Thioantimonates in geothermal waters

The formation of aqueous antimony sulfide complexes upon dissolution of stibnite (Sb2S3) and their importance for geothermal antimony transport has often been stressed. All the more surprising, up to date only laboratory studies and theoretical calculations support the existence of these thioantimony species1-5. We successfully applied alkaline chromatographic separation and detection by inductively coupled plasma mass spectrometry (AEC-ICP-MS) previously used for thioarsenates6, for the determination of two antimony-sulfur species in synthetic solutions and natural geothermal waters. Based on their S/Sb ratios of 3.08 ± 0.28 and 4.05 ± 0.32 they were provisionally assigned as tri- and tetrathioantimonate. Using X-ray absorption spectroscopy (XAS), the identity of tetrathioantimonate was confirmed based on shell fits by about 4 Sb-S paths (CN 4.2-4.3) and the characteristic pentavalent Sb-S binding length of 2.33-2.34 Å. Aqueous trithioantimonate concentrations were too low for structural characterization.
XAS analyses further confirmed that the initial species formed from antimonite in the presence of excess sulfide under anoxic conditions is not a pentavalent thioantimonate, but the trivalent trithioantimonite (CN 3.4-3.7, binding length 2.40-2.41 Å). However, this species is highly instable and rapidly transforms either to tetrathioantimonate in the presence of oxygen or antimonite at excess OH- versus SH- concentrations. Thioantimonites thus escape chromatographic detection even in complete absence of oxygen.
In natural geothermal waters from Yellowstone National Park, where oxygen concentrations > 0.2 mg/L render the presence of thioantimonites highly unlikely, tri- and tetrathioantimonate were detected. In accordance with our own laboratory studies and previous observations1-5 their share increased at increasingly alkaline pH and with increasing sulfide and decreasing oxygen concentrations to a maximum of 30 and 9% of total antimony, respectively. However, given the large S/Sb ratio (100 to 10,000) almost quantitative transformation of antimony to thioantimonates would have been expected based on results in synthetic pure antimony solutions. We postulate that the presence of arsenic and direct competition for a limited source of sulfide affects thioantimonate formation in natural waters. In the same samples, thioarsenate formation at S/As ratios of 2 to 4 is much higher (> 80% of total arsenic) and corresponds to results from synthetic pure arsenic solutions. Sulfur might therefore be a key species in helping to resolve different results and an ongoing controversy on similar 7, 8 or dissimilar 9, 10 behavior of arsenic and antimony in the environment.

References
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