Yujiro Watanabe

Ion Exchange Behavior of Natural Zeolites in Distilled Water, Hydrochloric Acid, and Ammonium Chloride Solution

This study presents basic research on the use of natural zeolites for the removal of ammonium ions from aqueous solutions. Three natural zeolites from Shimane, Japan, used in this study are shown to contain varying amounts of clinoptilolite and mordenite. The chemical behavior of these natural zeolites was investigated in distilled water, hydrochloric acid, and ammonium chloride solution. The amount of ammonium ions adsorbed by these zeolites was measured using an electrode specific to ammonium ion. The results were compared with published adsorption values for other zeolites. Hydrogen ions displaced the exchangeable cations (Na+, K+, Ca2+, and Mg2+) on the natural zeolites in distilled water and hydrochloric acid. With the decrease in equilibrium pH, the amounts of dissolved silica and alumina increased. This behavior indicates that the structures of the zeolites are damaged under this condition. Ammonium ions were preferentially adsorbed on the natural zeolites in ammonium chloride solution. The amount of ammonium ions adsorbed quantitatively corresponded to the theoretically expected amount at high ammonium concentrations. This result indicates that ammonium adsorption occurs by ion exchange of ammonium ions with Na+, K+, Mg2+, and Ca2+ in cation exchange sites on zeolites. The order of selectivity of ammonium ions for other cations on the zeolites was Na+≫K+>Ca2+>Mg2+. Ammonium adsorption was the highest in the zeolite containing the highest proportion of clinoptilolite of the three natural zeolites. The maximum adsorption was 1.28 mmol/g.

Uptake of Cesium and Strontium Ions by Artificially Altered Phlogopite

Potassium (K(+)) phlogopite was transformed to a vermiculite-like mineral through a topotactic reaction under acidic conditions (pH 2) followed by hydrothermal treatment with Na(+), Mg(2+), Ca(2+), and Al(3+) cations. The resulting Na(+)-, Mg(2+)-, Ca(2+)-, and Al(3+)-altered phlogopites (Phl) denoted as Na-Phl, Mg-Phl, Ca-Phl, and Al-Phl, respectively. Na-Phl, Mg-Phl, and Ca-Phl all exhibited the same high adsorption capacity as natural vermiculite and the absorption of Cs(+) and Sr(2+) ions on these materials followed the Langmuir model. High-angle annular dark-field scanning transmission electron microscopy showed that Cs(+) ions in the Mg-Phl layers were intercalated deep within the crystal structure, along specific interlayer regions. These adsorbed anhydrous Cs(+) ions were firmly fixed at the centers of hexagonal rings positioned simultaneously in the upper and lower tetrahedral silicate sheets, whereas Sr(2+) ions adsorb into the interlayer in the hydrous state. Al-Phl formed a hydroxyl-interlayered vermiculite and demonstrated significant selectivity for Cs(+) at very low concentrations of the isotope. Consequently, the artificially altered phlogopites prepared in this study showed controllable and versatile adsorption capabilities making them significantly more suitable than natural vermiculite for Cs and Sr decontamination.

Novel Long-Term Immobilization Method for Radioactive Iodine-129 Using a Zeolite/Apatite Composite Sintered Body

The amount of radioactive iodine generated from nuclear power plants is expected to increase with the proliferation of nuclear energy production, and long-term immobilization methods for such radioactive elements need to be developed to make nuclear energy sustainable. The standard immobilization method of radioactive elements, vitrification, is not very effective for radioactive iodine-129 because of the low solubility of iodine in silicate melts, its very high volatility at standard vitrification process temperatures, and its instability in the alkaline environment of deep geological layers below 300 m. We have developed a novel three-phase ceramic composite produced by a sintering process. Iodine adsorbed onto Ca-type zeolite A was covered with a hydroxyapatite nanolayer through the exchange reaction of ammonium with calcium. Clusters of iodine of 30 nm within the zeolite structure were found to be thermally stable up to 1253 K because of the partial blockage of the alpha-cage apertures by ammonium ions and the partial change from a crystalline phase to an amorphous phase at 473 K. No gasification of iodine molecules was found to occur during the sintering process. The outer phase was highly crystalline hydroxyfluorapatite in which the hydroxyapatite nanolayer plays an important role for successful sintering. The elution of iodine in low-dioxygen water, similar to that found within the Earths crust, was investigated and was found to occur only in the surface layer of the sintered body.

Geomaterials: their application to environmental remediation

Abstract Geomaterials are materials inspired by geological systems originating from the billion years long history of the Earth. This article reviews three important classes of geomaterials. The first one is smectites—layered silicates with a cation-exchange capacity. Smectites are useful for removing pollutants and as intercalation compounds, catalysts and polymer nanocomposites. The second class is layered double hydroxides (LDHs). They have an anion-exchange capacity and are used as catalysts, catalyst precursors, sorbents and scavengers for halogens. The third class of geomaterials is zeolites—microporous materials with a cation-exchange capacity which are used for removing harmful cations. Zeolite composites with LDHs can absorb ammonium and phosphate ions in rivers and lakes, whereas zeolite/apatite composites can immobilize the radioactive iodine. These geomaterials are essential for environmental remediation.

