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1.
A dated core from the profoundal zone in a pristine oligotrophic acidic lake was analyzed for Cd as well as for Al, Ca, Fe, Mg, Mn, Pb, Ti and total carbon and nitrogen. Overlying water and porewater samples were also obtained on six occasions at the same site, and yielded vertical profiles of pH and dissolved Cd, Ca, Fe, Mg, Mn, sulfide, SO4−2, organic and inorganic carbon concentrations. These extensive porewater and sediment geochemical data were used, together with information on infaunal benthos, to decipher the sedimentary record of Cd contamination. Depth variation of sediment Ca concentrations indicate that the lake suffered from progressive acidification starting about 1950. The present-day accumulation rate of Cd (JaccCd = 5.4 ± 0.4 × 10−11 mol cm−2 yr−1) in the sediments is the sum of the flux of Cd deposited with settling particles (JSCd = 3.3 ± 0.2 × 10−11 mol cm−2 yr−1) and the fluxes of dissolved Cd across the sediment-water interface due to molecular diffusion (JDCd = 1.8 ± 0.3 × 10−11 mol cm−2 yr−1), bioturbation (JBCd = 1.1 ± 0.2 × 10−14 mol cm−2 yr−1) and bioirrigation (JICd = 0.27 ± 0.05 × 10−11 mol cm−2 yr−1). Biological mixing of the sediments was negligible. The shape of the vertical profile of total Cd concentration with depth in the sediment appears to be determined more by its input history than by post-depositional mobilization and redistribution in the sediment column.  相似文献   

2.
The injection and recovery of oxic water into deep anoxic aquifers may help to alleviate short- and long-term imbalance between freshwater supply and demand. The extent and structure of physical and geochemical heterogeneity of the aquifer will impact the water quality evolution during injection, storage and recovery. Water–sediment interactions within the most permeable parts of the aquifer, where the bulk of the injectant will penetrate, may dominate, however, water quality may also be impacted by interactions within the finer-grained, less permeable but potentially highly reactive media. In this study, the heterogeneity of the reductive capacity of an aquifer selected for water reuse projects was characterised, the amount, type and reactivity of the sedimentary reductants present determined, and the relationship between reductive capacity and sedimentary lithologies quantified. The average potential reductive capacities (PRCTOT), based on total organic C and pyrite concentrations of the sediment, were quantified for sands (382 μmol O2 g−1), clays (1522 μmol O2 g−1), and silts (1957 μmol O2 g−1). Twenty-seven samples, spanning the three different lithologies, were then incubated for 50 days and the measured reductive capacities (MRC) determined for the sands (29.2 μmol O2 g−1), silts (136 μmol O2 g−1), and clays (143 μmol O2 g−1). On average, the MRC were 10% of the PRCTOT. The main consumers of O2 were pyrite (20–100%), sedimentary organic matter (SOM; 3–56%), siderite (3–28%) and Fe(II)-aluminosilicates (8–55%). The incubation data plus hydrogeochemical modelling, indicated that pH-buffering was controlled firstly by dissolution of trace level carbonates, followed by dissolution of feldspars. Zinc, Co, Ni, Cd and Pb were readily mobilized during incubation.  相似文献   

