Solubility Of Iodine Research Articles

Spatial variation in iodine content with relation to soil physicochemical properties in lower Himalayan region

Topography of a place has a significant impact on soil characteristics that ultimately influence soil iodine levels. Lower Himalayan region (LHR) in Pakistan has a wide range of climatic and geological variations. Hence, an investigation was conducted to analyze the iodine concentration and other physicochemical properties of soils in two LHR districts, Haripur and Mansehra. Spatial analysis indicated a decrease in iodine levels in the mountainous regions in comparison to the flat portions of LHR. Soil samples obtained from different locations across Haripur had a stronger affinity for iodine due to variations in solubility and adsorption of iodine to soil clay components, which can be attributed to lower pH, higher organic matter, and a higher cation exchange capacity (CEC). In contrast to the plains of Haripur, elevated locations in the Mansehra district had decreased levels of iodine, along with a higher soil pH and reduced soil organic matter. The soil erosion and depletion of soil micronutrients in the hilly region of Mansehra may be attributed to the unfavorable soil conditions and excessive precipitation. Presence of clay, iron (Fe), and aluminum (Al) in the soil led to a rise in iodine levels. Iodine concentrations exhibited an inverse relationship with soil acidity. Study revealed a direct correlation between soil iodine levels and their cation exchange capacity (CEC) and clay content. This study aims to gather fundamental data for the chosen regions of LHR to address illnesses caused by iodine deficiency.

A novel cationic β-cyclodextrin decorated with a choline-like pendant exhibits Iodophor, Mucoadhesive and bactericidal properties

Iodine is a vital microelement and a powerful antiseptic with a rapid and broad spectrum of action. The development of iodophor compounds to improve the solubility and stability of iodine is still challenging. Here, we report the synthesis of a novel cationic β-cyclodextrin bearing a choline-like pendant (β-CD-Chol) designed to complex and deliver iodine to bacterial cells. The characterization of β-CD-Chol and the investigation of the inclusion complex with iodine were performed by NMR spectroscopy, mass spectrometry, UV–vis spectrophotometry, isothermal titration calorimetry, and dynamic light scattering. The functionalization with the positively charged unit conferred improved water-solubility, mucoadhesivity, and iodine complexation efficiency to the β-CD scaffold. The water-soluble β-CD-Chol/iodine complex efficiently formed both in solution and by solid-vapor reaction. The solid complex exhibited a significant stability for months. Iodine release from the inclusion complex was satisfactory and the bactericidal activity was proved against a Staphylococcus epidermidis strain. The absence of cytotoxicity tested on human keratinocytes and the improved mucoadhesivity make β-CD-Chol a promising drug delivery system and an appealing iodophor candidate for iodine-based antisepsis including mucosa disinfection.

Environmentally Friendly Water-Based Electrolyte for Dye-Sensitized Solar Cells: Future Prospective and Outlook

The use of traditional Dye-sensitized solar cells (DSSCs) is limited due to the use of toxic and non-environmentally safe solvents. In this review, water is proposed as a viable alternative to developing green chemistry and sustainable materials for DSSCs. However, water-based DSSCs (WBDSSCs) require improvement in their photovoltaic parameters. The detachment of dye molecules from the semiconductor and the poor solubility of iodine in water are the primary reasons for their low efficiency. This review analyzes the best results achieved by 100% aqueous electrolytes containing synthetic, organic, and natural dyes to identify the best conditions to overcome these limitations. Developing a suitable photoanode/electrolyte interface and finding a compromise in the choice of dye are the main research goals in the coming years. WBDSSCs can contribute significantly to producing clean energy using sustainable and environmentally friendly materials. Furthermore, here we report the state of the art of the emerging technology of underwater dye-sensitized solar cells, which are a promising technology for generating renewable energy in aquatic environments. Recent advancements in material science and device engineering have shown promising results in enhancing their efficiency and durability. Further research and development can make these devices a viable alternative for sustainable energy generation in a wide range of underwater applications.

