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1.
It is important to understand how phosphate sorption dynamics of coastal carbonate aquifers are affected by seawater intrusion, because many coastal aquifers are composed of carbonate rocks and subject to an increase in saltwater intrusion during relative sea-level rise. Twelve carbonate rock and unconsolidated sediment specimens were acquired from a test corehole spanning the full thickness of the Biscayne aquifer in southeastern Florida. All 12 samples exhibit low phosphorus content but variable contents of iron. Column leaching experiments were conducted with two carbonate aquifer samples, alternating between freshwater and saltwater flow. With the first influx of saltwater, phosphate concentration in leachate increased rapidly from a freshwater value of approximately 0.2 μM to peaks of between 0.8 and 1.6 μM. The phosphate concentration began to diminish as saltwater continued to flow, but sustained desorption continued for over 2 h. Overall, seawater drove sorption behavior much more than chemical composition for the aquifer rocks and sediment from the seven rock samples for which we did isotherm sorption experiments. Our results indicate that an immediate and intense pulse of phosphate desorption from carbonate rock and sediment with low phosphorus content occurs in response to an influx of seawater and that the duration of desorption will vary by layer within a single aquifer. 相似文献
2.
Iron (III) oxides are ubiquitous in near-surface soils and sediments and interact strongly with dissolved phosphates via sorption, co-precipitation, mineral transformation and redox-cycling reactions. Iron oxide phases are thus, an important reservoir for dissolved phosphate, and phosphate bound to iron oxides may reflect dissolved phosphate sources as well as carry a history of the biogeochemical cycling of phosphorus (P). It has recently been demonstrated that dissolved inorganic phosphate (DIP) in rivers, lakes, estuaries and the open ocean can be used to distinguish different P sources and biological reaction pathways in the ratio of 18O/16O (δ18OP) in PO43−. Here we present results of experimental studies aimed at determining whether non-biological interactions between dissolved inorganic phosphate and solid iron oxides involve fractionation of oxygen isotopes in PO4. Determination of such fractionations is critical to any interpretation of δ18OP values of modern (e.g., hydrothermal iron oxide deposits, marine sediments, soils, groundwater systems) to ancient and extraterrestrial samples (e.g., BIF’s, Martian soils). Batch sorption experiments were performed using varied concentrations of synthetic ferrihydrite and isotopically-labeled dissolved ortho-phosphate at temperatures ranging from 4 to 95 °C. Mineral transformations and morphological changes were determined by X-Ray, Mössbauer spectroscopy and SEM image analyses.Our results show that isotopic fractionation between sorbed and aqueous phosphate occurs during the early phase of sorption with isotopically-light phosphate (P16O4) preferentially incorporated into sorbed/solid phases. This fractionation showed negligible temperature-dependence and gradually decreased as a result of O-isotope exchange between sorbed and aqueous-phase phosphate, to become insignificant at greater than ∼100 h of reaction. In high-temperature experiments, this exchange was very rapid resulting in negligible fractionation between sorbed and aqueous-phase phosphate at much shorter reaction times. Mineral transformation resulted in initial preferential desorption/loss of light phosphate (P16O4) to solution. However, the continual exchange between sorbed and aqueous PO4, concomitant with this mineralogical transformation resulted again in negligible fractionation between aqueous and sorbed PO4 at long reaction times (>2000 h). This finding is consistent with results obtained from natural marine samples. Therefore, 18O values of dissolved phosphate (DIP) in sea water may be preserved during its sorption to iron-oxide minerals such as hydrothermal plume particles, making marine iron oxides a potential new proxy for dissolved phosphate in the oceans. 相似文献
3.
