Effect of groundwater pH and ionic strength on strontium sorption in aquifer sediments: Implications for Sr mobility at contaminated nuclear sites     

Sarah H. Wallace  Samuel Shaw  Katherine Morris  Joe S. Small  Adam J. Fuller  Ian T. Burke 《Applied Geochemistry》2012

Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem contaminant at many nuclear facilities. Transport of ⁹⁰Sr in groundwaters is primarily controlled by sorption reactions with aquifer sediments. The extent of sorption is controlled by the geochemistry of the groundwater and sediment mineralogy. Here, batch sorption experiments were used to examine the sorption behaviour of ⁹⁰Sr in sediment–water systems representative of the UK Sellafield nuclear site based on groundwater and contaminant fluid compositions. In experiments with low ionic strength groundwaters (<0.01 mol L⁻¹), pH variation is the main control on sorption. The sorption edge for ⁹⁰Sr was observed between pH 4 and 6 with maximum sorption occurring (K_d ∼ 10³ L kg⁻¹) at pH 6–8. At ionic strengths above 10 mmol L⁻¹, and at pH values between 6 and 8, cation exchange processes reduced ⁹⁰Sr uptake to the sediment. This exchange process explains the lower ⁹⁰Sr sorption (K_d ∼ 40 L kg⁻¹) in the presence of artificial Magnox tank liquor (IS = 29 mmol L⁻¹). Strontium K-edge EXAFS spectra collected from sediments incubated with Sr²⁺ in either HCO₃-buffered groundwater or artificial Magnox tank liquor, revealed a coordination environment of ∼9 O atoms at 2.58–2.61 Å after 10 days. This is equivalent to the Sr²⁺ hydration sphere for the aqueous ion and indicates that Sr occurs primarily in outer sphere sorption complexes. No change was observed in the Sr sorption environment with EXAFS analysis after 365 days incubation. Sequential extractions performed on sediments after 365 days also found that ∼80% of solid associated ⁹⁰Sr was exchangeable with 1 M MgCl₂ in all experiments. These results suggest that over long periods, ⁹⁰Sr in contaminated sediments will remain primarily in weakly bound surface complexes. Therefore, if groundwater ionic strength increases (e.g. by saline intrusion related to sea level rise or by design during site remediation) then substantial remobilisation of ⁹⁰Sr is to be expected.  相似文献   

Sorption of Cs to micaceous subsurface sediments from the Hanford site, USA   总被引：1，自引：0，他引：1  

John M Zachara  Steven C SmithChongxuan Liu  James P McKinleyR.Jeffrey Serne  Paul L Gassman 《Geochimica et cosmochimica acta》2002,66(2):193-211

The sorption of Cs⁺ was investigated over a large concentration range (10⁻⁹−10⁻² mol/L) on subsurface sediments from a United States nuclear materials site (Hanford) where high-level nuclear wastes (HLW) have been accidentally released to the vadose zone. The sediment sorbs large amounts of radiocesium, but expedited migration has been observed when HLW (a NaNO₃ brine) is the carrier. Cs⁺ sorption was measured on homoionic sediments (Na⁺, K⁺, Ca²⁺) with electrolyte concentrations ranging from 0.01 to 1.0 mol/L. In Na⁺ electrolyte, concentrations were extended to near saturation with NaNO_3(s) (7.0 mol/L). The sediment contained nonexpansible (biotite, muscovite) and expansible (vermiculite, smectite) phyllosilicates. The sorption data were interpreted according to the frayed edge-planar site conceptual model. A four-parameter, two-site (high- and low-affinity) numeric ion exchange model was effective in describing the sorption data. The high-affinity sites were ascribed to wedge zones on the micas where particle edges have partially expanded due to the removal of interlayer cations during weathering, and the low-affinity ones to planar sites on the expansible clays. The electrolyte cations competed with Cs⁺ for both high- and low-affinity sites according to the trend K⁺ >> Na⁺ ≥ Ca²⁺. At high salt concentration, Cs⁺ adsorption occurred only on high-affinity sites. Na⁺ was an effective competitor for the high-affinity sites at high salt concentrations. In select experiments, silver-thiourea (AgTU) was used as a blocking agent to further isolate and characterize the high-affinity sites, but the method was found to be problematic. Mica particles were handpicked from the sediment, contacted with Cs⁺_(aq), and analyzed by electron microprobe to identify phases and features important to Cs⁺ sorption. The microprobe study implied that biotite was the primary contributor of high-affinity sites because of its weathered periphery. The poly-phase sediment exhibited close similarity in ion selectivity to illite, which has been well studied, although its proportion of high-affinity sites relative to the cation exchange capacity (CEC) was lower than that of illite. Important insights are provided on how Na⁺ in HLW and indigenous K⁺ displaced from the sediments may act to expedite the migration of strongly sorbing Cs⁺ in subsurface environments.  相似文献   

Competitive adsorption of strontium and fulvic acid at the muscovite-solution interface observed with resonant anomalous X-ray reflectivity     

Sang Soo Lee  Changyong Park  Neil C. Sturchio 《Geochimica et cosmochimica acta》2010,74(6):1762-9581