Predicting iodine solubility at high pressure in borosilicate nuclear waste glasses using optical basicity: an experimental study

Constraining the systematic evolution of iodine solubility in borosilicate glasses is required for the formulation of adequate glass matrices able to immobilize 129I representing a major troublesome radioisotopes produced by nuclear anthropic activities. We investigated experimentally the change in iodine solubility in a large series of borosilicate glasses synthesized under high-pressure conditions (1.5 GPa) and at 1350 °C. The nature of network modifying cation (Na, K, Rb, Ca, Ba and Sr) and the concentration of B2O3 (~ < 10 mol.% and ~ > 10 mol.%) have been tested. The XPS measurements showed that iodine speciation in glasses is mostly represented by iodide (I−) for a range of iodine solubility from 0.3 to 3.1 mol.% as determined by SEM EDS and LA-ICP-MS analyses. The iodine solubility is enhanced in glasses with lower B2O3 content and with a higher concentration in network modifying cation. Regardless of the cation nature: alkali or alkaline-earth; increasing the cation size appears to induce a decrease in iodine solubility. We used the optical basicity (ΛGlass) and iono-covalent parameter (ICPGlass) to express the large variety of investigated glass compositions; both relating to the electron donor capability of the glass. We show that iodine solubility is positively correlated with ΛGlass and accordingly negatively correlated with ICPGlass. It implies that more iodine will be dissolved in glass compositions having a stronger electron-donating capability. Evidence of the relationship between iodine solubility and oxygen network is shown by the iodine solubility positive trend with equilibrium constant of the oxygen speciation. Future work should take these parameters into consideration for modeling iodine solubility in borosilicate glasses providing additional information are collected such as the boron, aluminum and silicon speciation in glasses.

Efficient capture and stabilization of iodine via gas-solid reaction using cyclodextrin metal-organic frameworks

For treatment of wound infection with stabilized iodine, potassium iodide cyclodextrin metal-organic frameworks (KI-CD-MOF) was prepared to carry iodine via gas-solid reaction. Apart from highly ordered porous frameworks, KI-CD-MOF contains uniformly distributed iodide ions which stabilize iodine (I2). X-ray photoelectron spectroscopy and potentiometric titration were utilized to confirm the formation of I3− in the highly porous KI-CD-MOF as I2@KI-CD-MOF. Molecular simulation and characterizations of the synchrotron radiation Fourier transform infrared spectroscopy, differential scanning calorimeter, powder X-ray diffraction and N2 adsorption were conducted to illustrate the inclusion mechanism of iodine in I2@KI-CD-MOF. The apparent solubility of iodine in water was 3.86 times enhanced. The stability and antibacterial activity tests demonstrated that the highly-dispersed iodide ions in KI-CD-MOF are crucial in improvement to the solubility, stability, and bacteriostatic effects of active iodide. Therefore, KI-CD-MOF has broad application prospects and advantages in efficiently capturing and stabilizing iodine.

High pressure experimental study on iodine solution mechanisms in nuclear waste glasses

The 129I is a long-lived isotope, radiotoxic and highly mobile by-product of fission reactors. Its disposal in geological repositories requires matrix at least as durable as current nuclear waste glasses. The volatility of iodine at high temperature is a concern for nuclear waste glass processing, because its evaporation prior to vitrification, results in a low retention rate in the final glass. The present study investigates the use of pressure for immobilizing iodine during vitrification. For three different glass compositions: International Simple Glass (ISG), NH (a depolymerized glass similar to Low Activity Waste glasses, LAW) and boron-free trachytic glass; we conducted the experiments between 0.5 and 2.0 GPa for pressure and 1300–1500 °C for temperature. We determined the iodine solubility in glass compositions representative of the simplified nuclear waste glass simulants, and the influence of iodine on the structure of theses glasses.In comparison to ambient atmosphere experiments, increasing pressure (up to 1.5 GPa) induces an increase in the iodine concentration in ISG and NH glass compositions at ∼1 and ∼2 mol.%, respectively; whereas the iodine concentration remains almost constant for boron-free trachytic glass composition suggesting the influence of boron in the iodine solution mechanism. The glass structure investigated via 23Na NMR showed that sodium environment is modified by the presence of iodine in its surroundings. 11B NMR highlighted the structural changes induced by iodine: polymerized glasses (ISG) tend to depolymerize with iodine dissolution, depolymerized glasses (NH) tend to polymerize with iodine dissolution. Our high-pressure data suggest that both I− and IO3− are the solubilized forms of iodine with a preferential affinity to sodium atoms within the glass structure; and comparable to previous studies conducted at ambient pressure.

