Master Thesis

Polysulfides in geothermal waters of Yellowstone National Park

Axel Müller (08/2011-07/2013)

Support: Britta Planer-Friedrich, Regina Lohmayer

Polysulfides, Sn2-, are twofold negatively charged sulfur species that supposedly play an important role in the geochemical S-cycle of suboxic waters. Aim of this study was the comparison of polysulfide determination by HPLC-analysis after derivatization with methyltrifluoromethanesulfonate versus voltammetry for basic and acidic hot springs at Yellowstone National Park.

Laboratory pre-experiments revealed increasing temperature as accelerating factor for polysulfide formation from sulfide oxidation. The most abundant species was tetrasulfide. In the field, however, no correlation was found between total polysulfide concentration and temperature or pH. Disulfide was the most abundant species in all springs. Sampling along  the drainage channel showed no trend between exposure to the atmosphere and polysulfide speciation or total concentrations, which ranged from 3μM to 38μM with standard deviations between 17%and 103%, for individual chain lengths up to 200%.

Different voltammetric techniques were tested for their suitability for polysulfide detection in preexperiments. Cyclic voltammetry with a solid state gold-amalgam microelectrode calibrated by the pilot ion method was finally chosen for in-situ monitoring of sulfide, elemental sulfur, and polysulfides. Polysulfides were detected either by direct peak identification or indirectly by detection of sulfide originated from polysulfides decomposition after sample acidification. Voltammetry yielded polysulfide concentrations ranging from 370μM to 1450μM for direct, and 38μM to 560μM for indirect measurements. Large differences were observed in total concentrations and speciation for samples taken only minutes apart. Overall, the two methods did not show comparable results, but delivered for the first time qualitative evidence for the presence of polysulfides in basic and acidic hot springs.