Molecular-scale distributions of Sr²⁺ and fulvic acid (FA) adsorbed on the muscovite (0 0 1) surface were investigated using in situ specular X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR). The total amount of Sr²⁺ adsorbed from a 1 × 10⁻² mol/kg SrCl₂ and 100 mg/kg Elliott Soil Fulvic Acid II (ESFA II) solution at pH 5.5 compensated 81 ± 5% of the muscovite surface charge, less than previously measured (118 ± 5%) in an ESFA II-free solution with the same Sr concentration and pH. Inner-sphere (IS) and outer-sphere (OS) Sr²⁺ constituted 87% of the total adsorbed species in IS:OS proportions of 19:81 compared to 42:58 in the solution without FA, suggesting that adsorbed FA competes with the IS Sr²⁺ for surface sites. The coverage of both IS and OS Sr²⁺ decreased even more in a pH 3.5 solution containing the same concentration of FA and 0.5 × 10⁻² mol/kg Sr(NO₃)₂, whereas a significant amount of Sr²⁺ accumulated farther from the surface in the FA layer. The amount of Sr²⁺ incorporated in the ∼10 Å thick FA layer decreased by 79% with decreasing FA concentration (100 → 1 mg/kg) and increasing Sr²⁺ concentration (0.5 × 10⁻² → 1 × 10⁻² mol/kg) and pH (3.5 → 3.6). These results indicate not only that adsorbed FA molecules (and perhaps also H₃O⁺) displace Sr²⁺ near the muscovite surface, but also that the sorbed FA film provides binding sites for additional Sr²⁺ away from the surface. When a muscovite crystal pre-coated with FA after reaction in a 100 mg/kg ESFA II solution for 50 h was subsequently reacted with a 0.5 × 10⁻² mol/kg Sr(NO₃)₂ and 100 mg/kg ESFA II solution at pH 3.7, a significant fraction of Sr²⁺ was distributed in the outer part of the FA film similar to that observed on fresh muscovite reacted at pH 3.5 with a pre-mixed Sr-FA solution at the same concentrations. However, this Sr²⁺ sorbed in the pre-adsorbed organic film was more widely distributed and had a lower coverage, suggesting that pre-sorbed FA may undergo fractionation and/or conformational changes that diminish its capacity, and that of the muscovite (0 0 1) surface, for adsorbing the aqueous Sr cation.  相似文献   

Silicate weathering in anoxic marine sediments     

K. Wallmann  G. Aloisi  P. Tishchenko  J. Greinert  A. Eisenhauer 《Geochimica et cosmochimica acta》2008,72(12):2895-2918

Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50-150 mg dm⁻³) may promote the dissolution of silicate phases through complexation of Al³⁺ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4-99.2 mmol CO₂ m⁻² year⁻¹) are lower than the marine weathering rates calculated from the solid phase data (198-245 mmol CO₂ m⁻² year⁻¹) suggesting a decrease in marine weathering over time. The production of CO₂ through reverse weathering in surface sediments (4.22-15.0 mmol CO₂ m⁻² year⁻¹) is about one order of magnitude smaller than the weathering-induced CO₂ consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO₂ consumption through marine silicate weathering estimated here as 5-20 Tmol CO₂ year⁻¹ is as high as the global rate of continental silicate weathering.  相似文献   

Geochemical fluxes and weathering of volcanic terrains on high standing islands: Taranaki and Manawatu-Wanganui regions of New Zealand     

Steven T. Goldsmith  Anne E. Carey  W. Berry Lyons  D. Murray Hicks 《Geochimica et cosmochimica acta》2008,72(9):2248-2267

Sediment fluxes from high standing oceanic islands (HSIs) such as New Zealand are some of the highest known [Milliman J. D. and Syvitski J. P. M. (1992) Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. J. Geol.100, 525-544]. Recent geochemical work has suggested that along with their extremely high physical weathering yields, many New Zealand watersheds also have very high chemical weathering yields. In New Zealand, the magnitude of both the physical and chemical weathering yields is related to the lithology of the watershed. Most of the previous work on this topic has been undertaken in Southern Alps watersheds of schist and greywacke and in East Cape watersheds of semi-consolidated marine sediments and greywacke. We recently sampled North Island watersheds in the Taranaki and Manawatu-Wanganui regions which have been subjected to volcanism since the Miocene. We sampled watersheds that contain both volcanic and sedimentary rocks. A series of water and sediment samples was collected and analyzed for major, minor and trace elements. This was done to quantify the weathering intensities in the watersheds and to establish the relationship between physical and chemical weathering yields in volcanic lithologies. Our results reveal distinct chemical signatures for the different regions. Waters draining the Taranaki region volcanics are significantly enriched in K⁺, and depleted in Ca²⁺ and Sr²⁺ compared to waters draining the Manawatu-Wanganui region volcanics, which also traverse expanses of sedimentary siltstones and mudstones. The Ca²⁺ and Sr²⁺ depletions may reflect the relative absence of CaCO₃ in the Taranaki region watersheds. In addition, sediment samples from the Taranaki region show significant enrichment in Ti, Al, Ca, Fe, Mn, Mg, Ca, and P and depletion in Si and Rb compared to those of the Manawatu-Wanganui region. From total dissolved solids concentrations and mean annual water discharge, we calculate chemical weathering yields of 60-240 tons km⁻² a⁻¹. These weathering yields fall within the middle to upper range of those previously documented for the Southern Alps (93-480 tons km⁻² a⁻¹) and East Cape (62-400 tons km⁻² a⁻¹). Calculated silicate weathering yields of 12-33.6 tons km⁻² a⁻¹ and CO₂ consumption of 852-2390 × 10³ mol km⁻² a⁻¹ for the rivers draining the Taranaki volcanic region are higher than those previously reported for watersheds hosted in sedimentary and metamorphosed rock terrains on HSIs. CO₂ consumption is found to be within the range previously measured for the basaltic terrains of the Deccan Traps (580-2450 × 10³ mol km⁻² a⁻¹) and Réunion Island(1300-4400 × 10³ mol km⁻² a⁻¹). Our calculated chemical weathering yields demonstrate the importance of HSIs, particularly those with volcanic terrains, when considering global geochemical fluxes.  相似文献   

Desorption kinetics of radiocesium from subsurface sediments at Hanford Site, USA     

Chongxuan Liu  John M. Zachara  James P. McKinley 《Geochimica et cosmochimica acta》2003,67(16):2893-2912

