Diffusion Coefficients Of Metal Ions Research Articles

Determination of the Diffusion Coefficient in Electrodeposition Reactions by Electrochemical Impedance Spectroscopy: A Case Study of Cobalt in Molten LiCl-KCl Salt

Compared to direct current (DC) techniques, electrochemical impedance spectroscopy (EIS) allows for a more accurate determination of the diffusion coefficient of metal ions in electrodeposition reactions. However, its actual application has been less attempted due to the difficulty in determining complex parameters, such as the concentration of deposited species in the electrode matrix. Here, we introduce a simplified approach to facilitate the EIS application to electrodeposition reactions, using the Co(II)/Co(0) reaction in molten LiCl-KCl salt as a case study. The working electrode surface area was determined by the photography of the electrochemical cell taken in situ. The diffusion coefficients of Co(II) in the LiCl-KCl salt, determined by EIS, were found to be (3.56 ± 0.49) × 10−5 cm2/s and (1.61 ± 0.12) × 10−5 cm2/s for the tungsten and liquid bismuth electrodes, respectively. The possible reasons for the observed discrepancy in the diffusion coefficients of Co(II) obtained from the two different electrodes were discussed.

Analysis on Oxidation Behavior of HR3C Steel at 700°C in Water Vapor

HR3C is an advanced heat-resistant alloy, and its high-temperature oxidation resistance directly determines the service performance of the material. Therefore, exploring the high-temperature oxidation behavior of HR3C is of great significance for the theoretical research and practical application of the material. The oxidation kinetic curve, oxide film morphology, oxide film composition, and oxide film phase composition of HR3C samples in the supply state after being oxidized in water vapor at 700°C for 50h, 150h, 250h, 350h, 450h, 550h and 600h were analyzed by SEM (Scanning Electron Microscopy), EDS (Energy Dispersive Spectroscopy) and XRD (X-ray diffraction). The results show that the oxidation kinetic curve of the material conforms to the parabolic law, the oxide film first forms a lamellar oxide, and the lamellar oxide film grows alternately to form a loose and porous oxide film surface, the oxide is composed of three oxides: Fe₃O₄, FeCr₂O₄ and Fe₂O₃. The FeCr₂O₄ is a spinel compound, which can form a dense oxide film, and the diffusion coefficient of metal ions in it is small, which hinders the diffusion and mass transfer of ions, thereby improving the high temperature oxidation resistance of stainless steel. The element Cr is mainly enriched in the inner oxide layer, and the element Fe is relatively high in the outer oxide layer.

Isotropic Tuning of Electrochemical Etching for the Nanometric Finishing of Metals

Isotropic etching polishing (IEP) based on the merging of isotropic etch pits has been proposed as a generic metal finishing approach. In this work, the tuning of the etching isotropy of various metals, which is the key to realizing the finishing effect of IEP, is studied by theoretical analysis and etching experiments. The isotropic etching of various metals can be realized through mass transfer polarization by adjusting the electrochemical parameters. The addition of sulfuric acid in the electrolyte is the most effective for tuning the isotropy of electrochemical etching. It can decrease the diffusion coefficient of metal ions, thereby increasing the resistance of mass transfer and transforming the electrochemical dissolution of metal into mass transfer polarization. In this study, the atomic and close-to-atomic scale surface finishing of various metals and alloys has been successfully achieved through isotropic etching. After etching at a current of 1.5 A for 3 min, the surface Sa roughness of TA2 is drastically reduced from 242 to 3.98 nm. After etching for 1 min at a current of 3 A, the surface Sa roughness of pure tungsten, NiTi, and CoCrNi decreases from 9.33, 76.4, and 37.6 nm, respectively, to 1.16, 2.01, and 2.51 nm, respectively.