Solubility of Iodine in Sub/Supercritical Carbon Dioxide

Abstract—The solubility of iodine in sub- and supercritical carbon dioxide was studied using a flow-type extraction setup. It is shown that at 20–80°C and 60–350 atm, the solubility of iodine increases at rising temperature and CO2 pressure and the mole fraction of iodine in its saturated solution ranges from 0.12 × 10–3 to 1.42 × 10–3.

Xenon and iodine behaviour in magmas

Iodine (I) and xenon (Xe) are two key elements that trace Earth's differentiation (e.g. atmosphere formation) and dynamics (e.g. volcanism and recycling at subduction zones). Iodine and Xe abundances are linked through the decay of the extinct 129I that produced 129Xe, which is today depleted in the Earth's atmosphere compared to the composition of the solar system (i.e. chondrites). Iodine and Xe cycles and storage in the deep Earth are almost unknown, which is in large part due to the fact that their behaviour in magmas and fluids, key agents of mass transfer through planetary envelopes, are poorly known. Here, the solubility of Xe and I in melts is measured under high pressure (P) and temperature (T) conditions using large volume presses, and Xe and I behaviour in melts and fluids is monitored in situ under high P-T conditions using resistive heating diamond anvil cells combined with synchrotron x-ray fluorescence (XRF) and Raman spectroscopy. Xenon, I and H (H2O) contents were measured in quenched glasses by particle x-ray Emission (PIXE) and Elastic Recoil Detection Analysis (ERDA). Solubility, speciation and degassing processes are investigated for two different compositions: haplogranitic melt (HPG analogue for crustal melts) and basaltic melts (MORB and IAB). Experimentally measured solubilities for both elements are much higher than their natural abundances in terrestrial magmas. Xenon solubility at 3.5 GPa reaches 4.00 wt.% in HPG and 0.40 wt.% in basalts. Iodine solubility is 0.46 wt.% at 0.4 GPa on average in HPG, and reaches 1.42 wt.% in basalts at 2 GPa. The in situ Raman spectroscopic study shows that I forms I-I bonds in hydrous high P fluids/melts unlike Xe that was previously shown to oxidize in high P melts. The XRF monitoring of I and Xe partitioning between aqueous fluids and silicate melts during decompression (i.e. water degassing) shows that Xe degassing is strongly P-T dependent and can be retained in the melt at deep crust conditions, while I is totally washed out from the silicate melt by the aqueous phase. Xenon and I degassing processes are based on different mechanisms, which implies that the atmospheric isotopic signature of Xe cannot be inherited from a process involving volcanic water degassing. Instead, 129Xe depletion may originate from a separation of both elements at depth, by deep fluids, a proposition that agrees with a deep storage of Xe in minerals.

Iodine solubility and speciation in glasses

The study of iodine in glasses and melts is critical in many areas, from geosciences to materials science to waste management. Glasses in the ternary system Na2O-B2O3-SiO2 were studied with the goal of identifying a glass matrix able to dissolve large quantities of this element, and to identify the main parameters affecting the solubility of iodine. Two sets of experiments were carried out: the first one with the aim of determining the solubility limit of iodine, and the second one to identify the structural variations occurring within the glass network upon iodine incorporation, and to identify the parameters influencing the most both iodine solubility and speciation. We demonstrated that there is a strong dependence of iodine incorporation on bulk chemistry and glass physical properties. A solubility limit of ~5 mol% I has been assessed for B2O3-rich glasses and of ~1 mol% for SiO2-rich ones, and this composition dependence has been explained by considering the fragility parameter of the glass network. Structural variations in the iodine local environment and in the glass network were characterized by Raman, X-ray Absorption Spectroscopy, and 11B NMR. Spectroscopy data point out the coexistence of different I species within the glasses, with iodide being the predominant one, surrounded by Na+ ions.

