Chemisorption Of Ions Research Articles

Monotonic Surface Charging at Polycrystalline Platinum-Electrolyte Interface in the Presence of Chemisorbed Species Using Streaming Current Method

Surface charging characteristics at a polycrystalline Pt-electrolyte interface were studied using electrokinetic streaming current method integrated with in-situ electrochemical cell, which was developed by our group [1-3]. Experimentally, surface charging relations are determined from the potential-dependent capacitance data. For catalytic surfaces (e.g., platinum), this method is challenging to apply due to the strong chemisorption properties of the metal [4]. We developed a method to measure surface charge of a metal in presence of strong chemisorption of ions by incorporating electrokinetic streaming current technique with traditional electrochemical 3-electrode setup (Figure 1a).Generally, the metal charge increases monotonically with applied potential. Based on theoretical models, it was suggested that the Pt-electrolyte interface might deviate from this trend and have negative charge above the potential of zero charge (PZC) due to the metal’s strong chemisorption properties [5,6]. We attempted to experimentally determine the validity of this claim using our method. In addition, we also observed the effect of anion adsorption (oxide, chloride, sulfate, etc.) on surface charging. The experimental method also allowed us to observe the slow time-dependent growth of oxides on Pt surface.Since our first publication in 2019 [1], we were able to develop the setup to be more efficient by reducing noise, electrolyte resistance, current fluctuations, etc. that enabled us to apply higher potential on the metal, observe stronger ion adsorption effects on surface charging, and obtain more reliable data. Our recent experimental study shows that Pt charging remains monotonic and does not reverse due to chemisorption of oxides in the potential window where the charge reversal was speculated to occur (Figure 1b). Figure 1c shows that metal charge (at 1.2 V vs SHE) grows slowly over time to settle down to an equilibrium value. We hypothesize that it was caused by the slow growth of oxides at the interface. The time-dependent growth of oxides has already been observed [7]. The electrolyte used in our experiments was HClO4 where the anion (ClO4 -) is less adsorbing than Cl-, SO4 2-, Br- etc. The only adsorbing anions in this electrolyte solution are OH- and O2-. In this work, we will present a brief description of the improvements of the electrokinetic-electrochemical method and our experimental results showing monotonic charging behavior for Pt in the presence of ion adsorption and oxide growth.

Electrochemical properties of magnetite nanoparticles supported on carbon paste electrode in various electrolyte solutions: A study by cyclic voltammetry

Electrochemical properties of magnetite nanoparticles supported on carbon paste electrodes were investigated by means of cyclic voltammetry. The measurements were performed for bare and coated with citrate magnetite nanoparticles in aqueous solutions of various electrolytes: NaCl, NaClO4, and Na2HPO4. Cyclic voltammetry curves obtained on bare and citrate functionalized Fe3O4 nanoparticles are rather similar indicating that the electrochemical properties of the magnetite nanoparticles are not significantly affected by the citrate coating. Cyclic voltammetry scans reveal the formation of metallic iron below −1.2 V. The defective structure of the oxide phases formed by oxidation of metallic iron seems to play an important role in the chemisorption of chloride ions and their subsequent oxidation.Graphical

Efficient lithium extraction from aqueous solutions by MIL-100(Fe): A study on adsorption kinetics, thermodynamics and mechanism

MIL-100(Fe) is a kind of metal–organic frameworks (MOFs) with robust water and chemical stability. The adsorption kinetics, isotherms, thermodynamics and regeneration of MIL-100(Fe) were investigated to illustrate the process of efficient lithium adsorption. The adsorption kinetics of MIL-100(Fe) followed Pseudo-second-order model, and the lithium adsorption equilibrium was reached rapidly in 2 h. Langmuir model fit best with the adsorption isotherms, indicating chemisorption of lithium ions on MIL-100(Fe) and possibly a monolayer adsorption process. By calculating thermodynamic parameters (ΔGθ, ΔHθ and ΔS θ), the adsorption of lithium on MIL-100(Fe) can be illustrated by a spontaneous endothermic process. The adsorption mechanism involved electrostatic interaction, impact of crystal defect sites and coordinative interactions between metal active sites and lithium ions (Fe—F‧‧‧Li). MIL-100(Fe) was regenerated for four cycles, and there was only a slightly decrease in adsorption capacity and the morphology remained stable. Combined with extremely short equilibrium time and high stability, MIL-100(Fe) can be a promising adsorbent for lithium extraction.

