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1.
The mobility and toxicity of Cr within surface and subsurface environments is diminished by the reduction of Cr(VI) to Cr(III). The reduction of hexavalent chromium can proceed via chemical or biological means. Coupled processes may also occur including reduction via the production of microbial metabolites, including aqueous Fe(II). The ultimate pathway of Cr(VI) reduction will dictate the reaction products and hence the solubility of Cr(III). Here, we investigate the fate of Cr following a coupled biotic-abiotic reduction pathway of chromate under iron-reducing conditions. Dissimilatory bacterial reduction of two-line ferrihydrite indirectly stimulates reduction of Cr(VI) by producing aqueous Fe(II). The product of this reaction is a mixed Fe(III)-Cr(III) hydroxide of the general formula Fe1−xCrx(OH)3 · nH2O, having an α/β-FeOOH local order. As the reaction proceeds, Fe within the system is cycled (i.e., Fe(III) within the hydroxide reaction product is further reduced by dissimilatory iron-reducing bacteria to Fe(II) and available for continued Cr reduction) and the hydroxide products become enriched in Cr relative to Fe, ultimately approaching a pure Cr(OH)3 · nH2O phase. This Cr purification process appreciably increases the solubility of the hydroxide phases, although even the pure-phase chromium hydroxide is relatively insoluble.  相似文献   

2.
Young ochreous precipitations from Fe-bearing spring waters in Finland consist mainly of ferrihydrite. a poorly ordered Fe-oxide with a layer structure and the bulk composition 5 Fe2O3 ·9 H2O Crystallinity ranges from a reasonably well developed structure to a highly disordered one with only two prismatic reflections at 2.5 and 1.5 Å. In contrast to other Fe-oxides. ferrihydrite is highly soluble in oxalate. Electron microscopy shows spherical particles 2–5 nm in diameter forming aggregates of 100–300 nm. The specific surface ranges from 220 to 560 m2/g. During their formation, the ferrihydrites adsorb large quantities of silica, part of which is unpolymerized as indicated by Si-O-Fe bonds (i.r.), and part of which is polymerized. NaOH preferentially extracts polymerized silica causing a shift in the i.r. absorption band. Silica also causes a shift in the temperature at which ferrihydrite converts to hematite. ‘Hydrous Fe(III)-oxides’ with 0–15mol% Si prepared from Si containing Fe(III) salt solutions showed similar properties: Si-O-Fe bonds are shown by i.r. and increasing temperatures of transformation to hematite with increasing amount of Si. Adsorbed Si may also retard the transformation of ferrihydrite to the more stable goethite in nature.  相似文献   

3.
The mobilization of arsenic (As) to the groundwater was studied in a shallow Holocene aquifer on the Red River flood plain near Hanoi, Vietnam. The groundwater chemistry was investigated in a transect of 100 piezometers. Results show an anoxic aquifer featuring organic carbon decomposition with redox zonation dominated by the reduction of Fe-oxides and methanogenesis. Enhanced PCO2 pressure causes carbonate dissolution to take place but mainly in the soil and unsaturated zone. The concentration of As increases over depth to a concentration of up to 550 μg/L. Most As is present as As(III) but some As(V) is always found. Arsenic correlates well with NH4, relating its release to organic matter decomposition and the source of As appears to be the Fe-oxides being reduced. Part of the produced Fe(II) is apparently reprecipitated as siderite containing less As. Results from sediment extraction indicate most As to be related to the Fe-oxide fractions. The measured amount of sorbed As is low. In agreement, speciation calculations for a Fe-oxide surface suggest As(III) to constitute only 3% of the surface sites while the remainder is occupied by carbonate and silica species. The evolution in water chemistry over depth is homogeneous and a reactive transport model was constructed to quantify the geochemical processes along the vertical groundwater flow component. A redox zonation model was constructed using the partial equilibrium approach with organic carbon degradation in the sediment as the only rate controlling parameter. Apart from the upper meter a constant degradation rate of 0.15 C mmol/L/yr could explain the redox zonation throughout the aquifer. Modeling also indicates that the Fe-oxide being reduced is of a stable type like goethite or hematite. Arsenic is contained in the Fe-oxides and is first released during their dissolution. Our model further suggests that part of the released As is adsorbed on the surface of the remaining Fe-oxides and in this way may be retarded.  相似文献   

