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1.
Arsenite sorption on troilite (FeS) and pyrite (FeS2)   总被引：4，自引：0，他引：4  
Arsenic is a toxic metalloid whose mobility and availability are largely controlled by sorption on sulfide minerals in anoxic environments. Accordingly, we investigated reactions of As(III) with iron sulfide (FeS) and pyrite (FeS2) as a function of total arsenic concentration, suspension density, sulfide concentration, pH, and ionic strength. Arsenite partitioned strongly on both FeS and FeS2 under a range of conditions and conformed to a Langmuir isotherm at low surface coverages; a calculated site density of near 2.6 and 3.7 sites/nm2 for FeS and FeS2, respectively, was obtained. Arsenite sorbed most strongly at elevated pH (>5 to 6). Although solution data suggested the formation of surface precipitates only at elevated solution concentrations, surface precipitates were identified using X-ray absorption spectroscopy (XAS) at all coverages. Sorbed As was coordinated to both sulfur [d(As-S) = 2.35 Å] and iron [d(As-Fe) = 2.40 Å], characteristic of As coordination in arsenopyrite (FeAsS). The absorption edge of sorbed As was also shifted relative to arsenite and orpiment (As2S3), revealing As(III) reduction and a complete change in As local structure. Arsenic reduction was accompanied by oxidation of both surface S and Fe(II); the FeAsS-like surface precipitate was also susceptible to oxidation, possibly influencing the stability of As sorbed to sulfide minerals in the environment. Sulfide additions inhibit sorption despite the formation of a sulfide phase, suggesting that precipitation of arsenic sulfide is not occurring. Surface precipitation of As on FeS and FeS2 supports the observed correlation of arsenic and pyrite and other iron sulfides in anoxic sediments.  相似文献   

2.
Arsenate, As(V), sorption onto synthetic iron(II) monosulfide, disordered mackinawite (FeS), is fast. As(V) sorption decreases above the point of zero surface charge of FeS and follows the pH-dependent concentration of positively charged surface species. No redox reaction is observed between the As(V) ions and the mineral surface over the time span of the experiments. This observation shows that As(V) dominantly forms an outer-sphere complex at the surface of mackinawite. Arsenite, As(III), sorption is not strongly pH-dependent and can be expressed by a Freundlich isotherm. Sorption is fast, although slower than that of As(V). As(III) also forms an outer-sphere complex at the surface of mackinawite. In agreement with previous spectroscopic studies, complexation at low As(V) and As(III) concentration occurs preferentially at the mono-coordinated sulfide edge sites. The Kd (L g−1) values obtained from linear fits to the isotherm data are ∼9 for As(V) and ∼2 for As(III). Stronger sorption of As(V) than As(III), and thus a higher As(III) mobility, may be reflected in natural anoxic sulfidic waters when disordered mackinawite controls arsenic mobility.  相似文献   

3.
The mobility and availability of arsenite, As(III), in anoxic environments is largely controlled by adsorption onto iron sulfides and/or precipitation of arsenic in solid phases. The interaction of As(III) with synthetic mackinawite (FeSm) in pH 5 and 9 suspensions was investigated using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), STEM elemental mapping, high resolution TEM, and X-ray photoelectron spectroscopy (XPS). At pH 5, arsenic sulfide phases precipitate among the FeSm particles as discrete particles that are an amorphous hydrous phase of arsenic sulfide. The oxidation state of As in the surface layers of the arsenic sulfide precipitates is ‘realgar-like’ based on XPS results showing that > 75% of the As 3d peak area is due to As with oxidation states between 0 and 2+. Discrete, arsenic sulfide precipitates are absent at pH 9, but elemental mapping in STEM-EDX mode shows that arsenic is uniformly distributed on the FeSm, suggesting that uptake is caused by the sorption of As(III) oxyanions and/or the precipitation of highly dispersed arsenic sulfides on FeSm. XPS also revealed that the FeSm that equilibrated without As(III) has a more oxidized surface composition than the sample at pH 9, as indicated by the higher concentration of O ( three times greater than that at pH 9) and the larger fraction of Fe(III) species making up the total Fe (2p3/2) peak. These findings provide a better understanding of redox processes and phase transitions upon As(III) adsorption on iron sulfide substrates.  相似文献   

