Desorption Of Mg Research Articles

Improvement in hydrogen storage performance of Mg by mechanical grinding with molten salt etching Ti3C2Clx

Mg-based materials are currently a hot research topic as hydrogen storage materials due to their considerable theoretical hydrogen storage capacity. However, the kinetic performance of hydrogen absorption and desorption of Mg is too slow and requires high temperature, which seriously hinders the application of this material. MXene is a new type of two-dimensional material with significant role in improving thermodynamics and kinetics. In this experiment, a two-dimensional layered MXene containing Cl functional group was prepared by molten salt etching using the Ti-containing MAX phase as the raw material. Then different ratios of Ti3C2Clx were uniformly dispersed onto the surface of Mg by high energy ball milling. The samples were characterized by hydrogen absorption and desorption kinetics, SEM, XRD, XPS, and DSC to investigate the effect of Ti3C2Clx on the hydrogen absorption and desorption performance of Mg. The onset hydrogen absorption temperature can be reduced to room temperature and the hydrogen release temperature is reduced by 200 ​°C by doping Ti3C2Clx. And there is also 5.4 ​wt% hydrogen storage in the isothermal hydrogen absorption test at 400 ​°C. The results of DSC demonstrate that the Ea of Mg+15 ​wt% Ti3C2Clx was reduced by 12.6% compared to pristine Mg. The ΔH is almost invariable. The results of XPS show that the presence of multivalent Ti promotes electron transfer and thus improves the conversion between Mg2+/Mg and H−/H. This study provides a guideline for further improving the hydrogen absorption and desorption performance of Mg-based hydrogen storage materials.

Al and Zr addition to improve the hydrogen storage kinetics of Mg-based nanocomposites: Synergistic effects of multiphase nanocatalysts

Solid-state hydrogen storage is regarded as the key technology to develop commercial application of hydrogen energy, and Mg added with high efficiency catalysts is a promising candidate to break through the barrier of hydrogen storage materials. Herein, Mg−Al and Mg−Al−Zr nanocomposites have been prepared by adding the single-metal (Al) and bimetal (Al, Zr) into Mg nanoparticles by the hydrogen plasma-metal reaction (HPMR) method. The two kinds of nanocomposites have an approximate size of 100 nm. With the expansion of the plasma current, the content of Al and Zr in Mg−Al−Zr nanocomposites increases gradually, and the addition of Al and Zr conspicuously accelerates the kinetics of hydrogen absorption and desorption of Mg. Compared with the nanocomposites with the single-metal (Al), the addition of bimetal (Al, Zr) can further enhance the hydrogen absorption kinetics of Mg, and the content of Zr has a positive effect on the reduction of activation energy (Ea). The Ea of Mg88.3Al7.6Zr4.1 for hydrogen absorption is 68.6 kJ/mol, which is significantly lower than that of Mg92.6Al7.4 (80.6 kJ/mol) and other two Mg−Al−Zr nanocomposites with low Al and Zr (70.7 and 69.8 kJ/mol, respectively). Both the nanostructure of Mg and the synergistic catalytic effects of active “catalytic sites” and H “diffusion channels” provided by the multiphase nanocatalysts of MgAl and AlZr3 contribute to the reduction of Ea. This work provides guidance for the development of Mg-based hydrogen storage materials with high catalytic efficiency.

