Shingo Yokoyama

ATOMIC FORCE MICROSCOPY STUDY OF MONTMORILLONITE DISSOLUTION UNDER HIGHLY ALKALINE CONDITIONS

Montmorillonite dissolution under highly alkaline conditions (pH = 13.3; I = 0.3 M) was investigated by bulk dissolution methods and in situ atomic force microscopy (AFM). In bulk dissolution experiments, initial SiO2 concentrations were high, and a steady state was reached after 136 h. The dissolution rates derived from the edge surface area (ESA) at the steady-state condition at 30, 50 and 70°C were 3.39 x 10−12, 1.75 × 10−11 and 5.81 × 10−11 mol/m2 s, respectively. The AFM observations were conducted under three conditions: (Run I) short-term in situ batch dissolution at RT; (Run II) long-term in situ flow-through dissolution at RT; and (Run III) long-term batch dissolution at 50°C. The observed reductions in montmorillonite particle volume for Runs I and II were due primarily to edge-surface dissolution. The ESA-based dissolution rate for Run I (10−9 mol/m2 s) was three orders of magnitude faster than that for Run II (10−12 mol/m2 s). The rate obtained for Run II corresponded to the rate at the steady-state conditions in the bulk dissolution experiments. A small number of etch pits developed in Run III slightly increased the ESA of montmorillonite since most of the montmorillonite particles were separated into monolayers lacking three-dimensional periodicity. The ESA-based dissolution rate for Run III was 2.26 × 10−11 mol/m2 s. Dissolution rates based on long-term AFM observations could be directly compared with steady-state rates obtained from bulk dissolution experiments. The AFM observations indicated that dissolution occurred at edge surfaces; therefore, the ESA should be used to calculate the dissolution rate for montmorillonite under alkaline conditions. Dissolution rates of individual particles with different morphologies estimated by AFM were similar to rates estimated from bulk dissolution experiments.

Arsenic-bearing smectite from the geothermal environment

Abstract Arsenic-rich scales are widely associated with geothermal fields and constitute a potential hazard to human health. Such arsenic has hitherto been reported to be almost exclusively hosted by sulphide or oxide phases or occurring as surface species. We report here, however, the occurrence of an arsenic- rich (1500 to 4000 mg kg-1 As) smectite from geothermal precipitates from a geothermal field in northwestern Japan and present evidence that the arsenic is predominantly hosted within this silicate mineral. Consistently ~80% of the total arsenic determined in these geothermal precipitates was found by selective chemical extractions to be associated with an operationally defined clay mineral fraction, with lesser proportions being associated with operationally defined amorphous silica, Fe oxide and sulphide fractions. Analysis by XRD, ATR IR and XRF showed the clay fraction to be dominated by Mg-rich trioctahedral smectite. Arsenic K-edge XAS spectra of the smectite suggested the dominance of As(III)-O coordinated species with significant contributions from As(V)-O coordinated species. Both XPS and a magnesium chloride chemical extraction indicated minimal adsorption of arsenic on smectite surfaces suggesting that the arsenic was predominantly either dissolved within the smectite or occurred within mineral occlusions. No such occlusions greater than 1 μm in size were observed in the As-rich smectites. The potential occurrence of arsenic-bearing clays should be considered when determining remediation strategies for geothermal environments or evaluating risks associated with the industrial usage of geothermal precipitates.

Collective Structural Changes in Vermiculite Clay Suspensions Induced by Cesium Ions

Following the Fukushima Daiichi nuclear disaster in 2011, Cs radioisotopes have been dispersed over a wide area. Most of the Cs has remained on the surface of the soil because Cs+ is strongly adsorbed in the interlayer spaces of soil clays, particularly vermiculite. We have investigated the microscopic structure of an aqueous suspension of vermiculite clay over a wide length scale (1–1000 Å) by small-angle X-ray scattering. We determined the effect of the adsorption behavior of Cs+ on the structural changes in the clay. It was found that the abruption of the clay sheets was induced by the localization of Cs+ at the interlayer. This work provides important information for predicting the environmental fate of radioactive Cs in polluted areas, and for developing methods to extract Cs from the soil and reduce radioactivity.

Mesoscopic structures of vermiculite and weathered biotite clays in suspension with and without cesium ions.

The effect of cesium (Cs) adsorption on the mesoscopic structure of the clay minerals vermiculite and weathered biotite (WB) in suspensions was elucidated by small-angle X-ray scattering (SAXS). The clay minerals form multilayered structures, and the Cs cations (Cs(+)) are strongly adsorbed in the interlayer space of the soil clays, in particular vermiculite and WB. SAXS was used to monitor the relationship between Cs(+) adsorption at the clay interlayers and the structural changes at length scales from 1 to 1000 Å. The variation in the distance between the neighboring clay sheets and the spatial arrangement of the clay sheets with and without Cs(+) were clarified. Our quantitative analyses revealed that the number of stacked layers of pure vermiculite was decreased by Cs(+) addition, whereas that of WB increased. Moreover, the average distance between the neighboring layers of vermiculite in suspension was larger than that of WB, which reflects the different conditions of Cs(+) intercalation. These findings provide fundamental insights that are important for predicting the environmental fate of radioactive Cs in contaminated regions and for developing methods for extracting Cs from soil.

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.

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.