4.
The Fe(II) adsorption by non-ferric and ferric (hydr)oxides has been analyzed with surface complexation modeling. The CD model has been used to derive the interfacial distribution of charge. The fitted CD coefficients have been linked to the mechanism of adsorption. The Fe(II) adsorption is discussed for TiO2, γ-AlOOH (boehmite), γ-FeOOH (lepidocrocite), α-FeOOH (goethite) and HFO (ferrihydrite) in relation to the surface structure and surface sites. One type of surface complex is formed at TiO2 and γ-AlOOH, i.e. a surface-coordinated Fe2+ ion. At the TiO2 (Degussa) surface, the Fe2+ ion is probably bound as a quattro-dentate surface complex. The CD value of Fe2+ adsorbed to γ-AlOOH points to the formation of a tridentate complex, which might be a double edge surface complex. The adsorption of Fe(II) to ferric (hydr)oxides differs. The charge distribution points to the transfer of electron charge from the adsorbed Fe(II) to the solid and the subsequent hydrolysis of the ligands that coordinate to the adsorbed ion, formerly present as Fe(II). Analysis shows that the hydrolysis corresponds to the hydrolysis of adsorbed Al(III) for γ-FeOOH and α-FeOOH. In both cases, an adsorbed M(III) is found in agreement with structural considerations. For lepidocrocite, the experimental data point to a process with a complete surface oxidation while for goethite and also HFO, data can be explained assuming a combination of Fe(II) adsorption with and without electron transfer. Surface oxidation (electron transfer), leading to adsorbed Fe(III)(OH)2, is favored at high pH (pH > ∼7.5) promoting the deprotonation of two FeIII-OH2 ligands. For goethite, the interaction of Fe(II) with As(III) and vice versa has been modeled too. To explain Fe(II)-As(III) dual-sorbate systems, formation of a ternary type of surface complex is included, which is supposed to be a monodentate As(III) surface complex that interacts with an Fe(II) ion, resulting in a binuclear bidentate As(III) surface complex.  相似文献   

5.
The interaction of aqueous As(III) with magnetite during its precipitation from aqueous solution at neutral pH has been studied as a function of initial As/Fe ratio. Arsenite is sequestered via surface adsorption and surface precipitation reactions, which in turn influence the crystal growth of magnetite. Sorption samples were characterized using EXAFS spectroscopy at the As K-edge in combination with HRTEM observations, energy dispersive X-ray analysis at the nanoscale, electron energy loss spectroscopy at the Fe L3-edge, and XRD-Rietveld analyses of reaction products. Our results show that As(III) forms predominantly tridentate hexanuclear As(III)O3 complexes (3C), where the As(III)O3 pyramids occupy vacant tetrahedral sites on {1 1 1} surfaces of magnetite particles. This is the first time such a tridentate surface complex has been observed for arsenic. This complex, with a dominant As-Fe distance of 3.53 ± 0.02 Å, occurs in all samples examined except the one with the highest As/Fe ratio (0.33). In addition, at the two highest As/Fe ratios (0.133 and 0.333) arsenite tends to form mononuclear edge-sharing As(III)O3 species (2E) within a highly soluble amorphous As(III)-Fe(III,II)-containing precipitate. At the two lowest As/Fe ratios (0.007 and 0.033), our results indicate the presence of additional As(III) species with a dominant As-Fe distance of 3.30 ± 0.02 Å, for which a possible structural model is proposed. The tridentate 3C As(III)O3 complexes on the {1 1 1} magnetite surface, together with this additional As(III) species, dramatically lower the solubility of arsenite in the anoxic model systems studied. They may thus play an important role in lowering arsenite solubility in putative magnetite-based water treatment processes, as well as in natural iron-rich anoxic media, especially during the reductive dissolution-precipitation of iron minerals in anoxic environments.  相似文献   