Formation of Fe(III) oxyhydroxide colloids in freshwater and brackish seawater, with incorporation of phosphate and calcium 总被引：2，自引：0，他引：2
Anneli GunnarsSven Blomqvist Peter Johansson Christian Andersson 《Geochimica et cosmochimica acta》2002,66(5):745-758
The formation of Fe(III) oxyhydroxide colloids by oxidation of Fe(II) and their subsequent aggregation to larger particles were studied in laboratory experiments with natural water from a freshwater lake and a brackish coastal sea. Phosphate was incorporated in the solid phase during the course of hydrolysis of iron. The resulting precipitated amorphous Fe(III) oxyhydroxide phases were of varying composition, depending primarily on the initial dissolved Fe/P molar ratio, but with little influence by salinity or concentration of calcium ions. The lower limiting Fe/P ratio found for the solid phase suggests the formation of a basic Fe(III) phosphate compound with a stoichiometric Fe/P ratio of close to two. This implies that an Fe/P stoichiometry of ≈2 ultimately limits the capacity of precipitating Fe(III) to fix dissolved phosphate at oxic/anoxic boundaries in natural waters. In contrast to phosphorus, the uptake of calcium seemed to be controlled by sorption processes at the surface of the iron-rich particles formed. This uptake was more efficient in freshwater than in brackish water, suggesting that salinity restrains the uptake of calcium by newly formed Fe(III) oxyhydroxides in natural waters. Moreover, salinity enhanced the aggregation rate of the colloids formed. The suspensions were stabilised by the presence of organic matter, although this effect was less pronounced in seawater than in freshwater. Thus, in seawater of 6 to 33 ‰S, the removal of particles was fast (removal half time < 200 h), whereas the colloidal suspensions formed in freshwater were stable (removal half time > 900 h). Overall, oxidation of Fe(II) and removal of Fe(III) oxyhydroxide particles were much faster in seawater than in freshwater. This more rapid turnover results in lower iron availability in coastal seawater than in freshwater, making iron more likely to become a limiting element for chemical scavenging and biologic production. 相似文献
4.
Lead coprecipitation with iron oxyhydroxide nano-particles 总被引：1，自引：0，他引：1
Peng Lu Noel T. Nuhfer Qin Li Hiromi Konishi Erika Elswick Chen Zhu 《Geochimica et cosmochimica acta》2011,75(16):4547-4561
Pb2+ and Fe3+ coprecipitation was studied with sorption edge measurements, desorption experiments, sorbent aging, High Resolution Transmission and Analytical Electron Microscopy (HR TEM-AEM), and geochemical modeling. Companion adsorption experiments were also conducted for comparison. The macroscopic chemical and near atomic scale HRTEM data supplemented our molecule scale analysis with EXAFS (Kelly et al., 2008). Coprecipitation of Pb2+ with ferric oxyhydroxides occurred at ∼pH 4 and is more efficient than adsorption in removing Pb2+ from aqueous solutions at similar sorbate/sorbent ratios and pH. X-ray Diffraction (XRD) shows peaks of lepidocrocite and two additional broad peaks similar to fine particles of 2-line ferrihydrite (2LFh). HRTEM of the Pb-Fe coprecipitates shows a mixture of 2-6 nm diameter spheres and 8-20 by 200-300 nm needles, both uniformly distributed with Pb2+. Geochemical modeling shows that surface complexation models fit the experimental data of low Pb:Fe ratios when a high site density is used. Desorption experiments show that more Pb2+ was released from loaded sorbents collected from adsorption experiments than from Pb to Fe coprecipitates at dilute EDTA concentrations. Desorbed Pb2+ versus dissolved Fe3+ data show a linear relationship for coprecipitation (CPT) desorption experiments but a parabolic relationship for adsorption (ADS) experiments.Based on these results, we hypothesize that Pb2+ was first adsorbed onto the nanometer-sized, metastable, iron oxyhydroxide polymers of 2LFh with domain size of 2-3 nm. As these nano-particles assembled into larger particles, some Pb2+ was trapped in the iron oxyhydroxide structure and re-arranged to form solid solutions. Therefore, the CPT contact method produced more efficient removal of Pb2+ than the adsorption contact method, and Pb2+ bound in CPT solids represent a more stable sequestration of Pb2+ in the environment than Pb2+ adsorbed on iron oxyhydroxide surfaces. 相似文献
5.
The uptake and release of trace metals (Cu, Ni, Zn, Cd, and Co) in estuaries are studied using river and sea end-member waters
and suspended particulate matter (SPM) collected from the Changjiang Estuary, China. The kinetics of adsorption and desorption
were studied in terms of environmental factors (pH, SPM loading, and salinity) and metal concentrations. The uptake of the
metals studied onto SPM occurred mostly within 10 h and reached an asymptotic value within 40 h in the Changjiang Estuary.
