The Effect of Metal Cations on the Aqueous Behavior of Dopamine. Thermodynamic Investigation of the Binary and Ternary Interactions with Cd2+, Cu2+ and UO22+ in NaCl at Different Ionic Strengths and Temperatures

The interactions of dopamine [2-(3,4-Dihydroxyphenyl)ethylamine, (Dop−)] with cadmium(II), copper(II) and uranyl(VI) were studied in NaCl(aq) at different ionic strengths (0 ≤ I/mol dm−3 ≤ 1.0) and temperatures (288.15 ≤ T/K ≤ 318.15). From the elaboration of the experimental data, it was found that the speciation models are featured by species of different stoichiometry and stability. In particular for cadmium, the formation of only MLH, ML and ML2 (M = Cd2+; L = dopamine) species was obtained. For uranyl(VI) (UO22+), the speciation scheme is influenced by the use of UO2(acetate)2 salt as a chemical; in this case, the formation of ML2, MLOH and the ternary MLAc (Ac = acetate) species in a wide pH range was observed. The most complex speciation model was obtained for the interaction of Cu2+ with dopamine; in this case we observed the formation of the following species: ML2, M2L, M2L2, M2L2(OH)2, M2LOH and ML2OH. These speciation models were determined at each ionic strength and temperature investigated. As a further contribution to this kind of investigation, the ternary interactions of dopamine with UO22+/Cd2+ and UO22+/Cu2+ were investigated at I = 0.15 mol dm−3 and T = 298.15K. These systems have different speciation models, with the MM’L and M2M’L2OH [M = UO22+; M’ = Cd2+ or Cu2+, L = dopamine] common species; the species of the mixed Cd2+ containing system have a higher stability with respect the Cu2+ containing one. The dependence on the ionic strength of complex formation constants was modelled by using both an extended Debye–Hückel equation that included the Van’t Hoff term for the calculation of the formation enthalpy change values and the Specific Ion Interaction Theory (SIT). The results highlighted that, in general, the entropy is the driving force of the process. The quantification of the effective sequestering ability of dopamine towards the studied cations was evaluated by using a Boltzmann-type equation and the calculation of pL0.5 parameter. The sequestering ability was quantified at different ionic strengths, temperatures and pHs, and this resulted, in general, that the pL0.5 trend was always: UO22+ > Cu2+ > Cd2+.

Radionuclide geochemistry: solubility and thermodynamics in an HLW repository

Deep geological disposal is the internationally favoured option to isolate high-level nuclear waste (HLW) from the biosphere and to minimise the potential radiological risk for future generations. Potentially contacting aqueous solutions such as groundwater may, however, lead to the corrosion of the solid casks containing the nuclear waste, and the formation of aqueous radionuclide systems in the near-field of the emplacement rooms. As dissolved species, radionuclides can in principle further migrate into the far-field and finally reach the biosphere on medium and long timescales. Like all chemical species, the radionuclides are subject to fundamental (geo)chemical laws. Relevant reactions that control retention and release, and hence, the migration behaviour and fate of radionuclides in a repository, are solubility equilibria, formation of soluble complexes, redox reactions, sorption on and incorporation into mineral surfaces, transport phenomena etc. These processes depend directly on the (geo)chemical boundary conditions, and, consequently, can differ greatly for various host rock systems such as clay rock, rock salt, and crystalline rock. Many of the radionuclides in HLW are heavy metals that are sparingly soluble under various repository-relevant conditions, e.g. actinides, lanthanides, transition metals, so that only partial dissolution (mobilisation) from the solid waste matrices is expected. This underlines the importance of evaluating the radionuclide solubility within a geochemically based safety assessment for repositories as it provides reliable upper-limit concentrations of the mobile, potentially migrating radionuclide fraction in the near-field. In this contribution, we discuss relevant aspects related to the topic radionuclide solubility and thermodynamics in a HLW repository. This includes a summary of recent laboratory studies on the solubility behaviour and speciation of key radionuclides in repository-relevant solutions, which are an important basis for obtaining (geo)chemical information and models, and the corresponding fundamental thermodynamic constants on aqueous radionuclide systems. National and international thermodynamic database projects, where quality-assured thermodynamic data (solubility products, complex formation constants, and ion-interaction parameters) are evaluated and compiled, e.g. the Nuclear Energy Agency Thermochemical Database (http://www.oecd-nea.org, last access: 1 November 2021) or the Thermodynamic Reference Database (http://www.thereda.de, last access: 1 November 2021), are highlighted and the main remaining uncertainties discussed. The experimental information and the quantitative thermodynamic data are applied within a generic case study to demonstrate the impact of different geochemical solution conditions representing different host rock systems considered as HLW repositories in Germany on the solubility and speciation of selected radionuclides.

