Yuichi Niibori

Modeling of the complex formation of metal ions with humic acids

Summary Based on the conceptual model of cation-humic interactions considering the heterogeneous composition and polyelectrolyte nature of humic acids, the description of the interaction by the expression, log Kapp= log K - m log [M]+ a log α - b log [Na+], is proposed, where Kapp=[M··(zR)]/([M][R]), [M··(zR)] is the concentration of the cation bound to the humic acid, [M] is the concentration of the free cation with charge z, and [R] is the concentration of the free anionic sites of the humic acid, [R]=(CR-z[M··(zR)])α (CR is a total concentration of proton exchanging sites and α is a degree of dissociation). log K, m, a and b are the constant parameters depending on the cation and the humic acid. The expression is successfully applied to the experimental results on the complex formation (or protonation) of humate and polyacrylate (a homogeneous polymeric weak acid) with H+, NpO2+, Eu3+, Ca2+, Fe2+ and Fe3+ obtained under the condition where hydrolysis of cations and saturation of the reacting sites can be neglected.

Preparation of stable alginate microcapsules coated with chitosan or polyethyleneimine for extraction of heavy metal ions.

Stable alginate microcapsules in dried form containing bis(2,4,4-trimethylpentyl) monothiophosphinic acid (HA) were prepared by coating of fresh alginate microcapsules with chitosan or polyethyleneimine (PEI). The thickness of coatings was estimated by electron probe microanalysis (EPMA), along with electron microscopy (SEM), as well as comparison of uptake percentage of coated and uncoated hollow capsules. Characterization of microcapsules was carried out by Ag+ uptake experiments, destructive chemical analyses and thermogravimetric methods (TG and DTA). Chemical stability tests in HNO3 and NaNO3 media indicated that the coating with 4-double layer chitosan or mono-layer PEI led to an appreciable enhancement of impermeability in the range of pH > 1 or [Na+] < 1 M (mol dm−3). Unfortunately, multiple coating causes some extractant losses due to effect of physical stress during the coating; however, PEI-coated microcapsules nearly completely hold their extractant content. Stable extractive microcapsules have an appreciable potential for the selective removal of heavy metal ions.

Identification of geothermal reservoir structure analyzing tracer responses using the Two-Fractured-Layer model

Abstract A model to analyze tracer test data is proposed. It consists of a fractured layer and a slightly-fractured layer, where tracers move between the layers in proportion to their concentration gradients. It is shown that the slightly-fractured layer plays an important role in the movement of fluids and transport of tracers in fractured rock masses. The concepts behind the proposed model for identifying geothermal reservoir structure from the analysis of tracer returns and its application to field data are described. The results demonstrate that one main flow path can explain tracer responses showing two peaks or a long tail. The model allows us to calculate the flow fraction and the fluid flow velocity through the slightly-fractured layer from the tracer returns observed in geothermal wells.

Analysis of Water Injection in Fractured Reservoirs Using a Fractional-Derivative-Based Mass and Heat Transfer Model

This research proposes a numerical scheme for evaluating the effect of cold-water injection into a geothermal reservoir. A fractional heat transfer equation (fHTE) is derived based on the fractional advection–dispersion equation (fADE) that describes non-Fickian dispersion in a fractured reservoir. Numerical simulations are conducted to examine the applicability of the fADE and the fHTE in interpreting tracer and thermal responses in a fault-related subsidiary structure associated with fractal geometry. A double-peak is exhibited when the surrounding rocks have a constant permeability. On the other hand, the peak in the tracer response gradually decreases when the permeability varies with distance from the fault zone according to a power law, which can be described by the fADE. The temperature decline is more gradual when the permeability of surrounding rocks varies spatially than when they have a constant permeability. The fHTE demonstrates good agreement with the temperature profiles for the different permeabilities of surrounding rocks. The retardation parameters in the fADE and the fHTE increase with the permeability of the surrounding rocks. The orders of the temporal fractional derivatives in the fADE and the fHTE vary with the permeability patterns.

