Pourbaix Research Articles

Investigating trace metal precipitation in highly concentrated cell culture media with Pourbaix diagrams.

Commercial production of therapeutic proteins using mammalian cells requires complex process solutions, and consistency of these process solutions is critical to maintaining product titer and quality between batches. Inconsistencies between process solutions prepared at bench and commercial scale may be due to differences in mixing time, temperature, and pH which can lead to precipitation and subsequent removal via filtration of critical solution components such as trace metals. Pourbaix diagrams provide a useful tool to model the solubility of trace metals and were applied to troubleshoot the scale-up of nutrient feed preparation after inconsistencies in product titer were observed between bench- and manufacturing-scale batches. Pourbaix diagrams modeled the solubility of key metals in solution at various stages of the nutrient feed preparation and identified copper precipitation as the likely root cause of inconsistent medium stability at commercial scale. Copper precipitation increased proportionally with temperature in bench-scale preparations of nutrient feed and temperature was identified as the root cause of copper precipitation at the commercial scale. Additionally, cell culture copper titration studies performed in bench-scale bioreactors linked copper-deficient mammalian cell culture to inconsistent titers at the commercial scale. Pourbaix diagrams can predict when trace metals are at risk of precipitating and can be used to mitigate risk during the scale-up of complex medium preparations.

Spatial and temporal variations of Cu and Cd mobility and their controlling factors in pore water of contaminated paddy soil under acid mine drainage: A laboratory column study

Acid mine drainage (AMD) poses a potential threat to human health worldwide, due to its high content of inorganic contaminants including heavy metals. Nevertheless, AMD is commonly used for irrigation of paddy soils. To determine the extent to which AMD affects contaminant levels in such practices, the effect of continuous AMD flooding on pH, redox potential Eh and the migration of Cu and Cd in contaminated paddy soil was studied in column experiments. By means of simulated AMD, dynamic changes of Cu and Cd concentrations in pore water were measured and the controlling factors pH, Eh and presence of Fe, dissolved organic carbon and sulfate were determined over a period of 60 days. Minerals in the soil were assessed by means of an Eh-pH diagram and solid-phase mineral detection. During continuous flooding with AMD-simulated water the soil pH increased, while Eh decreased over time. After 60 days the soil pH stabilized. Cu and Cd concentrations in the pore water negatively correlated with pH and with sulfate concentrations. Five-step sequential extraction illustrated that the fraction of exchangeable Cu increased significantly during AMD flooding. The overall content of Cu increased from initially 0.29 mg/g to 0.41 mg/g, while the content of Cd decreased from 9.2 mg/g to approximately 7.2 mg/g. Mobility factors were calculated and these conformed that Cd mobility significantly increased in contaminated soils during continuous AMD flooding. Our findings indicate that the release of Cu and Cd under AMD flooding can increase potential environmental risks, even though they lead to formation of metal sulfide deposits under anaerobic conditions. The presented data improves our understanding of the impact of overlying water conditions on the mobility of toxic metals in contaminated paddy soils.

Investigations of the intrinsic corrosion and hydrogen susceptibility of metals and alloys using density functional theory

Abstract Mechanisms for materials degradation are usually inferred from electrochemical measurements and characterization performed before, during, and after exposure testing and/or failure analysis of service materials. Predicting corrosion and other materials degradation modes, such as hydrogen-assisted cracking, from first-principles has generally been limited to thermodynamic predictions from Pourbaix or Ellingham diagrams and the Galvanic series. Using electronic structure calculations, modern first-principles methods can predict ab initio the key rate-controlling processes for corrosion and hydrogen susceptibility as a function of pH, potential, and solution chemistry, and materials composition and microstructure. Herein we review density functional theory (DFT) approaches for studying the electrochemical reactions occurring on fresh metal and alloy surfaces related to environmentally assisted cracking and localized corrosion/pitting. Predicted changes in surface chemistry as a function of the environment were correlated against experimental crack growth rate data obtained for alloys 718, 725, and pipeline steel under electrochemical control. We also review the application of the method to study the effects of alloying on the chloride susceptibility of stainless steels and Ni–Cr-based corrosion-resistant alloys. Perspectives for improving the model are given, and extending it to future fields of application in corrosion science and engineering.

