Sorption sample

Results and discussion. Uranyl colloids are formed in the blank and sorption samples at... 

FIGURE 8.5 Normalized Zn-EXAFS k3-weighted chi spectra of reference materials and sorption samples used as empirical models for linear combination fitting. (Reprinted with permission from Roberts, D.R. et al., Environ. Sci. Technol., 36, 1742, copyright 2002. American Chemical Society.)... 

EXAFS Parameters for Zn Reference Minerals and Sorption Samples... 

Figure 4. Normalized, kl-weighted As-EXAFS spectra (a) and uncorrected Fourier transforms (FTs) (b) of scorodite (a crystalline ferric arsenate), an x-ray amorphous analog, and As(V) sorbed to amorphous hydrous ferric oxide (HFO), The EXAFS spectra can clearly be used to distinguish the coordination environment of arsenic in each of the materials. The highly symmetric local environment of arsenic in scorodite is shown in (c) each arsenic is surrounded by 4 Fe neighbors at a distance of 3.34 A. (see Table 2 for details of precipitate fits, and Table 3 for details of sorption sample fit). Reprinted from Foster (1999). The arrow highlights the region of particular difference among the three spectra. Peak positions in FTs are not corrected for phase-shift effects, and are therefore approximately 0.5 A shorter than the true distance.

Sorption samples were prepared for XAS using the same procedures described above for generating the pH edges and isotherm data. All samples were equilibrated for a minimum of 24 h prior to pH measurement and centrifugation. An aliquot of supernatant was then removed and stored in acid prior to subsequent analysis for metal ion content. Most of the remaining supernatant solution was then removed and the remaining wet pastes were placed into aluminum sample holders, sealed with mylar or Kapton tape windows for XAS analysis. 

Fig -8. Co XAS results for sorption to a-Al203 (A) background subtracted k3 Co(II) EXAFS spectra as a function of surface coverage, 7, (B) fast Fourier transformed radial structure functions of Co(II) EXAFS, uncorrected for phase shift. Uncorrected peaks at approximately 2600 and 5500 pm in the sorption samples are primarily due to Co-Co second shell and Co-Co fourth-shell interactions, respectively (after Hayes Katz, 1996). 

Fig. 7-13. Example fits of Sr(II) EXAFS data. Thick lines are the data and fine lines itv for all of the sorption samples.

Figure 23. Fc sorption on synthetic single quartz epi quality surfaces. GI-EXAFS Fourier transform functions. Bottom two functions show effect of sorption from silica-saturated solution, hor 90 refers to electric vector in the plane of the quartz surface at right angles to the mirror plane, vert is e-vector perpendicular to the surface. Middle two functions contrast analogous experiment done without silica saturation. Top two functions show the effect of diying (i.e., ex situ experiment) on the unsaturated sorption sample. The diy and washed ex situ sample has been cleaned with a high pressure jet of DI water to remove surface precipitates. It s function is consistent with soibed complexes with little precipitate signature.

Figure 1. Summary of the procedures used to prepare powder (left) and single crystal (right) sorption samples for spectroscopic analysis.

These results have important implications for surface complexation and reactive transport models as co-precipitation of nanoscale multicomponent phases may be a significant mode of sorption, particularly under conditions where the concentration of metal ions in the aqueous solution is fairly high. Such phases also provide new surfaces on which further sorption of aqueous ions can occur. However, detection of these phases and distinguishing them from the sorbate metal hydroxide phase requires very careful EXAFS analysis and HRTEM studies of sorption samples. 

Figure 9 illustrates radial structure fimetions (RSFs) produced by forward Fourier transforms of the XAFS spectra represented in Figure 8 (76). The spectra were uncorrected for phase shift. All spectra showed a peak of R 1.8A, which represents the first coordination shell of Ni. A second peak representing the second Ni shell can be observed at R 2.8A in the spectra of the Ni sorption samples and takovite (Figure 9). These spectra also showed peaks beyond the second shell at R == 5-6A (Figure 9) these peaks resulted from multiple scattering among Ni atoms (72).

The structural parameters derived from XAFS analysis are summarized in Table II (76). Least-square fits of filtered XAFS for the first RSF peak reveal that in the first coordination shell Ni is surrounded by six O atoms. This behavior indicates that Ni(II) is in an octahedral environment. The Ni-0 distances for the Ni sorption samples are 2.02-2.03A and are similar to those in takovite (2.03A). The Ni-0 distances in crystalline Ni(OH)2(s) are distinctly longer (2.06A). 

Nickel sorption on pyrophyllite, kaolinite, gibbsite, and montmorillonite at pH 7.5 results in formation of Ni-nucleation products from solutions which are undersaturated with respect to the thermodynamic solubility product of Ni(OH)2(s). An important finding of the study of Scheidegger et al. (16) is that the structural environment of Ni in all Ni sorption samples is similar. There is also an obvious similarity among the spectra of the Ni sorption samples and the spectrum of takovite, suggesting the presence of Ni phases of similar structure (Table II). 

Continuous flow systems for the automated pretreatment of biological fluids for GC-MS of drugs have seemingly been developed by the authors group only. A CFS relying on SPE constitutes the most simple, robust, cheap, and fruitful approach in this context. Sample, reagents, and eluents are introduced into a continuous module furnished with a sorbent column located in the loop of an injection valve. The CFS operation comprises three steps sorption (sample introduction), elution, and derivatization—the last two can occur simultaneously, however. The final extract obtained can be introduced into the GC-MS (Fig. 4). 

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