Synthesis of Intermetallic (Mg1−x,Alx)2Ca by Combinatorial Sputtering

The synthesis–composition–structure relationship in the Mg–Ca–Al system is studied using combinatorial magnetron sputtering. With increasing deposition temperature, a drastic decrease in Mg concentration is obtained. This behavior can be understood based on density functional theory calculations yielding a desorption energy of 1.9 eV/atom for Mg from a hexagonal Mg nanocluster which is far below the desorption energy of Mg from a Mg2Ca nanocluster (3.4 eV/atom) implying desorption of excess Mg during thin film growth at elevated temperatures. Correlative structural and chemical analysis of binary Mg–Ca thin films suggests the formation of hexagonal Mg2Ca (C14 Laves phase) in a wide Mg/Ca range from 1.7 to 2.2, expanding the to date reported stoichiometry range. Pronounced thermally-induced desorption of Mg is utilized to synthesize stoichiometric (Mg1−x,Alx)2Ca thin films by additional co-sputtering of elemental Al, exhibiting a higher desorption energy (6.7 eV/atom) compared to Mg (3.4 eV/atom) from Mg2Ca, which governs its preferred incorporation during synthesis. X-ray diffraction investigations along the chemical gradient suggest the formation of intermetallic C14 (Mg1–x,Alx)2Ca with a critical aluminum concentration of up to 23 at.%. The introduced synthesis strategy, based on the thermally-induced desorption of weakly bonded species, and the preferential incorporation of strongly bonded species, may also be useful for solubility studies of other phases within this ternary system as well as for other intermetallics with weakly bonded alloying constituents.

New mechanism and improved kinetics of hydrogen absorption and desorption of Mg(In) solid solution alloy milling with CeF3

This paper presents improving the hydrogen absorption and desorption of Mg(In) solid solution alloy through doped with CeF3. A nanocomposite of Mg0.95In0.05-5 wt% CeF3 was prepared by mechanical ball milling. The microstructures were systematically investigated by X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy. And the hydrogen storage properties were evaluated by isothermal hydrogen absorption and desorption, and pressure-composition-isothermal measurements in a temperature range of 230 °C–320 °C. The mechanism of hydrogen absorption and desorption of Mg0.95In0.05 solid solution is changed by the addition of CeF3. Mg0.95In0.05-5 wt% CeF3 nanocomposite transforms to MgH2, MgF2 and intermetallic compounds of MgIn and CeIn3 by hydrogenation. Upon dehydrogenation, MgH2 reacts with the intermetallic compounds of MgIn and CeIn3 forming a pseudo-ternary Mg(In, Ce) solid solution, which is a fully reversible reaction with a reversible hydrogen capacity～4.0 wt%. The symbiotic nanostructured CeIn3 impedes the agglomeration of MgIn compound, thus improving the dispersibility of element In, and finally improving the reversibility of hydrogen absorption and desorption of Mg(In) solution alloy. For Mg0.95In0.05-5 wt% CeF3 nanocomposite, the dehydriding enthalpy is reduced to about 66.1 ± 3.2 kJ⋅mol−1⋅H2, and the apparent activation energy of dehydrogenation is significantly lowered to 71.9 ± 10.0 kJ⋅mol−1⋅H2, a reduction of ～73 kJ⋅mol−1⋅H2 relative to that for Mg0.95In0.05 solid solution. As a result, Mg0.95In0.05-5 wt% CeF3 nanocomposite can release ～57% H2 in 10 min at 260 °C. The improvements of hydrogen absorption and desorption properties are mainly attributed to the reversible phase transition of Mg(In, Ce) solid solution combing with the multiphase nanostructure.

Tuning the de/hydriding thermodynamics and kinetics of Mg by mechanical alloying with Sn and Zn

Mg95Sn3Zn2 alloy was prepared by mechanical alloying. The phase constituents and phase transition were analyzed by X-ray diffraction (XRD) method. The microstructure was characterized by scanning electron microscope (SEM). The hydrogen storage properties were evaluated in detail by the measurements of isothermal hydrogen absorption and desorption, and pressure-composition isotherms (PCI) using the Sieverts method. The addition of Zn benefits to extend the solubility of Sn in the Mg lattice, as a result supersaturated Mg(Sn, Zn) ternary solid solution was synthesized by mechanical alloying, which decomposed to MgH2, Sn and MgZn2 in the hydrogenating process. The in situ formed nanostructure Mg2Sn and MgZn2 have positive effects on the hydrogen absorption and desorption of Mg. Mg95Sn3Zn2 alloy showed significantly improved kinetics with lowered hydrogen absorption and desorption activation energies of 38.1 kJ/mol and 86.6 kJ/mol respectively, and exhibited a reduced dehydriding enthalpy of 67.0 ± 1.9 kJ/(mol·H2).

