Seung Yeop Lee

Adsorption of naphthalene by HDTMA modified kaolinite and halloysite

The adsorption of the cationic surfactant hexadecyltrimethylammonium (HDTMA) by kaolinite and halloysite was studied. Fourier transformed infrared (FTIR) spectroscopic studies revealed a change of the surfactant arrangement with solution conditions. Naphthalene was partitioned into the organic phase created by the surfactant tails of the HDTMA modified kaolinite and halloysite. The adsorption isotherms for naphthalene were nearly linear, suggesting that adsorption could be described by a distribution process. The distribution coefficients were primarily affected by the amount of surfactant adsorbed. Adsorption of naphthalene, however, was particularly dependent on the arrangement of the surfactant cations. At high surface coverage (e.g., >60% cation exchange capacity, CEC), the bilayer surfactant structure formed on kaolinite adsorbed large amounts of naphthalene. On halloysite, HDTMA formed surfactant clusters, and no prominent increase of naphthalene adsorption even at high HDTMA loadings was observed. Thus, the clay mineral structure and morphology had a considerable influence on the surfactant arrangement responsible for partitioning hydrophobic organic contaminants (HOCs) such as naphthalene.

EXPANSION OF SMECTITE BY HEXADECYLTRIMETHYLAMMONIUM

The adsorption of hexadecyltrimethylammonium (HDTMA) in smectite was studied by adsorption isotherms, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Smectites that had reacted for 48 h with HDTMA cations equivalent to 0.2–3.0 times the cation exchange capacity (CEC) were converted to HDTMA-exchanged smectites with various d-spacings. Study of HDTMA-smectites by HRTEM suggests that the HDTMA adsorption results in interlayer expansion with various d-spacings and irregular wavy layer structures. We believe that HDTMA loading beyond the CEC of smectite affects the structure of clay by the additional adsorption of HDTMA-Br− via hydrophobic bonding. Surfactant orientation probably depends on the quantity of surfactant in the interlayer. Our TEM study shows that the structure of the adsorbed HDTMA layer in the interlayers of smectite depends on the charge distribution and chemical composition of smectite.

Expansion characteristics of organoclay as a precursor to nanocomposites

Smectite that has reacted for 48 h with hexadecyltrimethylammonium (HDTMA) cations equivalent to 1.0 and 2.5 times the cation exchange capacity (CEC) converts to HDTMA-smectite. The microstructure of this organoclay is observed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). When the Na cations in the interlayer of clay are exchanged with HDTMA ions, the change in internal external surface configuration is augmented by the intercalation of organic cations. The HRTEM image of the HDTMA-treated smectite (1.0 CEC) reveals stacks of slightly curved layers with an average basal spacing of 20–24 A. In such organoclay, the repetitive multilayer structure is well preserved, but as the loading amounts of HDTMA increase to 2.5 CEC, a peculiar expansion is observed in some silicate layers. The HDTMA-exchanged smectite shows not only morphological features such as the irregular, wavy surface with curled edges, but also structural features such as large d-spacings (>40 A) and lattice distortion. Such morphological characteristics seem to be related to the intercalation amounts of HDTMA cations into the interlayer of smectite. A close examination of almost any layer reveals a continuously variable spacing, and this seems to be due to a different interlayer charge. Some exfoliated layers or edges may be locally developed on the low charge interlayer regions by the sufficient adsorption of organic surfactants beyond the CEC due to the tendency of alkyl chain interaction.

Biogenic Formation and Growth of Uraninite (UO2)

Biogenic UO₂ (uraninite) nanocrystals may be formed as a product of a microbial reduction process in uranium-enriched environments near the Earths surface. We investigated the size, nanometer-scale structure, and aggregation state of UO₂ formed by iron-reducing bacterium, Shewanella putrefaciens CN32, from a uranium-rich solution. Characterization of biogenic UO₂ precipitates by high-resolution transmission electron microscopy (HRTEM) revealed that the UO₂ nanoparticles formed were highly aggregated by organic polymers. Nearly all of the nanocrystals were networked in more or less 100 nm diameter spherical aggregates that displayed some concentric UO₂ accumulation with heterogeneity. Interestingly, pure UO₂ nanocrystals were piled on one another at several positions via UO₂-UO₂ interactions, which seem to be intimately related to a specific step in the process of growing large single crystals. In the process, calcium that was easily complexed with aqueous uranium(VI) appeared not to be combined with bioreduced uranium(IV), probably due to its lower binding energy. However, when phosphate was added to the system, calcium was found to be easily associated with uranium(IV), forming a new uranium phase, ningyoite. These results will extend the limited knowledge of microbial uraniferous mineralization and may provide new insights into the fate of aqueous uranium complexes.

Combined analyses of chemometrics and kriging for identifying groundwater contamination sources and origins at the Masan coastal area in Korea

Hydrogeochemical analyses including the basic statistics of chemical components, Piper’s trilinear diagram, and Mazor’s compositional bivariate diagram revealed that the main source and origin of groundwater contamination was seawater intrusion in the study area. However, the other sources and origins of groundwater contamination could be found by the combined analyses of chemometrics and kriging. Cluster analysis was helpful for the classification on the basis of the contamination characteristics of groundwater quality; however, it was not sufficient for the apportionment of groundwater contamination sources. Factor analysis (FA) determined three factors with 81.07% in total variance: Factor 1 for seawater contamination, Factor 2 for nitrate contamination, and Factor 3 for iron contamination. Factor analysis determined the sources of groundwater contamination; however, it could not discover the origins of contaminants except Factor 1. In backward stepwise mode, discriminant analysis decreased the number of parameters from 18 to 6 in discriminating the contaminant type with 96.2% correctness. TDS, Ca, NO3, Mn, Fe, and Br were the most significant parameters for the discrimination of contaminants. Kriging analysis was very useful for the understanding of correlation and similarity between contaminants and factors of FA, and for the investigation of contaminant origins. It also showed that the similarity between factor scores and contaminant concentrations was proportional to the magnitudes of factor loadings for contaminants. This study represented that the combined analyses of chemometrics and kriging were very indispensable to the identification of groundwater contamination sources and origins, as well as for the spatial classification and assessment of groundwater quality.