3.
With a half-life of 15.7 Ma, a high mobility and the potential to accumulate in the biosphere, 129I is considered, in safety assessment calculations for radioactive waste repositories, to be one of the main contributors to the radiological dose. Several authors have reported that, at low concentration, I− is weakly retained on argillaceous rocks. This process is not yet well-understood and different hypotheses have been put forward as to whether reactive phases or experimental artifacts (e.g. pyrite oxidation) could be the reason for the retention of I− observed at low concentration. The aim of this study was to investigate the effect on I− mobility of (i) the redox conditions and (ii) the amount of pyrite and natural organic matter (NOM) contents of the rock. These questions were addressed by performing batch sorption, through-diffusion and out-diffusion experiments on rock samples of Toarcian argillaceous rock from Tournemire (Aveyron, France). One of the challenges faced during this study was to distinguish actual transport properties from experimental artifacts. A especially elaborate experimental set-up allowed limiting the (i) oxidation of both argillaceous rock and I−, and (ii) carbonate precipitation. A comparison of the batch sorption results obtained for two Toarcian clay specimens, that differed in their amount of pyrite and NOM, allowed relating I− sorption to pyrite oxidation. However, no evidence was found to associate the I− behavior to the NOM amounts. While the through-diffusion experiments showed a very slight sorption (distribution ratio (Rd) = 0.016 mL g−1) for the lowest I− concentration under oxic conditions, the out-diffusion tests performed after the through-diffusion experiments on the same cells showed significant sorption under both oxic and anoxic conditions, resulting in Rd ranging from 0.02 mL g−1 to 1.25 mL g−1. The range of Rd values was higher for the upstream reservoir under oxic conditions. The discrepancies observed between the through-diffusion and the out-diffusion experiments suggest a kinetic control of the I− uptake by argillaceous rocks under oxic and anoxic conditions.  相似文献   

4.
This work is devoted to the physico-chemical study of cadmium and lead interaction with diatom-water interfaces for two marine planktonic (Thalassiosira weissflogii, TW; Skeletonema costatum, SC) and two freshwater periphytic species (Achnanthidium minutissimum, AMIN; Navicula minima, NMIN) by combining adsorption measurements with surface complexation modeling. Adsorption kinetics was studied as a function of pH and initial metal concentration in sodium nitrate solution and in culture media. Kinetic data were consistent with a two-step mechanism in which the loss of a water molecule from the inner coordination sphere of the metal is rate limiting. Reversible adsorption experiments, with 3 h of exposure to metal, were performed as a function of pH (2-9), metal concentration in solution (10−9-10−3 M), and ionic strength (10−3-1.0 M). While the shape of pH-dependent adsorption edge is similar among all four diatom species, the constant-pH adsorption isotherm and maximal binding capacities differ. Measurements of electrophoretic mobilities (μ) revealed negative surface potential for AMIN diatom, however, the absolute value of μ decreases with increase of [Pb2+]aq suggesting the metal adsorption on negative surface sites. These observations allowed us to construct a surface complexation model (SCM) for cadmium and lead binding by diatom surfaces that postulates the Constant Capacitance of the electric double layer and considers Cd and Pb complexation with mainly carboxylic and, partially, silanol groups. In the full range of investigated Cd concentration, the SCM is able to describe the concentration of adsorbed metal as a function of [Cd2+]aq without implying the presence of high affinity, low abundance sites, that are typically used to model the metal interactions with natural multi-component organic substances. At the same time, Cd fast initial reaction requires the presence of “highly reactive sites” those concentration represents only 2.5-3% of the total amount of carboxylic sites. For reversible adsorption experiments, the dominating carboxylic groups, whose concentration is allowed to vary within the uncertainty of experimental acid-base titrations, are sufficient to reproduce the metal adsorption isotherms. Results of this study strongly suggest that laboratory experiments performed in a wide range of metal to biomass ratios, represent robust and relatively simple method for assessing the distribution of metals between aqueous solution and planktonic and periphytic biomass in natural settings.  相似文献   