6.
Permanganate (MnO4−) has widely been used as an effective oxidant for drinking water treatment systems, as well as for in situ treatment of groundwater impacted by various organic contaminants. The reaction stoichiometry of As(III) oxidation by permanganate has been assumed to be 1.5, based on the formation of solid product, which is putatively considered to be MnO2(s). This study determined the stoichiometric ratio (SR) of the oxidation reaction with varying doses of As(III) (3-300 μM) and MnO4− (0.5 or 300 μM) under circumneutral pH conditions (pH 4.5-7.5). We also characterized the solid product that was recovered ∼1 min after the oxidation of 2.16 mM As(III) by 0.97 mM MnO4− at pH 6.9 and examined the feasibility of secondary heterogeneous As(III) oxidation by the solid product. When permanganate was in excess of As(III), the SR of As(III) to Mn(VII) was 2.07 ± 0.07, regardless of the solution pH; however, it increased to 2.49 ± 0.09 when As(III) was in excess. The solid product was analogous to vernadite, a poorly crystalline manganese oxide based on XRD analysis. The average valence of structural Mn in the solid product corresponded to +III according to the splitting interval of the Mn3s peaks (5.5 eV), determined using X-ray photoelectron spectroscopy (XPS). The relative proportions of the structural Mn(IV):Mn(III):Mn(II) were quantified as 19:62:19 by fitting the Mn2p3/2 spectrum of the solid with the five multiplet binding energy spectra for each Mn valence. Additionally, the O1s spectrum of the solid was comparable to that of Mn-oxide but not of Mn-hydroxide. These results suggest that the solid product resembled a poorly crystalline hydrous Mn-oxide such as (MnII0.19MnIII0.62MnIV0.19)2O3·nH2O, in which Mn(II) and Mn(IV) were presumably produced from the disproportionation of aqueous phase Mn(III). Thermodynamic calculations also show that the formation of Mn(III) oxide is more favorable than that of Mn(IV) oxide from As(III) oxidation by permanganate under circumneutral pH conditions. Arsenic(III), when it remained in the solution after all of the permanganate was consumed, was effectively oxidized by the solid product. This secondary heterogeneous As(III) oxidation consisted of three steps: sorption to and oxidation on the solid surface and desorption of As(V) into solution, with the first step being the rate-limiting process as observed in As(III) oxidation by various Mn (oxyhydr)oxides reported elsewhere. We also discussed a potential reaction pathway of the permanganate oxidation of As(III).  相似文献   

7.
Natural attenuation of arsenic by simple adsorption on oxyhydroxides may be limited due to competing oxyanions, but uptake by coprecipitation may locally sequester arsenic. We have systematically investigated the mechanism and mode (adsorption versus coprecipitation) of arsenic uptake in the presence of carbonate and phosphate, from solutions of inorganic composition similar to many groundwaters. Efficient arsenic removal, >95% As(V) and ∼55% in initial As(III) systems, occurred over 24 h at pHs 5.5-6.5 when Fe(II) and hydroxylapatite (Ca5(PO4)3OH, HAP) “seed” crystals were added to solutions that had been previously reacted with HAP, atmospheric CO2(g) and O2(g). Arsenic adsorption was insignificant (<10%) on HAP without Fe(II). Greater uptake in the As(III) system in the presence of Fe(II) was interpreted as due to faster As(III) to As(V) oxidation by molecular oxygen in a putative pathway involving Fe(IV) and As(IV) intermediate species. HAP acts as a pH buffer that allows faster Fe(II) oxidation. Solution analyses coupled with high-resolution transmission electron microscopy (HRTEM), X-ray Energy-Dispersive Spectroscopy (EDS), and X-Ray Absorption Spectroscopy (XAS) indicated the precipitation of sub-spherical particles of an amorphous, chemically-mixed, nanophase, FeIII[(OH)3(PO4)(AsVO4)]·nH2O or FeIII[(OH)3( PO4)(AsVO4)(AsIIIO3)minor]·nH2O, where AsIIIO3 is a minor component.The mode of As uptake was further investigated in binary coprecipitation (Fe(II) + As(III) or P), and ternary coprecipitation and adsorption experiments (Fe(II) + As(III) + P) at variable As/Fe, P/Fe and As/P/Fe ratios. Foil-like, poorly crystalline, nanoparticles of FeIII(OH)3 and sub-spherical, amorphous, chemically-mixed, metastable nanoparticles of FeIII[(OH)3, PO4]·nH2O coexisted at lower P/Fe ratios than predicted by bulk solubilities of strengite (FePO4·2H2O) and goethite (FeOOH). Uptake of As and P in these systems decreased as binary coprecipitation > ternary coprecipitation > ternary adsorption.Significantly, the chemically-mixed, ferric oxyhydroxide-phosphate-arsenate nanophases found here are very similar to those found in the natural environment at slightly acidic to circum-neutral pHs in sub-oxic to oxic systems, such phases may naturally attenuate As mobility in the environment, but it is important to recognize that our system and the natural environment are kinetically evolving, and the ultimate environmental fate of As will depend on the long-term stability and potential phase transformations of these mixed nanophases. Our results also underscore the importance of using sufficiently complex, yet systematically designed, model systems to accurately represent the natural environment.  相似文献   