As low pH river water flows into the high pH seawater and the water become more alkaline as it approaches to the seaside of
estuary, metals adsorb more on SPM in higher pH water, thus, particulate phase transport of metal become increasingly important
in the seaward side of the estuary. The percentage of adsorption recovery and the distribution coefficients for trace metals
remained to be relatively invariable and a significant reduction only occurred in very high concentrations of metals (>0.1 mg L−1). The general effect of salinity on metal behavior was to decrease the degree of adsorption of Cu, Zn, Cd, Co, and Ni onto
SPM but to increase their adsorption equilibrium pH. The adsorption–desorption kinetics of trace metals were further investigated
using Kurbatov adsorption model. The model appears to be most useful for the metals showing the conservative behavior during
mixing of river and seawater in the estuary. Our work demonstrates that dissolved concentration of trace metals in estuary
can be modeled based on the metal concentration in SPM, pH and salinity using a Kurbatov adsorption model assuming the natural
SPM as a simple surfaced molecule. 相似文献
6.
Removal of Pb^2＋, Cu^2＋, Zn^2＋ and Cd^2＋ from aqueous solutions by sorption on a natural phosphate rock （FAP） was investigated. The effects of the contact time and initial metal concentration were examined in the batch method. The percentage sorption of heavy metals from solution ranges generally between 50% and 99%. The amount of sorbed metal ions follows the order Cu〉Pb〉Cd〉Zn. Heavy metal immobilization was attributed to both surface complexation of metal ions on the surface of FAP grains and partial dissolution and precipitation of a heavy metal-containing phosphate. The very low desorption ratio of heavy metals further supports the effectiveness of FAP as an alternative and low-cost material to remove toxic Pb^2＋, Cu^2＋, Zn^2＋ and Cd^2＋ from polluted waters. 相似文献
7.
Sulfate and selenate adsorption on iron oxides are important reactions in natural systems under a very wide range of pH values, ionic strengths, and electrolyte compositions. Under such conditions, spectroscopic and theoretical calculations have demonstrated the potential importance of a variety of surface species. Understanding the variations in the surface speciation of these oxyanions is fundamental to prediction of their partitioning between minerals and aqueous solutions. In the present study, published experimental spectroscopic and theoretical molecular evidence of the identity of sulfate/selenate surface species are integrated with a surface complexation model consistent with a wide variety of experimental adsorption, surface titration, and proton coadsorption data to define the surface speciation of sulfate and selenate on iron oxides under a wide range of conditions. The analysis was carried out with the extended triple layer model (ETLM) taking into account the electrostatics of water dipole desorption during ligand exchange reactions. On seven out of eight goethites studied, sulfate and selenate surface reactions can be represented by the formation of a monodentate-mononuclear inner-sphere and a bidentate-binuclear outer-sphere (or H-bonded) species according to
8.
W. Davison 《Geochimica et cosmochimica acta》1979,43(10):1693-1696
Recent stability constant data for ferrous complexation by chloride, hydroxide and sulphate ions are used to recalculate the equilibrium distribution of soluble ferrous species at pH 7 and 8 in fresh- and seawater. Contrary to previous calculations hydroxy complexes are negligible and the aquated Fe2+ ion is the dominant species for all conditions. In seawater up to 15% of the iron might be present as chloride complexes, and 9% as sulfate complexes. The implications of these results to mineral formation are discussed. 相似文献
9.
