Search Results (32)

In this article, we present experimental and theoretical studies of pyridine derivatives (pyDs) inspired by natural systems to investigate the electron transfer processes occurring in aqueous media and elaborate a theoretical model that adequately predicts the behavior of new derivatives. Our results might be relevant to scientific and technological applications, including energy storage, redox-active scaffolds for organic synthesis, photoredox catalysis, and new materials. The synthesis of eight pyDs is reported. To improve water solubility, six new compounds are hexafluorophosphate alkylammonium salts. The pyDs exhibit irreversible redox processes, with electron-donating substituents decreasing the cathodic peak potential while electron-withdrawing groups increase it; when both substituents are present, the latter effect prevails. A computational study was performed to investigate the electrochemical behavior of the synthesized compounds and design new electroactive pyDs. DFT calculations provided the predominant species’ redox potentials and acidity constants to elaborate Pourbaix diagrams for each compound. The synthesized molecules exhibit a two-electron-one-proton dismutation process in the water pH window. Beyond this range, stabilized radical species undergo one-electron exchange processes. We correlated experimental and calculated parameters, screening 22 additional derivatives to evaluate their electrochemical behavior, identifying potential candidates capable of performing a one-electron transfer process in the pH window of water, revealing new applications for pyDs. Full article

Copper shaft furnace (CSF) dust containing valuable metals with a composition of 44.02% Zn and 14.57% Pb, in the form of oxides (PbO and ZnO), was used for leaching in 1 mol/L sodium hydroxide lixiviant at a temperature of 80 °C. The leaching efficiency for lead removal was 98%. The leaching of CSF dust in sodium hydroxide was thermodynamically studied using Pourbaix diagrams for the Pb/Zn/-Na–H₂O system at temperatures of 25 °C and 80 °C. A suitable precipitating agent was 0.5 mol/L sulfuric acid at pH 3. The formation of lead sulfate as the final product was confirmed by SEM, EDX, and XRD analysis. Although increasing the temperature reduced the aging time required for the precipitation, it did not affect the amount of lead precipitated. The solution, after lead precipitation and containing zinc (Zn²⁺), was further treated with ammonium carbonate for zinc precipitation. Various analytical methods, including SEM, EDX, XRD, XRF, and AAS, were used to analyze the input samples and the final products obtained after alkali leaching of CSF dust and lead and zinc precipitation. Full article

Brasses are well-known structural materials, and their electrochemistry seems to be known. However, the formation of nanostructured anodic oxides on brasses is still not common and researched enough. Despite the electrochemical oxidation or anodization of copper and zinc being well-recognized and known in the scientific community, there is a lack of a satisfactory amount of research on brass anodizing. Both copper and zinc can passivate in neutral and alkaline electrolytes, and also the mechanism of the nanostructured oxide growth of both seems to be similar. In this review, much effort was put in to gather the information on the protocols on the electrochemical oxidations of brasses and their applications. Usually, the effects of electrochemical oxidation allow us to obtain nanostructured surfaces made of mixed Cu and Zn species. The formation of such composite nanostructures allows us to apply them in such emerging applications as photocatalytic organic pollutant decomposition, photoelectrochemical hydrogen generation, electrochemical carbon dioxide reduction reactions, or electrochemical methanol oxidation. Full article

A possible thermodynamic study of processing Cu (Ag, Au) and Fe sulfide concentrate as a by-product after the processing of tetrahedrite concentrate, applying pyrometallurgical and hydrometallurgical methods, was studied. The sample of sulfide concentrate, 34.7 wt. % Cu, 21.4% Fe, 12 g/t Au, and 7.317 g/t Ag was contained. Analytical technique AAS was used to analyze the sample before conducting a thermodynamic study of the leaching of sulfide concentrate by applying Pourbaix Eh–pH diagrams. The outcome of this thermodynamic research will provide essential data to support recent hydrometallurgical technologies. If its correctness can be verified experimentally, this result will be promoted to developing a new alternative copper-production technology. The minor components Sb, As, Hg, and Bi are also present in the concentrate in the form of sulfides Sb₂S₃, As₂S₃, Bi₂S₃, and HgS. This theoretical proposed hydrometallurgical technology shows that it is possible to obtain Fe in the form of Fe(OH)₃, and after its thermal decomposition, it can be prepared as Fe₂O₃ as a marketable product. In any case, the most economically advantageous would be complete hydrometallurgical processing, i.e., also Cu(Ag,Au)Fe sulfide concentrate, with the possibility of valorizing Cu, Ag, and Au in metallic form. Full article