Crystal Structure Analysis of Electrochemically Magnesiated MgxLi0.1Mn0.54Ni0.13Co0.13O2

INTRODUCTION Recently, the Mg secondary battery has been studied for the purpose of the development of the rechargeable battery with larger energy density than the Li-ion battery. In this study, we focused attention on the Li1.2Mn0.54Ni0.13Co0.13O2, which is a positive electrode material of the high capacity lithium ion battery, to examine the use for the cathode material capable of inserting Mg ions. We have reported the crystal and electronic structures and thermodynamic stability of the chemically delithiated Li1.2-xMn0.54Ni0.13Co0.13O2 (x = 0.3, 0.6, 0.9)1). Although there is no knowledge about the insertion and desorption of Mg for the delithiated material, the high capacity such as shown in the Li ion battery is expected for Mg secondary battery. The purpose of the present study was to evaluate the electrode property of delithiated Li1.2-xMn0.54Ni0.13Co0.13O2 in the Mg secondary battery, to analyze the crystal and electronic structure changes before and after Mg insertion, to reveal the valence change of transition metals, and furthermore to examine the charge/discharge mechanism. EXPERIMENTAL Li1.2Mn0.54Ni0.13Co0.13O2 was synthesized by using a solid-state reaction via co-precipitated precursors. Precipitation was obtained by dissolving each metal nitrate in distilled water and adding it dropwise to an aqueous solution of lithium hydroxide. Based on the predetermined metal ratio of the precipitate, we added lithium hydroxide to the precipitate again, and dried and heated at 650 ºC for 15h in air. After that, the powder was calcined at 900 ºC for 8h in air to obtain a sample. The obtained samples were identified by powder XRD. The metal composition was determined by the ICP-AES. The chemically delithiation treatment was pursued as the previously reported using NO2BF4 in an argon filled glovebox1). The electrochemical measurements were subjected to a charge-discharge cycle test (at 3.0 – 0.5 V vs. Mg/Mg2 +, the negative electrode: AZ31, electrolyte: 0.5 mol/L Mg(N(SO2CF3)2)2 in acetonitrile, separator: polypropylene) at 60 ºC by using the HS cell (Hosen Ltd.). The XAFS (BL14B2, SPring-8) and synchrotron XRD measurements (BL19B2, SPring-8) were performed on the Li1.2-xMn0.54Ni0.13Co0.13O2, i.e. before Mg insertion, and the MgxLi1.2-xMn0.54Ni0.13Co0.13O2 electrode after Mg insertion to evaluate the valence change of transition metals and to analyze the crystal structures by Rietveld analysis (Rietan-FP). RESULTS AND DISCUSSION Chemically delithiated Li0.13Mn0.54Ni0.13Co0.13O2 was synthesized using NO2BF4 for Li1.2Mn0.54Ni0.13Co0.13O2. In order to refine the crystal structure of this sample, synchrotron X-ray diffraction measurement was performed and Rietveld analysis was performed. As a result of the analysis, it was revealed that the 97 % of the pristine monoclinic (C2/m) structure converted to the trigonal (R m) structure after delithiation. As a result of discharging test of this sample with the counter electrode of Mg alloy (AZ31), the discharge capacity was 287 mAh g-1 (Fig.1) which corresponded to 0.52 apfu as the insertion amount of Mg and corresponded to approximately half of chemically eliminated Li. In order to refine the crystal structure of the electrode after electrochemical Mg insertion, synchrotron XRD measurement was performed and Rietveld analysis was performed. As a result of the analysis, it was revealed that the 91% of the delithiated phase remained as trigonal R m structure. In addition, it was revealed that unit cell volume expanded by 2.8% at C2/m and by 2.7% at R m after Mg insertion. From the results of XANES, it was also revealed that the valence states of Mn and Co decreased by about 1 valence after Mg insertion, respectively. Therefore, it is concluded that the discharge mechanism was explained by the reduction of transition metals and the intercalation of Mg, which is supported that the lattice volume expanded due to the reduction of the transition metal by increasing the ionic radius. References 1) N. Ishida, N. Tamura, N. Kitamura, and Y. Idemoto., J. Power Sources, 319, 255-261 (2016) . Figure 1

Saléeite, Mg(UO2)2(PO4)2, from the Wojcieszyce uranium deposit (Lower Silesia, Poland)

