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1.
《古地理学报》2016,(2)
利用普通薄片、铸体薄片、扫描电镜、物性分析、测井解释和粒度分析等资料,对松辽盆地大安油田下白垩统泉头组四段特低渗—超低渗储集层成岩作用类型、成岩作用强度及其影响因素等进行了研究。结果表明:(1)压实作用是导致大安油田泉四段特低渗—超低渗储集层物性变差最主要的成岩作用。研究区储集层压实程度为中等,压实破坏程度与储集层复合砂体厚度、分选系数、泥质含量等参数具有较好的相关性。(2)胶结作用是研究区储集层物性变差的又一重要因素。区内胶结作用强度较弱,胶结作用减孔率与相对构型界面的位置、石英绝对含量、碎屑组分含量和杂基含量等因素具有明显的相关性。(3)溶蚀作用对区内储集层物性的改善程度相对较小。溶蚀作用增孔率与复合砂体厚度、杂基含量具有一定的相关性。压实、胶结、溶蚀作用对储集层物性改造程度影响因素的分析结果,反映砂体的垂向叠置关系、河道厚度规模、水动力强度等沉积特征对成岩作用类型及其强度有一定的控制作用。 相似文献
2.
鄂尔多斯盆地东部子洲—清涧地区二叠系山西组23段石英砂岩区块单井产能与砂体结构类型具有一定相关性。基于单砂体垂向粒度韵律和自然伽马测井资料,将砂体结构划分出块状型砂体、正粒序型砂体和叠加型砂体3种类型。通过铸体薄片鉴定和粒度分析实验,对3种类型砂体储集层在成岩作用类型、特征、发育程度、产物、垂向变化及成岩孔隙演化的差异性进行表征。研究表明,3类砂体的溶蚀作用、压实作用强度及分布特征存在较大差异,粒度及其垂向分布是制约不同结构类型砂体成岩演化的直接因素,进而影响储集层物性,决定了含气层与致密层的垂向分布规律。块状型砂体储集层原生孔隙、次生溶孔共同发育,含气层厚度较大,局部致密。正粒序型砂体垂向物性差异较大,中—下部储集层溶蚀程度较弱,以原生孔隙为主,次生孔隙少量发育,为含气层;上部储集层压实、胶结作用强烈,相对致密。叠加型砂体储集层形成于河流控制作用逐渐增强的过渡环境,兼有正粒序型、块状型砂体的特征,多层含气且厚度大。 相似文献
3.
综合应用测井资料和铸体薄片、扫描电镜、X-衍射、粒度分析等资料,对志丹油田永宁探区长6储层的特低渗透成因和主控因素进行了分析。结果表明：研究区长6储层特低渗透成因主要受沉积作用和成岩作用的双重影响,主要表现为沉积微相对储层物性的影响较大,水下分流河道砂层的物性好于河口坝沉积砂体,河口坝沉积砂体的物性好于分流间湾砂体沉积;成岩压实作用和胶结作用使得储层物性变差,成岩溶蚀及蚀变作用和成岩缝的产生对储层的特低渗物性的改善有积极作用。 相似文献
4.
鄂尔多斯盆地陇东地区长82储集层平均孔隙度为8.8%,平均渗透率为0.64×10-3μm2,属于低孔低渗储集层。强烈的压实作用是储集层物性变差主要原因,碳酸盐胶结作用促使储集层物性进一步变差。尽管储集层物性整体较差,但在普遍低孔低渗背景下仍发育相对高孔渗的优质储集层。对优质储集层进行了分析,总结了优质储集层的典型特征,统计表明:优质储集层的塑性组分含量较低,孔隙类型以绿泥石胶结残余粒间孔和溶孔为主,孔隙结构以中小孔—中细喉为主。在此基础上,分析了优质储集层成因机理。研究认为:分流河道、河口坝等微相高能厚层砂体有利于优质储集层发育,相对低的塑性组分含量和绿泥石环边胶结有利于粒间孔隙的保存,溶蚀作用是次生孔隙形成的最主要成岩作用。 相似文献
5.
