Yoshitaka Minai

Comparison of adsorption behavior of multiple inorganic ions on kaolinite and silica in the presence of humic acid using the multitracer technique

Abstract The influence of humate formation on the adsorption of various ions on kaolinite and amorphous silica was studied using the multitracer technique, i.e., the simultaneous application of a variety of radioactive tracers. The technique simultaneously provided the solid-aqueous distributions of 32 elements, namely, Be, Na, Sc, V, Cr, Mn, Fe, Co, Zn, Ga, As, Se, Rb, Sr, Y, Zr, Tc, Ru, Rh, Ag, Te, Ba, Ce, Pm, Eu, Gd, Tm, Yb, Lu, Hf, Re, and Pt, in the absence and presence of humic acid. Speciation calculations under our experimental conditions were also conducted for 19 elements among them, using our previous results on the stability constants of their humate complexes. Comparison of the pH dependences of each element and humic acid dissolved in the aqueous phase enabled us to evaluate the interaction between them. With the aid of the speciation calculation, we discussed the predominant species that would control the environmental behavior of each ion. Among the elements studied, it was suggested that humate formation greatly affects the behavior of rare earth elements, which would indicate that the environmental behavior of these ions is influenced by humic substances. For alkaline earth metals (Be, Sr, Ba), divalent transition metals (Mn, Co, Zn), trivalent transition metals (Cr, Fe), and Ag, the influence of the formation of humate complexes on their distribution behavior was observed. It was estimated that the organic-inorganic complex made up of inorganic particles (e.g., kaolinite and silica in this study) coated with humic substances, plays an important role in the behavior of various ions in the environment. For the other ions of V V , Ga, Rb, Zr, Ru, Rh, Te, Hf, and Pt, humate formation was not important, but other factors such as hydrolysis and adsorption on solid surfaces were predominant factors. Based on the present results and an equilibrium model expressed by stability constants of hydroxides and carbonates or oxalates, the environmental behaviors of various cations are compared. For the oxoanions found in this study, such as As V , Se IV , Tc VII , and Re VII , humate formation also was not important. The inhibiting effect, on the adsorption of ions onto the solid surface, of the humic acid coating on the surface was observed for the distributions of V V , As V , Se IV , Zr, Ru, Rh, Hf, and Pt. These results show that the adsorption of humic substances alters the nature of inorganic particles in adsorbing metal ions.

Bimetallic promotion of alcohol production in CO hydrogenation and olefin hydroformylation on RhFe, PtFe, PdFe, and IrFe cluster-derived catalysts

Iron-containing bimetallic catalysts were prepared from carbonyl clusters as precursors deposited on SiO2. FeRh4 and Fe2Rh4 cluster-derived catalysts showed high activity and selectivity for formation of ethanol and methanol in CO hydrogenation. Fe3Pt3, Fe6Pd6 and FeIr4 cluster catalysts gave methanol in high selectivity, while Fe-rich Fe4Pt and Fe4Pd were not selective catalysts. The RhFe cluster catalysts showed improved activity in hydroformylation of olefins; C4-alcohols were substantially obtained from C3 + CO + H2. Mossbauer and EXAFS studies on the Fe2Rh4/SiO2 catalyst show that highly dispersed RhFe bimetallic particles are located on the SiO2 surface, where Fe atoms exist preferentially in the state of Fe3+ even after H2 reduction. FTIR spectra of CO chemisorbed on Fe2Rh4/SiO2 exhibit a low-frequency band possibly due to the G and O-bonded CO on Rh-Fe3+ sites. Bimetallic promotion of alcohol production in CO hydrogenation and olefin hydroformylation is proposed to originate from the two-site interaction of Rh-Fe3+ (or Pt-Fe3+, Pd-Fe3+, IrFe3+) sites with CO to enhance migratory CO insertion.

Direct observation of Cm(III)-fulvate species on fulvic acid-montmorillonite hybrid by laser-induced fluorescence spectroscopy

Particulate matter plays an important role in the removal of metal ions from water in natural aquifers. Some of the most important of these materials consist of associations of inorganic particles (clay minerals, oxides) with humic substances, associations that can form readily in such an environment due to the strong affinity between inorganic particles and humic substances. These associations are referred to in this paper as organic-inorganic hybrids. However, it is not clear whether the sorbed species of metal ions in such organic-inorganic hybrids are organic or inorganic species because of the complexity of such hybrids and the lack of appropriate methods for characterizing the trace metal ions incorporated in them. In this study, laser-induced fluorescence spectroscopy (LIF) was used successfully to characterize the Cm(III) species on an FA(fulvic acid)-montmorillonite hybrid, an example of such organic-inorganic hybrids. The LIF clearly showed that Cm(III) can be sorbed as Cm(III)-fulvate complex in the FA-montmorillonite hybrid. These results were consistent with those of experiments of solid-water partitioning of Cm(III) (or Eu(III) used as an analogue) and speciation calculations based on the stability constants of Cm(III)-fulvate complexes determined in this study. The results of LIF and the partitioning experiments showed that the solid-water distribution of humic substances governed that of Cm(III) under our experimental conditions. The Cm(III) preference for forming Cm(III)-fulvate complexes was also evident under a condition that would be found in a natural aquifer with a fairly low concentration of organic matter in freshwater (dissolved organic carbon: 2 mg/dm3), as determined by our speciation calculations. These findings on the importance of humic substances in the migration of Cm(III) indicate that the clarification of the environmental behavior of humic substances is necessary to understand fully the behavior of Cm(III), or actinide(III) and lanthanide(III) ions, in natural aquifers.