The desorption of ¹³⁷Cs⁺ was investigated on sediments from the United States Hanford site. Pristine sediments and ones that were contaminated by the accidental release of alkaline ¹³⁷Cs⁺-containing high level nuclear wastes (HLW, 2 × 10⁶ to 6 × 10⁷ pCi ¹³⁷Cs⁺/g) were studied. The desorption of ¹³⁷Cs⁺ was measured in Na⁺, K⁺, Rb⁺, and NH₄⁺electrolytes of variable concentration and pH, and in presence of a strong Cs⁺-specific sorbent (self-assembled monolayer on a mesoporous support, SAMMS). ¹³⁷Cs⁺ desorption from the HLW-contaminated Hanford sediments exhibited two distinct phases: an initial instantaneous release followed by a slow kinetic process. The extent of ¹³⁷Cs⁺ desorption increased with increasing electrolyte concentration and followed a trend of Rb⁺ ≥ K⁺ > Na⁺ at circumneutral pH. This trend followed the respective selectivities of these cations for the sediment. The extent and rate of ¹³⁷Cs⁺ desorption was influenced by surface armoring, intraparticle diffusion, and the collapse of edge-interlayer sites in solutions containing K⁺, Rb⁺, or NH₄⁺. Scanning electron microscopic analysis revealed HLW-induced precipitation of secondary aluminosilicates on the edges and basal planes of micaceous minerals that were primary Cs⁺ sorbents. The removal of these precipitates by acidified ammonium oxalate extraction significantly increased the long-term desorption rate and extent. X-ray microprobe analyses of Cs⁺-sorbed micas showed that the ¹³⁷Cs⁺ distributed not only on mica edges, but also within internal channels parallel to the basal plane, implying intraparticle diffusive migration of ¹³⁷Cs⁺. Controlled desorption experiments using Cs⁺-spiked pristine sediment indicated that the ¹³⁷Cs⁺ diffusion rate was fast in Na⁺-electrolyte, but much slower in the presence of K⁺ or Rb⁺, suggesting an effect of edge-interlayer collapse. An intraparticle diffusion model coupled with a two-site cation exchange model was used to interpret the experimental results. Model simulations suggested that about 40% of total sorbed ¹³⁷Cs⁺ was exchangeable, including equilibrium and kinetic desorbable pools. At pH 3, this ratio increased to 60-80%. The remainder of the sorbed ¹³⁷Cs⁺ was fixed or desorbed at much slower rate than our experiments could detect.  相似文献   

Benthic nitrogen cycling traversing the Peruvian oxygen minimum zone     

L. Bohlen  A.W. Dale  S. Sommer  T. Mosch  C. Hensen  A. Noffke  F. Scholz  K. Wallmann 《Geochimica et cosmochimica acta》2011,75(20):6094-7276

Benthic nitrogen (N) cycling was investigated at six stations along a transect traversing the Peruvian oxygen minimum zone (OMZ) at 11°S. An extensive dataset including porewater concentration profiles and in situ benthic fluxes of nitrate (NO₃⁻), nitrite (NO₂⁻) and ammonium (NH₄⁺) was used to constrain a 1-D reaction-transport model designed to simulate and interpret the measured data at each station. Simulated rates of nitrification, denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) by filamentous large sulfur bacteria (e.g. Beggiatoa and Thioploca) were highly variable throughout the OMZ yet clear trends were discernible. On the shelf and upper slope (80-260 m water depth) where extensive areas of bacterial mats were present, DNRA dominated total N turnover (?2.9 mmol N m⁻² d⁻¹) and accounted for ?65% of NO₃⁻ + NO₂⁻ uptake by the sediments from the bottom water. Nonetheless, these sediments did not represent a major sink for dissolved inorganic nitrogen (DIN = NO₃⁻ + NO₂⁻ + NH₄⁺) since DNRA reduces NO₃⁻ and, potentially NO₂⁻, to NH₄⁺. Consequently, the shelf and upper slope sediments were recycling sites for DIN due to relatively low rates of denitrification and high rates of ammonium release from DNRA and ammonification of organic matter. This finding contrasts with the current opinion that sediments underlying OMZs are a strong sink for DIN. Only at greater water depths (300-1000 m) did the sediments become a net sink for DIN. Here, denitrification was the major process (?2 mmol N m⁻² d⁻¹) and removed 55-73% of NO₃⁻ and NO₂⁻ taken up by the sediments, with DNRA and anammox accounting for the remaining fraction. Anammox was of minor importance on the shelf and upper slope yet contributed up to 62% to total N₂ production at the 1000 m station. The results indicate that the partitioning of oxidized N (NO₃⁻, NO₂⁻) into DNRA or denitrification is a key factor determining the role of marine sediments as DIN sinks or recycling sites. Consequently, high measured benthic uptake rates of oxidized N within OMZs do not necessarily indicate a loss of fixed N from the marine environment.  相似文献   

Adsorption of Rb and Sr at the orthoclase (0 0 1)-solution interface     

P. Fenter  C. Park 《Geochimica et cosmochimica acta》2008,72(7):1848-1863

Adsorption of Rb⁺ and Sr²⁺ at the orthoclase (0 0 1)-solution interface is probed with high-resolution X-ray reflectivity and resonant anomalous X-ray reflectivity. Specular X-ray reflectivity data for orthoclase in contact with 0.01 m RbCl solution at pH 5.5 reveal a systematic increase in electron density adjacent to the mineral surface with respect to that observed in contact with de-ionized water (DIW). Quantitative analysis indicates that Rb⁺ adsorbs at a height of 0.83 ± 0.03 Å with respect to the bulk K⁺ site with a nominal coverage of 0.72 ± 0.10 ions per surface unit mesh (55.7 Å²). These results are consistent with an ion-exchange reaction in which Rb⁺ occupies an inner-sphere adsorption (IS) site. In contrast, X-ray reflectivity data for orthoclase in contact with 0.01 m Sr(NO₃)₂ solution at pH 5.3 reveal few significant changes with respect to DIW. Resonant anomalous X-ray reflectivity was used to probe Sr²⁺ adsorption and to image its vertical distribution. This element-specific measurement reveals that Sr²⁺ adsorbs with a total coverage of 0.37 ± 0.02 ions per surface unit mesh, at a substantially larger height (3.28 ± 0.05 Å) than found for Rb⁺, and with a relatively broad density distribution (having a root-mean-square width of 1.88 ± 0.08 Å for a single-peak model), implying that Sr²⁺ adsorbs primarily as a fully-hydrated outer-sphere (OS), species. Comparison to a two-height model suggests that 13 ± 5% of the adsorbed Sr²⁺ may be present as an IS species. This partitioning implies a ∼5 kJ/mol difference in free energy between the IS and OS Sr²⁺ on orthoclase. Differences in the partitioning of Sr²⁺ between IS and OS species for orthoclase (0 0 1) and muscovite (0 0 1) suggest control by the geometry of the IS adsorption site. Results for the OS distribution are compared to predictions of the Poisson-Boltzmann equation in the strong coupling regime, which predicts an intrinsically narrow vertical diffuse ion distribution; the OS distribution might thus be thought of as the diffuse ion profile in the limit of high surface charge.  相似文献   

