Formation and stability of thiomolybdates - laboratory experiments and investigations at Lake Rogoznica, Croatia
Gloria Reithmaier (11/2013-08/2014)
Support: Britta Planer-Friedrich, Regina Lohmayer
Molybdenum serves as a sediment marker for anoxic paleo environments, because it precipitates under anoxic conditions as molybdenum sulfide. Recent studies predict, largely based on modeling, that formation of aqueous thiomolybdates, specifically tetrathiomolybdate, preceeds molybdenum sulfide precipitation. However, the underlying mechanisms are not completely understood. The aim of this study was to investigate natural thiomolybdate occurrence in the euxinic Lake Rogoznica, Croatia, and to conduct formation and stability experiments under controlled laboratory conditions. The laboratory experiments with analysis by HPLC-UV showed that tetrathiomolybdate could be fully preserved by flash-freezing. Surprisingly, tetrathiomolybdate was significantly less stable in the dark than under light conditions. The formation of thiomolybdates from molybdate and sulfide increased with increasing ionic strength and ammonium concentration or decreasing pH and in the absence of light. Formation kinetics were faster with increasing sulfide excess. Contrary to previous reports on the almost exclusive occurrence of tetrathiomolybdate, simultaneous occurrence of mono-, di-, tri- and tetrathiomolybdate was observed in synthetic mixtures over a period of three weeks. The simultaneous occurrence suggests that thiomolybdate intermediates are at least meta-stable and thus also increase aqueous molybdate sulfide concentrations. At Lake Rogoznica, samples were taken for thiomolybdate analysis by IC-ICP-MS but unfortunately could not be analyzed as their concentrations turned out to be too low and salt water matrix effects too high for the current analytical method. Nevertheless, all results of the laboratory experiments indicate that the presence of thiomolybdates in the lake is highly likely as natural conditions are found to be similar to those set for the laboratory experiments. In particular, all thiomolybdate intermediates may exist simultaneously near the chemocline, where sulfide concentrations are low and temporal oxygen supply leads to hydrolysis. Further improvements of the analytical methods are currently underway to confirm the hypothesis of the presence of thiomolybdates in the Lake Rogoznica.