Micro-cubic glass from pseudomorphism after thermal treatment of ammonium-exchanged zeolite A

Abstract Isothermal treatments of the ammonium-exchanged Na–zeolite A were carried out at 400 °C for different durations up to 1000 min in the air. The products were characterized by XRD, SEM, AFM, FT-IR, DTA and nitrogen adsorption measurement. The results confirmed that the amorphous pseudomorphism occurred, showing the tweed-like patterns on the surfaceof cubic solid. These characterizations supported the existence of thin surface layer of zeolite crystal as a microvessel,which was consistent with the previous hypothesis. The amorphous transformation was taking place by the mechanism, in which the zeolite framework reacted with hydrogen decomposed from ammonium ions. The obtained micro-cubic glass should be useful as an eco-functional material, such as the fixation of harmful ions.

Synthesis of Nano-sized Boehmites for Optimum Phosphate Sorption

Nano-sized boehmites with different crystallinity were synthesized at the temperature range of 25 to 200°C in order to produce phosphate absorbents with high capacity. The physicochemical property of boehmites was depended on the synthesis temperature: the particle size was increased and the surface area showed the maximum for the boehmite at 50°C. The phosphate sorptions into boehmites were analyzed at room temperature in the phosphoric acid solutions as a model of wastewater at the concentration of 0.1 to 3.0 mmol l−1 and the pH of 3 to 7. The boehmite synthesized at 50°C exhibited the highest amount of phosphate sorption (1.73 mmol g−1 at pH 3.3) compared with Al-bearing materials. The reaction mechanism during phosphate sorption was described by the anion exchange reaction between phosphate ions in sodium phosphate solution and hydroxide ions on boehmite surfaces. Therefore, the variation in the amount of phosphate sorption on the different boehmite samples depends strongly on the surface area in the specimens. In addition, the sample showed high stability in near-neutral-pH solutions, and an increase in pH toward neutral was found to lead to a decrease in phosphate sorption due to a change in the phosphate species and a reduction of the surface sites.

Alkali‐Hydrothermal Modification of Air‐Classified Korean Natural Zeolite and Their Ammonium Adsorption Behaviors

Abstract Korean natural zeolite in which clinoptilolite and mordenite coexisted with feldspar and illite as impurities, was treated with 1.0, 3.0, and 5.0 M NaOH solutions at 100, 150, and 200 °C under autogeneous pressure for 17 hours either with or without an air classification as pretreatment. Phillipsite, analcime, and hydroxycancrinite were identified as reaction products depending on the reaction temperature and NaOH concentration. The air classification of the starting material prior to alkali‐hydrothermal treatment effectively reduced the amount of feldspar, which hardly reacted to zeolite in the hydrothermal reaction. The ammonium adsorption behavior of the treated and untreated samples were investigated in solutions of between 10−3 M and 10−2 M NH4Cl. The amount of adsorbed ammonium ions in alkalihydrothermally treated product from air‐classified material was higher by about two times than was that of corresponding untreated zeolites. The air‐classified zeolite treated in 3 M NaOH solution at 100 °C showed the highest adsorption of ammonium ion among samples. It was explained by both the phase change of clinoptilolite and mordenite to phillipsite with higher cation exchange capacity and the reduction in the amount of feldspar that was less reactive under hydrothermal conditions for the formation of phillipsite. The results indicated that the combination of the air classification and alkali‐hydrothermal treatment effectively improved the adsorption behavior for ammonium ions on natural zeolites with impurities.

Adsorption Behavior of Metal Ions with β-Diketone Type Chelating Agents Supported on Hydrophobized Mesoporous Silicate MCM-41

To investigate the adsorption behavior of metal ions with β-diketone type chelating agents, thenoyltrifluoroacetone (Htta) and benzoyltrifluoroacetone (Hbfa) supported on mesoporous silicate MCM-41 that has been hydrophobized (hMCM-41) using trimethylchlorosilane was synthesized. The hexagonally arranged pore structure of MCM-41 was not significantly changed by the hydrophobic treatment and attachment of chelating agents, although the pore size, pore volume and surface area were slightly smaller than those of untreated MCM-41 as a result of the chemical modification of the surface. The amount of chelating agents adsorbed on hMCM-41 was 0.47 for Htta and 0.33 mmol g for Hbfa, respectively. All metal ions examined, except for Cd, were quantitatively adsorbed on hMCM-41 as metal complex, M(tta)2 or M(bfa)2. The order of the adsorption selectivity of the metal ions was similar to the stability constants of metal ions with Htta or Hbfa. Furthermore, most of the metal ions adsorbed onto hMCM-41 at lower pH compared with the results of having used MCM-41 as a support of chelating agents. This suggests that the adsorption ability of metal ions with chelating agents was improved by the hydrophobic treatment of MCM-41.