8.
Surface water samples for arsenic (As) concentration and speciation analysis were collected from organic matter-rich blackwaters of the Lake Drummond portion of the Great Dismal Swamp in southeastern Virginia, USA. Arsenic concentrations and speciation were determined by selective hydride generation, gas chromatography with photoionization detection. Surface waters from the Great Dismal Swamp are high in dissolved organic carbon (DOC) concentrations (445–9,600 μmol/kg) and of low pH (4.2–6.4). Total dissolved As concentrations [i.e., As(III) + As(V)], hereafter AsT, range from 2.2 nmol/kg to 21.4 nmol/kg. Arsenite, As(III), concentrations range from ∼1 nmol/kg to 17.7 nmol/kg, and As(V) ranges from ∼1 nmol/kg to 14.1 nmol/kg. Arsenate, As(V), is the predominant form of dissolved As in the inflow waters to the Great Dismal Swamp, whereas within the swamp proper arsenite, As(III), dominates. Arsenite accounts for 8–37% of AsT in inflow waters west of the Suffolk Scarp, and between 54% and 81% of AsT in Lake Drummond and Great Dismal Swamp waters east of the scarp. Arsenite is strongly correlated to DOC (r = 0.94) and inversely related to pH (r = −0.9), both at greater than the 99% confidence level. Arsenate is weakly related to pH and DOC (r = 0.4 and −0.37, respectively), and neither relationship is statistically significant. No statistical relationships exist between As(V) or As(III) and PO4 concentrations. The predominance of As(III) and its strong correlation with DOC in Great Dismal Swamp waters suggest that DOC may inhibit As(III) adsorption or form stable aqueous complexes with As(III) in these waters. Alternatively, phytoplankton and/or bacterially mediated reduction of As(V) may be important processes in the organic-rich blackwaters and/or sediment porewaters of the swamp, leading to the prevalence of As(III) in the water column.  相似文献   

9.
The influence of aqueous silica on the hydrolysis of iron(III) nitrate and chloride salts in dilute aqueous solutions (mFe ∼ 0.01 mol/kg) was studied at ambient temperature using X-ray absorption fine structure (XAFS) spectroscopy at the Fe K-edge. Results show that in Si-free iron nitrate and chloride solutions at acid pH (pH < 2.5), Fe is hexa-coordinated with 6 oxygens of H2O- and/or OH-groups in the first coordination sphere of the metal, at an Fe-O distance of 2.00 ± 0.01 Å. With increasing pH (2.7 < pH < 13), these groups are rapidly replaced by bridging hydroxyls (-OH-) or oxygens (-O-), and polymerized Fe hydroxide complexes form via Fe-(O/OH)-Fe bonds. In these polymers, the first atomic shell of iron represents a distorted octahedron with six O/OH groups and Fe-O distances ranging from 1.92 to 2.07 Å. The Fe octahedra are linked together by their edges (Fe-Fe distance 2.92-3.12 Å) and corners (Fe-Fe distance ∼3.47 ± 0.03 Å). The Fe-Fe coordination numbers (Nedge = 1-2; Ncorner = 0.5-0.7) are consistent with the dominant presence of iron dimers, trimers and tetramers at pH 2.5 to 2.9, and of higher-polymerized species at pH > 3.At pH > 2.5 in the presence of aqueous silica, important changes in Fe(III) hydrolysis are detected. In 0.05-m Si solutions (pH ∼ 2.7-3.0), the corner linkages between Fe octahedra in the polymeric complexes disappear, and the Fe-Fe distances corresponding to the edge linkages slightly increase (Fe-Feedge ∼ 3.12-3.14 Å). The presence of 1 to 2 silicons at 3.18 ± 0.03 Å is detected in the second atomic shell around iron. At basic pH (∼12.7), similar structural changes are observed for the iron second shell. The Fe-Si and Fe-Fe distances and coordination numbers derived in this study are consistent with (1) Fe-Si complex stoichiometries Fe2Si1-2 and Fe3Si2-3 at pH < 3; (2) structures composed of Fe-Fe dimers and trimers sharing one or two edges of FeO6-octahedra; and (3) silicon tetrahedra linked to two neighboring Fe octahedra via corners. At higher Si concentration (0.16 m, polymerized silica solution) and pH ∼ 3, the signal of the Fe second shell vanishes indicating the destruction of the Fe-Fe bonds and the formation of different Fe-Si linkages. Moreover, ∼20 mol.% of Fe is found to be tetrahedrally coordinated with oxygens in the first coordination shell (RFe-O = 1.84 Å). This new finding implies that Fe may partially substitute for Si in the tetrahedral network of the silica polymers in Si-rich solutions.The results of this study demonstrate that aqueous silica can significantly inhibit iron polymerization and solid-phase formation, and thus increase the stability and mobility of Fe(III) in natural waters. The silica “poisoning” of the free corner sites of iron-hydroxide colloids should reduce the adsorption and incorporation of trace elements by these colloids in Si-rich natural waters.  相似文献   

