reaction), ΔH_ML = −26 ± 4 kJ mol⁻¹, and ΔH_M2L2(OH)2 = 39 ± 2 kJ mol⁻¹. The results were that UO₂²⁺ complexation by Eph⁻ was an entropy-driven process. The dependence on the ionic strength of protonation and the complex formation constants was modeled using the extended Debye–Hückel, specific ion interaction theory (SIT), and Pitzer approaches. The sequestering ability of adrenaline toward the investigated cations was evaluated using the calculation of pL_0.5 parameters. The sequestering ability trend resulted in the following: UO₂²⁺ >> (CH₃)₂Sn²⁺ > CH₃Hg⁺. For example, at I = 0.15 mol dm⁻³ and pH = 7.4 (pH = 9.5 for CH₃Hg⁺), pL_0.5 = 7.68, 5.64, and 2.40 for UO₂²⁺, (CH₃)₂Sn²⁺, and CH₃Hg⁺, respectively. Here, the pH is with respect to ionic strength in terms of sequestration.

The acid–base properties of two bifunctional 3-hydroxy-4-pyridinone ligands and their chelating capacity towards Zn²⁺, an essential bio-metal cation, were investigated in NaCl aqueous solutions by potentiometric, UV-Vis spectrophotometric, and ¹H NMR spectroscopic titrations, carried out at 0.15 ≤ I/mol ⁻¹ ≤ 1.00 and 288.15 ≤ T/K ≤ 310.15. A study at I = 0.15 mol L⁻¹ and T = 298.15 K was also performed for other three Zn²⁺/L^z− systems, with ligands belonging to the same family of compounds. The processing of experimental data allowed the determination of protonation and stability constants, which showed accordance with the data obtained from the different analytical techniques used, and with those reported in the literature for the same class of compounds. ESI-MS spectrometric measurements provided support for the formation of the different Zn²⁺/ligand species, while computational molecular simulations allowed information to be gained on the metal–ligand coordination. The dependence on ionic strength and the temperature of equilibrium constants were investigated by means of the extended Debye–Hückel model, the classical specific ion interaction theory, and the van’t Hoff equations, respectively.

8-HQA is a good sequestering agent towards Fe²⁺and Fe³⁺over a wide pH range.

Landfill leachates can contaminate nearby aquifers. The hazards deriving from this contamination also depend on the chemical speciation of various contaminants. A novel approach is proposed here to face this problem from a chemical thermodynamics point of view. The complexing ability of the soluble fraction of a landfill leachate (collected from Bellolampo, Palermo, Italy) towards Pb²⁺, Cd²⁺ and Cu²⁺ has been investigated at T = 298.15 K in NaCl_aq at I = 0.1 mol dm⁻³. The soluble fraction of the landfill leachate was first characterized by different analytical techniques. Then, its acid–base properties were studied by ISE-H⁺ potentiometric titrations and modelled by the so-called diprotic-like model. Differential Pulse Anodic Stripping Voltammetry (DP-ASV) titrations of metal ion aqueous solutions with a diluted landfill leachate were carried out, successively, in order to determine the stability constants of the leachate-metal complexes. The stability of the Pb²⁺/OH⁻ and Pb²⁺/Cl⁻ complexes was also studied by the same technique. Finally, the sequestering ability of the leachate towards the investigated metal cations has been quantified by the calculations of various pL_0.5 values under different pH conditions. The results proved that the landfill leachate is a good sequestering agent toward those cations, reducing the fraction of the free cations in solution, and that it can be a selective sequestrant at different pH.

The complex formation between molybdate(VI) (MoO₄ ²⁻) and phytate [Phy, 1,2,3,4,5,6-hexakis(dihydrogen phosphate)myo-inositol] has been investigated at T = 298.15 K in NaCl_(aq) at different ionic strengths (0 < I/mol dm⁻³ ≤ 1.0). Under these experimental conditions, eight (MoO₄)(Phy)H_r protonated species are formed, with 2 ≤ r ≤ 9. The corresponding formation constants determined proved to be fairly dependent on ionic strength, and this dependence has been modelled by an extended Debye–Hückel equation. Furthermore, the stability (in terms of stepwise equilibria) of (MoO₄)(Phy)H_r complexes regularly increases with the number of protons in the species, probably due to the formation of intermolecular hydrogen bonds between molybdate and phytate in their protonated forms. This unusual behaviour affects the sequestering ability of phytate towards molybdate, which is higher at low rather than at high pH, as demonstrated by the analysis of the pL_0.5 values calculated for this system under various conditions. Finally, a series of empirical relationships have been proposed, with the aim of defining the speciation and the sequestration of molybdate and phytate in a wide number of systems.

The complex formation between a series of biologically, environmentally, and technologically relevant cations (namely Sn²⁺, Zn²⁺, Cu²⁺, Fe²⁺, Fe³⁺ and Al³⁺) and diethylenetriamine-N,N,N′,N′′,N′′-pentakis(methylenephosphonic) acid (DTPMPA) has been investigated in NaCl aqueous solutions at I = 0.1 and 0.3 mol dm⁻³ and T = 298.15 K. The ligand used has been obtained in sufficient purity by a new efficient synthetic procedure for the determination of accurate and reliable data on its acid–base properties and metal complex formation. The stability constants determined in this work under different experimental conditions have been then used to assess the speciation of many systems containing the above-cited cations, and to quantify the sequestering ability of DTPMPA toward them, by means of the calculation of several pL_0.5 values under various conditions simulating those of many real systems where this chelant is employed as a cationic sequestrant. Finally, some ¹H- and ³¹P-NMR studies have also been performed to gain a further insight into the binding mode of DTPMPA toward metal cations.