Determination of corrosion products for steam and flue gas injection environments using thermodynamic simulation

Thermal recovery processes using steam injection with combustion gases (flue gas) have shown positive results in the recovery of heavy crude oil over the conventional process, by integrating different recovery mechanisms. However, under the operating conditions, the injection of these fluids generates highly corrosive environments that have an impact on the deterioration of the materials, resulting in risks and operational costs. Therefore, it is necessary to determine the theoretical corrosion products that can be generated in these processes. This research focused on the study of API N-80 carbon steel exposed to a steam and flue gas atmosphere, at pressure and temperature conditions in the ranges of 800 psia - 1100 psia (55 bar - 75 bar) and 520 °F - 560 °F (270 °C - 290 °C) respectively. Based on this environment, to determine the theoretical corrosion products, a thermodynamic simulation stage was developed using HSC Chemistry software, which was used to generate Pourbaix, Ellingham and thermodynamic equilibrium diagrams. It was found that the main theoretical corrosion products corresponded to oxides, carbonates, and hydroxides, among which the significant presence of iron (III) oxide (Fe2O3), iron (II, III) oxide (Fe3O4) and iron carbonate (II) (FeCO3) was corroborated.

Leaching of Metals from e-Waste: From Its Thermodynamic Analysis and Design to Its Implementation and Optimization.

The aim of this study is to design and develop an efficient leaching process based on a fundamental and theoretical thermodynamic analysis and the optimization of the operation parameters via the response surface methodology (RSM). Using this methodology, the design of a leaching process for the recovery of copper, silver, and lead from highly metal-concentrated fractions of e-waste is presented. Thermodynamic predictions were performed through the construction and analysis of Pourbaix diagrams for the specific conditions of the leaching system. From this analysis, it was possible to determine the values of potential (E vs NHE) and pH at which the leaching reactions occur spontaneously. Additionally, RSM was useful to deduce a quadratic semiempirical model that predicts the copper leaching efficiencies as a function of two parameters involved in the leaching procedure, the stirring speed and the solid/liquid ratio, by which the response variable, the leaching efficiency, can be optimized.

Surface Pourbaix diagram of AgPd nanoalloys and its application in formate oxidation reaction

Surface Pourbaix diagrams for 38-atom AgPd nanoalloys with different Ag/Pd ratio are constructed based on the density functional theory calculations, with an increasing electrode potential, the adsorbed hydroxides increase on the surface of nanoparticle and convert to oxygen atoms partially sunk into subsurface. AgPd nanoalloys with a core-shell structure have an improved oxidation resistance compared with pure nanoparticle and Ag atoms on nanoalloys surface could suppress Pd dissolution and oxidation. It is newfound that a substitutional introduction of Pd atoms into the core of AgPd nanoalloys could suppress the oxide formations and increase the oxidation resistance, and a substitutional introduction of Ag atoms onto the surface of Pd38 nanoparticle could suppress Pd dissolution. Among all the calculated AgPd nanoalloys, Ag18Pd20 has an excellent durability against electrochemical dissolution and oxidation. Experimentally, the activity of as-synthesized Ag48Pd52 nanoalloy notably increases with increasing the upper limit potential from 0.3 to 0.5 V under cyclic voltammetry test of formate oxidation reaction (FOR). Increased oxidation state of Ag atoms on electrocatalyst surface, which is confirmed by Mulliken charge analysis, has been identified as the key to prompt the FOR activity enhancement.

Application of Eh-pH Diagrams on Acid Leaching Systems for the Recovery of REEs from Bastnaesite, Monazite and Xenotime

Bastnaesite, monazite and xenotime are rare earth minerals (REMs) that are typical sources for rare earth elements (REEs). To advance the understanding of their leaching and precipitation behavior in different hydrometallurgical processes, Eh-pH diagrams were constructed and modified using the HSC 9.9 software. The aqueous stability of rare earth elements in H2O and acid leaching systems, i.e., the REE-Ligands-H2O systems, were depicted and studied based on the Eh-pH diagrams. This study considers the most relevant lixiviants, their resulting equilibrium states and the importance in the hydrometallurgical recovery of rare earth elements (REMs). A literature review was performed summarizing relevant Eh-pH diagrams and associated thermodynamic data. Shifting stability regions for REEs were discovered with additions of acid ligands and a narrow stability region for soluble REE-(SO4/Cl/NO3) complexes under highly acidic conditions. As such, the recovery of REEs can be enhanced by adjusting pH and Eh values. In addition, the Eh-pH diagrams of the major contaminants (i.e., Fe, Ca and Al) in leaching systems were studied. The resulting Eh-pH diagrams provide possible insights into potential passivation on the particle surfaces due to the formation of an insoluble product layer.