10.
《Applied Geochemistry》2004,19(2):169-180
Arsenic is present in aqueous environments in +III and +V oxidation states. In oxidizing environments, the principle attenuation mechanism of As migration is its adsorption on Fe(III) oxide and hydroxides. The adsorption affinity is higher for As(V) under lower pH conditions and for As(III) under higher pH conditions. Ferric oxide and hydroxides can dissolve under low Eh and pH conditions releasing adsorbed As. Oxidation-reduction processes often involve high organic matter content in sediments and also contamination by organics such as BTEX. Arsenic may desorb under high pH conditions. Changes of pH can be related to some redox reactions, cation exchange reactions driving dissolution of carbonates, and dissolution of silicates. In very reducing environments, where SO4 reduction takes place, secondary sulfide minerals like As-bearing pyrite and orpiment, As2S3, can incorporate As. Geochemical modeling can be divided into two principal categories: (a) forward modeling and (b) inverse modeling. Forward modeling is used to predict water chemistry after completion of predetermined reactions. Inverse modeling is used to suggest which processes take place along a flowpath. Complex coupled transport and geochemistry programs, which allow for simulation of As adsorption, are becoming available. A common modeling approach is based on forward modeling with surface complexation modeling (SCM) of As adsorption, which can incorporate the effect of different adsorbent/As ratios, adsorption sites density, area available for adsorption, pH changes and competition of As for adsorption sites with other dissolved species such as phosphate. The adsorption modeling can be performed in both batch and transport modes in codes such as PHREEQC. Inverse modeling is generally used to verify hypotheses on the origin of As. Basic prerequisites of inverse modeling are the knowledge of flow pattern (sampling points used in model have to be hydraulically connected) and information about mineralogy including As mineral phases. Case studies of geochemical modeling including modeling of As adsorption are presented.  相似文献   

11.
Partial melting of sulfide ores during prograde metamorphism could have been more prevalent than generally accepted. However, identification of such melting is difficult as sulfide melts do not form glasses and the textures generated on quenching are obliterated due to the tendency of sulfides for ready recrystallization. The polymetallic base metal sulfide deposit at Rajpura–Dariba, Rajasthan, India is a typical stratiform ore metamorphosed to the middle amphibolite facies. The peak metamorphic temperature of 600 °C should have been sufficient to initiate sulfide melting as evident from experimental studies in the ZnS–PbS–Cu2S–FeS2–S system. Further, syn-metamorphic melting of the original SEDEX ore was abetted by the high fS2 condition that prevailed as a consequence of barite dissolution. A Zn–Fe–S melt containing minor Pb, Sb and Cu but no Ag fractionated from an initial melt in the above system resulting in a residual immiscible sulfosalt-bearing PbS melt. The final metallic melts, represented by formation of dyscrasite (Ag3Sb) from the sulfosalt-bearing melt and breithauptite (NiSb) or ullmannite (NiSbS) from the sulfosalt-absent melt, were a product of independent fractional crystallization of the immiscible sulfide and PbS–sulfosalt melts.  相似文献   

12.
Arsenopyrite (FeAsS) and enargite (Cu3AsS4) fractured in a nitrogen atmosphere were characterised after acidic (pH 1.8), oxidative dissolution in both the presence and absence of the acidophilic microorganism Leptospirillum ferrooxidans. Dissolution was monitored through analysis of the coexisting aqueous solution using inductively coupled plasma atomic emission spectroscopy and coupled ion chromatography-inductively coupled plasma mass spectrometry, and chemical changes at the mineral surface observed using X-ray photoelectron spectroscopy and environmental scanning electron microscopy (ESEM). Biologically mediated oxidation of arsenopyrite and enargite (2.5 g in 25 ml) was seen to proceed to a greater extent than abiotic oxidation, although arsenopyrite oxidation was significantly greater than enargite oxidation. These dissolution reactions were associated with the release of ∼917 and ∼180 ppm of arsenic into solution. The formation of Fe(III)-oxyhydroxides, ferric sulphate and arsenate was observed for arsenopyrite, thiosulphate and an unknown arsenic oxide for enargite. ESEM revealed an extensive coating of an extracellular polymeric substance associated with the L. ferrooxidans cells on the arsenopyrite surface and bacterial leach pits suggest a direct biological oxidation mechanism, although a combination of indirect and direct bioleaching cannot be ruled out. Although the relative oxidation rates of enargite were greater in the presence of L. ferrooxidans, cells were not in contact with the surface suggesting an indirect biological oxidation mechanism. Cells of L. ferrooxidans appear able to withstand several hundreds of ppm of As(III) and As(V).  相似文献   