Effect of Cobalt and Chromium Ions on the Chlorhexidine Digluconate as Seen by Intermolecular Diffusion

Metal ions such as cobalt (II) and chromium (III) might be present in the oral cavity, as a consequence of the corrosion of Co-Cr dental alloys. The diffusion of such metal ions into the organism, carried by saliva, can cause health problems as a consequence of their toxicity, enhanced by a cumulative effect in the body. The effect of the chlorhexidine digluconate, which is commonly used in mouthwash formulations, on the transport of these salts is evaluated in this paper by using the Taylor dispersion technique, which will allow an assessment of how the presence of chlorhexidine digluconate (either in aqueous solution or in a commercial formulation) may affect the diffusion of metal ions. The ternary mutual diffusion coefficients of metal ions (Co and Cr) in the presence of chlorhexidine digluconate, in an artificial saliva media, were measured. Significant coupled diffusion of CoCl2 (and CrCl3) and chlorhexidine digluconate is observed by analysis of the non-zero values of the cross-diffusion coefficients, D12 and D21. The observed interactions between metal ions and chlorhexidine digluconate suggest that the latter might be considered as an advantageous therapeutic agent, once they contribute to the reduction of the concentration of those ions inside the mouth.

Adsorption behaviors and mechanism of heavy metals onto attapulgite functionalized by polyamine silane

AbstractPollution from heavy‐metal ions has become a major challenge to the global fight against environmental pollution. Given the availability of various low‐cost and environmentally friendly adsorbents, adsorption has become the most efficient technology for the removal of heavy metals from water. In this study, attapulgite (ATP) was directly functionalized by coupling with an aminosilane agent. Analysis showed this maneuver provided a suitable adsorbent for the removal of lead ion (Pb2+) from an aqueous solution. The effects of several parameters including solution pH, contacting time, adsorbent dosage, and initial Pb2+ ion concentration were investigated. Batch sorption results showed that the adsorption process was rapid and over 98% of Pb2+ was removed within 30 min at the optimal pH 4.0. The maximum adsorption capacity at 25°C, calculated by the Langmuir isotherm, was 82.17, 78.80, 61.13, and 28.56 mg/g for γ‐divinyltriaminepropyl‐methyldimethoxylsilane‐grafted attapulgite (KH‐103‐ATP), γ‐aminopropyl‐methyldiethoxysilane‐grafted attapulgite (KH‐912‐ATP), N‐(β‐aminoethyl‐γ‐aminopropyl)‐methyl‐dimethoxysilane‐grafted attapulgite (KH‐602‐ATP), and ATP, respectively. Moreover, molecular dynamics simulations of adsorption behaviors of heavy‐metal ions at attapulgite surfaces (010) modified by aminosilane agents were carried out. Both the PMF value and diffusion coefficient of metal ions suggest that KH‐103‐ATP owns the highest rate constant and capacity compared with the other two. And the analysis of free energy and results of XPS characterization revealed that Pb2+ formed covalent bonds with the nitrogen atom of aminosilane agents.

Diffusion-Limited 1D Pit Growth of S13Cr in Brine at Elevated Temperature

Diffusion-limited pit growth of Super 13Cr stainless steel (S13Cr) was studied using one-dimensional (1D) pit electrodes in 0.1 – 3 M NaCl solutions at 85 °C. Applied potential (Eapp) was controlled in four different steps to achieve a target pit depth and measure the anodic current density for S13Cr dissolution under potentiostatic polarization and during a backward scan. For pits with similar depths, the diffusion-limited current density (ilim) of anodic dissolution decreased with increasing NaCl concentration. A characteristic potential, below which the current density started to decay from steady ilim, was found during the backward scan. This potential is associated with dissolution of a salt film at the pit bottom and was defined as the potential of salt film dissolution (Esfd). Esfdalso decreased with increasing NaCl concentration. To explain the dependence of ilim and Esfd on NaCl concentration, the diffusion coefficient of metal ions (D) and saturation concentration of FeCl2 (Csat), which are directly associated with the product of pit depth and ilim, were calculated using model concentration profiles in a pit and a thermodynamic database software. Both D and Csat were predicted to decrease as NaCl concentration increased, indicating slower diffusion and easier precipitation of salt film, which both increase the stability of a salt film. The product of the calculated D and Csat values was used to predict ilim for certain pit depths, and the predicted ilim values were in the range of experimentally-measured ilim in different NaCl concentrations, which validated the model.

