Comparison of nickel speciation in workplace aerosol samples using sequential extraction analysis and X-ray absorption near-edge structure spectroscopy.

There is a pressing need to further develop speciation knowledge of Ni workplace aerosols as the Zatka sequential extraction method used until now to speciate workplace Ni exposures has limitations. Here we compare the Zatka and XANES methods and evaluate XANES spectroscopy as a more appropriate and accurate technique for identifying nickel species in workplace aerosols. XANES spectroscopy is capable of identifying unique Ni species in the unaltered samples. Our findings indicate some significant departures in speciation assignment between the Zatka and XANES methods. In particular, the Zatka method can overestimate the soluble Ni fraction and it may underestimate the sulphidic and metallic fractions in some samples. Of particular importance, XANES is able to identify component sulphidic species. This information can lead to more accurate exposure matrices and more refined epidemiological analysis of respiratory cancer causation in sulphidic Ni processing.

Determination of nickel species in stack emissions from eight residual oil-fired utility steam-generating units.

XAFS spectroscopy has been used to determine the Ni species in particulate matter collected on quartz thimble filters in the stacks of eight residual (No. 6 fuel) oil-burning electric utility steam-generating units. Proper speciation of nickel in emitted particulate matter is necessary to correctly anticipate potential health risks. Analysis of the spectroscopic data using least-squares linear combination methods and a newly developed method specific for small quantities of Ni sulfide compounds in such emissions show that potentially carcinogenic Ni sulfide compounds are absent within the detection limits of the method (≤ 3% of the total Ni) in the particulate matter samples investigated. In addition to the major nickel sulfate phase (NiSO(4)·6H(2)O), lesser amounts of (Ni,Mg)O and/or NiFe(2)O(4) were also identified in most emission samples. On the basis of the results from these emission characterization studies, the appropriateness of the U.S. Environmental Protection Agency's assumption that the Ni compound mixture emitted from residual oil-fired power plants is 50% as carcinogenic as nickel subsulfide (Ni(3)S(2)) should be re-evaluated.

Effective diffusion coefficients for methanol in sulfuric acid solutions measured by Raman spectroscopy.

The diffusion of methanol into 0-96.5 wt % sulfuric acid solutions was followed using Raman spectroscopy. Because methanol reacts to form protonated methanol (CH 3OH 2 (+)) and methyl hydrogen sulfate in H 2SO 4 solutions, the reported diffusion coefficients, D, are effective diffusion coefficients that include all of the methyl species diffusing into H 2SO 4. The method was first verified by measuring D for methanol into water. The value obtained here, D = (1.4 +/- 0.6) x 10 (-5) cm (2)/s, agrees well with values found in the literature. The values of D in 39.2-96.5 wt % H 2SO 4 range from (0.11-0.3) x 10 (-5) cm (2)/s, with the maximum value of D occurring for 61.6 wt % H 2SO 4. The effective diffusion coefficients do not vary systematically with the viscosity of the solutions, suggesting that the speciation of both methanol and sulfuric acid may be important in determining these transport coefficients.

Uptake and surface reaction of methanol by sulfuric acid solutions investigated by vibrational sum frequency generation and Raman spectroscopies.

The uptake of methanol at the air-liquid interface of 0-96.5 wt % sulfuric acid (H2SO4) solutions has been observed directly using vibrational sum frequency generation (VSFG) spectroscopy. As the concentration of H2SO4 increases, the VSFG spectra reveal a surface reaction between methanol and H2SO4 to form methyl hydrogen sulfate. The surface is saturated with the methyl species after 15 min. The uptake of methyl species into the solutions by Raman spectroscopy was also observed and occurred on a much longer time scale. This suggests that uptake of methanol by sulfuric acid solutions is diffusion-limited.

Structure of butanol and hexanol at aqueous, ammonium bisulfate, and sulfuric acid solution surfaces investigated by vibrational sum frequency generation spectroscopy.

The organization of 1-butanol and 1-hexanol at the air-liquid interface of aqueous, aqueous ammonium bisulfate, and sulfuric acid solutions was investigated using vibrational broad bandwidth sum frequency generation spectroscopy. There is spectroscopic evidence supporting the formation of centrosymmetric structures at the surface of pure butanol and pure hexanol. At aqueous, ammonium bisulfate, and at most sulfuric acid solution surfaces, butanol molecules organize in all-trans conformations. This suggests that butanol self-aggregates. The spectrum for the 0.052 M butanol in 59.5 wt % sulfuric acid solution is different from the other butanol solution spectra, that is, the surface butanol molecules are observed to possess a significant number of gauche defects. Relative to surface butanol, surface hexanol chains are more disordered at the surface of their respective solutions. Statistically, an increase in the number of gauche defects is expected for hexanol relative to butanol, a six carbon chain vs a four carbon chain. Yet, self-aggregation of hexanol at its aqueous solution surfaces is not ruled out because the methylene spectral contribution is relatively small. The surface spectra for butanol and hexanol also show evidence for salting out from the ammonium bisulfate solutions.