5.
Bulk Cd adsorption isotherm experiments, thermodynamic equilibrium modeling, and Cd K edge EXAFS were used to constrain the mechanisms of proton and Cd adsorption to bacterial cells of the commonly occurring Gram-positive and Gram-negative bacteria, Bacillus subtilis and Shewanella oneidensis, respectively. Potentiometric titrations were used to characterize the functional group reactivity of the S. oneidensis cells, and we model the titration data using the same type of non-electrostatic surface complexation approach as was applied to titrations of B. subtilis suspensions by Fein et al. (2005). Similar to the results for B. subtilis, the S. oneidensis cells exhibit buffering behavior from approximately pH 3-9 that requires the presence of four distinct sites, with pKa values of 3.3 ± 0.2, 4.8 ± 0.2, 6.7 ± 0.4, and 9.4 ± 0.5, and site concentrations of 8.9(±2.6) × 10−5, 1.3(±0.2) × 10−4, 5.9(±3.3) × 10−5, and 1.1(±0.6) × 10−4 moles/g bacteria (wet mass), respectively. The bulk Cd isotherm adsorption data for both species, conducted at pH 5.9 as a function of Cd concentration at a fixed biomass concentration, were best modeled by reactions with a Cd:site stoichiometry of 1:1. EXAFS data were collected for both bacterial species as a function of Cd concentration at pH 5.9 and 10 g/L bacteria. The EXAFS results show that the same types of binding sites are responsible for Cd sorption to both bacterial species at all Cd loadings tested (1-200 ppm). Carboxyl sites are responsible for the binding at intermediate Cd loadings. Phosphoryl ligands are more important than carboxyl ligands for Cd binding at high Cd loadings. For the lowest Cd loadings studied here, a sulfhydryl site was found to dominate the bound Cd budgets for both species, in addition to the carboxyl and phosphoryl sites that dominate the higher loadings. The EXAFS results suggest that both Gram-positive and Gram-negative bacterial cell walls have a low concentration of very high-affinity sulfhydryl sites which become masked by the more abundant carboxyl and phosphoryl sites at higher metal:bacteria ratios. This study demonstrates that metal loading plays a vital role in determining the important metal-binding reactions that occur on bacterial cell walls, and that high affinity, low-density sites can be revealed by spectroscopy of biomass samples. Such sites may control the fate and transport of metals in realistic geologic settings, where metal concentrations are low.  相似文献   

6.
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   

7.
We have performed experiments to evaluate Au solubility in natural, water-saturated basaltic melts as a function of oxygen fugacity. Experiments were carried out at 1000 °C and 200 MPa, and oxygen fugacity was controlled at the fayalite-magnetite-quartz (FMQ) oxygen fugacity buffer and FMQ + 4. All experiments were saturated with a metal-chloride aqueous solution loaded initially as a 10 wt% NaCl eq. fluid. The stable phase assemblage at FMQ consists of basalt melt, olivine, clinopyroxene, a single-phase aqueous fluid, and metallic Au. The stable phase assemblage at FMQ + 4 consists of basalt melt, clinopyroxene, magnetite-spinel solid solution, a single-phase aqueous fluid, and metallic Au. Silicate glasses (i.e., quenched melt) and their contained crystalline material were analyzed by using both electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Measured Au concentrations in the quenched melt range from 4.8 μg g−1 to 0.64 μg g−1 at FMQ + 4, and 0.54 μg g−1 to 0.1 μg g−1 at FMQ. The measured solubility of Au in olivine and clinopyroxene was consistently below the LA-ICP-MS limit of detection (i.e., 0.1 μg g−1). These melt solubility data place important limitations on the dissolved Au content of water-saturated, Cl- and S-bearing basaltic liquids at geologically relevant fO2 values. The new data are compared to published, experimentally-determined values for Au solubility in dry and hydrous silicate liquids spanning the compositional range from basalt to rhyolite, and the effects of melt composition, oxygen fugacity, pressure and temperature are discussed.  相似文献   