综合利用岩心、铸体薄片、扫描电镜等多种资料,对鄂尔多斯盆地正宁地区三叠系延长组长9油层组储集层的岩石类型、成岩作用类型、成岩相分布特征及主控因素进行了系统研究。延长组长9油层组储集层的岩石类型为细—中粒岩屑质长石砂岩和长石质岩屑砂岩,成岩作用类型主要为压实作用,同时还有绿泥石、方解石、铁方解石及硅质胶结作用,方解石交代作用和少量的溶蚀作用,储集层目前处于中成岩A期。根据层理频率和塑性颗粒含量将岩石划分为高频层理富片状塑性颗粒粉—细砂岩岩石相和低频层理富刚性矿物细—中砂岩岩石相,前者分布于水下分流河道边部,发育压实致密成岩相;后者形成于水下分流河道主河道和河口坝。根据砂体结构可划分为厚层单砂体和厚层多砂体。厚层单砂体为单期形成的河道或河口坝砂体,上部以方解石充填胶结成岩相为特征,下部以绿泥石衬边胶结成岩相为特征;厚层多砂体为多期砂体冲刷叠置形成,发育方解石绿泥石充填胶结成岩相。塑性颗粒含量对压实作用强度影响最大,叠加碳酸盐胶结致使储集层质量变差,而早期绿泥石环边胶结抑制压实作用,是有利储集层发育的重要条件。综合评价表明,延长组长9油层组有利储集层发育于绿泥石衬边胶结相,其次为绿泥石和方解石充填胶结相,最差储集层为方解石充填胶结相和压实致密相。 相似文献
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7.
鄂尔多斯盆地中部姚店油田上三叠统长4＋5储层砂岩的成岩作用 总被引：1，自引：0，他引：1
根据砂岩薄片、铸体薄片、扫描电镜、X－衍射等分析,对姚店油田三叠系延长组长4+5油层砂体的岩石学特征、成岩作用阶段划分、成岩作用类型及其对孔隙的影响进行了研究。研究结果表明,该区长4+5储层储集砂体成分主要以长石砂岩为主。成岩作用类型包括压实作用、胶结作用和溶蚀作用。砂岩经历了早期成岩阶段A、B期和晚期成岩阶段A期（A1、A2 期）的成岩演化过程。早期的压实作用、多种矿物胶结作用是使物性变差的2个重要因素,但早成岩阶段形成的绿泥石膜保护了一些砂岩的原生孔隙;晚成岩阶段A期的溶蚀作用,特别是长石的溶蚀是储层物性变好的重要成岩控制因素 相似文献
8.
南海莺歌海盆地内地层普遍发育超压,东方区和乐东区是莺歌海盆地天然气成藏有利区的核心地带。其中,东方区位于底辟带中心,超压形成时间较早(9—5 Ma),主要物源来自西部昆嵩隆起区;乐东区位于莺东斜坡带,超压形成时间较晚(5—2 Ma),主要物源区为东部海南隆起区。本研究通过铸体薄片、荧光薄片、扫描电镜、电子探针、含烃包裹体微束荧光分析、流体包裹体激光拉曼成分与均一温度、X射线衍射黏土矿物等分析测试研究方法,对东方区和乐东区超压背景下黄流组砂岩储集层的岩石学与物性特征、烃类充注与成岩作用特征进行对比分析。结果显示: (1)东方区黄流组砂岩的压实作用、胶结作用较弱,物性较好(平均孔隙度17.68%,平均渗透率11.11×10-3 μm2),处于中成岩A期晚期;乐东区黄流组砂岩整体压实作用、胶结作用较强,物性较差(平均孔隙度8.94%,平均渗透率1.52×10-3 μm2),处于中成岩B期。(2)研究区储集层物性主要受沉积作用、超压背景和成岩作用的控制。物源、沉积中心与沉降中心、沉积相类型,超压形成时间和烃类充注规模共同影响了储集层的成岩作用特征及成岩—孔隙演化过程。(3)超压在一定程度上抵抗了压实作用强度,抑制了胶结作用,促进了溶蚀作用。 相似文献
9.