4.
The aim of this study was to design and test a new tool for (i) the quantitative in situ monitoring of Fe(III) reduction in soils and (ii) the tracking of the potential mineralogical changes of Fe-oxides. The tool consists of small (2 × 2 × 0.2 cm) striated polymer plates coated with synthetic pure ferrihydrite or As-doped ferrihydrite (Fh–As). These slides were then inserted within two different horizons (organo-mineral and albic) located in a wetland soil with alternating redox conditions. Dissolution was quantified by X-ray fluorescence (XRF) analyses of total metal contents before and after insertion into the soil. The crystallographic evolution of Fe-oxides was characterized by scanning electron microscope equipped with an energy-dispersive spectrometer (SEM–EDS). Over the months, the ferrihydrite progressively disappeared, at rates comparable to those previously measured in laboratory studies, i.e. in the 1–10 × 10−12 mol Fe m−2 s−1 range. SEM observations indicate that the supports were highly colonized by bacteria and biofilms in the organo-mineral horizon, suggesting a biological-mediated process, while the albic horizon appeared to be characterized by a mostly chemical-mediated process. In the albic horizon, Fe-sulphide and other micro-precipitates were formed after 7 months of incubation in balance with a quasi dissolution of initial Fe-oxides.  相似文献   

5.
新近发现的老挝石因与寿山石多个品种高度相似,给老挝石的科学定名和印材质量评价带来困难。本文采用常规宝玉石学测试结合电子微探针(EPMA)、X射线粉晶衍射(XRD)以及扫描电镜(SEM)等矿物谱学分析测试方法,系统解析其化学成分、矿物组成及显微结构特征。结果表明,老挝石由高岭石亚族矿物多型组成,其中主要为高岭石、地开石及其过渡矿物,少量为珍珠陶石。红、黄色老挝石颜色与铁含量呈正相关性,表明铁元素可能为老挝石常见红、黄两色的主要致色原因。老挝石主要组成矿物晶体以不规则鳞片状或假六方片状为主,其粒径越细小、镶嵌越紧密,相应表现为质地细腻和篆刻工艺性能优异,作为印章石的工艺价值也就越高。老挝石的密度(2.58~2.60 g/cm3)及折射率(1.561~1.570)等常规宝石学特征均与地开石型寿山石较为一致,其中颜色鲜艳、结构细腻者可作为优质的篆刻印材新资源。  相似文献   

6.
Reduction of As(V) and reductive dissolution and transformation of Fe (hydr)oxides are two dominant processes controlling As retention in soils and sediments. When developed within soils and sediments, Fe (hydr)oxides typically contain various impurities—Al being one of the most prominent—but little is known about how structural Al within Fe (hydr)oxides alters its biotransformation and subsequent As retention. Using a combination of batch and advective flow column studies with Fe(II) and Shewanella sp. ANA-3, we examined (1) the extent to which structural Al influences reductive dissolution and transformations of ferrihydrite, a highly reactive Fe hydroxide, and (2) the impact of adsorbed As on dissolution and transformation of (Al-substituted) ferrihydrite and subsequent As retention. Structural Al diminishes the extent of ferrihydrite reductive transformation; nearly three-orders of magnitude greater concentration of Fe(II) is required to induce Al-ferrihydrite transformation compared to pure two-line ferrihydrite. Structural Al decreases Fe(II) retention/incorporation on/into ferrihydrite and impedes Fe(II)-catalyzed transformation of ferrihydrite. Moreover, owing to cessation of Fe(II)-induced transformation to secondary products, Al-ferrihydrite dissolves (incongruently) to a greater extent compared to pure ferrihydrite during reaction with Shewanella sp. ANA-3. Additionally, adsorption of As(V) to Al-ferrihydrite completely arrests Fe(II)-catalyzed transformation of ferrihydrite, and it diminishes the difference in the rate and extent of ferrihydrite and Al-ferrihydrite reduction by Shewanella sp. ANA-3. Our study further shows that reductive dissolution of Al-ferrihydrite results in enrichment of Al sites, and As(V) reduction accelerates As release due to the low affinity of As(III) on these non-ferric sites.  相似文献   