8.
Pyrite dissolution and interaction with Fe(II), Co(II), Eu(III) and U(VI) have been studied under anoxic conditions by solution chemistry and spectroscopic techniques. Aqueous data show a maximal cation uptake above pH 5.5. Iron (II) uptake can explain the non-stoichiometric [S]aq/[Fe]aq ratios often observed during dissolution experiments. Protonation data corrected for pyrite dissolution resulted in a proton site density of 9 ± 3 sites nm−2. Concentration isotherms for Eu(III) and U(VI) sorption on pyrite indicate two different behaviours which can be related to the contrasted redox properties of these elements. For Eu(III), sorption can be explained by the existence of a unique site with a saturation concentration of 1.25 × 10−6 mol g−1. In the U(VI) case, sorption seems to occur on two different sites with a total saturation concentration of 4.5 × 10−8 mol g−1. At lower concentration, uranium reduction occurs, limiting the concentration of dissolved uranium to the solubility of UO2(s).Scanning electron microscopy and micro-Raman spectrometry of U(VI)-sorbed pyrite indicate a heterogeneous distribution of U at the pyrite surface and a close association with oxidized S. X-ray photoelectron spectroscopy confirms the partial reduction of U and the formation of a hyperstoichiometric UO2+x(s). Our results are consistent with a chemistry of the pyrite surface governed not by Fe(II)-bound hydroxyl groups, but by S groups which can either sorb cations and protons, or sorb and reduce redox-sensitive elements such as U(VI).  相似文献   

9.
Kinetics of microbial sulfate reduction in estuarine sediments   总被引：2，自引：0，他引：2  
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, Km, 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, Rmax, 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 Rmax 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 Corg/N ratios. Inflow solutions supplemented with lactate enhanced potential SRR at all three sites. Slurry incubations systematically yielded higher Rmax 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   

10.
Surface chemistry of disordered mackinawite (FeS)   总被引：1，自引：0，他引：1  
Disordered mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by Ksapp = {Fe2+} · {H2S(aq)} · {H+}−2 = 10+4.87±0.27. Acid-base titrations show that the point of zero charge (PZC) of disordered mackinawite lies at pH ∼7.5. The hydrated disordered mackinawite surface can be best described by strongly acidic mono-coordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH < PZC and has a concentration of 1.2 × 10−3 mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 × 10−3 mol/g FeS, determines surface charge changes. Total site density is 4 sites nm−2. The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.  相似文献   

11.
The interaction of the lanthanides (Ln) with humic substances (HS) was investigated with a novel chemical speciation tool, Capillary Electrophoresis-Inductively Coupled Plasma Mass Spectrometry (CE-ICP-MS). By using an EDTA-ligand competition method, a bi-modal species distribution of LnEDTA and LnHS is attained, separated by CE, and detected online by sector field ICP-MS. We quantified the binding of all 14 rare earth elements (REEs), Sc and Y with Suwannee river fulvic acid, Leonardite coal humic acid, and Elliot soil humic acid under environmental conditions (pH 6-9, 0.001-0.1 mol L−1 NaNO3, 1-1000 nmol L−1 Ln, 10-20 mg L−1 HS). Conditional binding constants for REE-HS interaction (Kc) ranged from 8.9 < log Kc < 16.5 under all experimental conditions, and display a lanthanide contraction effect, ΔLKc: a gradual increase in Kc from La to Lu by 2-3 orders of magnitude as a function of decreasing ionic radius. HS polyelectrolyte effects cause Kc to increase with increasing pH and decreasing ionic strength. ΔLKc increases significantly with increasing pH, and likely with decreasing ionic strength. Based on a strong correlation between ΔLKc values and denticity for organic acids, we suggest that HS form a range of tri- to tetra-dentate complexes under environmental conditions. These results confirm HS to be a strong complexing agent for Ln, and show rigorous experimental evidence for potential REE fractionation by HS complexation.  相似文献   

12.
The solubility of baddeleyite (ZrO2) and the speciation of zirconium have been investigated in HF-bearing aqueous solutions at temperatures up to 400 °C and pressures up to 700 bar. The data obtained suggest that in HF-bearing solutions zirconium is transported mainly in the form of the hydroxyfluoride species ZrF(OH)3° and ZrF2(OH)2°. Formation constants determined for these species (Zr4+ + nF− + mOH− = ZrFn(OH)m°) range from 43.7 at 100 °C to 46.41 at 400 °C for ZrF(OH)3°, and from 37.25 at 100 °C to 43.88 at 400 °C for ZrF2(OH)2°.Although the solubility of ZrO2 is retrograde with respect to temperature, the measured concentrations of Zr are orders of magnitude higher than those predicted from theoretical extrapolations based on simple fluoride species (ZrF3+-ZrF62−). Model calculations performed for zircon show that zirconium can be transported by aqueous fluids in concentrations sufficient to account for the concentration of this metal at conditions commonly encountered in fluoride-rich natural hydrothermal systems.  相似文献   