Evidence of microbially mediated arsenic mobilization from sediments of the Aquia aquifer, Maryland, USA     

Christine A. PearcyDarren A. Chevis  T. Jade HaugHolly A. Jeffries  Ningfang YangJianwu Tang  Deborah A. GrimmKaren H. Johannesson 《Applied Geochemistry》2011,26(4):575-586

Sediments from the Aquia aquifer in coastal Maryland were collected as part of a larger study of As in the Aquia groundwater flow system where As concentration are reported to reach levels as high as 1072 nmol kg⁻¹, (i.e., ∼80 μg/L). To test whether As release is microbially mediated by reductive dissolution of Fe(III) oxides/oxyhydroxides within the aquifer sediments, the Aquia aquifer sediment samples were employed in a series of microcosm experiments. The microcosm experiments consisted of sterilized serum bottles prepared with aquifer sediments and sterilized (i.e., autoclaved), artificial groundwater using four experimental conditions and one control condition. The four experimental conditions included the following scenarios: (1) aerobic; (2) anaerobic; (3) anaerobic + acetate; and (4) anaerobic + acetate + AQDS (anthraquinone-2,6-disulfonic acid). AQDS acts as an electron shuttle. The control condition contained sterilized aquifer sediments kept under anaerobic conditions with an addition of AQDS. Over the course of the 27 day microcosm experiments, dissolved As in the unamended (aerobic and anaerobic) microcosms remained constant at around ∼28 nmol kg⁻¹ (2 μg/L). With the addition of acetate, the amount of As released to the solution approximately doubled reaching ∼51 nmol kg⁻¹ (3.8 μg/L). For microcosm experiments amended with acetate and AQDS, the dissolved As concentrations exceeded 75 nmol kg⁻¹ (5.6 μg/L). The As concentrations in the acetate and acetate + AQDS amended microcosms are of similar orders of magnitude to As concentrations in groundwaters from the aquifer sediment sampling site (127-170 nmol kg⁻¹). Arsenic concentrations in the sterilized control experiments were generally less than 15 nmol kg⁻¹ (1.1 μg/L), which is interpreted to be the amount of As released from Aquia aquifer sediments owing to abiotic, surface exchange processes. Iron concentrations released to solution in each of the microcosm experiments were higher and more variable than the As concentrations, but generally exhibited similar trends to the As concentrations. Specifically, the acetate and acetate + AQDS amended microcosm typically exhibited the highest Fe concentrations (up to 1725 and 6566 nmol kg⁻¹, respectively). The increase in both As and Fe in the artificial groundwater solutions in these amended microcosm experiments strongly suggests that microbes within the Aquia aquifer sediments mobilize As from the sediment substrate to the groundwaters via Fe(III) reduction.  相似文献   

Contributions from glacially derived sediment to the global iron (oxyhydr)oxide cycle: Implications for iron delivery to the oceans     

Rob Raiswell  Martyn Tranter  Martin Siegert  Philippe Huybrechts 《Geochimica et cosmochimica acta》2006,70(11):2765-2780

Estimates of glacial sediment delivery to the oceans have been derived from fluxes of meltwater runoff and iceberg calving, and their sediment loads. The combined total (2900 Tg yr⁻¹) of the suspended sediment load in meltwaters (1400 Tg yr⁻¹) and the sediment delivered by icebergs (1500 Tg yr⁻¹) are within the range of earlier estimates. High-resolution microscopic observations show that suspended sediments from glacial meltwaters, supraglacial, and proglacial sediments, and sediments in basal ice, from Arctic, Alpine, and Antarctic locations all contain iron (oxyhydr)oxide nanoparticles, which are poorly crystalline, typically ∼5 nm in diameter, and which occur as single grains or aggregates that may be isolated or attached to sediment grains. Nanoparticles with these characteristics are potentially bioavailable. A global model comparing the sources and sinks of iron present as (oxyhydr)oxides indicates that sediment delivered by icebergs is a significant source of iron to the open oceans, beyond the continental shelf. Iceberg delivery of sediment containing iron as (oxyhydr)oxides during the Last Glacial Maximum may have been sufficient to fertilise the increase in oceanic productivity required to drawdown atmospheric CO₂ to the levels observed in ice cores.  相似文献   

Kinetics of microbial sulfate reduction in estuarine sediments   总被引：2，自引：0，他引：2  

Céline Pallud  Philippe Van Cappellen 《Geochimica et cosmochimica acta》2006,70(5):1148-1162

