Clayton T. Wagner, Megan M. Maloney, Redeat Dadi , and Trent P. Vorlicek

1. Verifying the Speciation of Molybdenum in Sulfidic and PolysulfidicNatural Waters Using Ion Chromatography with Suppressed Conductivity Detection Clayton T. Wagner, Megan M. Maloney, RedeatDadi, and Trent P. Vorlicek Department of Chemistry and Geology, Minnesota State University Mankato Abstract Areas of Novel Inquiry Anoxia: Making a Dead Zone Within sulfidic waters, Mo speciation is characterized by the formation of thiomolybdates (MoOnS4-n2- n=1-4). Using thermodynamic constants, Mo speciation in sulfidic basins has been calculated. However, actual speciation in natural waters has not been verified because a suitable analytical method remains elusive. Zero-valent sulfur has been shown to influence Mo speciation and sequestration by pyrite via formation of anionic Mo-polysulfido complex(es). Unfortunately, optical means were not able to identify the complex(es) definitively. We have demonstrated that ion chromatography with suppressed conductivity detection gives linear response down to at least 10-7M MoO42-. IC methods are developed to quantify contrived mixtures of MoO42- and thiomolybdates at SMo=10-7 in simulated seawater. IC-MS is used to identify and quantify Mo-polysulfido complex(es) formed in polysulfidic test solutions; kinetic and equilibrium constants will be calculated. This research will better define Mo speciation and its relation to Mo fixation in anoxic waters. A.) Chromatographic Separation Proxies of Anoxia Figure 4: Hypothetical ion chromatogram for a solution of thiomolybdates. Figure 5: Series of chromatograms (left panel) for 25 μL injections of deionized water and 0.1 μM to 10 μM MoO42- solutions prepared in deionized water. The chromatograms indicate that MoO42- elutes with a retention time of 4.5 +/- 0.1 min. The right panel displays the resulting calibration curve for MoO42- derived from MoO42- peak areas of the chromatographs shown in the left panel. • Because Mo exists as an anion in solution, ion chromatography (IC) provides a plausible means for separating these species. • Optical detection has been used2,16 extensively for studying Mo sulfide reactions. However, conductivity detection should provide lower detection limits, allowing us to study reactions at reduced concentrations (<10-5 M). Chromatographic data will be used to quantify thiomolybdate stability constants and lay the foundation for mass spectroscopy (MS) work with stable isotopes. • IC chromatograms of MoO42- have demonstrated linearity down to 10-7 M (See Fig. 5) with 25 mL injections. • Unfortunately, further inquiry has been impossible owing to the IC being down for several months. • Similar work with ReO4- suggests linearity can be achieved down to ~10-8 M using IC. Injection loops as large as 1000 μL are available to further enhance sensitivity. Figure 24: Sediment pore water concentrations of O2, Re, and Mo in the sediment of Buzzards Bay off the coast of Massachusetts during August 2003. Sediments often become anoxic because mixing with the overlying water column is severely limited (See Fig. 1 for a demonstration of water column anoxia.). Here, anoxia persists at >0.3 cm in sediment depth. Below this depth, H2S(aq) concentration rises. Dashed vertical lines indicate the constant concentration of Re and Mo in the overlying oxic water column. Note that Re and Mo removal occurs immediately below the sediment-water interface. These elements are now “fixed” in the solid-phase sediment record. By quantifying the solid-phase concentrations of these elements in dated sediments, the history of anoxia in the recent1 and distant5,6 past may be reconstructed. Figure created using published data4. Figure 1: Cross section of an anoxic coastal basin. The “Dead Zone” is a prime example of an estuarine system experiencing anoxia. This region, aptly named because of the nefarious impacts of anoxia, extends from the mouth of the Mississippi River into the Gulf of Mexico. Here, warm, buoyant, and nutrient-rich fresh water meets cold, dense seawater. As the summer sun heats the surface water, large algal blooms occur and intensify the organic matter flux to the deep water. A combination of reduced freshwater inputs and density gradients limits mixing between surface and deep waters. Because of the intense organic matter flux, the biological oxygen demand (BOD) outweighs the resupply of O2(g) to the deep waters; anoxia ensues, and H2S(aq) concentrations rise. Many coastal systems experience anoxia1,4. Figure prepared with the help of Elise Chandler Vorlicek. Geochemistry of Mo • In oxic seawater, Mo is present as geochemically inert, MoO42-, with a constant concentration at ~10-7 M. Under anoxic conditions, Mo converts to the thiomolybdates2 (MoO3S2-, MoO2S22-, MoOS32-, and MoS42-); all anions maintain the MoVI oxidation state: • MoVIO42-(aq) + nH2S(aq) ⇌ MoVIO4-nSn2-(aq) + nH2O(l) • Because Mo is only removed under anoxic and sulfidic conditions (See Fig. 2.), the thiomolybdates likely lie in the pathway to Mo fixation. • X-ray absorption data1 indicates that Mo may be fixed by adhering to iron sulfide7,8 (e.g., FeS2(s)) particles as extremely stable Fe-Mo-S cubane clusters. • Water column data from sulfidic basins suggeststhat Mo removal involves precipitation of an Fe-Mo-S mineral phase (possibly, Fe5Mo3S14) 11. • Evidencesuggests that S0 (present as the polysulfides, S42- and S52-) may play a role in Mo speciation9,10 and fixation9 through formation of reduced MoIV or V-polysulfido species (e.g., MoIVS52-) (See Fig. 6.). B.) Influence of S0 on Mo Speciation • Near the anoxic-oxic boundary, S0 will be present17 via oxidation of H2S or incomplete reduction of SO42-. S0 can react further to form polysulfides (Sn2- ): • (n-1)S0(s) + HS-(aq) ⇌ Sn2-(aq) + H+(aq) • S0 has been shown9to affect Mo speciation and promote Mo uptake by pyrite (see Fig. 6). Figure 69: Uptake of Mo by pyrite for solutions containing initially MoOS32- and relatively high or low Sn2- concentrations at pH 8.5. Figure reproduced9. Research Questions • Experiments in this area will involve Mo reactions in the absence and presence of polysulfides. IC-MS will ultimately be used to identify and quantify Mo-polysulfido complex(es) responsible for the enhanced uptake by pyrite (see Fig. 6). • IC-MS will be used to quantify MoO42-, MoO3S2-, MoO2S22-, MoOS32-, and MoS42- down to ~10-9 M ΣMo in synthetic Sea H2O. 1.) Can the thiomolybdates be separated chromatographically with non-optical detection? 2.) Can thiomolybdates be quantified in natural water samples using IC? 3.) How does S0 affect the speciation of Mo, and what role do these S0-containing species play in fixation? Figure 3: World map of coastal dead zones1. Global climate change is expected to promote and intensify coastal anoxic events. Certain trace metals (e.g., Mo, Re, V, U) may serve as proxies of geohistorical anoxia. Defining the geochemical behavior of Mo and Re is important for improving the utility of these chemical tools. Geohistorical information gleaned from these proxies could prove critical toward developing strategies for the mitigation of or adaptation to the consequences of climate change upon aquatic environments. (Figure taken from www.coastalcare.org)