13.
Published solubility data for amorphous ferric arsenate and scorodite have been reevaluated using the geochemical code PHREEQC with a modified thermodynamic database for the arsenic species. Solubility product calculations have emphasized measurements obtained under conditions of congruent dissolution of ferric arsenate (pH < 3), and have taken into account ion activity coefficients, and ferric hydroxide, ferric sulfate, and ferric arsenate complexes which have association constants of 104.04 (FeH2AsO42+), 109.86 (FeHAsO4+), and 1018.9 (FeAsO4). Derived solubility products of amorphous ferric arsenate and crystalline scorodite (as log Ksp) are −23.0 ± 0.3 and −25.83 ± 0.07, respectively, at 25 °C and 1 bar pressure. In an application of the solubility results, acid raffinate solutions (molar Fe/As = 3.6) from the JEB uranium mill at McClean Lake in northern Saskatchewan were neutralized with lime to pH 2-8. Poorly crystalline scorodite precipitated below pH 3, removing perhaps 98% of the As(V) from solution, with ferric oxyhydroxide (FO) phases precipitated starting between pH 2 and 3. Between pH 2.18 and 7.37, the apparent log Ksp of ferric arsenate decreased from −22.80 to −24.67, while that of FO (as Fe(OH)3) increased from −39.49 to −33.5. Adsorption of As(V) by FO can also explain the decrease in the small amounts of As(V)(aq) that remain in solution above pH 2-3. The same general As(V) behavior is observed in the pore waters of neutralized tailings buried for 5 yr at depths of up to 32 m in the JEB tailings management facility (TMF), where arsenic in the pore water decreases to 1-2 mg/L with increasing age and depth. In the TMF, average apparent log Ksp values for ferric arsenate and ferric hydroxide are −25.74 ± 0.88 and −37.03 ± 0.58, respectively. In the laboratory tests and in the TMF, the increasing crystallinity of scorodite and the amorphous character of the coexisting FO phase increases the stability field of scorodite relative to that of the FO to near-neutral pH values. The kinetic inability of amorphous FO to crystallize probably results from the presence of high concentrations of sulfate and arsenate.  相似文献   

14.
The re-vegetation of soils contaminated by potentially harmful metals is generally considered a suitable option to reduce the metal dispersion in surrounding environments. A continuous flow experiment was conducted to quantitatively assess the effect of Italian ryegrass (Lolium multiflorum Lam.) root activity on the weathering of smithsonite (ZnCO3), a common Zn mineral. At the end of the experiment (10 days), the total amount of Zn released by smithsonite was increased by a factor of 2.25 in the presence of plants. This increase was due not only to plant uptake but also to the enhancement of the Zn release into leachates. The rate of Zn release from smithsonite to leachates was 2.9 × 10−4 μg g−1 s−1 and 1.5 × 10−4 μg g−1 s−1 in the presence and the absence of plants, respectively. The strong correlation (r = 0.95; p < 0.001) between concentrations of Zn and dissolved organic C (DOC) produced by the rhizosphere activity in leachates indicated that organic root exudates and secretions were closely involved in smithsonite weathering. Although the results are derived from laboratory study, and further in situ investigations over the long term are needed, they clearly highlighted that plants can enhance metal release into the environment by accelerating mineral weathering. Therefore, it is suggested that the ability of plants to alter metal phases in soils should be further taken into account when re-vegetation strategies are proposed for the rehabilitation of metal-polluted soils.  相似文献   

