Childhood leukemia clusters in e.g. Fallon, NV have been tentatively linked to the proximity to tungsten (W)-bearing ore deposits and ore-processing operations. Because residents of Fallon were shown to have high body-burdens of W in their systems, it has been suggested that W may be responsible for the high incidence of childhood leukemia. Studies have since shown that W can be toxic and may be carcinogenic. The need to understand W in the environment is also important due to its increasing use as a replacement for lead in ammunition and in fishing weights. Here, the use of W was originally thought to be a non-toxic, inert metal of low environmental mobility, it was thus a manner in which to limit the addition of toxic Pb to the environment. However, W is readily mobilized in the environment following oxidation and very little is actually known about its biogeochemistry in the environment, and in particular, its mobility and transport in real groundwater flow systems.
This project will investigate the biogeochemistry of W reaction and transport in the environment. Specifically, the project will evaluate how W concentrations evolve along groundwater flow paths as biogeochemical reactions between groundwaters, aquifer minerals, organic matter, and in situ microbial communities, modify the groundwater solution composition and redox conditions. To conduct the study, the project will: 1) measure W concentrations along flow paths in well characterized aquifers along with redox sensitive parameters [e.g., Fe species, S(-II)], and other geochemical constituents to determine how changing solution composition and redox conditions affect W in aquifers; 2) examine solid-phase W speciation in aquifer sediments (e.g., XANES, EXAFS, sequential extractions); 3) measure stability constants for thiotungstate complexes in sulfidic aqueous solutions to develop a solution complexation model that will allow prediction of W speciation in aerobic and anaerobic groundwaters; and 4) assemble a conceptual, biogeochemical model that incorporates the stability constants for thiotungstate complexes along with the currently available thermodynamic data for the tungstate oxyanion to probe the biogeochemical behavior of W along groundwater flow paths. The project focus on pristine aquifers that are important drinking water sources to investigate the natural geochemical cycling of W in aquifers. The resulting 'baseline' data and conceptual model will provide an important resource for other investigators studying the effects of anthropogenic sourced W in the environment.
The project produced a number of significant results. First, the study confirmed earlier investigations that tungsten is relatively mobile in oxic natural waters as the tungstate oxyanion, and that the mobility of tungstate is also a function of pH and the amount of available Fe(III)/Mn(IV) oxides/oxyhydroxides in aquifer sediments. Specifically, aquifers with groundwater with pH > 8 and low contents of Fe(III)/Mn(IV) oxides/oxyhydroxides in the aquifer sediments both favor tungstate mobilization and transport in groundwater flow systems (Johannesson et al., 2013). Moreover, our work in groundwaters from the Bengal Basin in West Bengal, India strongly suggests that although W shares some geochemical similarities to arsenic (As), such as mobilization to groundwaters during reductive dissolution of Fe(III)/Mn(IV) oxides/oxyhydroxides within aquifer sediments, W probably has other sources in aquifer sediments that likely include clay minerals and/or apatite (Mohajerin et al., 2014a). The project also successfully allowed for the first determination of all four equilibrium constants that describe the step-wise thiolation of the tungstate oxyanion to the tetrathiotungstate anion (Mohajerin et al., 2014b). This work revealed that thiotungstate anions form in sulfidic natural waters, including anoxic groundwaters, where sulfate reduction is the chief redox reaction buffering the redox conditions. Furthermore, we showed that the thiolation reactions are first order, acid catalyzed, and that the empirical rate constants demonstrate the formation of thiotungstate is kinetically more "sluggish" than the analogous formation of thiomolybdates. The combination of our thermodynamic and kinetic investigations indicate that thiotungstate anions form in sulfidic waters, but generally require very high dissolved sulfide concentrations for the thiolation reaction (tungstate to tetrathiotungstate) to reach completion. Therefore, in sulfidic waters where molybdate is completely thiolated to tetrathiomolybdate [i.e., S(-II) ~ 11 uM], intermediate thiotungstate anions (dithiothungstate, trithiotungstate) will predominate (Mohajerin et al., 2014b). The project has also demonstrated that the tungstate oxyanion is more particle reactive than the molybdate oxyanion in oxic natural waters, whereas thiotungstate anions appear to be less particle reactive than the corresponding thiomolydate anions. This result has broad consequences regarding our understanding of the mobility of tungsten and molybdenum in the environment, including groundwaters, but also has implications towards the biochemistry of these two transition metals. For example, molybdoenzymes are common in many organisms and catalyze numerous important biochemical and metabolic processes. Analogous tungstoenzymes, although they do exist, are comparatively rare and found in a limited number of organisms, most of which are extremophiles (e.g., hyperthermophiles) and/or members of the Archaea (e.g., methanogens) domain of life. Questions that had dogged researchers now for decades regarding tungstoenzymes and molybdoenzymes include: 1) why are tungstoenzymes rare, whereas molybdoenzymes are ubiquitous; and 2) which came first during the evolution of life. More specifically, do tungstoenzymes represent an ancient strategy for early life that evolved when conditions on the Earth were dominated by low, or no, free oxygen (e.g., early in the Archean before the much later "Great Oxidation Event"), when bioavailable molybdenum in aqueous solutions would have been very low owing to thiolation of molybdate to particle reactive tetrathiomolybdate, or are thiotungstates a more modern development? We submit that our work on tungsten as part of this NSF award can provide some critical evidence towards answering this question. Briefly, as our work suggests that thiotungstate anions are more "soluble" than thiomolybdate anions in anoxic, sulfidic waters, it seems likely that tungsten would have been more available as a dissolved constituent for earlier microbes to incorporate and, hence, use for synthesizing enzymes to help catalyze metabolic processes necessary for life. After the "Great Oxidation Event", tungsten concentrations in the Proterozoic ocean would have decreased and the molybdenum concentrations would have increased in solution as the corresponding thioanions disappeared from solution in favor of the oxyanions, which exhibit the opposite sense of particle reactively from the thioanions. Future work that I hope to conduct will be directed towards investigating tungsten in highly sulfidic natural waters. Johannesson K. H., Dave H. B., Mohajerin T. J., and Datta S. (2013) Controls on tungsten concentrations in groundwater flow systems: The role of adsorption, aquifer sediment Fe(III) oxide/oxyhydroxide content, and thiotungstate formation. Chemical Geology 351, 76-94. Mohajerin T. J., Neal A. W., Telfeyan K., Sasihharan S. M., Ford S., Yang N., Chevis D. A., Grimm D. A., Datta S., White C. D., and Johannesson K. H. (2014a) Geochemistry of tungsten and arsenic in aquifer systems: A comparative study of groundwaters from West Bengal, India and Nevada, USA. Water, Air, and Soil Pollution 225: 1792, doi: 10.1007/s11270-013-1792-x. Mohajerin T. J., Helz G. R., White C. D., and Johannesson K. H. (2014b) Tungsten speciation in sulfidic waters: Determination of thiotungstate formation constants and modeling their distribution in natural waters. Geochimica et Cosmochimica Acta 144, 157-172.