Frank Millero Fen Huang Xiaorong Zhu Xuewu Liu Jia-Zhong Zhang 《Aquatic Geochemistry》2001,7(1):33-56
The adsorption and desorption of phosphate on calcite and aragonite were investigated as a function of temperature (5–45 °C)and salinity (0–40) in seawater pre-equilibrated with CaCO3. An increase in temperature increased the equilibrium adsorption; whereas an increase in salinity decreased the adsorption. Adsorption measurements made in NaCl were lower than the results in seawater. The higher values in seawater were due to the presence of Mg2+ and Ca2+ ions. The increase was 5 times greater for Ca2+ than Mg2+. The effects ofCa2+ and Mg2+ are diminished with the addition of SO4
2- apparently due to the formation of MgSO4 and CaSO4 complexes in solution and/or SO4
2- adsorption on the surface of CaCO3. The adsorbed Ca2+ and Mg2+ on CaCO3 (at carbonate sites) may act as bridges to PO4
3- ions. The bridging effect of Ca2+is greater than Mg2+ apparently due to the stronger interactions of Ca2+ with PO4
3-.The apparent effect of salinity on the adsorption of PO4 was largely due to changes in the concentration of HCO3
- in the solutions. An increase in the concentration of HCO3
- caused the adsorption of phosphate to decrease, especially at low salinities. The adsorption at the same level of HCO3
- (2 mM) was nearly independent of salinity. All of the adsorption measurements were modeled empirically using a Langmuir-type adsorption isotherm[ [PO4]ad = KmCm[PO4]T/(1 +Km [PO4]T) , ]where [PO4]ad and [PO4]T are the adsorbed and total dissolved phosphate concentrations, respectively. The values of Cm (the maximum monolayer adsorption capacity, (mol/g) and Km (the adsorption equilibrium constant, g/(mol) over the entire temperature (t, °C) and salinity (S) range were fitted to[ Cm = 17.067 + 0.1707t - 0.4693S + 0.0082S2 ( = 0.7) ][ ln Km = - 2.412 + 0.0165t - 0.0004St - 0.0008S2 ( = 0.1) ]These empirical equations reproduce all of our measurements of[PO4]ad up to 14 mol/g and within ±0.7 mol/g.The kinetic data showed that the phosphate uptake on carbonate minerals appears to be a multi-step process. Both the adsorption and desorption were quite fast in the first stage (less than 30 min) followed by a much slower process (lasting more than 1 week). Our results indicate that within 24 hours aragonite has a higher sorption capacity than calcite. The differences between calcite and aragonite become smaller with time. Consequently, the mineral composition of the sediments may affect the short-term phosphate adsorption and desorption on calcium carbonate. Up to 80 % of the adsorbed phosphate is released from calcium carbonate over one day. The amount of PO4 left on the CaCO3 is close to the equilibrium adsorption. The release of PO4 from calcite is faster than from aragonite. Measurements with Florida Bay sediments produced results between those for calcite and aragonite. Our results indicate that the calcium carbonate can be both a sink and source of phosphate in natural waters. 相似文献
10.
The adsorption and desorption of arsenate (As(V)) and arsenite (As(III)) on calcite was investigated in a series of batch experiments in calcite-equilibrated solutions. The solutions covered a broad range of pH, alkalinity, calcium concentration and ionic strength. The initial arsenic concentrations were kept low (<33 μM) to avoid surface precipitation. The results show that little or no arsenite sorbs on calcite within 24 h at an initial As concentration of 0.67 μM. In contrast, arsenate sorbs readily and quickly on calcite. Likewise, desorption of arsenate from calcite is fast and complete within hours, indicating that arsenate is not readily incorporated into the calcite crystal lattice. The degree of arsenate sorption depends on the solution chemistry. Sorption increases with decreasing alkalinity, indicating a competition for sorption sites between arsenate and (bi)carbonate. pH also affects the sorption behavior, likely in response to changes in arsenate speciation or protonation/deprotonation of the adsorbing arsenate ion. Finally, sorption is influenced by the ionic strength, possibly due to electrostatic effects. The sorption of arsenate on calcite was modeled successfully using a surface complexation model comprising strong and weak sites. In the model, the adsorbing arsenate species were and . The model was able to correctly predict the adsorption of arsenate in the wide range of calcite-equilibrated solutions used in the batch experiments and to describe the non-linear shape of the sorption isotherms. Extrapolation of the experimental results to calcite bearing aquifers suggests a large variability in the mobility of arsenic. Under reduced conditions, arsenite, which does not sorb on calcite, will dominate and, hence, As will be highly mobile. In contrast, when conditions are oxidizing, arsenate is the predominant species and, because arsenate adsorbs strongly on calcite, As mobility will be significantly retarded. The estimated retardation factors for arsenate in carbonate aquifers range from 25 to 200. 相似文献
11.
12.
《Applied Geochemistry》1998,13(1):77-88
There is an increasing awareness that rate-limited sorption reactions can play an important role in the transport of solutes in groundwater. The rate and extent of reactions between aqueous metals and mineral surfaces are affected by many factors, including the temperature, the presence of organic chelating agents, and adsorbent mineralogy. Cobalt sorption was investigated in terms of temperature, citrate concentration, and silica sand surface coating. The kinetic sorption data were described well by two simultaneous second-order reactions. The results suggested that decreasing temperature or the presence of citrate resulted in a slower approach to equilibrium for Co sorption to the uncoated silica sand that contained small amounts of secondary minerals. Using the same sand coated with an amorphous Fe(III) oxide, increasing temperature or the presence of citrate resulted in a faster approach to equilibrium for Co sorption. The equilibrium adsorption isotherms were described well by a generalized two-layer surface complexation model. Citrate decreased the extent of Co sorption to the uncoated silica; the effect was most pronounced at low temperature. Conversely, citrate increased the extent of Co sorption to the Fe-coated silica. These results suggest that citrate decreased the rate and extent of adsorption to the uncoated silica through the formation of a stable anionic aqueous complex that has a lower affinity for the surface than Co2+. Conversely, the higher anion sorption capacity of the Fe-coated silica increased the rate and extent of Co sorption with citrate present, presumably through the formation of an organo-metallic ternary surface complex. 相似文献
13.