13.
On the basis of experimental works in the FeS-FeO-SiO2(-Fe3O4 or -Na2O) system with synthetic ZnS or PbS, the partition of zinc and lead between silicate and sulfide liquids is shown to be affected by the oxygen content of the sulfide liquids. The partition coefficients K, metal wt. % in sulfide liquid / metal wt. % in silicate liquid, for zinc and lead go through a minimum at relatively low oxygen contents of the sulfide liquids. KZn and KPb for natural sulfide liquids in equilibrium with basic magmas near the earth's surface are estimated at 0.1–0.5 and greater than 10, respectively. Although KZn and KPb change appreciably with oxygen content of the sulfide liquids, they never become sufficiently high to concentrate zinc and lead in economic amounts in magmatic sulfide ores.  相似文献   

14.
Arsenic sulfide (AsS (am), As2S3 (am), orpiment, and realgar) oxidation rates increase with increasing pH values. The rates of arsenic sulfide oxidation at higher pH values relative to those at pH∼2 are in the range of 26-4478, 3-17, 8-182, and 4-10 times for As2S3 (am), orpiment, AsS (am), and realgar, respectively.Numerical simulations of orpiment and realgar oxidation kinetics were conducted using the geochemical reaction path code EQ3/6 to evaluate the effects of variable DO concentrations and mineral reactivity factors on water chemistry evolution during orpiment and realgar oxidation. The results show that total As concentrations increase by ∼1.14 to 13 times and that pH values decrease by ∼0.6 to 4.2 U over a range of mineral reactivity factors from 1% to 50% after 2000 days (5.5 yr). The As release from orpiment and realgar oxidation exceeds the current U.S. National Drinking Water Standard (0.05 ppm) approximately in 200-300 days at the lowest initial dissolved oxygen concentration (3 ppm) and a reactivity factor of 1%. The results of simulations of orpiment oxidation in the presence of albite and calcite show that calcite can act as an effective buffer to the acid water produced from orpiment oxidation within relatively short periods (days/months), but the release of As continues to increase.Pyrite oxidation rates are faster than orpiment and realgar from pH 2.3 to 8; however, pyrite oxidation rates are slower than As2S3 (am) and AsS (am) at pH 8. The activation energies of arsenic sulfide oxidation range from 16 to 124 kJ/mol at pH∼8 and temperature 25 to 40°C, and pyrite activation energies are ∼52 to 88 kJ/mol, depending on pH and temperature range. The magnitude of activation energies for both pyrite and arsenic sulfide solids indicates that the oxidation of these minerals is dominated by surface reactions, except for As2S3 (am). Low activation energies of As2S3 (am) indicate that diffusion may be rate controlling.Limestone is commonly mixed with sulfide minerals in a mining environment to prevent acid water formation. However, the oxidation rates of arsenic sulfides increase as solution pH rises and result in a greater release of As. Furthermore, the lifetimes of carbonate minerals (i.e., calcite, aragonite, and dolomite) are much shorter than those of arsenic sulfide and silicate minerals. Thus, within a geologic frame time, carbonate minerals may not be present to act as a pH buffer for acid mine waters. Additionally, the presence of silicate minerals such as pyroxenes (wollastonite, jadeite, and spodumene) and Ca-feldspars (labradorite, anorthite, and nepheline) may not be important for buffering acid solutions because these minerals dissolve faster than and have shorter lifetimes than sulfide minerals. However, other silicate minerals such as Na and K-feldspars (albite, sanidine, and microcline), quartz, pyroxenes (augite, enstatite, diopsite, and MnSiO3) that have much longer lifetimes than arsenic sulfide minerals may be present in a system. The results of our modeling of arsenic sulfide mineral oxidation show that these minerals potentially can release significant concentrations of dissolved As to natural waters, and the factors and mechanisms involved in arsenic sulfide oxidation warrant further study.  相似文献   