南海莺歌海盆地内地层普遍发育超压,东方区和乐东区是莺歌海盆地天然气成藏有利区的核心地带。其中,东方区位于底辟带中心,超压形成时间较早(9—5 Ma),主要物源来自西部昆嵩隆起区;乐东区位于莺东斜坡带,超压形成时间较晚(5—2 Ma),主要物源区为东部海南隆起区。本研究通过铸体薄片、荧光薄片、扫描电镜、电子探针、含烃包裹体微束荧光分析、流体包裹体激光拉曼成分与均一温度、X射线衍射黏土矿物等分析测试研究方法,对东方区和乐东区超压背景下黄流组砂岩储集层的岩石学与物性特征、烃类充注与成岩作用特征进行对比分析。结果显示: (1)东方区黄流组砂岩的压实作用、胶结作用较弱,物性较好(平均孔隙度17.68%,平均渗透率11.11×10-3 μm2),处于中成岩A期晚期;乐东区黄流组砂岩整体压实作用、胶结作用较强,物性较差(平均孔隙度8.94%,平均渗透率1.52×10-3 μm2),处于中成岩B期。(2)研究区储集层物性主要受沉积作用、超压背景和成岩作用的控制。物源、沉积中心与沉降中心、沉积相类型,超压形成时间和烃类充注规模共同影响了储集层的成岩作用特征及成岩—孔隙演化过程。(3)超压在一定程度上抵抗了压实作用强度,抑制了胶结作用,促进了溶蚀作用。 相似文献
10.
鄂尔多斯盆地东部子洲—清涧地区二叠系山西组23段石英砂岩区块单井产能与砂体结构类型具有一定相关性。基于单砂体垂向粒度韵律和自然伽马测井资料,将砂体结构划分出块状型砂体、正粒序型砂体和叠加型砂体3种类型。通过铸体薄片鉴定和粒度分析实验,对3种类型砂体储集层在成岩作用类型、特征、发育程度、产物、垂向变化及成岩孔隙演化的差异性进行表征。研究表明,3类砂体的溶蚀作用、压实作用强度及分布特征存在较大差异,粒度及其垂向分布是制约不同结构类型砂体成岩演化的直接因素,进而影响储集层物性,决定了含气层与致密层的垂向分布规律。块状型砂体储集层原生孔隙、次生溶孔共同发育,含气层厚度较大,局部致密。正粒序型砂体垂向物性差异较大,中—下部储集层溶蚀程度较弱,以原生孔隙为主,次生孔隙少量发育,为含气层;上部储集层压实、胶结作用强烈,相对致密。叠加型砂体储集层形成于河流控制作用逐渐增强的过渡环境,兼有正粒序型、块状型砂体的特征,多层含气且厚度大。 相似文献
11.
Kunaljeet S. Tanwar Sarah C. Petitto Peter J. Eng 《Geochimica et cosmochimica acta》2009,73(15):4346-2587
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
12.