Diagrams with multiple metals in CHNOSZ

BackgroundCHNOSZ is an open-source R package that includes a thermodynamic database for thermodynamic calculations at 25 ​°C and higher temperatures and functions to make equilibrium chemical activity diagrams including Eh–pH diagrams and logfO2–pH diagrams. The main workflow supports the creation of basic diagrams where the reactions are balanced on a single element. Therefore, basic diagrams are limited to single-metal systems, but the ability to make diagrams for multiple metals is a common feature request. MethodsNew functions have been added to the package to make diagrams for multiple metals. ResultsThis paper describes the methods and demonstrates their use to reproduce published Eh–pH and chemical activity diagrams for oxide and sulfide minerals in two bimetallic systems (Fe–V–O–H and Cu–Fe–S–O–H). New logfO2–pH diagrams are presented to show dissolved species in the Cu–Fe–S–O–H–Cl system and the effects of Fe:Cu ratio on sulfide mineral assemblages. ConclusionsThe functions greatly expand the thermodynamic systems that can be diagrammed in CHNOSZ and should be useful for researchers in different fields including hydrothermal geochemistry and materials science.

Thermodynamics of Copper Arsenious Raw Materials Dissolution in Nitric Acid

This article describes thermodynamic study of hydrometallurgical method for processing of copper arsenious gold-containing raw material. Chemical and phase composition of the material were researched. Calculations of the Gibbs energy change were conducted for possible reactions of the main minerals, present in the raw material, with nitric acid. Eh-pH diagrams and solid/liquid equilibrium distribution diagrams, which were constructed in order to confirm the possibility of obtaining the required reaction products through nitric acid leaching of the studied raw material.

Thermodynamic stability diagram of the copper/water/amylxanthate flotation system

The purpose of this work was to obtain the thermodynamic stability diagram that characterizes the copper/water/amylxanthate flotation system, as part of the theoretical foundation necessary for the analysis of the copperion flotation process with the flotation collector reagent potassium amylxanthate. From the system of fundamental chemical reactions and with the help of the Medusa software, the Eh-pH diagram was obtained, in which the stability zones of the different chemical species are defined and it is established that in the pH range from 4 to 13, the xanthogenic species of copper(I) and (II) coexist, whose ratio decreases with the increase of pH.

Potential-Dependent Mechanistic Study of Ethanol Electro-oxidation on Palladium.

We herein used the density functional theory (DFT) method and the implicit continuum solvation model to study the potential-dependent mechanism of ethanol oxidation reaction (EOR) on palladium (Pd). Energy evolutions of the EOR on low-index Pd surfaces, including (111), (110), and (100), were obtained as a function of the electrode potential. Moreover, the onset potentials for key intermediates and products were calculated. In addition, the potential range for adsorbed ethanol as the most stable adsorption state for proceeding the EOR was determined to be between 0.15 and 0.78 V via the calculated Pourbaix diagrams when considering hydrogen underpotential deposition and Pd(II) oxide formation as competing reactions. Specifically, the behavior of Pd(111) as the dominating facet decided the overall activity of the EOR with onset potentials to acidic acid/acetate at 0.40 V, to carbon dioxide at 0.71 V, and to oxide formation at 0.78 V. Pd(110) was predicted to exhibit the optimal activity toward the EOR with the lowest onset potentials to both the first dehydrogenation process and carbon dioxide at 0.08 and 0.60 V, respectively. A computational potential-dependent mechanism of the EOR was proposed, which agrees well with the experimental curve of linear sweeping voltammetry on the commercial Pd/C electrocatalyst. Our study suggests that targeted control of products can be tuned with proper overpotential and thus provides a foundation for the future development of EOR electrocatalysts.