A First-Principle Study Towards Understanding Surface Reactions in Li-I2 Batteries

Large-scale stationary energy storage urgently demands to develop rechargeable batteries with low cost, high energy density, and enhanced cycling stability. Beyond conventional Li-ion batteries, secondary Li-I2 batteries based on the conversion reaction mechanism is considered as a promising next-generation energy storage device. However, Li-I2 batteries also suffer shuttle effect caused by the high solubility of iodine in the organic electrolyte. A first-principles approach is employed to probe the effect of I2 molecules on corroding the metallic Li anode surface. It is found that the I2 molecule always tends to dissociate to two I atoms on the Li surface. Electrons in the metallic substrate spontaneously migrate to I atoms, resulting in the self-discharge. According to the ab-initio molecular dynamics simulation, the metallic Li surface can be oxidized to LiI species. LiI species cannot binding to the metallic substrate strongly and will be exfoliated easily, leading to the loss of active materials and irreversible capacity fade. According to the present simulation, LiNO3 additive can also prevent the Li surface from the iodization corrosion. The NO3 - group can completely dissociate and penetrate into the Li substrate, result in forming a protective ternary compound layer. This layer can provide a strong affinity to LiI species, leading to alleviating the loss of active materials. Beyond LiNO3 additive, using N-doped graphene is another strategy to improve the battery performance. The N dopant can provide a strong attraction to trap I2 molecules. In addition, the N heteroatom can shift the Fermi energy level above the pi-antibonding states, which can enhance the reduction and dissociation of the I2 molecule.

The Influence of Glass Composition on Iodine Solubility

AbstractTwo glass series in the ternary systems K2O-B2O3-SiO2 (KBS) and Na2O-B2O3-SiO2 (NBS) were studied in order to identify the main factors influencing the solubility of iodine. We established that iodine incorporation is strongly linked to the bulk chemistry, i.e. the SiO2/(B2O3+SiO2) molar ratio, and to the physical properties of the glasses, and we assessed three different solubility limits. Iodine in Si-rich glasses has a low solubility (≤1 mol% I) regardless of the alkali ion present. On the contrary, in B-rich glasses, the solubility is five times higher than in Si-rich glasses for Na-glasses, and more than six times higher for K-glasses. The strong dependence of iodine solubility on the bulk chemistry is related to the adaptability of the glass network. Furthermore, our data suggest that iodine is stable with different redox states in the glasses here analyzed.

Statistical thermodynamics of regular solutions and solubility parameters

Solubility parameters, developed originally for regular solutions, have been applied to solutions beyond the presumed weak non-ideality, implying that the true foundation of the solubility parameters may be more general than the regular solution theory. To assess the root of regularity on rigorous statistical thermodynamics, here we re-examine the classical iodine dissolution experiments by Shinoda and Hildebrand, who concluded that the entropy of mixing is ideal regardless of solute-solvent size ratio. We show that iodine solubility is concerned with the limit of infinite dilution, while the regular solution theory is a scheme to describe the dependence on the solute concentration. This means that the solubility of iodine cannot be a foundation of the regular solution; it is further shown that the differences in the solvation free energy among organic solvents are dominated by enthalpy with negligible role of the entropic component. In addition, the validity of the regular solution concept, i.e., the enthalpic nature of the solution non-ideality, can now be examined quantitatively by expressing the Margules model in terms of the Kirkwood-Buff integrals, which incorporate the excluded volume effects and the potential of mean force nature of interactions that were beyond the reach of the classical thermodynamic models. Such insights into the physical basis of solubility parameters may be useful for improving solubility prediction.

Iodine-water-alkanol and potassium iodide-water-alkanol systems: Phase diagrams and regularities of association according to IR and NMR spectroscopic data

The variation of the iodine and potassium iodide solubilities in water-monoatomic alcohol (ethanol, propanol, isopropanol) solvents is considered from the standpoint of IR spectroscopic and chemometric data on association in water-alkanol binary mixtures. The iodine and potassium iodide solubilities in the mixed solvents vary nonlinearly with solvent composition because of the formation of 1 : 1 and 1 : 3 water-alcohol heteroassociates and alcohol homoassociates. Different kinds of phase diagram are observed for the iodine-water-alcohol systems: the I2-H2O-1-C3H7OH and the I2-H2O-2-C3H7OH diagrams have a phase separation region, while the I2-H2O-C2H5OH diagram does not. This fact is explained in terms of the interaction between the components of the systems. The variation of the potassium iodide solubility in the mixed solvent is discussed: a decrease in the KI solubility is symbatic to an increase in the relative concentration of 1 : 1 associates in the water-alcohol solution. The run of the iodine and potassium iodide solubility curves at low alcohol concentrations is explained on the basis of NMR spectroscopic data on association in aqueous solutions of the monoatomic alcohols.