A Survey of Trace Elements in Pteridophytes

Concentration of 11 trace elements (Ca, Sc, Cr, Fe, Co, Zn, Rb, Cs, Ba, La, and Ce) in 96 pteridophytes (fern and fern ally species) was determined by instrumental neutron activation analysis to evaluate a concentration range for each element and also to find species characteristic in the uptake of trace elements. Asplenium trichomanes was found to accumulate Sc, Cr, and Co to the highest concentrations among 96 pteridophytes. The highest concentration of Ca and Zn was observed for Asplenium obscurum. The other Pteridophytes exhibited only one element whose concentration was the highest. A positive correlation was found between the concentrations of Fe and Sc, and also between the concentrations of Cr and Co. The remarkable accumulation of lanthanides (La and Ce) was observed mainly in diversifying genera (Polystichum and Dryopteris in Dryopteridaceae, Diplazium in Woodsiaceae, and Asplenium in Aspleniaceae).

Adsorption of europium(III) and americium(III) on kaolinite and montmorillonite in the presence of humic acid

Distribution of trace amount of Eu(III), or Am(III), in the aqueous/solid system containing humic acid and kaolinite, or montmorillonite, was studied by batch experiments. Humic acid was also adsorbed on the clay minerals and its adsorption isotherm can be regarded as a Langmuir type. It is shown that Eu(III), or Am(III), exists as humate complex either in the aqueous or on solid phase in the system including kaolinite, or montmorillonite. These results suggest that the organic-inorganic complex like clay minerals coated with humic substances is important as metal reservoir in the environment.

Beneficial effect of rare earth elements on the growth of Dryopteris erythrosora

Summary Lanthanum was demonstrated to be beneficial to the growth of Dryopteris erythrosora , a fern species which accumulates rare earth elements (REEs) under natural conditions. The enhancement of its growth by La was much greater than that by Ca. The uptake rates of REEs were evaluated to be higher than that of Zn and almost equal to those of Sr and Co. The region in the mesophyll where REEs were concentrated was not the same as that of Ca. In mesophyll cells, REEs were observed mainly in chloroplasts. These observations suggest that REEs do not behave as a ‹super Ca› as was suggested for some plants, but that they are transferred to chloroplasts by special mechanisms, possibly for fulfilling yet unknown functions or being stored as a mechanism of detoxification.

Ionic strength and pH dependence of binding constants of Am(III)- and Eu(III)-humates

Binding constants of Eu(III)- and Am(III)-complexes with soil-derived humic acid were determined by solvent extraction at various pH and ionic strength. Based on the dependence of binding constants on pH and ionic strength, stabilities of the humate complexes in land water and seawater were estimated. Speciation calculation based on the binding constants indicated that Am(III) could combine with humic substances in natural water system.

Characterization of silica supported Rh-Fe catalysts by Mössbauer technique

Silica supported Rh−Fe catalysts were characterized by means of in situ57Fe Mössbauer spectroscopy. The Mössbauer spectra indicated that iron on the silica support existed either as Fe/O/ in the Rh−Fe alloy or as Fe3+ in Rh−Fe metal cluster compounds. The (Fe3+/Fe/O/) ratio and Mössbauer parameters were found to depend on the (Fe/Rh) atomic ratio in the catalysts. Such dependence corresponded to the change of catalytic properties of the supported Rh−Fe catalyst with the varying (Fe/Rh) ratio.

Multitracer study on the uptake mechanism of yttrium and rare earth elements by autumn fern

Summary Irrespective of low bioavailability, some plant species accumulate Y and rare earth elements (REEs) to a great extent (accumulator species). The uptake mechanisms of Y and REEs were investigated for autumn fern, one of accumulator species. For comparison, plant species which accumulated poorly REEs (non-accumulator species) were also studied. In the present investigation, two noticeable phenomena were observed. (I) Autumn fern showed no ionic-radius dependence of Y-REE uptake by leaves, while non-accumulator species showed an extremely high uptake for Y compared with REEs. (II) Y-REE uptake by autumn fern was influenced by the addition of chelating reagents to the uptake solution, while no effect was observed for non-accumulator species.

MULTITRACER STUDY ON THE EFFECT OF HUMATE FORMATION ON THE ADSORPTION BEHAVIOR OF METAL IONS ON KAOLINITE AND SILICA GEL

The multitracer technique was applied to elucidate of influence of humate formation on adsorption behavior of ultratrace elements. Dissolved fractions of Co, As, Rb, Sr, Y, Zr, Ba, Ce, Eu, Gd, Tb, Yb, Lu, Hf, Re and Pt in contact with kaolinite or silica gel were determined simultaneously either in the presence or absence of humic acid, which was partly adsorbed on the solid. Percentage of dissolved fraction of rare earth elements was identical to that of humic acid, indicating high stability of the rare earth-humate complex. Hydrolysis was the most important factor controlling the behavior of Zr and Hf. Both hydrolysis and humate complexation influenced the adsorption of Co, Sr, Ba and Pt, whereas neither affected the distribution of As, Rb and Re.