Removal of Pb2 ,Cu~(2 ),Zn~(2 )and Cd~(2 )from aqueous solutions by sorption on a natural phosphate rock(FAP)was investigated.The effects of the contact time and initial metal concentration were examined in the batch method.The percentage sorption of heavy metals from solution ranges generally between 50% and 99%.The amount of sorbed metal ions follows the order Cu>Pb>Cd>Zn. Heavy metal immobilization was attributed to both surface complexation of metal ions on the surface of FAP grains and partial dissolution and precipitation of a heavy metal-containing phosphate.The very low desorption ratio of heavy metals further supports the effectiveness of FAP as an alternative and low-cost material to remove toxic Pb~(2 ),Cu~(2 ),Zn~(2 )and Cd~(2 )from polluted waters. 相似文献
14.
Sorption of Ni(II) onto chlorite surfaces was studied as a function of pH (5–10), ionic strength (0.01–0.5 M) and Ni concentration (10−8–10−6 M) in an Ar atmosphere using batch sorption with radioactive 63Ni as tracer. Such studies are important since Ni(II) is one of the major activation products in spent nuclear fuel and sorption data on minerals such as chlorite are lacking. The sorption of Ni(II) onto chlorite was dependent on pH but not ionic strength, which indicates that the process primarily comprises sorption by surface complexation. The maximum sorption was at pH ∼ 8 (Kd = ∼10−3 cm3/g). Desorption studies over a period of 1–2 weeks involving replacement of the aqueous solution indicated a low degree of desorption. The acid–base properties of the chlorite mineral were determined by titration and described using a non-electrostatic surface complexation model in FITEQL. A 2-pK NEM model and three surface complexes, Chl_OHNi2+, Chl_OHNi(OH)+ and Chl_OHNi(OH)2, gave the best fit to the sorption results using FITEQL. The high Kd values and low degree of desorption observed indicate that under expected groundwater conditions, a large fraction of Ni(II) that is potentially leachable from spent nuclear fuel may be prevented from migrating by sorption onto chlorite surfaces. 相似文献
15.
Apparent overall equilibrium constants for the adsorption of Cd, Cu, Ni, Pb and Zn onto natural iron oxyhydroxides have been calculated from the partitioning of these trace metals in oxic lake sediments and the in situ measurement of trace metal concentrations in the associated pore waters. Such values obtained from lakes of various pH located on the Precambrian Shield, in the area of Sudbury, Ontario, are compared with equilibrium constants obtained for the adsorption of the trace metals onto iron oxyhydroxides in well-defined media.The field data are consistent with laboratory experiments reported in the literature and with theory. Both the influence of pH upon adsorption and the binding strength sequence observed for the field data agree with theory. At high sediment pH values, the partitioning of Cd, Ni and Zn between the pore waters and the natural iron oxyhydroxides is similar to those reported in the literature for the adsorption of these metals at low surface coverage onto amorphous iron oxyhydroxides in a NaNO3 medium; deviation from this simple model is however observed for Cu and Pb, presumably due to the competitive action of dissolved ligands. At low sediment pH values, the adsorption is much higher than predicted by the simple model and can be explained by the formation of ternary complexes with the iron oxyhydroxide surface. 相似文献
16.
