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1.
A strategy to neutralize acidic pit lakes was tested in an upscaling process using field mesocosms of 26 to ca. 4500 m3 volume in the acidic pit Mining Lake 111 in Germany. After addition of the substrates Carbokalk and straw a neutral sediment layer formed, in which microbial sulfate and iron reduction as well as sulfide precipitation occurred. The net rate of neutralization was limited by the precipitation of iron sulfides rather than by microbial reactions. Oxidation of H2S by ferric iron in the anoxic sediment lowered the net sulfate reduction rate. Seasonal fluctuations of iron sulfides in the sediment showed that the reaction products were not necessarily stable. The long-term success of the approach depends on the net partition of the precipitated iron-(mono-/di-) sulfide that is permanently buried in the anoxic sediment. It could be shown by field experiments that the long-term success of the neutralization depends on the spatial scale and duration of the experiments. Volumes from 26 to 4500 m3, exposition times from 4 months to 5 years, and increasingly thick coverings of the sediments with straw, from zero to 40 cm, were used. Net neutralization rates decreased from 41 meq m− 2 d− 1 in laboratory microcosms to a mean rate of 2.3 meq m− 2 d− 1 in the 4500 m3 field experiment. The results show that the success of the microbial treatment of acid pit lakes lastly depends on the limnological conditions in the lake that cannot be simulated by upscaling of simple laboratory experiments.  相似文献   

2.
《地学前缘(英文版)》2020,11(4):1369-1380
This paper reports new geochronological (U–Pb) and isotope (C, O, and S) data to investigate the timing of mineralization and mode of ore genesis for the recently discovered Changtuxili Mn–Ag–Pb–Zn deposit, located on the western slopes of the southern Great Hinggan Range in NE China. The mineralization is hosted by intermediate–acidic lavas and pyroclastic rocks of the Baiyingaolao Formation. Three stages of mineralization are identified: quartz–pyrite (Stage I), galena–sphalerite–tetrahedrite–rhodochrosite (Stage II), and quartz–pyrite (Stage III). δ13C and δ18O values for carbonate from the ore vary from −8.51‰ to −4.96‰ and 3.97‰ to 15.90‰, respectively, which are indicative of a low-temperature alteration environment. δ34SV-CDT values of sulfides range from −1.77‰ to 4.16‰ and show a trend of equilibrium fractionation (δ34SPy ​> ​δ34SSp ​> ​δ34SGn). These features indicate that pyrite, sphalerite, and galena precipitated during the period of mineralization. The alteration mineral assemblage and isotope data indicate that the weakly acidic to weakly alkaline ore-forming fluid was derived largely from meteoric water and the ore-forming elements C and S originated from magma. During the mineralization, a geochemical barrier was formed by changes in the pH of the ore-forming fluid, leading to the precipitation of rhodochrosite. On the basis of the mineralization characteristics, new isotope data, and comparison with adjacent deposits, we propose that the Changtuxili Mn–Ag–Pb–Zn deposit is an intermediate-to low-sulfidation epithermal deposit whose formation was controlled by fractures and variability in the pH of the ore-forming fluid. The surrounding volcanic rocks yield zircon U–Pb ages of 160−146 ​Ma (Late Jurassic), indicating that the mineralization is younger than 146 ​Ma.  相似文献   