15.
This study used batch reactors to quantify the mechanisms and rates of calcite dissolution in the presence and absence of a single heterotrophic bacterial species (Burkholderia fungorum). Experiments were conducted at T = 28°C and ambient pCO2 over time periods spanning either 21 or 35 days. Bacteria were supplied with minimal growth media containing either glucose or lactate as a C source, NH4+ as an N source, and H2PO4− as a P source. Combining stoichiometric equations for microbial growth with an equilibrium mass-balance model of the H2O-CO2-CaCO3 system demonstrates that B. fungorum affected calcite dissolution by modifying pH and alkalinity during utilization of ionic N and C species. Uptake of NH4+ decreased pH and alkalinity, whereas utilization of lactate, a negatively charged organic anion, increased pH and alkalinity. Calcite in biotic glucose-bearing reactors dissolved by simultaneous reaction with H2CO3 generated by dissolution of atmospheric CO2 (H2CO3 + CaCO3 → Ca2+ + 2HCO3−) and H+ released during NH4+ uptake (H+ + CaCO3 → Ca2+ + HCO3−). Reaction with H2CO3 and H+ supplied ∼45% and 55% of the total Ca2+ and ∼60% and 40% of the total HCO3−, respectively. The net rate of microbial calcite dissolution in the presence of glucose and NH4+ was ∼2-fold higher than that observed for abiotic control experiments where calcite dissolved only by reaction with H2CO3. In lactate bearing reactors, most H+ generated by NH4+ uptake reacted with HCO3− produced by lactate oxidation to yield CO2 and H2O. Hence, calcite in biotic lactate-bearing reactors dissolved by reaction with H2CO3 at a net rate equivalent to that calculated for abiotic control experiments. This study suggests that conventional carbonate equilibria models can satisfactorily predict the bulk fluid chemistry resulting from microbe-calcite interactions, provided that the ionic forms and extent of utilization of N and C sources can be constrained. Because the solubility and dissolution rate of calcite inversely correlate with pH, heterotrophic microbial growth in the presence of nonionic organic matter and NH4+ appears to have the greatest potential for enhancing calcite weathering relative to abiotic conditions.  相似文献   

16.
Time-dependent sorption and desorption of Cd on calcite was studied over 210 days utilizing 109Cd as a tracer to distinguish between ‘labile’ and ‘non-labile’ forms of sorbed Cd. Stabilizing the calcite suspensions for 12 months under atmospheric PCO2 and controlled temperature was necessary to reliably follow Cd dynamics following initial sorption. Results revealed time-dependant Cd sorption and marked desorption hysteresis by calcite under environmentally relevant conditions. Data obtained were fitted to a first-order kinetic model and a concentric shell diffusion model. Both models described the progressive transfer of Cd2+ to a less reactive form within calcite and subsequent desorption of Cd subject to different initial contact times. The kinetic model provided a better fit to the combined sorption and desorption data (R2 = 0.992). It differentiates between two ‘pools’ of sorbed Cd2+ on calcite, ‘labile’ and ‘non-labile’, in which labile sorbed Cd is in immediate equilibrium with the free Cd2+ ion activity in solution whereas non-labile Cd is kinetically restricted. For the diffusion model (R2 = 0.959), the rate constants describing Cd dynamics in calcite produced a half-life for Cd desorption of ∼175 d, for release to a ‘zero-sink’ solution. Results from this study allow comment on the likely mechanisms occurring at the calcite surface following long-term Cd sorption.  相似文献   