Abiotic reduction of uranium by mackinawite (FeS) biogenerated under sulfate-reducing condition

Sulfate-reducing bacteria and their by-products, such as iron sulfides, are widely distributed in groundwater and sediments, and can affect subsurface aqueous chemistry. Here we show the catalytic reduction of hexavalent uranium by FeS particles, which were largely generated by the activities of Desulfovibrio desulfuricans and D. vulgaris in anaerobic condition. Characterization of FeS particles by X-ray diffraction and high-resolution transmission electron microscopy revealed the presence of mackinawite having thin and flexible platy sheets with 0.5-nm lamellar spacing. This biogenic phase mediated abiotic reduction of U(VI) to U(IV) which was confirmed by UV–Vis absorption spectroscopy. The U conversion occurred through surface catalysis that involved adsorption of aqueous U(VI)–carbonate complexes (predominantly UO2(CO3)34−) onto the mackinawite, but the transformed uranium was then released and remained in suspended form in the solution phase. This surface catalysis and subsequent U(IV) remobilization has not been reported as a pathway to occur under sulfate-reducing conditions. Our results suggest that the iron sulfide solid, which is characteristic of conductive property, is very sensitive and variable depending on the electron supplying and transferring environment, negatively affecting the surface uranium to be strongly stabilized and fixed on the FeS surface.

Study on the exchange reaction of HDTMA with the inorganic cations in reference montmorillonites

The adsorption of hexadecyltrimethylammonium (HDTMA) cations on swelling layer silicates (montmorillonites) was studied by adsorption isotherms, X-ray diffraction, and electron microscopy. At low HDTMA concentrations, HDTMA ions started to be adsorbed on interlayer sites of SWy montmorillonite, causing a preferential release of interlayer Na+ compared with Ca2+, while the lateral attraction between adsorbed HDTMA cations at edges or external surfaces highly prevailed in SAz montmorillonite. Montmorillonite clay surfaces appeared as foliated and irregular aggregate structures at high HDTMA loadings. Besides the clay surface charge, the interlayer inorganic cations appeared to have a substantial influence on the expansion behavior of silicate layers as well as the evolution of the surface aggregates as a function of HDTMA surface coverage.

Transmission electron microscopy of hexadecyltrimethylammonium-exchanged smectite

Abstract The morphological and structural changes in smectite caused by hexadecyltrimethylammonium (HDTMA) treatment were studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The HDTMA-exchanged smectite shows not only morphological features such as irregular, wavy surface with curled edges, but also structural features such as large d-spacings (20 – 26 Å) and lattice distortions. The surface morphological heterogeneity is assumed to be related to the inhomogeneous intercalation of HDTMA cations into the interlayer of smectite.

Uranium and other trace elements’ distribution in Korean granite: implications for the influence of iron oxides on uranium migration

To understand trace radionuclide (uranium) migration occurring in rocks, a granitic batholith located at the Korea Atomic Energy Research Institute (KAERI) site was selected and investigated. The rock samples obtained from this site were examined using mineralogical methods, including scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The changes in the distribution pattern of uranium (U) and small amounts of trace elements, and the mineralogical textures affected by weathering, were examined. Based on the element distribution analyses, it was found that Fe2+ released from fresh biotite is oxidized in short geological time, forming amorphous iron oxides, such as ferrihydrite, around silicate minerals. In that case, the amorphous ferrihydrite does not show distinct adsorption for U. However, as it gradually crystallizes to goethite or hematite, the most U-rich phases were found to be associated with the secondary iron oxides having granular forms. This evidence suggests that the geological subsurface environment is favorable for the crystallized iron oxides to keep their structures more stable for a long time as compared with the amorphous phases. There is a possibility that the long residence of U which is in contact with the stable crystalline phases of iron may finally lead to the partial sequestration of U in their structure. Consequently, it seems that Fe-oxide crystallization can be a dominating mechanism for U uptake and controls long-term U transport in granites with low U contents.

Sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions.

The sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions were investigated as a function of pH, Se(IV) concentration, and Fe(II) concentration under an anoxic condition. The sorption of Se(IV) onto chlorite surfaces followed the Langmuir isotherm regardless of the presence of Fe(II) ions in the solution. The Se(IV) sorption was observed to be very low at all pH values when the solution was Fe(II)-free or the concentration of Fe(II) ions was as low as 0.5 mg/L. However, the Se(IV) sorption was enhanced at a pH > 6.5 when the Fe(II) concentration was higher than 5 mg/L because of the increased sorption of Fe(II) onto the chlorite surfaces. XANES (X-ray absorption near edge structure) spectra of the Se K-edge showed that most of the sorbed Se(IV) was reduced to Se(0) by Fe(II) sorbed onto the chlorite surfaces, especially at pH > 9. The combined results of field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) also showed that elemental selenium and goethite were formed and precipitated on the chlorite surfaces during the sorption of selenite. Consequently it can be concluded that Se(IV) can be reduced to Se(0) in the presence of Fe(II) ions by the surface catalytic oxidation of Fe(II) into Fe(III) and the formation of goethite at neutral and particularly alkaline conditions. Thus the mobility of selenite in groundwater is expected to be reduced by the presence of a relatively higher concentration of Fe(II) in subsurface environments.