Selective adsorption and stable solidification of radioactive cesium ions by porous silica gels loaded with insoluble ferrocyanides

Development of highly functional cesium selective adsorbents for the decontamination of high-activity-level water (HALW) from the Fukushima NPP-1 accident is very urgent. In order to selectively adsorb the radioactive cesium, three kinds of novel porous silica gels loaded with insoluble ferrocyanides (SLFC) were prepared using a successive impregnation/precipitation method. Based on the results of previous research, the SLFC composites have relatively large uptake ratio above 95%, distribution coefficients (Kd) above 103 cm3/g, and excellent adsorption kinetics even in seawater. The solidification results also indicate that zeolites have an excellent Cs immobilization characteristic, gas-trapping and self-sintering abilities, and low leachability. We chose three kinds of SLFC composites to achieve the optimization of solidification by mixing with nine kinds of additives at high temperatures (up to 1200 °C). The Cs contents in the three composites were estimated to be below 30% of the initial contents and decreased with the three stages at calcination temperatures ranging from 25 to 1200 °C. By contrast, the Cs immobilization ratio was markedly lowered by mixing with additives: of those, allophane had the best immobilization result. By increasing the additive ratio to 50 wt%, the Cs immobilization ratio became almost 100% and no volatilization of Cs was detected even after calcination at 1200 °C. This result indicates that calcination of the mixture of SLFC composites after adsorbing Cs+ ions and specific additives under appropriate ratio is effective for stable solidification.

Dynamic Behavior of Colloidal Silica in the Presence of Solid Phase

Since silica undergoes polymerization, precipitation, and dissolution depending on the change in pH or temperature, the chemical behavior of silica would be much complicated when cement for the construction of geological disposal system greatly changes the pH (8 to 13) of groundwater. To clarify the dynamic behavior of silica in such an alkaline solution, the concentrations of silica in both soluble and colloidal form in the supersaturated solution in the presence of solid phase have been traced over a 40-day period. In the experiment, the concentration of silica in a soluble form was determined by the silicomolybdenum-yellow method, and the concentration of silica in soluble plus colloidal forms was determined by adjusting the pH of the solution to 13, where all the silica changes into a soluble form (mainly monomeric). In order to examine the dynamic behavior of colloidal silica with solid phase of silica, this study has used natural quartz and pure commercial amorphous silica, both in a size fraction of 74–149 μm, whose specific surface-area (BET, N 2 gas) were respectively 1.0 m 2 /g and 400 m 2 /g. The Na 2 SiO 3 solution (250 ml, pH>10, 298 K) was poured into a polyethylene vessel containing quartz or amorphous silica (0.1 g or 0.5 g), HNO 3 and a buffer solution. The pH of the solution was set to 8. The silica initially in a soluble form at pH>10 (6.8×10 -3 M or 1.2×10 -2 M) became supersaturated and either deposited on the solid surface or changed into the colloidal form. The ratio of silica in those form depended both on the initial concentration of soluble-silica and the surface area of the solid. The concentration of colloidal-silica gradually decreased, where the logarithm of its concentration decreased linearly against time after the concentration of soluble-silica decreased to a metastable concentration slightly higher than the solubility of soluble-silica.

Separation of Am and Cm by using TODGA and DOODA(C8) adsorbents with hydrophilic ligand-nitric acid solution

Extraction chromatographic separation of trivalent minor actinides (MA: Am and Cm) was investigated by using solid adsorbents impregnating the lipophilic diamide-type ligands, TODGA (N,N,N′,N′-tetraoctyl-3-oxapentane-1,5-diamide) and DOODA(C8) (N,N,N′,N′-tetraoctyl-3,6-dioxaoctane-1,8-diamide). In nitric acid eluents, hydrophilic diamide-type ligands, DOODA(C2) (N,N,N′,N′-tetraethyl-3,6-dioxaoctanediamide) and TEDGA (N,N,N’,N’-tetraethyl-diglycolamide) were included for the respective adsorbents as masking agents in order to produce a synergistic effect in mutual separation of Am and Cm. The adsorption tendency of Am and Cm for the adsorbents was opposite each other, that was similar to the case of lanthanides.