Kinetic parameters of microbial sulfate reduction in intertidal sediments from a freshwater, brackish and marine site of the Scheldt estuary (Belgium, the Netherlands) were determined. Sulfate reduction rates (SRR) were measured at 10, 21, and 30 °C, using both flow-through reactors containing intact sediment slices and conventional sediment slurries. At the three sites, and for all depth intervals studied (0-2, 2-4, 4-6 and 6-8 cm), the dependence of potential SRR on the sulfate concentration followed the Michaelis-Menten rate equation. Apparent sulfate half-saturation concentrations, K_m, measured in the flow-through reactor experiments were comparable at the freshwater and marine sites (0.1-0.3 mM), but somewhat higher at the brackish site (0.4-0.9 mM). Maximum potential SRR, R_max, in the 0-4 cm depth interval of the freshwater sediments were similar to those in the 0-6 cm interval of the marine sediments (10-46 nmol cm⁻³ h⁻¹ at 21 °C), despite much lower in situ sulfate availability and order-of-magnitude lower densities of sulfate-reducing bacteria (SRB), at the freshwater site. Values of R_max in the brackish sediments were lower (3.7-7.6 nmol cm⁻³ h⁻¹ at 21 °C), probably due to less labile organic matter, as inferred from higher C_org/N ratios. Inflow solutions supplemented with lactate enhanced potential SRR at all three sites. Slurry incubations systematically yielded higher R_max values than flow-through reactor experiments for the freshwater and brackish sediments, but similar values for the marine sediments. Transport limitation of potential SRR at the freshwater and brackish sites may be related to the lower sediment porosities and SRB densities compared to the marine site. Multiple rate controls, including sulfate availability, organic matter quality, temperature, and SRB abundance, modulate in situ sulfate-reducing activity along the estuarine salinity gradient.  相似文献   

Rare earth elements (REE) and yttrium in stream waters, stream sediments, and Fe-Mn oxyhydroxides: Fractionation, speciation, and controls over REE + Y patterns in the surface environment   总被引：1，自引：0，他引：1  

Matthew I. Leybourne  Karen H. Johannesson 《Geochimica et cosmochimica acta》2008,72(24):5962-5983

We have collected ∼500 stream waters and associated bed-load sediments over an ∼400 km² region of Eastern Canada and analyzed these samples for Fe, Mn, and the rare earth elements (REE + Y). In addition to analyzing the stream sediments by total digestion (multi-acid dissolution with metaborate fusion), we also leached the sediments with 0.25 M hydroxylamine hydrochloride (in 0.05 M HCl), to determine the REE + Y associated with amorphous Fe- and Mn-oxyhydroxide phases. We are thus able to partition the REE into “dissolved” (<0.45 μm), labile (hydroxylamine) and detrital sediment fractions to investigate REE fractionation, and in particular, with respect to the development of Ce and Eu anomalies in oxygenated surface environments. Surface waters are typically LREE depleted ([La/Sm]_NASC ranges from 0.16 to 5.84, average = 0.604, n = 410; where the REE are normalized to the North America Shale Composite), have strongly negative Ce anomalies ([Ce/Ce^∗]_NASC ranges from 0.02 to 1.25, average = 0.277, n = 354), and commonly have positive Eu anomalies ([Eu/Eu^∗]_NASC ranges from 0.295 to 1.77, average = 0.764, n = 84). In contrast, the total sediment have flatter REE + Y patterns relative to NASC ([La/Sm]_NASC ranges from 0.352 to 1.12, average = 0.778, n = 451) and are slightly middle REE enriched ([Gd/Yb]_NASC ranges from 0.55 to 3.75, average = 1.42). Most total sediments have negative Ce and Eu anomalies ([Ce/Ce^∗]_NASC ranges from 0.097 to 2.12, average = 0.799 and [Eu/Eu^∗]_NASC ranges from 0.39 to 1.43, average = 0.802). The partial extraction sediments are commonly less LREE depleted than the total sediments ([La/Sm]_NASC ranges from 0.24 to 3.31, average = 0.901, n = 4537), more MREE enriched ([Gd/Yb]_NASC ranges from 0.765 to 6.28, average = 1.97) and Ce and Eu anomalies (negative and positive) are more pronounced.The partial extraction recovered, on average ∼20% of the Fe in the total sediment, ∼80% of the Mn, and 21-29% of the REEs (Ce = 19% and Y = 32%). Comparison between REEs in water, partial extraction and total sediment analyses indicates that REEs + Y in the stream sediments have two primary sources, the host lithologies (i.e., mechanical dispersion) and hydromorphically transported (the labile fraction). Furthermore, Eu appears to be more mobile than the other REE, whereas Ce is preferentially removed from solution and accumulates in the stream sediments in a less labile form than the other REEs + Y. Despite poor statistical correlations between the REEs + Y and Mn in either the total sediment or partial extractions, based on apparent distribution coefficients and the pH of the stream waters, we suggest that either sediment organic matter and/or possibly δ-MnO₂/FeOOH are likely the predominant sinks for Ce, and to a lesser extent the other REE, in the stream sediments.  相似文献   

Multicomponent diffusion of a suite of tracers (HTO, Cl, Br, I, Na, Sr, Cs) in a single sample of Opalinus Clay   总被引：1，自引：0，他引：1  

C.A.J. Appelo  L.R. Van Loon 《Geochimica et cosmochimica acta》2010,74(4):1201-57

Diffusion experiments with HTO, ³⁶Cl⁻, Br⁻, I⁻, ²²Na⁺, ⁸⁵Sr²⁺ and ¹³⁴Cs⁺ at trace concentrations in a single sample of Opalinus Clay are modeled with PHREEQC’s multicomponent diffusion module. The model is used first in a classical approach to derive accessible porosities, geometrical factors (the ratio of pore tortuosity and constrictivity) and sorption behavior of the individual tracers assuming that the clay is homogeneous. The accessible porosity for neutral species and cations is obtained from HTO, the anion exclusion volume from ³⁶Cl⁻ and Br⁻, and the cation exchange capacity from ²²Na⁺. The homogeneous model works well for tritium, the anions and ²²Na⁺. However, the ⁸⁵Sr²⁺ and ¹³⁴Cs⁺ experiments show an early arrival of the tracer and a front-form that suggest a dual porosity structure. A model with 10% dead-end pores, containing 19% of the total exchange capacity, can satisfactorily calculate all the experimental data. The Cs⁺ diffusion model builds on a 3-site exchange model, constructed from batch sorption data. The excellent agreement of modeled and measured data contradicts earlier reports that the exchange capacity for Cs⁺ would be smaller in diffusion than in batch experiments.The geometrical factors for the anions are 1.5 times larger than for HTO, and for the cations 2-4 times smaller than for HTO. The different behavior is explained by a tripartite division of the porespace in free porewater, diffuse double layer (DDL) water, and interlayer water in montmorillonite. Differences between estimated and observed geometrical factors for cations are attributed to increased ion-pairing of the divalent cations in DDL water as a result of the low relative dielectric permittivity. Interlayer and/or surface diffusion contributes significantly to the diffusive flux of Cs⁺ but is negligible for the other solutes. The geometrical factors for anions are higher than estimated, because pore constrictions with overlapping double layers force the anions to take longer routes than HTO and the cations. Small differences among the anions can also be attributed to different ion-pairing in DDL water.  相似文献   