7.
Systematic changes in mineralogy, enrichment and depletion of selected elements, and mineralogical speciation of selected elements in fly ash and bottom ash samples from the Lingan Power Plant were compared to run-of-mine and pulverized feed coal from the Sydney coalfield, Nova Scotia, eastern Canada. The analytical techniques used were an electron microprobe equipped with energy and wavelength X-ray dispersive spectrometers, X-ray diffraction, neutron activation, scanning electron microscopy with energy dispersive X-ray and incident light petrography. Three types of glasses (Fe/O-rich, Fe/Al/Si/O-rich and or Al/Si/O-rich) were identified in the combustion residues; they were formed as a result of the interaction of melted pyrite and clay minerals. Compared to the feed coal, most elements were enriched 10 to20 times in the fly ash. The concentration of the elements in both the fly ash and bottom ash are comparable to coal ash that is generated by the low temperature asher in the laboratory. Some chalcophile elements such as arsenic and lead occurred as a solid solution in pyrite in the feed coal and were concentrated in the float fraction (density: <2.81 g/cm3) of the fly ash with non-crystalline Fe-oxides. X-ray mapping of arsenic in the fly ash and bottom ash indicates that arsenic was evenly distributed as oxide within the Fe/O- and Fe/Al/Si/O-rich glass and crystalline phases in the fly ash, possibly in solid solution. Arsenic is associated with Fe/O and Fe/S crystalline phases in the bottom ash.  相似文献   

8.
Evidence for a simple pathway to maghemite in Earth and Mars soils   总被引：1，自引：0，他引：1  
Soil magnetism is greatly influenced by maghemite (γ-Fe2O3), the presence of which is usually attributed to the following: (1) heating of goethite in the presence of organic matter; (2) oxidation of magnetite (Fe3O4); or (3) dehydroxylation of lepidocrocite (γ-FeOOH). Formation of the latter two minerals in turn requires the presence of Fe(II) in the system. No laboratory experiment or soil study to date has shown whether maghemite can form from ferrihydrite, a poorly crystalline Fe(III) oxide [∼Fe4.5(O,OH,H2O)13.5], below 250°C. However, ferrihydrite is the usual precursor of goethite (α-FeOOH) and hematite (α-Fe2O3), the most frequently occurring crystalline Fe(III) oxides in soils. Here is presented in vitro evidence that ferryhidrite can partly transform into maghemite at 150°C. This transformation occurs upon aging of ferrihydrite precipitated in the presence of phosphate or other ligands capable of ligand exchange with Fe-OH surface groups. This maghemite coexists with hematite and is a transient phase in the transformation of ferrihydrite to hematite, which is apparently stabilized by the adsorbed ligands. Its particle size is small (10 to 30 nm), and its X-ray diffraction pattern exhibits superstructure reflections. The possible formation of maghemite in Mars and in different Earth soils can partly be explained in the light of this pathway with minimal ad hoc assumptions.  相似文献   

9.
对抚顺油页岩样品的容重测定、工业分析、低温干馏、元素分析、发热量测定、灰分成分分析及灰熔点测定,可知油页岩容重为1.4～2.7 t/m3、含油率为1.17%～20.70%、发热量为0.45～11.18 MJ/kg。油页岩的颜色多为浅褐-深褐色,条痕为褐色,颜色越深含油率越高。低温干馏时,温度控制在510℃,焦油率最高。油页岩有机质主要由C、H、O、N元素组成,无机物主要由Si、Al、Fe、Ca、Mg等元素组成,其伴生元素主要为Ti、Zr、Ge和Ga;油页岩灰分主要由SiO2、Al2O3、Fe2O3、CaO、MgO和SO3组成,其中SiO2的含量最高,Al2O3、Fe2O3次之。  相似文献   