17.
The anionic charge of humic substances (HS) plays a major role in the interaction of HS with other components. Therefore, the potential of the polyelectrolyte titration technique to obtain the charge density of HS in simple 1-1 electrolyte solutions has been investigated. Titrations are carried out with an automatic titrator combined with the “Mütek particle charge detector” which allows determination of the Mütek potential and the pH as a function of the added amount of titrant which is a solution of poly-diallyldimethylammonium chloride (polyDADMAC), a cationic strong polyelectrolyte. When the Mütek potential reverses its sign the iso-electric point (IEP) of the polyDADMAC-HS complex is reached. The polyDADMAC/HS mass ratio at the IEP gives information on the HS charge density and from the pH changes in solution an estimate of the charge regulation in the HS-polyDADMAC complex can be obtained. In general, for polyDADMAC-HS complexes an increase in the dissociation of the acid groups of HS is found (charge regulation). The charge regulation decreases with increasing concentration of 1-1 background electrolyte. Cation incorporation can be neglected at 1-1 electrolyte concentrations ? 1 mmol L−1 and a 1-1 stoichiometry exists between the polyDADMAC and HS charge. However, at these low salt concentrations the charge regulation is substantial. A detailed analysis of purified Aldrich humic acid (PAHA) at pH 5 and a range of KCl concentrations reveals that the anionic charge of PAHA in the complex increases at 5 mmol L−1 KCl by 30% and at 150 mmol L−1 KCl by 12%. On the other hand, increasing amounts of K+ become incorporated in the complex: at 5 mmol L−1 KCl 5% and at 150 mmol L−1 KCl 24% of the PAHA charge is balanced by K+. By comparing at pH 5 the mass ratios polyDADMAC/PAHA in the complex at the IEP with the theoretical mass ratios of polyDADMAC/PAHA required to neutralize PAHA in the absence of charge regulation and K+ incorporation, it is found that at 50 mmol L−1 KCl the extra negative charge due to the interaction between polyDADMAC and PAHA is just compensated by K+ incorporation in the complex. Therefore, a pseudo 1-1 stoichiometry exists at about 50 mmol L−1 1-1 electrolyte concentration and only at this salt concentration polyDADMAC titrations and conventional proton titrations give identical results. Most likely this is also true for other HA samples and other pH values. For FA further study is required to reveal the conditions for which polyDADMAC and proton titrations give identical results.  相似文献   

18.
Aluminium has received great attention in the second half of the 20th century, mainly in the context of the acid rain problem mostly in forest soils. In this research the effect of land use and depth of the groundwater on Al, pH and DOC concentration in groundwater under Dutch sandy soils has been studied. Both pH and DOC concentration play a major role in the speciation of Al in solution. Furthermore, the equilibrium with mineral phases like gibbsite, amorphous Al(OH)3 and imogolite, has been considered. Agricultural and natural land use were expected to have different effects on the pH and DOC concentration, which in turn could influence the total Al concentration and the speciation of Al in groundwater at different depths (phreatic, shallow and deep). An extensive dataset (n = 2181) from the national and some provincial monitoring networks on soil and groundwater quality was used. Land use type and groundwater depth did influence the pH, and Al and DOC concentrations in groundwater samples. The Al concentration ranged from <0.4 μmol L−1 at pH > 7 to 1941 μmol L−1 at pH < 4; highest Al concentrations were found for natural-phreatic groundwater. The DOC concentration decreased and the median pH increased with depth of the groundwater. Natural-phreatic groundwater showed lower pH than the agricultural-phreatic groundwater. Highest DOC concentrations were found for the agricultural-phreatic groundwater, induced by the application of organic fertilizers. Besides inorganic complexation, the NICA-Donnan model was used to calculate Al3+ concentrations for complexation with DOC. Below pH 4.5 groundwater samples were mainly in disequilibrium with a mineral phase. This disequilibrium is considered to be the result of kinetic constraints or equilibrium with organic matter. Log K values were derived by linear regression and were close to theoretical values for Al(OH)3 minerals (e.g. gibbsite or amorphous Al(OH)3), except for natural-phreatic groundwater for which lower log K values were found. Complexation of Al with DOC is shown to be an important factor for the Al concentrations, especially at high DOC concentrations as was found for agricultural-phreatic groundwater.  相似文献   