Synthesis and Characterization of Citric Acid-Modified Iron Oxide Nanoparticles Prepared with Electrohydraulic Discharge Treatment.

Chemical co-precipitation from ferrous and ferric salts at a 1:1.9 stoichiometric ratio in NH4OH base with ultrasonication (sonolysis) in a low vacuum environment has been used for obtaining colloidal suspensions of Fe3O4 nanoparticles coated with citric acid. Before coating, the nanoparticles were processed by electrohydraulic discharges with a high discharge current (several tens of amperes) in a water medium using a pulsed direct current. Magnetite nanoparticles were obtained with an average crystallite diameter D = 25-28 nm as obtained by XRD and particle sizes of 25 nm as measured by small-angle X-ray scattering. Magnetometry showed that all samples were superparamagnetic. The saturation magnetization for the citric acid covered samples after electrohydraulic processing showed higher value (58 emu/g) than for the directly coated samples (50 emu/g). Ultraviolet-visible spectroscopy and Fourier transform infrared spectroscopy showed the presence and binding of citric acid to the magnetite surface by chemisorption of carboxylate ions. Hydrodynamic sizes obtained from DLS and zeta potentials were 93 and 115 nm, -26 and -32 mV for the citric acid covered nanoparticles and 226 nm and 21 mV for the bare nanoparticles, respectively. The hydraulic discharge treatment resulted in a higher citric acid coverage and better particle dispersion. The developed method can be used in nanoparticle synthesis for biomedical applications.

Gd-Co-Fe perovskite mixed oxides as catalysts for dry reforming of methane

The effect of substitution of cobalt by iron in perovskite-type oxides GdCoxFe1-xO3 (х = 0; 0.5 and 1) on their physical properties and catalytic performance in dry reforming of methane (DRM) was investigated. The perovskites were synthesized by sol-gel technology and characterized by XRD, DTG/TG, BET specific area, TPR and SEM. It was established that the introduction of cobalt into the B site of the perovskite-type structure of ferrite leads to an increase in catalytic performance, and, consequently, to a decrease in the process temperature. It was found that catalytic activity depends on the proportion of cobalt in the B site and increases in the row: GdFeO3 < GdCo0.5Fe0.5O3 < GdCoO3. Probably, such behavior of Co-containing catalysts is associated with the formation and stabilization of a more catalytically active Co–Gd2O3 system under the action of the reaction mixture. It has been assumed that the element in A-site (Gd) is responsible for dissociative chemisorption of CO2 and ions in site B (Fe and Co) are responsible for the formation of atomic hydrogen.

Achieving strong Pb-Cr complexation in Mg/Al LDHs for ultrafast chromate ions separation and chrome recovery from complex water matrices

Layered double hydroxides with high anion exchange capacity and positive surface charge were being explored to remove metal oxo-anions. Environmental complexities such as the presence of other major interfering anions, organic matter, etc., drastically hampers their performance and limit their applicability. Based on the fundamental understanding of strong Pb-Cr complexation, in this work, Pb2+ incorporated LDH (LDH-Pb) was designed for selective chemisorption of chromate ions overcoming existing environmental limitations of LDH for chromium removal. LDH-Pb was designed by substituting Mg2+ ions of Mg/Al LDH with Pb2+ during crystallization process. Pb2+ have shown efficient incorporation at MII crystal sites of Mg/Al LDH and also sorbed in form of oxy-hydroxide on the surface. Pertaining to faster Pb-Cr complexation, strong binding, and formation of insoluble PbCrO4, LDH-Pb has shown efficient and higher removal of chromate ions than LDHs. Despite environmental complexities such as varying pH, competitive ions, organic matter, and natural matrices, efficiency of LDH-Pb remained less altered. Direct application (proof-of-concept) has been shown for extraction of chromate ions and chrome recovery from collected chromite ore processing residue leachate contaminated groundwater samples. Higher-order of removal kinetics and sorption capacity of LDH-Pb (n = 9.2, and 188.7 mg/g) further justified its candidature. Strong Pb-Cr complexation in LDH-Pb further resulted in faster and efficient continuous chromate filtration (47.3 mg/g) with adsorbent loaded syringe filters. Chrome recovery was achieved through easy acid dissolution of LDH from LDH-Pb-Cr waste (pH <3) resulting in chrome particles (monocrystalline micro rods) that have several potential industrial applications in various sectors.