3.
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4.
《Applied Geochemistry》1998,13(2):269-280
A slow flow, plug-through reactor was developed for measuring equilibrium and kinetic parameters of biogeochemical reactions on intact sections of sediment cores. The experimental approach was designed to preserve the structural, geochemical and microbiological integrity of the sediment sections and, hence, retrieve reaction parameters that apply to in-situ conditions.Inert tracer breakthrough experiments were performed on a variety of unconsolidated surface sediments from lacustrine, estuarine and marine depositional environments. The sediments studied cover wide ranges of composition, porosity (46–83%) and mean grain size (10−4−10−2 cm). Longitudinal dispersion coefficients were determined from the breakthrough curves of Br−. The curves were also used to check for early breakthrough or trailing, that is, features indicative of non-ideal flow conditions. Sediment plugs that exhibited these features were eliminated from further experiments.Dimensionless equilibrium adsorption coefficients (K) of NH4+, were calculated from measured retardation times between the breakthrough of NH4+ and Br−. The values of K at 5°C vary between 0.3 and 2.3, with the highest value obtained in a fine-grained marine sediment, the lowest in a coarse-grained lake sediment. The values for the marine and estuarine sediments agree with values reported in the literature. The dependencies of K on ionic strength (range 0.2-0.7m) and temperature (range 5–25°C) in an estuarine sediment confirm that the main sorption mechanism for NH4+ is ion exchange.The reactor was used in recirculation mode to measure steady-state rates of dissimilatory SO42− reduction in a salt-marsh sediment. Recirculation homogenizes solute concentrations within the reactor, hence facilitating the derivation of reaction rate expressions that depend on solution composition. The rate of microbial S04− reduction was found to be nearly independent of the dissolved SO42− concentration in the range of 2.2−1 mM. Fitting of the experimental rates to a Monod relationship resulted in a maximum estimate of the half-saturation concentration, Ks, of 240 μM. This value is comparable to those reported for a pure culture of SO42−-reducing bacteria, but is significantly smaller than the only other Ks value reported in the literature for SO42− utilization in a natural marine sediment.  相似文献   

5.
The oxygen isotope fractionation factor of dissolved oxygen gas has been measured during inorganic reduction by aqueous FeSO4 at 10−54 °C under neutral (pH 7) and acidic (pH 2) conditions, with Fe(II) concentrations ranging up to 0.67 mol L−1, in order to better understand the geochemical behavior of oxygen in ferrous iron-rich groundwater and acidic mine pit lakes. The rate of oxygen reduction increased with increasing temperature and increasing Fe(II) concentration, with the pseudo-first-order rate constant k ranging from 2.3 to 82.9 × 10−6 s−1 under neutral conditions and 2.1 to 37.4 × 10−7 s−1 under acidic conditions. The activation energy of oxygen reduction was 30.9 ± 6.6 kJ mol−1 and 49.7 ± 13.0 kJ mol−1 under neutral and acidic conditions, respectively. Oxygen isotope enrichment factors (ε) become smaller with increasing temperature, increasing ferrous iron concentration, and increasing reaction rate under acidic conditions, with ε values ranging from −4.5‰ to −11.6‰. Under neutral conditions, ε does not show any systematic trends vs. temperature or ferrous iron concentration, with ε values ranging from −7.3 to −10.3‰. Characterization of the oxygen isotope fractionation factor associated with O2 reduction by Fe(II) will have application to elucidating the process or processes responsible for oxygen consumption in environments such as groundwater and acidic mine pit lakes, where a number of possible processes (e.g. biological respiration, reduction by reduced species) may have taken place.  相似文献   

6.
The accumulation and mobility of Fe, Mn, Al, Cu, Ni and Pb in the sediments of two lakes (Clearwater, pH 4.5; and McFarlane, pH 7.5) near Sudbury, Ontario have been investigated. The Al, Cu and Ni concentrations are expectedly relatively high in the overlying waters of Clearwater Lake and much lower for Al and Cu in McFarlane Lake. The low trace metal concentrations found in the anoxic porewaters of Clearwater Lake could be explained by a sharp increase in porewater pH concomitant with SO42 reduction and H2S production within the first 1–2 cm of the sediments, which has conceivably led to the precipitation of mineral phases such as AL(OH)3, NiS, and CuS. In both lakes, Fe concentrations in anoxic porewaters appear to be controlled by FeS and/or FeCO3 formation. Solubility calculations also indicate MnCO3 precipitation in McFarlane Lake. In Clearwater Lake, however, both porewater and total Mn were relatively low, a possible result of the continuous loss of Mn(II) through the acidic interface. It is suggested that upwardly decreasing total Mn profiles resulting from the removal of Mn from the top sediment layers under acidic conditions may constitute a reliable symptom of recent lake acidification.The downward diffusion of AI, Cu and Ni from the overlying water to the sediments has been estimated from their concentration gradients at the interface and compared to their total accumulation rates in the sediments. In both lakes the diffusion of Al is negligible compared to its accumulation rate. However, diffusion accounts for 24–52% of the accumulation of Cu in the sediments of Clearwater Lake, but appears negligible in McFarlane Lake. The downward diffusive flux of Ni is important and may explain 76–161% of the estimated Ni accumulation rate in Clearwater Lake, and 59% in McFarlane Lake. The porewater Cu and Ni profiles suggest that the subsurface sedimentary trace metal peaks observed in Clearwater Lake (as in other acid lakes) may not be caused by sediment leaching or by a recent reduction in sedimentation but may have a diagenetic origin instead. Diffusion to the sediments thus appears to be an important and previously overlooked trace metal deposition mechanism, particularly in acid lakes.  相似文献   

