Thiomolybdate occurrence at high temperatures –laboratory experiments and field studies at geothermal waters of Iceland
Friedrich Boeing (11/2014-12/2015)
Support: Britta Planer-Friedrich, Regina Lohmayer
Thiomolybdates were measured in the natural environment for the first time in geothermal waters in Yellowstone Nationalpark (YNP). However, the predominant species was molybdate. In contrast, thiomolybdates formed in laboratory experiments at 25 °C as the dominant species. Hence, the objective of the study was to test, whether temperature has an influence on the formation and stability of thiomolybdates. For this purpose, laboratory experiments were conducted with molybdate-sulfide solutions and tetrathiomolybdate (TetraTM) solutions at 24 °C, 50 °C, and 90 °C with pH 7 and 8.5. Moreover, geothermal waters were sampled in Iceland with different pH, temperature, sulfide concentrations and ionic strength. The laboratory experiments showed that at increased temperatures thiolation of molybdate was less complete and TetraTM transformed faster. During these transformation processes, intermediate thiomolybdates were formed more at increased temperatures over a limited time range. The results of the field samples showed that thiomolybdates reach shares of 80% in Icelandic geothermal waters. The average share of thiomolybdates in all Icelandic geothermal water samples was 29%. Intermediate thiomolybdates accounted for higher shares (MonoTM 6%, DiTM 9%, TriTM 10% on average) than the more stable TetraTM (4%). The comparison of the laboratory experiments with the results of the field sampling in Iceland suggests that temperatures around 90 °C and pH 8.5 promotes the occurrence of thiomolybdates in geothermal waters. The lower shares of intermediate thiomolybdates in the samples from YNP in comparison to the samples from Iceland may be connected to lower pH and higher ionic strength in YNP.