Selective capture uranium from acidic solution by potassium manganese ferrocyanide

Rapid and effective uranium removal and enrichment from water is necessary for sustainable economic development and ecological protection. In this paper, an efficient adsorbent Prussian blue compound, potassium manganese ferrocyanide (KMnFC), was synthesized and characterized for the separation of uranium. Its effectiveness in U(VI) removal and adsorption amount were evaluated using batch static adsorption experiments. The adsorption effect of the compound on U(VI) in aqueous solution was investigated, and the maximum adsorption capacity of the compound was 469.48 mg g−1. The adsorption-diffusion kinetic study showed that liquid film diffusion was the main rate control step. The adsorption process of uranium (VI) on KMnFC is a heat absorption reaction and chemisorption of uranium ions in exchange with potassium ions occurs. Most importantly, the compound is uranium selective, and its uranium removal ability is relatively effective even under high acidity circumstances.

Controlled growth of TiO2/Zeolite nanocomposites for simultaneous removal of ammonium and phosphate ions to prevent eutrophication

In the modern era, problems like eutrophication caused by increased nutrients such as ammonia and phosphorous in freshwater bodies have become the cause of freshwater ecosystem deterioration. To save freshwater by reducing eutrophication, new cost-effective strategies and methods are urgently needed. In this study, titanium oxide nanoparticles dispersed on zeolite were chemically synthesized for the simultaneous removal of phosphate and ammonium ions from aqueous solutions. SEM and XRD analysis were used to characterize the synthesized TiO2/zeolite nanocomposites, which revealed that the synthesized material was more stable and dispersed than zeolite. The nanocomposites removed 38.8% NH4+ and 98.1% PO43− from an initial concentration of both ions of 20 mg 100 ml−1. The removal of both ions was investigated under various conditions including different concentrations of nanocomposites, initial concentration of the solution, temperature, time, and pH. The maximum adsorption of nanocomposites for PO43- was 38.63 mg g−1 at optimal conditions, and 3.75 mg g−1 for NH4+. Kinematics studies showed that both the ions were adsorbed by a pseudo-second-order model. Ion chemisorption occurred as a result of ligand exchange or electrostatic adsorption between ions and nanocomposites. Overall, it was determined that this strategy is a viable and efficient method for simultaneously removing both ions (anionic phosphate and cationic ammonium) from eutrophic waters.

Synthesis and Corrosion Inhibition Potential of Cerium/Tetraethylenepentamine Dithiocarbamate Complex on AA2024-T3 in 3.5% NaCl.

In this work, a cerium/tetraethylenepentamine dithiocarbamate complex was synthesized and evaluated for the corrosion inhibition capability on an AA2024-T3 Al alloy in a 3.5% NaCl medium. The synthesized compounds were characterized via spectroscopic techniques. The corrosion inhibition behaviour of the complex was elucidated by electrochemical measurements and surface analysis techniques. Based on electrochemical test results, the corrosion inhibition efficiency of the complex increases with the immersion time of aluminium alloy in the test solution. The corrosion inhibition reaches 96.80% when the aluminium is immersed in a 3.5% NaCl solution containing a corrosion inhibitor for 120 h. The potentiodynamic polarization test results show that the complex acts as a mixed-type corrosion inhibitor and the passive range is widened. The surface analysis methods reveal that the corrosion inhibition ability of the complex originated from the formation of a protective layer on the Al surface. This film is created from the physisorption and chemisorption of cerium ions and organic parts simultaneously released from the complex molecules.

Sorption properties of Zr-CaMg and Ti-Ca-Mg phosphates in relation to Sr(II) ions

The synthesis of Zr-Ca-Mg and Ti-Ca-Mg phosphates of different composition was carried out and their sorption properties to Sr(II) ions were studied. The effect of pH of aqueous solutions, as well as of background electrolytes NaCl, CaCl2 , the seawater solution on the sorption-selective properties of phosphate sorbents was studied. The sorption isotherms and kinetic regularities of the Sr(II) ion removal were studied, the mathematical modeling of the experimental data using various models was carried out. It was found that Ti-containing sorbents had the greatest selectivity to Sr(II) ions. The maximum sorption capacity for the Ti-2 sample reached 196.5 mg/g (2.2 mmol/g). Samples of Zr-Ca-Mg and Ti-Ca-Mg phosphates had a high sorption capacity in the 0.1 M NaCl solution and in seawater. The sorption kinetics for all sorbents with a high degree of confidence (R2 ≥ 0.99) was described by a pseudo-second-order equation, due to the chemical interaction during the Sr(II) ions sorption. The study of the phase composition of sorption products indicated the ion exchange and chemisorption mechanism of the Sr(II) ions sorption.

