摘要:

IntroductionIn hydrothermal solutions, As and Sb are often present in appreciable concentration,1often occurring in association with Ag, Au and Hg, but the identities of the As and Sb species present are not well understood. In neutral to alkaline sulfidic waters at low temperature, thio- species are believed to predominate.2The speciation of Sb in sulfidic solutions has been studied for some time, but new results are still emerging. The main questions concern the oxidation state (iiiorv), the coordination number and the degree of oligomerization of the species. Typically Sb(iii) compounds, which essentially have a 5 s2lone pair orbital, will be trigonal three-coordinate, while Sb(v) compounds, without the lone pair, will be tetrahedral four-coordinate. By 1990 a consensus seemed to emerge that in alkaline sulfidic solutions Sb existed as Sb(iii), based on numerous solubility studies2–4and Raman studies.5However, recent EXAFS studies6,7have presented evidence for the presence of Sb(v) species in such solutions. The Sb–S distances determined by EXAFS were more consistent with those for model compounds with four-coordinate Sb(v) than for those with three-coordinate Sb(iii) and the coordination numbers from the model fits to the data were close to 4. Recently Helz and coworkers8reported the results of a solubility study for stibnite, Sb2S3, and elemental S in equilibrium with alkaline sulfidic solutions, which could be best interpreted in terms of a number of dimeric species, including the mixed Sb(iii,v) and the Sb(v,v) dimers, Sb2S52−and Sb2S62−, which were new species, not previously considered. They also presented visible-UV absorption spectra which showed a broad peak around 4.4 eV, consistent with the limited experimental data available on Sb(v) sulfides. In recent work we have calculated energetics9for the formation of such oxidized dimer species which are in good agreement with the experimental data of Helzet al.8We had previously calculated structures, energetics and spectra for various Sb(iii) monomers and oligomers,10assigning the spectra of Wood5to a more protonated Sb(iii) dimer than in the original work. At that time procedures recently developed to calculate pKas for such species11were not yet available. We also noted inref. 10that three-coordinate Sb(iii) and four-coordinate Sb(v) had very similar Sb–S stretching frequencies so that information in addition to the Raman spectra was necessary to exclude the presence of Sb(v) species in the solutions studied. Additional Raman spectral data has since been presented for the As–S system,12but the spectra seem so complex that assigning species based just on the Raman seems very difficult. Additional information has also recently become available from ion-exchange mass spectrometry, but only limited information on atom ratios can be obtained using this method.13Recently, UV spectroscopy has been used to study acid dissociation in solution, first for H2S14and then for As(OH)3.15Although the concentrations of the different species were determined primarily through changes in spectral intensities at energies lower than the absorption maximum (on the low energy side of the band), the maxima themselves were determined for As(OH)3and AsO(OH)2−, and showed a difference of around 0.5 eV. We calculated the UV absorption spectra for both bare gas-phase As(OH)3, its conjugate base and these same species microsolvated with water.16The calculations were also extended to some of the oligomers of As(OH)3and to related species derived from thioarsenious acid As(SH)3. The calculated energies were in good agreement with experiment and it was clear that both protonation state and degree of oligomerization had observable effects upon the spectrum. This indicated that visible-UV spectroscopy could be a useful new technique for assessing speciation in solutions containing metalloid sulfides, and perhaps for polysulfides as well. It therefore became important to establish whether visible-UV spectra could be accurately calculated for a range of such anionic sulfide species in aqueous solution and whether simple recognizable trends in spectral energy were present, which could be used to determine speciation. Although visible-UV spectra have not been extensively studied for As sulfides, the instruments needed to perform such measurements are readily available. The main impediment to such experimental studies is the paucity of studies calculating and interpreting such data using accurate quantum chemical techniques. This paper represents part of our effort to remedy this deficiency.A detailed knowledge of As and Sb speciation is important for a number of reasons. First, although several different speciation models may be able to explain a limited set of experimental solubility data, extrapolating into new regimes of concentration,PandTcan reveal significant differences in both species concentrations and total element concentrations. Second, thermodynamic models for mineral stability and solubility can only be accurately constructed from experimental data if correct speciations are known. Third, the speciation of an element also influences the interaction of that element with mineral surfaces. For example, to understand the well known association between Au and the As and Sb sulfides16it is important to understand both the speciation of As and Sb sulfides and the characteristics of the mineral surfaces. Helzet al.8have noted that theiii,vandv,vSb sulfide dimers they have characterized will be anionic, rather than neutral as for As hydroxides, and that they will interact unfavorably with mineral surfaces carrying negative charges, leading to desorption of Sb. This is important since Sb2S3in the presence of S is soluble enough to exceed the drinking water standards for total Sb concentration.As discussed in more detail inref. 10determination of speciation from solubility data alone is often ambiguous. Even if spectral data such as EXAFS or Raman is also available, determining speciation based on comparison with model compound data is often difficult. Using quantum mechanical methods we can evaluate a number of different properties, including structure, energy and spectral properties for a number of candidate species, and search for the best overall fit to the available experimental data. In this case our primary goal is to determine if the visible-UV spectra of polysulfide and metalloid sulfide species show changes with oxidation state, protonation state or degree and type of oligomerization which are diagnostic of their structure.

ISSN:1467-4866    

DOI:10.1039/b305086f

出版商:RSC    

年代:2003

数据来源: RSC