Sebastien Kerisit Chongxuan Liu Eugene S. Ilton 《Geochimica et cosmochimica acta》2008,72(6):1481-1497
Molecular dynamics simulations of water in contact with the (0 0 1) and (0 1 0) surfaces of orthoclase (KAlSi3O8) were carried out to investigate the structure and dynamics of the feldspar-water interface, contrast the intrinsic structural properties of the two surfaces, and provide a basis for future work on the diffusion of ions and molecules in microscopic mineral fractures. Electron density profiles were computed from the molecular dynamics trajectories and compared with those derived experimentally from high-resolution X-ray reflectivity measurements by Fenter and co-workers [Fenter P., Cheng L., Park C., Zhang H. and Sturchio N. C. (2003a) Structure of the orthoclase (0 0 1)- and (0 1 0)-water interfaces by high-resolution X-ray reflectivity. Geochim. Cosmochim. Acta67, 4267-4275]. For each surface, three scenarios were considered whereby the interfacial species is potassium, water, or a hydronium ion. Excellent agreement was obtained for the (0 0 1) surface when potassium is the predominant interfacial species; however, some discrepancies in the position of the interfacial peaks were obtained for the (0 1 0) surface. The two surfaces showed similarities in the extent of water ordering at the interface, the activation energies for water and potassium desorption, and the adsorption localization of interfacial species. However, there are also important differences between the two surfaces in the coordination of a given adsorbed species, adsorption site densities, and the propensity for water molecules in surface cavities and those in the first hydration layer to coordinate to surface bridging oxygen atoms. These differences may have implications for the extent of dissolution in the low-pH regime since hydrolysis of Si(Al)OSi(Al) bonds is a major dissolution mechanism. 相似文献
13.
In situ Atomic Force Microscopy study of dissolution of the barite (0 0 1) surface in water at 30 °C
Yoshihiro Kuwahara 《Geochimica et cosmochimica acta》2011,75(1):41-51
The dissolution behavior of the barite (0 0 1) surface in pure water at 30 °C was investigated using in situ Atomic Force Microscopy (AFM), to better understand the dissolution mechanism and the microtopographical changes that occur during the dissolution, such as steps and etch pits. The dissolution of the barite (0 0 1) surface started with the slow retreat of steps, after which, about 60 min later, the <hk0> steps of one unit cell layer or multi-layers became two-step fronts (fast “f” and slow “s” steps) with a half-unit cell layer showing different retreat rates. The “f” step had a fast retreat rate (≈(14 ± 1) × 10−2 nm/s) and tended to have a jagged step edge, whereas the “s” step (≈(1.8 ± 0.1) × 10−2 nm/s) had a relatively straight front. The formation of the “f” steps led to the formation of a new one-layer step, where the front of the “s” step was overtaken by that of the immediate underlying “f” step. The “f” steps also led to the decrease of the <hk0> steps and the increase in the percentage of stable steps parallel to the [0 1 0] direction during the dissolution.Etch pits, which could be observed after about 90 min, were of three types: triangular etch pits with a depth of a half-unit cell, shallow etch pits, and deep etch pits. The triangular etch pits were bounded by the step edges parallel to [0 1 0], [1 2 0], and [] and had opposite orientations in the upper half and lower half layers. Shallow etch pits that had a depth of two or more half-unit cell layers had any two consecutive pits pointing in the opposite direction of each other. The triangular etch pit appeared to grow by simultaneously removal of a row of ions parallel to each direction from the three step edges. At first, deep etch pits were elongated in the [0 1 0] direction with a curved outline and then gradually developed to an angular form bounded by the {1 0 0}, {3 1 0}, and (0 0 1) faces. The retreat rate of the (0 0 1) face was much slower than those of the {1 0 0} and {3 1 0} and tended to separate into two rates ((0.13 ± 0.01) × 10−2 nm/s for the deep etch pits derived from a screw dislocation and (0.07 ± 0.01) × 10−2 nm/s for those from other line defects).The changes in the dissolution rate of a barite (0 0 1) surface during the dissolution were estimated using the retreat rates and densities of the various steps as well as the growth rates, density, and areas of the lateral faces of the deep etch pits that were obtained from this AFM analysis. Our results showed that the dissolution rate of the barite (0 0 1) surface gradually increased and approached the bulk dissolution rate because of the change in the main factor determining the dissolution rate from the density of the steps to the growth and the density of the deep etch pits on the surface. 相似文献
14.