The sorption of ferrous iron to a clay mineral, nontronite (NAu-2, a ferruginous smectite), was investigated under strictly anoxic conditions as a function of pH (3-10), Fe2+ concentration (0.01-50 mM), equilibration time (1-35 days), and ionic strength (0.01-0.5 M NaClO4). The surface properties of NAu-2 were independently characterized to determine its fixed charge and amphoteric site density in order to interpret the Fe2+ sorption data. Fe2+ sorption to NAu-2 was strongly dependent on pH and ionic strength, reflecting the coupled effects of Fe2+ sorption through ion exchange and surface complexation reactions. Fe2+ sorption to NAu-2 increased with increasing pH from pH 2.5 to 4.5, remained constant from pH 4.5 to 7.0, increased again with further increase of pH from pH 7.0 to 8.5, and reached a maximum above pH 8.5. The Fe2+ sorption below pH 7.0 increased with decreasing ionic strength. The differences of Fe2+ sorption at different ionic strengths, however, diminished with increasing equilibration time. The Fe2+ sorption from pH 4.5 to 7.0 increased with increasing equilibration time up to 35 days and showed stronger kinetic behavior in higher ionic strength solutions. The kinetic uptake of Fe2+ onto NAu-2 is consistent with a surface precipitation mechanism although our measurements were not able to identify secondary precipitates. An equilibrium model that integrates ion exchange, surface complexation and aqueous speciation reactions reasonably well describes the Fe2+ sorption data as a function of pH, ionic strength, and Fe2+ concentration measured at 24 h of equilibration. Model calculations show that the species Fe(OH)+ was required to describe Fe2+ sorption above pH 8.0 satisfactorily. Overall, this study demonstrated that Fe2+ sorption to NAu-2 is affected by complex equilibrium and kinetic processes, likely caused by surface precipitation reactions. 相似文献
17.
《Applied Geochemistry》1994,9(1):23-36
The reactivity of iron(III) oxyhydroxides as reflected by their tendency to dissolve is of great importance in the redox cycling of iron and the bioavailability of iron to phytoplankton in natural waters. In this study, various iron(III) oxyhydroxides were produced by oxygenation of iron(II) in the presence of solutes, such as phosphate, sulfate, bicarbonate, valeric acid, TRIS, humic and fulvic acids, and in the presence of minerals, such as bentonite and δ-Al2O3 under conditions encountered in aquatic systems. The reactivity of the different iron(III) oxyhydroxides was subsequently assessed by means of a reductive dissolution using ascorbate and non-reductive dissolution using HQS (8-hydroxyquinoline-5-sulfonic acid) or oxalate. The experimental results show that the iron(III) oxyhydroxides with a low degree of polymerization exhibit higher reactivity than those with a high degree of polymerization or with high crystallinity. The quantity of active surface sites and the coordination arrangement of the functional groups at the surface of the iron(III) oxyhydroxides, especially the extent of the endstanding -OH groups per iron(III) ion determine the reactivity of iron(III) oxyhydroxides toward dissolution.Surfaces, such as clay and aluminum oxides, not only accelerate the oxygenation reaction of iron(II), but also induce the formation of iron(III) oxyhydroxides which are more active toward the dissolution reactions. Polymerization of iron(III) oxyhydroxides on the surfaces occurs predominantly in two dimensions rather than in three dimensions.In a laboratory experiment, the iron(III) oxyhydroxide formed in the presence of TRIS can be reduced by fulvic acid in a closed system under the following conditions: Fe(OH)3(s) 0.01 g/l, fulvic acid 5 mg/l, pH 7.5, 20°C. The kinetics of the reaction depend on the reactivity of iron(III) oxyhydroxide and reducing power of fulvic acid. Although reductants other than fulvic acid may be of importance in antural waters, this result provides the laboratory evidence that the >FeIII-OH/Fe(II) is able to act as an electron transfer mediator for the oxidation of natural organic substances, such as fulvic acid, by molecular oxygen either in the absence of microorganisms or as a supplement to microbial activity. 相似文献
18.
Synergistic effect of calcium and bicarbonate in enhancing arsenate release from ferrihydrite 总被引：1，自引：0，他引：1
Many groundwater systems contain anomalously high arsenic concentrations, associated with less than expected retention of As by adsorption to iron (hydr)oxides. Although carbonates are ubiquitous in aquifers, their relationship to arsenate mobilization is not well characterized. This research examines arsenate release from poorly crystalline iron hydroxides in abiotic systems containing calcium and magnesium with bicarbonate under conditions of static and dynamic flow (pH 7.5-8). Aqueous arsenic levels remained low when arsenate-bearing ferrihydrite was equilibrated with artificial groundwater solution containing Ca, Mg, and HCO3−. In batch titrations in which a solution of Ca and HCO3− was added repeatedly, the ferrihydrite surface became saturated with adsorbed Ca and HCO3−, and aqueous As levels increased by 1-2 orders of magnitude. In columns containing Ca or Mg and HCO3−, As solubility initially mimicked titrations, but then rapidly increased by an additional order of magnitude (reaching 12 μM As). Separately, calcium chloride and other simple salts did not induce As release, although sodium bicarbonate and lactate facilitated minor As release under flow. Results indicate that adsorption of calcium or magnesium with bicarbonate leads to As desorption from ferrihydrite, to a degree greater than expected from competitive effects alone, especially under dynamic flow. This desorption may be an important mechanism of As mobilization in As-impacted, circumneutral aquifers, especially those undergoing rapid mineralization of organic matter, which induces calcite dissolution and the production of dissolved calcium and bicarbonate. 相似文献
19.
