Hydrous ferric oxide: evaluation of Cd-HFO surface complexation models combining Cd(K) EXAFS data,potentiometric titration results,and surface site structures identified from mineralogical knowledge

The surface properties of ferrihydrite were studied by combining wet chemical data, Cd(K) EXAFS data, and a surface structure and protonation model of the ferrihydrite surface. Acid-base titration experiments and Cd(II)-ferrihydrite sorption experiments were performed within 3<-log[H(+)]<10.5 and 0.5<[Cd(t)]<12 mM in 0.3 M NaClO(4) at 25 degrees C, where [Cd(t)] refers to total Cd concentration. Measurements at -5.5triple bond Fe-OH(-1/2),logk((int))=-8.29, assuming the existence of a unique intrinsic microscopic constant, logk((int)), and consequently the existence of a single significant type of acid-base reactive functional groups. The surface structure model indicates that these groups are terminal water groups. The Cd(II) data were modeled assuming the existence of a single reactive site. The model fits the data set at low Cd(II) concentration and up to 50% surface coverage. At high coverage more Cd(II) ions than predicted are adsorbed, which is indicative of the existence of a second type of site of lower affinity. This agrees with the surface structure and protonation model developed, which indicates comparable concentrations of high- and low-affinity sites. The model further shows that for each class of low- and high-affinity sites there exists a variety of corresponding Cd surface complex structure, depending on the model crystal faces on which the complexes develop. Generally, high-affinity surface structures have surface coordinations of 3 and 4, as compared to 1 and 2 for low-affinity surface structures.     

Thermo-cleavable polymers: Materials with enhanced photochemical stability

Photochemical stability of three thermo-cleavable polymers was investigated as thin films under atmospheric conditions. A significant increase in lifetime was observed once the side-chain was cleaved emphasizing the detrimental effect of solubilizing groups on the photochemical stability of conjugated polymers. In addition to their ease of processing, thermo-cleavable polymers thus also offer a greater intrinsic stability under illumination.     

Optically implemented broadband blueshift switch in the terahertz regime

We experimentally demonstrate, for the first time, an optically implemented blueshift tunable metamaterial in the terahertz (THz) regime. The design implies two potential resonance states, and the photoconductive semiconductor (silicon) settled in the critical region plays the role of intermediary for switching the resonator from mode 1 to mode 2. The observed tuning range of the fabricated device is as high as 26% (from 0.76 THz to 0.96 THz) through optical control to silicon. The realization of broadband blueshift tunable metamaterial offers opportunities for achieving switchable metamaterials with simultaneous redshift and blueshift tunability and cascade tunable devices. Our experimental approach is compatible with semiconductor technologies and can be used for other applications in the THz regime.     

Evidence for the Formation of Trioctahedral Clay upon Sorption of Co(2+) on Quartz

The sorption mechanism of Co on quartz at room temperature has been investigated by an in-depth analysis of published extended X-ray absorption fine structure (EXAFS) spectroscopy and solution chemistry data. In particular, the 3.5-5 ? mid-range atomic environment of Co has been determined with unprecedented precision by combining ad initio FEFF7.02 calculations and results obtained by polarized EXAFS on the mid-distance structure of sheet silicate minerals. The local atomic environment around sorbed Co atoms is identical to that of Co in trioctahedral clays and substantially different from that in the cobalt hydroxide Co(OH)(2(s)). Neoformation of a trioctahedral clay is consistent with calculated thermodynamic solubilities, which indicate that 2:1 and 1:1 Co-rich hydrous silicates, similar to kerolite and chrysotile, are less soluble than Co(OH)(2(s)). Consequently, precipitation of Co-rich clay is favored over that of Co(OH)(2(s)) at pH values below 9 and for a dissolved Si concentration equal to quartz solubility. New experimental data show that dissolved Si concentrations can approach, and even exceed, that of quartz solubility during the short times of sorption experiments. Based on the available data, it is not possible to conclude unequivocally if the Co layer silicate grew epitaxially on the quartz surface, topotactically in a surface amorphous layer, or independently of the quartz framework structure. The structural and chemical interpretation is supported by recent published studies in which sorption of a hydrolyzable cation leads to the neoformation of a mixed layer phase formed from the sorbate species and the sorbent metal. This surface-induced precipitation mechanism is a general phenomenon that may account for the formation of secondary clays as coatings on silicates. Copyright 1999 Academic Press.     

Sorption of Metal Ions on Clay Minerals

The mechanism of Co uptake from aqueous solution onto hectorite (a magnesian smectite) and its impact on the stability of this clay mineral were investigated as a function of Co concentration (TotCo = 20 to 200 μM, 0.3 M NaNO(3)) and ionic strength (0.3 and 0.01 M NaNO(3), TotCo = 100 μM) by combining kinetics measurements and Co K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The morphology of the sorbent phase was characterized by atomic force microscopy (AFM) and consists of lath-type particles bounded by large basal planes and layer edges. At low ionic strength (0.01 M NaNO(3)), important Co uptake occurred within the first 5 min of reaction, consistent with Co adsorption on exchange sites of hectorite basal planes. Thereafter, the sorption rate dramatically decreased. In contrast, at high ionic strength (0.3 M NaNO(3)), Co uptake rate was much slower within the first 5 min and afterward higher than at 0.01 M NaNO(3), consistent with Co adsorption on specific surface sites located on the edges of hectorite. Time-dependent isotherms for Co uptake at high ionic strength indicated the existence of several sorption mechanisms having distinct equilibration times. The dissolution of hectorite was monitored before and after Co addition. A congruent dissolution regime was observed prior to Co addition. Just after Co addition, an excess release of Mg relatively to congruent dissolution rates occurred at both high and low ionic strengths. At high ionic strength, this excess release nearly equaled the amount of sorbed Co. The dissolution rate of hectorite then decreased at longer Co sorption times. EXAFS spectra of hectorite reacted with Co at high and low ionic strengths and for reaction times longer than 6 h, exhibited similar features, suggesting that the local structural environments of Co atoms are similar. Spectral simulations revealed the occurrence of approximately 2 Mg and approximately 2 Si neighboring cations at interatomic distances characteristic of edge-sharing linkages between Co and Mg octahedra and corner-sharing linkages between Co octahedra and Si tetrahedra, respectively. This local structure is characteristic of inner sphere mononuclear surface complexes at layer edges of hectorite platelets. The occurrence of these complexes even at low ionic strength apparently conflicts with kinetics results, as exchangeable divalent cations are known to form outer sphere surface complexes. To clarify this issue, the amount of Co adsorbed on exchange sites was calculated from the solute Co concentration, assuming that cation exchange was always at equilibrium. These calculations showed that sorbed Co was transferred within 48 h from exchange sites to edge sorption sites. Copyright 1999 Academic Press.     