15.
Past mining, processing, and waste disposal activities have left a legacy of uranium-contaminated soil and groundwater. Phosphate addition to subsurface environments can potentially immobilize U(VI) in-situ through interactions with uranium at mineral-water interfaces. Phosphate can induce the precipitation of low solubility U(VI)-phosphates, and it may enhance or inhibit U(VI) adsorption to iron(III) (oxy)hydroxide surfaces. Such surfaces may also facilitate the heterogeneous nucleation of U(VI)-phosphate precipitates. The interactions among phosphate, U(VI), and goethite (α-FeOOH) were investigated in a year-long series of experiments at pH 4. Reaction time, total U(VI), total phosphate, and the presence and absence of goethite were systematically varied to determine their effects on the extent of U(VI) uptake and the dominant uranium immobilization mechanism. Dissolved U(VI) and phosphate concentrations were interpreted within a reaction-based modeling framework that included dissolution-precipitation reactions and a surface complexation model to account for adsorption. The best available thermodynamic data and past surface complexation models were integrated to form an internally consistent framework. Additional evidence for the uptake mechanisms was obtained using scanning electron microscopy and X-ray diffraction. The formation and crystal growth of a U(VI)-phosphate phase, most likely chernikovite, UO2HPO4·4H2O(s), occurred rapidly for initially supersaturated suspensions both with and without goethite. Nucleation appears to occur homogeneously for almost all conditions, even in the presence of goethite, but heterogeneous nucleation was likely at one condition. The U(VI)-phosphate solids exhibited metastability depending on the TOTU:TOTP ratio. At the highest phosphate concentration studied (130 μM), U(VI) uptake was enhanced due to the likely formation of a ternary surface complex for low (∼1 μM) to intermediate (∼10 μM) TOTU concentrations and to U(VI)-phosphate precipitation for high TOTU (∼100 μM) concentrations. For conditions favoring precipitation, the goethite surface acted as a sink for dissolved phosphate that resulted in higher dissolved U(VI) concentrations relative to goethite-free conditions. Based on the total uranium and available sorption sites, a critical phosphate concentration between 15 μM and 130 μM was required for preferential precipitation of uranium phosphate over U(VI) adsorption.  相似文献   

16.
Bacteria are very efficient sorbents of trace metals, and their abundance in a wide variety of natural aqueous systems means biosorption plays an important role in the biogeochemical cycling of many elements. We measured the adsorption of Cu(II) to Bacillus subtilis as a function of pH and surface loading. Adsorption edge and XAS experiments were performed at high bacteria-to-metal ratio, analogous to Cu uptake in natural geologic and aqueous environments. We report significant Cu adsorption to B. subtilis across the entire pH range studied (pH ∼2-7), with adsorption increasing with pH to a maximum at pH ∼6. We determine directly for the first time that Cu adsorbs to B. subtilis as a (CuO5Hn)n−8 monodentate, inner-sphere surface complex involving carboxyl surface functional groups. This Cu-carboxyl complex is able to account for the observed Cu adsorption across the entire pH range studied. Having determined the molecular adsorption mechanism of Cu to B. subtilis, we have developed a new thermodynamic surface complexation model for Cu adsorption that is informed by and consistent with EXAFS results. We model the surface electrostatics using the 1pK basic Stern approximation. We fit our adsorption data to the formation of a monodentate, inner-sphere RCOOCu+ surface complex. In agreement with previous studies, this work indicates that in order to accurately predict the fate and mobility of Cu in complex biogeochemical systems, we must incorporate the formation of Cu-bacteria surface complexes in reactive transport models. To this end, this work recommends log K RCOOCu+ = 7.13 for geologic and aqueous systems with generally high B. subtilis-to-metal ratio.  相似文献   

17.
Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Bangladesh was amended with labile water-soluble organic C (molasses) and MgSO4 to stimulate metabolism of indigenous SO4-reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO4, and contained <10 μg/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Bangladesh, groundwater was under moderately reducing conditions and contained ∼10 mg/L Fe and ∼100 μg/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 μg/L) under geochemical conditions consistent with bacterial Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO4 reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS2. By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO4 reduction commenced but increased to pre-experiment level once SO4 reduction ended. In the ongoing experiment, both SO4 and Fe(II) were amended to groundwater to evaluate if FeS/FeS2 formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist’s Workbench, its stability field completely dominates in reducing Eh–pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions.  相似文献   