10.
The quantitative determination of the main compounds Al (OH)3 (gibbsite) as Al2O3 and hydrated Fe-oxides as Fe2O3 in natural and beneficiated ferruginous bauxites (low-grade bauxites) by X-ray diffraction (XRD) is presented employing the external standard method. The calibration parameters for the analysis by XRD were ascertained by using ten synthetic mixtures of the pure oxides Al2O3 and Fe2O3 which were calcined at 1200°C. The concentration ranges for Fe2O3 were 10–70 wt.% and those for alumina 30–90 wt.%. The standard deviation for the calibrations of the two oxides was 3.33 wt.%. The results obtained by XRD for both oxides were compared with those attained by X-ray fluorescence. The precision of the determinations by XRD was ascertained by analysing a natural sample consecutively 5 times. The accuracy was verified by the analysis of two international standard reference materials with low silica contents.  相似文献   

11.
Groundwater arsenic (As) concentrations above 10 μg/L (World Health Organization; WHO standard) are frequently found in the Titas Upazila in Bangladesh. This paper evaluates the groundwater chemistry and the mechanisms of As release acting in an underground aquifer in the middle-northeast part of the Titas Upazila in Bangladesh. Previous measurements and analyses of 43 groundwater samples from the region of interest (ROI) are used. Investigation is based on major ions and important trace elements, including total As and Fe in groundwater samples from shallow (8–36 m below ground level: mbgl) and deep (85–295 mbgl) tube wells in the aforementioned ROI. Principal hydrochemical facies are Ca–HCO3, with circumneutral pH. The different redox-sensitive constituents (e.g., As, Fe, Mn, NH4, and SO4) indicate overlapping redox zones, leading to differences regarding the redox equilibrium. Multivariate statistical analysis (factor analysis) was applied to reduce 20 chemical variables to four factors but still explain 81% of the total variance. The component loadings give hints as to the natural processes in the shallow aquifers, in which organic matter is a key reactant. The observed chemistry of As, Fe, and Mn can be explained by simultaneous equilibrium between Fe-oxide and SO4 reduction and an equilibrium of rhodochrosite precipitation/dissolution. A correlation test indicates the likeliness of As release by the reductive dissolution of Fe-oxides driven by the degradation of sediments organic matter. Other mechanisms could play a role in As release, albeit to a lesser extent. Reactive transport modeling using PHREEQC reproduced the observed chemistry evolution using simultaneous equilibrium between Fe-oxide and SO4 reduction and the equilibrium of rhodochrosite dissolution/precipitation alongside organic matter oxidation.  相似文献   

12.
The speciation of Mn, Fe, As, and Zn in a fast-growing (0.02mm/yr), shallow-marine, ferromanganese nodule has been examined by micro X-ray fluorescence, micro X-ray diffraction, and micro X-ray absorption spectroscopy. This nodule exhibits alternating Fe-rich and Mn-rich layers reflecting redox variations in water chemistry. Fe occurs as two-line ferrihydrite. The As is strictly associated with Fe and is mostly pentavalent, with an environment similar to that of As sorbed on or coprecipitated with synthetic ferrihydrite. The Mn is in the form of turbostratic birnessite with ∼10% trivalent manganese in the layers and probably ∼8% corner-sharing metal octahedra in the interlayers. The Zn is enriched on the rim of the nodule, associated with Mn. The Zn is completely (>90%) tetrahedrally coordinated and sorbed in the interlayers of birnessite on vacant layer Mn sites. The Zn and Mn species are similar to ones found in soils, suggesting common structural principles despite the differing formation conditions in these systems.  相似文献   