Mathematical modeling of heavy metal ions (Cd 2+, Zn 2+and Pb 2+) biosorption by chemically modified short hemp fibers

Biosorption, an innovative and low-cost effective method for removal of heavy metal ions from wastewaters, primarily depends on the diffusion of metal ions through the porous structure of sorbent and the resistance effects arise as the result of electrostatic repulsive interactions between ions within the sorbent. Herein, the biosorption of heavy metal ions (Pb 2+, Cd 2+ and Zn 2+) from aqueous solution by lignocellulosic material (chemically modified hemp fibers) was investigated, in order to develop a mathematical model that would describe the phenomenon of different ions transport through porous fiber matrices. Obtained results show that the biosorption process is very fast at the beginning and mainly determined by diffusion of the ions through the porous fiber structure. However, over a period of time the biosorption slows down due to the increase of the resistance to the further transport of metal ions through the fibers (i.e. the electrostatic repulsive interactions between ions into the fibers increase with the increase of the ion concentration in the fibers). The effective diffusion coefficient of metal ions through the fibers, the profile of heavy metal ion concentration in the fibers and the effectiveness of sorption were determined by a mathematical model based on the second Fick's low. Proposed mathematical model is used to optimize the biosorption of heavy metal ions by connecting the model parameters with the fiber performances.

Ion-exchange kinetics of Cu(II) and Zn(II) from aqueous solutions with two chelating resins

The Na form of two chelating ion exchange resins Chelex 100 (Bio-Rad) and Amberlite IRC 748, both of which have iminodiacetic acid function group, was used to exchange Cu 2+ and Zn 2+ from aqueous solutions. Kinetic experiments were performed at 298 K in a batch mode by varying the initial concentration of metal ions (3–9 mol/m 3 ) and initial aqueous pH value (2.0–6.5). The kinetic data were first treated by conventional unreacted-core model and by the Fick’s first law that takes into account diffusion of metal ions within resin pores. The effective diffusion coefficients of metal ions were evaluated. The kinetic data were also treated and compared by a second-order reversible reaction model based on exchange reaction equilibrium. It was shown that the rate constants for Cu 2+ and Zn 2+ exchange with Amberlite IRC 748 resin were larger than with Chelex 100 resin under comparable conditions. © 2007 Elsevier B.V. All rights reserved.

Diffusion coefficients of metals and metal complexes in hydrogels used in diffusive gradients in thin films

Diffusion coefficients of metal ions and metal–ligand complexes in hydrogels were measured at ionic strengths of 0.1–100 mmol L −1 using a diffusion cell. The three different types of hydrogel were all based on polyacrylamide and have been commonly used in the technique of diffusive gradients in thin films (DGT), where accurate diffusion coefficients are essential for the assessment of the concentrations of labile metal in solution. There was little difference in the diffusion coefficients of either Cu and Cd measured in gels with thicknesses of 0.4, 0.8 and 1.6 mm, showing that any diffusion boundary layer in solution was negligibly small. The diffusion coefficients of Pb, Ni, Cu and Cd in simple inorganic solutions was independent of ionic strength (1–100 mmol L −1), except at very low ionic strengths (0.1 mmol L −1) where the value of D was 50% lower. This observation is consistent with the gel having a positive charge that creates a Donnan partitioning of cations at the gel surface. Diffusion coefficient of Pb complexes decreased with increasing size of the ligand, in the order of diglycolic acid (DGA), nitrilotriacetic acid (NTA), fulvic acid and humic acid. In freshwaters the contribution of fulvic acid species to measured DGT mass should be considered. No significant difference was found between the rate of diffusion of Cu and Cd through the mostly commonly used gel and the filter membrane at the face of the DGT device. Diffusion coefficients obtained using a gel with small pore size (restricted gel) plus filter membrane were higher than those obtained using the restricted gel alone. It may be necessary to modify the standard DGT equation for restricted gel deployments, to take into account the difference in diffusion between the gel and the filter membrane.