7.
Light-induced reduction of dissolved and particulate Fe(III) has been observed to occur in the surface waters of the acidic mine pit lake of San Telmo (143,600 m2, pH 2.8, Fetotal = 2.72 mM). This photochemical production of Fe(II) is directly related to the intensity of solar radiation and competes with biologically catalyzed reactions (i.e., bacterial re-oxidation of Fe(II)) and physical processes (including ionic diffusion, advection, and convection, which tend to homogenize the epilimnetic concentration of Fe(II) at every moment). Therefore, diel cycles of Fe(II) concentration are observed at the lake surface, with minimum values of 10–20 μM Fe(II) (0.35–0.70% Fetotal) at the sunrise and sunset, and maximum values of 90 μM Fe(II) (3.2% Fetotal) at midday in August 2005. Field and experimental work conducted in San Telmo and other pit lakes of the Iberian Pyrite Belt (IPB) (pH 2.3–3.1, Fetotal = 0.34–17 mM) indicate that the kinetics of the photoreductive reaction is zero-order and is independent of the Fe(III) concentration, but highly dependent on the intensity of solar radiation and temperature. Experimental work conducted with natural Fe(III) minerals (schwertmannite, goethite, and lepidocrocite) suggests that dissolved organic matter is an important factor contributing to the photochemical production of Fe(II). The wavelengths involved in the photoreduction of Fe(III) include not only the spectrum of UV-A radiation (315–400 nm), but also part of the photosynthetically active radiation (PAR, 400–700 nm). This finding is of prime importance for the understanding of the photoreduction processes in the pit lakes of the IPB, because the photo-reactive depth is not limited to the penetration depth of UV-A radiation (upper 1–10 cm of the water column depending on the TDS content), but it is approximately equal to the penetration depth of PAR (e.g., first 4–6 m of the water column in San Telmo on July 2007); thus, increasing the importance of photochemical processes in the hydro(bio)geochemistry of pit lakes.  相似文献   

8.
《Applied Geochemistry》2004,19(8):1217-1232
Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO2 (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m−2 h−1 at pH 2.78 in the batch experiments to 0.009 μmol Si m−2 h−1 at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H+ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO2 demonstrated that dissolution rates were independent of CO2 within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO2 have been elevated over the last decade. This release of magmatic CO2 has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO2 may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO2 can be excluded.  相似文献   

9.
《Applied Geochemistry》1988,3(3):333-344
The sediments of Lake Anna, Virginia, act as a major sink for incoming acid mine drainage (AMD) pollutants (Fe, SO42−, H+) due to bacterial sulfate reduction (SR). Acid-volatile sulfide (AVS), elemental S, and pyrite concentrations in the sediments of the polluted arm of the lake are significantly greater than those in unpolluted sections of the lake. Measurements of SR using 35SSO42− showed that AVS and S0 are the major short-term (48 h) products of SR in these sediments. Inorganic forms of S(AVS, S0, and FeS2) made up from 60 to 100% of the total sediment S concentration. Pyrite concentrations in the sediment were high but decreased exponentially with distance from the AMD source, suggesting that the pyrite was deposited as stream detritus from the abandoned mines. Iron monosulfide and elemental S concentrations were highest at a station 1 km away from the AMD inflow, indicating formation in situ. There was no evidence for the formation of organic S species. The results suggest that in Fe- and S-rich locations such as those contaminated with acid mine drainage, the distribution of end products of SR may vary substantially from those reported for more moderate environments.  相似文献   