The secondary uranium mineral, saleeite, was found in dumps of the former Wojcieszyce Uranium Mine. It occurs as tiny tabular crystals, up to 0.3 cm in size. The chemical formula, which was calculated on the basis of a 12 spot analyses is: (Mg 0.73 Fe 0.05 K 0.04 Ca 0.04 ) S0.86 (UO 2 ) 2.16 (PO 4 ) 2.00 ·10H 2 O. The cation position is occupied mainly by Mg. The Mg ions are partially substituted by Fe, K and Ca. The tetrahedral position is occupied by the PO 4 3- ion. The interaction between acidic pore solutions (originating from the reaction of pyrite oxidation), uraninite and minerals of the host rocks (chlorite, apatite) leads to the release of Mg 2+ , UO 2 2+ and PO 4 3- ions. These ions are adsorbed by amorphous Fe oxyhydroxides and poorly ordered ferrihydrite. Crystallization and recrystallization of these phases leads to the release of adsorbed ions and favours the crystallization of saleeite. The coexistence of saleeite and goethite, which forms by recrystallization of ferrihydrite, points to an important role of sorption and desorption of Mg 2+ , PO 4 3- and UO 2 2+ ions in the process of crystallization of this mineral species. Crystallization of saleeite occurs at a pH of 5.5–6.0. The supergene zone of the Wojcieszyce deposit is the first occurrence of saleeite in Poland.

Using stable Mg isotopes to distinguish dolomite formation mechanisms: A case study from the Peru Margin

The magnesium isotope composition of diagenetic dolomites and their adjacent pore fluids were studied in a 250m thick sedimentary section drilled into the Peru Margin during Ocean Drilling Program (ODP) Leg 201 (Site 1230) and Leg 112 (Site 685). Previous studies revealed the presence of two types of dolomite: type I dolomite forms at ~6m below seafloor (mbsf) due to an increase in alkalinity associated with anaerobic methane oxidation, and type II dolomite forms at focused sites below ~230mbsf due to episodic inflow of deep-sourced fluids into an intense methanogenesis zone. The pore fluid δ26Mg composition becomes progressively enriched in 26Mg with depth from values similar to seawater (i.e. −0.8‰, relative to DSM3 Mg reference material) in the top few meters below seafloor (mbsf) to 0.8±0.2‰ within the sediments located below 100mbsf. Type I dolomites have a δ26Mg of −3.5‰, and exhibit apparent dolomite-pore fluid fractionation factors of about −2.6‰ consistent with previous studies of dolomite precipitation from seawater. In contrast, type II dolomites have δ26Mg values ranging from −2.5 to −3.0‰ and are up to −3.6‰ lighter than the modern pore fluid Mg isotope composition. The enrichment of pore fluids in 26Mg and depletion in total Mg concentration below ~200mbsf is likely the result of Mg isotope fractionation during dolomite formation, The 26Mg enrichment of pore fluids in the upper ~200mbsf of the sediment sequence can be attributed to desorption of Mg from clay mineral surfaces. The obtained results indicate that Mg isotopes recorded in the diagenetic carbonate record can distinguish near surface versus deep formed dolomite demonstrating their usefulness as a paleo-diagenetic proxy.

Effect of carbon type (graphite, CFs and diamond) on the hydrogen desorption of Mg–C powder mixtures under microwave irradiation

Binary MgH2–xwt% C mixtures (x=0, 2, 5 and 10wt% for carbon fibers (CFs), 10wt% for graphite (G) and diamond (D)) were prepared by hand mixing and by ball milling (5h) for MgH2–10wt% CFs. The aims of the work were: – to investigate the dehydriding reaction of MgH2 catalyzed by carbon under microwave irradiation; – to assess the effect of presence of different carbon type and content on the desorption reaction; – to evaluate the influence microwave power on the rate of hydrogen release; to study the influence of milling process on the dehydrogenation time. The amount of hydride and Mg phases (e.g. hydrogen content) was estimated from XRD pattern matching (Eva and Fullprof). The morphology of the powders and the particle size distribution were observed by Scanning Electron Microscopy (SEM) and laser granulometry respectively. Carbon distribution in the MgH2 matrix was investigated by electron probe microanalyses.The different carbon type transfer the heat, generated from absorbed microwave irradiation, to the MgH2 matrix which induces the hydrogen release. It was found that the dehydrogenation rate and time were enhanced with rising carbon content, microwave power and with milling time. For the MgH2–10wt% CFs mixtures, under 1500W of microwave, more than 90% of conversion MgH2 to Mg phases was obtained after 20s for hand mixing mixture and after only 10s for 5h ball milled one.