Trace Metal Measurements in Low Ionic Strength Synthetic Solutions by Diffusive Gradients in Thin Films

In view of conflicting reports regarding the performance of DGT in low ionic strength solutions (I < 1 mM), further investigations have been carried out. Minimal washing of the diffusive gel and deployment in 1.0 and 10 mM NaNO3 solutions containing Cu and Cd gave the theoretical response of 1 for [C](DGT)/[C](SOLN), where [C](DGT) is the concentration of metal measured by DGT and [C](SOLN) is the concentration of metal measured directly in the solution by an appropriate analytical method. Erroneously high values for [C](DGT)/[C](SOLN) were obtained when these same gels were deployed at I = 0.1 mM, presumably due to a net negative charge on the gel, attributable to the presence of initiation products of polymerization. However, washing the diffusive gels completely, where the storage solution pH equaled that of deionized water, gave values of approximately 0.5 for [C](DGT)/[C](SOLN) from deployments at I = 0.1 mM, consistent with the lower measured value of the diffusion coefficients at this ionic strength. These results can be explained by the presence of a net positive charge on the gel when it is exhaustively washed, which reduces the effective diffusion coefficient of metal ions by changing their concentration at the gel-solution interface (Donnan partitioning). Diffusive gel equilibration experiments showed the presence of low capacity sites capable of binding metals irrespective of ionic strength. This binding within the diffusive gel does not affect most DGT measurements, as short (4 h) deployments at concentrations of 10 ppb gave theoretical results. Incomplete washing of the resin-gel caused a 5-15% measurement error and a decrease in precision, even at ionic strengths of 10 mM. A high level of accuracy and precision (typically <5%) was maintained during all aspects of this work, even at ionic strengths of 0.1 mM, in contrast to previous results. This is attributable to three factors: (1) exhaustive washing and conditioning protocols, (2) improvements to the DGT sampling device, and (3) low and reproducible blanks due to ultraclean handling procedures. Provided effective diffusion coefficients measured at the same ionic strength are used, the established DGT theory is obeyed irrespective of ionic strength.

Special Features of Heavy Metal Ion Reduction from Liquid Electrolytes

The ion exchange reduction of heavy metal ions from electrolytes has been studied by theoretical and experimental methods. It is established that high efficiency of the ion exchange is provided by metal ion diffusion through a thin subsurface layer of an ion exchanger with a thickness of \(\bar \delta = 1.6 \times 10^{ - 4} \) under conditions of the optimum solution flow velocity U0, such that the time of ion penetration through the layer of water molecules adsorbed on the surface of pores in this layer is on the order of 10−2 s. In this case, the effective diffusion coefficient of metal ions in the subsurface layer is 4.5 × 10−7 m2/s, which is more than two orders of magnitude higher than the value in solution.

Computer simulation of diffusion of metal ions in resins – phenomenological analysis of diffusion process

In this study, a series of photographs of a Cu(NH 3) 4 2+ loaded resin contacted with EDTA solutions at different concentrations was taken by an instrument set which consisted of a microscope with a Panasonic colour digital camera and a PC matching with Moviestar software. The evaluation of the photographs provides evidence for the development of ion exchange kinetic process occurring in the investigated system and the state of swelling can be followed exactly. A three-dimensional dynamic photograph model of the ion exchange was fitted to the diffusion process of the Cu(NH 3) 4 2+ load resin ∼NH 4 + using Matlab 5.1 software. The dynamic photograph model of the ion exchange process supports the `moving-boundary' process hypothesis in ion exchange kinetics. In the process of the exchange, the bulk of the resin showed no significant increase and the exchanging boundary of Cu(NH 3) 4 2+ loaded resin with NH 4 + exchange gradually shrank as the exchange proceeded. The results show that the Cu(NH 3) 4 2+ loaded resin with NH 4 + exchange process may consist of the following four consecutive steps: (1) detention period; (2) film diffusion; (3) combination of film diffusion and inner diffusion and (4) inner diffusion. The exchange rate of Cu(NH 3) 4 2+ loaded resin with NH 4 + increase with increasing EDTA concentration. In addition, it was demonstrated that the boundary profiles recorded by single particle microphotograph technology can be used for evaluating the diffusion coefficients of metal ions in ion exchange process.

Performance study of diffusive gradients in thin films for 55 elements.