Thermodynamic Study of Oxidovanadium(IV) with Kojic Acid Derivatives: A Multi-Technique Approach

The good chelating properties of hydroxypyrone (HPO) derivatives towards oxidovanadium(IV) cation, VIVO2+, constitute the precondition for the development of new insulin-mimetic and anticancer compounds. In the present work, we examined the VIVO2+ complex formation equilibria of two kojic acid (KA) derivatives, L4 and L9, structurally constituted by two kojic acid units linked in position 6 through methylene diamine and diethyl-ethylenediamine, respectively. These chemical systems have been characterized in solution by the combined use of various complementary techniques, as UV-vis spectrophotometry, potentiometry, NMR and EPR spectroscopy, ESI-MS spectrometry, and DFT calculations. The thermodynamic approach allowed proposing a chemical coordination model and the calculation of the complex formation constants. Both ligands L4 and L9 form 1:1 binuclear complexes at acidic and physiological pHs, with various protonation degrees in which two KA units coordinate each VIVO2+ ion. The joined use of different techniques allowed reaching a coherent vision of the complexation models of the two ligands toward oxidovanadium(IV) ion in aqueous solution. The high stability of the formed species and the binuclear structure may favor their biological action, and represent a good starting point toward the design of new pharmacologically active vanadium species.

The Solution Behavior of Dopamine in the Presence of Mono and Divalent Cations: A Thermodynamic Investigation in Different Experimental Conditions

The interactions of dopamine [2-(3,4-Dihydroxyphenyl)ethylamine, (Dop-)] with methylmercury(II) (CH3Hg+), magnesium(II), calcium(II), and tin(II) were studied in NaCl(aq) at different ionic strengths and temperatures. Different speciation models were obtained, mainly characterized by mononuclear species. Only for Sn2+ we observed the formation of binuclear complexes (M2L2 and M2LOH (charge omitted for simplicity); M = Sn2+, L = Dop−). For CH3Hg+, the speciation model reported the ternary MLCl (M = CH3Hg+) complex. The dependence on the ionic strength of complex formation constants was modeled by using both an extended Debye–Hückel equation that included the Van’t Hoff term for the calculation of enthalpy change values of the formation and the Specific Ion Interaction Theory (SIT). The results highlighted that, in general, the entropy is the driving force of the process. The sequestering ability of dopamine towards the investigated cations was evaluated using the calculation of pL0.5 parameter. The sequestering ability trend resulted to be: Sn2+ > CH3Hg+ > Ca2+ > Mg2+. For example, at I = 0.15 mol dm−3, T = 298.15 K and pH = 7.4, pL0.5 = 3.46, 2.63, 1.15, and 2.27 for Sn2+, CH3Hg+, Ca2+ and Mg2+ (pH = 9.5 for Mg2+), respectively. For the Ca2+/Dop- system, the precipitates collected at the end of the potentiometric titrations were analyzed by thermogravimetry (TGA). The thermogravimetric calculations highlighted the formation of solid with stoichiometry dependent on the different metal:ligand ratios and concentrations of the starting solutions.

Nucleotide Interaction with Chitosan Layer Deposited on Fumed Silica Surface: Practice and Theory towards Establishing the Mechanism at the Molecular Level

The growing interest in gene therapy is coupled to the strong need for the development of safe and efficient gene transfection vectors. A composite based on chitosan and fumed silica has been found to be a prospective gene delivery carrier. This study presents an investigation of the nature of the bonds between a series of mono-, di- and triphosphate nucleotides with a chitosan layer deposited on a fumed silica surface. It was observed that the adsorption of most of the studied nucleotides is determined by the formation of one surface complex. Experimentally measured surface complex formation constants (logK) of the nucleotides were found to be in range 2.69–4.02 which is higher than that for the orthophosphate (2.39). Theoretically calculated nucleotide complexation energies for chitosan deposited on the surface range from 11.5 to 23.0 kcal·mol^–1 in agreement with experimental data. The adsorption of nucleotides was interpreted using their calculated speciation in aqueous solution. Based on the structures of all optimized complexes determined from quantum-chemical PM6 calculations, electrostatic interactions between the surface-located NH₃⁺ groups and –PO₃H^––/–PO₃^2- fragments of the nucleotides was identified to play the decisive role in the adsorption mechanism. The saccharide fragment of monophosphates also plays an important role in the binding of the nucleotides to chitosan through the creation of hydrogen bonds; in the case of di- and triphosphates the role of the saccharide fragment decreases significantly.