Experimental study of iodine removal efficiency in self-priming venturi scrubber

The objective of present experimental study is to examine the iodine removal efficiency of a self-priming venturi scrubber for submerged operating condition. The venturi scrubber is used in Containment Filtered Venting System of nuclear power plants to remove the gaseous pollutants from contaminated gas during severe accidents. The experiment consists of mixing the iodine vapours with the air using suction venturi and pressure cooker system. The purpose of iodine mixing with air is to examine scrubbing performance of the designed venturi scrubber with water as scrubbing liquid. The performance parameters of venturi scrubber are expressed mainly in terms of pressure drop and iodine removal efficiency. The iodine removal efficiency of venturi scrubber is estimated for a series of two experiments by measuring the quantity of iodine in water from iodometric titration with four distinct pH of water. It has been experimentally observed that iodine removal efficiency is improved by using higher pH value of scrubbing liquid since solubility of iodine gets improved at higher pH.

Iodine solubility in a low-activity waste borosilicate glass at 1000 °C

The purpose of this study was to determine the solubility of iodine in a low-activity waste borosilicate glass when heated inside an evacuated and sealed fused quartz ampoule. The iodine was added to glass frit as KI in quantities of 99.4–24,005ppm iodine (by mass). Each mixture was added to an ampoule, heated at 1000°C for 2h, and then air quenched. In samples with ⩾11,999ppm iodine, low viscosity salt phases were observed on the surface of the melts that solidified into a white coating upon cooling. These salts were identified by X-ray diffraction as mixtures of KI, NaI, and Na2SO4. Iodine concentrations in glass specimens were analyzed with inductively-coupled plasma mass spectrometry, and the overall iodine solubility was determined to be 10,000ppm. Several crystalline inclusions of iodine sodalite, Na8(AlSiO4)6I2, were observed in the 24,005ppm specimen.

Metal–silicate partitioning of iodine at high pressures and temperatures: Implications for the Earth's core and 129⁎Xe budgets

The partition coefficients of iodine Dmet/sil between molten metal and molten silicate were investigated using a Laser Heated Diamond Anvil Cell (LHDAC) at pressures between 2 and 20GPa and at ~2800K. No pressure dependence of Dmet/sil was observed within this range, but the composition of the Fe–Ni alloy liquid phase was shown to have an effect. When the metallic liquid was alloyed with S, O and Si, there was an increase in iodine solubility in the metal. Iodine exhibited mildly siderophile behaviour across all the investigated conditions, with Dmet/sil=1.25±0.65 (2 s.d.) (Fe metal system) and Dmet/sil=4.33±1.41 (2 s.d.) (Fe-alloy). In conjunction with a revised bulk silicate Earth (BSE) concentration, it is calculated that the core could be a significant reservoir for iodine, with up to 82% of the bulk Earth's iodine budget in the core, depending on the light element content of the metal phase and the process of core formation. The composition of the metal phase appears to have a greater effect on the partitioning and sequestration of iodine than the style of core segregation.As the core likely formed while 129I was still extant, the core can also be a reservoir for radiogenic 129Xe from the decay system 129I–129Xe (T1/2=15.7Myr). Preliminary modelling indicates that the decay of 129I in the core has the potential to generate radiogenic 129Xe concentrations that are at least two orders of magnitude greater than what has been estimated for the depleted mantle. While this may have a significant impact on the isotopic signatures of the overlying mantle, it is not yet clear how flux from the core fits within the overall picture of mantle noble gas evolution.

Phase separation characteristics of HI–I2–H2SO4–H2O mixture at elevated temperatures

The iodine–sulfur (IS) thermochemical water splitting cycle is one of the most promising massive hydrogen production approaches using nuclear or solar energy. Bunsen reaction is a crucial reaction of the process by which sulfuric and hydriodic acids form. The simultaneously phase separation of the Bunsen production products, composed of HI, I2, H2SO4, and H2O are essential for the smooth operation of the process. In this work, the phase separation characteristics of the quaternary mixtures were studied at elevated temperatures of 40–80°C to clarify the phase separation characteristics. The critical conditions of the phase separation and iodine solubility in the mixture, as well as the effects of the composition and temperature on the purity of the two phases, were investigated. Favorable concentration range of each component in the quaternary solutions at various temperatures was obtained and presented in a tetrahedron diagram. The solution separates into two liquid phases spontaneously without iodine precipitation within the range. The amount of I2 should be as high as possible, with no I2 precipitation, to keep impurities at minimum content in the two phases. The results of this study offer technical basis for the design and operation of the IS process.