Jarosite is an important mineral on Earth, and possibly on Mars, where it controls the mobility of iron, sulfate and potentially toxic metals. Atomistic simulations have been used to study the incorporation of Al3+, and the M2+ impurities Cd, Cu and Zn, in the (0 1 2) and (0 0 1) surfaces of jarosite. The calculations show that the incorporation of Al on an Fe site is favorable on all surfaces in which terminal Fe ions are exposed, and especially on the (0 0 1) [Fe3(OH)3]6+ surface. Incorporation of Cd, Cu or Zn on a K site balanced by a K vacancy is predicted to stabilize the surfaces, but calculated endothermic solution energies and the high degree of distortion of the surfaces following incorporation suggest that these substitutions will be limited. The calculations also suggest that incorporation of Cd, Cu and Zn on an Fe site balanced by an OH vacancy, or by coupled substitution on both K and Fe sites, is unfavorable, although this might be compensated for by growth of a new layer of jarosite or goethite, as predicted for bulk jarosite. The results of the simulations show that surface structure will exert an influence on uptake of impurities in the order Cu > Cd > Zn, with the most favorable surfaces for incorporation being (0 1 2) [KFe(OH)4]0 and (0 0 1) [Fe3(OH)3]6+. 相似文献
15.
Michel L. Schlegel Kathryn. L. Nagy Paul Fenter Neil C. Sturchio 《Geochimica et cosmochimica acta》2006,70(14):3549-3565
The structure and mechanism of cation sorption at the (0 0 1) muscovite-water interface were investigated in 0.01 and 0.5 m KCl, CsCl, and CaCl2 and 0.01 m BaCl2 solutions at slightly acidic pH by high-resolution X-ray reflectivity. Structural relaxations of atom positions in the 2M1 muscovite were small (?0.07 Å) and occurred over a distance of 30 to 40 Å perpendicular to the interface. Cations in all solutions were sorbed dominantly in the first and second solution layers adjacent to the mineral surface. The derived heights of the first solution layer in KCl and CsCl solutions, 1.67(6)-1.77(7) and 2.15(9)-2.16(2) Å, respectively, differ in magnitude by the approximate difference in crystallographic radii between K and Cs, and correspond closely to the interlayer cation positions in bulk K- and Cs-mica structures. The first solution layer heights in CaCl2 and BaCl2 solutions, 2.46(5)-2.56(11) and 2.02(5) Å, respectively, differ in a sense opposite to that expected based on crystallographic or hydrated radii of the divalent cations. The derived ion heights in all solutions imply that there is no intercalated water layer between the first solution layer and the muscovite surface. Molecular compositions were assigned to the first two solution layers in the electron density profiles using models that constrain the number density of sorbed cations, water molecules, and anions by considering the permanent negative charge of the muscovite and average solution density. The models result in partial charge balance (at least 50%) by cations sorbed in the first two layers in the 0.01 m solutions and approximately full charge balance in the 0.5 m solutions. Damped oscillations of model water density away from the first two solution layers agree with previous X-ray reflectivity results on the muscovite (0 0 1) surface in pure water. 相似文献
16.
Talc is one of the weakest minerals that is associated with fault zones. Triaxial friction experiments conducted on water-saturated talc gouge at room temperature yield values of the coefficient of friction, μ (shear stress, τ/effective normal stress, σ′N) in the range 0.16–0.23, and μ increases with increasing σ′N. Talc gouge heated to temperatures of 100°–400 °C is consistently weaker than at room temperature, and μ < 0.1 at slow strain rates in some heated experiments. Talc also is characterized by inherently stable, velocity-strengthening behavior (strength increases with increasing shear rate) at all conditions tested. The low strength of talc is a consequence of its layered crystal structure and, in particular, its very weak interlayer bond. Its hydrophobic character may be responsible for the relatively small increase in μ with increasing σ′N at room temperature compared to other sheet silicates.Talc has a temperature–pressure range of stability that extends from surficial to eclogite-facies conditions, making it of potential significance in a variety of faulting environments. Talc has been identified in exhumed subduction zone thrusts, in fault gouge collected from oceanic transform and detachment faults associated with rift systems, and recently in serpentinite from the central creeping section of the San Andreas fault. Typically, talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica-saturated hydrothermal fluids. This mode of formation of talc is a prime example of a fault-zone weakening process. Because of its velocity-strengthening behavior, talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern California that are associated with ophiolitic rocks. 相似文献
17.