13.
硫化纳米铁（S-nZVI）是一种具有壳核结构的新型纳米铁（nZVI）改性材料,在多种污染物的去除上表现出超越nZVI的反应活性。本文采用两步合成法制备了S-nZVI,并采用透射电镜-能量色散X射线（TEM-EDX）、X射线衍射（XRD）和X射线光电子能谱分析（XPS）方法对S-nZVI和nZVI进行表征,探讨了不同硫铁摩尔比（n（S）/n（Fe））、初始pH值、试剂投加量和地下水化学成分对nZVI及S-nZVI去除Cr（Ⅵ）的影响。结果表明：S-nZVI具有明显的壳核结构,其Fe0核外层包覆着非晶的硫化亚铁和多硫化物;S-nZVI去除Cr（Ⅵ）的最佳n（S）/n（Fe）为0.14;增加S-nZVI投加量会提高其对Cr（Ⅵ）的去除率,投加量相同时,S-nZVI对Cr（Ⅵ）的去除率显著高于nZVI;提高初始pH值时,S-nZVI和nZVI对Cr（Ⅵ）的去除率均逐渐降低,但在相同pH值条件下,S-nZVI对Cr（Ⅵ）的去除率和去除速率始终高于nZVI,尤其是在pH=5时,S-nZVI仍能去除100%的Cr（Ⅵ）,而nZVI只能去除85%;K+、Na+、Ca2+、Mg2+、SO42-、NO3-和Cl-对S-nZVI和nZVI去除Cr（Ⅵ）均有促进作用,但对S-nZVI体系的促进作用更强;HCO3-的存在会使溶液的pH值升高从而抑制S-nZVI和nZVI对Cr（Ⅵ）的去除,对nZVI的抑制作用强于S-nZVI。总体来说,S-nZVI对Cr（Ⅵ）的去除率在不同pH值和多种地下水化学组分影响条件下均高于nZVI,因此具有更广泛的应用前景。  相似文献   

14.
Sorption of U(VI) on Hanford fine sand (HFS) with varying Fe-oxide (especially ferrihydrite) contents showed that U(VI) sorption increased with the incremental addition of synthetic ferrihydrite into HFS, consistent with ferrihydrite being one of the most reactive U(VI) sorbents present in natural sediments. Surface complexation model (SCM) calculations for U(VI) sorption, using only U(VI) surface-reaction constants obtained from U(VI) sorption data on freshly synthesized ferrihydrite at different pHs, were similar to the measured U(VI) sorption results on pure synthetic ferrihydrite and on HFS with high contents of ferrihydrite (5 wt%) added. However, the SCM prediction using only U(VI) sorption reactions and constants for synthetic ferrihydrite overestimated U(VI) sorption on the natural HFS or HFS with addition of low amounts of added ferrihydrite (1 wt% added). Over-predicted U(VI) sorption was attributed to reduced reactivity of natural ferrihydrite present in Hanford Site sediments, compared to freshly prepared synthetic ferrihydrite. Even though the SCM general composite (GC) approach is considered to be a semi-quantitative estimation technique for contaminant sorption, which requires systematic experimental data on the sorbent–sorbate system being studied to obtain credible SCM parameters, the general composite SCM model was still found to be a useful technique for describing U(VI) sorption on natural sediments. Based on U(VI) batch sorption results, two simple U(VI) monodentate surface species, SO_UO2HCO3 and SO_UO2OH on ferrihydrite and phyllosillicate in HFS, respectively, can be successfully used to describe U(VI) sorption onto Hanford Site sediment contacting varying geochemical solutions.  相似文献   