10.
An enrichment of light rare earth elements (LREE) is characteristic for most of the acidic, Fe- and SO4-rich pit lakes and groundwaters in the lignite mining area of Lower Lusatia (Germany). One of these acidic lakes – the pit lake “RL 1223” – has a strong thermal and chemical stratification. The upper water layer (0–9 m) shows pH values of about 3 during all seasons. The monimolimnion (10–17 m) of the lake is anoxic and has pH values of about 7. The rare earth element (REE) patterns of the upper lake water show enriched LREE (LaN/YbN = 1.6) whereas the opposite patterns (depletion of LREE, LaN/YbN = 0.4) are found in the anoxic water of the monimolimnion. Experiments were conducted to observe the behaviour of REE during Fe oxidation in water from the monimolimnion (depth 14 m). The sampled monimolimnion water was placed in plastic bottles, and the changing water chemistry was observed for 40 weeks after sampling. Due to the initial anoxic conditions almost all Fe precipitated in the investigated water, and the pH value decreased from about 7 to 3 during the oxidation. The Fe precipitates are identified as ferrihydrite which is transformed into goethite within the oxidation process. Stable pH conditions (pH 3.0) were reached after about 10 weeks of oxidation.The original REE patterns of the investigated water are generally reflected in the Fe precipitates collected at the beginning of the experiment as well as after up to 40 weeks of oxidation. However, in the corresponding water LREE were temporally enriched with a maximum LaN/YbN ratio of 1.0 and a maximum LaN/SmN ratio of 2.3 after 6 weeks of oxidation time (pH 3.8–4.9). Although complex geochemical changes took place between the start and the end of the experiment REE patterns observed at these points in time are nearly identical. These differences of the REE pattern can be explained by the sampling procedure. The experimental findings can be transmitted to the mining dump aquifers of the study area where geochemical conditions comparable to the experimental oxidation time from 3 to 6 weeks are found and hydrous ferric oxides are precipitating. Groundwater passing through the mining dumps can preferentially desorb LREE from the Fe precipitates and display the typical LREE enrichment and carry it to the epilimnion of the acidic pit lakes in Lower Lusatia.  相似文献   

11.
Chemical composition and equilibrium trends in mine pit lakes were examined to provide guidance for the application of geochemical models in predicting future lake water quality at prospective open pit mines. Composition trends show that elevated solute levels generally occur only at the extremes of acidic and alkaline pH conditions. Concentrations of cationic metals (Al, Cd, Cu, Fe, Mn, Pb, and Zn) are elevated only in acidic pit lakes, whereas anionic metalloids (As and Se) are generally elevated only in alkaline pit lakes. These trends are indicative of sulfide mineral oxidation and evapoconcentration for acidic and alkaline conditions, respectively.For nearly all pit lakes, SO4 is the dominant solute, but is limited by gypsum solubility. Fluorite, calcite, and barite are also important solubility controls. Well-defined solubility controls exist for the major metals (Al, Fe, Mn), including jurbanite and alunite for Al, ferrihydrite for Fe, and manganite, birnessite, and, possibly, rhodochrosite for Mn. Determinations of definite controls for the minor metals are less distinct, but may include otavite for Cd, brochantite and malachite for Cu, cerrusite and pyromorphite for Pb, and hydrozincite and Zn silicates for Zn. Concentrations of As and Se appear to be limited only by adsorption, but this control is sharply diminished by increased pH and SO4 concentration. In general, the concentrations of minor metals in pit lakes are not well represented by the theoretical solubilities of pure-phase minerals contained in the thermodynamic databases. Hence, modeling efforts will generally have to rely on empirical data on the leaching characteristics of pit wall-rocks to predict the concentrations of minor metals (Cd, Cu, Pb, Zn) in mine pit lakes.Methodologies for predicting pit lake water chemistry are still evolving. Geochemical and equilibrium trends in existing pit lakes can provide valuable information for guiding the development and application of predictive models. However, mineralogical studies of pit lake sediments, suspended particles, and alteration assemblages and studies of redox transformations are still needed to validate and refine the representations of geochemical processes in water quality models of mine pit lakes.  相似文献   