Jeffrey G. Catalano 《Geochimica et cosmochimica acta》2011,75(8):2062-2061
Ordering of interfacial water at the hematite and corundum (0 0 1)-water interfaces has been characterized using in situ high resolution specular X-ray reflectivity measurements. The hematite (0 0 1) surface was prepared through an annealing process to produce a surface isostructural with corundum (0 0 1), facilitating direct comparison. Interfacial water was found to display a similar structure on this pair of isostructural surfaces. A single layer of adsorbed water having a large vibrational amplitude was present on each surface and additional ordering of water extended at least 1 nm into the bulk fluid, with the degree of ordering decreasing with increasing distance from the surfaces. Consistent with prior studies of the (0 1 2) and (1 1 0) surfaces of hematite and corundum, the configuration of water above the (0 0 1) surfaces is primarily controlled by the surface structure, specifically the arrangement of surface functional groups. However, interfacial water at the (0 0 1) surfaces displayed significantly larger vibrational amplitudes throughout the interfacial region than at other isostructural sets of hematite and corundum surfaces, indicating weaker ordering. Comparison of the vibrational amplitudes of adsorbed water on a series of oxide, silicate, and phosphate mineral surfaces suggests that the presence or absence of a substantial interfacial electrostatic field is the primary control on water ordering and not the surface structure itself. On surfaces for which charge originates dominantly through protonation-deprotonation reactions the controlling factor appears to be whether conditions exist where most functional groups are uncharged as opposed to the net surface charge. The doubly coordinated functional groups on hematite and corundum (0 0 1) surfaces are largely uncharged under slightly acidic to circumneutral pH conditions, leading to weak ordering, whereas singly coordinated groups on (0 1 2) and (1 1 0) surfaces of these phases are always charged, even when the net surface charge is zero, and induce strong water ordering. Surfaces lacking structural charge can thus be divided into two distinct classes that induce either strong or weak ordering of interfacial water. Surface functional group coordination is the ultimate control on this division as it determines the charge state of such groups under different protonation configurations. Ion adsorption and electron transfer processes may differ between these classes of surfaces because of the effect of water ordering strength on interfacial capacitances and hydrogen bonding. 相似文献
18.
Adsorption of Rb+ and Sr2+ at the orthoclase (0 0 1)-solution interface is probed with high-resolution X-ray reflectivity and resonant anomalous X-ray reflectivity. Specular X-ray reflectivity data for orthoclase in contact with 0.01 m RbCl solution at pH 5.5 reveal a systematic increase in electron density adjacent to the mineral surface with respect to that observed in contact with de-ionized water (DIW). Quantitative analysis indicates that Rb+ adsorbs at a height of 0.83 ± 0.03 Å with respect to the bulk K+ site with a nominal coverage of 0.72 ± 0.10 ions per surface unit mesh (55.7 Å2). These results are consistent with an ion-exchange reaction in which Rb+ occupies an inner-sphere adsorption (IS) site. In contrast, X-ray reflectivity data for orthoclase in contact with 0.01 m Sr(NO3)2 solution at pH 5.3 reveal few significant changes with respect to DIW. Resonant anomalous X-ray reflectivity was used to probe Sr2+ adsorption and to image its vertical distribution. This element-specific measurement reveals that Sr2+ adsorbs with a total coverage of 0.37 ± 0.02 ions per surface unit mesh, at a substantially larger height (3.28 ± 0.05 Å) than found for Rb+, and with a relatively broad density distribution (having a root-mean-square width of 1.88 ± 0.08 Å for a single-peak model), implying that Sr2+ adsorbs primarily as a fully-hydrated outer-sphere (OS), species. Comparison to a two-height model suggests that 13 ± 5% of the adsorbed Sr2+ may be present as an IS species. This partitioning implies a ∼5 kJ/mol difference in free energy between the IS and OS Sr2+ on orthoclase. Differences in the partitioning of Sr2+ between IS and OS species for orthoclase (0 0 1) and muscovite (0 0 1) suggest control by the geometry of the IS adsorption site. Results for the OS distribution are compared to predictions of the Poisson-Boltzmann equation in the strong coupling regime, which predicts an intrinsically narrow vertical diffuse ion distribution; the OS distribution might thus be thought of as the diffuse ion profile in the limit of high surface charge. 相似文献
19.