19.
Experimental studies on the stability of several Mg-sulfate hydrates including epsomite (MgSO4·7H2O), hexahydrite (MgSO4·6H2O), starkeyite (MgSO4·4H2O), and kieserite (MgSO4·H2O) as a function of temperature and relative humidity are in poor agreement with calculations based on thermodynamic properties of these substances taken from the literature. Therefore, we synthesized four different MgSO4 hydrates and measured their enthalpies of formation by solution calorimetry at T = 298.15 K. The resulting enthalpies of formation from the elements are:
•
ΔfH0298 (epsomite) = −3387.7 ± 1.3 kJmol−1
•
ΔfH0298 (hexahydrite) = −3088.1 ± 1.1 kJmol−1
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ΔfH0298 (sanderite, MgSO4·2H2O) = −1894.9 ± 1.3 kJmol−1
•
ΔfH0298 (kieserite) = −1612.4 ± 1.3 kJmol−1
Using mathematical programming (MAP) techniques, standard thermodynamic values consistent both with our calorimetric data and previously published humidity brackets could be derived:
•
Epsomite: ΔfH0298 = −3388.7 kJmol−1, S0298 = 371.3 Jmol−1 K−1, ΔfG0298 = −2871.0 kJmol−1
•
Hexahydrite: ΔfH0298 = −3087.3 kJmol−1, S0298 = 348.5 Jmol−1 K−1, ΔfG0298 = −2632.3 kJmol−1
•
Starkeyite: ΔfH0298 = −2496.1 kJmol−1, S0298 = 259.9 Jmol−1 K−1, ΔfG0298 = −2153.8 kJmol−1
•
Kieserite: ΔfH0298 = −1611.5 kJmol−1, S0298 = 126.0 Jmol−1 K−1, ΔfG0298 = −1437.9 kJmol−1
Additionally, heat capacity measurements and standard entropy determinations of several magnesium sulfate hydrate minerals from the literature are analyzed and judged against estimates obtained from a linear combination of the heat capacities of MgSO4 and hexagonal ice. The results of the MAP analysis are compared to these estimates to conclude that heat capacity and entropy correlate well with the number of waters of hydration. However, even the good correlation is not good enough to capture the fine variations in these properties. Consequently, their experimental measurement is inevitable if reliable thermodynamic data are sought. Our MAP thermodynamic data show that epsomite, hexahydrite, and kieserite have stability fields in the T-%RH space. Starkeyite is metastable. Although no MAP data could have been derived for pentahydrite (MgSO4·5H2O) and sanderite, their transient existence suggest that both of them are metastable as well.  相似文献   

20.
The availability of dissolved O2 can limit biodegradation of organic compounds in aquifers. Where O2 is depleted, biodegradation proceeds via anaerobic processes, including NO3-, Mn(IV)-, Fe(III)- and SO4-reduction and fermentation/methanogenesis. The environmental controls on these anaerobic processes must be understood to support implementation of management strategies such as monitored natural attenuation (MNA). In this study stable isotope analysis is used to show that the relative significance of two key anaerobic biodegradation processes (bacterial SO4 reduction (BSR) and methanogenesis) in a phenol-contaminated sandstone aquifer is sensitive to spatial and temporal changes in total dissolved phenols concentration (TPC) (= phenol + cresols + dimethylphenols) over a 5-a period. In general, 34SO4-enrichment (characteristic of bacterial SO4 reduction) is restricted spatially to locations where TPC < 2000 mg L−1. In contrast, 13C-depleted CH4 and 13C-enriched CO2 isotope compositions (characteristic of methanogenesis) were measured at TPC up to 8000 mg L−1. This is consistent with previous studies that demonstrate suppression of BSR at TPC of >500 mg L−1, and suggests that methanogenic microorganisms may have a higher tolerance for TPC in this contaminant plume. It is concluded that isotopic enrichment trends can be used to identify conditions under which in situ biodegradation may be limited by the properties of the biodegradation substrate (in this case TPC). Such data may be used to deduce the performance of MNA for contaminated groundwater in similar settings.  相似文献   

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