Coupling between Pb_ex and organic matter in sediments of a nutrient-enriched lake: An example from Lake Chenghai, China     

G.J. Wan  J.A. Chen  S.Q. Xu  E.Y. Wan  R.G. Huang  P.H. Santschi 《Chemical Geology》2005,224(4):223-236

Sediment cores were collected from deep-water areas of Lake Chenghai, China in June 1997. The vertical profile of ¹³⁷Cs activity gives reliable geochronological results. The results also indicate that sediment accumulation rates in deep-water areas of Lake Chenghai were relatively constant in recent decades, averaging 0.43 g cm^− 2 y^− 1, despite a variable organic carbon influx. ²¹⁰Pb_eq (= ²²⁶Ra) activity was relatively constant also, with an average value of 54.3 ± 3.2 Bq kg^− 1. Vertical profiles of ²¹⁰Pb_ex (= ²¹⁰Pb_total − ²²⁶Ra) decreased exponentially, resulting in somewhat lower sediment accumulation rates (0.3 g cm^− 2 y^− 1). These lower rates are likely less reliable, as the relatively large fluctuations in ²¹⁰Pb_ex activities correlate closely to the organic carbon (C_org) content of the sediments. For example, the vertical profile of ²¹⁰Pb_ex activity displays peaks at mass depths of 3.7-4.7 g cm^− 2 (10-12 cm) and 10-11 g cm^− 2(25-28 cm), similar to the maxima in the vertical profile of C_org. This phenomenon must be related to the delivery of particulate organic matter (POM) from the water to the sediments, or to watershed soil erosion. Since the mean atomic ratios of H_org / C_org and C_org / N_org in Lake Chenghai sediments are 5.5 and 7.0, respectively, indicating that POM was predominantly derived from the remains of authigenic algae, this eliminates watershed erosion rates as a primary control on lake sedimentation rates as resolved by ²¹⁰Pb_ex. Sedimentation fluxes (F(C_org)) of particulate organic carbon since 1970 varied between 60 to 160 g m^− 2 y^− 1, and appeared to closely influence variations in ²¹⁰Pb_ex concentrations. For example, sedimentation fluxes of ²¹⁰Pb_ex (F(²¹⁰Pb_ex)) showed maxima in the years 1972-1974 and 1986-1989, likely reflecting historical variations of lake biological productivity or carbon preservation.  相似文献   

Impact of highly basic solutions on sorption of Cs to subsurface sediments from the Hanford site, USA     

C.C. Ainsworth  J.M. Zachara  K. Wagnon  S. McKinley  C. Liu  S.C. Smith  H.T. Schaef  P.L. Gassman 《Geochimica et cosmochimica acta》2005,69(20):4787-4800

The effect of caustic NaNO₃ solutions on the sorption of ¹³⁷Cs to a Hanford site micaceous subsurface sediment was investigated as a function of base exposure time (up to 168 d), temperature (10°C or 50°C), and NaOH concentration (0.1 mol/L to 3 mol/L). At 10°C and 0.1 M NaOH, the slow evolution of [Al]_aq was in stark contrast to the rapid increase and subsequent loss of [Al]_aq observed at 50°C (regardless of base concentration). Exposure to 0.1 M NaOH at 10°C for up to 168 d exhibited little if any measurable effect on sediment mineralogy, Cs⁺ sorption, or Cs⁺ selectivity; sorption was well described with a two-site ion exchange model modified to include enthalpy effects. At 50°C, dissolution of phyllosilicate minerals increased with [OH]. A zeolite (tetranatrolite; Na₂Al₂Si₃O₁₀·2H₂O) precipitated in 0.1 M NaOH after about 7 days, while an unnamed mineral phase (Na₁₄Al₁₂Si₁₃O₅₁·6H₂O) precipitated after 4 and 2 days of exposure to 1 M and 3 M NaOH solutions, respectively. Short-term (16 h) Cs⁺ sorption isotherms (10⁻⁹-10⁻² mol/L) were measured on sediment after exposure to 0.1 M NaOH for 56, 112, and 168 days at 50°C. There was a trend toward slightly lower conditional equilibrium exchange constants (Δlog _Na^CsK_c ∼ 0.25) over the entire range of surface coverage, and a slight loss of high affinity sites (15%) after 168 days of pretreatment with 0.1 M base solution. Cs⁺ sorption to sediment over longer times was also measured at 50°C in the presence of NaOH (0.1 M, 1 M, and 3 M NaOH) at Cs⁺ concentrations selected to probe a range of adsorption densities. Model simulations of Cs⁺ sorption to the sediment in the presence of 0.1 M NaOH for 112 days slightly under-predicted sorption at the lower Cs⁺ adsorption densities. At the higher adsorption densities, model simulations under-predicted sorption by 57%. This under-prediction was surmised to be the result of tetranatrolite precipitation, and subsequent slow Na → Cs exchange. At higher OH concentrations, Cs⁺ sorption in the presence of base for 112 days was unexpectedly equal to, or greater than that expected for pristine sediment. The precipitation of secondary phases, coupled with the fairly unique mica distribution and quantity across all size-fractions in the Hanford sediment, appears to mitigate the impact of base dissolution on Cs⁺ sorption.  相似文献   