Alena Kremleva 《Geochimica et cosmochimica acta》2011,75(3):706-718
We report density functional investigations of kaolinite edge surfaces and uranyl adsorption thereon. Applying periodic slab models, we studied the (0 1 0) surface of kaolinite as an example of kaolinite edge facets which are expected to be highly reactive and to adsorb preferentially metal ions. Among the four terminations of the (0 1 0) surface, we selected the two most likely ones and determined their structures to be affected by solvation. On these modified surfaces, we explored bidentate inner-sphere adsorption complexes of uranyl, at single metal center sites, Al(O,OH), and sites of mixed type, AlOH-SiO. On one of the terminations hydrolysis of uranyl was found to occur. Comparison of key calculated structure parameters with available experimental data suggests an extension of the prevailing interpretation and implies that a set of uranyl complexes may coexist on edge surfaces. 相似文献
20.
The fate of the oxoanion arsenate in diverse systems is strongly affected by its adsorption on the surfaces of iron (oxyhydr)oxide minerals. Predicting this behavior in the environment requires an understanding of the mechanisms of arsenate adsorption. In this study, the binding site and adsorption geometry of arsenate on the hematite (0 1 2) surface is investigated. The structure and termination of the hematite (0 1 2)-water interface were determined by high resolution X-ray reflectivity, revealing that two distinct terminations exist in a roughly 3:1 proportion. The occurrence of multiple terminations appears to be a result of sample preparation, and is not intrinsic to the hematite (0 1 2) surface. X-ray standing wave (XSW) measurements were used to determine the registry of adsorbed arsenate to the hematite structure, and thus the binding site and geometry of the resulting surface complex. Arsenate forms a bridging bidentate complex on two adjacent singly coordinated oxygen groups on each of the two distinct terminations present at the hematite surface. Although this geometry is consistent with that seen in past studies, the derived As-Fe distances are longer, the result of the topology of the FeO6 octahedra on the (0 1 2) surface. As EXAFS-derived As-Fe distances are often used to determine the adsorption mechanism in environmental samples (e.g., mine tailings, contaminated sediments), this demonstrates the importance of considering the possible sorbent surface structures and arrangements of adsorbates when interpreting such data.As multiple functional groups are present and multiple binding geometries are possible on the hematite (0 1 2) surface, the XSW data suggest that formation of bridging bidentate surface complexes on singly coordinated oxygen sites is the preferred adsorption mechanism on this and most other hematite surfaces (provided those surfaces contain adjacent singly coordinated oxygen groups). These measurements also constrain the likely reaction stoichiometry, with only the protonation state of the surface complex undetermined. Although bridging bidentate inner-sphere surface complexes comprised the majority of the adsorbed arsenate present on the hematite (0 1 2) surface, there is an additional population of sorbed arsenate species that could not be characterized by the XSW measurements. These species are likely more disordered, and thus more weakly bound, than the bridging bidentate complexes, and may play a role in determining the fate, transport, and bioavailability of arsenate in the environment. Finally, the possibility of obtaining species-specific XSW measurements by tuning the incident beam energy to specific features in a XANES spectrum is described. 相似文献