A theoretical and experimental study of the effects of NaCl and the competitive chemisorption of ions at the surface sites in the context of galena flotation

• The recovery of galena by collector-less flotation improves as a result of NaCl addition. • Our galena surface complexation model is a “two-site / not amphoteric” model. • The theoretical predictions of the models support the experimental results. • The decreasing repulsion at higher ionic strengths corresponds to a higher galena recovery. In this study we have investigated the effects of increasing the NaCl concentration on the flotation of galena. Experiments were carried out using a Hallimond tube in NaCl solutions with concentrations of 1, 10 and 100 mM, at pH 9. It was found that the recovery of galena by collector-less flotation improved for higher NaCl concentrations. Zeta potential measurements made on galena particles conditioned in NaCl solutions were used to calibrate the parameters for describing a chemisorption model representing charge regulation at the galena binding sites. Our galena chemisorption model presented here is a “two-site/not amphoteric” surface complexation model. A chemisorption model representing the charge regulation process on the surface of an air bubble has also been applied. The zeta potentials calculated using these models were in agreement with the measured values, indicating that these models can be applied to predict the potentials on the surface of galena and an air bubble for a range of NaCl concentrations and pH. To investigate the mechanisms of particle-bubble interactions for each NaCl concentration, the total interaction free energy as a function of the separation distance between galena and an air bubble was determined. It was found that at a NaCl concentration of 1 mM, due to adsorption of ions at the galena surface, repulsion dominated the interaction, and the lowest galena recovery was reported. With an increase in NaCl concentration in solution to 100 mM, the total interaction between galena and an air bubble was represented by a purely attractive total interaction free energy curve. The theoretical predictions of the models supported the experimental results, with a stronger attraction predicted by the models at higher ionic concentrations, corresponding to a higher galena recovery during flotation.

Tuning the reversible chemisorption of hydroxyl ions to promote the electrocatalysis on ultrathin metal-organic framework nanosheets

Interfacial engineering to alter the configuration of active sites in heterogeneous catalysts is a potential strategy for activity enhancement, but it remains unelucidated for metal-organic frameworks (MOFs). Here, we demonstrate that the surface of two-dimensional Co-based MOF is modified by decorating Ag quantum dots (QDs) simply through in-situ reduction of Ag+ ions. Toward oxygen evolution reaction (OER), it reveals that the catalysis is mediated by the reversible redox of Co sites between Co3+ and Co4+ states coupling with transfer of OH− ions. The decoration of Ag QDs decreases the redox potential of Co sites, and thus effectively decreases the overpotential of OER. The TOFs of Co sites are increased by 77 times to reach 5.4 s−1 at an overpotential of 0.35 V. We attribute the activity enhancement to the tuning of the coupling process between Co sites and OH− ions during the redox of Co sites by Ag QDs decoration based on Pourbaix analysis.

Silver(I) Ion Removal Efficiency of Activated Carbon Prepared from Terminalia-bellerica (Barro) Seed Stone

Phosphoric acid-activated Terminalia-bellerica (Barro) seed stone powder was carbonized in a muffle furnace at three different temperatures (300, 400, and 500oC). The activated carbons (BAC-300, BAC-400, and BAC-500) were characterized by using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), methylene blue number, and iodine number. The iodine number (357 mg/g) and specific surface area (537 m2/g) were a maximum for BAC-400. The BACs followed Langmuir adsorption isotherm and the maximum methylene blue adsorption capacity was 212.77 mg/g. The silver ion removal efficiency was a maximum at pH 6, 3 mg/L of adsorbent dose, and 20 mg/L of silver ion concentration. The BAC-400 could adsorb 40 % of silver ion within 5 mins with the initial Ag(I) ion concentration of 20 mg/L and an adsorbent dose of 1 mg/L. The percentage of adsorption enhanced to 100% with the increment of adsorbent doseto3 g/L.The adsorption kinetics of silver (I) ion on BAC-400 was well fitted to pseudo-second-order kinetics suggesting the chemisorption of silver ions. All the results attributed that low-cost viable adsorbent can be prepared from Barro seed stone for the efficient removal of silver ion from aqueous solution.