Terahertz time domain spectroscopy for the analysis of cultural heritage related materials

A powerful broadband femtosecond laser-driven terahertz source was built and used to analyze cultural heritage and art-related materials in the terahertz region of the electromagnetic spectrum. The high signal-to-noise ratio available in our system revealed details in the absorption spectra which are inaccessible with traditional FTIR spectrometers. Clear spectral signatures were recorded from organic binding media thus demonstrating the potential of the approach for applications in the monitoring and analysis of cultural heritage materials. PACS 42.65.Tg; 87.50.U-; 87.64.K-     

Sensitivity of a Lamb wave sensor with 2 microm AlN membrane

Anti-symmetrical Lamb wave mode A0 presents a large sensitivity to mass loading and can be used in contact with liquids with a small attenuation. The advantages of this system are the possibility to get a large mass sensitivity. The sensitivity increases when the thickness of membrane decreases. Therefore the problem is to obtain thin piezoelectric membranes. A membrane of AlN with a thickness of 2 microm has been made. The measured mass sensitivity with a fluid is 200 cm(2) g(-1). In a practical use point of view, the problem in this kind of sensor is its temperature sensitivity. In order to reduce effective temperature sensitivity, a device with thin metallic strips is presented. On the same membrane two different waves with perpendicular propagating directions are produced. Experimentally, temperature sensitivity is rather different depending on the propagation direction but mass sensitivity is almost the same, this allows distinguishing temperature effects from those due to mass loading on the frequency shift measurements.     

Stathmin interaction with HSC70 family proteins

Stathmin is a ubiquitous cytosolic phosphoprotein participating in the relay and integration of diverse intracellular signaling pathways involved in the control of cell proliferation, differentiation, and activities. It is phosphorylated in response to diverse extracellular signals including hormones and growth factors, and it is highly expressed during development and in diverse tumoral cells and tissues. Stathmin interacts with tubulin and other potential protein partners such as BiP, KIS, CC1 and CC2/tsg101. In our present search for further functional partners of stathmin, we identified proteins in the Hsp70 family, and in particular Hsc70, as interacting with stathmin in vitro. Hsc70 is among the proteins coimmunoprecipitated with stathmin, and it is the main protein retained specifically on stathmin-Sepharose beads identified by one- and two-dimensional electrophoresis and immunoblots. Bovine serum albumin (BSA)-Sepharose did not bind Hsc70, and anti-stathmin antisera specifically inhibited the interaction of Hsc70 with stathmin-Sepharose. The binding of Hsc70 to stathmin is dependent on the phosphorylation status of stathmin, as it did not occur with a "pseudophosphorylated" mutant form of stathmin. This interaction is further dependent on the ATP status of Hsc70. It was inhibited in the presence of ATP-Mg++ but not in the presence of ATP-Mg++ and ethylenediaminetetraacetic acid (EDTA) or of ADP. Our results suggest that the interaction of stathmin with Hsc70 is specific in both proteins and most likely biologically relevant in the context of their functional implication in the control of numerous intracellular signaling and regulatory pathways, and hence of normal cell growth and differentiation.     

Influence of Anionic Layer Structure of Fe-Oxyhydroxides on the Structure of Cd Surface Complexes

The nature of crystallographic reactive sites on the lepidocrocite (gammaFeOOH) surface has been determined by atomic force microscopy (AFM) and extended X-ray absorption fine structure (EXAFS) spectroscopy and compared to the surface bonding properties of goethite. To this end, the specific surface areas of lepidocrocite particles, and of their crystal faces, were calculated from the size and shape of individual particles determined by AFM, and the structure of Cd surface complexes was determined from Cd-Fe EXAFS distances. The combined results show that Cd forms solely mononuclear surface complexes, even at 100% surface coverage, and that hydrated Cd octahedra sorb on basal {010} and lateral {hk0}, {h0l} faces of lepidocrocite platelets by sharing edges with surface Fe octahedra. The absence, or scarcity, of corner-sharing linkage between Fe and Cd octahedra on the surface of lepidocrocite is in contrast to goethite (alphaFeOOH), where this type of complex is predominant. The explanation for the observed difference of Cd sorption mechanism on these two polymorphs lies not in the shape and relative surface area of their crystallographic faces, but in their different bulk structures and, specifically, in the stacking mode of anion layers (O(2-), OH(-)) which is hexagonal in alphaFeOOH and cubic in gammaFeOOH. This study demonstrates that the stacking mode of anions in the sorbent solid is a key factor in determining the structure of surface complexes on mineral surfaces. Copyright 2000 Academic Press.