15.
Discharge of Fe(II)-rich groundwaters into surface-waters results in the accumulation of Fe(III)-minerals in salinized sand-bed waterways of the Hunter Valley, Australia. The objective of this study was to characterise the mineralogy, micromorphology and pore-water geochemistry of these Fe(III) accumulations. Pore-waters had a circumneutral pH (6.2–7.2), were sub-oxic to oxic (Eh 59–453 mV), and had dissolved Fe(II) concentrations up to 81.6 mg L−1. X-ray diffraction (XRD) on natural and acid-ammonium-oxalate (AAO) extracted samples indicated a dominance of 2-line ferrihydrite in most samples, with lesser amounts of goethite, lepidocrocite, quartz, and alumino-silicate clays. The majority of Fe in the samples was bound in the AAO extractable fraction (FeOx) relative to the Na-dithionite extractable fraction (FeDi), with generally high FeOx:FeDi ratios (0.52–0.92). The presence of nano-crystalline 2-line ferrihydrite (Fe5HO3·4H2O) with lesser amounts of goethite (α-FeOOH) was confirmed by scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED). In addition, it was found that lepidocrocite (γ-FeOOH), which occurred as nanoparticles as little as ∼5 lattice spacings thick perpendicular to the (0 2 0) lattice plane, was also present in the studied Fe(III) deposits. Overall, the results highlight the complex variability in the crystallinity and particle-size of Fe(III)-minerals which form via oxidation of Fe(II)-rich groundwaters in sand-bed streams. This variability may be attributed to: (1) divergent precipitation conditions influencing the Fe(II) oxidation rate and the associated supply and hydrolysis of the Fe(III) ion, (2) the effect of interfering compounds, and (3) the influence of bacteria, especially Leptothrix ochracea.  相似文献   

16.
Lead sorption efficiencies of natural and synthetic Mn and Fe-oxides   总被引：1，自引：0，他引：1  
Lead sorption efficiencies (sorption per specific surface area) were measured for a number of natural and synthetic Mn and Fe-oxides using a flow-through reactor. The Mn-oxide phases examined included synthetic birnessite, natural and synthetic cryptomelane, and natural and synthetic pyrolusite; the Fe-oxides studied were synthetic akaganéite, synthetic ferrihydrite, natural and synthetic goethite, and natural and synthetic hematite. The sorption flow study experiments were conducted with 10 ppm Pb with an ionic strength of either 0.01 M NaNO3 or 0.01 M KNO3, both at pH 5.5. The experimental effluent solution was analyzed using aqueous spectroscopic methods and the reacted solids were analyzed using microscopy (field emission scanning electron microscopy, FE-SEM), structure analysis (powder X-ray diffraction, XRD), bulk chemical spectroscopy (energy dispersive spectroscopy, EDS), and surface sensitive spectroscopy (X-ray photoelectron spectroscopy, XPS). Overall and under these conditions, the synthetic Mn-oxides have higher sorption efficiencies than the natural Mn-oxides, which in turn are higher than the natural and synthetic Fe-oxides. Only natural pyrolusite had a sorption efficiency as low as the Fe-oxides. Most of the natural and synthetic Fe-oxides examined in this study removed about the same amount of Pb from solution once normalized to BET N2 surface area, although synthetic akaganéite and hematite were significantly less reactive than the rest.It is suggested that the observed efficiency of Mn-oxides for Pb sorption is directly related to internal reactive sites in the structures that contain them (birnessite and cryptomelane, in the case of this study). Comparisons of solution data to XPS data indicated that Pb went into the interlayer of the birnessite, which was supported by XRD; similarly some Pb may go into the tunnels of the cryptomelane structure. Layer structures such as birnessite have the highest Pb sorption efficiency, while the 2 × 2 tunnel structure of cryptomelane has lower efficiencies than birnessite, but higher efficiencies than other Mn- or Fe-oxide structures without internal reactive sites.  相似文献   