The technique of diffusive gradients in thin films (DGT) is a fairly new and useful tool for in situ measurements of labile metal ions in water. The applicability of DGTs was investigated by comparing independently determined or estimated diffusion coefficients with DGT effective diffusion coefficients (D(DGT)) for 55 elements. The DGTs were exposed at a controlled fluid velocity of 0.1 m s(-1) and a concentration of 1 ng mL(-1) at four pH levels between 4.7 and 6.0, and the D(DGT) values were determined from the uptake by the sampler. The measured D(DGT) values for the elements Co, Ni, Cu, Zn, Cd, Pb, Al, Mn, and Ga were close to previously published values with some deviations for Pb and Zn. The uptake of V, Cr, Fe, U, Mo, Ti, Ba, and Sr varied with pH, and there were some experimental problems that require further investigations. A novel set of D(DGT) values for the lanthanides (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Yb, Lu, Y) was established. The D(DGT) values for these were about 10-15% lower than for free ions in water and indicate that diffusion coefficients of metal ions in the agarose polyacrylamide diffusive hydrogel are 10-15% lower than in water. The high consistency of the data for the lanthanides establishes these elements as new performance test metals for the DGT sampler. The accumulation of the elements Li, Na, K, Rb, Mg, Ca, B, Tl, P, S, As, Bi, Se, Si, Sn, Sb, Te, Zr, Nb, Hf, Ta, W, Th, and Ag was low (D(DGT) lower than 10% of theoretical values). A more efficient elution procedure using concentrated nitric acid for the absorbent gel was established, with elution efficiencies between 95 and 100% for most metals. For deployment times of 24 h, detection limits from 0.001 to 1 ng mL(-1) were achieved with moderate precautions to prevent contamination.

Electrochemical Considerations Regarding General Corrosion of Materials in a BWR Primary Circuit

Electrochemical calculations of 3d-transition metal (Fe, Ni, Co and Cr)-water systems at 558 K were performed to evaluated the general corrosion behavior of materials in a BWR primary circuit. Thermodynamics expressed by Pourbaix diagrams, the solubilities of oxides and the equilibrium activities of H2, O2 and H2O2 in oxide formations, and kinetics determining the corrosion potential and corrosion rate of the materials were evaluated. The results assist recognition of the following corrosion behaviors: 1. Good corrosion resistance of type 304 SS under various water chemistry conditions is due to the properties of NiFe2O4 and (Fe, Ni)Cr2O4; namely wide stable regions and low diffusion coefficients of metal ions and the ability of these oxides to compensate for each others weakness. 2. A typical increase of the corrosion rate of Alloy X750 and Stellite #6 in an oxygenated aqueous environment containing H2O2 is due to the properties of NiO and CoO; namely high solubilities and high diffusion coefficients of ...

Electrochemical Considerations Regarding General Corrosion of Materials in a BWR Primary Circuit.

Electrochemical calculations of 3d-transition metal (Fe, Ni, Co and Cr)-water systems at 558 K were performed to evaluated the general corrosion behavior of materials in a BWR primary circuit. Thermodynamics expressed by Pourbaix diagrams, the solubilities of oxides and the equilibrium activities of H2, O2 and H2O2 in oxide formations, and kinetics determining the corrosion potential and corrosion rate of the materials were evaluated. The results assist recognition of the following corrosion behaviors: 1. Good corrosion resistance of type 304 SS under various water chemistry conditions is due to the properties of NiFe2O4 and (Fe, Ni)Cr2O4; namely wide stable regions and low diffusion coefficients of metal ions and the ability of these oxides to compensate for each others weakness.2. A typical increase of the corrosion rate of Alloy X750 and Stellite #6 in an oxygenated aqueous environment containing H2O2 is due to the properties of NiO and CoO; namely high solubilities and high diffusion coefficients of metal ions.

Adsorption of heavy metal cations from aqueous solutions by wool fibers

AbstractAdsorption of metal cations by kivircik wool from aqueous NiCl2, CuCl2, ZnCl2, CdCl2, HgCl2 and Pb(NO3)2 solutions at 25°C and 50°C was investigated using atomic absorption spectroscopy. A fiber diffusion controlled adsorption rate model was used to predict the effective diffusion coefficients of metal ions in wool. It has been shown that wool is a potential adsorbent for removing toxic metal ions from contaminated water.

Diffusion Coefficients, Oxygen Activities, and Defect Concentrations Across a p‐Type Oxide Layer on a Metal

Oxygen activities, diffusion coefficients of metal ions, and defect concentrations across an oxide layer on nickel, cobalt, iron, and manganese have been determined from the parabolic rate constants for growth of the oxide on the metal in different pressures of oxygen. Theoretical expressions are presented which account for the observed results.