Long-range electron transfer (ET) is an essential component of all biological systems. Reactions of metalloproteins are important in this context. Recent work on protein “charge ladders” has revealed how the redox state of embedded metal ions can influence the ionization of amino acid residues at protein surface sites. Inspired by these observations, we carried out a variable pH investigation of intramolecular ET reactions in a ruthenium-modified protein system built on azurin from Pseudomonas aeruginosa. We also generate a Pourbaix diagram that describes the variable pH redox behavior of a Ru model complex, Ru(2,2′-bipyridyl)₂(imidazole)₂(PF₆)₂. The intramolecular ET rate constants for the oxidation of azurin-Cu⁺ by flash-quench-generated Ru³⁺ oxidants do not follow the predictions of the semi-classical ET rate expression with fixed values of reorganization energy (λ) and electronic coupling (H_DA). Based on the pH dependence of the Ru^3+/2+ redox couple, we propose a model where pure ET is operative at acidic pH values (≤ 7) and the mechanism changes to proton-coupled electron transfer at pH ≥ 7.5. The implications of this mechanistic proposal are discussed in the context of biological redox reactions and with respect to other examples of intramolecular ET reactions in the literature. Full article

The presented article deals with the thermodynamic study of copper converter flue dust (CCFD) treatment by hydrometallurgical process. The investigated sample of CCFD contains 38.31 wt.% Zn, 11.35 wt.% Pb, and 2.92 wt.% Sn in the form of oxides (PbO, ZnO, and SnO₂). The leaching of CCFD in sulphuric acid, acetic acid, nitric acid, and sodium hydroxide was thermodynamically studied. Using Pourbaix diagrams for individual metal–S/C/N/Na/–H₂O systems, the possibility of leaching oxides in selected leaching agents was confirmed. A sustainable and environmentally friendly method of processing the sample of CCFD using a hydrometallurgical method is proposed. A suitable selective leaching agent is the acetic acid solution. When leaching in an acetic acid solution, zinc and lead are chemically dissolved to form acetates Zn(CH₃COO)_2(aq) in the form of Zn²⁺_(aq) at a temperature of 20 °C with a pH range of 0–3.5 and at a temperature of 80 °C with a pH range of 0–2.95, as well as Pb(CH₃COO)_2(aq) as Pb²⁺_(aq) at a temperature of 20 °C with a pH range of 0–1.95 and at a temperature of 80 °C with a pH range of 0–2, respectively, while tin remains as a solid residue in the form of SnO_2(s) at the temperatures of 20 °C and 80 °C throughout the whole acidic pH range. Various analytical techniques, such as SEM, EDX, XRD, and AAS, were used to analyse samples before a thermodynamic study of the leaching of CCFD was conducted by applying Pourbaix Eh–pH diagrams. Full article