Experimental study on hydrogen storage properties of Li–Mg–N–H based tank

Cylindrical lab-scale tank based on 98.0g of the Mg(NH2)2–2LiH–0.07KOH compacts was designed. Hydrogen storage properties of the tank, including bed temperature, hydrogen pressure and flow rate in the hydrogen absorption and desorption process were investigated under a variety of conditions. In the hydrogen absorption process of which the applied hydrogen pressure is 6.0MPa, the central temperature of the hydride bed increases obviously from 453K to 524K. Furthermore, the increase of the applied hydrogen pressure can lead to greater changes in the bed temperature. The hydrogen desorption process exhibits 3 stages under the constant flow rate mode, and the hydrogen desorption of Mg(NH2)2–2LiH–0.07KOH mainly carries out at stage II accompanied with a significant temperature drop. As the flow rate increases, the endothermic dehydrogenation of the hydride degrades the bed temperature to a lower value, and the amount of hydrogen released also decreases at this stage. The results indicate that an efficient improvement of the heat transfer property for the Li–Mg–N–H based tank is needed to improve its sorption behavior.

Hydrogen desorption of Mg–Mg2Ni hypo-eutectic alloys in air, Ar, CO2, N2 and H2

Abstract The hypo-eutectic Mg–Mg 2 Ni system can be modified by trace elemental additions of Na which change the microstructure and the functional properties of the alloy. The modified microstructure results in improvements to the hydrogen storage properties. In alloys of optimal composition, it has been shown that the reversible storage of 6.5–7 wt% H 2 is possible at a rate of reaction that is realistic for industrial applications. This research investigates the release of H 2 in air under atmospheric pressure as well as under 0.2 MPa atmospheres of Ar, CO 2 , N 2 , and H 2 . The release kinetics were characterised using in situ crystallographic phase transformation analysis obtained by synchrotron Powder X-ray diffraction (PXRD) at the Australian Synchrotron. The mole fraction of MgH 2 , Mg, Mg 2 Ni, and Mg 2 NiH 4 was determined by Rietveld refinement using RIETAN-FP. It was found that the hydrogen release temperature largely depends on the atmosphere.

Adsorption- and desorption-controlled magnesium isotope fractionation during extreme weathering of basalt in Hainan Island, China

Magnesium isotopic compositions of a set of clay-rich saprolites developed on the Neogene tholeiitic basalt from Hainan Island in southern China have been measured in order to document the behavior of Mg isotopes during continental weathering. Compared with unaltered basalts (δ26Mg=−0.36‰), the overlying saprolites are strongly depleted in Mg (i.e., τTh,Mg=−99.1% to −92.9%), and display highly variable δ26Mg, ranging from −0.49‰ to +0.40‰. Magnesium concentration and δ26Mg value of the saprolites display a general increasing trend upwards in the lower part of the profile, but a decreasing trend towards the surface in the upper part. The variations of Mg concentration and isotopic composition in this weathering profile can be explained through adsorption and desorption processes: (1) adsorption of Mg to kaolin minerals (kaolinite and halloysite), with preferential uptake of heavy Mg isotopes onto kaolin minerals; and (2) desorption of Mg through cation exchange of Mg with the relatively lower hydration energy cations in the upper profile. Evidence for adsorption is supported by the positive correlation between δ26Mg and the modal abundance of kaolin minerals in saprolite of the lower profile, while negative correlations between δ26Mg and concentrations of lower hydration energy cations (e.g., Sr and Cs) in the upper profile support the desorption process. Our results highlight that adsorption and desorption of Mg on clay minerals play an important role in behavior of Mg isotopes during extreme weathering, which may help to explain the large variation in Mg isotopic composition of river waters.