We present rock mechanical test results and analytical calculations which demonstrate that a negative surface charge, resulting from sulfate adsorption from the pore water, impacts the rock mechanical behavior of high-porosity chalk. Na2SO4 brine flooded into chalk cores at 130 °C results in significantly reduced bulk modulus and yield point compared with that of NaCl brine at the same conditions. The experimental results have been interpreted using a surface complexation model combined with the Gouy-Chapman theory to describe the double layer. The calculated sulfate adsorption agrees well with the measured data. A sulfate adsorption of about 0.3 μmol/m2 and 0.7–1 μmol/m2 was measured at 50 and 130 °C, respectively. Relative to a total sites of 5 sites/nm2 these values correspond to an occupation of 4 % and 8–13 % which sufficiently explains the negative charging of the calcite surfaces. The interaction between charged surfaces specifically in the weak overlaps of electrical double layer gives rise to the total disjoining pressure in granular contacts. The net repulsive forces act as normal forces in the grains vicinity, counteracting the cohesive forces and enhance pore collapse failure during isotropic loading, which we argue to account for the reduced yield and bulk modulus of chalk cores. The effect of disjoining pressure is also assessed at different sulfate concentrations in aqueous solution, temperatures, as well as ionic strength of solution; all together remarkably reproduce similar trends as observed in the mechanical properties. 相似文献
20.
An assessment of the origin and variation of groundwater salinity in southeastern Ghana 总被引：1，自引：1，他引：0
Groundwater from the major aquifers in southeastern part of Ghana was sampled to determine the main controls on groundwater
salinity in the area. This paper uses multivariate statistical methods, conventional graphical methods and stable isotope
data to determine spatial relationships among groundwaters from the different hydrogeologic units in the area on the basis
of salinity. Q-mode hierarchical cluster analysis (HCA) was used to spatially classify the samples, whilst R-mode factor analysis
was used to reduce the dataset into two major principal components representing the sources of variation in the hydrochemistry.
Analysis of the major chemical parameters suggests that the principal component responsible for salinity increment in the
area is the weathering of minerals in the aquifers. This factor is especially more significant in the upland areas away from
the coast. The second factor responsible for salinity in the area is the combined effects of seawater intrusion, and anthropogenic
activities. This study finds that four major spatial groundwater groups exist in the area: low salinity, acidic groundwaters
which are mainly derived from the Birimian and Togo Series aquifers; low salinity, moderate to neutral pH groundwaters which
are mainly from the Voltaian, Buem and Cape Coast granitoids; very high salinity waters which are not suitable for most domestic
and irrigation purposes and are mainly from the Keta aquifers; and intermediate salinity groundwaters comprising groundwater
from the Keta basin aquifers with minor contributions from the other major terrains. The major water type identified in this
study is the Ca–Mg–HCO3 type, which degrades into predominantly Na–Cl–SO4 more saline groundwaters toward the coast. Stable isotope data analyses suggest that groundwater in the Voltaian aquifers
is largely of recent meteoric origin. The Birimian and Togo aquifers receive a component of recharge from the tributaries
of the Densu and Volta Rivers, after the waters have undergone evaporative enrichment of the heavier isotopes. In the Keta
basin, recharge is mainly from precipitation but an observed enrichment of 2H and 18O isotopes is probably due to seawater and evaporative effects since the water table there is very shallow. An analysis of
the irrigation quality of groundwater from the six aquifers in the study area using sodium adsorption ratio and electrical
conductivity suggests that most of the aquifers supply groundwater of acceptable quality for irrigation. The only exception
is the Keta Basin area, where extremely high salinities and SAR values render groundwater from this basin unsuitable for irrigation
purposes. 相似文献