Investigation of the process of fluoride ions adsorption by natural sorbents

A detailed analysis of literature sources on the content of fluorides in the environment, methods of conditioning and defluoridation of water was held. The mechanism of interaction of fluorides with zeolite in the process of adsorption treatment of wastewaters and domestic waters was clarified. It was first established that in the process of adsorption there is the formation of magnesium fluoride compounds due to the binding of cations extracted from the zeolite by the mechanism of ion exchange. The mechanism of chemisorption of fluoride ions by the formation of insoluble CaF2 was also proved. According to the results of experimental research, a technological scheme has been developed, which provides the preliminary treatment of fluoride ions in the form of complexes with aluminium. In order to prevent the permeation of aluminium fluoride into the treated water, a stage of adsorption water purification was proposed. Calculations of the amount of reagent necessary to ensure the required degree of purification of water from fluoride ions were done. The statistical parameters of the process and the significance of the obtained results were calculated. The coefficient of determination of experimental data was 0.87… 0.99, the standard deviation was 0.017… 0.026.

Revealing the sodium‐storage performance enhancement of adsorption‐type carbon materials after ammonia treatment: Active nitrogen dopants or specific surface area?

Adsorption-type carbon materials are promising candidates for fast sodium-ion storage via surface physisorption or chemisorption of sodium ions. Post-treatment of carbon such as ammonia can simultaneously strengthen both adsorption effects by increasing the specific surface area for physisorption, and by introducing active nitrogen dopants for chemisorption. Which factor, however, the increment of specific surface area or the introduction of active nitrogen dopants, predominantly contributes to sodium-storage capacity increment, remains a question. Answering this question is of great importance for understanding the sodium storage mechanism of adsorption-type carbon materials and thereby optimizing their sodium storage capabilities. In this work, pristine carbon is thermally treated in ammonia at temperatures from 600 to 1200°C, resulting in a simultaneous increase of specific surface area (8.6-1155 m2 g−1) and active nitrogen dopants (0.75-6.47 wt%). Correlations between sodium storage capacity and specific surface area/active nitrogen dopants are established. It is found that as the post-treatment temperature increases, the capacity increment is contributed first by sodium physisorption on active surface, and then by sodium chemisorption on nitrogen dopants. Our findings enrich the mechanistic understanding of sodium storage in adsorption-type carbon materials, which may guide the rational designs of carbon materials for high-rate sodium-based energy storage systems.

Direct Measurement of Water-Assisted Ion Desorption and Solvation on Isolated Carbon Nanotubes.

We have investigated the change in mean residence time of gaseous ions adsorbed on the surface of suspended carbon nanotube field-effect transistors (CNT-FETs) with and without native surface water layers that exists in atmospheric conditions. Devices were characterized electrically before and after dehydration by thermal, dry gas, and vacuum desiccation and in each scenario were found to have substantially higher mean ion residence times. It is proposed that water molecules native to the CNT surface in ambient conditions provide a reduction pathway for incoming gaseous ions, yielding hydronium ions (H3O+). This is supported by the appearance of frequent clustered readsorption events in the presence of surface water, caused by the rapid hopping of H+ between the device surface and the lowest water layer, which are not present in data collected from desiccated devices. After desiccation of the device, a thermal trial was conducted to determine the adsorption energy of N2+ ions on the CNT surface. This work has profound implications for our understanding of wetting in one-dimensional systems and the chemistry of ion chemisorption and solvation on the surfaces of materials in general.