Effects of TiO2 and Nb2O5 on Hydrogen Desorption of Mg(BH4)2

Effects of TiO2 and Nb2O5 on Hydrogen Desorption of Mg(BH4)2

Photoemission studies of the initial interface formation of ultrathin MgO dielectric layers on the Si(111) surface

This study investigates the interface formation between a magnesium oxide dielectric overlayer and an ultrathin SiO 2 layer (∼ 0.3 nm) grown on the atomically clean p-type Si(111) surface in ultra high vacuum. Both soft X-ray synchrotron radiation based photoemission and conventional X-ray photoelectron spectroscopy have been used to characterise the evolution of the interface and monitor the change in the interfacial oxide thickness. As the MgO film grows, there is an increase in the intensity of the silicon oxide features indicating the growth of the interfacial oxide which saturates at a thickness of approximately 0.7 nm. Spectra acquired at the surface sensitive 130 eV photon energy, reveal the emergence of a chemically shifted component on the low binding energy side of the substrate peak which is attributed to atomic displacement of silicon atoms from the substrate to the interfacial oxide at room temperature. This evidence of atomic disruption at the high dielectric constant material (high-κ) and silicon interface would be expected to contribute to charge carrier scattering mechanisms in the silicon and could account for the generally observed mobility degradation in high-κ stacks. Thermal annealing studies of deposited MgO films show that dissociation begins to occur above 600 °C with desorption of Mg and the growth of a silicon oxide.

Surfactant-modified alumina: An efficient adsorbent for malachite green removal from water environment

Surface of alumina was modified with sodium dodecyl sulfate (SDS), an anionic surfactant. The surfactant-modified alumina (SMA) was characterized by FTIR and thermal analysis. The SMA was then used for the removal of malachite green (MG; Basic Green 4), a well-known toxic cationic dye from aqueous environment. The removal of MG takes place in the micellar structure formed on alumina surface, and the process is called adsolubilization. All the studies were carried out in batch mode. The kinetic studies showed that 1 h contact time was sufficient to attain equilibrium. SMA was very efficient to remove MG up to 99% under optimum conditions. The concentration range of MG was 20–100 mg/L. The isotherm studies showed that it follows Langmuir model better than the Freundlich model. The maximum adsorption capacity was 185 mg/g. The effects of various parameters such as pH, presence of interfering ions (Cl−, NO3 −, H2PO4 −, SO4 2−, Fe2 +, Ca2 +) and organics (pesticides such as 2,4-dichlorophenoxyacetic acid, atrazine, endosulfan, and humic acid) are evaluated. It was observed that H2PO4 −, Fe2 +, endosulfan, and humic acid have maximum interference. Desorption of MG from exhausted SMA using acetone, and its reuse was studied. The regenerated adsorbent shows ∼ 80% efficiency on the removal of MG. The usability of SMA for the removal of MG from real wastewater was also examined. The kinetic equilibrium was attained within 1 h and the removal could be achieved up to ∼ 95% at a dose of 20 g/L. The adsorption followed Freundlich isotherm model better than the Langmuir model.

Sorption of Zinc by Clinoptilolite–Fe(NO3)3 Systems

Many factors may affect the zinc (Zn) adsorption and desorption on natural clinoptilolite and iron (Fe) oxides. Among them are ionic strength and pH. In this article, the Zn retention behavior on two clinoptilolite–Fe(NO3)3 systems as a function of ionic strength and pH were evaluated. Two clinoptilolite–Fe(NO3)3 systems were obtained from the mixture of clinoptilolite and iron nitrate in different proportions and conditions and were characterized using Attenuated Total Reflectance (ATR) and X‐ray Diffraction (XRD). The clinoptilolite–Fe(NO3)3 systems have specific surface area equal to 151 and 140 m2/g respectively and were fully iron exchanged (Fe/Al = 3.17 and 1.23 respectively). The adsorption of Zn and desorption of Zn, magnesium (Mg), and Fe were examined after mixing 1.00 g from each of the two systems with five different Zn concentrations, three different pH values, and two different electrolyte concentrations. The Zn adsorption increases as electrolyte concentration decreases and pH increases. The percentage of Zn adsorption decreases as initial Zn concentration increases. Zinc, Mg, and Fe desorption increases as pH decreases and electrolyte concentration increases. The desorption of Mg from the second system is higher than that of the first system, but the desorption of Fe from the first system is higher than that of the second. The statistical analysis reveals a strong relationship between A and B systems concerning the desorption of Mg and Fe. Consequently, the clinoptilolite–Fe(NO3)3 systems could be considered as two satisfactory detergents of high soil Zn concentration.