Dissolution Rate of Colloidal Silica in Highly Alkaline Solution

For the performance assessment of the radioactive waste repository, it is important to clarify the dynamic behavior of silica (silicic acid and hydrous or unhydrous silicon dioxides). The behavior of silica would be complicated when cement is used for the construction of the repository, since silica takes various forms due to polymerization, precipitation and dissolution with change of pH or temperature. In order to know the fundamental kinetic property of silica, this study has examined the dissolution rate of colloidal-silica. In the experiment, the concentration of silica in a soluble form was determined by the silicomolybdenum-yellow method. In this study, soluble-silica was defined as silica reacting with molybdate reagent and coloring yellow, and colloidal-silica was defined as silica in liquid phase except for soluble-silica. Colloidal-silica was obtained through the polymerization process, where the pH value of silica solution was brought down from over 10 by HNO 3 solution. This study examined dissolution rate of colloidal-silica again by setting to 10 or 13 in pH-value and 288 K, 298 K or 313 K in temperature. In the experimental results, the dissolution reaction of colloidal-silica proceeded linearly with time, when the dissolution of colloidal silica was not restricted by the solubility of silica. To estimate the dissolution rate, we assumed df /dt = k * , where f is the soluble-silica fraction defined as the amount of soluble-silica divided by the silica amount introduced into the sample solution, t the time (s) and k * the rate constant (s -1 ). The activation energy for the dissolution of the colloidal-silica at pH 13 was estimated to be approximately 80 kJ·mol -1 which was similar to that for amorphous silica (solid phase) at pH 13. This suggests the same reaction mechanism for the dissolution of colloidal-silica and amorphous silica in highly alkaline solution.

An Experimental Approach on the Effect of Rock Alteration on Sorption Behavior

In the geological disposal of radioactive wastes, cement used for the repository construction alters the condition of groundwater to highly alkaline of pH about 13. Such alkaline groundwater around the repository would alter the surfaces of the rocks in the flow paths to the amorphous phase. Once the surface is altered, it takes a geological period (so much long time) to restore the surface to its former condition. This study examined the sorption behavior of europium (Eu-152 (tracer), Eu-151 (5x10 -5 M, carrier) onto some silica minerals with polymeric silicic acid. Polymeric silicic acid also affects the alteration of the solid surface. The results showed that the kinds of silica minerals strongly affect the sorption behavior in the range of 5<pH<8. The main difference was due to the degree of crystallization of the solid phase, the specific surface area and the concentration of polymeric silicic acid. Since the amorphous silica or polymeric silicic acid takes a loose structure, the isoelectric point is high compared to the crystal ones such as opal-CT and cristobalite, which in turn decreases the sorption. The crystalline silica with polymeric silicic acid remarkably decreased the sorption of europium. This suggested that the surface of solid phase was altered by polymeric silicic acid.

Redox speciation method for neptunium in a wide range of concentrations

Summary The adsorption method for determination of the distribution of neptunium among its oxidation states by the use of solvent impregnated resins and bismuth phosphate precipitates as adsorbents is described. For neptunium in concentration ranging from ∼10-10 to ∼10-2.5M, Np(IV) and Np(VI) were quantitatively adsorbed on the impregnated resin in a solution of the acidity 1.0-0.1M HClO4, while Np(V) remained in the liquid phase. In the same condition, the precipitate of bismuth phosphate absorbed Np(IV) selectively, leaving Np(V) and Np(VI) in the liquid phase. By carrying out these adsorption methods in Ultrafree-MC Microporous Centrifugal Filter Units (Millipore Corporation), the oxidation state distribution of neptunium in acid solution can be determined in a simple manner with a small sample volume (500μl) regardless of its concentration.