An investigation has been carried out to understand the solution chemistry of the Cu-NH⁻-SO₄⁻² system, focusing on the effect of pH on the solubility of copper in the solution and maximizing the Cu(I):Cu(II) ratio. A Pourbaix diagram for the Cu-N-S system has also been created using the HSC Chemistry software for a wide range of Cu-NH₃ species, unlike most other studies that focused only on Cu(NH₃)₄²⁺ and Cu(NH₃)₅²⁺ (Cu(II)) as the dominant species. The Pourbaix diagram demonstrated that the Cu(I) exists as Cu(NH₃)₂⁺, while the Cu(II) species are present in the system as Cu(NH₃)₄²⁺ and Cu(NH₃)₅²⁺, depending upon the Eh and pH of the solution. Copper precipitation was observed in the electrolyte at pH values less than 8.0, and the precipitation behavior increased as the pH became acidic. The highest Cu(I):Cu(II) ratio was observed at higher pH values of 10.05 due to the higher solubility of copper at higher alkaline pH. The maximum Cu(II) concentration can be achieved at 4.0 M NH₄OH and 0.76 M (NH₄)₂SO₄. In the case of low pH, the highest Cu(I):Cu(II) ratio obtained was 0.91 against the 4.0 M and 0.25 M concentrations of NH₄OH and (NH₄)₂SO₄, respectively. Meanwhile, at high pH, the maximum Cu(I):Cu(II) ratio was 15.11 against the 0.25 M (NH₄)₂SO₄ and 4.0 M NH₄OH. Furthermore, the low pH experiments showed the equilibrium constant (K) K < 1, and the high pH experiments demonstrated K > 1, which justified the lower and higher copper concentrations in the solution, respectively. Full article

This review details both the conventional and emerging methods of extracting tin from cassiterite. The emerging methods reviewed include sulphuric acid leaching of SnO, cooling crystallization of SnO, sulphide leaching, alkaline leaching, and dry chlorination. From these methods, the conventional approach (direct reduction smelting) stands out as the sole method that is suitable for industrial application, with none of the emerging ones being promising enough to be a contender. The thermodynamics involved in the hydrometallurgical extraction of tin from the mineral are also discussed. ${\Delta G}^o$ values calculated at 25 °C for the reduction–dissolution of SnO₂ using reducing gases revealed feasibility only when carbon monoxide was used. An indication of the possible species produced during the hydrolysis of the oxide of the metal (SnO₂ and SnO) as a function of pH (ranging from −2 to 14 and 0 to 14 for SnO₂ and SnO, respectively) was noted and highlighted to link a Pourbaix diagram generated from literature data. This diagram suggests that the solubility of SnO₂ in both strongly acidic and alkaline media is possible, but with a small dissolution window in each. The purification and recovery routes of the various processing techniques were then envisaged. Full article

Recycling lithium-ion batteries provides sustainable raw materials. Crushing and separation are necessary for extracting metals, like lithium, from batteries. Crushing a battery carries a risk of fire or explosion. Fully discharging the battery is crucial for safe production. Discharging batteries in a salt solution is a simple and cost-effective large-scale process. However, it is important to note that there is a potential risk of corrosion and loss of battery elements when batteries are immersed in a salt solution. The purpose of this study is to investigate the effectiveness of two distinct methodologies at enhancing the voltage drop of a cylindrical battery when immersed in a salt solution while preventing corrosion. These techniques involve the application of iron and copper accelerators. A 20 wt.% salt water solution was chosen based on the research of several researchers. As the current flows through the metal parts, it encounters electrical resistance and forms an electric circuit with the electrolyte solution. This interaction converts electrical energy into various physical–electrical–electrochemical phenomena, leading to a decrease in battery voltage. Research revealed that the battery can be discharged up to 100% within 4 h without causing corrosion to its components. Another point to note is that if copper conductors are used, it is possible to decrease the battery voltage by around 90% within 8 h. The gap between the copper conductor and the battery had a direct impact on the battery’s discharge rate. Reducing the distance significantly increased the discharge rate, as confirmed by experimental evidence. This discharge mechanism was thoroughly described in a schematic, and, to further explain the electrochemical reaction, the Pourbaix diagram was utilized for both the Fe-Na-Cl and Cu-Na-Cl systems. Moreover, our theoretical predictions were validated through a chemical and mineralogical analysis of the precipitates that formed in the solution. Full article