18.
The adsorption of five toxic metallic cations, Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II), onto montmorillonite was investigated as a function of pH and ionic strength and a two-site surface complexation model was used to predict the adsorption data. The results showed that in the lower pH range, 3∼6 for Cd, Cu, Ni and Zn, and 3∼4.5 for Pb, the adsorption was greatly affected by ionic strength, while in the higher pH range, the adsorption was not. In the lower pH range, the metallic cations were mainly bound through the formation of outer-sphere surface on the permanently charged basal surface sites (≡X−), while in the higher pH range the adsorption occurred mainly on the variably charged edge sites (≡SOH) through the formation of inner-sphere surface complexes. Acid-base surface constants and metal binding constants for the two sites were optimized using FITEQL. The adsorption affinity of the five metallic cations to the permanently charged sites of montmorillonite was Pb > Cu > Ni ≈ Zn ≈ Cd, while that to the variable charged sites was Pb ? Cu > Zn > Cd > Ni.  相似文献   

19.
X-ray absorption fine structure (XAFS) spectroscopic analysis at the As, Se, and Mn K-edges was used to study arsenate [As(V)O43−] and selenite [Se(IV)O32−] sorption complexes on the synthetic hydrous manganese oxides (HMOs) vernadite (δ-MnO2) and K-birnessite (nominal composition: K4Mn14O27 · 9H2O). No significant changes were observed in sorption complex structure as a function of sorbent, pH (5 to 8), surface coverage (0.04 to 0.73 μmol/m2), or reaction time (5 to 22 h) in the arsenate or selenite systems. In the arsenate/HMO system, extended XAFS parameters indicate an average second-neighbor As(V) coordination of 2.0 ± 0.4 Mn at an average distance of 3.16 ± 0.01 Å, which is consistent with formation of As(V)O4 sorption complexes sharing corners with two adjacent Mn(IV)O6 surface species (i.e., bidentate, binuclear). In the selenite/HMO system, selenite surface complexes are surrounded by two shells of Mn atoms, which could represent two different adsorption complexes or a precipitate. The first shell consists of 1.6 ± 0.4 Mn at 3.07 ± 0.01 Å, which is consistent with the selenite anion forming bidentate (mononuclear) edge-sharing complexes with Mn(II)O6 or Mn(III)O6 octahedra. The second shell consists of 1.4 ± 0.4 Mn at 3.49 ± 0.03 Å, consistent with selenite forming monodentate, corner-sharing complexes with Mn(II)O6 or Mn(III)O6 octahedra. Pauling bond valence analysis that uses the extended XAFS-derived bond lengths for As(V)-O, Se(IV)-O, and Mn-O bonds indicates that the proposed surface complexes of selenite and arsenate on HMOs should be stable. Although a nearly identical Se(IV) coordination environment is found in a crystalline Mn(II)-Se(IV) precipitate (which has a structure similar to that of MnSeO3 · H2O), there are significant differences in the X-ray absorption near-edge structure and extended XAFS spectra of this precipitate and the selenite/HMO sorption samples. These differences coupled with transmission electron microscopy results suggest that if a precipitate is present it lacks long-range order characteristic of crystalline MnSeO3 · H2O.  相似文献   

20.
The surface structure of α-Fe2O3(0 0 0 1) was studied using crystal truncation rod (CTR) X-ray diffraction before and after reaction with aqueous Fe(II) at pH 5. The CTR results show the unreacted α-Fe2O3(0 0 0 1) surface consists of two chemically distinct structural domains: an O-layer terminated domain and a hydroxylated Fe-layer terminated domain. After exposing the α-Fe2O3(0 0 0 1) surface to aqueous Fe(II), the surface structure of both co-existing structural domains was modified due to adsorption of Fe at crystallographic lattice sites of the substrate, resulting in six-coordinated adsorbed Fe at the surface. The average Fe-O bond lengths of the adsorbed Fe are consistent with typical Fe(III)-O bond lengths (in octahedral coordination), providing evidence for the oxidation of Fe(II) to Fe(III) upon adsorption. These results highlight the important role of substrate surface structure in controlling Fe(II) adsorption. Furthermore, the molecular scale structural characterization of adsorbed Fe provides insight into the process of Fe(II) induced structural modification of hematite surfaces, which in turn aids in assessing the effective reactivity of hematite surfaces in Fe(II) rich environments.  相似文献   

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