Potential kinetic availability of metals in sulphidic freshwater sediments.
ABSTRACT The insolubility of metal sulphides is believed to limit the bioavailability of trace metals in sulphidic sediments. However, if non-equilibrium conditions are important, metals may be more available than simple thermodynamic calculations suggest. To investigate the possible dynamic supply of Cu, Ni and Zn in a sulphidic freshwater sediment, they were measured, along with iron, manganese and sulphide, by the technique of diffusive gradients in thin-films (DGT). DGT measures the supply of solute from sediment to solution in response to a local solute sink. Release of Mn, Cu, Zn and Ni was observed at the sediment surface and attributed to the supply from reductive dissolution of manganese oxides. The depth profile of simultaneously extractable metals (SEM) for Cu and Ni followed the shape of the Mn profile more closely than the profiles of either acid volatile sulphur (AVS) or Fe, again consistent with supply from Mn oxides. Solubility calculations for a mesocosm of homogenised sediment indicated supersaturation with respect to the sulphides of Fe, Cu, Ni and Zn, yet DGT measurements demonstrated a substantial supply of both trace metals and sulphide from the solid phase to the pore waters. Ratios of metals measured in pore waters by DGT were consistent with their release from iron and manganese oxides, indicating that supply, as much as removal processes, determines the pseudo-steady state concentrations in the pore waters. The observations suggest that trace metals are not immediately bound in an insoluble, inert form when they are in contact with sulphide. This has consequences for modelling metal processes in sediment, as well as for uptake by some biota.
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ABSTRACT: This thesis examines some effects of surface water and groundwater hydrology on the mobility of trace elements and phosphorus in natural environments. Three separate field sites are studied: 1) the shoreline of Lake Powell, a large reservoir on the Colorado River in Utah and Arizona where the surface elevation fluctuates on yearly and multi-yearly timescales, 2) the Colorado River inflow region to Lake Powell, where the sediment delta has been exposed due to low water levels, and 3) the lower Merced River, which is located in the San Joaquin Valley, California, amidst extensive agricultural development. On the shoreline of Lake Powell, depth profiles of manganese and uranium were used to estimate the redox state of sediment porewater. Samples were collected before and after a fluctuation in reservoir level exposed two sampling locations to air and then resubmerged them. Results indicate that reducing conditions are re-established at different rates in two nearby shoreline locations, and that manganese reduction occurs more rapidly than uranium reduction upon resubmergence. In the Colorado River inflow region of Lake Powell, sediment samples were collected from the lakebed and shoreline. Measurements indicate that particle size anticorrelates with the concentrations of most elements and clay minerals and explains much, but not all, of the variation in trace elements. Spatial trends of particle size imply that low reservoir levels may induce resuspension of fine sediment, a process that may lead to increased primary productivity observed in monitoring data. Sequential extractions performed on these sediment samples suggest that phosphorus, the limiting nutrient in Lake Powell, is primarily associated with calcite and biogenic apatite. Sorption experiments indicate that fine particles sorb much more phosphorus than coarse particles, and that only a small amount of the sediment-associated phosphorus is desorbed during sediment resuspension. When reservoir levels are low, measurements of dissolved phosphorus suggest that sediment resuspended by the Colorado River may supply phosphorus to the photic zone under specific hydrologic conditions. Samples of groundwater collected from beneath the Merced River were analyzed for a suite of trace elements. Statistical analyses suggest that hydrologic processes generally influence the transport of trace solutes more than redox chemistry, and results vary between strontium, barium, uranium, and phosphorus.
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ABSTRACT: One of the most powerful predictive tools in sediment biogeochemistry is the electron acceptor layering model, which describes the order in which oxidised compounds are reduced by successions of respiring microbial populations, and how this layering is influenced by benthic macro-organism activity. However, techniques allowing convenient determination of heterogeneous distributions of reduced substances, such as iron(II) and sulfide, have been lacking. A combined diffusive gradients in thin films–diffusive equilibrium in thin films technique was used to quantitatively measure the two-dimensional iron(II) and sulfide distributions at high resolution in the vicinity of various sediment features, including macrofauna burrows, particulate organic matter and macrophyte roots. Substantial heterogeneity was observed for both analytes in all probes, especially in the vicinity of seagrass roots and particulate organic matter. Measured distributions tended to follow the general patterns predicted by the tertiary electron acceptor layering model. However, there was unexpected overlap of sulfide and iron(II) distributions at the millimetre to centimetre scale in several samples from different sediments, notably the more complex sediments containing particulate organic matter and seagrass roots. The cause of such overlap is unclear and further study is necessary to elucidate how such distributions can occur. Yes YesEnvironmental Chemistry 01/2009; DOI:10.1071/EN08059 · 3.04 Impact Factor
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ABSTRACT: The behaviour of Mn, Fe, Co, Zn, Cd, Pb and Ni has been studied during early diagenesis in three different riverine sediments (Spierre, Lys and Sheldt). For that purpose (1) pore waters were extracted from sediment cores by centrifugation under nitrogen and further analyzed for the determination of total dissolved metal concentrations and (2) DET and DGT probes have been deployed in situ for the determination of high resolution profiles of labile and total dissolved metal concentrations. Furthermore, sulfidization processes have been examined; they revealed a production of pyrite near the water-sediment interface at Helkijn and Wervik sampling sites, probably due to a partial re-oxidation of reduced sulphur species. In Spierre sediments, where Eh values are the most negative, pyrite production should be mainly due to strict anaerobic processes. Concentrations of AVS in Spierre sediments are also very high and result in low TI values and low trace metal concentrations in the pore waters. Otherwise, in Wervik sediments, the low pH values combined to a TI value close to 0 results in the highest observed dissolved trace metal levels. DOS remains low at the three sites, since it does not exceed 0.4. In Wervik and Helkijn, the limitation is probably due to low sedimentary inputs of sulphate. In Spierre, sulphate is never exhausted in the pore water, suggesting a limitation of the DOS by a lack of bio-degradable organic matter. Values of Cd, Cu and Pb DGT concentrations remain low in pore waters whatever the site, due to their strong affinity with the reduced sulphur pool. It has also been demonstrated that the labile fractions of Pb and Cd are the lowest and do not exceed 0.5, while Co and Ni are the most available metals.Science of The Total Environment 10/2008; 407(1):447-59. DOI:10.1016/j.scitotenv.2008.08.033 · 4.10 Impact Factor