The recovery and recycling of metals that generate toxic ions in the environment is of particular importance, especially when these are tungsten and, in particular, thorium. The radioactive element thorium has unexpectedly accessible domestic applications (filaments of light bulbs and electronic tubes, welding electrodes, and working alloys containing aluminum and magnesium), which lead to its appearance in electrical and electronic waste from municipal waste management platforms. The current paper proposes the simultaneous recovery of waste containing tungsten and thorium from welding electrodes. Simultaneous recovery is achieved by applying a hybrid membrane electrolysis technology coupled with nanofiltration. An electrolysis cell with sulphonated polyether–ether–ketone membranes (sPEEK) and a nanofiltration module with chitosan–polypropylene membranes (C–PHF–M) are used to carry out the hybrid process. The analysis of welding electrodes led to a composition of W (tungsten) 89.4%; Th 7.1%; O₂ 2.5%; and Al 1.1%. Thus, the parameters of the electrolysis process were chosen according to the speciation of the three metals suggested by the superimposed Pourbaix diagrams. At a constant potential of 20.0 V and an electrolysis current of 1.0 A, the pH is varied and the possible composition of the solution in the anodic workspace is analyzed. Favorable conditions for both electrolysis and nanofiltration were obtained at pH from 6 to 9, when the soluble tungstate ion, the aluminum hydroxide, and solid thorium dioxide were formed. Through the first nanofiltration, the tungstate ion is obtained in the permeate, and thorium dioxide and aluminum hydroxide in the concentrate. By adding a pH 13 solution over the two precipitates, the aluminum is solubilized as sodium aluminate, which will be found after the second nanofiltration in the permeate, with the thorium dioxide remaining integrally (within an error of ±0.1 ppm) on the C–PHF–M membrane. Full article

In electrocatalytic processes, traditional powder/film electrodes inevitably suffer from damage or deactivation, reducing their catalytic performance and stability. In contrast, self-healing electrocatalysts, through special structural design or composition methods, can automatically repair at the damaged sites, restoring their electrocatalytic activity. Here, guided by Pourbaix diagrams, foam metal was activated by a simple cyclic voltammetry method to synthesize metal clusters dispersion solution (MC/KOH). The metal clusters-modified hydroxylated Ni-Fe oxyhydroxide electrode (MC/Ni_xFe_yOOH) by a facile Ni-Fe metal–organic framework-reconstructed strategy, exhibiting superior performance toward the oxygen evolution reaction (OER) in the mixture of MC/KOH and saline–alkali water (MC/KOH+SAW). Specifically, using a nickel clusters-modified hydroxylated Ni-Fe oxyhydroxide electrode (NC/Ni_xFe_yOOH) for OER, the NC/Ni_xFe_yOOH catalyst has an ultra-low overpotential of 149 mV@10 mA cm⁻², and durable stability of 100 h at 500 mA cm⁻². By coupling this OER catalyst with an efficient hydrogen evolution reaction catalyst, high activity and durability in overall SAW splitting is exhibited. What is more, benefiting from the excellent fluidity, flexibility, and enhanced catalytic activity effect of the liquid NC, we demonstrate a self-healing electrocatalysis system for OER operated in the flowing NC/(KOH+SAW). This strategy provides innovative solutions for the fields of sustainable energy and environmental protection. Full article

Chromium nitrides such as CrN and Cr₂N are often used for corrosion and wear resistant applications. In order to understand the thermodynamic stability of the nitrides, Pourbaix diagrams will be extremely useful. In this paper, Pourbaix diagrams are constructed for CrN and Cr₂N using thermodynamical data for species at 298 K (25 °C) and at a concentration of 10⁻⁶ M for aqueous species. These diagrams are useful indicators for the stable regions in which these compounds can be used. The diagrams show that passive Cr₂O₃ films form on the surfaces where chromium nitride was present. It is argued that the formation of Cr₂O₃ films will degrade chromium nitride and make it much less useful as a wear resistant layer. However, the presence of nitrogen in solid solution is better for the stability of passive films. Full article