12.
The geochemistry of dissolved and suspended loads in river catchments of two low mountain ranges in Central Europe allows comparison of pertinent chemical weathering rates. Distinct differences in lithology, i.e. granites prevailing in the Black Forest compared to Palaeozoic sediments in the Rhenish Massif, provide the possibility to examine the influence of lithology on weathering. Here we determine the origin of river water using the stable isotope ratio δ18OH2O and we quantify the geogenic proportions of sulphate from stable isotope ratios δ34SSO4 and δ18OSO4. Particularly in catchments with abundant pyrite, determination of the geogenic amount of sulphate is important, since oxidation of pyrite leads to acidity, which increases weathering. Our results show that spatially averaged silicate weathering rates are higher for the river catchments Acher and Gutach in the Black Forest (10–12 t/km2/yr) compared to the river catchments of the Möhne dam and the Aabach dam in the Rhenish Massif (2–6 t/km2/yr). Correspondingly, the CO2 consumption by silicate weathering in the Black Forest (334–395 × 103 mol/km2/yr) is more than twice as high as in the Rhenish Massif (28–151 × 103 mol/km2/yr). These higher rates for watersheds of the Black Forest are likely due to steeper slopes leading to higher mechanical erosion with respective higher amounts of fresh unweathered rock particulates and due to the fact that the sediments in the Rhenish Massif have already passed through at least one erosion cycle. Carbonate weathering rates vary between 12 and 38 t/km2/yr in the catchments of the Rhenish Massif. The contribution of sulphuric acid to the silicate weathering is higher in the catchments of the Rhenish Massif (9–16%) than in the catchments of the Black Forest (5–7%) due to abundant pyrite in the sediments of the Rhenish Massif. Three times higher long-term erosion rates derived from cosmogenic nuclides compared to short-term erosion rates derived from river loads in Central Europe point to three times higher CO2 consumption during the past 103 to 104 years.  相似文献   

13.
The Cu hydroxy mineral, atacamite, is commonly associated with saline environments and is generally thought to dissolve rapidly in the presence of fresh water. A Cu contaminated soil from the arid Namaqualand region, South Africa, shows atacamite as the dominant Cu containing mineral. The stability of the Cu phase in this soil was determined through equilibrium and leaching studies using both deionised water (DI) and a concentrated (0.5 M) NaCl solution. Initially a high concentration of exchangeable Cu was released from the soils leached with NaCl. Continued leaching with NaCl resulted in a substantial decrease in Cu release as atacamite equilibria started to control dissolved Cu. This suggests that an initial spike of Cu laden water will leach from the soils at the onset of a large rainfall event. Further additions of water will result in a lower but sustained release of Cu from the soil. The Cu contaminated soils are exposed to acidic sulphate leachate thus the dissolution kinetics of synthetic atacamite in the acidic range (pH 5.5–4.0) was determined in both NaCl and DI solutions. The kinetic data showed that atacamite dissolution rates are significantly higher in DI than in NaCl but the rates converge at pH 4. In comparison to common acid soluble minerals, atacamite displays a moderate dissolution rate (10−9.55–10−7.14 mol m−2 s−1) within the acid range (pH 5.5–4.0). The atacamite dissolution reaction order with respect to pH is 1.3 and 1.6 in DI and NaCl solutions, respectively, suggesting that dissolution rates of atacamite are highly pH dependent in the acid range. The type of acid used to lower the pH had no effect on the reaction kinetics, with HNO3 and H2SO4 resulting in comparable dissolution rates of atacamite at pH 4.5.  相似文献   

14.
 Within the next 10–15 years, over 35 mines in Nevada will have a lake in their open pit mines after dewatering and cessation of mining. Of the ten past or existing pit lakes at eight different gold mines for which temporal data are available, most had near neutral pH, yet most had at least one constituent (e.g., As, SO4, TDS) that exceeded drinking water standards for at least one sampling event. Most samples from pit lakes had TDS exceeding drinking water standards, but lower than that in the natural Pyramid (TDS≈5,500 mg/l) and Walker (TDS≈14,000 mg/l) Lakes. In the past century, salinity increased in both natural, terminal lakes, in part due to irrigation withdrawals and evapoconcentration. The salinity in the pit lakes may also increase through time via evapoconcentration. However, water balance models indicate that up to 132% (Walker Lake) of the total yearly inflow evaporates from the terminal lakes, whereas steady-state may be reached in the pit lakes modelled, where evaporative losses account for only ≈6% of the total pit lake volume annually and ≈100% of the net inflow (groundwater inflow minus outflow, precipitation and runoff into the lake). The effects of evapoconcentration are expected to be less significant at most pit lakes than at the natural, terminal lakes because (1) evaporation rates are lower at many pit lakes because they are located at higher elevations than the terminal lakes, and (2) the surface area to depth ratio of the pit lakes is >1000 times smaller than that of the terminal lakes. Received: 1 March 1999 · Accepted: 13 April 1999  相似文献   