17.
Mining and metallurgical processing of gold and base metal ores can lead to the release of arsenic into the aqueous environment as a result of the weathering and leaching of As-bearing minerals during processing and following disposal. Arsenic in process solutions and mine drainage can be effectively stabilized through the precipitation of ferrihydrite. However, under anaerobic conditions imposed by burial and waste cover systems, ferrihydrite is susceptible to microbial reduction. This research, stimulated by the paucity of information and limited understanding of the microbial reduction of arsenical ferrihydrite, was conducted on synthetic adsorbed and co-precipitated arsenical 6-line ferrihydrite (Fe/As molar ratio of 10/1) using Shewanella sp. ANA-3 and Shewanella putrefaciens CN32 in a chemically defined medium containing 0.045 mM phosphate concentration. Both bacteria were equally effective in their reducing abilities around pH 7, resulting in initial rates of formation of dissolved As(III) of 0.10 μM/h for the adsorbed, and 0.08 μM/h for the co-precipitated arsenical 6-line ferrihydrite samples. The solid phases in the post-reduction samples were characterized by powder X-ray diffraction (XRD), micro-XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron microprobe and X-ray absorption spectroscopy (XAS) techniques. The results indicate the formation of secondary phases such as a biogenic Fe(II)–As(III) compound, akaganeite, goethite, hematite and possibly magnetite during bacterial reduction experiments. Holes and bacterial imprints measuring about 1–2 μm were observed on the surfaces of the secondary phases formed after 1200 h of reduction. This study demonstrates the influence of Fe and As reducing bacteria on the release of significant concentrations of more mobile and toxic As(III) species from arsenical 6-line ferrihydrite, more readily from the adsorbed than from the co-precipitated ferrihydrite.  相似文献   

18.
Uranium co-precipitation with iron oxide minerals   总被引：2，自引：0，他引：2  
In oxidizing environments, the toxic and radioactive element uranium (U) is most soluble and mobile in the hexavalent oxidation state. Sorption of U(VI) on Fe-oxides minerals (such as hematite [α-Fe2O3] and goethite [α-FeOOH]) and occlusion of U(VI) by Fe-oxide coatings are processes that can retard U transport in environments. In aged U-contaminated geologic materials, the transport and the biological availability of U toward reduction may be limited by coprecipitation with Fe-oxide minerals. These processes also affect the biological availability of U(VI) species toward reduction and precipitation as the less soluble U(IV) species by metal-reducing bacteria.To examine the dynamics of interactions between U(VI) and Fe oxides during crystallization, Fe-oxide phases (containing 0.5 to 5.4 mol% U/(U + Fe)) were synthesized by means of solutions of U(VI) and Fe(III). Wet chemical (digestions and chemical extractions) and spectroscopic techniques were used to characterize the synthesized Fe oxide coprecipitates after rinsing in deionized water. Leaching the high mol% U solids with concentrated carbonate solution (for sorbed and solid-phase U(VI) species) typically removed most of the U, leaving, on average, about 0.6 mol% U. Oxalate leaching of solids with low mol% U contents (about 1 mol% U or less) indicated that almost all of the Fe in these solids was crystalline and that most of the U was associated with these crystalline Fe oxides. X-ray diffraction and Fourier-transform infrared (FT-IR) spectroscopic studies indicate that hematite formation is preferred over that of goethite when the amount of U in the Fe-oxides exceeds 1 mol% U (∼4 wt% U). FT-IR and room temperature continuous wave luminescence spectroscopic studies with unleached U/Fe solids indicate a relationship between the mol% U in the Fe oxide and the intensity or existence of the spectra features that can be assigned to UO22+ species (such as the IR asymmetric υ3 stretch for O = U = O for uranyl). These spectral features were undetectable in carbonate- or oxalate-leached solids, suggesting solid phase and sorbed U(VI)O22+ species are extracted by the leach solutions. Uranium L3-edge x-ray absorption spectroscopic (XAFS) analyses of the unleached U-Fe oxide solids with less than 1 mol% U reveal that U(VI) exists with four O atoms at radial distances of 2.19 and 2.36 Å and second shell Fe at a radial distance at 3.19 Å.Because of the large ionic radius of UO22+ (∼1.8 Å) relative to that of Fe3+ (0.65 Å), the UO22+ ion is unlikely to be incorporated in the place of Fe in Fe(III)-oxide structures. Solid-phase U(VI) can exist as the uranyl [U(VI)O22+] species with two axial U-O double bonds and four or more equatorial U-O bonds or as the uranate species (such as γ-UO3) without axial U-O bonds. Our findings indicate U6+ (with ionic radii of 0.72 to 0.8 Å, depending on the coordination environment) is incorporated in the Fe oxides as uranate (without axial O atoms) until a point of saturation is reached. Beyond this excess in U concentration, precipitating U(VI) forms discrete crystalline uranyl phases that resemble the uranyl oxide hydrate schoepite [UO2(OH)2·2H2O]. Molecular modeling studies reveal that U6+ species could bond with O atoms from distorted Fe octahedra in the hematite structure with an environment that is consistent with the results of the XAFS. The results provide compelling evidence of U incorporation within the hematite structure.  相似文献   