Surface complexation model for strontium sorption to amorphous silica and goethite     

Susan A Carroll  Sarah K Roberts  Louise J Criscenti  Peggy A O'Day 《Geochemical transactions》2008,9(1):2

Strontium sorption to amorphous silica and goethite was measured as a function of pH and dissolved strontium and carbonate concentrations at 25°C. Strontium sorption gradually increases from 0 to 100% from pH 6 to 10 for both phases and requires multiple outer-sphere surface complexes to fit the data. All data are modeled using the triple layer model and the site-occupancy standard state; unless stated otherwise all strontium complexes are mononuclear. Strontium sorption to amorphous silica in the presence and absence of dissolved carbonate can be fit with tetradentate Sr²⁺ and SrOH⁺ complexes on the β-plane and a monodentate Sr²⁺complex on the diffuse plane to account for strontium sorption at low ionic strength. Strontium sorption to goethite in the absence of dissolved carbonate can be fit with monodentate and tetradentate SrOH⁺ complexes and a tetradentate binuclear Sr²⁺ species on the β-plane. The binuclear complex is needed to account for enhanced sorption at hgh strontium surface loadings. In the presence of dissolved carbonate additional monodentate Sr²⁺ and SrOH⁺ carbonate surface complexes on the β-plane are needed to fit strontium sorption to goethite. Modeling strontium sorption as outer-sphere complexes is consistent with quantitative analysis of extended X-ray absorption fine structure (EXAFS) on selected sorption samples that show a single first shell of oxygen atoms around strontium indicating hydrated surface complexes at the amorphous silica and goethite surfaces.  相似文献   

50-year record and solid state speciation of mercury in natural and contaminated reservoir sediment     

Sabine Castelle  Jörg SchäferGérard Blanc  Stéphane AudryHenri Etcheber  Jean-Pierre Lissalde 《Applied Geochemistry》2007

Contaminated fluvial sediments represent both temporary sinks for river-borne pollutants and potential sources in case of natural and/or anthropogenic resuspension. Reservoir lakes play a very important role in sediment dynamics of watersheds and may offer great opportunities to study historical records of river-borne particles and associated elements transported in the past. The fate and potential environmental impact of Hg depends on its abundance, its carrier phases and its chemical speciation. Historical Hg records and solid state Hg speciation were compared in sediments from two contrasting reservoirs of the Lot River (France) upstream and downstream from a major polymetallic pollution (e.g. Cd, Zn) source. Natural (geochemical background) and anthropogenic Hg concentrations and their relationships with predominant carrier phases were determined. The results reveal important historical Hg contamination (up to 35 mg kg⁻¹) of the downstream sediment, reflecting the historical evolution of industrial activity at the point source, i.e. former coal mining, Zn ore treatment and post-industrial remediation work. Single chemical extractions (ascorbate, H₂O₂, KOH) suggest that at both sites most (∼75%) of the Hg is bound to organic and/or reactive sulphide phases. Organo-chelated (KOH-extracted) Hg, representing an important fraction in the uncontaminated sediment, shows similar concentrations (∼0.02 mg kg⁻¹) at both sites and may be mainly attributed to natural inputs and/or processes. Although, total Hg concentrations in recent surface sediments at both sites are still very different, similar mono-methylmercury concentrations (up to 4 μg kg⁻¹) and vertical distributions were observed, suggesting comparable methylation-demethylation processes. High mono-methylmercury concentrations (4–15 μg kg⁻¹) in 10–40 a-old, sulphide-rich, contaminated sediment suggest long-term persistence of mono-methylmercury. Beyond historical records of total concentrations, the studied reservoir sediments provided new insights in solid state speciation and carrier phases of natural and anthropogenic Hg. In case of sediment resuspension, the major part of the Hg historically stored in the Lot River sediments will be accessible to biogeochemical recycling in the downstream fluvial-estuarine environment.  相似文献   

Pathways and regulation of carbon, sulfur and energy transfer in marine sediments overlying methane gas hydrates on the Opouawe Bank (New Zealand)     

A.W. Dale  S. Sommer  K. Wallmann  G. Wegener 《Geochimica et cosmochimica acta》2010,74(20):5763-5784

This study combines sediment geochemical analysis, in situ benthic lander deployments and numerical modeling to quantify the biogeochemical cycles of carbon and sulfur and the associated rates of Gibbs energy production at a novel methane seep. The benthic ecosystem is dominated by a dense population of tube-building ampharetid polychaetes and conspicuous microbial mats were unusually absent. A 1D numerical reaction-transport model, which allows for the explicit growth of sulfide and methane oxidizing microorganisms, was tuned to the geochemical data using a fluid advection velocity of 14 cm yr⁻¹. The fluids provide a deep source of dissolved hydrogen sulfide and methane to the sediment with fluxes equal to 4.1 and 18.2 mmol m⁻² d⁻¹, respectively. Chemosynthetic biomass production in the subsurface sediment is estimated to be 2.8 mmol m⁻² d⁻¹ of C biomass. However, carbon and oxygen budgets indicate that chemosynthetic organisms living directly above or on the surface sediment have the potential to produce 12.3 mmol m⁻² d⁻¹ of C biomass. This autochthonous carbon source meets the ampharetid respiratory carbon demand of 23.2 mmol m⁻² d⁻¹ to within a factor of 2. By contrast, the contribution of photosynthetically-fixed carbon sources to ampharetid nutrition is minor (3.3 mmol m⁻² d⁻¹ of C). The data strongly suggest that mixing of labile autochthonous microbial detritus below the oxic layer sustains high measured rates of sulfate reduction in the uppermost 2 cm of the sulfidic sediment (100-200 nmol cm⁻³ d⁻¹). Similar rates have been reported in the literature for other seeps, from which we conclude that autochthonous organic matter is an important substrate for sulfate reducing bacteria in these sediment layers. A system-scale energy budget based on the chemosynthetic reaction pathways reveals that up to 8.3 kJ m⁻² d⁻¹ or 96 mW m⁻² of catabolic (Gibbs) energy is dissipated at the seep through oxidation reactions. The microorganisms mediating sulfide oxidation and anaerobic oxidation of methane (AOM) produce 95% and 2% of this energy flux, respectively. The low power output by AOM is due to strong bioenergetic constraints imposed on the reaction rate by the composition of the chemical environment. These constraints provide a high potential for dissolved methane efflux from the sediment (12.0 mmol m⁻² d⁻¹) and indicates a much lower efficiency of (dissolved) methane sequestration by AOM at seeps than considered previously. Nonetheless, AOM is able to consume a third of the ascending methane flux (5.9 mmol m⁻² d⁻¹ of CH₄) with a high efficiency of energy expenditure (35 mmol CH₄ kJ⁻¹). It is further proposed that bioenergetic limitation of AOM provides an explanation for the non-zero sulfate concentrations below the AOM zone observed here and in other active and passive margin sediments.  相似文献   