Nucleotide Interaction with Chitosan Layer Deposited on Fumed Silica Surface: Practice and Theory towards Establishing the Mechanism at the Molecular Level

The growing interest in gene therapy is coupled to the strong need for the development of safe and efficient gene transfection vectors. A composite based on chitosan and fumed silica has been found to be a prospective gene delivery carrier. This study presents an investigation of the nature of the bonds between a series of mono-, di- and triphosphate nucleotides with a chitosan layer deposited on a fumed silica surface. It was observed that the adsorption of most of the studied nucleotides is determined by the formation of one surface complex. Experimentally measured surface complex formation constants (logK) of the nucleotides were found to be in range 2.69–4.02 which is higher than that for the orthophosphate (2.39). Theoretically calculated nucleotide complexation energies for chitosan deposited on the surface range from 11.5 to 23.0 kcal·mol^–1 in agreement with experimental data. The adsorption of nucleotides was interpreted using their calculated speciation in aqueous solution. Based on the structures of all optimized complexes determined from quantum-chemical PM6 calculations, electrostatic interactions between the surface-located NH₃⁺ groups and –PO₃H^––/–PO₃^2- fragments of the nucleotides was identified to play the decisive role in the adsorption mechanism. The saccharide fragment of monophosphates also plays an important role in the binding of the nucleotides to chitosan through the creation of hydrogen bonds; in the case of di- and triphosphates the role of the saccharide fragment decreases significantly.

Understanding the Solution Behavior of Epinephrine in the Presence of Toxic Cations: A Thermodynamic Investigation in Different Experimental Conditions

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 NaClaq 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: ΔHHL = −39 ± 1 kJ mol−1, ΔHH2L = −67 ± 1 kJ mol−1 (overall reaction), ΔHML = −26 ± 4 kJ mol−1, and ΔHM2L2(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 pL0.5 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+), pL0.5 = 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.

Determination of complex formation constants of neptunium(V) with propionate and lactate in 0.5–2.6 m NaCl solutions at 22–60°C using a solvent extraction technique

Natural clay rocks like Opalinus (OPA) and Callovo-Oxfordian (COx) clay rock are considered as potential host rocks for deep geological disposal of nuclear waste. However, small organic molecules such as propionate and lactate exist in clay rock pore water and might enhance Np mobility through a complexation process. Therefore, reliable complex formation data are required in the frame of the Safety Case for a nuclear waste repository. A solvent extraction technique was applied for the determination of ${\rm{NpO}}_2^ + $ complexation with propionate and lactate. Extraction was conducted from isoamyl alcohol solution containing 10−3 M TTA and 5 · 10−4 M 1,10-phenanthroline. Experiments were performed in 0.5–2.6 m NaCl solutions at temperatures ranging from 22 to 60 °C. Formation of 1:1 Np(V) complexes for propionate and lactate was found under the studied conditions. The SIT approach was applied to calculate equilibrium constants β°(T) at zero ionic strength from the experimental data. Log β°(T) is found linearly correlated to 1/T for propionate and lactate, evidencing that heat capacity change is near 0. Molal reaction enthalpy and entropy ( ${\Delta _{\rm{r}}}H_{\rm{m}}^ \circ $ and ${\Delta _{\rm{r}}}S_{\rm{m}}^ \circ $ ) could therefore be derived from the integrated van’t Hoff equation. Data for log β° (298.15 K) are in agreement with literature values for propionate and lactate. Np(V) speciation was calculated for concentrations of acetate, propionate and lactate measured in clay pore waters of COx. In addition, the two site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model was applied to quantitatively describe the influence of Np(V) complexation on its uptake on Na-illite, a relevant clay mineral of OPA and COx.

Determination of Distribution Equilibrium-Potential Differences Based on Extraction with Several Crown Ethers by Nitrobenzene, 1,2-Dichloroethane and Dichloromethane

Extraction constants (Kex± & Kex) were determined at 298 K for the extraction of sodium picrate (NaPic) by nitrobenzene (NB), 1,2-dichloroethane (DCE) and dichloromethane using 3m-crown-m ethers and their benzo-derivatives (m = 5, 6; abbreviated as L) together with the determination of conditional distribution constants (KD,Pic) of picrate ion, Pic-, into these diluents. The K1,org (= [NaLPic]org/[NaL+]org[Pic-]org) values at the organic (org) phases, such as NB & DCE, were calculated from the relation Kex/Kex± = K1,org. Distribution equilibrium-potential differences (Dfeq) at extraction equilibria were evaluated from the equation Dfeq = -0.05916´(log KD,Pic - constant) at 298 K. Correlations of the above equilibrium constants, particularly Kex±, with Dfeq were examined. Furthermore, the standard formal potentials for Na+ transfers across the interfaces were briefly evaluated from calculated Dfeq, [Na+] and [Pic-]org. The above extraction systems were characterized by K1,org and the complex formation constants of Na+ with L in the org phases.