Study on the phase separation characteristics of HI–I2–H2SO4–H2O mixture at 20 °C

The phase separation of the Bunsen products mixture containing HI, I2, H2SO4, and H2O is an essential subsection of the iodine–sulfur hydrogen production process (I–S process). It is also the pre-condition of the continuous operation of the I–S process. This experimental study was conducted at 20°C using jacketed glass vessels to clarify the characteristics of the phase separation of the quaternary mixture. The critical conditions of the phase separation, iodine solubility in the mixture, and intermiscibility of the constituents in the solution were investigated. The favorable concentration range of each component in the Bunsen products mixture was illustrated using a tetrahedron diagram. Within the concentration range, the solution separates spontaneously into two liquid phases without iodine precipitation. In addition, a computer program was developed to easily ascertain the phase state of the quaternary mixture with a given composition.

Preparation and Antimicrobial Properties of Gemini Surfactant-Supported Triiodide Complex System

Iodine is an effective, simple, and inexpensive bactericide in disinfection. However, the poor solubility and stability of iodine in water limit its applications. In addition, the active iodine content in the commercial iodophors is quite low, and the reported triiodide complex is unstable. In this work, a long-term stable triiodide complex antimicrobial system was prepared by mixing iodine and a cationic gemini surfactant into lauryldimethylamine oxide (LDAO) aqueous solution, and its stability was examined by means of UV-vis spectrophotometry. It was found that the content of LDAO, cationic gemini surfactant and H(2)SO(4) played crucial roles in stabilizing antimicrobial system, and the active iodine (i.e., triiodide complex) content of the optimum formulation can remain stable for 150 days, as iodine is encapsulated by the mixed vesicles assembled by the protonated LDAO and the added gemini surfactant. However, the active iodine reduced rapidly when NaCl was added or the pH was increased in the environment. Furthermore, the antimicrobial efficacy of the optimized formulation was studied against Candida albicans, and more than 4 log reduction in viable cell after 5 min of contact was obtained.

Iodine dynamics in soils

We investigated changes in iodine (129I) solubility and speciation in nine soils with contrasting properties (pH, Fe/Mn oxides, organic carbon and iodine contents), incubated for nine months at 10 and 20°C. The rate of 129I sorption was greater in soils with large organic carbon contents (%SOC), low pH and at higher temperatures. Loss of iodide (I−) from solution was extremely rapid, apparently reaching completion over minutes–hours; iodate (IO3-) loss from solution was slower, typically occurring over hours–days. In all soils an apparently instantaneous sorption reaction was followed by a slower sorption process for IO3-. For iodide a faster overall reaction meant that discrimination between the two processes was less clear. Instantaneous sorption of IO3- was greater in soils with high Fe/Mn oxide content, low pH and low SOC content, whereas the rate of time-dependent sorption was greatest in soils with higher SOC contents. Phosphate extraction (0.15M KH2PO4) of soils, ∼100h after 129I spike addition, indicated that concentrations of sorbed inorganic iodine (129I) were very low in all soils suggesting that inorganic iodine adsorption onto oxide phases has little impact on the rate of iodine assimilation into humus. Transformation of dissolved inorganic 129IO3- and 129I− to sorbed organic forms was modelled using a range of reaction- and diffusion-based approaches. Irreversible and reversible first order kinetic models, and a spherical diffusion model, adequately described the kinetics of both IO3- and I− loss from the soil solution but required inclusion of a distribution coefficient (kd) to allow for instantaneous adsorption. A spherical diffusion model was also collectively parameterised for all the soils studied by using pH, soil organic carbon concentration and combined Fe+Mn oxide content as determinants of the model parameters (kd and D/r2). The kinetic model parameters were not directly related to a single soil parameter; inclusion of pH, SOC, oxide content and temperature was necessary to describe the observed behaviour. From the temperature-dependence of the sorption data the activation energy (Ea) for 129IO3- transformation to organic forms was estimated to be ∼43kJmol−1. The Ea value was independent of %SOC and was consistent with a reaction mechanism slower than pore diffusion or physical adsorption, but faster than most surface reactions.