Chemical stability diagrams as a powerful tool to the synthesis of Cu2SnS3 thin films by chemical bath deposition

Cu2SnS3 films are produced by chemical bath method employing low toxicity citrate precursor taking advantage of the strong complexes formed with Cu(II) and Sn(II). The deposition conditions for Cu2SnS3 thin films were obtained through a thermodynamic analysis of the effect of tin concentration on Cu(II)–Sn(II)-Citrate complexes and Cu(II)–Sn(II)-Citrate-Thiourea system using a species distribution and potential–pH (Eh-pH) analysis. Theoretical analysis of the speciation fraction of Cu(II) and Sn(II) shown that the predominant species at pH equal to 4.5 at 90 °C are [Cu2(Cit)2OH]3− and [Sn2(Cit)2]2−. For the Cu(II)–Sn(II)-Citrate-Thiourea system at 90 °C, the Eh-pH diagrams established that binary precursors, Cu2S and SnS2, would be obtained under basic pH conditions. Subsequently, Cu2SnS3 films were synthetized at 90 °C at pH equal to 8 by keeping fixed the molar relationships complexing agent-metal and sulfur-metal. The redox potentials were measured in solution and chemical equilibrium diagrams were constructed, confirming the existence of binary precursors under reductive conditions. Characterization of structural, electronic and transport properties indicated that Cu2SnS3 fabricated by chemical bath deposition have monoclinic crystal structure, a direct band gap energy of 1.70 eV and p-type conductivity.

Constructing the Pourbaix diagram of Fe-Cl−-H2O ternary system under supercritical water conditions

Supercritical water oxidation (SCWO) technology, which has outstanding performance, is impeded commercial development due to serious corrosion problems. In this work, the first Pourbaix diagrams of the Fe-Cl−-H2O ternary system under supercritical water conditions was established to predict the evolution process of metals in thermodynamics equilibrium. The fifteen species were rationally screened by the ion distribution coefficient, and thermodynamics parameters under supercritical conditions were calculated by the extrapolation method of the revised Helgeson-Kirkham Flowers (R-HKF) model. The results show that hematite and magnetite passivation film and ferrous iron are continuously eroded by FeCl3 (aq) corrosion zone with the increase of chloride concentration, which greatly expands the area. Interestingly, the range of the immunization zone is almost unchanged. On the other hand, most of the Fe(OH)- 3 corrosion zone, passivation zone, and immunization zone to be converted to ferrous iron caused by temperature rise when FeCl3 (aq) reaches a certain concentration. Metals often form a hematite and magnetite passivation film to reduce the dissolution rate of the substrate under alkaline conditions with relatively high potentials, so the pre-neutralization method can be used as an anti-corrosion measure.

Study on the surface morphology and corrosion rates of electroplated coatings based on binary alloys of refractory metals after accelerated acid tests

ABSTRACT The corrosion rates of Ni-W, Co-W, Ni-Mo, and Co-Mo electrodeposited coatings after two-hour immersion tests in HNO3, H2SO4 and HCl environments were studied by the weight loss method. The best corrosion resistance was revealed for Ni-W alloy, and the worst for Co-Mo. For all tested samples (36 in total), the maximal corrosion rate was observed in nitric acid, and the minimal one, with an exception of Ni-W, in hydrochloric acid. It was found that for the W-containing coatings corrosion evenly affected each individual crystallite of the alloy structure changing its shape and volume, and the corrosion of Mo alloy-based coatings showed mixed character. According to the Pourbaix diagram, for Mo alloys (Ni-Mo and Co-Mo) the lower corrosion resistance compared to the coatings containing W can be explained by a narrower region of MoO3 formation, as well as by their higher solubility in comparison with WO3.

Hydrometallurgical Leaching of Copper Flash Furnace Electrostatic Precipitator Dust for the Separation of Copper from Bismuth and Arsenic

Flash furnace electrostatic precipitator dust (FF-ESP dust) is a recycle stream in some primary copper production facilities. This dust contains high amounts of copper. In some cases, the FF-ESP dust contains elevated levels of bismuth and arsenic, both of which cause problems during the electrorefining stages of copper production. Because of this, methods for separation of copper from bismuth and arsenic in FF-ESP dust are necessary. Hydrometallurgical leaching using a number of lixiviants, including sulfuric acid, sulfurous acid, sodium hydroxide, and water, were explored. Pourbaix diagrams of copper, bismuth, and arsenic were used to determine sets of conditions which would thermodynamically separate copper from bismuth and arsenic. The data indicate that water provides the best overall separation between copper and both bismuth and arsenic. Sodium hydroxide provided a separation between copper and arsenic. Sulfurous acid provided a separation between copper and bismuth. Sulfuric acid did not provide any separations between copper and bismuth or copper and arsenic.