This paper aims to clarify the effect of polymer coating damage of dual polymer–zinc coating used to protect rebar reinforcement from corrosion. The coating damage can result in crevices between the two materials. At these crevices, corrosion–passivation plays an important role in the integrity of the intended coating and the protectiveness of the steel base metal. An experimental design was developed to replicate a crevice of Zn-2%Al alloy. This alloy is commonly used for the dual coating protection of rebars. Experiments in this investigation were performed to test several crevice sizes and conditions to assess the state of crevice corrosion. Prepared electrodes were submerged in a 1 M NaOH solution and connected to a data logger to monitor the potential. A special reference electrode was prepared using activated titanium, against which the potential was measured. Additionally, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization were carried out to assess the corrosion rate of the different specimens. The results suggest that, in the absence of a crevice, corrosion occurred on bare specimens. However, whenever a crevice was present, a shift in potential and corrosion rate values indicated that the specimen shifted from an active corrosion regime to a passive one. This shift (gain) in potential was measured as approximately 0.9 V, resulting in a shift in the electrode potential to −0.6 V (SCE). The analysis showed that the solution inside the crevice shifted toward lower pH values, with pH = 12 suggested as the level that supports more passivity inside the crevice according to the Pourbaix diagram. Full article

Faced with the new stage of water oxidation by molecular catalysts (MCs) in artificial photosynthesis to overcome the bottle neck issue, the “Photon-flux density problem of sunlight,” a two-electron oxidation process forming H₂O₂ in place of the conventional four-electron oxidation evolving O₂ has attracted much attention. The molecular characteristics of tin(IV)-tetrapyridylporphyrin (SnTPyP), as one of the most promising MCs for the two-electron water oxidation, has been studied in detail. The protolytic equilibria among nine species of SnTPyP, with eight pK_a values on the axial ligands’ water molecules and peripheral pyridyl nitrogen atoms in both the ground and excited states, have been clarified through the measurements of UV-vis, fluorescence, ¹H NMR, and dynamic fluorescence decay behaviour. The oxidation potentials in the Pourbaix diagram and spin densities by DFT calculation of the one-electron oxidized form of each nine species have predicted that the fully deprotonated species ([SnTPyP(O⁻)₂]²⁻) and the singly deprotonated one ([SnTPyP(OH)(O⁻)]⁻) serve as the most favourable MCs for visible light-induced two-electron water oxidation when they are adsorbed on TiO₂ for H₂ formation or SnO₂ for Z-scheme CO₂ reduction in the molecular catalyst sensitized system of artificial photosynthesis. Full article

Galena is the most important mineral for lead production, as it is the main source of lead in the world. Currently, the concentrates of this mineral are mainly treated using pyrometallurgical methods, creating several environmental problems, such as the generation of toxic and greenhouse gases. In addition, these processes involve high energy consumption, which limits their applicability. Hydrometallurgical routes are proposed as alternative processes for obtaining some metals such as silver, copper, gold, etc. The drawback of these processes is that the minerals tend to be passive in aqueous media. To mitigate this issue, researchers have used extreme conditions of pressure and temperature (6 atm. and 155 °C) or the use of very corrosive conditions. In this sense, the use of complexing agents that dissolve the metals of interest has been proposed. Citrate ion is one of the most promising complexing agents for galena leaching, obtaining high percentages of dissolution in relatively short times. Unfortunately, there has not been enough investigation about the concentration optimization of the complexing in the pH range from 5 to 9. In this sense, thermodynamic diagrams, such as the Pourbaix diagrams, are very useful for this purpose. Therefore, in this work, the effects of pH and temperature on the leaching of galena in citrate ion solutions are studied thermodynamically and experimentally. The experimental work was carried out with pure galena samples with a particle size of +149 − 74 µm (−100 + 200 mesh). The results show that higher recoveries were obtained working at a pH of 8 and at temperatures of 30 and 40 °C. The thermodynamic and experimental data demonstrated that the existence of an optimal concentration of citrate ion, due the extraction of lead from galena, has a greater reaction rate at a relatively low initial concentration of 0.3 M. This is due the formation of the complex lead citrate 1 ($\mathrm{Pb}{\left(\mathrm{cit}\right)}^{-}$). Full article