Geochemistry of trace metals associated with reduced sulfur in freshwater sediments |
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| Institution: | 1. College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97330, USA;2. College of Agricultural and Environmental Sciences, The University of Georgia, Athens, GA 30602, USA;3. Department of Geography, University of California, Santa Barbara, CA 93106, USA;4. Department of Crop and Soil Sciences, Oregon State University, Corvallis, OR 97330, USA;1. Instituto de Investigaciones Oceanológicas, Universidad Autónoma de Baja California, Campus Punta Morro, Km. 103 Carr. Tijuana-Ensenada, Ensenada, Baja California CP 22860, Mexico;2. Posgrado en Oceaografía Costera, Facultad de Ciencias Marinas/Instituto de Investigaciones Oceanológicas, Universidad Autónoma de Baja California, Campus Punta Morro, Km. 103 Carr. Tijuana-Ensenada, Ensenada, Baja California CP 22860, Mexico;3. Departamento de Edafoloxía e Química Agícola, Facultade de Bioloxía, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain;1. GeoZentrum Nordbayern, Universität Erlangen-Nürnberg, Erlangen 91054, Germany;2. Department of Geology, University of Leicester, Leicester LE1 7RH, UK;3. Institut für Geologie und Paläontologie, Universität Münster, Münster 48149, Germany |
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| Abstract: | Porewater concentration profiles were determined for Fe, trace elements (As, Cd, Co, Cu, Mn, Ni, Pb, Zn), sulfide, SO4 and pH in two Canadian Shield lakes (Chevreuil and Clearwater). Profiles of pyrite, sedimentary trace elements associated with pyrite and AVS were also obtained at the same sites. Thermodynamic calculations are used, for the anoxic porewaters where sulfide was measured, to characterize diagenetic processes involving sulfide and trace elements and to illustrate the importance of sulfide, and possibly polysulfides and thiols, in binding trace elements. The ion activity products (IAP) of Fe sulfide agree with the solubility products (Ks) of greigite or mackinawite. For Co, Ni and Zn, IAP values are close to the KS values of their sulfide precipitates; for Cu and Pb, IAP/Ks indicate large oversaturations, which can be explained by the presence of other ligands (not measured) such as polysulfides (Cu) and thiols (Pb). Cobalt, Cu, Ni and Zn porewater profiles generally display a decrease in concentration with increasing ΣH2S, as expected for transition metals, whereas Cd, Pb and Zn show an increase (mobilisation). The results suggest that removal of trace elements from anoxic porewaters occurs by coprecipitation (As and Mn) with FeS(s) and/or adsorption (As and Mn) on FeS(s), and by formation of discrete solid sulfides (Cd, Cu, Ni, Pb, Zn and Co). Reactive Fe is extensively sulfidized (51–65%) in both lakes, mostly as pyrite, but also as AVS. Similarities between As, Co, Cu and Ni to Fe ratios in pyrite and their corresponding mean diffusive flux ratios suggest that pyrite is an important sink at depth for these trace elements. High molar ratios of trace elements to Fe in pyrite from Clearwater Lake correspond chronologically to the onset of smelting activities. AVS can be an important reservoir of reactive As, Cd and Ni and, to a lesser extent, of Co, Cu and Pb. Overall, the trace elements most extensively sulfidized were Ni, Cd and As (maximum of 100%, 81% and 49% of the reactive fraction, respectively), whereas Co, Cu, Mn, Pb and Zn were only moderately sulfidized (11–16%). |
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