Simulation of the Ion Modification Processes in the Silicate Dust of Protoplanetary Disks

The modification of crystalline olivine under irradiation with 1.43-MeV deuterons was studied experimentally. For the first time, the partial destruction of olivine in the surface (100–200 nm) layer was found at a current density of 5–10 μA/cm2. This effect is caused by ionization and desorption of Mg and Fe atoms at deuteron fluences (3–8 × 1016cm–2) which are substantially below the amorphization threshold. It was suggested that the negative charge of the anion group SiO4 4–is neutralized by the interaction with the deuterium ions and Frenkel pairs. The mechanism of sputtering of the crystalline olivine surfaces under irradiation with H, D, He, and Ar ions was studied using of the TRIM 98 program.

Peculiarities of the Dynamics of Ion Exchange in Supersaturated Solutions and Colloid Systems

This paper reports the results obtained by experimental and theoretical study of the dynamics of ion exchange accompanied by the ion-exchange isothermal supersaturation (IXISS) effect. This effect is observed for a numberof ion-exchange systems where the frontal or the reverse frontal separation is accompanied by the formation of extremely stable supersaturated solutions of low solubility substances in the interstitial space of ion-exchange columns. After leaving the column, a supersaturated solution crystallizes spontaneously, which allows for designing a practically ideal ion-exchange process where a crystalline product is obtained directly after the ion-exchange treatment cycle. The paper comprises results on the experimental investigation of IXISS of magnesium carbonate on carboxylic resins, which is observed in desorption of Mg 2 + from the resin in the Mg form with Na 2 CO 3 solutions or with solutions of Na 2 CO 3 -NaHCO 3 mixtures. The physical and mathematical models of the kinetics and the dynamics of the ion-exchange processes proceeding in a colloid system comprising a supersaturated solution and an ion exchanger, which surface is partially blocked by adsorbed precrystalline aggregated (micelles), are proposed, and their validity is experimentally confirmed.

Dissolution and hydration kinetics of MgO

The dissolution and hydration kinetics of MgO single crystals and powder samples were investigated with regard to the H + and Mg 2+ concentrations and the temperature. The rate of dissolution of rotating MgO discs in buffered solutions was determined from measurements of [Mg 2+] and those of the crystals and powder fractions were determined by pH and conductivity analysis. The degree of hydration was analysed by means of a thermogravimetric method. Several rate-controlling processes depending on pH were present at room temperature. (1) At pH < 5 the rate-controlling step was proton attack followed by desorption of Mg 2+ of OH - depending on the value of [Mg 2+]. The rate was proportional to either -pH or pMg-pH. These processes are part of the overall neutralization reaction. MgO + 2H +→Mg 2+ + H 2 O (2) At pH ≈ 5 the rate-controlling step was a diffusion-limitation process due to protons. The rate was proportional to the proton concentration. (3) At pH > 7 the rate-controlling step was OH - adsorption followed by Mg 2+ and OH - desorption leading to a rate maximum. These processes are part of the overall dissolution reaction. MgO + H 2 O→Mg 2+ + 2OH - The neutralization processes are interpreted in terms of irreversible thermodynamics yielding a linear dependence of the rate on pH or pMg-pH. It is concluded from conductivity and scanning electron microscopy measurements during and after hydration experiments that the hydration rate is controlled by the dissolution rate under given conditions. After a supersaturation period Mg(OH) 2 precipitates preferentially at the MgO surface, so that an MgO lattice reaction can be excluded. All processes undergo an Arrhenius acceleration with increasing temperature (activation energy, 70 kJ mol -1) and the overall reactions are then limited by proton and OH - diffusion.