15.
《Applied Geochemistry》2000,15(8):1203-1218
Ca6[Al(OH)6]2(CrO4)3·26H2O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca6[Al(OH)6]2(CrO4)3·26H2O⇌6Ca2++2Al(OH)−4+3CrO2−4+4OH−+26H2O varies with pH unless a CaCrO4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca2++CrO2−4=CaCrO4(aq) was obtained by minimizing the variance in the IAP for Ca6[Al(OH)6]2(CrO4)3·26H2O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log KSP) of Ca6[Al(OH)6]2(CrO4)3·26H2O at 25°C is −41.46±0.30. The temperature dependence of the log KSP is log KSP=A−B/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG0r,298 and ΔH0r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol−1. the values of ΔC0P,r,298 and ΔS0r,298 are −1506±140 and −534±83 J mol−1 K−1. Using these values and published standard state partial molal quantities for constituent ions, ΔG0f,298=−15,131±19 kJ mol−1, ΔH0f,298=−17,330±8.6 kJ mol−1, ΔS0298=2.19±0.10 kJ mol−1 K−1, and ΔC0Pf,298=2.12±0.53 kJ mol−1 K−1, were calculated.  相似文献   

16.
《Applied Geochemistry》2006,21(5):849-857
The determination of reduced S species in natural waters is particularly difficult due to their high instability and chemical and physical interferences in the current analytical methods. In this paper a new, rapid and reliable analytical procedure is presented, named the Cd–IC method, for their determination as ΣS2− via oxidation to SO42- after chemical trapping with an ammonia–cadmium solution that allows precipitation of all the reduced S species as CdS. The S2−–SO4 is analysed by ion-chromatography. The main advantages of this method are: low cost, high stability of CdS precipitate, absence of interferences, low detection limit (0.01 mg/L as SO4 for 10 mL of water) and low analytical error (about 5%). The proposed method has been applied to more than 100 water samples from different natural systems (water discharges and cold wells from volcanic and geothermal areas, crater lakes) in central-southern Italy.  相似文献   

17.
The effect of citrate and oxalate on tremolite dissolution rate was measured at 37 °C in non-stirred flow-through reactors, using modified Gamble’s solutions at pH 4 (macrophages), 7.4 (interstitial fluids) and 5.5 (intermediate check point) containing 0, 0.15, 1.5 and 15 mmol L−1 of citrate or oxalate. The dissolution rates calculated from Si concentration in the output solutions without organic ligands depend on pH, decreasing when the pH increases from −13.00 (pH 4) to −13.35 (pH 7.4) mol g−1 s−1 and following a proton-promoted mechanism. The presence of both ligands enhances dissolution rates at every pH, increasing this effect when the ligand concentration increases. Citrate produces a stronger effect as a catalyst than oxalate, mainly at more acidic pHs and enhances dissolution rates until 20 times for solutions with 15 mmol L−1 citrate. However, at pH 7.4 the effect is lighter and oxalate solutions (15 mmol L−1) only enhances dissolution rates eight times respect to free organic ligand solutions. Dissolution is promoted by the attack to protons and organic ligands to the tremolite surface. Magnesium speciation in oxalate and citrate solutions shows that Mg citrate complexes are more effective than oxalate ones during the alteration of tremolite in magrophages, but this tendency is the opposite for interstitial fluids, being oxalate magnesium complexes stronger. The biodurability estimations show that the destruction of the fibers is faster in acidic conditions (macrophages) than in the neutral solutions (interstitial fluid). At pH 4, both ligands oxalate and citrate reduce the residence time of the fibers with respect to that calculated in absence of ligands. Nevertheless, at pH 7.4 the presence of ligands does not reduce significantly the lifetime of the fibers.  相似文献   