Phosphogypsum and clay mineral/phosphogypsum ceramic composites as useful adsorbents for uranium uptake

We analyzed the sorption capacity of phosphogypsum and clay mineral-based ceramic composites. Phosphogypsum is a waste by-product generated from apatite in the production process of phosphoric acid and phosphate fertilizers. On the one hand, the chemical composition of phosphogypsum poses a considerable problem to production plants due to its acidic properties. On the other hand, phosphogypsum can constitute an excellent sorbent material for cations that prospectively create insoluble phosphoric mineral phases. To examine and study the above issues, a series of ceramic composites with differing phosphogypsum content were prepared and examined to evaluate their uranyl sorption capacity. The isotherms of uranyl adsorption onto the composite surface were determined and kinetic experiments were conducted to determine the sorption properties and kinetic parameters. Experiments were conducted under both static and dynamic conditions. Furthermore, diffraction studies and chemical composition analyses were carried out using a scanning electron microscope with an energy dispersive spectrometry feature. We found that the maximum uranium (VI) adsorption reached nearly 0.09 mol kg−1, which equaled 21 g of uranium adsorbed per 1 kg of phosphogypsum at 298 K. The results indicated efficient uranyl ion chemisorption onto the examined composites. In addition, the occurrence of phases corresponding to the composition of autunite group minerals were observed. The composites have potential application in water treatment technologies and hazardous waste landfills.

Chemisorption of copper ions in aqueous acidic solutions by modified chitosan

Sorption of copper ions by composite sorbents based on cross-linked chitosan such as chitosan/colloid silicon dioxide, chitosan/polymethylsiloxane and chitosan/ polymethylsiloxane/gelatin have been studied. Sorption capacity of the sorbents is increased the pristine chitosan 1.5–3.85 times. Eliminating of Cu2+ is based on competing mechanism of salt formation between metal ions and amino-groups in a two-phase system “aqueous solution of acids and salts – chitosan”, which involve metal ions (M2+), protons (H+) and their carriers – anions (X−) on the amino-groups of composite sorbent with final formation of chelates of the following structure CS[(-NH2)2M(X)2] and CS[-NH3X]2.The role of pH on sorption of copper ions and protons is explained by competing mechanism of metal cations and protons for sorption sites. Sorption of protons is accompanies by transformation of amino-groups into inactive form, H-form, which is resulted in decreasing the capacity of sorbents for ions of heavy metals in aqueous acidic solutions.

Functionalization of the Parylene C Surface Enhances the Nucleation of Calcium Phosphate: Combined Experimental and Molecular Dynamics Simulations Approach.

Interactions at the solid–body fluid interfaces play a vital role in bone tissue formation at the implant surface. In this study, fully atomistic molecular dynamics (MD) simulations were performed to investigate interactions between the physiological components of body fluids (Ca2+, HPO42–, H2PO4–, Na+, Cl–, and H2O) and functionalized parylene C surface. In comparison to the native parylene C (−Cl surface groups), the introduction of −OH, −CHO, and −COOH surface groups significantly enhances the interactions between body fluid ions and the polymeric surface. The experimentally observed formation of calcium phosphate nanocrystals is discussed in terms of MD simulations of the calcium phosphate clustering. Surface functional groups promote the clustering of calcium and phosphate ions in the following order: −OH > −CHO > −Cl (parent parylene C) ≈ −COO–. This promoting role of surface functional groups is explained as stimulating the number of Ca2+ and HPO42– surface contacts as well as ion chemisorption. The molecular mechanism of calcium phosphate cluster formation at the functionalized parylene C surface is proposed.

A thermodynamic correction to the theory of competitive chemisorption of ions at surface sites with nonelectrostatic physisorption.

We resolve a thermodynamic inconsistency in previous theoretical descriptions of the free energy of chemisorption (charge regulation) under conditions where nonelectrostatic physisorption is included, as applied to surface forces and particle-particle interactions. We clarify the role of nonelectrostatic ion physisorption energies and show that a term previously thought to represent physisorbed ion concentrations (activities) should instead be interpreted as a "partial ion activity" based solely on the electrostatic physisorption energy and bulk concentration, or alternatively on the nonelectrostatic physisorption energy and surface concentration. Second, the chemisorption energy must be understood as the change in chemical potential after subtracting the electrostatic energy, not subtracting the physisorption energy. Consequently, a previously reported specific ion nonelectrostatic physisorption contribution to the chemisorption free energy is annulled. We also report a correction to the calculation of surface charge. The distinction in "partial ion activity" evaluated from bulk concentration or from surface concentration opens a way to study nonequilibrium forces where chemisorption is in equilibrium with physisorbed ions but not in equilibrium with bulk ions, e.g., by a jump in ion concentrations.