19.
石墨尾矿作为一类二次资源,当云母类矿物的含量在10%以上时,具有回收利用价值。黑龙江某地石墨尾矿中含有10%以上的云母类矿物,但其细度(500~800目)低于常规的云母矿,本文采用X射线衍射(XRD)分析矿物组成,再结合化学分析、扫描电镜-能谱、光学显微镜与纯矿物进行对比,由此鉴定云母类矿物种属。XRD研究初步表明石墨尾矿含有的云母族矿物属于白云母亚类或黑云母亚类。扫描电镜分析表明尾矿中的云母矿物与纯绢云母矿物均以片状和鳞片状为主要存在形态,而纯黑云母主要以片状形式存在。尾矿的Si O2含量为5%~13%,Al2O3含量为4%~8%,与纯绢云母接近。光学显微镜鉴定显示在正交偏光下石墨尾矿中的云母颜色和纯绢云母类似,而与纯黑云母截然不同。综合以上结果最终可确定该地石墨尾矿中的云母族矿物为白云母亚类中的绢云母。本研究为后续石墨尾矿的选矿工艺奠定了基础,也可应用于类似细度绢云母的鉴定。  相似文献   

20.
Scorodite, ferric arsenate and arsenical ferrihydrite are important arsenic carriers occurring in a wide range of environments and are also common precipitates used by metallurgical industries to control arsenic in effluents. Solubility and stability of these compounds are controversial because of the complexities in their identification and characterization in heterogeneous media. To provide insights into the formation of scorodite, ferric arsenate and ferrihydrite, series of synthesis experiments were carried out at 70 °C and pH 1, 2, 3 and 4.5 from 0.2 M Fe(SO4)1.5 solutions also containing 0.02-0.2 M Na2HAsO4. The precipitates were characterized by transmission electron microscopy, X-ray diffraction and X-ray absorption fine structure techniques. Ferric arsenate, characterized by two broad diffuse peaks on the XRD pattern and having the structural formula of FeAsO4·4-7H2O, is a precursor to scorodite formation. As defined by As XAFS and Fe XAFS, the local structure of ferric arsenate is profoundly different than that of scorodite. It is postulated that the ferric arsenate structure is made of single chains of corner-sharing Fe(O,OH)6 octahedra with bridging arsenate tetrahedra alternating along the chains. Scorodite was precipitated from solutions with Fe/As molar ratios of 1 over the pH range of 1-4.5. The pH strongly controls the kinetics of scorodite formation and its transformation from ferric arsenate. The scorodite crystallite size increased from 7 to 33 nm by ripening and aggregation. Precipitates, resulting from continuous synthesis at pH 4.5 from solutions having Fe/As molar ratios ranging from 1 to 4 and resembling the compounds referred to as ferric arsenate, arsenical ferrihydrite and As-rich hydrous ferric oxide in the literature, represent variable mixtures of ferric arsenate and ferrihydrite. When the Fe/As ratio increases, the proportion of ferrihydrite increases at the expense of ferric arsenate. Arsenate adsorption appears to retard ferrihydrite growth in the precipitates with molar Fe/As ratios of 1-4, whereas increased reaction gradually transforms two-line ferrihydrite to six-line ferrihydrite at Fe/As ratios of 5 and greater.  相似文献   

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