18.
Monsoon-induced coastal upwelling, land run-off, benthic and atmospheric inputs make the western Indian shelf waters biologically productive that is expected to lead to high rates of mineralisation of organic matter (OM) in the sediments. Dissimilatory sulphate reduction (SR) is a major pathway of OM mineralisation in near-shore marine sediments owing to depletion of other energetically more profitable electron acceptors (O2, NO3 ?, Mn and Fe oxides) within few millimetres of the sediment-water interface. We carried out first ever study to quantify SR rates in the inner shelf sediments off Goa (central west coast of India) using the 35S radiotracer technique. The highest rates were recorded in the upper 10 cm of the sediment cores and decreased gradually thereafter below detection. Despite significant SR activity in the upper ～12 to 21 cm at most of the sites, pore water sulphate concentrations generally did not show much variation with depth. The depth integrated SR rate (0.066–0.46 mol m?2 year?1) decreased with increasing water depth. Free sulphide was present in low concentrations (0–3 μM) in pore waters at shallow stations (depth <30 m). However, high build-up of sulphide (100–600 μM) in pore waters was observed at two deeper stations (depths 39 and 48 m), 7–11 cm below the sediment-water interface. The total iron content of the sediment decreased from ～7 to 5 % from the shallowest to the deepest station. The high pyrite content indicates that the shelf sediments act as a sink for sulphide accounting for the low free sulphide levels in pore water. In the moderately organic rich (2–3.5 %) sediments off Goa, the measured SR rates are much lower than those reported from other upwelling areas, especially off Namibia and Peru. The amount of organic carbon remineralised via sulphate reduction was ～0.52 mol m?2 year?1. With an estimated average organic carbon accumulation rate of ～5.6 (±0.5) mol m?2 year?1, it appears that the bulk of organic matter gets preserved in sediments in the study region.  相似文献   

19.
Significant resources are spent on counteracting the effects of acidification, mainly by liming. Due to lower S and N deposition in Europe and North America, authorities are changing directives and strategies for remediation and reducing liming. However, as the acid–base buffer capacity differs in different water bodies, the desirable reduction of the lime dose is variable. In this study, a geochemical model is used to predict pH and inorganic monomeric Al (Ali) when liming is reduced and finally terminated in the 3000 Swedish lakes currently treated with lime. To estimate Ca and Mg concentrations not affected by liming for use in the model, the Ca/Mg ratio in nearby unlimed reference lakes was used. For the modelling of pH and inorganic Al the Visual MINTEQ program including the Stockholm Humic Model recently calibrated for Swedish fresh water was used. The predictions were validated with modelling results from six monitored lakes, in which liming had been terminated. The use of geochemical modelling appeared to be a promising tool for the calculation of accurate lime requirements in acid waters. For simulations in which liming was completely terminated, the pH value decreased by, on average, 1 pH unit to pH 5.7, whereas Ali increased by 17 μg L−1 to 32 μg L−1. If liming was reduced by half, the pH would drop only 0.3 pH units and Ali would increase by 2 μg L−1. Lakes in the south-western part of Sweden were predicted to reach a lower pH and higher Ali, which would be expected due to their greater historical S deposition. The results indicate that liming can be terminated in certain areas and in other areas be reduced without increases in the lake acidity.  相似文献   

20.
The quantification of carbon burial in lake sediments, and carbon fluxes derived from different origins are crucial to understand modern lacustrine carbon budgets, and to assess the role of lakes in the global carbon cycle. In this study, we estimated carbon burial in the sediment of Lake Qinghai, the largest inland lake in China, and the carbon fluxes derived from different origins. We find that: (1) The organic carbon burial rate in lake sediment is approximately 7.23 g m−2 a−1, which is comparable to rates documented in many large lakes worldwide. We determined that the flux of riverine particulate organic carbon (POC) is approximately 10 times higher than that of dissolved organic carbon (DOC). Organic matter in lake sediments is primarily derived from POC in lake water, of which approximately 80% is of terrestrial origin. (2) The inorganic carbon burial rate in lake sediment is slightly higher than that of organic carbon. The flux of riverine dissolved inorganic carbon (DIC) is approximately 20 times that of DOC, and more than 70% of the riverine DIC is drawn directly and/or indirectly from atmospheric CO2. (3) Both DIC and DOC are concentrated in lake water, suggesting that the lake serves as a sink for both organic and inorganic carbon over long term timescales. (4) Our analysis suggests that the carbon burial rates in Lake Qinghai would be much higher in warmer climatic periods than in cold ones, implying a growing role in the global carbon cycle under a continued global warming scenario.  相似文献   

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