Estimating specific surface area of fine stream bed sediments from geochemistry     

B.G. Rawlins  G. TurnerI. Mounteney  G. Wildman 《Applied Geochemistry》2010

Specific surface area (SSA) of headwater stream bed sediments is a fundamental property which determines the nature of sediment surface reactions and influences ecosystem-level, biological processes. Measurements of SSA – commonly undertaken by BET nitrogen adsorption – are relatively costly in terms of instrumentation and operator time. A novel approach is presented for estimating fine (<150 μm) stream bed sediment SSA from their geochemistry – after removal of organic matter – for agricultural headwater catchments across 15,400 km² of central England, UK. From a regional set of 1972 stream bed sediment sites with common characteristics for which geochemical data were available, 60 samples were selected – based on maximising their variation in Al concentrations – and their BET SSA measured by N₂ adsorption. After careful selection of potential regression predictors following a principal component analysis and removal of a subset of samples with the largest Mo concentrations (>2.5 mg kg⁻¹), four elements were identified as significant predictors of SSA (ordered by decreasing predictive power): V > Ca > Al > Rb. The optimum model from these four elements accounted for 73% of the variation in bed sediment SSA (range 6–46 m² g⁻¹) with a root mean squared error of prediction – based on leave-one-out cross-validation – of 6.3 m² g⁻¹. It is believed that V is the most significant predictor because its concentration is strongly correlated both with the quantity of Fe-oxides and clay minerals in the stream bed sediments, which dominate sediment SSA. Sample heterogeneity in SSA – based on triplicate measurements of sub-samples – was a substantial source of variation (standard error = 2.2 m² g⁻¹) which cannot be accounted for in the regression model.  相似文献   

Between carbonatite and lamproite—Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes     

Sebastian Tappe  Stephen F. Foley  Rolf L. Romer  Andreas Stracke 《Geochimica et cosmochimica acta》2008,72(13):3258-3286

New U-Pb perovskite ages reveal that diamondiferous ultramafic lamprophyre magmas erupted through the Archean crust of northern Labrador and Quebec (eastern Canada) between ca. 610 and 565 Ma, a period of strong rifting activity throughout contiguous Laurentia and Baltica. The observed Torngat carbonate-rich aillikite/carbonatite and carbonate-poor mela-aillikite dyke varieties show a large spread in Sr-Nd-Hf-Pb isotope ratios with pronounced correlations between isotope systems. An isotopically depleted component is identified solely within aillikites (⁸⁷Sr/⁸⁶Sr_i = 0.70323-0.70377; εNd_i = +1.2-+1.8; εHf_i = +1.4-+3.5; ²⁰⁶Pb/²⁰⁴Pb_i = 18.2-18.5), whereas some aillikites and all mela-aillikites range to more enriched isotope signatures (⁸⁷Sr/⁸⁶Sr_i = 0.70388-0.70523; εNd_i = −0.5 to −3.9; εHf_i = −0.6 to −6.0; ²⁰⁶Pb/²⁰⁴Pb_i = 17.8-18.2). These contrasting isotopic characteristics of aillikites/carbonatites and mela-aillikites, along with subtle differences in their modal carbonate, SiO₂, Al₂O₃, Na₂O, Cs-Rb, and Zr-Hf contents, are consistent with two distinctive metasomatic assemblages of different age in the mantle magma source region.Integration of petrologic, geochemical, and isotopic information leads us to propose that the isotopically enriched component originated from a reduced phlogopite-richterite-Ti-oxide dominated source assemblage that is reminiscent of MARID suite xenoliths. In contrast, the isotopically depleted component was derived from a more oxidized phlogopite-carbonate dominated source assemblage. We argue that low-degree CO₂-rich potassic silicate melts from the convective upper mantle were preferentially channelled into an older, pre-existing MARID-type vein network at the base of the North Atlantic craton lithosphere, where they froze to form new phlogopite-carbonate dominated veins. Continued stretching and thinning of the cratonic lithosphere during the Late Neoproterozoic remobilized the carbonate-rich vein material and induced volatile-fluxed fusion of the MARID-type veins and the cold peridotite substrate. Isotopic modelling suggests that only 5-12% trace element contribution from such geochemically extreme MARID-type material is required to produce the observed compositional shift from the isotopically most depleted aillikites/carbonatites towards enriched mela-aillikites.We conclude that cold cratonic mantle lithosphere can host several generations of contrasting vein assemblages, and that each may have formed during past tectonic and magmatic events under distinctively different physicochemical conditions. Although cratonic MARID-type and carbonate-bearing veins in peridotite can be the respective sources for lamproite and carbonatite magmas when present as the sole metasome, their concomitant fusion in a complex source region may give rise to a whole new variety of deep volatile-rich magmas and we suggest that orangeites (formerly Group 2 kimberlites), kamafugites, and certain types of ultramafic lamprophyre are formed in this manner.  相似文献