The Combined Role of Faceting and Heteroatom Doping for Hydrogen Evolution on a WC Electrocatalyst in Aqueous Solution: A Density Functional Theory Study

Tungsten carbide (WC) is an established model electrocatalyst for the hydrogen evolution reaction (HER) in aqueous solutions. In spite of extensive interest in and work on this material, a systematic atomistic understanding of the combined role of faceting and heteroatom doping for hydrogen evolution on WC electrocatalysts in aqueous solution is not available. To fill this knowledge gap, here we explore the interplay of these parameters for the electrocatalytic performance of WC by means of density functional theory (DFT) simulations. In our simulations, we explicitly account for solvent (water) effects and related electrochemical potential-dependent oxygen or hydroxyl (co)adsorption on the WC surfaces as determined by the analysis of surface Pourbaix diagrams. Among the several low-index WC surfaces screened, the DFT simulations indicate that the W-terminated (−100) and (111) surfaces of WC show comparable exchange currents of 3.908 and 0.133 mA/cm2, respectively. The simulations also suggest that V or N doping can substantially increase the HER activity of some WC surfaces to levels comparable to commercial Pt/C electrocatalysts, with current densities as large as 10.555, 1.730, and 0.636 mA/cm2 for the V-doped W-terminated (001), V-doped (101), and N-doped (101) WC surfaces, respectively. The improved HER activities are rationalized in terms of the d-band or p-band center, which are both shown to be related to the capability of tuning the hydrogen adsorption, and thence the HER performance. The calculated strong dependence of the HER activity on the presence of the electrochemical adsorption of oxygen and hydroxyls reiterates the importance of modeling realistic electrochemical conditions for WC pristine or doped surfaces. We expect the DFT results presented to provide timely and original guidelines for further theoretical and experimental studies of HER activity on WC-related electrocatalysts.

Building Ab Initio Interface Pourbaix diagrams to Investigate Electrolyte Stability in the Electrochemical Double Layer: Application to Magnesium Batteries.

Insights into the electrochemical processes occurring at the electrode-electrolyte interface are a crucial step in most electrochemistry domains and in particular in the optimization of the battery technology. However, studying potential-dependent processes at the interface is one of the biggest challenges, both for theoreticians and experimentalists. The challenge is pushed further when stable species also depend on the concentration of specific ligands in the electrolyte, such as chlorides. Herein, we present a general theoretical ab initio methodology to compute a Pourbaix-like diagram of complex electrolytes as a function of electrode potential and anion's chemical potential, that is, concentration. This approach is developed not only for the bulk properties of the electrolytes but also for electrode-electrolyte interfaces. In the case of chlorinated magnesium complexes in dimethoxyethane, we show that the stability domains of the different species are strongly shifted at the interface compared to the bulk of the electrolyte because of the strong local electric fields and charges occurring in the double layer. Thus, as the interfacial stability domains are strongly modified, this approach is necessary to investigate all interface properties that often govern the reaction kinetics, such as solvent degradation at the electrode. Interface Pourbaix diagram is used to give some insights into the improved stability at the Mg anode induced by the addition of chloride. Because of its far-reaching insights, transferability, and wide applicability, the methodology presented herein should serve as a valuable tool not only for the battery community but also for the wider electrochemical one.

Employing speciation of metals to assess photo-assisted electrochemical efficiency for improving rainwater quality in Tehran, Iran

Photo-assisted electrochemical process is a promising technology for a clean ecosystem. It is largely utilized for recovering metals and oxidizing recalcitrant organic compounds. This paper aims to discuss the performance of the photo-oxidation and reduction by applying semiconductor photo-catalysis as the anode and graphite as the cathode for treating rainwater in Tehran, Iran. Organic pollutants and different metals were investigated to study the probable roles of the anodic potential, the gap between electrodes, and the concentration of electrolytes in the possible mechanisms. The laboratory experiments which lasted for 120 min exhibited that the best performance for organic, iron, manganese, and lead removal was at the anodic potential of 10 V and gap of 6 cm. But, for zinc and cadmium, the best performance occurred at the anodic potential of 5.5 V and sodium chloride of 0.65 g/L. The presented reactions and mechanisms demonstrated incidents using the treatment process. They were identical to the presented dendrogram and Pourbaix diagrams. The results of the diagrams showed iron, manganese, and lead were removed by oxidation at the photo-anode. Zinc and cadmium were mainly removed by precipitation. Chemical oxygen demand, iron, manganese, and lead were significantly related and behaved similarly in the treatment process. Their removal was controlled by all the analyzed parameters. This paper may present a cost-effective approach for removing organic and metallic pollutants by photo-assisted electrochemical process. It can be suggested for future evolution of photo-assisted electrochemical on the treatment applications of rainwater.

Hydrogen evolution on different facets of δ1- MoN and δ3-MoN: Considering the adsorbed oxygen and hydroxyl by Surface Pourbaix diagrams

δ1-and δ3-MoN, the two most stable phases of molybdenum nitride, show a potential application in hydrogen evolution as their excellent corrosion resistance and high conductivity. However, we still lack the theoretical study about HER on their different surfaces. In order to simulate the realistic condition during heterogeneous catalysis, detailed atomic structure of each MoN surface (the ∗O and ∗OH) is determined by surface Pourbaix diagram. Further exploration of hydrogen evolution shows that the ∗O and ∗OH could change the catalytic site of one MoN surface, and weak the hydrogen adsorption ability. This is attributed to the downshift of Mo d-band center in top layer caused by the ∗O and ∗OH. And the hydrogen adsorption ability on catalytic sites of N and O atoms also follow the p-band center theory, respectively. It is interesting that several δ1-and δ3-MoN surfaces demonstrate comparable exchange current density, e.g., (101) and (001)N of δ1-MoN, and (110) of δ3-MoN with 1.76, 1.97 and 0.17 mA cm−2, respectively. This work is expected to contribute to the theoretical understanding of HER on different δ1-and δ3-MoN surfaces, and provide guidance for corresponding experiments on them.

On the Catalytic Activity and Corrosion Behavior of Polycrystalline Nickel in Alkaline Media in the Presence of Neutral and Reactive Gases

Nickel (Ni) is an important catalytic metal that finds application in electrochemical energy technologies, such as alkaline water electrolyzers (AWEs) and alkaline fuel cells (AFCs). An essential property that determines nickel’s applicability in AWE and AFC technologies is its stability under operating conditions, i.e., performance at extreme potential (E) conditions and in the presence of reactive gases. However, the electrochemical and electrocatalytic behavior of nickel is not as well understood as that of noble electrocatalytic metals such as platinum. This lack of knowledge needs to be addressed prior to dedicating significant efforts to the design, fabrication, and characterization of Ni-based materials for electrochemical energy technologies. We report experimental data on the catalytic activity and corrosion behavior of polycrystalline Ni in the presence of different dissolved gases. Cyclic voltammetry (CV) and potentiodynamic polarization (PDP) measurements are conducted at room temperature in 0.10 M aqueous NaOH solution saturated with either N2(diss), H2(diss), or O2(diss). CV measurements are performed using different potential scan rates in the regions of α-Ni(OH)2 and NiOOH formation/reduction. PDP measurements are conducted in the same electrolyte and in the presence of different dissolved gases but over a much broader potential range (−0.40 V ≤ E ≤ 2.20 V) and at a very low potential scan rate (s = 0.10 mV s−1) to achieve steady-state conditions. The influence of the state of the electrode’s surface (metallic versus oxidized) on the catalytic activity and corrosion behavior of nickel is also investigated by applying two different types of conditioning. The results demonstrate that the electrochemical behavior of Ni changes depending on the nature of the dissolved gas. The corrosion behavior is shown to depend on the polarization direction, thus the surface state of the electrode, and the nature of the dissolved gas. A graph, which is an extension of the Pourbaix diagram for Ni, summarizes the main interfacial and faradaic processes occurring at the surface of polycrystalline Ni in relation to the potential. The results and their analysis are expected to benefit renewable electrochemical energy technologies, such as alkaline water electrolyzers and fuel cells. In addition, they will serve as standards in electrochemical and electrocatalytic characterization of monocrystalline Ni materials and Ni-based nanomaterials.